Network Working Group                                          M. Arango
Request for Comments: 2705                                       RSL COM
Category: Informational                                         A. Dugan
                                                              I. Elliott
                                                   Level3 Communications
                                                              C. Huitema
                                                               Telcordia
                                                              S. Pickett
                                                       Vertical Networks
                                                            October 1999

Media Gateway Control Protocol (MGCP)
Version 1.0

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

IESG NOTE:

   This document is being published for the information of the
   community.  It describes a protocol that is currently being deployed
   in a number of products.  Implementers should be aware of
   developments in the IETF Megaco Working Group and ITF-T SG16 who are
   currently working on a potential successor to this protocol.

Abstract

   This document describes an application programming interface and a
   corresponding protocol (MGCP) for controlling Voice over IP (VoIP)
   Gateways from external call control elements. MGCP assumes a call
   control architecture where the call control "intelligence" is outside
   the gateways and handled by external call control elements.

   The document is structured in 6 main sections:

   *  The introduction presents the basic assumptions and the relation
      to other protocols such as H.323, RTSP, SAP or SIP.

   *  The interface section presents a conceptual overview of the MGCP,
      presenting the naming conventions, the usage of the session
      description protocol SDP, and the procedures that compose MGCP:
      Notifications Request, Notification, Create Connection, Modify
      Connection, Delete Connection, AuditEndpoint, AuditConnection and
      RestartInProgress.

   *  The protocol description section presents the MGCP encodings,
      which are based on simple text formats, and the transmission
      procedure over UDP.

   *  The security section presents the security requirement of MGCP,
      and its usage of IP security services (IPSEC).

   *  The event packages section provides an initial definition of
      packages and event names.

   *  The description of the changes made in combining SGCP 1.1 and IPDC
      to create MGCP 1.0.

   1.  Introduction ..............................................  5
      1.1.  Relation with the H.323 standards ....................  7
      1.2.  Relation with the IETF standards .....................  8
      1.3.  Definitions ..........................................  9
   2.  Media Gateway Control Interface ...........................  9
      2.1.  Model and naming conventions. ........................ 10
         2.1.1.  Types of endpoints .............................. 10
            2.1.1.1.  Digital channel (DS0) ...................... 11
            2.1.1.2.  Analog line ................................ 11
            2.1.1.3.  Annoucement server access point ............ 12
            2.1.1.4.  Interactive Voice Response access point .... 12
            2.1.1.5.  Conference bridge access point ............. 13
            2.1.1.6.  Packet relay ............................... 13
            2.1.1.7.  Wiretap access point ....................... 14
            2.1.1.8.  ATM "trunk side" interface. ................ 14
         2.1.2.  Endpoint identifiers ............................ 15
         2.1.3.  Calls and connections ........................... 17
            2.1.3.1.  Names of calls ............................. 20
            2.1.3.2.  Names of connections ....................... 20
            2.1.3.3.  Management of resources, attributes of ..... 20
            2.1.3.4.  Special case of local connections .......... 23
         2.1.4.  Names of Call Agents and other entities ......... 23
         2.1.5.  Digit maps ...................................... 24
         2.1.6.  Names of events ................................. 26
      2.2.  Usage of SDP ......................................... 29
      2.3.  Gateway Control Commands ............................. 30

         2.3.1.  EndpointConfiguration ........................... 32
         2.3.2.  NotificationRequest ............................. 33
         2.3.3.  CreateConnection ................................ 38
         2.3.4.  ModifyConnection ................................ 44
         2.3.5.  DeleteConnection (from the Call Agent) .......... 46
         2.3.6.  DeleteConnection (from the VoIP gateway) ........ 51
         2.3.7.  DeleteConnection (multiple connections, from the  51
         2.3.8.  Audit Endpoint .................................. 52
         2.3.9.  Audit Connection ................................ 55
         2.3.10.  Restart in progress ............................ 56
      2.4.  Return codes and error codes. ........................ 58
      2.5.  Reason Codes ......................................... 61
   3.  Media Gateway Control Protocol ............................ 61
      3.1.  General description .................................. 62
      3.2.  Command Header ....................................... 62
         3.2.1.  Command line .................................... 62
            3.2.1.1.  Coding of the requested verb ............... 63
            3.2.1.2.  Transaction Identifiers .................... 63
            3.2.1.3.  Coding of the endpoint identifiers and ..... 64
            3.2.1.4.  Coding of the protocol version ............. 65
         3.2.2.  Parameter lines ................................. 65
            3.2.2.1.  Response Acknowledgement ................... 68
            3.2.2.2.  Local connection options ................... 68
            3.2.2.3.  Capabilities ............................... 70
            3.2.2.4.  Connection parameters ...................... 71
            3.2.2.5.  Reason Codes ............................... 72
            3.2.2.6.  Connection mode ............................ 73
            3.2.2.7.  Coding of event names ...................... 73
            3.2.2.8.  RequestedEvents ............................ 74
            3.2.2.9.  SignalRequests ............................. 76
            3.2.2.10.  ObservedEvent ............................. 76
            3.2.2.11.  RequestedInfo ............................. 76
            3.2.2.12.  QuarantineHandling ........................ 77
            3.2.2.13.  DetectEvents .............................. 77
            3.2.2.14.  EventStates ............................... 77
            3.2.2.15.  RestartMethod ............................. 78
            3.2.2.16.  Bearer Information ........................ 78
      3.3.  Format of response headers ........................... 78
      3.4.  Formal syntax description of the protocol ............ 81
      3.5.  Encoding of the session description .................. 86
         3.5.1.  Usage of SDP for an audio service ............... 86
         3.5.2.  Usage of SDP in a network access service ........ 87
         3.5.3.  Usage of SDP for ATM connections ................ 90
         3.5.4.  Usage of SDP for local connections .............. 91
      3.6.  Transmission over UDP ................................ 91
         3.6.1.  Providing the At-Most-Once functionality ........ 91
         3.6.2.  Transaction identifiers and three ways handshake. 92
         3.6.3.  Computing retransmission timers ................. 93

         3.6.4.  Piggy backing ................................... 94
         3.6.5.  Provisional responses ........................... 94
   4.  States, failover and race conditions. ..................... 95
      4.1.  Basic Asumptions ..................................... 95
      4.2.  Security, Retransmission, and Detection of Lost ...... 96
      4.3.  Race conditions ...................................... 99
         4.3.1.  Quarantine list ................................. 99
         4.3.2.  Explicit detection ..............................103
         4.3.3.  Ordering of commands, and treatment of disorder .104
         4.3.4.  Fighting the restart avalanche ..................105
         4.3.5.  Disconnected Endpoints ..........................107
   1.   A "disconnected" timer is initialized to a random value, .107
   2.   The gateway then waits for either the end of this timer, .107
   3.   When the "disconnected" timer elapses, when a command is .107
   4.   If the "disconnected" procedure still left the endpoint ..107
   5.  Security requirements .....................................108
      5.1.  Protection of media connections ......................109
   6.  Event packages and end point types ........................109
      6.1.  Basic packages .......................................110
         6.1.1.  Generic Media Package ...........................110
         6.1.2.  DTMF package ....................................112
         6.1.3.  MF Package ......................................113
         6.1.4.  Trunk Package ...................................114
         6.1.5.  Line Package ....................................116
         6.1.6.  Handset emulation package .......................119
         6.1.7.  RTP Package .....................................120
         6.1.8.  Network Access Server Package ...................121
         6.1.9.  Announcement Server Package .....................122
         6.1.10.  Script Package .................................122
      6.2.  Basic endpoint types and profiles ....................123
   7.  Versions and compatibility ................................124
      7.1.  Differences between version 1.0 and draft 0.5 ........124
      7.2.  Differences between draft-04 and draft-05 ............125
      7.3.  Differences between draft-03 and draft-04 ............125
      7.4.  Differences between draft-02 and draft-03 ............125
      7.5.  Differences between draft-01 and draft-02 ............126
      7.6.  The making of MGCP from IPDC and SGCP ................126
      7.7.  Changes between MGCP and initial versions of SGCP ....126
   8.  Security Considerations ...................................128
   9.  Acknowledgements ..........................................128
   10. References ................................................129
   11. Authors' Addresses ........................................130
   12. Appendix A: Proposed "MoveConnection" command .............132
      12.1.  Proposed syntax modification ........................133
   13. Full Copyright Statement ..................................134

1. Introduction

   This document describes an abstract application programming interface
   and a corresponding protocol (MGCP) for controlling Telephony
   Gateways from external call control elements called media gateway
   controllers or call agents. A telephony gateway is a network element
   that provides conversion between the audio signals carried on
   telephone circuits and data packets carried over the Internet or over
   other packet networks.  Example of gateways are:

   *  Trunking gateways, that interface between the telephone network
      and a Voice over IP network. Such gateways typically manage a
      large number of digital circuits.

   *  Voice over ATM gateways, which operate much the same way as voice
      over IP trunking gateways, except that they interface to an ATM
      network.

   *  Residential gateways, that provide a traditional analog (RJ11)
      interface to a Voice over IP network. Examples of residential
      gateways include cable modem/cable set-top boxes, xDSL devices,
      broad-band wireless devices

   *  Access gateways, that provide a traditional analog (RJ11) or
      digital PBX interface to a Voice over IP network. Examples of
      access gateways include small-scale voice over IP gateways.

   *  Business gateways, that provide a traditional digital PBX
      interface or an integrated "soft PBX" interface to a Voice over IP
      network.

   *  Network Access Servers, that can attach a "modem" to a telephone
      circuit and provide data access to the Internet. We expect that,
      in the future, the same gateways will combine Voice over IP
      services and Network Access services.

   *  Circuit switches, or packet switches, which can offer a control
      interface to an external call control element.

   MGCP assumes a call control architecture where the call control
   "intelligence" is outside the gateways and handled by external call
   control elements. The MGCP assumes that these call control elements,
   or Call Agents, will synchronize with each other to send coherent
   commands to the gateways under their control. MGCP does not define a
   mechanism for synchronizing Call Agents. MGCP is, in essence, a
   master/slave protocol, where the gateways are expected to execute
   commands sent by the Call Agents.  In consequence, this document
   specifies in great detail the expected behavior of the gateways, but

   only specify those parts of a call agent implementation, such as
   timer management, that are mandated for proper operation of the
   protocol.

   MGCP assumes a connection model where the basic constructs are
   endpoints and connections. Endpoints are sources or sinks of data and
   could be physical or virtual. Examples of physical endpoints are:

   *  An interface on a gateway that terminates a trunk connected to a
      PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that
      terminates trunks is called a trunk gateway.

   *  An interface on a gateway that terminates an analog POTS
      connection to a phone, key system, PBX, etc. A gateway that
      terminates residential POTS lines (to phones) is called a
      residential gateway.

   An example of a virtual endpoint is an audio source in an audio-
   content server. Creation of physical endpoints requires hardware
   installation, while creation of virtual endpoints can be done by
   software.

   Connections may be either point to point or multipoint. A point to
   point connection is an association between two endpoints with the
   purpose of transmitting data between these endpoints. Once this
   association is established for both endpoints, data transfer between
   these endpoints can take place. A multipoint connection is
   established by connecting the endpoint to a multipoint session.

   Connections can be established over several types of bearer networks:

   *  Transmission of audio packets using RTP and UDP over a TCP/IP
      network.

   *  Transmission of audio packets using AAL2, or another adaptation
      layer, over an ATM network.

   *  Transmission of packets over an internal connection, for example
      the TDM backplane or the interconnection bus of a gateway. This is
      used, in particular, for "hairpin" connections, connections that
      terminate in a gateway but are immediately rerouted over the
      telephone network.

   For point-to-point connections the endpoints of a connection could be
   in separate gateways or in the same gateway.

1.1. Relation with the H.323 standards

1.2. Relation with the IETF standards

   While H.323 is the recognized standard for VoIP terminals, the IETF
   has also produced specifications for other types of multi-media
   applications. These other specifications include:

   *  the Session Description Protocol (SDP), RFC 2327,

   *  the Session Announcement Protocol (SAP),

   *  the Session Initiation Protocol (SIP),

   *  the Real Time Streaming Protocol (RTSP), RFC 2326.

   The latter three specifications are in fact alternative signaling
   standards that allow for the transmission of a session description to
   an interested party. SAP is used by multicast session managers to
   distribute a multicast session description to a large group of
   recipients, SIP is used to invite an individual user to take part in
   a point-to-point or unicast session, RTSP is used to interface a
   server that provides real time data. In all three cases, the session
   description is described according to SDP; when audio is transmitted,
   it is transmitted through the Real-time Transport Protocol, RTP.

   The distributed gateway systems and MGCP will enable PSTN telephony
   users to access sessions set up using SAP, SIP or RTSP. The Call
   Agent provides for signaling conversion, according to the following
   table:

1.3. Definitions

Trunk: A communication channel between two switching systems. E.g., a
DS0 on a T1 or E1 line.

2. Media Gateway Control Interface

   The interface functions provide for connection control and endpoint
   control. Both use the same system model and the same naming
   conventions.

2.1. Model and naming conventions

   The MGCP assumes a connection model where the basic constructs are
   endpoints and connections. Connections are grouped in calls. One or
   more connections can belong to one call. Connections and calls are
   set up at the initiative of one or several Call Agents.

2.1.1. Types of endpoints

   In the introduction, we presented several classes of gateways.  Such
   classifications, however, can be misleading.  Manufacturers can
   arbitrarily decide to provide several types of services in a single
   packaging.  A single product could well, for example, provide some
   trunk connections to telephony switches, some primary rate
   connections and some analog line interfaces, thus sharing the
   characteristics of what we described in the introduction as
   "trunking", "access" and "residential" gateways.   MGCP does not make
   assumptions about such groupings.  We simply assume that media
   gateways support collections of endpoints.  The type of the endpoint
   determines its functionalities. Our analysis, so far, has led us to
   isolate the following basic endpoint types:

   *    Digital channel (DS0),

   *    Analog line,

   *    Annoucement server access point,

   *    Interactive Voice Response access point,

   *    Conference bridge access point,

   *    Packet relay,

   *    Wiretap access point,

   *    ATM "trunk side" interface.

   In this section, we will develop the expected behavior of such end
   points.

   This list is not limitative.  There may be other types of endpoints
   defined in the future, for example test endpoint that could be used
   to check network quality, or frame-relay endpoints that could be used
   to managed audio channels multiplexed over a frame-relay virtual
   circuit.

2.1.1.1. Digital channel (DS0)

   Digital channels provide an 8Khz*8bit service.  Such channels are
   found in trunk and ISDN interfaces.  They are typically part of
   digital multiplexes, such as T1, E1, T3 or E3 interfaces. Media
   gateways that support such channels are capable of translating the
   digital signals received on the channel, which may be encoded
   according to A or mu-law, using either the complete set of 8 bits or
   only 7 of these bits, into audio packets.  When the media gateway
   also supports a NAS service, the gateway shall be capable of
   receiving either audio-encoded data (modem connection) or binary data
   (ISDN connection) and convert them into data packets.

                                         +-------
                           +------------+|
              (channel) ===|DS0 endpoint| -------- Connections
                           +------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint support is a characteristic of the
   gateway, and may in fact vary according with the allocation of
   resource within the gateway.

   In some cases, digital channels are used to carry signalling.  This
   is the case for example of SS7 "F" links, or ISDN "D" channels.
   Media gateways that support these signalling functions shall be able
   to send and receive the signalling packets to and from a call agent,
   using the "back haul" procedures defined by the SIGTRAN working group
   of the IETF.  Digital channels are sometimes used in conjunction with
   channel associated signalling, such as "MF R2".  Media gateways that
   support these signalling functions shall be able to detect and
   produce the corresponding signals, such as for example "wink" or "A",
   according to the event signalling and reporting procedures defined in
   MGCP.

2.1.1.2. Analog line

   Analog lines can be used either as a "client" interface, providing
   service to a classic telephone unit, or as a "service" interface,
   allowing the gateway to send and receive analog calls.  When the
   media gateway also supports a NAS service, the gateway shall be
   capable of receiving audio-encoded data (modem connection) and
   convert them into data packets.

                                         +-------
                        +---------------+|
              (line) ===|analog endpoint| -------- Connections
                        +---------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The audio signals
   originating from these connections shall be mixed according to the
   connection "mode", as specified later in this document.  The precise
   number of connections that an endpoint support is a characteristic of
   the gateway, and may in fact vary according with the allocation of
   resource within the gateway.  A typical gateway should however be
   able to support two or three connections per endpoint, in order to
   provide services such as "call waiting" or "three ways calling".

2.1.1.3. Annoucement server access point

   An announcement server endpoint provides acces to an announcement
   service. Under requests from the call agent, the announcement server
   will "play" a specified announcement.  The requests from the call
   agent will follow the event signalling and reporting procedures
   defined in MGCP.

             +----------------------+
             | Announcement endpoint| -------- Connection
             +----------------------+

   A given announcement endpoint is not supposed to support more than
   one connection at a time. If several connections were established to
   the same endpoint, then the same announcements would be played
   simultaneously over all the connections.

   Connections to an announcement server are typically oneway, or "half
   duplex" -- the announcement server is not expected to listen the
   audio signals from the connection.

2.1.1.4. Interactive Voice Response access point

   An Interactive Voice Response (IVR) endpoint provides acces to an IVR
   service. Under requests from the call agent, the IVR server will
   "play" announcements and tones, and will "listen" to responses from
   the user.  The requests from the call agent will follow the event
   signalling and reporting procedures defined in MGCP.

                      +-------------+
                      | IVR endpoint| -------- Connection
                      +-------------+

   A given IVR endpoint is not supposed to support more than one
   connection at a time. If several connections were established to the
   same endpoint, then the same tones and announcements would be played
   simultaneously over all the connections.

2.1.1.5. Conference bridge access point

   A conference bridge endpoint is used to provide access to a specific
   conference.

                                         +-------
             +--------------------------+|
             |Conference bridge endpoint| -------- Connections
             +--------------------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document. The precise number of
   connections that an endpoint support is a characteristic of the
   gateway, and may in fact vary according with the allocation of
   resource within the gateway.

2.1.1.6. Packet relay

   A packet relay endpoint is a specific form of conference bridge, that
   typically only supports two connections.  Packets relays can be found
   in firewalls between a protected and an open network, or in
   transcoding servers used to provide interoperation between
   incompatible gateways, for example gateways that do not support
   compatible compression algorithms, or gateways that operate over
   different transmission networks such as IP and ATM.

                                          +-------
                  +---------------------+ |
                  |Packet relay endpoint|  2 connections
                  +---------------------+ |
                                          +-------

2.1.1.7. Wiretap access point

   A wiretap access point provides access to a wiretap service,
   providing either a recording or a life playback of a connection.

                  +-----------------+
                  | Wiretap endpoint| -------- Connection
                  +-----------------+

   A given wiretap endpoint is not supposed to support more than one
   connection at a time. If several connections were established to the
   same endpoint, then the recording or playback would mix the audio
   signals received on this connections.

   Connections to an wiretap endpoint are typically oneway, or "half
   duplex" -- the wiretap server is not expected to signal its presence
   in a call.

2.1.1.8. ATM "trunk side" interface.

   ATM "trunk side" endpoints are typically found when one or several
   ATM permanent virtual circuits are used as a replacement for the
   classic "TDM" trunks linking switches.  When ATM/AAL2 is used,
   several trunks or channels are multiplexed on a single virtual
   circuit; each of these trunks correspond to a single endpoint.

                                         +-------
                     +------------------+|
         (channel) = |ATM trunk endpoint| -------- Connections
                     +------------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint support is a characteristic of the
   gateway, and may in fact vary according with the allocation of
   resource within the gateway.

2.1.2. Endpoint identifiers

   Endpoints identifiers have two components that both are case
   insensitive:

   *  the domain name of the gateway that is managing the endpoint,

   *  a local name within that gateway,

   The syntax of the local name depends on the type of endpoint being
   named. However, the local name for each of these types is naturally
   hierarchical, beginning with a term which identifies the physical
   gateway containing the given endpoint and ending in a term which
   specifies the individual endpoint concerned. With this in mind,  the
   following rules for construction and interpretation of the Entity
   Name field for these entity types MUST be supported:

   1) The individual terms of the naming path MUST be separated by a
      single slash ("/", ASCII 2F hex).

   2) The individual terms are character strings composed of letters,
      digits or other printable characters, with the exception of
      characters used as delimitors ("/", "@"), characters used for
      wildcarding ("*", "$") and white spaces.

   3) Wild-carding is represented either by an asterisk ("*") or a
      dollar sign ("$") for the terms of the naming path which are to be
      wild-carded. Thus, if the full naming path looks like

             term1/term2/term3

      then the Entity Name field looks like this depending on which
      terms are wild-carded:

             */term2/term3 if term1 is wild-carded
             term1/*/term3 if term2 is wild-carded
             term1/term2/* if term3 is wild-carded
             term1/*/* if term2 and term3 are wild-carded,
              etc.

      In each of these examples a dollar sign could have appeared
      instead of an asterisk.

   4) A term represented by an asterisk is to be interpreted as: "use
      ALL values of this term known within the scope of the Media
      Gateway".  A term represented by a dollar sign is to be
      interpreted as: "use ANY ONE value of this term known within the
      scope of the Media Gateway".  The description of a specific
      command may add further criteria for selection within the general
      rules given here.

   If the Media Gateway controls multiple physical gateways, the first
   term of the naming MUST identify the physical gateway containing the
   desired entity.  If the Media Gateway controls only a single physical
   gateway, the first term of the naming string MAY identify that
   physical gateway, depending on local practice.  A local name that is
   composed of only a wildcard character refers to either all (*) or any
   ($) endpoints within the media gateway.

   In the case of trunking gateways, endpoints are trunk circuits
   linking a gateway to a telephone switch. These circuits are typically
   grouped into a digital multiplex, that is connected to the gateway by
   a physical interface. Such circuits are named in three contexts:

   *  In the ISUP protocol, trunks are grouped into trunk groups,
      identified by the SS7 point codes of the switches that the group
      connects. Circuits within a trunk group are identified by a
      circuit number (CIC in ISUP).

   *  In the gateway configuration files, physical interfaces are
      typically identified by the name of the interface, an arbitrary
      text string. When the interface multiplexes several circuits,
      individual circuits are typically identified by a circuit number.

   *  In MGCP, the endpoints are identified by an endpoint identifier.

   The Call Agents use configuration databases to map ranges of circuit
   numbers within an ISUP trunk group to corresponding ranges of
   circuits in a multiplex connected to a gateway through a physical
   interface. The gateway will be identified, in MGCP, by a domain name.
   The local name will be structured to encode both the name of the
   physical interface, for example X35V3+A4, and the circuit number
   within the multiplex connected to the interface, for example 13. The
   circuit number will be separated from the name of the interface by a
   fraction bar, as in:

        X35V3+A4/13

   Other types of endpoints will use different conventions. For example,
   in gateways were physical interfaces by construction only control one
   circuit, the circuit number will be omitted. The exact syntax of such
   names should be specified in the corresponding server specification.

2.1.3. Calls and connections

   Connections are created on the call agent on each endpoint that will
   be involved in the "call."  In the classic example of a connection
   between two "DS0" endpoints (EP1 and EP2), the call agents
   controlling the end points will establish two connections (C1 and
   C2):

                 +---+                            +---+
   (channel1) ===|EP1|--(C1)--...        ...(C2)--|EP2|===(channel2)
                 +---+                            +---+

   Each connection will be designated locally by a connection
   identifier, and will be characterized by connection attributes.

   When the two endpoints are located on gateways that are managed by
   the same call agent, the creation is done via the three following
   steps:

   1) The call agent asks the first gateway to "create a connection" on
      the first endpoint.  The gateway allocates resources to that
      connection, and respond to the command by providing a "session
      description."  The session description contains the information
      necessary for a third party to send packets towards the newly
      created connection, such as for example IP address, UDP port, and
      packetization parameters.

   2) The call agent then asks the second gateway to "create a
      connection" on the second endpoint.  The command carries the
      "session description" provided by the first gateway. The gateway
      allocates resources to that connection, and respond to the command
      by providing its own "session description."

   3) The call agent uses a "modify connection" command to provide this
      second "session description" to the first endpoint.  Once this is
      done, communication can proceed in both directions.

   When the two endpoints are located on gateways that are managed by
   the different call agents, these two call agents shall exchange
   information through a call-agent to call-agent signalling protocol,
   in order to synchronize the creation of the connection on the two
   endpoints.

   Once established, the connection parameters can be modified at any
   time by a "modify connection" command.  The call agent may for
   example instruct the gateway to change the compression algorithm used
   on a connection, or to modify the IP address and UDP port to which
   data should be sent, if a connection is "redirected."

   The call agent removes a connection by sending to the gateway a
   "delete connection" command.  The gateway may also, under some
   circumstances, inform a gateway that a connection could not be
   sustained.

   The following diagram provides a view of the states of a connection,
   as seen from the gateway:

             Create connection
                received
                    |
                    V
           +-------------------+
           |resource allocation|-(failed)-+
           +-------------------+          |
                    |           (connection refused)
              (successful)
                    |
                    v
       +----------->+
       |            |
       |   +-------------------+
       |   |  remote session   |
       |   |   description     |----------(yes)--------+
       |   |    available ?    |                       |
       |   +-------------------+                       |
       |            |                                  |
       |          (no)                                 |
       |            |                                  |
       |      +-----------+                         +------+
       | +--->| half open |------> Delete   <-------| open |<----------+
       | |    |  (wait)   |      Connection         |(wait)|           |
       | |    +-----------+       received          +------+           |
       | |          |                 |              |                 |
       | |   Modify Connection        |         Modify Connection      |
       | |      received              |            received            |
       | |          |                 |                |               |
       | | +--------------------+     |       +--------------------+   |
       | | |assess modification |     |       |assess modification |   |
       | | +--------------------+     |       +--------------------+   |
       | |    |             |         |          |             |       |
       | |(failed)     (successful)   |      (failed)     (successful) |
       | |    |             |         |          |             |       |
       | +<---+             |         |          +-------------+-------+
       |                    |         |
       +<-------------------+         |
                                      |
                             +-----------------+
                             | Free connection |
                             | resources.      |
                             | Report.         |
                             +-----------------+
                                      |
                                      V

2.1.3.1. Names of calls

   One of the attributes of each connection is the "call identifier."

   Calls are identified by unique identifiers, independent of the
   underlying platforms or agents. These identifiers are created by the
   Call Agent. They are treated in MGCP as unstructured octet strings.

   Call identifiers are expected to be unique within the system, or at a
   minimum, unique within the collection of Call Agents that control the
   same gateways. When a Call Agent builds several connections that
   pertain to the same call, either on the same gateway or in different
   gateways, these connections that belong to the same call share the
   same call-id.  This identifier can then be used by accounting or
   management procedures, which are outside the scope of MGCP.

2.1.3.2. Names of connections

   Connection identifiers are created by the gateway when it is
   requested to create a connection. They identify the connection within
   the context of an endpoint. They are treated in MGCP as unstructured
   octet strings.  The gateway should make sure that a proper waiting
   period, at least 3 minutes, elapses between the end of a connection
   that used this identifier and its use in a new connection for the
   same endpoint.  (Gateways may decide to use identifiers that are
   unique within the context of the gateway.)

2.1.3.3. Management of resources, attributes of connections

   Many types of resources will be associated to a connection, such as
   specific signal processing functions or packetization functions.
   Generally, these resources fall in two categories:

   1) Externally visible resources, that affect the format of "the bits
      on the network" and must be communicated to the second endpoint
      involved in the connection.

   2) Internal resources, that determine which signal is being sent over
      the connection and how the received signals are processed by the
      endpoint.

   The resources allocated to a connection, and more generally the
   handling of the connection, are chosen by the gateway under
   instructions from the call agent.  The call agent will provide these
   instructions by sending two set of parameters to the gateway:

   1) The local directives instruct the gateway on the choice of
      resources that should be used for a connection,

   2) When available, the "session description" provided by the other
      end of the connection.

   The local directives specify such parameters as the mode of the
   connection (e.g. send only, send-receive), preferred coding or
   packetization methods, usage of echo cancellation or silence
   suppression.  (A detailed list can be found in the specification of
   the LocalConnectionOptions parameter of the CreateConnection
   command.) For each of these parameters, the call agent can either
   specify a value, a range of value, or no value at all.  This allow
   various implementations to implement various level of control, from a
   very tight control where the call agent specifies minute details of
   the connection handling to a very loose control where the call agent
   only specifies broad guidelines, such as the maximum bandwidth, and
   let the gateway choose the detailed values.

   Based on the value of the local directives, the gateway will
   determine the resources allocated to the connection.  When this is
   possible, the gateway will choose values that are in line with the
   remote session description - but there is no absolute requirement
   that the parameters be exactly the same.

   Once the resource have been allocated, the gateway will compose a
   "session description" that describes the way it intends to receive
   packets.  Note that the session description may in some cases present
   a range of values.  For example, if the gateway is ready to accept
   one of several compression algorithm, it can provide a list of these
   accepted algorithms.

2.1.3.4. Special case of local connections

   Large gateways include a large number of endpoints which are often of
   different types.  In some networks, we may often have to set-up
   connections between endpoints that are located within the same
   gateway.  Examples of such connections may be:

   *  Connecting a trunk line to a wiretap device,

   *  Connecting a call to an Interactive Voice-Response unit,

   *  Connecting a call to a Conferencing unit,

   *  Routing a call from on endpoint to another, something often
      described as a "hairpin" connection.

   Local connections are much simpler to establish than network
   connections. In most cases, the connection will be established
   through some local interconnecting device, such as for example a TDM
   bus.

   When two endpoints are managed by the same gateway, it is possible to
   specify the connection in a single command that conveys the name of
   the two endpoints that will be connected.  The command is essentially
   a "Create Connection" command which includes the name of the second
   endpoint in lieu of the "remote session description."

2.1.4. Names of Call Agents and other entities

   The media gateway control protocol has been designed to allow the
   implementation of redundant Call Agents, for enhanced network
   reliability.  This means that there is no fixed binding between
   entities and hardware platforms or network interfaces.

   Reliability can be improved by the following precautions:

   *  Entities such as endpoints or Call Agents are identified by their
      domain name, not their network addresses. Several addresses can be
      associated with a domain name. If a command or a response cannot
      be forwarded to one of the network addresses, implementations
      should retry the transmission using another address.

   *  Entities may move to another platform. The association between a
      logical name (domain name) and the actual platform are kept in the
      domain name service. Call Agents and Gateways should keep track of
      the time-to-live of the record they read from the DNS. They should
      query the DNS to refresh the information if the time to live has
      expired.

   In addition to the indirection provided by the use of domain names
   and the DNS, the concept of "notified entity" is central to
   reliability and fail-over in MGCP. The "notified entity" for an
   endpoint is the Call Agent currently controlling that endpoint. At
   any point in time, an endpoint has one, and only one, "notified
   entity" associated with it, and when the endpoint needs to send a
   command to the Call Agent, it MUST send the command to the current
   "notified entity" for which endpoint(s) the command pertains. Upon
   startup, the "notified entity" MUST be set to a provisioned value.
   Most commands sent by the Call Agent include the ability to
   explicitly name the "notified entity" through the use of a
   "NotifiedEntity" parameter. The "notified entity" will stay the same
   until either a new "NotifiedEntity" parameter is received or the
   endpoint reboots. If the "notified entity" for an endpoint is empty
   or has not been set explicitly, the "notified entity" will then
   default to the source address of the last connection handling command
   or notification request received for the endpoint. Auditing will thus
   not change the "notified entity."

2.1.5. Digit maps

   The Call Agent can ask the gateway to collect digits dialed by the
   user.  This facility is intended to be used with residential gateways
   to collect the numbers that a user dials; it may also be used with
   trunking gateways and access gateways alike, to collect the access
   codes, credit card numbers and other numbers requested by call
   control services.

   An alternative procedure is for the gateway to notify the Call Agent
   of the dialed digits, as soon as they are dialed. However, such a
   procedure generates a large number of interactions. It is preferable
   to accumulate the dialed numbers in a buffer, and to transmit them in
   a single message.

   The problem with this accumulation approach, however, is that it is
   hard for the gateway to predict how many numbers it needs to
   accumulate before transmission. For example, using the phone on our
   desk, we can dial the following numbers:

        _______________________________________________________
       |  0                     |  Local operator             |
       |  00                    |  Long distance operator     |
       |  xxxx                  |  Local extension number     |
       |  8xxxxxxx              |  Local number               |
       |  #xxxxxxx              |  Shortcut to local number at|
       |                        |  other corporate sites      |
       |  *xx                   |  Star services              |
       |  91xxxxxxxxxx          |  Long distance number       |
       |  9011 + up to 15 digits|  International number       |
       |________________________|_____________________________|

   The solution to this problem is to load the gateway with a digit map
   that correspond to the dial plan. This digit map is expressed using a
   syntax derived from the Unix system command, egrep. For example, the
   dial plan described above results in the following digit map:

      (0T| 00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)

   The formal syntax of the digit map is described by the DigitMap rule
   in the formal syntax description of the protocol (section 3.4).  A
   Digit-Map, according to this syntax, is defined either by a "string"
   or by a list of strings. Each string in the list is an alternative
   numbering scheme, specified either as a set of digits or timers, or
   as regular expression. A gateway that detects digits, letters or
   timers will:

   1) Add the event parameter code as a token to the end of an internal
      state variable called the "current dial string"

   2) Apply the current dial string to the digit map table, attempting a
      match to each regular expression in the Digit Map in lexical order

   3) If the result is under-qualified (partially matches at least one
      entry in the digit map), do nothing further.

   If the result matches, or is over-qualified (i.e. no further digits
   could possibly produce a match), send the current digit string to the
   Call Agent. A match, in this specification, can be either a "perfect
   match," exactly matching one of the specified alternatives, or an
   impossible match, which occur when the dial string does not match any
   of the alternative. Unexpected timers, for example, can cause
   "impossible matches."  Both perfect matches and impossible matches
   trigger notification of the accumulated digits.

   Digit maps are provided to the gateway by the Call Agent, whenever
   the Call Agent instructs the gateway to listen for digits.

2.1.6. Names of events

   The concept of events and signals is central to MGCP. A Call Agent
   may ask to be notified about certain events occurring in an endpoint,
   e.g.  off-hook events, and a call agent may request certain signals
   to be applied to an endpoint, e.g. dial-tone.

   Events and signals are grouped in packages within which they share
   the same namespace which we will refer to as event names in the
   following.  Packages are groupings of the events and signals
   supported by a particular type of endpoint. For instance, one package
   may support a certain group of events and signals for analog access
   lines, and another package may support another group of events and
   signals for video lines. One or more packages may exist for a given
   endpoint-type.

   Event names are case insensitive and are composed of two logical
   parts, a package name and an event name. Both names are strings of
   letters, hyphens and digits, with the restriction that hyphens shall
   never be the first or last characters in a name. Package or event
   names are not case sensitive - values such as "hu", "Hu", "HU" or
   "hU" should be considered equal.

   Examples of package names are "D" (DTMF), "M" (MF), "T" (Trunk) or
   "L" (Line). Examples of event names can be "hu" (off hook or "hang-
   up" transition), "hf" (flash hook) or "0" (the digit zero).

   In textual representations, the package name, when present, is
   separated from the event name by a slash ("/").  The package name is
   in fact optional. Each endpoint-type has a default package associated
   with it, and if the package name is excluded from the event name, the
   default package name for that endpoint-type is assumed. For example,
   for an analog access line, the following two event names are equal:

   l/dl dial-tone in the line package for an analog access line.

   dl   dial-tone in the line package (default) for an analog access
        line.

   This document defines a basic set of package names and event names.
   Additional package names and event names can be registered with the
   IANA. A package definition shall define the name of the package, and
   the definition of each event belonging to the package. The event
   definition shall include the precise name of the event (i.e., the
   code used in MGCP), a plain text definition of the event, and, when
   appropriate, the precise definition of the corresponding signals, for
   example the exact frequencies of audio signal such as dial tones or
   DTMF tones.

   In addition, implementers can gain experience by using experimental
   packages. The names of experimental packages must start with the two
   characters "x-"; the IANA shall not register package names that start
   with these characters.

   Digits, or letters, are supported in many packages, notably "DTMF"
   and "MF". Digits and letters are defined by the rules "Digit" and
   "Letter" in the definition of digit maps. This definition refers to
   the digits (0 to 9), to the asterisk or star ("*") and orthotrope,
   number or pound sign ("#"), and to the letters "A", "B", "C" and "D",
   as well as the timer indication "T". These letters can be combined in
   "digit string" that represent the keys that a user punched on a dial.
   In addition, the letter "X" can be used to represent all digits, and
   the sign "$" can be used in wildcard notations. The need to easily
   express the digit strings has a consequence on the form of event
   names:

     An event name that does not denote a digit should always contain at
     least one character that is neither a digit, nor one of the letters
     A, B, C, D, T or X. (Such names should not contain the special
     signs "*", "#", "/" or "$".)

   A Call Agent may often have to ask a gateway to detect a group of
   events. Two conventions can be used to denote such groups:

   *  The wildcard convention can be used to detect any event belonging
      to a package, or a given event in many packages, or event any
      event in any package supported by the gateway.

   *  The regular expression Range notation can be used to detect a
      range of digits.

   The star sign (*) can be used as a wildcard instead of a package
   name, and the keyword "all" can be used as a wildcard instead of an
   event name:

     A name such as "foo/all" denotes all events in package "foo"
     A name such as "*/bar" denotes the event "bar" in any package
     supported by the gateway
     The names "*" or "*/all" denote all events supported by the
     gate way.

   The call agent can ask a gateway to detect a set of digits or letters
   either by individually describing those letters, or by using the
   "range" notation defined in the syntax of digit strings. For example,
   the call agent can:

     Use the letter "x" to denote "any letter or digit."
     Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
     sign.

   In some cases, Call Agents will request the gateway to generate or
   detect events on connections rather than on the end point itself.
   For example, gateways may be asked to provide a ringback tone on a
   connection.  When an event shall be applied on a connection, the name
   of the connection is added to the name of the event, using an "at"
   sign (@) as a delimiter, as in:

     G/rt@0A3F58

   The wildcard character "*" (star) can be used to denote "all
   connections". When this convention is used, the gateway will generate
   or detect the event on all the connections that are connected to the
   endpoint. An example of this convention could be:

     R/qa@*

   The wildcard character "$" can be used to denote "the current
   connection." It should only be used by the call agent, when the event
   notification request is "encapsulated" within a command creation or
   modification command. When this convention is used, the gateway will
   generate or detect the event on the connection that is currently
   being created or modified. An example of this convention is:

     G/rt@$

   The connection id, or a wildcard replacement, can be used in
   conjunction with the "all packages" and "all events" conventions.
   For example, the notation:

     */all@*

   can be used to designate all events on all connections.

   Events and signals are described in packages. The package description
   must provide, for each events, the following informations:

   *  The description of the event and its purpose, which should mean
      the actual signal that is generated by the client (i.e., xx ms FSK
      tone) as well as the resulting user observed result (i.e., MW
      light on/off).

   *  The detailed characteristics of the event, such as for example
      frequencies and amplitude of audio signals, modulations and
      repetitions,

   *  The typical and maximum duration of the event.

   Signals are divided into different types depending on their behavior:

   *  On/off (OO) Once applied, these signals last forever until they
      are turned off.  This may happen either as the result of an event
      or a new SignalRequests (see later).

   *  Time-out (TO) Once applied, these signals last until they are
      either turned off (by an event or SignalRequests) or a signal
      specific period of time has elapsed. Depending on package
      specifications, a signal that times out may generate an "operation
      complete" event.

   *  Brief (BR) The duration of these signals is so short, that they
      stop on their own. If an event occurs the signal will not stop,
      however if a new SignalRequests is applied, the signal will stop.
      (Note: this point should be debated.  One could make a case that
      events such as strings of DTMF digits should in fact be allowed to
      complete.)

      TO signals are normally used to alert the endpoints' users, to
      signal them that they are expected to perform a specific action,
      such as hang down the phone (ringing). Transmission of these
      signals should typically be interrupted as soon as the first of
      the requested events has been produced.

      Package descriptions should describe, for all signals, their type
      (OO, TO, BR). They should also describe the maximum duration of
      the TO signals.

2.2. Usage of SDP

   The Call Agent uses the MGCP to provision the gateways with the
   description of connection parameters such as IP addresses, UDP port
   and RTP profiles. These descriptions will follow the conventions
   delineated in the Session Description Protocol which is now an IETF
   proposed standard, documented in RFC 2327.

   SDP allows for description of multimedia conferences. This version
   limits SDP usage to the setting of audio circuits and data access
   circuits.  The initial session descriptions contain the description
   of exactly one media, of type "audio" for audio connections, "nas"
   for data access.

2.3. Gateway Control Commands

   This section describes the commands of the MGCP. The service consists
   of connection handling and endpoint handling commands. There are nine
   commands in the protocol:

   *  The Call Agent can issue an EndpointConfiguration command to a
      gateway, instructing the gateway about the coding characteristics
      expected by the "line-side" of the endpoint.

   *  The Call Agent can issue a NotificationRequest command to a
      gateway, instructing the gateway to watch for specific events such
      as hook actions or DTMF tones on a specified endpoint .

   *  The gateway will then use the Notify command to inform the Call
      Agent when the requested events occur.

   *  The Call Agent can use the CreateConnection command to create a
      connection that terminates in an "endpoint" inside the gateway.

   *  The Call Agent can use the ModifyConnection command to change the
      parameters associated to a previously established connection.

   *  The Call Agent can use the DeleteConnection command to delete an
      existing connection. The DeleteConnection command may also be used
      by a gateway to indicate that a connection can no longer be
      sustained.

   *  The Call Agent can use the AuditEndpoint and AuditConnection
      commands to audit the status of an "endpoint" and any connections
      associated with it. Network management beyond the capabilities
      provided by these commands are generally desirable, e.g.
      information about the status of the gateway. Such capabilities are
      expected to be supported by the use of the Simple Network
      Management Protocol (SNMP) and definition of a MIB which is
      outside the scope of this specification.

   *  The Gateway can use the RestartInProgress command to notify the
      Call Agent that the gateway, or a group of endpoints managed by
      the gateway, is being taken out of service or is being placed back
      in service.

   These services allow a controller (normally, the Call Agent) to
   instruct a gateway on the creation of connections that terminate in
   an "endpoint" attached to the gateway, and to be informed about
   events occurring at the endpoint. An endpoint may be for example:

   *  A specific trunk circuit, within a trunk group terminating in a
      gateway,

   *  A specific announcement handled by an announcement server.

   Connections are grouped into "calls". Several connections, that may
   or may not belong to the same call, can terminate in the same
   endpoint .  Each connection is qualified by a "mode" parameter, which
   can be set to "send only" (sendonly), "receive only" (recvonly),
   "send/receive" (sendrecv), "conference" (confrnce), "data",
   "inactive" (inactive), "loopback", "continuity test" (conttest),
   "network loop back" (netwloop) or "network continuity test"
   (netwtest).

   The handling of the audio signals received on these connections is
   determined by the mode parameters:

   *  Audio signals received in data packets through connections in
      "receive", "conference" or "send/receive" mode are mixed and sent
      to the endpoint.

   *  Audio signals originating from the endpoint are transmitted over
      all the connections whose mode is "send", "conference" or
      "send/receive."

   *  In addition to being sent to the endpoint, audio signals received
      in data packets through connections in "conference" mode are
      replicated to all the other connections whose mode is
      "conference."

   The "loopback" and "continuity test" modes are used during
   maintenance and continuity test operations. There are two flavors of
   continuity test, one specified by ITU and one used in the US. In the
   first case, the test is a loopback test. The originating switch will
   send a tone (the go tone) on the bearer circuit and expect the
   terminating switch to loopback the circuit. If the originating switch
   sees the same tone returned (the return tone), the COT has passed. If
   not, the COT has failed. In the second case, the go and return tones
   are different. The originating switch sends a certain go tone. The
   terminating switch detects the go tone, it asserts a different return
   tone in the backwards direction. When the originating switch detects
   the return tone, the COT is passed. If the originating switch never
   detects the return tone, the COT has failed.

   If the mode is set to "loopback", the gateway is expected to return
   the incoming signal from the endpoint back into that same endpoint.
   This procedure will be used, typically, for testing the continuity of
   trunk circuits according to the ITU specifications.

   If the mode is set to "continuity test", the gateway is informed that
   the other end of the circuit has initiated a continuity test
   procedure according to the GR specification. The gateway will place
   the circuit in the transponder mode required for dual-tone continuity
   tests.

   If the mode is set to "network loopback", the audio signals received
   from the connection will be echoed back on the same connection.

   If the mode is set to "network continuity test", the gateway will
   process the packets received from the connection according to the
   transponder mode required for dual-tone continuity test, and send the
   processed signal back on the connection.

2.3.1. EndpointConfiguration

   The EndpointConfiguration commands are used to specify the encoding
   of the signals that will be received by the endpoint.  For example,
   in certain international telephony configurations, some calls will
   carry mu-law encoded audio signals, while other will use A-law.  The
   Call Agent will use the EndpointConfiguration command to pass this
   information to the gateway. The configuration may vary on a call by
   call basis, but can also be used in the absence of any connection.

           ReturnCode
           <-- EndpointConfiguration( EndpointId,
                                      BearerInformation)

   EndpointId is the name for the endpoint in the gateway where
   EndpointConfiguration executes, as defined in section 2.1.1.  The
   "any of" wildcard convention shall not be used.  If the "all of"
   wildcard convention is used, the command applies to all the endpoint
   whose name matches the wildcard.

   BearerInformation is a parameter defining the coding of the data
   received from the line side.  These information is encoded as a list
   of sub-parameters.  The only sub-parameter defined in this version of
   the specification is the encoding method, whose values can be set to
   "A-law" and "mu-law".

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.2. NotificationRequest

   The NotificationRequest commands are used to request the gateway to
   send notifications upon the occurrence of specified events in an
   endpoint.  For example, a notification may be requested for when a
   gateway detects that an endpoint is receiving tones associated with
   fax communication.  The entity receiving this notification may decide
   to use a different type of encoding method in the connections bound
   to this endpoint.

       ReturnCode
       <-- NotificationRequest( EndpointId,
                                [NotifiedEntity,]
                                [RequestedEvents,]
                                RequestIdentifier,
                                [DigitMap,]
                                [SignalRequests,]
                                [QuarantineHandling,]
                                [DetectEvents,]
                                [encapsulated EndpointConfiguration])

   EndpointId is the name for the endpoint in the gateway where
   NotificationRequest executes, as defined in section 2.1.1.

   NotifiedEntity is an optional parameter that specifies where the
   notifications should be sent. When this parameter is absent, the
   notifications should be sent to the originator of the
   NotificationRequest.

   RequestIdentifier is used to correlate this request with the
   notifications that it triggers.

   RequestedEvents is a list of events that the gateway is requested to
   detect and report. Such events include, for example, fax tones,
   continuity tones, or on-hook transition.  To each event is associated
   an action, which can be:

   *  Notify the event immediately, together with the accumulated list
      of observed events,

   *  Swap audio,

   *  Accumulate the event in an event buffer, but don't notify yet,

   *  Accumulate according to Digit Map,

   *  Keep Signal(s) active,

   *  process the Embedded Notification Request,

   *  Ignore the event.

   Some actions can be combined.  In particular:

   *  The "swap audio" action can be combined with "Notify",
      "Accumulate" and "Ignore."

   *  The "keep signal active" action can be combined with "Notify",
      "Accumulate", "Accumulate according to Digit Map", "Ignore" and
      "Embedded Notification Request."

   *  The "Embedded Notification Request" can be combined with
      "Accumulate" and with "Keep signals active." It can also be
      combined with Notify, if the gateway is allowed to issue several
      Notify commands in response to a single Notification request.

   In addition to the requestedEvents parameter specified in the
   command, some profiles of MGCP have introduced the concept of
   "persistent events." According to such profiles, the persistent event
   list is configured in the endpoint, by means outside the scope of
   MGCP. The basic MGCP specification does not specify any persistent
   event.

   If a persistent event is not included in the list of RequestedEvents,
   and the event occurs, the event will be detected anyway, and
   processed like all other events, as if the persistent event had been
   requested with a Notify action. Thus, informally, persistent events
   can be viewed as always being implicitly included in the list of
   RequestedEvents with an action to Notify, although no glare
   detection, etc., will be performed.

   Non-persistent events are those events explicitly included in the
   RequestedEvents list. The (possibly empty) list of requested events
   completely replaces the previous list of requested events. In
   addition to the persistent events, only the events specified in the
   requested events list will be detected by the endpoint. If a
   persistent event is included in the RequestedEvents list, the action
   specified will then replace the default action associated with the
   event for the life of the RequestedEvents list, after which the
   default action is restored. For example, if "Ignore off-hook" was
   specified, and a new request without any off-hook instructions were
   received, the default "Notify off-hook" operation then would be
   restored. A given event MUST NOT appear more than once in a
   RequestedEvents.

   The gateway will detect the union of the persistent events and the
   requested events. If an event is not specified in either list, it
   will be ignored.

   The Swap Audio action can be used when a gateway handles more than
   one active connection on an endpoint. This will be the case for
   three-way calling, call waiting, and possibly other feature
   scenarios. In order to avoid the round-trip to the Call Agent when
   just changing which connection is attached to the audio functions of
   the endpoint, the NotificationRequest can map an event (usually hook
   flash, but could be some other event) to a local function swap audio,
   which selects the "next" connection in a round robin fashion. If
   there is only one connection, this action is effectively a no-op.

   If signal(s) are desired to start when an event being looked for
   occurs, the "Embedded NotificationRequest" action can be used. The
   embedded NotificationRequest may include a new list of
   RequestedEvents, SignalRequests and a new digit map as well. The
   semantics of the embedded NotificationRequest is as if a new
   NotificationRequest was just received with the same NotifiedEntity,
   and RequestIdentifier. When the "Embedded NotificationRequest" is
   activated, the "current dial string" will be cleared; the list of
   observed events and the quarantine buffer will be unaffected.

   MGCP implementations shall be able to support at least one level of
   embedding.  An embedded NotificationRequest that respects this
   limitation shall not contain another Embedded NotificationRequest.

   DigitMap is an optional parameter that allows the Call Agent to
   provision the gateways with a digit map according to which digits
   will be accumulated. If this optional parameter is absent, the
   previously defined value is retained. This parameter must be defined,
   either explicitly or through a previous command, if the
   RequestedEvent parameters contain an request to "accumulate according
   to the digit map." The collection of these digits will result in a
   digit string. The digit string is initialized to a null string upon
   reception of the NotificationRequest, so that a subsequent
   notification only returns the digits that were collected after this
   request. Digits that were accumulated according to the digit map are
   reported as any other accumulated event, in the order in which they
   occur. It is therefore possible that other events be accumulated may
   be found in between the list of digits.

   SignalRequests is a parameter that contains the set of signals that
   the gateway is asked to apply to the endpoint, such as, for example
   ringing, or continuity tones. Signals are identified by their name,
   which is an event name, and may be qualified by parameters.

   The action triggered by the SignalRequests is synchronized with the
   collection of events specified in the RequestedEvents parameter. For
   example, if the NotificationRequest mandates "ringing" and the event
   request ask to look for an "off-hook" event, the ringing shall stop
   as soon as the gateway detect an off hook event. The formal
   definition is that the generation of all "Time Out" signals shall
   stop as soon as one of the requested events is detected, unless the
   "Keep signals active" action is associated to the specified event.

   The specific definition of actions that are requested via these
   SignalRequests, such as the duration of and frequency of a DTMF
   digit, is out side the scope of MGCP. This definition may vary from
   location to location and hence from gateway to gateway.

   The RequestedEvents and SignalRequests refer to the same event
   definitions. In one case, the gateway is asked to detect the
   occurrence of the event, and in the other case it is asked to
   generate it. The specific events and signals that a given endpoint
   can detect or perform are determined by the list of event packages
   that are supported by that end point.  Each package specifies a list
   of events and actions that can be detected or performed.  A gateway
   that is requested to detect or perform an event belonging to a
   package that is not supported by the specified endpoint shall return
   an error. When the event name is not qualified by a package name, the
   default package name for the end point is assumed.  If the event name
   is not registered in this default package, the gateway shall return
   an error.

   The Call Agent can send a NotificationRequest whose requested signal
   list is empty. It will do so for example when tone generation should
   stop.

   The optional QuarantineHandling parameter specifies the handling of
   "quarantine" events, i.e. events that have been detected by the
   gateway before the arrival of this NotificationRequest command, but
   have not yet been notified to the Call Agent.  The parameter provides
   a set of handling options:

   *  whether the quarantined events should be processed or discarded
      (the default is to process them.)

   *  whether the gateway is expected to generate at most one
      notification (step by step), or multiple notifications (loop), in
      response to this request (the default is exactly one.)

   When the parameter is absent, the default value is assumed.

   We should note that the quarantine-handling parameter also governs
   the handling of events that were detected but not yet notified when
   the command is received.

   DetectEvents is an optional parameter that specifies a list of events
   that the gateway is requested to detect during the quarantine period.
   When this parameter is absent, the events that should be detected in
   the quarantine period are those listed in the last received
   DetectEvents list.  In addition, the gateway should also detect the
   events specified in the request list, including those for which the
   "ignore" action is specified.

   Some events and signals, such as the in-line ringback or the quality
   alert, are performed or detected on connections terminating in the
   end point rather than on the endpoint itself.  The structure of the
   event names allow the Call Agent to specify the connection (or
   connections) on which the events should be performed or detected.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the NotificationRequest, with the
   exception of the EndpointId, which is not replicated.

   The encapsulated EndpointConfiguration command shares the fate of the
   NotificationRequest command.  If the NotificationRequest is rejected,
   the EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary. .NH 3 Notifications

   Notifications are sent via the Notify command and are sent by the
   gateway when the observed events occur.

               ReturnCode
               <-- Notify( EndpointId,
                           [NotifiedEntity,]
                           RequestIdentifier,
                           ObservedEvents)

   EndpointId is the name for the endpoint in the gateway which is
   issuing the Notify command, as defined in section 2.1.1. The
   identifier should be a fully qualified endpoint identifier, including
   the domain name of the gateway.  The local part of the name shall not
   use the wildcard convention.

   NotifiedEntity is an optional parameter that identifies the entity to
   which the notifications is sent. This parameter is equal to the last
   received value of the NotifiedEntity parameter.  The parameter is
   absent if there was no such parameter in the triggering request. The
   notification is sent to the "current notified entity" or, if no such
   entity was ever specified, to the address from which the request was
   received.

   RequestIdentifier is parameter that repeats the RequestIdentifier
   parameter of the NotificationRequest that triggered this
   notification.  It is used to correlate this notification with the
   request that triggered it.

   ObservedEvents is a list of events that the gateway detected. A
   single notification may report a list of events that will be reported
   in the order in which they were detected. The list may only contain
   the identification of events that were requested in the
   RequestedEvents parameter of the triggering NotificationRequest. It
   will contain the events that were either accumulated (but not
   notified) or treated according to digit map (but no match yet), and
   the final event that triggered the detection or provided a final
   match in the digit map.

   ReturnCode is a parameter returned by the call agent. It indicates
   the outcome of the command and consists of an integer number
   optionally followed by commentary.

2.3.3. CreateConnection

   This command is used to create a connection between two endpoints.

            ReturnCode,
            ConnectionId,
            [SpecificEndPointId,]
            [LocalConnectionDescriptor,]
            [SecondEndPointId,]
            [SecondConnectionId]
            <--- CreateConnection(CallId,
                                  EndpointId,
                                  [NotifiedEntity,]
                                  [LocalConnectionOptions,]
                                  Mode,
                                  [{RemoteConnectionDescriptor |
                                    SecondEndpointId}, ]
                                  [Encapsulated NotificationRequest,]
                                  [Encapsulated EndpointConfiguration])

   A connection is defined by its endpoints. The input parameters in
   CreateConnection provide the data necessary to build a gateway's
   "view" of a connection.

   CallId is a globally unique parameter that identifies the call (or
   session) to which this connection belongs. Connections that belong to
   the same call share the same call-id. The call-id can be used to
   identify calls for reporting and accounting purposes. It does not
   affect the handling of connections by the gateway.

   EndpointId is the identifier for the connection endpoint in the
   gateway where CreateConnection executes. The EndpointId can be
   fully-specified by assigning a value to the parameter EndpointId in
   the function call or it may be under-specified by using the "anyone"
   wildcard convention. If the endpoint is underspecified, the endpoint
   identifier will be assigned by the gateway and its complete value
   returned in the SpecificEndPointId parameter of the response.

   The NotifiedEntity is an optional parameter that specifies where the
   Notify or DeleteConnection commands should be sent. If the parameter
   is absent, the Notify or DeleteConnection commands should be sent to
   the last received Notified Entity, or to originator of the
   CreateConnection command if no Notified Entity was ever received for
   the end point.

   LocalConnectionOptions is a parameter used by the Call Agent to
   direct the handling of the connection by the gateway.  The fields
   contained in LocalConnectionOptions are the following:

   *  Encoding Method,

   *  Packetization period,

   *  Bandwidth,

   *  Type of Service,

   *  Usage of echo cancellation,

   *  Usage of silence suppression or voice activity detection,

   *  Usage of signal level adaptation and noise level reduction, or
      "gain control."

   *  Usage of reservation service,

   *  Usage of RTP security,

   *  Type of network used to carry the connection.

   This set of field can be completed by vendor specific optional or
   mandatory extensions. The encoding of the first three fields, when
   they are present, will be compatible with the SDP and RTP profiles:

   *  The encoding method shall be specified by using one or several
      valid encoding names, as defined in the RTP AV Profile or
      registered with the IANA.

   *  The packetization period is encoded as either the length of time
      in milliseconds represented by the media in a packet, as specified
      in the "ptime" parameter of SDP, or as a range value, specifying
      both the minimum and maximum acceptable packetization periods.

   *  The bandwidth is encoded as either a single value or a range,
      expressed as an integer number of kilobit per seconds.

   For each of the first three fields, the Call Agent has three options:

   *  It may state exactly one value, which the gateway will then use
      for the connection,

   *  It may provide a loose specification, such as a list of allowed
      encoding methods or a range of packetization periods,

   *  It may simply provide a bandwidth indication, leaving the choice
      of encoding method and packetization period to the gateway.

   The bandwidth specification shall not contradict the specification of
   encoding methods and packetization period. If an encoding method is
   specified, then the gateway is authorized to use it, even if it
   results in the usage of a larger bandwidth than specified.

   The LocalConnectionOptions parameter may be absent in the case of a
   data call.

   The Type of Service specifies the class of service that will be used
   for the connection. When the connection is transmitted over an IP
   network, the parameters encodes the 8-bit type of service value
   parameter of the IP header. When the Type of Service is not
   specified, the gateway shall use a default or configured value.

   The gateways can be instructed to perform a reservation, for example
   using RSVP, on a given connection.  When a reservation is needed, the
   call agent will specify the reservation profile that should be used,
   which is either "controlled load" or "guaranteed service."  The

   absence of reservation can be indicated by asking for the "best
   effort" service, which is the default value of this parameter. When
   reservation has been asked on a connection, the gateway will:

   *  start emitting RSVP "PATH" messages if the connection is in
      "send-only", "send-receive", "conference", "network loop back" or
      "network continuity test" mode (if a remote connection descriptor
      has been received,)

   *  start emitting RSVP "RESV" messages as soon as it receives "PATH"
      messages if the connection is in "receive-only", "send-receive",
      "conference", "network loop back" or "network continuity test"
      mode.

   The RSVP filters will be deduced from the characteristics of the
   connection. The RSVP resource profiles will be deduced from the
   connection's bandwidth and packetization period.

   By default, the telephony gateways always perform echo cancellation.
   However, it is necessary, for some calls, to turn off these
   operations.  The echo cancellation parameter can have two values,
   "on" (when the echo cancellation is requested) and "off" (when it is
   turned off.)

   The telephony gateways may perform gain control, in order to adapt
   the level of the signal.  However, it is necessary, for example for
   modem calls, to turn off this function.  The gain control parameter
   may either be specified as "automatic", or as an explicit number of
   decibels of gain.  The default is to not perform gain control, which
   is equivalent to specifying a gain of 0 decibels.

   The telephony gateways may perform voice activity detection, and
   avoid sending packets during periods of silence.  However, it is
   necessary, for example for modem calls, to turn off this detection.
   The silence suppression parameter can have two values, "on" (when the
   detection is requested) and "off" (when it is turned off.) The
   default is "off."

   The Call agent can request the gateway to enable encryption of the
   audio Packets.  It does so by providing an key specification, as
   specified in RFC 2327. By default, encryption is not used.

   The Call Agent may instruct the gateway to prepare the connection on
   a specified type of network.  The type of network is encoded as in
   the "connection-field" parameter of the SDP standard.  Possible
   values are IN (Internet), ATM and LOCAL. The parameter is optional;
   if absent, the network is determined by the type of gateway.

   RemoteConnectionDescriptor is the connection descriptor for the
   remote side of a connection, on the other side of the IP network. It
   includes the same fields as in the LocalConnectionDescriptor, i.e.
   the fields that describe a session according to the SDP standard.
   This parameter may have a null value when the information for the
   remote end is not known yet. This occurs because the entity that
   builds a connection starts by sending a CreateConnection to one of
   the two gateways involved in it. For the first CreateConnection
   issued, there is no information available about the other side of the
   connection. This information may be provided later via a
   ModifyConnection call. In the case of data connections (mode=data),
   this parameter describes the characteristics of the data connection.

   The SecondEndpointId can be used instead of the
   RemoteConnectionDescriptor to establish a connection between two
   endpoints located on the same gateway.  The connection is by
   definition a local connection. The SecondEndpointId can be fully-
   specified by assigning a value to the parameter SecondEndpointId in
   the function call or it may be under-specified by using the "anyone"
   wildcard convention. If the secondendpoint is underspecified, the
   second endpoint identifier will be assigned by the gateway and its
   complete value returned in the SecondEndPointId parameter of the
   response.

   Mode indicates the mode of operation for this side of the connection.
   The mode are "send", "receive", "send/receive", "conference", "data",
   "inactive", "loopback", "continuity test", "network loop back" or
   "network continuity test." The expected handling of these modes is
   specified in the introduction of the "Gateway Handling Function"
   section. Some end points may not be capable of supporting all modes.
   If the command specifies a mode that the endpoint cannot support, and
   error shall be returned.

   The gateway returns a ConnectionId, that uniquely identifies the
   connection within one endpoint, and a LocalConnectionDescriptor,
   which is a session description that contains information about
   addresses and RTP ports, as defined in SDP. The
   LocalConnectionDescriptor is not returned in the case of data
   connections. The SpecificEndPointId is an optional parameter that
   identifies the responding endpoint. It can be used when the
   EndpointId argument referred to a "any of" wildcard name. When a
   SpecificEndPointId is returned, the Call Agent should use it as the
   EndpointId value is successive commands referring to this call.

   When a SecondEndpointId is specified, the command really creates two
   connections that can be manipulated separately through
   ModifyConnection and DeleteConnection commands.  The response to the
   creation provides a SecondConnectionId parameter that identifies the
   second connection.

   After receiving a "CreateConnection" request that did not include a
   RemoteConnectionDescriptor parameter, a gateway is in an ambiguous
   situation. Because it has exported a LocalConnectionDescriptor
   parameter, it can potentially receive packets. Because it has not yet
   received the RemoteConnectionDescriptor parameter of the other
   gateway, it does not know whether the packets that it receives have
   been authorized by the Call Agent. It must thus navigate between two
   risks, i.e. clipping some important announcements or listening to
   insane data. The behavior of the gateway is determined by the value
   of the Mode parameter:

   *  If the mode was set to ReceiveOnly, the gateway should accept the
      voice signals and transmit them through the endpoint.

   *  If the mode was set to Inactive, Loopback, Continuity Test, the
      gateway should refuse the voice signals.

   *  If the mode was set to Network Loopback or Network Continuity
      Test, the gateway should perform the expected echo or Response.

   Note that the mode values SendReceive, Conference, Data and SendOnly
   don't make sense in this situation. They should be treated as errors,
   and the command should be rejected (Error code 517).

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter will be
   present, as well as, optionally, the RequestedEvents DigitMap,
   SignalRequests, QuarantineHandling and DetectEvents parameters. The
   encapsulated NotificationRequest is executed simultaneously with the
   creation of the connection. For example, when the Call Agent wants to
   initiate a call to an residential gateway, it should:

   *  ask the residential gateway to prepare a connection, in order to
      be sure that the user can start speaking as soon as the phone goes
      off hook,

   *  ask the residential gateway to start ringing,

   *  ask the residential gateway to notify the Call Agent when the
      phone goes off-hook.

   This can be accomplished in a single CreateConnection command, by
   also transmitting the RequestedEvent parameters for the off hook
   event, and the SignalRequest parameter for the ringing signal.

   When these parameters are present, the creation and the
   NotificationRequests should be synchronized, which means that
   bothshould be accepted, or both refused. In our example, the
   CreateConnection may be refused if the gateway does not have
   sufficient resources, or cannot get adequate resources from the local
   network access, and the off-hook Notification-Request can be refused
   in the glare condition, if the user is already off-hook. In this
   example, the phone should not ring if the connection cannot be
   established, and the connection should not be established if the user
   is already off hook.

   The NotifiedEntity parameter, if present, applies to both the
   CreateConnection and the NotificationRequest command. It defines the
   new "notified entity" for the endpoint.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the CreateConnection with the
   exception of the EndpointId, which is not replicated. The
   EndpointConfiguration command may be encapsulated together with an
   encapsulated NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   CreateConnection command.  If the CreateConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.4. ModifyConnection

   This command is used to modify the characteristics of a gateway's
   "view" of a connection. This "view" of the call includes both the
   local connection descriptors as well as the remote connection
   descriptor.

      ReturnCode,
      [LocalConnectionDescriptor]
       <--- ModifyConnection(CallId,
                             EndpointId,
                             ConnectionId,
                             [NotifiedEntity,]
                             [LocalConnectionOptions,]
                             [Mode,]
                             [RemoteConnectionDescriptor,]
                             [Encapsulated NotificationRequest,]
                             [Encapsulated EndpointConfiguration])

   The parameters used are the same as in the CreateConnection command,
   with the addition of a ConnectionId that identifies the connection
   within the endpoint. This parameter is returned by the
   CreateConnection function, as part of the local connection
   descriptor. It uniquely identifies the connection within the context
   of the endpoint.

   The EndpointId should be a fully qualified endpoint identifier.  The
   local name shall not use the wildcard convention.

   The ModifyConnection command can be used to affect parameters of a
   connection in the following ways:

   *  Provide information about the other end of the connection, through
      the RemoteConnectionDescriptor.

   *  Activate or deactivate the connection, by changing the value of
      the Mode parameter. This can occur at any time during the
      connection, with arbitrary parameter values.

   *  Change the sending parameters of the connection, for example by
      switching to a different coding scheme, changing the packetization
      period, or modifying the handling of echo cancellation.

   Connections can only be activated if the RemoteConnectionDescriptor
   has been provided to the gateway. The receive only mode, however, can
   be activated without the provision of this descriptor.

   The command will only return a LocalConnectionDescriptor if the local
   connection parameters, such as RTP ports, were modified. (Usage of
   this feature is actually for further study.)

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter will be
   present, as well as, optionnally, the RequestedEvents DigitMap,
   SignalRequests, QuarantineHandling and DetectEvents parameters. The

   encapsulated NotificationRequest is executed simultaneously with the
   modification of the connection. For example, when a connection is
   accepted, the calling gateway should be instructed to place the
   circuit in send-receive mode and to stop providing ringing tones.

   This can be accomplished in a single ModifyConnection command, by
   also transmitting the RequestedEvent parameters, for the on hook
   event, and an empty SignalRequest parameter, to stop the provision of
   ringing tones.

   When these parameters are present, the modification and the
   NotificationRequests should be synchronized, which means that both
   should be accepted, or both refused.  The NotifiedEntity parameter,
   if present, applies to both the ModifyConnection and the
   NotificationRequest command.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the ModifyConnection with the
   exception of the EndpointId, which is not replicated. The
   EndpointConfiguration command may be encapsulated together with an
   encapsulated NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   ModifyConnection command.  If the ModifyConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.5. DeleteConnection (from the Call Agent)

   This command is used to terminate a connection. As a side effect, it
   collects statistics on the execution of the connection.

        ReturnCode,
        Connection-parameters
        <-- DeleteConnection(CallId,
                             EndpointId,
                             ConnectionId,
                             [Encapsulated NotificationRequest,]
                             [Encapsulated EndpointConfiguration])

   The endpoint identifier, in this form of the DeleteConnection
   command, shall be fully qualified.  Wildcard conventions shall not be
   used.

   In the general case where a connection has two ends, this command has
   to be sent to both gateways involved in the connection. Some
   connections, however, may use IP multicast. In this case, they can be
   deleted individually.

   After the connection has been deleted, any loopback that has been
   requested for the connection should be cancelled. When all
   connections to an endpoint have been deleted, that endpoint should be
   placed in inactive mode.

   In response to the DeleteConnection command, the gateway returns a
   list of parameters that describe the status of the connection. These
   parameters are:

   Number of packets sent:

   The total number of RTP data packets transmitted by the sender since
   starting transmission on this connection. The count is not reset if
   the sender changes its synchronization source identifier (SSRC, as
   defined in RTP), for example as a result of a Modify command. The
   value is zero if the connection was set in "receive only" mode.

   Number of octets sent:

   The total number of payload octets (i.e., not including header or
   padding) transmitted in RTP data packets by the sender since starting
   transmission on this connection. The count is not reset if the sender
   changes its SSRC identifier, for example as a result of a
   ModifyConnection command. The value is zero if the connection was set
   in "receive only" mode.

   Number of packets received:

   The total number of RTP data packets received by the sender since
   starting reception on this connection. The count includes packets
   received from different SSRC, if the sender used several values. The
   value is zero if the connection was set in "send only" mode.

   Number of octets received:

   The total number of payload octets (i.e., not including header or
   padding) transmitted in RTP data packets by the sender since starting
   transmission on this connection. The count includes packets received
   from different SSRC, if the sender used several values. The value is
   zero if the connection was set in "send only" mode.

   Number of packets lost:

   The total number of RTP data packets that have been lost since the
   beginning of reception. This number is defined to be the number of
   packets expected less the number of packets actually received, where
   the number of packets received includes any which are late or
   duplicates.  The count includes packets received from different SSRC,
   if the sender used several values. Thus packets that arrive late are
   not counted as lost, and the loss may be negative if there are
   duplicates. The count includes packets received from different SSRC,
   if the sender used several values. The number of packets expected is
   defined to be the extended last sequence number received, as defined
   next, less the initial sequence number received. The count includes
   packets received from different SSRC, if the sender used several
   values. The value is zero if the connection was set in "send only"
   mode. This parameter is omitted if the connection was set in "data"
   mode.

   Interarrival jitter:

   An estimate of the statistical variance of the RTP data packet
   interarrival time measured in milliseconds and expressed as an
   unsigned integer. The interarrival jitter J is defined to be the mean
   deviation (smoothed absolute value) of the difference D in packet
   spacing at the receiver compared to the sender for a pair of packets.
   Detailed computation algorithms are found in RFC 1889. The count
   includes packets received from different SSRC, if the sender used
   several values. The value is zero if the connection was set in "send
   only" mode. This parameter is omitted if the connection was set in
   "data" mode.

   Average transmission delay:

   An estimate of the network latency, expressed in milliseconds. This
   is the average value of the difference between the NTP timestamp
   indicated by the senders of the RTCP messages and the NTP timestamp
   of the receivers, measured when this messages are received. The
   average is obtained by summing all the estimates, then dividing by
   the number of RTCP messages that have been received. This parameter
   is omitted if the connection was set in "data" mode.
   When the gateway's clock is not synchronized by NTP, the latency
   value can be computed as one half of the round trip delay, as
   measured through RTCP.
   When the gateway cannot compute the one way delay or the round trip
   delay, the parameter conveys a null value.

   For a detailed definition of these variables, refer to RFC 1889.

   When the connection was set up over an ATM network, the meaning of
   these parameters may change:

   Number of packets sent:  The total number of ATM cells transmitted
      since starting transmission on this connection.

   Number of octets sent:
      The total number of payload octets transmitted in ATM cells.

   Number of packets received:
      The total number of ATM cells received since starting reception on
      this connection.

   Number of octets received:
      The total number of payload octets received in ATM cells.

   Number of packets lost:
      Should be determined as the number of cell losts, or set to zero
      if the adaptation layer does not enable the gateway to assess
      losses.

   Interarrival jitter:
      Should be understood as the interarrival jitter between ATM cells.

   Average transmission delay:
      The gateway may not be able to assess this parameter over an ATM
      network.  It could simply report a null value.

   When the connection was set up over an LOCAL interconnect, the
   meaning of these parameters is defined as follows:

   Number of packets sent:
     Not significant.

   Number of octets sent:
     The total number of payload octets transmitted over the local
     connection.

   Number of packets received:
     Not significant.

   Number of octets received:
     The total number of payload octets received over the connection.

   Number of packets lost:
     Not significant.  A value of zero is assumed.

   Interarrival jitter:
     Not significant.  A value of zero is assumed.

   Average transmission delay:
     Not significant.  A value of zero is assumed.

   The standard set of connection parameters can be extended by the
   creation of extension parameters.

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter will be
   present, as well as, optionnally, the RequestedEvents DigitMap,
   SignalRequests, QuarantineHandling and DetectEvents parameters. The
   encapsulated NotificationRequest is executed simultaneously with the
   deletion of the connection. For example, when a user hang-up is
   notified, the gateway should be instructed to delete the connection
   and to start looking for an off hook event.

   This can be accomplished in a single DeleteConnection command, by
   also transmitting the RequestedEvent parameters, for the off hook
   event, and an empty SignalRequest parameter.

   When these parameters are present, the DeleteConnection and the
   NotificationRequests should be synchronized, which means that both
   should be accepted, or both refused.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the DeleteConnection with the
   exception of the EndpointId, which is not replicated. The
   EndpointConfiguration command may be encapsulated together with an
   encapsulated NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   DeleteConnection command.  If the DeleteConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.6. DeleteConnection (from the VoIP gateway)

   In some circumstances, a gateway may have to clear a connection, for
   example because it has lost the resource associated with the
   connection, or because it has detected that the endpoint no longer is
   capable or willing to send or receive voice. The gateway terminates
   the connection by using a variant of the DeleteConnection command:

            ReturnCode,
            <-- DeleteConnection( CallId,
                                  EndpointId,
                                  ConnectionId,
                                  Reason-code,
                                  Connection-parameters)

   In addition to the call, endpoint and connection identifiers, the
   gateway will also send the call's parameters that would have been
   returned to the Call Agent in response to a DeleteConnection command.
   The reason code indicates the cause of the disconnection.

   ReturnCode is a parameter returned by the call agent. It indicates
   the outcome of the command and consists of an integer number
   optionally followed by commentary.

2.3.7. DeleteConnection (multiple connections, from the Call Agent)

   A variation of the DeleteConnection function can be used by the Call
   Agent to delete multiple connections at the same time. The command
   can be used to delete all connections that relate to a Call for an
   endpoint:

            ReturnCode,
            <-- DeleteConnection( CallId,
                                  EndpointId)

   It can also be used to delete all connections that terminate in a
   given endpoint:

            ReturnCode,
            <-- DeleteConnection( EndpointId)

   Finally, Call Agents can take advantage of the hierarchical naming
   structure of endoints to delete all the connections that belong to a
   group of endpoints.  In this case, the "local name" component of the
   EndpointID will be specified using the "all value" wildcarding
   convention. The "any value" convention shall not be used.  For
   example, if endpoints names are structured as the combination of a
   physical interface name and a circuit number, as in "X35V3+A4/13",

   the Call Agent may replace the circuit number by a wild card
   character "*", as in "X35V3+A4/*".  This "wildcard" command instructs
   the gateway to delete all the connections that where attached to
   circuits connected to the physical interface "X35V3+A4".

   After the connections have been deleted, the endpoint should be
   placed in inactive mode. Any loopback that has been requested for the
   connections should be cancelled.

   This command does not return any individual statistics or call
   parameters.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.8. Audit Endpoint

   The AuditEndPoint command can be used by the Call Agent to find out
   the status of a given endpoint.

              ReturnCode,
                EndPointIdList|{
                [RequestedEvents,]
                [DigitMap,]
                [SignalRequests,]
                [RequestIdentifier,]
                [NotifiedEntity,]
                [ConnectionIdentifiers,]
                [DetectEvents,]
                [ObservedEvents,]
                [EventStates,]
                [BearerInformation,]
                [RestartReason,]
                [RestartDelay,]
                [ReasonCode,]
                [Capabilities]}
                        <--- AuditEndPoint(EndpointId,
                                                 [RequestedInfo])

   The EndpointId identifies the endpoint that is being audited. The
   "all of" wildcard convention can be used to start auditing of a group
   of endpoints. If this convention is used, the gateway should return
   the list of endpoint identifiers that match the wildcard in the
   EndPointIdList parameter. It shall not return any parameter specific
   to one of these endpoints.

   When a non-wildcard EndpointId is specified, the (possibly empty)
   RequestedInfo parameter describes the information that is requested
   for the EndpointId specified. The following endpoint info can be
   audited with this command:

   RequestedEvents, DigitMap, SignalRequests, RequestIdentifier,
   NotifiedEntity, ConnectionIdentifiers, DetectEvents, ObservedEvents,
   EventStates, RestartReason, RestartDelay, ReasonCode, and
   Capabilities.

   The response will in turn include information about each of the items
   for which auditing info was requested:

   *  RequestedEvents: The current value of RequestedEvents the endpoint
      is using including the action associated with each event.
      Persistent events are included in the list.

   *  DigitMap: the digit map the endpoint is currently using.

   *  SignalRequests: A list of the; Time-Out signals that are currently
      active, On/Off signals that are currently "on" for the endpoint
      (with or without parameter), and any pending Brief signals. Time-
      Out signals that have timed-out, and currently playing Brief
      signals are not included.

   *  RequestIdentifier, the RequestIdentifier for the last Notification
      Request received by this endpoint (includes NotificationRequest
      encapsulated in Connection handling primitives). If no
      notification request has been received, the value zero will be
      returned.

   *  QuarantineHandling, the QuarantineHandling for the last
      NotificationRequest received by this endpoint.

   *  DetectEvents, the list of events that are currently detected in
      quarantine mode.

   *  NotifiedEntity, the current notified entity for the endpoint.

   * ConnectionIdentifiers, the list of ConnectionIdentifiers for all
      connections that currently exist for the specified endpoint.

   *  ObservedEvents: the current list of observed events for the
      endpoint.

   *  EventStates: For events that have auditable states associated with
      them, the event corresponding to the state the endpoint is in,
      e.g., off-hook if the endpoint is off-hook. The definition of the
      individual events will state if the event in question has an
      auditable state associated with it.

   *  BearerInformation: the value of the last received
      BearerInformation parameter for this endpoint.

   *  RestartReason: the value of the restart reason parameter in the
      last RestartInProgress command issued by the endpoint, "restart"
      indicating a fully functional endpoint.

   *  RestartDelay: the value of the  restart delay parameter if a
      RestartInProgress command was issued by the endpoint at the time
      of the response, or zero if the command would not include this
      parameter.

   *  ReasonCode:the value of the Reason-Code parameter in the last
      RestartInProgress or DeleteConnection command issued by the
      gateway for the endpoint, or the special value 000 if the
      endpoint's state is nominal.

   *  The capabilities for the endpoint similar to the
      LocalConnectionOptions parameter and including event packages and
      connection modes.  If there is a need to specify that some
      parameters, such as e.g., silence suppression, are only compatible
      with some

   *  codecs,  then the gateway will return several capability sets:

         Compression Algorithm: a list of supported codecs. The rest of
         the parameters will apply to all codecs specified in this list.

         Packetization Period: A single value or a range may be
         specified.

         Bandwidth: A single value or a range corresponding to the range
         for packetization periods may be specified (assuming no silence
         suppression).

         Echo Cancellation: Whether echo cancellation is supported or
         not.

         Silence Suppression: Whether silence suppression is supported
         or not.

         Type of Service: Whether type of service is supported or not.

         Event Packages: A list of event packages supported. The first
         event package in the list will be the default package.

         Modes: A list of supported connection modes.

   The Call Agent may then decide to use the AuditConnection command to
   obtain further information about the connections.

   If no info was requested and the EndpointId refers to a valid
   endpoint, the gateway simply returns a positive acknowledgement.

   If no NotifiedEntity has been specified in the last
   NotificationRequest, the notified entity defaults to the source
   address of the last NotificationRequest command received for this
   connection.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.9. Audit Connection

   The AuditConnection command can be used by the Call Agent to retrieve
   the parameters attached to a connection:

              ReturnCode,
              [CallId,]
              [NotifiedEntity,]
              [LocalConnectionOptions,]
              [Mode,]
              [RemoteConnectionDescriptor,]
              [LocalConnectionDescriptor,]
              [ConnectionParameters]
                        <--- AuditConnection(EndpointId,
                                         ConnectionId,
                                         RequestedInfo)

   The EndpointId parameter specifies the endpoint that handles the
   connection. The wildcard conventions shall not be used.

   The ConnectionId parameter is the identifier of the audited
   connection, within the context of the specified endpoint.

   The (possibly empty) RequestedInfo describes the information that is
   requested for the ConnectionId within the EndpointId specified. The
   following connection info can be audited with this command:

      CallId, NotifiedEntity, LocalConnectionOptions, Mode,
      RemoteConnectionDescriptor, LocalConnectionDescriptor,
      ConnectionParameters

   The AuditConnectionResponse will in turn include information about
   each of the items auditing info was requested for:

   *  CallId, the CallId for the call the connection belongs to.

   *  NotifiedEntity, the current notified entity for the Connection.

   *  LocalConnectionOptions, the LocalConnectionOptions that was
      supplied for the connection.

   *  Mode, the current mode of the connection.

   *  RemoteConnectionDescriptor, the RemoteConnectionDescriptor that
      was supplied to the gateway for the connection.

   *  LocalConnectionDescriptor, the LocalConnectionDescriptor the gate-
      way supplied for the connection.

   *  ConnectionParameters, the current value of the connection
      parameters for the connection.

   If no info was requested and the EndpointId is valid, the gateway
   simply checks that the connection exists, and if so returns a
   positive acknowledgement.

   If no NotifiedEntity has been specified for the connection, the
   notified entity defaults to the source address of the last connection
   handling command received for this connection.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.10. Restart in progress

   The RestartInProgress command is used by the gateway to signal that
   An endpoint, or a group of endpoint, is taken in or out of service.

          ReturnCode,
          [NotifiedEntity]
                <------- RestartInProgress ( EndPointId,
                                             RestartMethod,
                                             [RestartDelay,]
                                             [Reason-code])

   The EndPointId identifies the endpoint that are taken in or out of
   service.  The "all of" wildcard convention may be used to apply the
   command to a group of endpoint, such as for example all endpoints
   that are attached to a specified interface, or even all endpoints
   that are attached to a given gateway.  The "any of" wildcard
   convention shall not be used.

   The RestartMethod parameter specified the type of restart.  Three
   values have been defined:

   *  A "graceful" restart method indicates that the specified endpoints
      will Be taken out of service after the specified delay. The
      established connections are not yet affected, but the Call Agent
      should refrain to establish new connections, and should try to
      gracefully tear down the existing connections.

   *  A "forced" restart method indicates that the specified endpoints
      are taken abruptely out of service. The established connections,
      if any, are lost.

   *  A "restart" method indicates that service will be restored on the
      endpoints after the specified "restart delay." There are no
      connections that are currently established on the endpoints.

   *  A "disconnected" method indicates that the endpoint has become
      disconnected and is now trying to establish connectivity. The
      "restart delay" specifies the number of seconds the endpoint has
      been disconnected. Established connections are not affected.

   *  A "cancel-graceful" method indicates that a gateway is canceling a
      previously issued "graceful" restart command.

   The optional "restart delay" parameter is expressed as a number of
   seconds. If the number is absent, the delay value should be
   considered null.  In the case of the "graceful" method, a null delay
   indicates that the call agent should simply wait for the natural
   termination of the existing connections, without establishing new
   connections. The restart delay is always considered null in the case
   of the "forced" method.

   A restart delay of null for the "restart" method indicates that
   service has already been restored. This typically will occur after
   gateway startup/reboot.

   The optional reason code parameter the cause of the restart.

   Gateways SHOULD send a "graceful" or "forced" RestartInProgress
   message as a courtesy to the Call Agent when they are taken out of
   service, e.g., by being shutdown, or taken out of service by a
   network management system, although the Call Agent cannot rely on
   always receiving such messages. Gateways MUST send a "restart"
   RestartInProgress message with a null delay to their Call Agent when
   they are back in service according to the restart procedure specified
   in Section 4.3.4 - Call Agents can rely on receiving this message.
   Also, gateways MUST send a "disconnected" RestartInProgress message
   to their current "notified entity" according to the "disconnected"
   procedure specified in Section 4.3.5.  The "restart delay" parameter
   MUST NOT be used with the "forced" restart method.

   The RestartInProgress message will be sent to the current notified
   entity for the EndpointId in question. It is expected that a default
   Call Agent, i.e., notified entity, has been provisioned for each
   endpoint so, after a reboot, the default Call Agent will be the
   notified entity for each endpoint. Gateways should take full
   advantage of wild- carding to minimize the number of
   RestartInProgress messages generated when multiple endpoints in a
   gateway restart and the endpoints are managed by the same Call Agent.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   A NotifiedEntity may additionally be returned with the response from
   the Call Agent:

   *  If the response indicated success (return code 200 - transaction
      executed), the restart procedure has  completed, and the
      NotifiedEntity returned is the new "notified entity" for the
      endpoint(s).

   *  If the response from the Call Agent indicated an error, the
      restart procedure is not yet complete, and must therefore be
      initiated again. If a NotifiedEntity parameter was returned, it
      then specifies the new "notified entity" for the endpoint(s),
      which must consequently be used when retrying the restart
      procedure.

2.4. Return codes and error codes.

   All MGCP commands are acknowledged. The acknowledgment carries a
   return code, which indicates the status of the command. The return
   code is an integer number, for which four ranges of values have been
   defined:

   *  values between 100 and 199 indicate a provisional response,

   *  values between 200 and 299 indicate a successful completion,

   *  values between 400 and 499 indicate a transient error,

   *  values between 500 and 599 indicate a permanent error.

   The values that have been already defined are listed in the following
   list:

100 The transaction is currently being executed. An actual
completion message will follow on later.

200 The requested transaction was executed normally.

250 The connection was deleted.

400 The transaction could not be executed, due to a transient error.

401 The phone is already off hook

402 The phone is already on hook

403 The transaction could not be executed, because the endpoint does
not have sufficient resources at this time

404 Insufficient bandwidth at this time

500 The transaction could not be executed, because the endpoint is
unknown.

01 The transaction could not be executed, because the endpoint is
not ready.

502 The transaction could not be executed, because the endpoint does
not have sufficient resources

510 The transaction could not be executed, because a protocol error
was detected.

11 The transaction could not be executed, because the command
contained an unrecognized extension.

512 The transaction could not be executed, because the gateway is
not equipped to detect one of the requested events.

513 The transaction could not be executed, because the gateway is
not equipped to generate one of the requested signals.

514 The transaction could not be executed, because the gateway
cannot send the specified announcement.

515 The transaction refers to an incorrect connection-id (may have
been already deleted)

516 The transaction refers to an unknown call-id.

517 Unsupported or invalid mode.

518 Unsupported or unknown package.

519 Endpoint does not have a digit map.

520 The transaction could not be executed, because the endpoint is
"restarting".

521 Endpoint redirected to another Call Agent.

522 No such event or signal.

523 Unknown action or illegal combination of actions

524 Internal inconsistency in LocalConnectionOptions

525 Unknown extension in LocalConnectionOptions

526 Insufficient bandwidth

527 Missing RemoteConnectionDescriptor

528 Incompatible protocol version

529 Internal hardware failure

530 CAS signaling protocol error.

531 failure of a grouping of trunks (e.g. facility failure).

2.5. Reason Codes

   Reason-codes are used by the gateway when deleting a connection to
   inform the Call Agent about the reason for deleting the connection.
   They may also be used in a RestartInProgress command, to inform the
   gateway of the Restart's reason. The reason code is an integer
   number, and the following values have been defined:

000 Endpoint state is nominal. (This code is used only in response
to audit requests.)

900 Endpoint malfunctioning

901 Endpoint taken out of service

902 Loss of lower layer connectivity (e.g., downstream sync)

3. Media Gateway Control Protocol

   The MGCP implements the media gateway control interface as a set of
   transactions. The transactions are composed of a command and a
   mandatory response. There are eight types of command:

   *  CreateConnection

   *  ModifyConnection

   *  DeleteConnection

   *  NotificationRequest

   *  Notify

   *  AuditEndpoint

   *  AuditConnection

   *  RestartInProgress

   The first four commands are sent by the Call Agent to a gateway. The
   Notify command is sent by the gateway to the Call Agent. The gateway
   may also send a DeleteConnection as defined in 2.3.6. The Call Agent
   may send either of the Audit commands to the gateway.  The Gateway
   may send a RestartInProgress command to the Call Agent.

3.1. General description

   All commands are composed of a Command header, optionally followed by
   a session description.

   All responses are composed of a Response header, optionally followed
   by a session description.

   Headers and session descriptions are encoded as a set of text lines,
   separated by a carriage return and line feed character (or,
   optionnally, a single line-feed character). The headers are separated
   from the session description by an empty line.

   MGCP uses a transaction identifier to correlate commands and
   responses.  The transaction identifier is encoded as a component of
   the command header and repeated as a component of the response header
   (see section 3.2.1, 3.2.1.2 and 3.3).

3.2. Command Header

   The command header is composed of:

   *  A command line, identifying the requested action or verb, the
      transaction identifier, the endpoint towards which the action is
      requested, and the MGCP protocol version,

   *  A set of parameter lines, composed of a parameter name followed by
      a parameter value.

   Unless otherwise noted or dictated by other referenced standards,
   each component in the command header is case insensitive. This goes
   for verbs as well as parameters and values, and all comparisons MUST
   treat upper and lower case as well as combinations of these as being
   equal.

3.2.1. Command line

   The command line is composed of:

   *  The name of the requested verb,

   *  The identification of the transaction,

   *  The name of the endpoint that should execute the command (in
      notifications or restarts, the name of the endpoint that is
      issuing the command),

   *  The protocol version.

   These four items are encoded as strings of printable ASCII
   characters, separated by white spaces, i.e. the ASCII space (0x20) or
   tabulation (0x09) characters. It is recommended to use exactly one
   ASCII space separator.

3.2.1.1. Coding of the requested verb

3.2.1.2. Transaction Identifiers

   MGCP uses a transaction identifier to correlate commands and
   responses.  A gateway supports two separate transaction identifier
   name spaces:

     a transaction identifier name space for sending transactions, and

     a transaction identifier name space for receiving transactions.

   At a minimum, transaction identifiers for commands sent to a given
   gateway MUST be unique for the maximum lifetime of the transactions
   within the collection of Call Agents that control that gateway. Thus,

   regardless of the sending Call Agent, gateways can always detect
   duplicate transactions by simply examining the transaction
   identifier. The coordination of these transaction identifiers between
   Call Agents is outside the scope of this specification though.

   Transaction identifiers for all commands sent from a given gateway
   MUST be unique for the maximum lifetime of the transactions
   regardless of which Call Agent the command is sent to. Thus, a Call
   Agent can always detect a duplicate transaction from a gateway by the
   combination of the domain-name of the endpoint and the transaction
   identifier.

   The transaction identifier is encoded as a string of up to nine
   decimal digits. In the command lines, it immediately follows the
   coding of the verb.

   Transaction identifiers have values between 1 and 999999999. An MGCP
   entity MUST NOT reuse a transaction identifier more quickly than
   three minutes after completion of the previous command in which the
   identifier was used.

3.2.1.3. Coding of the endpoint identifiers and entity names

   The endpoint identifiers and entity names are encoded as case
   insensitive e-mail addresses, as defined in RFC 821. In these
   addresses, the domain name identifies the system where the endpoint
   is attached, while the left side identifies a specific endpoint on
   that system.

   Examples of such addresses can be:

    ______________________________________________________________________
   | hrd4/56@gw23.example.net     |  Circuit number 56 in                |
   |                              |  interface "hrd4" of the Gateway 23  |
   |                              |  of the "Example" network            |
   | Call-agent@ca.example.net    |  Call Agent for the                  |
   |                              |  "example" network                   |
   | Busy-signal@ann12.example.net|  The "busy signal" virtual           |
   |                              |  endpoint in the announcement        |
   |                              |  server number 12.                   |
   |______________________________|______________________________________|

   The name of notified entities is expressed with the same syntax, with
   the possible addition of a port number as in:

     Call-agent@ca.example.net:5234

In case the port number is omitted, the default MGCP port (2427) will
be used.

3.2.1.4. Coding of the protocol version

   The protocol version is coded as the key word MGCP followed by a
   white space and the version number, and optionally followed by a
   profile name.. The version number is composed of a major version,
   coded by a decimal number, a dot, and a minor version number, coded
   as a decimal number. The version described in this document is
   version 1.0.

   The profile name, if present, is represented by a white-space
   separated strings of  visible (printable) characters extending to the
   end of the line. Profile names may be defined for user communities
   who want to apply restrictions or other profiling to MGCP.

   In the initial messages, the version will be coded as:

        MGCP 1.0

3.2.2. Parameter lines

   Parameter lines are composed of a parameter name, which in most cases
   is composed of a single upper case character, followed by a colon, a
   white space and the parameter value. The parameter that can be
   present in commands are defined in the following table:

   The parameters are not necessarily present in all commands. The
   following table provides the association between parameters and
   commands. The letter M stands for mandatory, O for optional and F for
   forbidden.

   ___________________________________________________________________
  | Parameter name      |  EP|  CR|  MD|  DL|  RQ|  NT|  AU|  AU|  RS|
  |                     |  CF|  CX|  CX|  CX|  NT|  FY|  EP|  CX|  IP|
  |_____________________|____|____|____|____|____|____|____|____|____|
  | ResponseAck         |  O |  O |  O |  O |  O |  O |  O |  O |  O |
  | BearerInformation   |  M |  O |  O |  O |  O |  F |  F |  F |  F |
  | CallId              |  F |  M |  M |  O |  F |  F |  F |  F |  F |
  | ConnectionId        |  F |  F |  M |  O |  F |  F |  F |  M |  F |
  | RequestIdentifier   |  F |  O+|  O+|  O+|  M |  M |  F |  F |  F |
  | LocalConnection     |  F |  O |  O |  F |  F |  F |  F |  F |  F |
  | Options             |    |    |    |    |    |    |    |    |    |
  | Connection Mode     |  F |  M |  M |  F |  F |  F |  F |  F |  F |
  | RequestedEvents     |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
  | SignalRequests      |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
  | NotifiedEntity      |  F |  O |  O |  O |  O |  O |  F |  F |  F |
  | ReasonCode          |  F |  F |  F |  O |  F |  F |  F |  F |  O |
  | ObservedEvents      |  F |  F |  F |  F |  F |  M |  F |  F |  F |
  | DigitMap            |  F |  O |  O |  O |  O |  F |  F |  F |  F |
  | Connection          |  F |  F |  F |  O |  F |  F |  F |  F |  F |
  | parameters          |    |    |    |    |    |    |    |    |    |
  | Specific Endpoint ID|  F |  F |  F |  F |  F |  F |  F |  F |  F |
  | Second Endpoint ID  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
  | RequestedInfo       |  F |  F |  F |  F |  F |  F |  M |  M |  F |
  | QuarantineHandling  |  F |  O |  O |  O |  O |  F |  F |  F |  F |
  | DetectEvents        |  F |  O |  O |  O |  O |  F |  F |  F |  F |
  | EventStates         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
  | RestartMethod       |  F |  F |  F |  F |  F |  F |  F |  F |  M |
  | RestartDelay        |  F |  F |  F |  F |  F |  F |  F |  F |  O |
  | SecondConnectionID  |  F |  F |  F |  F |  F |  F |  F |  F |  F |
  | Capabilities        |  F |  F |  F |  F |  F |  F |  F |  F |  F |
  |_____________________|____|____|____|____|____|____|____|____|____|
  | RemoteConnection    |  F |  O |  O |  F |  F |  F |  F |  F |  F |
  | Descriptor          |    |    |    |    |    |    |    |    |    |
  | LocalConnection     |  F |  F |  F |  F |  F |  F |  F |  F |  F |
  | Descriptor          |    |    |    |    |    |    |    |    |    |
  |_____________________|____|____|____|____|____|____|____|____|____|

   Note (+) that the RequestIdentifier parameter is optional in
   connection creation, modification and deletion commands, but that it
   becomes mandatory if the command contains an encapsulated
   notification request.

   Note (*) that the RequestedEvents and SignalRequests parameters are
   optional in the NotificationRequest. If these parameters are omitted,
   the corresponding lists will be considered empty.

   If implementers need to experiment with new parameters, for example
   when developing a new application of MGCP, they should identify these
   parameters by names that start with the string "X-" or "X+", such as
   for example:

             X-FlowerOfTheDay: Daisy

   Parameter names that start with "X+" are critical parameter
   extensions.  An MGCP entity that receives a critical parameter
   extension that it cannot understand should refuse to execute the
   command.  It should respond with an error code 511 (Unrecognized
   extension).

   Parameter names that start with "X-" are non critical parameter
   extensions. An MGCP entity that receives a non critical parameter
   extension that it cannot understand can safely ignore that parameter.

3.2.2.1. Response Acknowledgement

   The response acknowledgement attribute is used to managed the "at-
   most-once" facility described in the "transmission over UDP" section.
   It contains a comma separated list of "confirmed transaction-id
   ranges".

   Each "confirmed transaction-id ranges" is composed of either one
   decimal number, when the range includes exactly one transaction, or
   two decimal numbers separated by a single hyphen, describing the
   lower and higher transaction identifiers included in the range.

   An example of response acknowledgement is:

K: 6234-6255, 6257, 19030-19044

3.2.2.2. Local connection options

   The local connection options describe the operational parameters that
   the Call Agent suggests to the gateway. These parameters are:

   *  The packetization period in milliseconds, encoded as the keyword
      "p", followed by a colon and a decimal number. If the Call Agent
      specifies a range of values, the range will be specified as two
      decimal numbers separated by an hyphen.

   *  The preferred type of compression algorithm, encoded as the
      keyword "a", followed by a colon and a character string. If the
      Call Agent specifies a list of values, these values will be
      separated by a semicolon.

   *  The bandwidth in kilobits per second (1000 bits per second),
      encoded as the keyword "b", followed by a colon and a decimal
      number. If the Call Agent specifies a range of values, the range
      will be specified as two decimal numbers separated by an hyphen.

   *  The echo cancellation parameter, encoded as the keyword "e",
      followed by a colon and the value "on" or "off".

   *  The gain control parameter, encoded as the keyword "gc", followed
      by a colon a value which can be either the keyword "auto" or a
      decimal number (positive or negative) representing the number of
      decibels of gain.

   *  The silence suppression parameter, encoded as the keyword "s",
      followed by a colon and the value "on" or "off".

   *  The type of service parameter, encoded as the keyword "t",
      followed by a colon and the value encoded as two hexadecimal
      digits.

   *  The resource reservation parameter, encoded as the keyword "r",
      followed by a colon and the value "g" (guaranteed service), "cl"
      (controlled load) or "be" (best effort).

   *  The encryption key, encoded as the keyword "k" followed by a colon
      and a key specification, as defined for the parameter "K" of SDP
      (RFC 2327).

   *  The type of network, encoded as the keyword "nt" followed by a
      colon and the type of network encoded as the keyword "IN", "ATM"
      or "LOCAL".

   Each of the parameters is optional. When several parameters are
   present, the values are separated by a comma.

   Examples of connection descriptors are:

             L: p:10, a:PCMU
             L: p:10, a:G726-32
             L: p:10-20, b:64
             L: b:32-64, e:off

These set of attributes may be extended by extension attributes.

   Extension attributes are composed of an attribute name, followed by a
   semi-colon and by an attribute value. The attribute name should start
   by the two characters "x+", for a mandatory extensions, or "x-", for
   a non mandatory extension.  If a gateway receives a mandatory
   extension attribute that it does not recognize, it should reject the
   command with an error code 525 (Unknown extension in
   LocalConnectionOptions).

3.2.2.3. Capabilities

   Capabilities inform the Call Agent about endpoints' capabilities when
   audited. The encoding of capabilities is based on the Local
   Connection Options encoding for the parameters that are common to
   both. In addition, capabilities can also contain a list of supported
   packages, and a list of supported modes.

   The parameters used are:

   *
      A list of supported codecs. The following parameters will apply to
      all codecs specified in this list.  If there is a need to specify
      that some parameters, such as e.g. silence suppression, are only
      compatible with some codecs, then the gateway will return several
      LocalConnectionOptions parameters, one for each set of codecs.

   Packetization Period:
      A range may be specified.

   Bandwidth:
      A range corresponding to the range for packetization periods may
      be specified (assuming no silence suppression). If absent, the
      values will be deduced from the codec type.

   Echo Cancellation:
      "on" if echo cancellation is supported for this codec, "off"
      otherwise. The default is support.

   Silence Suppression:
      "on" if silence suppression is supported for this codec, "off"
      otherwise. The default is support.

   Gain Control:
      "0" if gain control is not supported.  The default is support.

   Type of Service:
      The value "0" indicates no support for type of service, all other
      values indicate support for type of service. The default is
      support.

   Resource Reservation:
      The parameter indicates the reservation services that are
      supported, in addition to best effort.  The value "g" is encoded
      when the gateway supports both the guaranteed and the controlled
      load service, "cl" when only the controlled load service is
      supported.  The default is "best effort."

   Encryption Key:
      Encoding any value indicates support for encryption.  Default is
      no support.

   Type of network:
      The keyword "nt", followed by a colon and a semicolon separated
      list of supported network types.  This parameter is optional.

   Event Packages
      The event packages supported by this endpoint encoded as the
      keyword "v", followed by a colon and a character string. If a list
      of values is specified, these values will be separated by a
      semicolon.  The first value specified will be the default package
      for that endpoint.

   Modes
      The modes supported by this endpoint encoded as the keyword "m",
      followed by a colon and a semicolon-separated list of supported
      connection modes for this endpoint.

3.2.2.4. Connection parameters

   Connection parameters are encoded as a string of type and value
   pairs, where the type is a either letter identifier of the parameter
   or an extension type, and the value a decimal integer. Types are
   separated from value by an `=' sign. Parameters are encoded from each
   other by a comma.

P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48

3.2.2.5. Reason Codes

   Reason codes are three-digit numeric values. The reason code is
   optionally followed by a white space and commentary, e.g.:

      900 Endpoint malfunctioning

   A list of reason-codes can be found in Section 2.5.

3.2.2.6. Connection mode

3.2.2.7. Coding of event names

   Event names are composed of an optional package name, separated by a
   slash (/) from the name of the actual event.  The event name can
   optionally be followed by an at sign (@) and the identifier of a
   connection on which the event should be observed. Event names are
   used in the RequestedEvents, SignalRequests and ObservedEvents
   parameter.

   Each signal has one of the following signal-types associated with:
   On/Off (OO), Time-out (TO), Brief (BR). (These signal types are
   specified in the package definitions, and are not present in the
   messages.)  On/Off signals can be parameterized with a "+" to turn
   the signal on, or a "-" to turn the signal off. If an on/off signal
   is not parameterized, the signal is turned on. Both of the following
   will turn the vmwi signal on:

      vmwi(+), vmwi

   The following are valid examples of event names:

       ____________________________________________________________
      | L/hu        |   on-hook transition, in the line package   |
      | F/0         |   digit 0 in the MF package                 |
      | fh          |   Flash-hook, assuming that the line package|
      |             |   is a default package for the end point.   |
      | G/rt@0A3F58 |   Ring back signal on                       |
      |             |   connection "0A3F58".                      |
      |_____________|_____________________________________________|

   In addition, the range and wildcard notation of events can be used,
   instead of individual names, in the RequestedEvents and DetectEvents
   parameters. The star sign can be used to denote "all connections",
   and the dollar sign can be used to denote the "current" connection.
   The following are valid examples of such notations:

       __________________________________________________________
      | M/[0-9]   |   Digits 0 to 9 in the MF package           |
      | fh        |   Flash-hook, assuming that the line package|
      |           |   is a default package for the end point.   |
      | [0-9*#A-D]|   All digits and letters in the DTMF        |
      |           |   packages (default for endpoint).          |
      | T/$       |   All events in the trunk packages.         |
      | R/qa@*    |   The quality alert event in all            |
      |           |   connections                               |
      | R/rt@$    |   Ringback on current connection            |
      |___________|_____________________________________________|

3.2.2.8. RequestedEvents

   The RequestedEvent parameter provides the list of events that have
   been requested. The event codes are described in the previous
   section.

   Each event can be qualified by a requested action, or by a list of
   actions. The actions, when specified, are encoded as a list of
   keywords, enclosed in parenthesis and separated by commas. The codes
   for the various actions are:

                ______________________________________
               | Action                       |  Code|
               |______________________________|______|
               | Notify immediately           |  N   |
               | Accumulate                   |  A   |
               | Treat according to digit map |  D   |
               | Swap                         |  S   |
               | Ignore                       |  I   |
               | Keep Signal(s) active        |  K   |
               | Embedded Notification Request|  E   |
               |______________________________|______|

   When no action is specified, the default action is to notify the
   event.  This means that, for example, ft and ft(N) are equivalent.
   Events that are not listed are ignored.

   The digit-map action can only be specified for the digits, letters
   and interdigit timers in the MF and DTMF packages, or in other
   packages that would define the encoding of digits and timers.

   The requested list is encoded on a single line, with event/action
   groups separated by commas. Examples of RequestedEvents encoding are:

R: hu(N), hf(S,N)
R: hu(N), [0-9#T](D)

   In the case of the "enable" action, the embedded notification request
   parameters are encoded as a list of up to three parameter groups,
   separated by commas.  Each group start by a one letter identifier,
   followed by a list of parameters enclosed between parenthesis.  The
   first optional parameter group, identified by the letter "R", is the
   enabled value of the RequestedEvents parameter.  The second optional
   group, identified by the letter "S", is the enabled value of the
   SignalRequests parameter.  The third optional group, identified by
   the letter "D", is the enabled value of the DigitMap. (Note that some
   existing implementation may encode these three components in a
   different order.)

   If the RequestedEvents is not present, the parameter will be set to a
   null value.  If the SignalRequest is not present, the parameter will
   be set to a null value. If the DigitMap is absent, the current value
   should be used. The following are valid examples of embedded
   requests:

         R: hd(E(R([0-9#T](D),hu(N)),S(dl),D([0-9].[#T])))
         R: hd(E(R([0-9#T](D),hu(N)),S(dl)))

3.2.2.9. SignalRequests

   The SignalRequests parameter provides the name of the signals that
   have been requested. Each signal is identified by a name, as
   indicated in the previous section.

   Several signals, such as for example announcement or ADSI display,
   can be qualified by additional parameters:

   *  the name and parameters of the announcement,

   *  the string that should be displayed.

   These parameters will be encoded as a set of UTF8 character strings,
   spearated by comams and enclosed within parenthesis, as in:
      S: adsi("123456 Francois Gerard")
      S: ann(no-such-number, 1234567)

   When several signals are requested, their codes are separated by a
   comma, as in:

S: asdi(123456 Your friend), rg

3.2.2.10. ObservedEvent

   The observed event parameters provides the list of events that have
   been observed. The event codes are the same as those used in the
   NotificationRequest. Events that have been accumulated according to
   the digit map may be grouped in a single string; they should be
   reported as lists of isolated events if other events where detected
   during the digit accumulation. Examples of observed actions are:

        O: L/hu
        O: 8295555T
        O: 8,2,9,5,5,L/hf,5,5,T
        O: L/hf, L/hf, L/hu

3.2.2.11. RequestedInfo

   The RequestedInfo parameter contains a comma separated list of
   parameter codes, as defined in the "Parameter lines" section.  For
   example, if one wants to audit the value of the NotifiedEntity,
   RequestIdentifier, RequestedEvents, SignalRequests, DigitMap,
   QuarantineHandling and DetectEvents parameters, The value of the
   RequestedInfo parameter will be:

         F:N,X,R,S,D,Q,T

   The capabilities request, in the AuditEndPoint command, is encoded by
   the keyword "A", as in:

         F:A

3.2.2.12. QuarantineHandling

   The quarantine handling parameter contains a list of comma separated
   keywords:

   *  The keyword "process" or "discard" to indicate the treatment of
      quarantined events.  If neither process or discard is present,
      process is assumed.

   *  The keyword "step" or "loop" to indicate whether exactly at most
      one notification is expected, or whether multiple notifications
      are allowed. If neither step or loop is present, step is assumed.
      The following values are valid examples:

               Q:loop
               Q:process
               Q:discard,loop

3.2.2.13. DetectEvents

The DetectEvent parameter is encoded as a comma separated list of
events, such as for example:

T: hu,hd,hf,[0-9#*]

It should be noted, that no actions can be associated with the
events.

3.2.2.14. EventStates

   The EventStates parameter is encoded as a comma separated list of
   events, such as for example:

      ES: hu

   It should be noted, that no actions can be associated with the
   events.

3.2.2.15. RestartMethod

   The RestartMethod parameter is encoded as one of the keywords
   "graceful", "forced", "restart", "disconnected" or "cancel-graceful"
   as for example:

         RM:restart

3.2.2.16. Bearer Information

   The values of the bearer informations are encoded as a comma
   separated list of attributes, represented by an attribute name,
   separated by a colon from an attribute value.

   The only attribute that is defined is the "encoding" (code "e"),
   whose defined values are "A" (A-law) and "mu" (mu-law).

   An example of bearer information encoding is:

B: e:mu

3.3. Format of response headers

   The response header is composed of a response line, optionally
   followed by headers that encode the response parameters.

   An example of response header could be:

         200 1203 OK

   The response line starts with the response code, which is a three
   digit numeric value. The code is followed by a white space, the
   transaction identifier, and an optional commentary preceded by a
   white space.

   The following table describe the parameters whose presence is
   mandatory or optional in a response header, as a function of the
   command that triggered the response. The letter M stands for
   mandatory, O for optional and F for forbidden.

    ___________________________________________________________________
   | Parameter name      |  EP|  CR|  MD|  DL|  RQ|  NT|  AU|  AU|  RS|
   |                     |  CF|  CX|  CX|  CX|  NT|  FY|  EP|  CX|  IP|
   |_____________________|____|____|____|____|____|____|____|____|____|
   | ResponseAck         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | BearerInformation   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | CallId              |  F |  F |  F |  F |  F |  F |  F |  O |  F |
   | ConnectionId        |  F |  O*|  F |  F |  F |  F |  F |  F |  F |
   | RequestIdentifier   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | LocalConnection     |  F |  F |  F |  F |  F |  F |  O |  O |  F |
   | Options             |    |    |    |    |    |    |    |    |    |
   | Connection Mode     |  F |  F |  F |  F |  F |  F |  F |  O |  F |
   | RequestedEvents     |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | SignalRequests      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | NotifiedEntity      |  F |  F |  F |  F |  F |  F |  F |  F |  O |
   | ReasonCode          |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | ObservedEvents      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | DigitMap            |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | Connection          |  F |  F |  F |  O |  F |  F |  F |  O |  F |
   | Parameters          |    |    |    |    |    |    |    |    |    |
   | Specific Endpoint ID|  F |  O |  F |  F |  F |  F |  F |  F |  F |
   | RequestedInfo       |  F |  F |  F |  F |  F |  F |  F |  F |  F |
   | QuarantineHandling  |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | DetectEvents        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | EventStates         |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | RestartMethod       |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | RestartDelay        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | Capabilities        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
   | SecondConnectionId  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
   | SecondEndpointID    |  F |  O |  F |  F |  F |  F |  F |  F |  F |
   |_____________________|____|____|____|____|____|____|____|____|____|
   | LocalConnection     |  F |  M |  O |  F |  F |  F |  F |  O*|  F |
   | Descriptor          |    |    |    |    |    |    |    |    |    |
   | RemoteConnection    |  F |  F |  F |  F |  F |  F |  F |  O*|  F |
   | Descriptor          |    |    |    |    |    |    |    |    |    |
   |_____________________|____|____|____|____|____|____|____|____|____|

   In the case of a CreateConnection message, the response line is
   followed by a Connection-Id parameter. It may also be followed a
   Specific-Endpoint-Id parameter, if the creation request was sent to a
   wildcarded Endpoint-Id. The connection-Id parameter is marked as
   optional in the Table.  In fact, it is mandatory with all positive
   responses, when a connection was created, and forbidden when the
   response is negative, when no connection as created.

   In the case of a DeleteConnection message, the response line is
   followed by a Connection Parameters parameter, as defined in section
   3.2.2.2.

   A LocalConnectionDescriptor should be transmitted with a positive
   response (code 200) to a CreateConnection. It may be transmitted in
   response to a ModifyConnection command, if the modification resulted
   in a modification of the session parameters. The
   LocalConnectionDescriptor is encoded as a "session description," as
   defined in section 3.4. It is separated from the response header by
   an empty line.

   When several session descriptors are encoded in the same response,
   they are encoded one after each other, separated by an empty line.
   This is the case for example when the response to an audit connection
   request carries both a local session description and a remote session
   description, as in:

         200 1203 OK
         C: A3C47F21456789F0
         N: [128.96.41.12]
         L: p:10, a:PCMU;G726-32
         M: sendrecv
         P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48

         v=0
         c=IN IP4 128.96.41.1
         m=audio 1296 RTP/AVP 0

         v=0
         c=IN IP4 128.96.63.25
         m=audio 1296 RTP/AVP 0 96
         a=rtpmap:96 G726-32/8000

   In this example, according to the SDP syntax, each description starts
   with a "version" line, (v=...).  The local description is always
   transmitted before the remote description. If a connection descriptor
   is requested, but it does not exist for the connection audited, that
   connection descriptor will appear with the SDP protocol version field
   only.

3.4. Formal syntax description of the protocol

   In this section, we provided a formal description of the protocol
   syntax, following the "Augmented BNF for Syntax Specifications"
   defined in RFC 2234.

MGCPMessage = MGCPCommand / MGCPResponse

MGCPCommand = MGCPCommandLine 0*(MGCPParameter) [EOL *SDPinformation]

MGCPCommandLine = MGCPVerb 1*(WSP) <transaction-id> 1*(WSP)
                        <endpointName> 1*(WSP) MGCPversion EOL

MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"
         / "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb

extensionVerb = "X" 3(ALPHA / DIGIT)

transaction-id = 1*9(DIGIT)

endpointName =  localEndpointName "@" DomainName
LocalEndpointName = LocalNamePart 0*("/" LocalNamePart)
LocalNamePart = AnyName / AllName / NameString
AnyName = "$"
AllNames = "*"
NameString = 1*(range-of-allowed-characters)
DomainName = 1*256(ALPHA / DIGIT / "." / "-") ; as defined in RFC 821

MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)
              [1*(WSP) ProfileName]
ProfileName = 1*(range-of-allowed-characters)

MGCPParameter = ParameterValue EOL

ParameterValue = ("K" ":" 0*WSP <ResponseAck>) /
                 ("B" ":" 0*WSP <BearerInformation>) /
                 ("C" ":" 0*WSP <CallId>) /
                 ("I" ":" 0*WSP <ConnectionId>) /
                 ("N" ":" 0*WSP <NotifiedEntity>) /
                 ("X" ":" 0*WSP <RequestIdentifier>) /
                 ("L" ":" 0*WSP <LocalConnectionOptions>) /
                 ("M" ":" 0*WSP <ConnectionMode>) /
                 ("R" ":" 0*WSP <RequestedEvents>) /
                 ("S" ":" 0*WSP <SignalRequests>) /
                 ("D" ":" 0*WSP <DigitMap>) /
                 ("O" ":" 0*WSP <ObservedEvents>) /
                 ("P" ":" 0*WSP <ConnectionParameters>) /
                 ("E" ":" 0*WSP <ReasonCode>) /

                 ("Z" ":" 0*WSP <SpecificEndpointID>) /
                 ("Z2" ":" 0*WSP <SecondEndpointID>) /
                 ("I2" ":" 0*WSP <SecondConnectionID>) /
                 ("F" ":" 0*WSP <RequestedInfo>) /
                 ("Q" ":" 0*WSP <QuarantineHandling>) /
                 ("T" ":" 0*WSP <DetectEvents>) /
                 ("RM" ":" 0*WSP <RestartMethod>) /
                 ("RD" ":" 0*WSP <RestartDelay>) /
                 ("A" ":" 0*WSP <Capabilities>) /
                 ("ES" ":" 0*WSP <EventStates>) /
                     (extensionParameter ":" 0*WSP <parameterString>)

ResponseAck =  confirmedTransactionIdRange
               *[ ","  confirmedTransactionIdRange ]

confirmedTransactionIdRange = 1*9DIGIT [ "-" 1*9DIGIT ]

BearerInformation = BearerAttribute 0*("," 0*WSP BearerAttribute)
BearerAttribute = ("e" ":" <BearerEncoding>)
BearerEncoding = "A" / "mu"

CallId = 1*32(HEXDIG)

// The audit request response may include a list of identifiers
ConnectionId = 1*32(HEXDIG) 0*("," 1*32(HEXDIG))
SecondConnectionID = ConnectionId

NotifiedEntity = [LocalName "@"] DomainName [":" portNumber]
LocalName = 1*32(suitableCharacter)
portNumber = 1*5(DIGIT)

RequestIdentifier = 1*32(HEXDIG)

LocalConnectionOptions = [ LocalOptionValue 0*(WSP)
                 0*("," 0*(WSP) LocalOptionValue 0*(WSP)) ]
LocalOptionValue = ("p" ":" <packetizationPeriod> )
                 / ("a" ":" <compressionAlgorithm> )
                 / ("b" ":" <bandwidth> )
                 / ("e" ":" <echoCancellation> )
                 / ("gc" ":" <gainControl> )
                 / ("s" ":" <silenceSuppression> )
                 / ("t" ":" <typeOfService> )
                 / ("r" ":" <resourceReservation> )
                 / ("k" ":" <encryptionmethod>[":"<encryptionKey>])
                 / ("nt" ":" <typeOfNetwork> )
                 / (localOptionExtensionName ":"
                 / localOptionExtensionValue)

Capabilities = [ CapabilityValue 0*(WSP)
                 0*("," 0*(WSP) CapabilityValue 0*(WSP)) ]

CapabilityValue = LocalOptionValue
                / ("v" ":" <supportedPackages>)
                / ("m" ":" <supportedModes> )

packetizationPeriod = 1*4(DIGIT)["-" 1*4(DIGIT)]
compressionAlgorithm = algorithmName 0*(";" algorithmName)
algorithmName = 1*32(SuitableCharacter)
bandwidth = 1*4(DIGIT)["-" 1*4(DIGIT)]
echoCancellation = "on" / "off"
gainControl = "auto" / ["-"]1*4(DIGIT)
silenceSuppression = "on" / "off"
typeOfService = 2HEXDIG
resourceReservation = "g" / "cl" / "be"

;encryption parameters are coded as in SDP (RFC 2327)
encryptiondata = ( "clear" ":" <encryptionKey> )
               / ( "base64" ":" <encodedEncryptionKey> )
               / ( "uri" ":" <URItoObtainKey> )
               / ( "prompt" ) ; defined in SDP, not usable in MGCP!
encryptionKey = 1*(SuitableCharacter / SP)
encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=")
URItoObtainKey = 1*(SuitableCharacter) / quotedString

typeOfNetwork = "IN" / "ATM" / "LOCAL"
supportedModes= ConnectionMode 0*(";" ConnectionMode)
supportedPackages = packageName 0*(";" packageName)

localOptionExtensionName = "x" ("+"/"-") 1*32(SuitableCharacter)
localOptionExtensionValue = 1*32(SuitableCharacter) / quotedString

ConnectionMode = "sendonly" / "recvonly" / "sendrecv" /
                 "confrnce" / "inactive" / "loopback" /
                 "conttest" / "netwloop" / "netwtest" / "data"

RequestedEvents = [requestedEvent 0*("," 0*(WSP) requestedEvent)]
requestedEvent = eventName [ "(" requestedActions ")" ]

eventName = [ (packageName / "*") "/" ] (eventId / "all" / eventRange)
            [ "@" (ConnectionId / "$" / "*") ]
packageName = 1*(ALPHA / DIGIT / HYPHEN)
eventId = 1*(SuitableCharacter)
eventRange = "[" 1*(DIGIT / DTMFLetter / "*" / "#" /

                        (DIGIT "-" DIGIT)/(DTMFLetter "-"
                         DTMFLetter)) "]"

requestedActions = requestedAction 0*("," 0*(WSP) requestedAction)
requestedAction = "N" / "A" / "D" / "S" / "I" / "K" /
                  "E" "(" EmbeddedRequest ")"

EmbeddedRequest =   (      "R" "(" EmbeddedRequestList ")"
                      ["," "S" "(" EmbeddedSignalRequest ")" ]
                      ["," "D" "(" EmbeddedDigitMap ")" ] )
                /   (      "S" "(" EmbeddedSignalRequest ")"
                      ["," "D" "(" EmbeddedDigitMap ")" ] )
                /   (      "D" "(" EmbeddedDigitMap ")" )

EmbeddedRequestList = RequestedEvents
EmbeddedSignalRequest = SignalRequests
EmbeddedDigitMap = DigitMap

SignalRequests = [ SignalRequest 0*("," 0*(WSP) SignalRequest ) ]
SignalRequest = eventName [ "(" eventParameters ")" ]
eventParameters = eventParameter 0*("," 0*(WSP) eventParameter)
eventParameter = eventParameterString / quotedString
eventParameterString = 1*(SuitableCharacter)

DigitMap = DigitString  / "(" DigitStringList ")"
DigitStringList = DigitString 0*( "|" DigitString )
DigitString = 1*(DigitStringElement)
DigitStringElement = DigitPosition ["."]
DigitPosition = DigitMapLetter / DigitMapRange
DigitMapLetter = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"
DigitMapRange =  "x" / "[" 1*DigitLetter "]"
DigitLetter ::= *((DIGIT "-" DIGIT ) / DigitMapLetter)

ObservedEvents = SignalRequests
EventStates = SignalRequests

ConnectionParameters = [ConnectionParameter
                        0*( "," 0*(WSP) ConnectionParameter )
ConnectionParameter = ( "PS" "=" packetsSent )
                    / ( "OS" "=" octetsSent )
                    / ( "PR" "=" packetsReceived )
                    / ( "OR" "=" octetsReceived )
                    / ( "PL" "=" packetsLost )
                    / ( "JI" "=" jitter )
                    / ( "LA" "=" averageLatency )
                    / ( ConnectionParameterExtensionName "="
                        ConnectionParameterExtensionValue )
packetsSent = 1*9(DIGIT)

octetsSent = 1*9(DIGIT)
packetsReceived = 1*9(DIGIT)
octetsReceived = 1*9(DIGIT)
packetsLost = 1*9(DIGIT)
jitter = 1*9(DIGIT)
averageLatency = 1*9(DIGIT)
ConnectionParameterExtensionName = "X" "-" 2*ALPHA
ConnectionParameterExtensionValue = 1*9(DIGIT)

ReasonCode = 3DIGIT [SPACE 1*(%x20-7E)]

SpecificEndpointID = endpointName
SecondEndpointID = endpointName

RequestedInfo = [infoCode 0*("," infoCode)]

infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" /
           "R" / "S" / "D" / "O" / "P" / "E" / "Z" /
           "Q" / "T" / "RC" / "LC" / "A" / "ES" / "RM" / "RD"

QuarantineHandling = loopControl / processControl /
              (loopControl "," processControl )
loopControl = "step" / "loop"
processControl = "process" / "discard"

DetectEvents = [eventName 0*("," eventName)]

RestartMethod = "graceful" / "forced" / "restart" / "disconnected"

RestartDelay = 1*6(DIGIT)

extensionParameter = "X" ("-"/"+") 1*6(ALPHA / DIGIT)
parameterString = 1*(%x20-7F)

MGCPResponse = MGCPResponseLine 0*(MGCPParameter)
                [EOL *SDPinformation]

MGCPResponseLine = (<responseCode> 1*(WSP) <transaction-id>
                          [1*(WSP) <responseString>] EOL)
responseCode = 3DIGIT
responseString = *(%x20-7E)

SuitableCharacter= DIGIT / ALPHA / "+" / "-" / "_" / "&" /
                   "!" / "'" / "|" / "=" / "#" / "?" / "/" /
                   "." / "$" / "*" / ";" / "@" / "[" / "]" /
                   "^" / "`" / "{" / "}" / "~"

quotedString = DQUOTE visibleString

0*(quoteEscape visibleString) DQUOTE
quoteEscape = DQUOTE DQUOTE
visibleString = (%x00-21 / %x23-FF)
EOL = CRLF / LF

SDPinformation = ;See RFC 2327

3.5. Encoding of the session description

   The session description is encoded in conformance with the session
   description protocol, SDP. MGCP implementations are expected to be
   fully capable of parsing any conformant SDP message, and should send
   session descriptions that strictly conform to the SDP standard. The
   usage of SDP actually depends on the type of session that is being,
   as specified in the "mode" parameter:

   *  if the mode is set to "data", the session description describes
      the configuration of a data access service.

   *  if the mode is set to any other value, the session description is
      for an audio service.

   For an audio service, the gateway will consider the information
   provided in SDP for the "audio" media. For a data service, the
   gateway will consider the information provided for the "network-
   access" media.

3.5.1. Usage of SDP for an audio service

   In a telephony gateway, we only have to describe sessions that use
   exactly one media, audio. The parameters of SDP that are relevant for
   the telephony application are:

      At the session description level:

      *  The IP address of the remote gateway (in commands) or of the
         local gateway (in responses), or multicast address of the audio
         conference, encoded as an SDP "connection data" parameter. This
         parameter specifies the IP address that will be used to
         exchange RTP packets.

      For the audio media:

      *  Media description field (m) specifying the audio media, the
         transport port used for receiving RTP packets by the remote
         gateway (commands) or by the local gateway (responses), the

         RTP/AVP transport, and the list of formats that the gateway
         will accept. This list should normally always include the code
         0 (reserved for PCMU).

      *  Optionally, RTPMAP attributes that define the encoding of
         dynamic audio formats,

      *  Optionally, a packetization period (packet time) attribute
         (Ptime) defining the duration of the packet,

      *  Optionally, an attribute defining the type of connection
         (sendonly, recvonly, sendrecv, inactive). Note that this
         attribute does not have a direct relation with the "Mode"
         parameter of MGCP.  In fact, the SDP type of connection will
         most of the time be set to "sendrecv", regardless of the value
         used by MGCP.  Other values will only be used rarely, for
         example in the case of information or announcement servers that
         need to establish one way connections.

      *  The IP address of the remote gateway (in commands) or of the
         local gateway (in responses), if it is not present at the
         session level.

      An example of SDP specification for an audio connection could be:

            v=0
            c=IN IP4 128.96.41.1
            m=audio 3456 RTP/AVP 0 96
            a=rtpmap:96 G726-32/8000

   There is a request, in some environments, to use the MGCP to
   negotiate connections that will use other transmission channels than
   RTP over UDP and IP. This will be detailed in an extension to this
   document.

3.5.2. Usage of SDP in a network access service

   The parameters of SDP that are relevant for a data network access
   application are:

      For the data media:

      *  Media description field (m) specifying the network access
         media, identified by the code "m=nas/xxxx", where "xxxx"
         describes the access control method that should be used for
         parametrizing the network access, as specified below. The field
         may also specify the port that should be used for contacting
         the server, as specified in the SDP syntax.

      *  Connection address parameter (c=) specifying the address, or
         the domain name, of the server that implement the access
         control method. This parameter may also be specified at the
         session level.

      *  Optionally, a bearer type attribute (a=bearer:) describing the
         type of data connection to be used, including the modem type.

      *  Optionally, a framing type attribue (a=framing:) describing the
         type of framing that will be used on the channel.

      *  Optionally, attributes describing the called number
         (a=dialed:), the number to which the call was delivered
         (a=called:) and the calling number (a=dialing:).

      *  Optionally, attributes describing the range of addresses that
         could be used by the dialup client on its LAN (a=subnet:).

      *  Optionally, an encryption key, encoded as specified in the SDP
         protocol(k=).

   The connection address shall be encoded as specified in the SDP
   standard. It will be used in conjunction with the port specified in
   the media line to access a server, whose type will one of:

       __________________________________________________________
      | Method name|  Method description                        |
      |____________|____________________________________________|
      | radius     |  Authentication according                  |
      |            |  to the Radius protocol.                   |
      | tacacs     |  Authentication according                  |
      |            |  to the TACACS+ protocol.                  |
      | diameter   |  Authentication according                  |
      |            |  to the Diameter protocol.                 |
      | l2tp       |  Level 2 tunneling protocol.               |
      |            |  The address and port are those of the LNS.|
      | login      |  Local login. (There is normally           |
      |            |  no server for that method.)               |
      | none       |  No authentication required.               |
      |            |  (The call was probably vetted             |
      |            |  by the Call Agent.)                       |
      |____________|____________________________________________|

   If needed, the gateway may use the key specified in the announcement
   to access the service. That key, in particular, may be used for the
   establishment of an L2TP tunnel.

   The bearer attribute is composed of a bearer name and an optional
   extension.  The bearer type specifies the type of modulation (modem
   name) or, in the case of digital connections, the type of ISDN
   service (8 bits, 7 bits).  When an extension is present, it is
   separated from the bearer name by a single slash (/).  The valid
   values of the bearer attribute are defined in the following table:

    ____________________________________________________________________
   | Type of bearer description      |  Example of values              |
   |_________________________________|_________________________________|
   | ITU modem standard              |  V.32, V.34, V.90.              |
   | ITU modem standard qualified    |  v.90/3com,                     |
   | by a manufacturer name          |  v.90/rockwell,                 |
   |                                 |  v.90/xxx                       |
   | Well known modem types          |  X2, K56flex                    |
   | ISDN transparent access, 64 kbps|  ISDN64                         |
   | ISDN64 + V.110                  |  ISDN64/V.110                   |
   | ISDN64 + V.120                  |  ISDN64/V.120                   |
   | ISDN transparent access, 56 kbps|  ISDN56                         |
   | Informal identification         |  (Requires coordination between |
   |                                 |  the Call Agent and the gateway)|
   |_________________________________|_________________________________|

   The valid values of the framing attribute are defined in the
   following table:

             _________________________________________________
            | Type of framing description|  Example of values|
            |____________________________|___________________|
            | PPP, asynchronous framing  |  ppp-asynch       |
            | PPP, HDLC framing          |  ppp-hdlc         |
            | SLIP, asynchronous         |  slip             |
            | Asynchronous, no framing   |  asynch           |
            |____________________________|___________________|

   The network access authentication parameter provides instructions on
   the access control that should be exercized for the data call. This
   optional attribute is encoded as:

        "a=subnet:" <network type> <address type>
           <connection address> "/" <prefix length>

   Where the parameters "network type", "address type", and "connection
   address" are formatted as defined for the connection address
   parameter (c=) in SDP, and where the "prefix length" is a decimal
   representation of the number of bits in the prefix.

   Examples of SDP announcement for the network access service could be:

         v=0
         m=nas/radius
         c=IN IP4 radius.example.net
         a=bearer:v.34
         a=framing:ppp-asynch
         a=dialed:18001234567
         a=called:12345678901
         a=dialing:12340567890

         v=0
         m=nas/none
         c=IN IP4 128.96.41.1
         a=subnet:IN IP4 123.45.67.64/26
         a=bearer:isdn64
         a=framing:ppp-sync
         a=dialed:18001234567
         a=dialing:2345678901

         v=0
         c=IN IP4 access.example.net
         m=nas/l2tp
         k=clear:some-shared-secret
         a=bearer:v.32
         a=framing:ppp-asynch
         a=dialed:18001234567
         a=dialing:2345678901

3.5.3. Usage of SDP for ATM connections

   The specification of the SDP payload for ATM connections will be
   described in a companion document, "Usage of MGCP to control Voice
   over ATM gateways." The following text is indicative.

   The SDP payload will specify:

   *  That the connection is to be established over an ATM interface,
      using the "c=" parameter of SDP to specify an address in the ATM
      family, the ATM addressing variant (NSAP, UNI, E.164) and the ATM
      address.

   *  The "m=audio" parameter will specify the audio encoding and, if
      needed, the VPI and VCI.

   *  Additional attributes parameters (a=) will be used to specify the
      ATM coding variants, such as the type of adaptation layer and the
      error correction or loss compenmsation algorithms.

   An example of SDP payload for an ATM connection could be:

         v=0 c=ATM NSAP
         47.0091.8100.0000.0060.3e64.fd01.0060.3e64.fd01.fe m=audio
         5/1002 ATM/AVP PCMU a=connection_type:AAL2

3.5.4. Usage of SDP for local connections

   When MGCP is used to set up internal connections within a single
   gateway, the SDP format is used to encode the parameters of that
   connection.  The following parameters will be used:

   *  The connection parameter (C=) will specify that the connection is
      local, using the keyword "LOCAL" as network type space, the
      keyword "EPN" (endpoint name) as  address type, and the name of
      the endpoint as the connection-address.

   *  The "m=audio" parameter will specify a port number, which will
      always be set to 0, the type of protocol, always set to the
      keyword LOCAL, and the type of encoding, using the same
      conventions used for RTP (RTP payload numbers.) The type of
      encoding should normally be set to 0 (PCMU).

   An example of local SDP payload could be:

         v=0
         c=LOCAL EPN X35V3+A4/13
         m=audio 0 LOCAL 0

3.6. Transmission over UDP

   MGCP messages are transmitted over UDP. Commands are sent to one of
   the IP addresses defined in the DNS for the specified endpoint . The
   responses are sent back to the source address of the commands.

   When no port is specified for the endpoint, the commands should be
   sent:

   *  by the Call Agents, to the default MGCP port for gateways, 2427.

   *  by the Gateways, to the default MGCP port for Call Agents, 2727.

3.6.1. Providing the At-Most-Once functionality

   MGCP messages, being carried over UDP, may be subject to losses. In
   the absence of a timely response, commands are repeated. Most MGCP
   commands are not idempotent.  The state of the gateway would become

   unpredictable if, for example, CreateConnection commands were
   executed several times.  The transmission procedures must thus
   provide an "At-Most-Once" functionality.

   MGCP entities are expected to keep in memory a list of the responses
   that they sent to recent transactions and a list of the transactions
   that are currently being executed. The transaction identifiers of
   incoming commands are compared to the transaction identifiers of the
   recent responses. If a match is found, the MGCP entity does not
   execute the transaction, but simply repeats the response. The
   remaining commands will be compared to the list of current
   transaction. If a match is found, the MGCP entity does not execute
   the transaction, which is simply ignored.

   The procedure use a long timer value, noted LONG-TIMER in the
   following.  The timer should be set larger than the maximum duration
   of a transaction, which should take into account the maximum number
   of repetitions, the maximum value of the repetition timer and the
   maximum propagation delay of a packet in the network.  A suggested
   value is 30 seconds.

   The copy of the responses can be destroyed either LONG-TIMER seconds
   after the response is issued, or when the gateway (or the call agent)
   receives a confirmation that the response has been received, through
   the "Response Acknowledgement attribute". For transactions that are
   acknowledged through this attribute, the gateway shall keep a copy of
   the transaction-id for LONG-TIMER seconds after the response is
   issued, in order to detect and ignore duplicate copies of the
   transaction request that could be produced by the network.

3.6.2. Transaction identifiers and three ways handshake

   Transaction identifiers are integer numbers in the range from 0 to
   999,999,999.  Call-agents may decide to use a specific number space
   for each of the gateways that they manage, or to use the same number
   space for all gateways that belong to some arbitrary group.  Call
   agents may decide to share the load of managing a large gateway
   between several independent processes.  These processes will share
   the same transaction number space.  There are multiple possible
   implementations of this sharing, such as having a centralized
   allocation of transaction identifiers, or pre-allocating non-
   overlapping ranges of identifiers to different processes.  The
   implementations must guarantee that unique transaction identifiers
   are allocated to all transactions that originate from a logical call
   agent, as defined in the "states, failover and race conditions"
   section. Gateways can simply detect duplicate transactions by looking
   at the transaction identifier only.

   The Response Acknowledgement Attribute can be found in any command.
   It carries a set of "confirmed transaction-id ranges."

   MGCP gateways may choose to delete the copies of the responses to
   transactions whose id is included in "confirmed transaction-id
   ranges" received in the Response Confirmation messages. They should
   silently discard further commands from that Call Agent when the
   transaction-id falls within these ranges.

   The "confirmed transaction-id ranges" values shall not be used if
   more than LONG-TIMER seconds have elapsed since the gateway issued
   its last response to that call agent, or when a gateway resumes
   operation.  In this situation, commands should be accepted and
   processed, without any test on the transaction-id.

   Commands that carry the "Response Acknowledgement attribute" may be
   transmitted in disorder.  The gateway shall retain the union of the
   "confirmed transaction-id ranges" received in recent commands.

3.6.3. Computing retransmission timers

   It is the responsibility of the requesting entity to provide suitable
   time outs for all outstanding commands, and to retry commands when
   time outs have been exceeded. Furthermore, when repeated commands
   fail to be acknowledged, it is the responsibility of the requesting
   entity to seek redundant services and/or clear existing or pending
   connections.

   The specification purposely avoids specifying any value for the
   retransmission timers. These values are typically network dependent.
   The retransmission timers should normally estimate the timer by
   measuring the time spent between the sending of a command and the
   return of a response. One possibility is to use the algorithm
   implemented in TCP-IP, which uses two variables:

   *  the average acknowledgement delay, AAD, estimated through an
      exponentially smoothed average of the observed delays,

   *  the average deviation, ADEV, estimated through an exponentially
      smoothed average of the absolute value of the difference between
      the observed delay and the current average

   The retransmission timer, in TCP, is set to the sum of the average
   delay plus N times the average deviation. In MGCP, the maximum value
   of the timer should however be bounded, in order to guarantee that no
   repeated packet will be received by the gateways after LONG-TIMER
   seconds.  A suggested maximum value is 4 seconds.

   After any retransmission, the MGCP entity should do the following:

   *  It should double the estimated value of the average delay, AAD

   *  It should compute a random value, uniformly distributed between
      0.5 AAD and AAD

   *  It should set the retransmission timer to the sum of that random
      value and N times the average deviation.

   This procedure has two effects. Because it includes an exponentially
   increasing component, it will automatically slow down the stream of
   messages in case of congestion. Because it includes a random
   component, it will break the potential synchronization between
   notifications triggered by the same external event.

3.6.4. Piggy backing

   There are cases when a Call Agent will want to send several messages
   at the same time to the same gateways.  When several MGCP messages
   have to be sent in the same UDP packets, they should be separated by
   a line of text that contain a single dot, as in for example:

         200 2005 OK
         DLCX 1244 card23/21@trgw-7.example.net MGCP 1.0
         C: A3C47F21456789F0
         I: FDE234C8

   The piggy-backed messages should be processed exactly has if they had
   been received in several simultaneous messages.

3.6.5. Provisional responses

   Executing some transactions may require a long time. Long execution
   times may interact with the timer based retransmission procedure.
   This may result either in an inordinate number of retransmissions, or
   in timer values that become too long to be efficient.

   Gateways that can predict that a transaction will require a long
   execution time may send a provisional response, with response code
   100.  They should send this response if they receive a repetition of
   a transaction that is still being executed.

   MGCP entities that receive a provisional response shall switch to a
   longer repetition timer for that transaction.

4. States, failover and race conditions.

   In order to implement proper call signalling, the Call Agent must
   keep track of the state of the endpoint, and the gateway must make
   sure that events are properly notified to the call agent.  Special
   conditions exist when the gateway or the call agent are restarted:
   the gateway must be redirected to a new call agent during "failover"
   procedures, the call agent must take special action when the gateway
   is taken offline, or restarted.

4.1. Basic Asumptions

   The support of "failover" is based on the following assumptions:

   *  Call Agents are identified by their domain name, not their network
      addresses, and several addresses can be associated with a domain
      name.

   *  An endpoint has one NotifiedEntity associated with it any given
      point in time.

   *  The NotifiedEntity is the last value of the "NotifiedEntity"
      parameter received for this endpoint (including wild-carded end-
      point-names). If no explicit "NotifiedEntity" parameter has been
      received, the "NotifiedEntity" defaults to the provisioned
      NotifiedEntity value, or if no value was provisioned to the source
      address of the last command received for the endpoint,

   *  Responses to commands are always sent to the source address of the
      command, regardless of the NotifiedEntity.

   *  When the "notified entity" refers to a domain name that resolves
      to multiple IP- address, endpoints are capable of switching
      between different interfaces on the same  logical call agent,
      however they cannot switch to other (backup) call agent(s) on
      their own. A backup call agent can however instruct them to
      switch, either directly or indirectly.

   *  If an entire call agent becomes unavailable, the endpoints managed
      by that call agent will eventually become "disconnected". The only
      way for these endpoints to become connected again is either for
      the failed call agent to become available, or for a backup call
      agent to contact the affected endpoints.

   *  When a backup call agent has taken over control of a group of
      endpoints, it is assumed that the failed call agent will
      communicate and synchronize with the backup call agent in order to

      transfer control of the affected endpoints back to the original
      call agent (if that's even desired - maybe the failed call agent
      should simply become the backup call agent now).

   We should note that handover conflict resolution between separate
   CA's is not in place - we are relying strictly on the CA's knowing
   what they are doing and communicating with each other (although
   AuditEndpoint can be used to learn about the current NotifiedEntity).

4.2. Security, Retransmission, and Detection of Lost Associations:

   The media gateway control protocol is organized as a set of
   transactions, each of which is composed of a command and a response,
   commonly referred to as an acknowledgement.  The MGCP messages, being
   carried over UDP, may be subject to losses. In the absence of a
   timely response, commands are repeated. MGCP entities are expected to
   keep in memory a list of the responses that they sent to recent
   transactions, i.e. a list of all the responses they sent over the
   last LONG-TIMER seconds, and a list of the transactions that are
   currently being executed.

   The transaction identifiers of incoming commands are compared to the
   transaction identifiers of the recent responses. If a match is found,
   the MGCP entity does not execute the transaction, but simply repeats
   the response. The remaining commands will be compared to the list of
   current transaction. If a match is found, the MGCP entity does not
   execute the transaction, which is simply ignored - a response will be
   provided when the execution of the command is complete.

   The repetition mechanism is used to guard against four types of
   possible errors:

   *  transmission errors, when for example a packet is lost due to
      noise on a line or congestion in a queue,

   *  component failure, when for example an interface to a call agent
      becomes unavailable,

   *  call agent failure, when for example an entire call agent becomes
      unavailable,

   *  failover, when a new call agent is "taking over" transparently.

   The elements should be able to derive from the past history an
   estimate of the packet loss rate due to transmission errors.  In a
   properly configured system, this loss rate should be kept very low,
   typically less than 1%.  If a call agent or a gateway has to repeat a
   message more than a few times, it is very legitimate to assume that

   something else than a transmission error is occurring.  For example,
   given a loss rate of 1%, the probability that 5 consecutive
   transmission attempts fail is 1 in 100 billion, an event that should
   occur less than once every 10 days for a call agent that processes
   1,000 transactions per second. (Indeed, the number of repetition that
   is considered excessive should be a function of the prevailing packet
   loss rate.) We should note that the "suspicion threshold", which we
   will call "Max1", is normally lower than the "disconnection
   threshold", which should be set to a larger value.

      Command issued: N=0
              |
       transmission: N++
              |  +------------ retransmission: N++ -----------+
              |  |                                            |
              |  |       transmission                         |
              |  |  +---to new address -+<--------------------|--+
              |  |  |        N=0        |                     |  |
              V  V  V                   |                     |  |
        +-----------+                   |                     |  |
        | awaiting  |- new call agent ->+  +------------+     |  |
        |  response |--- timer elapsed --->| N > Max1 ? |-(no)+  |
        +-----------+ <----------+         +------------+     ^  |
              |   |              |               |            |  |
              |   +- wrong key? -+             (yes)          |  |
              |                                  |            |  |
      response received                    (if N=Max1,        |  |
              |                             or N=Max2         |  |
              |                             check DNS)        |  |
              v                                  |            |  |
            (end)                       +---------------+     |  |
                                        |more addresses?|(yes)|--+
                                        +---------------+     |
                                                 |            |
                                               (no)           |
                                                 |            |
                                           +------------+     |
                                           | N > Max2 ? |(no)-+
                                           +------------+
                                                 |
                                               (yes)
                                                 |
                                                 v
                                          (disconnected)

   A classic retransmission algorithm would simply count the number of
   successive repetitions, and conclude that the association is broken
   after re-transmitting the packet an excessive number of times

   (typically between 7 and 11 times.) In order to account for the
   possibility of an undetected or in-progress "failover", we modify the
   classic algorithm as follows:

   *  We request that the gateway always checks for the presence of a
      new call agent. It can be noticed either by

      -  receiving a valid multicast message announcing a failover, or

      -  receiving a command where the NotifiedEntity points to the new
         call agent, or

      -  receiving a redirection response pointing to a new Call Agent.

      If a new call agent is detected, the gateway starts transmitting
      outstanding commands to that new agent.  Responses to commands are
      still transmitted to the source address of the command.

   *  we request that if the number of repetitions for this Call Agent
      is larger than "Max1", that the gateway actively queries the name
      server in order to detect the possible change of the call agent
      interfaces.

   *  The gateway may have learned several IP addresses for the call
      agent. If the number of repetitions is larger than "Max1" and
      lower than "Max2", and there are more interfaces that have not
      been tried, then the gateway should direct the retransmissions to
      alternate addresses.

   *  If there are no more interfaces to try, and the number of
      repetitions is Max2, then the gateway contacts the DNS one more
      time to see if any other interface should have become available.
      If not, the gateway is now disconnected.

   The procedure will maximize the chances of detecting an ongoing
   failover. It poses indeed two very specific problems, the potentially
   long delays of a timer based procedure and the risk of confusion
   caused by the use of cryptographic protections.

   In order to automatically adapt to network load, MGCP specifies
   exponentially increasing timers.  If the initial timer is set to 200
   milliseconds, the loss of a fifth retransmission will be detected
   after about 6 seconds.  This is probably an acceptable waiting delay
   to detect a failover.   The repetitions should continue after that
   delay not only in order to perhaps overcome a transient connectivity
   problem, but also in order to allow some more time for the execution
   of a failover - waiting a total delay of 30 seconds is probably
   acceptable.

   It is however important that the maximum delay of retransmissions be
   bounded.  Prior to any retransmission, it is checked that the time
   elapsed since the sending of the initial datagram is no greater than
   T- MAX. If more than T-MAX time has elapsed, the endpoint becomes
   disconnected. The value T-MAX is related to the LONG-TIMER value: the
   LONG-TIMER value is obtained by adding to T-MAX the maximum
   propagation delay in the network.

   Another potential cause of connection failure would be the reception
   of a "wrong key" message, sent by a call agent that could not
   authenticate the command, presumably because it had lost the security
   parameters of the association.  Such messages are actually not
   authorized in IPSEC, and they should in fact not be taken at face
   value: an attacker could easily forge "wrong key" messages in order
   to precipitate the loss of a control connection.  The current
   algorithm ignores these messages, which translates into a strict
   reliance on timers.  The algorithm could in fact be improved, maybe
   by executing a check with the key server of the call agent after
   "Max1" repetitions.

4.3. Race conditions

   MGCP deals with race conditions through the notion of a "quarantine
   list" and through explicit detection of desynchronization.

   MGCP does not assume that the transport mechanism will maintain the
   order of command and responses.  This may cause race conditions, that
   may be obviated through a proper behavior of the call agent. (Note
   that some race conditions are inherent to distributed systems; they
   would still occur, even if the commands were transmitted in strict
   order.)

   In some cases, many gateways may decide to restart operation at the
   same time.  This may occur, for example, if an area loses power or
   transmission capability during an earthquake or an ice storm.  When
   power and transmission are reestablished, many gateways may decide to
   send "RestartInProgress" commands simultaneously, leading to very
   unstable operation.

4.3.1. Quarantine list

   MGCP controlled gateways will receive "notification requests" that
   ask them to watch for a list of "events."  The protocol elements that
   determine the handling of these events are the "Requested Events"
   list, the "Digit Map" and the "Detect Events" list.

   When the endpoint is initialized, the requested events list and the
   digit map are empty.  After reception of a command, the gateway
   starts observing the endpoint for occurrences of the events mentioned
   in the list.

   The events are examined as they occur. The action that follows is
   determined by the "action" parameter associated to the event in the
   list of requested events, and also by the digit map.  The events that
   are defined as "accumulate" or "treat according to digit map" are
   accumulated in a list of events, the events that are marked as
   "treated according to the digit map" will additionally be accumulated
   in the dialed string. This will go on until one event is encountered
   that triggers a Notification to the "notified entity."

   The gateway, at this point, will transmit the notification command
   and will place the endpoint in a "notification" state. As long as the
   endpoint is in this notification state, the events that are to be
   detected on the endpoint are stored in a "quarantine" buffer for
   later processing.  The events are, in a sense, "quarantined." All
   events that are specified by the union of the RequestedEvents
   parameter and the most recently received DetectEvent parameter or, in
   the absence of the latter, all events that are referred to in the
   RequestedEvents, should be detected and quarantined, regardless of
   the action associated to the event.

   The endpoint exits the "notification state" when the acknowledgement
   of the Notify  command is received. The Notify command may be
   retransmitted in the "notification state", as specified in section
   3.5. When the endpoint exits the "notification state" it resets the
   list of observed events and the "current dial string" of the endpoint
   to a null value.

   Following that point, the behavior of the gateway depends on the
   value of The QuarantineHandling parameter in the notification
   request.  If the Call Agent specified that it expected at most one
   notification in response to the notification request command, then
   the gateway should simply keep on accumulating events in the
   quarantine list until it receives the next notification request
   command.

   If the gateway is authorized to send multiple successive Notify
   commands, it will proceed as follows.  When the gateway exits the
   "notification state", it resets the list of observed events and the
   "current dial string" of the endpoint to a null value and starts
   processing the list of quarantined events, using the already received
   list of requested events and digit map. When processing these events,

   the gateway may encounter an event which requires a Notify command to
   be sent. If that is the case, the gateway can adopt one of the two
   following behaviors:

   *  it can immediately transmit a Notify command that will report all
      events that were accumulated in the list of observed events until
      the triggering event, included, leaving the unprocessed events in
      the quarantine list,

   *  or it can attempt to empty the quarantined list and transmit a
      single Notify command reporting several sets of events and
      possibly several dial strings. The dial string is reset to a null
      value after each triggering event. The events that follow the last
      triggering event are left in the quarantine list.

   If the gateway transmits a Notify command, the end point will remain
   in the "notification state" until the acknowledgement is received. If
   the gateway does not find a quarantined event that requests a Notify
   command, it places the end point in a normal state.  Events are then
   processed as they come, in exactly the same way as if a Notification
   Request command had just been received.

   A gateway may receive at any time a new Notification Request command
   for the end point. When a new notification request is received in the
   notification state, the gateway shall ensure that the pending
   notification is received by the Call Agent prior to a successful
   response to the new NotificationRequest. It does so by using the
   "piggy-backing" functionality of the protocol. The messages will then
   be sent in a single packetto the source of the new
   NotificationRequest, regardless of respectively the source and
   "notified entity" for the old and new command. The steps involved are
   the following:

   a) the gateway builds a message that carries in a single packet a
      repetition of the old pending Notify command and the
      acknowledgement of the new notification request.

   b) the endpoint is then taken out of the "notification state" without
      waiting for the acknowledgement of the notification command.

   c) a copy of the unacknowledged Notify command command is kept until
      an acknowledgement is received.  If a timer elapses, the
      notification will be repeated, in a packet that will also carry a
      repetition of the acknowledgement of the notification request.

   d) if the acknowledgement is lost, the Call Agent will retransmit the
      Notification Request.  The gateway will reply to this repetition
      by retransmitting in a single packet the unacknowledged Notify and
      the acknowledgement of the notification request.

   e) if the gateway has to transmit a Notify before the previous Notify
      is acknowledged, it should construct a packet that piggybacks a
      repetition of the old Notify, a repetition of the acknowledgement
      of the last notification request and the new Notify.

   f) Gateways that cannot piggyback several packets in the same message
      should elect to leave the endpoint in the "notification" state as
      long as the last notification is not acknowledged.

   After receiving the Notification Request command, the requested
   events list and digit map (if a new one was provided) are replaced by
   the newly received parameters, and the list of observed events and
   accumulated dial string are reset to a null value.  The behavior is
   conditioned by the value of the QuarantineHandling parameter. The
   parameter may specify that quarantined events, or previously observed
   events, should be discarded, in which case they will be. If the
   parameter specifies that the quarantined events should be processed,
   the gateway will start processing the list of quarantined events or
   previously observed events, using the newly received list of
   requested events and digit map. When processing these events, the
   gateway may encounter an event which requires a Notify command to be
   sent. If that is the case, the gateway will immediately transmit a
   Notify command that will report all events that were accumulated in
   the list of observed events until the triggering event, included,
   leaving the unprocessed events in the quarantine buffer, and will
   enter the "notification state".

   A new notification request may be received while the gateway has
   accumulated events according to the previous notification requests,
   but has not yet detected a notification-triggering events.  The
   handling of not-yet-notified events is determined, as with the
   quarantined events, by the quarantine handling parameters:

   *  If the quarantine-handling parameter specifies that quarantined
      events shall be ignored, the observed event list is simply reset.

   *  If the quarantine-handling parameter specifies that quarantined
      events shall be processed, the observed event list is transferred
      to the quarantined event list.  The observed event list is then
      reset, and the quarantined event list is processed.

   Call Agents SHOULD provide the response to a successful Notify
   message and the new NotificationRequest in the same datagram using
   the piggy-backing mechanism.

4.3.2. Explicit detection

   A key element of the state of several endpoints is the position of
   the hook. A race condition may occur when the user decides to go
   off-hook before the Call Agent has the time to ask the gateway to
   notify an off hook event (the "glare" condition well known in
   telephony), or if the user goes on-hook before the Call Agent has the
   time to request the event's notification.

   To avoid this race condition, the gateway should check the condition
   of the endpoint before acknowledging a NotificationRequest. It should
   return an error:

   1- If the gateway is requested to notify an "off hook" transition
      while the phone is already off hook,

   2- If the gateway is requested to notify an "on hook" or "flash hook"
      condition while the phone is already on hook.

   It should be noted, that the condition check is performed at the time
   the notification request is received, where as the actual event that
   caused the current condition may have either been reported, or
   ignored earlier, or it may currently be quarantined.

   The other state variables of the gateway, such as the list of
   RequestedEvent or list of requested signals, are entirely replaced
   after each successful NotificationRequest, which prevents any long
   term discrepancy between the Call Agent and the gateway.

   When a NotificationRequest is unsuccessful, whether it is included in
   a connection-handling command or not, the gateway will simply
   continue as if the command had never been received. As all other
   transactions, the NotificationRequest should operate as an atomic
   transaction, thus any changes initiated as a result of the command
   should be reverted.

   Another race condition may occur when a Notify is issued shortly
   before the reception by the gateway of a NotificationRequest. The
   RequestIdentifier is used to correlate Notify commands with
   NotificationRequest commands.

4.3.3. Ordering of commands, and treatment of disorder

   MGCP does not mandate that the underlying transport protocol
   guarantees the sequencing of commands sent to a gateway or an
   endpoint.  This property tends to maximize the timeliness of actions,
   but it has a few draw backs.  For example:

   *  Notify commands may be delayed and arrive to the call agent after
      the transmission of a new Notification Request command,

   *  If a new NotificationRequest is transmitted before a previous one
      is acknowledged, there is no guarantee that the previous one will
      not be received in second position.

   Call Agents that want to guarantee consistent operation of the end
   points can use the following rules:

   1) When a gateway handles several endpoints, commands pertaining to
      the different endpoints can be sent in parallel, for example
      following a model where each endpoint is controlled by its own
      process or its own thread.

   2) When several connections are created on the same endpoint,
      commands pertaining to different connections can be sent in
      parallel.

   3) On a given connection, there should normally be only one
      outstanding command (create or modify).  However, a
      DeleteConnection command can be issued at any time.  In
      consequence, a gateway may sometimes receive a ModifyConnection
      command that applies to a previously deleted connection.  Such
      commands should be ignored, and an error code should be returned.

   4) On a given endpoint, there should normally be only one outstanding
      NotificationRequest command at any time.  The RequestId parameter
      should be used to correlate Notify commands with the triggering
      notification request.

   5) In some cases, an implicitly or explicitly wildcarded
      DeleteConnection command that applies to a group of endpoints can
      step in front of a pending CreateConnection command.  The Call
      Agent should individually delete all connections whose completion
      was pending at the time of the global DeleteConnection command.
      Also, new CreateConnection commands for endpoints named by the
      wild-carding cannot be sent until the wild-carded DeleteConnection
      command is acknowledged.

   6) When commands are embedded within each other, sequencing
      requirements for all commands must be adhered to. For example a
      Create Connection command with a Notification Request in it must
      adhere to the sequencing for CreateConnection and
      NotificationRequest at the same time.

   7) AuditEndpoint and AuditConnection is not subject to any
      sequencing.

   8) RestartInProgress must always be the first command sent by an
      endpoint as defined by the restart procedure. Any other command or
      response must be delivered after this RestartInProgress command
      (piggy-backing allowed).

   9) When multiple messages are piggy-backed in a single packet, the
      messages are always processed in order.

   These rules do not affect the gateway, which should always respond to
   commands.

4.3.4. Fighting the restart avalanche

   Let's suppose that a large number of gateways are powered on
   simultaneously.  If they were to all initiate a RestartInProgress
   transaction, the call agent would very likely be swamped, leading to
   message losses and network congestion during the critical period of
   service restoration. In order to prevent such avalanches, the
   following behavior is suggested:

   1) When a gateway is powered on, it should initiate a restart timer
      to a random value, uniformly distributed between 0 and a maximum
      waiting delay (MWD). Care should be taken to avoid synchronicity
      of the random number generation between multiple gateways that
      would use the same algorithm.

   2) The gateway should then wait for either the end of this timer, the
      reception of a command from the call agent, or the detection of a
      local user activity, such as for example an off-hook transition on
      a residential gateway.

   3) When the timer elapses, when a command is received, or when an
      activity is detected, the gateway should initiate the restart
      procedure.

   The restart procedure simply requires the endpoint to guarantee that
   the first message (command or response) that the Call Agent sees from
   this endpoint is a RestartInProgress message informing the Call Agent
   about the restart. The endpoint is free to take full advantage of
   piggy-backing to achieve this.

   It is expected that each endpoint in a gateway will have a
   provisionable Call Agent, i.e., "notified entity", to direct the
   initial restart message towards. When the collection of endpoints in
   a gateway is managed by more than one Call Agent, the above procedure
   must be performed for each collection of endpoints managed by a given
   Call Agent. The gateway MUST take full advantage of wild-carding to
   minimize the number of RestartInProgress messages generated when
   multiple endpoints in a gateway restart and the endpoints are managed
   by the same Call Agent.

   The value of MWD is a configuration parameter that depends on the
   type of the gateway. The following ]reasoning can be used to
   determine the value of this delay on residential gateways.

   Call agents are typically dimensioned to handle the peak hour traffic
   load, during which, in average, 10% of the lines will be busy,
   placing calls whose average duration is typically 3 minutes.  The
   processing of a call typically involves 5 to 6 MGCP transactions
   between each end point and the call agent.  This simple calculation
   shows that the call agent is expected to handle 5 to 6 transactions
   for each end point, every 30 minutes on average, or, to put it
   otherwise, about one transaction per end point every 5 to 6 minutes
   on average.  This suggest that a reasonable value of MWD for a
   residential gateway would be 10 to 12 minutes.  In the absence of
   explicit configuration, residential gateways should adopt a value of
   600 seconds for MWD.

   The same reasoning suggests that the value of MWD should be much
   shorter for trunking gateways or for business gateways, because they
   handle a large number of endpoints, and also because the usage rate
   of these endpoints is much higher than 10% during the peak busy hour,
   a typical value being 60%.  These endpoints, during the peak hour,
   are this expected to contribute about one transaction per minute to
   the call agent load. A reasonable algorithm is to make the value of
   MWD per "trunk" endpoint six times shorter than the MWD per
   residential gateway, and also inversely proportional to the number of
   endpoints that are being restarted. for example MWD should be set to
   2.5 seconds for a gateway that handles a T1 line, or to 60
   milliseconds for a gateway that handles a T3 line.

4.3.5. Disconnected Endpoints

   In addition to the restart procedure, gateways also have a
   "disconnected" procedure, which is initiated when an endpoint becomes
   "disconnected" as described in Section 3.4.2. It should here be
   noted, that endpoints can only become disconnected when they attempt
   to communicate with the Call Agent. The following steps are followed
   by an endpoint that becomes "disconnected":

   1. A "disconnected" timer is initialized to a random value, uniformly
      distributed between 0 and a provisionable "disconnected" initial
      waiting delay (Tdinit), e.g., 15 seconds.  Care MUST be taken to
      avoid synchronicity of the random number generation between
      multiple gateways and endpoints that would use the same algorithm.

   2. The gateway then waits for either the end of this timer, the
      reception of a command from the call agent, or the detection of a
      local user activity for the endpoint, such as for example an off-
      hook transition.

   3. When the "disconnected" timer elapses, when a command is received,
      or when a local user activity is detected, the gateway initiates
      the "disconnected" procedure for the endpoint. In the case of
      local user activity, a provisionable "disconnected" minimum
      waiting delay (Tdmin) must furthermore have elapsed since the
      gateway became disconnected or the last time it initiated the
      "disconnected" procedure in order to limit the rate at which the
      procedure is performed.

   4. If the "disconnected" procedure still left the endpoint
      disconnected, the "disconnected" timer is then doubled, subject to
      a provisionable "disconnected" maximum waiting delay (Tdmax),
      e.g., 600 seconds, and the gateway proceeds with step 2 again.

   The "disconnected" procedure is similar to the restart procedure in
   that it now simply states that the endpoint MUST send a
   RestartInProgress command to the Call Agent informing it that the
   endpoint was disconnected and furthermore guarantee that the first
   message (command or response) that the Call Agent now sees from this
   endpoint MUST be this RestartInProgress command. The endpoint MUST
   take full advantage of piggy-backing in achieving this. The Call
   Agent may then for instance decide to audit the endpoint, or simply
   clear all connections for the endpoint.

   This specification purposely does not specify any additional behavior
   for a disconnected endpoint. Vendors MAY for instance choose to
   provide silence, play reorder tone, or even enable a downloaded wav
   file to be played.

The default value for Tdinit is 15 seconds, the default value for
Tdmin, is 15 seconds, and the default value for Tdmax is 600 seconds.

5. Security requirements

   If unauthorized entities could use the MGCP, they would be able to
   set-up unauthorized calls, or to interfere with authorized calls. We
   expect that MGCP messages will always be carried over secure Internet
   connections, as defined in the IP security architecture as defined in
   RFC 2401, using either the IP Authentication Header, defined in RFC
   2402, or the IP Encapsulating Security Payload, defined in RFC 2406.
   The complete MGCP protocol stack would thus include the following
   layers:

                ________________________________
               |              MGCP             |
               |_______________________________|
               |              UDP              |
               |_______________________________|
               |          IP security          |
               | (authentication or encryption)|
               |_______________________________|
               |              IP               |
               |_______________________________|
               |       transmission media      |
               |_______________________________|

   Adequate protection of the connections will be achieved if the
   gateways and the Call Agents only accept messages for which IP
   security provided an authentication service. An encryption service
   will provide additional protection against eavesdropping, thus
   forbidding third parties from monitoring the connections set up by a
   given endpoint

   The encryption service will also be requested if the session
   descriptions are used to carry session keys, as defined in SDP.

   These procedures do not necessarily protect against denial of service
   attacks by misbehaving gateways or misbehaving call agents. However,
   they will provide an identification of these misbehaving entities,
   which should then be deprived of their authorization through
   maintenance procedures.

5.1. Protection of media connections

   MGCP allows call agent to provide gateways with "session keys" that
   can be used to encrypt the audio messages, protecting against
   eavesdropping.

   A specific problem of packet networks is "uncontrolled barge-in."
   This attack can be performed by directing media packets to the IP
   address and UDP port used by a connection. If no protection is
   implemented, the packets will be decompressed and the signals will be
   played on the "line side".

   A basic protection against this attack is to only accept packets from
   known sources, checking for example that the IP source address and
   UDP source port match the values announced in the "remote session
   description."  But this has two inconveniences: it slows down
   connection establishment and it can be fooled by source spoofing:

   *  To enable the address-based protection, the call agent must obtain
      the remote session description of the e-gress gateway and pass it
      to the in-gress gateway.  This requires at least one network round
      trip, and leaves us with a dilemma: either allow the call to
      proceed without waiting for the round trip to complete, and risk
      for example "clipping" a remote announcement, or wait for the full
      round trip and settle for slower call-set-up procedures.

   *  Source spoofing is only effective if the attacker can obtain valid
      pairs of source destination addresses and ports, for example by
      listening to a fraction of the traffic. To fight source spoofing,
      one could try to control all access points to the network.  But
      this is in practice very hard to achieve.

   An alternative to checking the source address is to encrypt and
   authenticate the packets, using a secret key that is conveyed during
   the call set-up procedure. This will no slow down the call set-up,
   and provides strong protection against address spoofing.

6. Event packages and end point types

This section provides an initial definition of packages and event
names. More packages can be defined in additional documents.

6.1. Basic packages

   The list of basic packages includes the following:

                _________________________________________
               | Package                      |   name  |
               |______________________________|_________|
               | Generic Media Package        |   G     |
               | DTMF package                 |   D     |
               | MF Package                   |   M     |
               | Trunk Package                |   T     |
               | Line Package                 |   L     |
               | Handset Package              |   H     |
               | RTP Package                  |   R     |
               | Network Access Server Package|   N     |
               | Announcement Server Package  |   A     |
               | Script Package               |   Script|
               |______________________________|_________|

   In the tables of events for each package, there are five columns:

      Symbol: the unique symbol used for the event
      Definition: a short description of the event

      R: an x appears in this column is the event can be Requested by
         the call agent.

      S: if nothing appears in this column for an event, then the event
         cannot be signaled on command by the call agent. Otherwise, the
         following symbols identify the type of event:

      OO On/Off signal.  The signal is turned on until commanded by the
         call agent to turn it off, and vice versa.

      TO Timeout signal.  The signal lasts for a given duration unless
         it is superseded by a new signal.

      BR Brief signal.  The event has a short, known duration.

      Duration: specifies the duration of TO signals.

6.1.1. Generic Media Package

   Package Name: G

   The generic media package group the events and signals that can be
   observed on several types of endpoints, such as trunking gateways,
   access gateways or residential gateways.

  _____________________________________________________________________
| Symbol   |   Definition               |   R |   S      Duration    |
|__________|____________________________|_____|______________________|
| mt       |   Modem detected           |   x |                      |
| ft       |   Fax tone detected        |   x |                      |
| ld       |   Long duration connection |   x |                      |
| pat(###) |   Pattern ### detected     |   x |   OO                 |
| rt       |   Ringback tone            |     |   TO                 |
| rbk(###) |   ring back on connection  |     |   TO     180 seconds |
| cf       |   Confirm tone             |     |   BR                 |
| cg       |   Network Congestion tone  |     |   TO                 |
| it       |   Intercept tone           |     |   OO                 |
| pt       |   Preemption tone          |     |   OO                 |
| of       |   report failure           |   x |                      |
|__________|____________________________|_____|______________________|

   The signals are defined as follows:

      The pattern definition can be used for specific algorithms such as
      answering machine detection, tone detection, and the like.

   Ring back tone (rt)
      an Audible Ring Tone, a combination of two AC tones with
      frequencies of 440 and 480 Hertz and levels of -19 dBm each, to
      give a combined level of -16 dBm.  The cadence for Audible Ring
      Tone is 2 seconds on followed by 4 seconds off. See GR- 506-CORE -
      LSSGR:  SIGNALING, Section 17.2.5.

   Ring back on connection
      A ring back tone, applied to the connection whose identifier is
      passed as a parameter.

   The "long duration connection" is detected when a connection has been
   established for more than 1 hour.

6.1.2. DTMF package

   Package name: D

    _______________________________________________________________
   | Symbol |   Definition              |   R |   S      Duration |
   |________|___________________________|_____|___________________|
   | 0      |   DTMF 0                  |   x |   BR              |
   | 1      |   DTMF 1                  |   x |   BR              |
   | 2      |   DTMF 2                  |   x |   BR              |
   | 3      |   DTMF 3                  |   x |   BR              |
   | 4      |   DTMF 4                  |   x |   BR              |
   | 5      |   DTMF 5                  |   x |   BR              |
   | 6      |   DTMF 6                  |   x |   BR              |
   | 7      |   DTMF 7                  |   x |   BR              |
   | 8      |   DTMF 8                  |   x |   BR              |
   | 9      |   DTMF 9                  |   x |   BR              |
   | #      |   DTMF #                  |   x |   BR              |
   | *      |   DTMF *                  |   x |   BR              |
   | A      |   DTMF A                  |   x |   BR              |
   | B      |   DTMF B                  |   x |   BR              |
   | C      |   DTMF C                  |   x |   BR              |
   | D      |   DTMF D                  |   x |   BR              |
   | L      |   long duration indicator |   x |          2 seconds|
   | X      |   Wildcard, match         |   x |                   |
   |        |   any digit 0-9           |     |                   |
   | T      |   Interdigit timer        |   x |          4 seconds|
   | of     |   report failure          |   x |                   |
   |________|___________________________|_____|___________________|

   The "interdigit timer" T is a digit input timer that can be used in
   two ways:

   *  When timer T is used with a digit map, the timer is not started
      until the first digit is entered, and the timer is restarted after
      each new digit is entered until either a digit map match or
      mismatch occurs. In this case, timer T functions as an inter-digit
      timer.

   *  When timer T is used without a digit map, the timer is started
      immediately and simply cancelled (but not restarted) as soon as a
      digit is entered. In this case, timer T can be used as an
      interdigit timer when overlap sending is used.

      When used with a digit map, timer T takes on one of two values,
      T(partial) or T(critical). When at least one more digit is
      required for the digit string to match any of the patterns in the
      digit map, timer T takes on the value T(partial), corresponding to

      partial dial timing. If a timer is all that is required to produce
      a match, timer T takes on the value T(critical) corresponding to
      critical timing. When timer T is used without a digit map, timer T
      takes on the value T(critical).  The default value for T(partial)
      is 16 seconds and the default value for T(critical) is 4 seconds.
      The provisioning process may alter both of these.

      The "long duration indicator" is observed when a DTMF signal is
      produced for a duration larger than two seconds.  In this case,
      the gateway will detect two successive events: first, when the
      signal has been recognized, the DTMF signal, and then, 2 seconds
      later, the long duration signal.

6.1.3. MF Package

      Package Name: M

       ________________________________________________________
      | Symbol |   Definition       |   R |   S      Duration |
      |________|____________________|_____|___________________|
      | 0      |   MF 0             |   x |   BR              |
      | 1      |   MF 1             |   x |   BR              |
      | 2      |   MF 2             |   x |   BR              |
      | 3      |   MF 3             |   x |   BR              |
      | 4      |   MF 4             |   x |   BR              |
      | 5      |   MF 5             |   x |   BR              |
      | 6      |   MF 6             |   x |   BR              |
      | 7      |   MF 7             |   x |   BR              |
      | 8      |   MF 8             |   x |   BR              |
      | 9      |   MF 9             |   x |   BR              |
      | X      |   Wildcard, match  |   x |                   |
      |        |   any digit 0-9    |     |                   |
      | T      |   Interdigit timer |   x |          4 seconds|
      | K0     |   MF K0 or KP      |   x |   BR              |
      | K1     |   MF K1            |   x |   BR              |
      | K2     |   MF K2            |   x |   BR              |
      | S0     |   MF S0 or ST      |   x |   BR              |
      | S1     |   MF S1            |   x |   BR              |
      | S2     |   MF S2            |   x |   BR              |
      | S3     |   MF S3            |   x |   BR              |
      | wk     |   Wink             |   x |   BR              |
      | wko    |   Wink off         |   x |   BR              |
      | is     |   Incoming seizure |   x |   OO              |
      | rs     |   Return seizure   |   x |   OO              |
      | us     |   Unseize circuit  |   x |   OO              |
      | of     |   report failure   |   x |                   |
      |________|____________________|_____|___________________|

   The definition of the MF package events is as follows:

   Wink
      A transition from unseized to seized to unseized trunk states
      within a specified period.  Typical seizure period is 100-350
      msec.)

   Incoming seizure
      Incoming indication of call attempt.

   Return seizure:
      Seizure in response to outgoing seizure.

   Unseize circuit:
      Unseizure of a circuit at the end of a call.

   Wink off:
      A signal used in operator services trunks.  A transition from
      seized to unseized to seized trunk states within a specified
      period of 100-350 ms. (To be checked)

6.1.4. Trunk Package

   Package Name: T

   _____________________________________________________________________
  | Symbol |   Definition                   |   R |   S      Duration  |
  |________|________________________________|_____|____________________|
  | co1    |   Continuity tone (single tone,|   x |   OO               |
  |        |   or return tone)              |     |                    |
  | co2    |   Continuity test (go tone,    |   x |   OO               |
  |        |   in dual tone procedures)     |     |                    |
  | lb     |   Loopback                     |     |   OO               |
  | om     |   Old Milliwatt Tone (1000 Hz) |   x |   OO               |
  | nm     |   New Milliwatt Tone (1004 Hz) |   x |   OO               |
  | tl     |   Test Line                    |   x |   OO               |
  | zz     |   No circuit                   |   x |   OO               |
  | as     |   Answer Supervision           |   x |   OO               |
  | ro     |   Reorder Tone                 |   x |   TO     30 seconds|
  | of     |   report failure               |   x |                    |
  | bl     |   Blocking                     |     |   OO               |
  |________|________________________________|_____|____________________|

   The definition of the trunk package signal events is as follows:

   Continuity Tone (co1):
      A tone at 2010 + or - 30 Hz.

   Continuity Test (co2):
      A tone at the 1780 + or - 30 Hz.

   Milliwatt Tones:
      Old Milliwatt Tone (1000 Hz), New Milliwatt Tone (1004 Hz)

   Line Test:
      105 Test Line test progress tone (2225 Hz + or - 25 Hz at -10 dBm0
      + or -- 0.5dB).

   No circuit:
      (that annoying tri-tone, low to high)

   Answer Supervision:

   Reorder Tone:
      Reorder tone is a combination of two AC tones with frequencies of
      480 and 620 Hertz and levels of -24 dBm each, to give a combined
      level of -21 dBm.  The cadence for Station Busy Tone is 0.25
      seconds on followed by 0.25 seconds off, repeating continuously.
      See GR-506-CORE - LSSGR: SIGNALING, Section 17.2.7.

   Blocking:
      The call agent can place the circuit in a blocked state by
      applying the "bl(+)" signal to the endpoint.  It can unblock it by
      applying the "bl(-)" signal.

   The continuity tones are used when the call agent wants to initiate a
   continuity test. There are two types of tests, single tone and dual
   tone. The Call agent is expected to know, through provisioning
   information, which test should be applied to a given endpoint. For
   example, the call agent that wants to initiate a single frequency
   test will send to the gateway a command of the form:

         RQNT 1234 epx-t1/17@tgw2.example.net
         X: AB123FE0
         S: co1
         R: co1

   If it wanted instead to initiate a dual-tone test, it would send the
   command:

         RQNT 1234 epx-t1/17@tgw2.example.net
         X: AB123FE0
         S: co2
         R: co1

   The gateway would send the requested signal, and in both cases would
   look for the return of the 2010 Hz tone (co1).  When it detects that
   tone, it will send the corresponding  notification.

   The tones are of type OO: the gateway will keep sending them until it
   receives a new notification request.

6.1.5. Line Package

   Package Name: L

________________________________________________________________________
|Symbol       |   Definition                 |   R |   S    Duration   |
|_____________|______________________________|_____|___________________|
|adsi(string) |   adsi display               |     |   BR              |
|vmwi         |   visual message             |     |   OO              |
|             |   waiting indicator          |     |                   |
|hd           |   Off hook transition        |   x |                   |
|hu           |   On hook transition         |   x |                   |
|hf           |   Flash hook                 |   x |                   |
|aw           |   Answer tone                |   x |   OO              |
|bz           |   Busy tone                  |     |   TO   30 seconds |
|ci(ti,nu,na) |   Caller-id                  |     |   BR              |
|wt           |   Call Waiting tone          |     |   TO   30 seconds |
|wt1, wt2,    |   Alternative call           |     |                   |
|wt3, wt4     |   waiting tones              |     |                   |
|dl           |   Dial tone                  |     |   TO   16 seconds |
|mwi          |   Message waiting ind.       |     |   TO   16 seconds |
|nbz          |   Network busy               |   x |   OO              |
|             |   (fast cycle busy)          |     |                   |
|ro           |   Reorder tone               |     |   TO   30 seconds |
|rg           |   Ringing                    |     |   TO   180 seconds|
|r0, r1, r2,  |   Distinctive ringing        |     |   TO   180 seconds|
|r3, r4, r5,  |                              |     |                   |
|r6 or r7     |                              |     |                   |
|rs           |   Ringsplash                 |     |   BR              |
|p            |   Prompt tone                |   x |   BR              |
|e            |   Error tone                 |   x |   BR              |
|sl           |   Stutter dialtone           |     |   TO   16 seconds |
|v            |   Alerting Tone              |     |   OO              |
|y            |   Recorder Warning Tone      |     |   OO              |
|sit          |   SIT tone                   |     |                   |
|z            |   Calling Card Service Tone  |     |   OO              |
|oc           |   Report on completion       |   x |                   |
|ot           |   Off hook warning tone      |     |   TO   indefinite |
|s(###)       |   Distinctive tone pattern   |   x |   BR              |
|of           |   report failure             |   x |                   |
|_____________|______________________________|_____|___________________|

   The definition of the tones is as follows:

   Dial tone:
      A combined 350 + 440 Hz tone.

   Visual Message Waiting Indicator
      The transmission of the VMWI messages will conform to the
      requirements in Section 2.3.2, "On-hook Data Transmission Not
      Associated with Ringing" in TR-H-000030 and the CPE guidelines in
      SR-TSV-002476. VMWI messages will only be sent from the SPCS when
      the line is idle. If new messages arrive while the line is busy,
      the VMWI indicator message will be delayed until the line goes
      back to the idle state. The CA should periodically refresh the
      CPE's visual indicator. See TR-NWT-001401 - Visual Message Waiting
      Indicator Generic Requirements; and GR- 30-CORE - Voiceband Data
      Transmission Interface.

   Message waiting Indicator
      See GR-506-CORE, 17.2.3.

   Alerting Tone:
      a 440 Hz Tone of 2 second duration followed by 1/2 second of tone
      every 10 seconds.

   Ring splash
      Ringsplash, also known as "Reminder ring" is a burst of ringing
      that may be applied to the physical forwarding line (when idle) to
      indicate that a call has been forwarded and to remind the user
      that a CF subfeature is active.  In the US, it is defined to be a
      0.5(-0,+0.1) second burst of power ringing. See TR-TSY-000586 -
      Call Forwarding Subfeatures.

   Call waiting tone
      Call Waiting tone is defined in GR-506-CORE, 14.2. Call Waiting
      feature is defined in TR-TSY-000571. By defining "wt" as a TO
      signal you are really defining the feature which seems wrong to me
      (given the spirit of MGCP), hence the definition of "wt" as a BR
      signal in ECS, per GR-506-CORE. Also, it turns out that there is
      actually four different call waiting tone patterns (see GR-506-
      CORE, 14.2) so we have wt1, wt2, wt3, wt4.

   Caller Id (ci(time, number, name)):
      The caller-id event carries three parameters, the time of the
      call, the calling number and the calling name. Each of the three
      fields are optional, however each of the commas will always be
      included.  See TR-NWT-001188, GR-30-CORE, and TR-NWT-000031.

   Recorder Warning Tone:
      1400 Hz of Tone of 0.5 second duration every 15 seconds.

   SIT tone:
      used for indicating a line is out of service.

   Calling Card Service Tone:
      60 ms of 941 + 1477 Hz and 940 ms of 350 + 440 Hz (dial tone),
      decaying exponentially with a time constant of 200 ms.

   Distinctive tone pattern:
      where ### is any number between 000 and 999, inclusive.  Can be
      used for distinctive ringing, customized dial tone, etc.

   Report on completion
      The report on completion event is detected when the gateway was
      asked to perform one or several signals of type TO on the
      endpoint, and when these signals were completed without being
      stopped by the detection of a requested event such as off-hook
      transition or dialed digit.  The completion report may carry as
      parameter the name of the signal that came to the end of its live
      time, as in:

            O: L/oc(L/dl)

   Ring back on connection
      A ring back tone, applied to the connection wghose identifier is
      passed as a parameter.

   We should note that many of these definitions vary from country to
   country.  The frequencies listed above are the one in use in North
   America.  There is a need to accommodate different tone sets in
   different countries, and there is still an ongoing debate on the best
   way to meet that requirement:

   *  One solution is to define different event packages specifying for
      example the German dialtone as "L-DE/DL".

   *  Another solution is to use a management interface to specify on an
      endpoint basis which frequency shall be associated to what tone.

6.1.6. Handset emulation package

   Package Name: H

________________________________________________________________________
|Symbol       |   Definition                 |   R |   S    Duration   |
|_____________|______________________________|_____|___________________|
|adsi(string) |   adsi display               |   x |   BR              |
|tdd          |                              |     |                   |
|vmwi         |                              |     |                   |
|hd           |   Off hook transition        |   x |   OO              |
|hu           |   On hook transition         |   x |   OO              |
|hf           |   Flash hook                 |   x |   BR              |
|aw           |   Answer tone                |   x |   OO              |
|bz           |   Busy tone                  |   x |   OO              |
|wt           |   Call Waiting tone          |   x |   TO   30 seconds |
|dl           |   Dial tone (350 + 440 Hz)   |   x |   TO   120 seconds|
|nbz          |   Network busy               |   x |   OO              |
|             |   (fast cycle busy)          |     |                   |
|rg           |   Ringing                    |   x |   TO   30 seconds |
|r0, r1, r2,  |   Distinctive ringing        |   x |   TO   30 seconds |
|r3, r4, r5,  |                              |     |                   |
|r6 or r7     |                              |     |                   |
|p            |   Prompt tone                |   x |   BR              |
|e            |   Error tone                 |   x |   BR              |
|sdl          |   Stutter dialtone           |   x |   TO   16 seconds |
|v            |   Alerting Tone              |   x |   OO              |
|y            |   Recorder Warning Tone      |   x |   OO              |
|t            |   SIT tone                   |   x |                   |
|z            |   Calling Card Service Tone  |   x |   OO              |
|oc           |   Report on completion       |   x |                   |
|ot           |   Off hook warning tone      |   x |   OO              |
|s(###)       |   Distinctive tone pattern   |   x |   BR              |
|of           |   report failure             |   x |                   |
|_____________|______________________________|_____|___________________|

   The handset emulation package is an extension of the line package, to
   be used when the gateway is capable of emulating a handset.  The
   difference with the line package is that events such as "off hook"
   can be signalled as well as detected.

6.1.7. RTP Package

   Package Name: R

    ____________________________________________________________________
   | Symbol  |   Definition                   |   R |   S      Duration|
   |_________|________________________________|_____|__________________|
   | UC      |   Used codec changed           |   x |                  |
   | SR(###) |   Sampling rate changed        |   x |                  |
   | JI(###) |   Jitter buffer size changed   |   x |                  |
   | PL(###) |   Packet loss exceeded         |   x |                  |
   | qa      |   Quality alert                |   x |                  |
   | co1     |   Continuity tone (single tone,|   x |   OO             |
   |         |   or return tone)              |     |                  |
   | co2     |   Continuity test (go tone,    |   x |   OO             |
   |         |  in dual tone procedures)      |     |                  |
   | of      |   report failure               |   x |                  |
   |_________|________________________________|_____|__________________|

   Codec Changed:
      Codec changed to hexadecimal codec number enclosed in parenthesis,
      as in UC(15), to indicate the codec was changed to PCM mu-law.
      Codec Numbers are specified in RFC 1890, or in a new definition of
      the audio profiles for RTP that replaces this RFC.  Some
      implementations of media gateways may not allow the codec to be
      changed upon command from the call agent.  codec changed to codec
      hexadecimal ##.

   Sampling Rate Changed:
      Sampling rate changed to decimal number in milliseconds enclosed
      in parenthesis, as in SR(20), to indicate the sampling rate was
      changed to 20 milliseconds.  Some implementations of media
      gateways may not allow the sampling rate to be changed upon
      command from a call agent.

   Jitter Buffer Size Changed:
      When the media gateway has the ability to automatically adjust the
      depth of the jitter buffer for received RTP streams, it is useful
      for the media gateway controller to receive notification that the
      media gateway has automatically increased its jitter buffer size
      to accomodate increased or decreased variability in network
      latency.  The syntax for requesting notification is "JI", which
      tells the media gateway that the controller wants notification of
      any jitter buffer size changes.  The syntax for notification from
      the media gateway to the controller is "JI(####)", where the ####
      is the new size of the jitter buffer, in milliseconds.

   Packet Loss Exceeded:
      Packet loss rate exceed the threshold of the specified decimal
      number of packets per 100,000 packets, where the packet loss
      number is contained in parenthesis.  For example, PL(10) indicates
      packets are being dropped at a rate of 1 in 10,000 packets.

   Quality alert
      The packet loss rate or the combination of delay and jitter exceed
      a specified quality threshold.

   The continuity tones are the same as those defined in the Trunk
   package.  They can be use in conjunction with the Network LoopBack or
   Network Continuity Test modes to test the continuity of an RTP
   circuit.

   The "operation failure" code can be used to report problems such as
   the loss of underlying connectivity.  The observed event can include
   as parameter the reason code of the failure.

6.1.8. Network Access Server Package

   Package Name: N

       ____________________________________________________________
      | Symbol |   Definition             |   R |   S     Duration|
      |________|__________________________|_____|_________________|
      | pa     |  Packet arrival          |  x  |                 |
      | cbk    |  Call back request       |  x  |                 |
      | cl     |  Carrier lost            |  x  |                 |
      | au     |   Authorization succeeded|  x  |                 |
      | ax     |   Authorization denied   |  x  |                 |
      | of     |   Report failure         |  x  |                 |
      |________|__________________________|_____|_________________|

   The packet arrival event is used to notify that at least one packet
   was recently sent to an Internet address that is observed by an
   endpoint.  The event report includes the Internet address, in
   standard ASCII encoding, between parenthesis:

         O: pa(192.96.41.1)

   The call back event is used to notify that a call back has been
   requested during the initial phase of a data connection. The event
   report includes the identification of the user that should be called
   back, between parenthesis:

         O: cbk(user25)

6.1.9. Announcement Server Package

   Package Name: A

    ___________________________________________________________________
   | Symbol         |   Definition           |   R |   S      Duration|
   |________________|________________________|_____|__________________|
   | ann(url,parms) |   Play an announcement |     |   TO     variable|
   | oc             |   Report on completion |   x |                  |
   | of             |   Report failure       |   x |                  |
   |________________|________________________|_____|__________________|

   The announcement action is qualified by an URL name and by a set of
   initial parameters as in for example:

         S: ann(http://scripts.example.net/all-lines-busy.au)

   The "operation complete" event will be detected when the announcement
   is played out. If the announcement cannot be played out, an operation
   failure event can be returned.  The failure may be explained by a
   commentary, as in:

         O: A/of(file not found)

6.1.10. Script Package

   Package Name: Script

    ______________________________________________________________
   | Symbol    |   Definition           |   R |   S  |   Duration|
   |___________|________________________|_____|______|___________|
   | java(url) |   Load a java script   |     |   TO |   variable|
   | perl(url) |   Load a perl script   |     |   TO |   variable|
   | tcl(url)  |   Load a TCL script    |     |   TO |   variable|
   | xml(url)  |   Load an XML script   |     |   TO |   variable|
   | oc        |   Report on completion |   x |      |           |
   | of        |   Report failure       |   x |      |           |
   |___________|________________________|_____|______|___________|

   The "language" action define is qualified by an URL name and by a set
   of initial parameters as in for example:

         S: script/java(http://scripts.example.net/credit-
            card.java,long,1234)

   The current definition defines keywords for the most common
   languages.  More languages may be defined in further version of this
   documents.  For each language, an API specification will describe how
   the scripts can issue local "notificationRequest" commands, and
   receive the corresponding notifications.

   The script produces an output which consists of one or several text
   string, separated by commas.  The text string are reported as a
   commentary in the report on completion, as in for example:

         O: script/oc(21223456794567,9738234567)

   The failure report may also return a string, as in:

         O: script/oc(21223456794567,9738234567)

   The definition of the script environment and the specific actions in
   that environment are for further study.

6.2. Basic endpoint types and profiles

   We define the following basic endpoint types and profiles:

   *  Trunk gateway (ISUP)

   *  Trunk gateway (MF)

   *  Network Access Server (NAS)

   *  Combined NAS/VOIP gateway

   *  Access Gateway

   *  Residential Gateway

   *  Announcement servers

7. Versions and compatibility

7.1. Differences between version 1.0 and draft 0.5

   Draft 0-5 was issued in February 1999, as the last update of draft
   version 0.1. Version 1.0 benefits from implementation experience, and
   also aligns as much as possible with the CableLabs' NCS project. The
   main differences between the February draft and version 1.0 are:

   *  Specified more clearly that the encoding of three
      LocalConnectionOptions parameters, Encoding Method, Packetization
      Period and Bandwidth, shall follow the conventions laid out in
      SDP.

   *  Specified how the quarantine handling parameter governs the
      handling of detected but not yet specified events.

   *  Specified that unexpected timers or digits should trigger
      transmission of the dialed string.

   *  Removed the digit map syntax description from section 2.1.5 (it
      was redundant with section 3.4.)

   *  Corrected miscellaneous bugs in the formal syntax description.

   *  Aligned specification of commands with the CableLabs NCS
      specification.  This mostly affects the AuditEndpoint and

      RestartInProgress commands.

   *  Aligned the handling of retransmission with the CableLabs NCS
      specification.

   *  Added the provisional response return code and corresponding
      behavior description.

   *  Added an optional reason code parameter to restart in progress.

   *  Added the possibility to audit the restart method, restart delay
      and reason code.

7.2. Differences between draft-04 and draft-05

Differences are minor: corrected the copyright statement, and
corrected a bug in the formal description.

7.3. Differences between draft-03 and draft-04

Draft 04 corrects a number of minor editing mistakes that were
pointed out during the review of draft 03, issued on February 1.

7.4. Differences between draft-02 and draft-03

   The main differences between draft-02, issued in January 22 1998, and
   draft 03 are:

   *  Introduced a discussion on endpoint types,

   * Introduced a discussion of the connection set-up procedure, and of
      the role of connection parameters,

   *  Introduced a notation of the connection identifier within event
      names,

   *  Documented the extension procedure for the LocalConnectionOptions
      parameter and for the ConnectionParameters parameter,

   *  Introduced a three-way handshake procedure, using a ResponseAck
      parameter, in order to allow gateways to delete copies of old
      responses without waiting for a 30 seconds timer,

   *  Expanded the security section to include a discussion of
      "uncontrolled barge-in."

   *  Propsed a "create two connections" command, as an appendix.

7.5. Differences between draft-01 and draft-02

   The main differences between draft-01, issued in November 1998, and
   draft 02 are:

   *  Added an ABNF description of the protocol.

   *  Specification of an EndpointConfiguration command,

   *  Addition of a "two endpoints" mode in the create connection
      command,

   *  Modification of the package wildcards from "$/$" to "*/all" at the
      Request of early implementors,

   *  Revision of some package definitions to better align with external
      specifications.

   *  Addition of a specification for the handling of "failover."

   *  Revision of the section on race conditions.

7.6. The making of MGCP from IPDC and SGCP

MGCP version 0.1 results from the fusion of the SGCP and IPDC
proposals.

7.7. Changes between MGCP and initial versions of SGCP

   MGCP version 0.1 (which subsumes SGCP version 1.2) introduces the
   following changes from SGCP version 1.1:

   *  Protocol name changed to MGCP.

   *  Introduce a formal wildcarding structure in the name of endpoints,
      inspired from IPDC, and detailed the usage of wildcard names in
      each operation.

   *  Naming scheme for events, introducing a package structure inspired
      from IPDC.

   *  New operations for audit endpoint, audit connection (requested by
      the Cablelabs) and restart (inspired from IPDC).

   *  New parameter to control the behavior of the notification request.

   *  Improved text on the detection and handling of race conditions.

   *  Syntax modification for event reporting, to incorporate package
      names.

   *  Definition of basic event packages (inspired from IPDC).

   *  Incorporation of mandatory and optional extension parameters,
      inspired by IPDC.

   SGCP version 1.1 introduces the following changes from version SGCP
      1.0:

   *  Extension parameters (X-??:)

   *  Error Code 511 (Unrecognized extension).

   *  All event codes can be used in RequestEvent, SignalRequest and
      ObservedEvent parameters.

   *  Error Code 512 (Not equipped to detect requested event).

   *  Error Code 513 (Not equipped to generate requested signal).

   *  Error Code 514 (Unrecognized announcement).

   *  Specific Endpoint-ID can be returned in creation commands.

   *  Changed the code for the ASDI display from "ad" to "asdi" to avoid
      conflict with the digits A and D.

   *  Changed the code for the answer tone from "at" to "aw" to avoid
      conflict with the digit A and the timer mark T

   *  Changed the code for the busy tone from "bt" to "bz" to avoid
      conflict with the digit B and the timer mark T

   *  Specified that the continuity tone value is "co" (CT was
      incorrectly used in several instances; CT conflicts with .)

   *  Changed the code for the dial tone from "dt" to "dl" to avoid
      conflict with the digit D and the timer mark T

   *  Added a code point for announcement requests.

   *  Added a code point for the "wink" event.

   *  Set the "octet received" code in the "Connection Parameters" to
      "OR" (was set to RO, but then "OR" was used throughout all
      examples.)

   *  Added a "data" mode.

   *  Added a description of SDP parameters for the network access mode
      (NAS).

   *  Added four flow diagrams for the network access mode.

   *  Incorporated numerous editing suggestions to make the description
      easier to understand. In particular, cleared the confusion between
      requests, queries, functions and commands.

   *  Defined the continuity test mode as specifying a dual-tone
      transponder, while the loopback mode can be used for a single tone
      test.

   *  Added event code "OC", operation completed.

   *  Added the specification of the "quarantine list", which clarifies
      the expected handling of events and notifications.

   *  Added the specification of a "wildcard delete" operation.

8. Security Considerations

Security issues are discussed in section 5.

9. Acknowledgements

   We want to thank here the many reviewers who provided us with advice
   on the design of SGCP and then MGCP, notably Flemming Andreasen,
   Sankar Ardhanari, Francois Berard, David Auerbach, Bob Biskner, David
   Bukovinsky, Jerry Kamitses, Oren Kudevitzki, Barry Hoffner, Troy
   Morley, Dave Oran, Jeff Orwick, John Pickens, Lou Rubin, Chip Sharp,
   Paul Sijben, Kurt Steinbrenner, Joe Stone and Stuart Wray.

   The version 0.1 of MGCP is heavily inspired by the "Internet Protocol
   Device Control" (IPDC) designed by the Technical Advisory Committee
   set up by Level 3 Communications.  Whole sets of text have been
   retrieved from the IP Connection Control protocol, IP Media Control
   protocol, and IP Device Management.  The authors wish to acknowledge
   the contribution to these protocols made by Ilya Akramovich, Bob
   Bell, Dan Brendes, Peter Chung, John Clark, Russ Dehlinger, Andrew
   Dugan, Isaac Elliott, Cary FitzGerald, Jan Gronski, Tom Hess, Geoff
   Jordan, Tony Lam, Shawn Lewis, Dave Mazik, Alan Mikhak, Pete
   O'Connell, Scott Pickett, Shyamal Prasad, Eric Presworsky, Paul
   Richards, Dale Skran, Louise Spergel, David Sprague, Raj Srinivasan,
   Tom Taylor and Michael Thomas.

10. References

   *  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
      A Transport Protocol for Real-Time Applications", RFC 1889,
      January 1996.

   *  Schulzrinne, H., "RTP Profile for Audio and Video Conferences with
      Minimal Control", RFC 1890, January 1996.

   *  Handley, M and V. Jacobson, "SDP: Session Description Protocol",
      RFC 2327, April 1998.

   *  Handley, M., "SAP - Session Announcement Protocol", Work in
      Progress.

   *  Handley, M., Schulzrinne, H. and E. Schooler, "Session Initiation
      Protocol (SIP)", RFC 2543, March 1999.

   *  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
      Protocol (RTSP)", RFC 2326, April 1998.

   *  ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
      USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-Torremolinos, 1984;
      modified at Helsinki, 1993)

   *  ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND
      SIGNALS OF THE ISDN USER PART OF SIGNALLING SYSTEM No. 7",
      (MalagaTorremolinos, 1984; modified at Helsinki, 1993)

   *  ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA
      COMMUNICATIONS SYSTEMS."

   *  ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
      Stream Packetization for Packet Based Multimedia Communications
      Systems."

   *  ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR
      MULTIMEDIA COMMUNICATION."

   *  Kent, S. and  R. Atkinson, "Security Architecture for the Internet
      Protocol", RFC 2401, November 1998.

   *  Kent, S. and  R. Atkinson, "IP Authentication Header", RFC 2402,
      November 1998.

   *  Kent, S. and  R. Atkinson, "IP Encapsulating Security Payload
      (ESP)", RFC 2406, November 1998.

   *  Crocker, D. and  P. Overell, "Augmented BNF for Syntax
      Specifications:  ABNF", RFC 2234, November 1997.

11.  Authors' Addresses

   Mauricio Arango
   RSL COM Latin America
   6300 N.W. 5th Way, Suite 100
   Ft. Lauderdale, FL 33309

   Phone: (954) 492-0913
   EMail: marango@rslcom.com

   Andrew Dugan
   Level3 Communications
   1450 Infinite Drive
   Louisville, CO 80027

   Phone: (303)926 3123
   EMail: andrew.dugan@l3.com

   Isaac Elliott
   Level3 Communications
   1450 Infinite Drive
   Louisville, CO 80027

   Phone: (303)926 3123
   EMail: ike.elliott@l3.com

   Christian Huitema
   Telcordia Technologies
   MCC 1J236B
   445 South Street
   Morristown, NJ 07960
   U.S.A.

   Phone: +1 973-829-4266
   EMail: huitema@research.telcordia.com

   Scott Pickett
   Vertical Networks
   1148 East Arques Ave
   Sunnyvale, CA 94086

   Phone: (408) 523-9700 extension 200
   EMail: ScottP@vertical.com

   Further information is available on the SGCP web site:

           http://www.argreenhouse.com/SGCP/

12. Appendix A: Proposed "MoveConnection" command

   It has been proposed to create a new command, that would move an
   existing connection from one endpoint to another, on the same
   gateway.  This command would be specially useful for handling certain
   call services, such as call forwarding between endpoints served by
   the same gateway.

         [SecondEndPointId,]
         [ConnectionId,]
         [LocalConnectionDescriptor]
          <--- ModifyConnection(CallId,
                                EndpointId,
                                ConnectionId,
                                SecondEndPointId,
                                [NotifiedEntity,]
                                [LocalConnectionOptions,]
                                [Mode,]
                                [RemoteConnectionDescriptor,]
                                [Encapsulated NotificationRequest,]
                                [Encapsulated EndpointConfiguration])

   The parameters used are the same as in the ModifyConnection command,
   with the addition of a SecondEndpointId that identifies the endpoint
   towards which the connection is moved.

   The EndpointId should be the fully qualified endpoint identifier of
   the endpoint on which the connection has been created. The local name
   shall not use the wildcard convention.

   The SecondEndpointId shall be the endpoint identifier of the endpoint
   towards which the connection has been created. The "any of" wildcard
   convention can be used, but not the "all of" convention.  If the
   SecondEndpointId parameter is unqualified, the gateway will choose a
   value, that will be returned to the call agent as a response
   parameter.

   The command will result in the "move" of the existing connection to
   the second endpoint.  Depending on gateway implementations, the
   connection identifier of the connection after the move may or may not
   be the same as the connection identifier before the move.  If it is
   not the same, the new value is returned as a response parameter.

   The intent of the command is to effect a local relocation of the
   connection, without having to modify such transmission parameters as
   IP addresses and port, and thus without forcing the call agent to
   signal the change of parameters to the remote gateway, at the other

   end of the connection.  However, gateway architectures may not always
   allow such transparent moves.  For example, some architectures could
   allow specific IP addresses to different boards that handles specific
   group of endpoints.  If for any reason the transmission parameters
   have to be changed as a result of the move, the new
   LocalConnectionDescriptor is returned as a response parameter.

   The LocalConnectionOptions, Mode, and RemoteConnectionDescriptor,
   when present, are applied after the move.

   The RequestedEvents, RequestIdentifier, DigitMap, SignalRequests,
   QuarantineHandling and DetectEvents parameters are optional.  They
   can be used by the Call Agent to transmit a NotificationRequest that
   is executed simultaneously with the move of the connection. When
   these parameters are present, the NotificationRequest applies to the
   second endpoint.

   When these parameters are present, the move and the
   NotificationRequests should be synchronized, which means that both
   should be accepted, or both refused.  The NotifiedEntity parameter,
   if present, applies to both the ModifyConnection and the
   NotificationRequest command.

   The command may carry an encapsulated EndpointConfiguration command,
   that will also apply to the second endpoint.  When this command is
   present, the parameters of the EndpointConfiguration command are
   inserted after the normal parameters of the MoveConnection with the
   exception of the SecondEndpointId, which is not replicated. The End-
   pointConfiguration command may be encapsulated together with an
   encapsulated NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   MoveConnection command.  If the MoveConnection is rejected, the End-
   pointConfiguration is not executed.

12.1. Proposed syntax modification

   The only syntax modification necessary for the addition of the
   moveConnection command is the addition of the keyword MOVE to the
   authorized values in the MGCPVerb clause of the formal syntax.

13. Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.