Network Working Group                                          S. Hanna
Requests for Comments: 2730                      Sun Microsystems, Inc.
Category: Standards Track                                      B. Patel
                                                            Intel Corp.
                                                                M. Shah
                                                        Microsoft Corp.
                                                          December 1999

Multicast Address Dynamic Client Allocation Protocol (MADCAP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Abstract

   This document defines a protocol, Multicast Address Dynamic Client
   Allocation Protocol (MADCAP), that allows hosts to request multicast
   addresses from multicast address allocation servers.

1. Introduction

   Multicast Address Dynamic Client Allocation Protocol (MADCAP) is a
   protocol that allows hosts to request multicast address allocation
   services from multicast address allocation servers. This protocol is
   part of the Multicast Address Allocation Architecture being defined
   by the IETF Multicast Address Allocation Working Group. However, it
   may be used separately from the rest of that architecture as
   appropriate.

1.1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [9].

   Constants used by this protocol are shown as [NAME-OF-CONSTANT], and
   summarized in Appendix B.

1.2. Definitions

   This specification uses a number of terms that may not be familiar to
   the reader. This section defines some of these and refers to other
   documents for definitions of others.

   MADCAP client or client
     A host requesting multicast address allocation services via MADCAP.

   MADCAP server or server
     A host providing multicast address allocation services via MADCAP.

   Multicast
     IP Multicast, as defined in [11] and modified in [12].

   Multicast Address
     An IP multicast address or group address, as defined in [11] and
     [13].  An identifier for a group of nodes.

   Multicast Scope
     A range of multicast addresses configured so that traffic sent to
     these addresses is limited to some subset of the internetwork. See
     [3] and [13].

   Scope ID
     The lowest numbered address in a multicast scope. This definition
     applies only within this document.

   Scope Zone
     One multicast scope may have several instances, which are known as
     Scope Zones or zones, for short.

     For instance, an organization may have multiple sites. Each site
     might have its own site-local Scope Zone, each of which would be an
     instance of the site-local Scope. However, a given interface on a
     given host would only ever be in at most one instance of a given
     scope.  Messages sent by a host in a site-local Scope Zones to an
     address in the site-local Scope would be limited to the site-local
     Scope Zone containing the host.

   Zone Name
     A human readable name for a Scope Zone. An ISO 10646 character
     string with an RFC 1766 [6] language tag. One zone may have several
     zone names, each in a different language. For instance, a zone for
     use within IBM's locations in Switzerland might have the names "IBM
     Suisse", "IBM Switzerland", "IBM Schweiz", and "IBM Svizzera" with
     language tags "fr", "en", "de", and "it".

   Multicast Scope List
     A list of multicast scope zones.

     Since it can be difficult to determine which multicast scope zones
     are in effect, MADCAP clients can ask MADCAP servers to supply a
     Multicast Scope List listing all of the zones available to the
     client. For each scope zone, the list includes the range of
     multicast addresses for this scope, a maximum TTL or hop count to
     be used for this scope, and one or more zone names for this scope
     zone.

     This definition applies only within this document.

1.3. Motivation and Protocol Requirements

   For multicast applications to be deployed everywhere, there is a need
   to define a protocol that any host may use to allocate multicast
   addresses. Here are the requirements for such a protocol.

   Quick response: The host should be able to allocate a multicast
   address and begin to use it promptly.

   Low network load: Hosts that are not allocating or deallocating
   multicast addresses at the present time should not need to send or
   receive any network traffic.

   Support for intermittently connected or power managed systems: Hosts
   should be able to be disconnected from the network, powered off, or
   otherwise inaccessible except during the brief period during which
   they are allocating a multicast address.

   Multicast address scopes: The protocol must be able to allocate both
   the administratively scoped and globally scoped multicast addresses.

   Efficient use of address space: The multicast address space is fairly
   small. The protocol should make efficient use of this scarce
   resource.

   Authentication: Because multicast addresses are scarce, it is
   important to protect against hoarding of these addresses. One way to
   do this is by authenticating clients. This is also a key prerequisite
   for establishing policies.

   Policy neutral: Allocation policies (such as who can allocate
   addresses) should not be dictated by the protocol.

   Conferencing support: When allocating an address for use in a
   conferencing environment, members of the conference should be able to
   modify a multicast address lease used for the conference.

1.4. Relationship with DHCP

   MADCAP was originally based on DHCP. There are still some
   similarities and it may be possible to share some code between a DHCP
   implementation and a MADCAP implementation. However, MADCAP is
   completely separate from DHCP, with no dependencies between the two
   and many significant differences.

1.5. Protocol Overview

   MADCAP is built on a client-server model, where hosts request address
   allocation services from address allocation servers. When a MADCAP
   client wishes to request a service, it unicasts or multicasts a
   message to one or more MADCAP servers, each of which optionally
   responds with a message unicast to the client.

   All messages are UDP datagrams. The UDP data contains a fixed length
   header and a variable length options field. Options are encoded in a
   type-length-value format with two octets type and value fields.  The
   fixed fields are version, msgtype (message type), addrfamily (address
   family), and xid (transaction identifier).

   Retransmission is handled by the client. If a client sends a message
   and does not receive a response, it may retransmit its request a few
   times using an exponential backoff. To avoid executing the same
   client request twice when a retransmitted request is received,
   servers cache responses for a short period of time and resend cached
   responses upon receiving retransmitted requests.

   Each request contains a msgtype, an xid, and a Lease Identifier
   option.  Clients must ensure that this triple is probably unique
   across all MADCAP messages received by a MADCAP server over a period
   of [XID-REUSE-INTERVAL] (10 minutes). This allows the MADCAP server
   to use this triple as the key in its response cache.

   Messages sent by servers include the xid included in the original
   request so that clients can match up responses with requests.

   The msgtype field is a single octet that defines the "type" of a
   MADCAP message. Currently defined message types are listed in Table
   2. They are: DISCOVER, OFFER, REQUEST, RENEW, ACK, NAK, RELEASE, and

   GETINFO.  DISCOVER, REQUEST, RENEW, RELEASE, and GETINFO messages are
   only sent by a client. OFFER, ACK, and NAK messages are only sent by
   a server.

   The REQUEST, RENEW, and RELEASE messages are used to request, renew,
   or release a lease on one or more multicast addresses. A client
   unicasts one of these messages to a server and the server responds
   with an ACK or a NAK.

   The GETINFO message is used to request information, such as the
   multicast scope list, or to find MADCAP servers. A client may unicast
   an GETINFO message to a MADCAP server. However, it may not know the
   IP address of any MADCAP server. In that case, it will multicast an
   GETINFO message to a MADCAP Server Multicast Address and all servers
   that wish to respond will send a unicast ACK or NAK back to the
   client.

   Each multicast scope has an associated MADCAP Server Multicast
   Address.  This address has been reserved by the IANA as the address
   with a relative offset of -1 from the last address of a multicast
   scope. MADCAP clients use this address to find MADCAP servers.

   The DISCOVER message is a message used to discover MADCAP servers
   that can probably satisfy a REQUEST. DISCOVER messages are always
   multicast.  Servers that can probably satisfy a REQUEST corresponding
   to the parameters supplied in the DISCOVER message temporarily
   reserve the addresses needed and send a unicast OFFER back to the
   client. The client selects a server with which to continue and sends
   a multicast REQUEST including the server's Server Identifier to the
   same multicast address used for the DISCOVER. The chosen server
   responds with an ACK or NAK and the other servers stop reserving the
   addresses they were temporarily holding.

   For detailed descriptions of typical protocol exchanges, consult
   Appendix A.

   MADCAP is a mechanism rather than a policy. MADCAP allows local
   system administrators to exercise control over configuration
   parameters where desired. For example, MADCAP servers may be
   configured to limit the number of multicast addresses allocated to a
   single client. Properly enforcing such a limit requires cryptographic
   security, as described in the Security Consideration section.

   MADCAP requests from a single host may be sent on behalf of different
   applications with different needs and requirements. MADCAP servers
   MUST NOT assume that because one request from a MADCAP client
   supports a particular optional feature (like Retry After), future
   requests from that client will also support that optional feature.

2. Protocol Description

   The MADCAP protocol is a client-server protocol. In general, the
   client unicasts or multicasts a message to one or more servers, which
   optionally respond with messages unicast to the client.

   A reserved port number dedicated for MADCAP is used on the server
   (port number 2535, as assigned by IANA). Any port number may be used
   on client machines. When a MADCAP server sends a message to a MADCAP
   client, it MUST use a destination port number that matches the source
   port number provided by the client in the message that caused the
   server to send its message.

   The next few sections describe the MADCAP message format and message
   types. A full list of MADCAP options is provided in section 3.

2.1. Message Format

   Figure 1 gives the format of a MADCAP message and Table 1 describes
   each of the fields in the MADCAP message. The numbers in parentheses
   indicate the size of each field in octets. The names for the fields
   given in the figure will be used throughout this document to refer to
   the fields in MADCAP messages.

   All multi-octet quantities are in network byte-order.

   Any message whose UDP data is too short to hold this format (at least
   12 bytes) MUST be ignored.

                +-+-+-+-+-+-+-+-+
                |  version (1)  |
                +---------------+
                |  msgtype (1)  |
                +---------------+
                |  addrfamily   |
                |     (2)       |
                +---------------+
                |               |
                |    xid (4)    |
                |               |
                |               |
                +---------------+
                |               |
                |   options     |
                |  (variable)   |
                |      ...      |
                +---------------+

        Figure 1:  Format of a MADCAP message

  FIELD      OCTETS       DESCRIPTION
  -----      ------       -----------

  version       1  Protocol version number (zero for this specification)
  msgtype       1  Message type (DISCOVER, GETINFO, etc.)
  addrfamily    2  Address family (IPv4, IPv6, etc.)
  xid           4  Transaction ID
  options     var  Options field

          Table 1:  Description of fields in a MADCAP message

2.1.1. The version field

   The version field must always be zero for this version of the
   protocol.  Any messages that include other values in this field MUST
   be ignored.

2.1.2. The msgtype field

The msgtype field defines the "type" of the MADCAP message.

For more information about this field, see section 2.2.

2.1.3. The addrfamily field

   The addrfamily field defines the default address family (such as IPv4
   or IPv6) for this MADCAP message, using the address family numbers
   defined in by the IANA (including those defined in [10]). Unless
   otherwise specified, all addresses included in the message will be
   from this family.

2.1.4. The xid field

   The xid field is a transaction identifier. This number MUST be chosen
   by the client so that the combination of xid, msgtype, and Lease
   Identifier is unique across all MADCAP messages received by a MADCAP
   server over a period of [XID-REUSE-INTERVAL] (10 minutes).

   The xid field is used by the client and server to associate messages
   and responses between a client and a server. Before a client sends a
   message, it chooses a number to use as an xid. The technique used to
   choose an xid is implementation-dependent, but whatever technique is
   used MUST make it unlikely that the same combination of xid, msgtype,
   and Lease Identifier will be used for two different messages within
   [XID-REUSE-INTERVAL] (even across multiple clients which do not
   communicate among themselves).  This allows enough time for the
   message to be dropped from all server response caches (as described
   in the next few paragraphs) and for any network delays to be
   accomodated.

   The RECOMMENDED technique for choosing an xid is to choose a random
   four octet number as the first xid in a session and increment this
   value each time a new xid is needed.  The random number chosen need
   not be cryptographically random. The random number may be chosen via
   any suitable technique, such as the one described in section A.6 of
   RFC 1889 [14].

   When a server responds to a client message, it MUST use the same xid
   value in the response that the client used in the request. This
   allows the client to associate responses with the message that they
   are responding to.

   When retransmitting messages (as described in section 2.3), the
   client MUST retransmit them without changing them, thereby using the
   same xid and and Lease Identifier.

   If a server receives a message with the same xid, msgtype, and Lease
   Identifier as one received within [RESPONSE-CACHE-INTERVAL], it MUST
   treat this message as a retransmission of the previously received one
   and retransmit the response, if any. After [RESPONSE-CACHE-INTERVAL],
   the server may forget about the previously received message and treat

   any retransmissions of this message as if they were new messages. Of
   course, a server need not cache a message if it ends up ignoring that
   message. However, performance gains may be achieved by doing so.

   This avoids retransmissions causing multiple allocations, since
   requests are not idempotent.  An appropriate value for [RESPONSE-
   CACHE-INTERVAL] would be sixty seconds, but it may have any value
   from zero seconds to 300 seconds (five minutes) and may be adjusted
   dynamically according to resource constraints on the server.
   However, using a value less than sixty seconds is NOT RECOMMENDED
   because this is the normal client retransmission period.

2.1.5. The options field

   The options field consists of a list of tagged parameters that are
   called "options". All options consist of a two octet option code and
   a two octet option length, followed by the number of octets specified
   by the option length. In the case of some options, the length field
   is a constant but must still be specified.

   The option field MUST contain a sequence of options with the last one
   being the End option (option code 0). Any message whose options field
   does not conform to this syntax MUST be ignored.

   Any MADCAP client or server sending a MADCAP message MAY include any
   of the options listed in section 3, subject to the restrictions in
   Table 5 and elsewhere in this document. They MAY also include other
   MADCAP options that are defined in the future. A MADCAP client or
   server MUST NOT include more than one option with the same option
   type in one MADCAP message.

   All MADCAP clients and servers MUST recognize all options listed in
   this document and behave in accordance with this document when
   receiving and processing any of these options. Any unrecognized
   options MUST be ignored and the rest of the message processed as if
   the unknown options were not present. If a MADCAP server receives a
   message that does not conform to the requirements of this document
   (for instance, not including all required options), an Invalid
   Request error MUST be generated and processed in the manner described
   in section 2.6. If a MADCAP client receives a message that does not
   conform to the requirements of this document, it MUST ignore the
   message.

   The order of options within a message has no significance and any
   order MUST be supported in an equivalent manner, with the exception
   that the End option must occur once per message, as the last option
   in the option field.

New MADCAP option codes may only be defined by IETF Consensus, as
described in section 5.

2.2. Message Types

   The msgtype field defines the "type" of a MADCAP message. Legal
   values for this field are:

           Value   Message Type
           -----   ------------
             1     DISCOVER
             2     OFFER
             3     REQUEST
             4     RENEW
             5     ACK
             6     NAK
             7     RELEASE
             8     GETINFO

      Table 2:  MADCAP message types

   Throughout this document, MADCAP messages will be referred to by the
   type of the message; e.g., a MADCAP message with a message type of 8
   will be referred to as an GETINFO message.

   Here are descriptions of the MADCAP message types.  Table 5, which
   appears at the beginning of section 3, summarizes which options are
   allowed with each message type.

   MADCAP clients and servers MUST handle all MADCAP message types
   defined in this document in a manner consistent with this document.

   If a MADCAP server receives a message whose message type it does not
   recognize, an Invalid Request error MUST be generated and processed
   in the manner described in section 2.6. If a MADCAP client receives a
   message whose message type it does not recognize, it MUST ignore the
   message.

   Note, however, that under some circumstances this document requires
   or suggests that clients or servers ignore messages with certain
   message types even though they may be recognized. For instance,
   clients that do not send DISCOVER messages SHOULD ignore OFFER
   messages.  Also, secure servers SHOULD ignore DISCOVER messages and
   all servers SHOULD ignore DISCOVER messages that they cannot satisfy.

   New MADCAP message types may only be defined by IETF Consensus, as
   described in section 5.

2.2.1. GETINFO

   The GETINFO message is used by a MADCAP client that wants to acquire
   configuration parameters, especially a multicast scope list.  This
   message also allows a client to determine which servers are likely to
   be able to handle future requests.

   The MADCAP client sends out an GETINFO message. The message may be
   unicast to a particular MADCAP server or multicast to a MADCAP Server
   Multicast Address. For more details about the MADCAP Server Multicast
   Address, see section 2.10.

   If a server receives an GETINFO message and it can process the
   request successfully, it MUST unicast an ACK message to the client.
   All GETINFO messages MUST include an Option Request List option. The
   server SHOULD try to include the specified options in its response,
   but is not required to do so (especially if it does not recognize
   them).

   If a server receives an GETINFO message and it does not process the
   request successfully, it MUST generate and process an error in the
   manner described in section 2.6.

   If a client sends an GETINFO message and does not receive any ACK
   messages in response, it SHOULD resend its GETINFO message, as
   described in section 2.3.

   When a MADCAP client sends an GETINFO message, it MAY include the
   Requested Language option, which specifies which language the client
   would prefer for the zone names in the Multicast Scope List. The
   proper way to handle this tag with respect to zone names is discussed
   in the definition of the Multicast Scope List option.

2.2.2. DISCOVER

   The DISCOVER message is a multicast message sent by a MADCAP client
   that wants to discover MADCAP servers that can probably satisfy a
   REQUEST.

   MADCAP clients are not required to use the DISCOVER message.  They
   MAY employ other methods to find MADCAP servers, such as sending a
   multicast GETINFO message, caching an IP address that worked in the
   past or being configured with an IP address. Using the DISCOVER
   message has the particular advantage that it allows clients to
   receive responses from all servers that can satisfy the request.

   The MADCAP client begins by sending a multicast DISCOVER message to a
   MADCAP Server Multicast Address. Any servers that wish to assist the
   client respond by sending a unicast OFFER message to the client. If a
   server can only process the request with a shorter lease time or
   later start time than the client requested, it SHOULD send an OFFER
   message with the lease time or start time that it can offer.
   However, it MUST NOT offer a lease time shorter than the minimum
   lease time specified by the client or a start time later than the
   maximum start time specified by the client.

   For more details about the MADCAP Server Multicast Address, see
   section 2.10.

   If a client sends a DISCOVER message and does not receive any OFFER
   messages in response, the client SHOULD retransmit its DISCOVER
   message, as described in section 2.3.

   If a client sends a DISCOVER message and receives one or more OFFER
   messages in response, it SHOULD select the server it wants to use (if
   any) and send a multicast REQUEST message identifying that server
   within [DISCOVER-DELAY] after receiving the first OFFER message.  See
   section 2.2.4 for more information about the REQUEST message.

   The mechanism used by the client in selecting the server it wants to
   use is implementation dependent.  The client MAY choose the first
   acceptable response or it MAY wait some period of time (no more than
   [DISCOVER-DELAY]) and choose the best response received in that
   period of time (if the first response has a smaller lease time than
   requested, for instance).

   The value of [DISCOVER-DELAY] is also implementation dependent, but
   the RECOMMENDED value is the current retransmit timer, as specified
   in section 2.3. Waiting too long (approaching [OFFER-HOLD]) may cause
   servers to drop the addresses they have reserved.

   When a MADCAP client sends a DISCOVER message, it MAY include the
   Lease Time, Minimum Lease Time, Start Time, Maximum Start Time,
   Number of Addresses Requested, and List of Address Ranges options,
   describing the addresses it wants to receive. However, it need not
   include any of these options. If one of these options is not
   included, the server will provide the appropriate default (maximum
   available for Lease Time, no minimum for Minimum Lease Time, as soon
   as possible for Start Time, no maximum for Maximum Start Time, one
   for Number of Addresses Requested, and any addresses available for
   List of Address Ranges).  The Multicast Scope option MUST be included
   in the DISCOVER message so that the server knows what scope should be
   used. The Current Time option MUST be included if the Start Time or
   Maximum Start Time options are included. The Lease Identifier option

MUST always be included.

2.2.3. OFFER

   The OFFER message is a unicast message sent by a MADCAP server in
   response to a DISCOVER message that it can probably satisfy.

   A MADCAP server is never required to send an OFFER message in
   response to a DISCOVER message. For instance, it may not be able to
   satisfy the client's request or it may have been configured to
   respond only to certain types of DISCOVER messages or not to respond
   to DISCOVER messages at all.

   If a MADCAP server decides to send an OFFER message, it MUST include
   the Lease Time and Multicast Scope options, describing the addresses
   it is willing to provide. However, it need not include the List of
   Address Ranges option. If the List of Address Ranges Allocated option
   is not included, it is assumed that the server is willing to provide
   the number of addresses that the client requested. If the Start Time
   option is not included, it is assumed that the server is willing to
   provide the start time requested by the client (if any). The Current
   Time option MUST be included if the Start Time option is included.

   If a server can process the request with a shorter lease time or
   later start time than the client requested, it SHOULD send an OFFER
   message with the lease time or start time that it can offer.
   However, it MUST NOT offer a lease time shorter than the minimum
   lease time specified by the client or a start time later than the
   maximum start time specified by the client.

   If the server sends an OFFER message, it SHOULD attempt to hold
   enough addresses to complete the transaction. If it receives a
   multicast REQUEST message with the same Lease Identifier option as
   the DISCOVER message for which it is holding these addresses and a
   Server Identifier option that does not match its own, it SHOULD stop
   holding the addresses.  The server SHOULD also stop holding the
   addresses after an appropriate delay [OFFER-HOLD] if the transaction
   is not completed. The value of this delay is implementation-specific,
   but a value of at least 60 seconds is RECOMMENDED.

   As with all messages sent by the server, the xid field MUST match the
   xid field included in the client request to which this message is
   responding. The Lease Identifier option MUST be included, with the
   value matching the one included in the client request. The Server
   Identifier option MUST be included, with the value being the server's
   IP address. And the packet MUST NOT be retransmitted.

2.2.4. REQUEST

   The REQUEST message is used by a MADCAP client that wants to allocate
   one or more multicast addresses. It is not used for renewing an
   existing lease. The RENEW message is used for that.

   If a REQUEST message is completing a transaction initiated by a
   DISCOVER message, the following procedure MUST be followed so that
   all MADCAP servers know which server was selected. The client MUST
   multicast a REQUEST message to the same MADCAP Server Multicast
   Address that the DISCOVER message was sent to. The same Lease
   Identifier used in the DISCOVER message MUST be used in the REQUEST
   message.  Also, the Server Identifier option MUST be included, using
   the Server Identifier of the server selected.

   If a REQUEST message is not completing a transaction initiated by a
   DISCOVER message, the REQUEST message MUST be unicast to the MADCAP
   server that the client wants to use. In this case, the Server
   Identifier option MAY be included, but need not be.

   If the selected server can process the request successfully, it
   SHOULD unicast an ACK message to the client. Otherwise, it SHOULD
   generate and process an error in the manner described in section 2.6.
   If a server can process the request with a shorter lease time or
   later start time than the client requested, it SHOULD send an ACK
   message with the lease time or start time that it can offer. However,
   it MUST NOT offer a lease time shorter than the minimum lease time
   specified by the client or a start time later than the maximum start
   time specified by the client.

   When a MADCAP client sends a REQUEST message, it MAY include the
   Lease Time, Minimum Lease Time, Start Time, Maximum Start Time,
   Number of Addresses Requested, and List of Address Ranges options,
   describing the addresses it wants to receive. However, it need not
   include any of these options. If one of these options is not
   included, the server will provide the appropriate default (maximum
   available for Lease Time, no minimum for Minimum Lease Time, as soon
   as possible for Start Time, no maximum for Maximum Start Time, one
   for Number of Addresses Requested, and any addresses available for
   List of Address Ranges). The Multicast Scope option MUST be included
   in the REQUEST message so that the server knows what scope should be
   used. The Current Time option MUST be included if the Start Time or
   Maximum Start Time options are included.

   If a client sends a REQUEST message and does not receive any ACK or
   NAK messages in response, the client SHOULD resend its REQUEST
   message, as described in section 2.3.

   If the server responds with a NAK or fails to respond within a
   reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
   client MAY try to find another server by sending a DISCOVER message
   with another xid or sending a REQUEST message with another xid to
   another server. The RECOMMENDED value for [NO-RESPONSE-DELAY] is 60
   seconds.

2.2.5. ACK

   The ACK message is used by a MADCAP server to respond affirmatively
   to an GETINFO, REQUEST, or RELEASE message. The server unicasts the
   ACK message to the client from which it received the message to which
   it is responding.

   The set of options included with an ACK message differs, depending on
   what sort of message it is responding to.

   If the ACK message is responding to an GETINFO message, it SHOULD
   include any options requested by the client using the Option Request
   List option.

   If the ACK message is responding to a REQUEST message, it MUST
   include Lease Time, Multicast Scope, and List of Address Ranges
   options.  It MAY include a Start Time option. If a Start Time option
   is included, a Current Time option MUST also be included. If no Start
   Time option is included, the lease is assumed to start immediately.

   If the ACK message is responding to a RENEW message, it MUST include
   Lease Time, Multicast Scope, and List of Address Ranges options.  It
   MAY include a Start Time option. If a Start Time option is included,
   a Current Time option MUST also be included. If no Start Time option
   is included, the lease is assumed to start immediately.

   If the ACK message is responding to a RELEASE message, it MUST only
   include Server Identifier and Lease Identifier options.

   As with all messages sent by the server, the xid field MUST match the
   xid field included in the client request to which this message is
   responding. The Lease Identifier option MUST be included, with the
   value matching the one included in the client request. The Server
   Identifier option MUST be included, with the value being the server's
   IP address. And the packet MUST NOT be retransmitted.

2.2.6. NAK

   The NAK message is used by a MADCAP server to respond negatively to a
   message. The server unicasts the NAK message to the client from which
   it received the message to which it is responding.

   As with all messages sent by the server, the xid field MUST match the
   xid field included in the client request to which this message is
   responding. The Lease Identifier option MUST be included, with the
   value matching the one included in the client request. The Server
   Identifier option MUST be included, with the value being the server's
   IP address. The Error option MUST be included with an error code
   indicating what went wrong. And the packet MUST NOT be retransmitted.

2.2.7. RENEW

   The RENEW message is used by a MADCAP client that wants to renew a
   multicast address lease, changing the lease time or start time.

   The client unicasts the RENEW message to a MADCAP server. If the
   server can process the request successfully, it SHOULD unicast an ACK
   message to the client. Otherwise, it MUST generate and process an
   error in the manner described in section 2.6.

   The lease to be renewed is whichever one was allocated with a Lease
   Identifier option matching the one provided in the RENEW message.

   When a MADCAP client sends a RENEW message, it MAY include the Lease
   Time, Minimum Lease Time, Start Time, and Maximum Start Time options,
   describing the new lease it wants to receive. However, it need not
   include any of these options. If one of these options is not
   included, the server will provide the appropriate default (maximum
   available for Lease Time, no minimum for Minimum Lease Time, as soon
   as possible for Start Time, and no maximum for Maximum Start Time).
   The Current Time option MUST be included if the Start Time or Maximum
   Start Time options are included.

   If a client sends a RENEW message and does not receive any ACK or NAK
   messages in response, the client SHOULD resend its RENEW message, as
   described in section 2.3.

   If the server responds with a NAK or fails to respond within a
   reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
   client MAY send a RENEW message with another xid to another server,
   provided that the Server Mobility feature was used in the original
   REQUEST message and that this feature is required for the subsequent
   RENEW message sent to another server. For more information about the
   Server Mobility feature, see section 2.13.1. The RECOMMENDED value
   for [NO-RESPONSE-DELAY] is 60 seconds.

2.2.8. RELEASE

   The RELEASE message is used by a MADCAP client that wants to
   deallocate one or more multicast addresses before their lease
   expires.

   The client unicasts the RELEASE message to the MADCAP server from
   which it allocated the addresses. If the selected server can process
   the request successfully, it MUST unicast an ACK message to the
   client. Otherwise, it MUST generate and process an error in the
   manner described in section 2.6.

   The lease to be released is whichever one was allocated with a Lease
   Identifier option matching the one provided in the RELEASE message.
   It is not possible to release only part of the addresses in a single
   lease.

   If a client sends a RELEASE message and does not receive any ACK or
   NAK messages in response, the client SHOULD resend its RELEASE
   message, as described in section 2.3.

   If the server responds with a NAK or fails to respond within a
   reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
   client MAY send a RELEASE message with another xid to another server,
   provided that the Server Mobility feature was used in the original
   REQUEST message and that this feature is required for the subsequent
   RELEASE message sent to another server. For more information about
   the Server Mobility feature, see section 2.13.1. The RECOMMENDED
   value for [NO-RESPONSE-DELAY] is 60 seconds.

2.3. Retransmission

   MADCAP clients are responsible for all message retransmission. The
   client MUST adopt a retransmission strategy that incorporates an
   exponential backoff algorithm to determine the delay between
   retransmissions. The delay between retransmissions SHOULD be chosen
   to allow sufficient time for replies from the server to be delivered
   based on the characteristics of the internetwork between the client
   and the server.

   The RECOMMENDED algorithm is to use a 4 second delay before the first
   retransmission and to double this delay for each successive
   retransmission, with a maximum delay of 16 seconds and a maximum of
   three retransmissions. If an initial transmission was sent at time
   (in seconds) t and no responses were received, subsequent
   transmissions would be at t+4, t+12, and t+28. If no response has
   been received by t+60, the client would stop retransmitting and take
   another course of action (such as logging an error or sending a

   message to another address.

   The client MAY provide an indication of retransmission attempts to
   the user as an indication of the progress of the process. The client
   MAY halt retransmission at any point.

2.4. The Lease Identifier

   The Lease Identifier option is included in each MADCAP message.  Its
   value is used to identify a lease and MUST be unique across all
   leases requested by all clients in a multicast address allocation
   domain.

   The first octet of the Lease Identifier is the Lease Identifier type.
   Table 3 lists the Lease Identifier types defined at this time and
   sections 2.4.1 and 2.4.2 describe these Lease Identifier types.

   New MADCAP Lease Identifier types may only be defined by IETF
   Consensus, as described in section 5.

           Lease Identifier Type   Name
           ---------------------   ----
                     0             Random Lease Identifier
                     1             Address-Specific Lease Identifier

      Table 3:  MADCAP Lease Identifier Types

   The MADCAP server does not need to parse the Lease Identifier. It
   SHOULD use the Lease Identifier only as an opaque identifier, which
   must be unique for each lease. The purpose of defining different
   Lease Identifier types is to allow MADCAP clients that already have a
   globally unique address to avoid the possibility of Lease Identifier
   collisions by using this address together with a client-specific
   identifier. MADCAP clients that do not have a globally unique address
   SHOULD use Lease Identifier type 0.

   In addition to associating client and server messages (along with the
   msgtype and xid fields, as described in the next section), the Lease
   Identifier is used to determine which lease a RENEW or RELEASE
   request refers to. MADCAP servers SHOULD match the Lease Identifier
   included in a RENEW or RELEASE message with the Lease Identifier used
   in an initial REQUEST message. If the Lease Identifier does not
   match, a MADCAP server MUST generate and process a Lease Identifier
   Not Recognized error in the manner described in section 2.6.

   For conferencing applications, it may be desirable to allow
   conference participants to modify a lease used for the conference.
   The Shared Lease Identifier feature code is used to support this
   requirement.  If this feature code was requested by the client and
   implemented by the server when the lease was allocated, the server
   SHOULD disable any authentication requirements and allow any client
   that knows the Lease Identifier to modify the lease.

   As described in the Security Considerations section, MADCAP security
   is not terribly useful without admission control in the multicast
   routing infrastructure. However, if MADCAP security is desired when
   using the Shared Lease Identifier feature, the confidentiality of the
   Lease Identifier MUST be maintained by encrypting all messages that
   contain it. A Lease Identifier that includes a long cryptographically
   random number (at least eight octets in length) MUST be used in this
   circumstance so that it is not easy to guess the Lease Identifier.

2.4.1. Random Lease Identifier

   The first octet of a Random Lease Identifier is the Lease Identifier
   type (0 to indicate Random Lease Identifier). After this come a
   sequence of octets, which SHOULD represent a long random number (at
   least 16 octets) from a decent random number generator.

   A Random Lease Identifier does not include any indication of its
   length.  It is assumed that this may be determined by external means,
   such as a length field preceding the Lease Identifier.

    Lease ID
      Type    Random Number
   +---------+-------------...
   |    0    |
   +---------+-------------...

2.4.2. Address-Specific Lease Identifier

   The first octet of an Address-Specific Lease Identifier is the Lease
   Identifier type (1 to indicate Address-Specific Lease Identifier).
   After this comes a two octet IANA-defined address family number
   (including those defined in [10]), an address from the specified
   address family, and a client-specific identifier (such as a sequence
   number or the current time).

   An Address-Specific Lease Identifier does not include any indication
   of its length. It is assumed that this may be determined by external
   means, such as a length field preceding the Lease Identifier.

    Lease ID     Address Family      Address    Client-specific
      Type           Number                       Identifier
   +---------+---------+---------+-----...-----+-----...-----+
   |    1    |     addrfamily    |   address   | cli-spec id |
   +---------+---------+---------+-----...-----+-----...-----+

2.5. Associating Client and Server Messages

   Messages between clients and servers are associated with one another
   using the Lease Identifier and the xid field. As described in section
   2.1.4, the client MUST choose an xid so that it is unlikely that the
   same combination of xid, msgtype, and Lease Identifier will be used
   for two different messages within [XID-REUSE-INTERVAL] (even across
   multiple clients which do not communicate among themselves).  The
   Lease Identifier option, msgtype, and xid field MUST be included in
   each message sent by the client or the server.

   The client MUST check the Lease Identifier option and xid field in
   each incoming message to ensure that they match the Lease Identifier
   and xid for an outstanding transaction. If not, the message MUST be
   ignored. The server MUST check the Lease Identifier option and xid
   field in each incoming message to establish the proper context for
   the message. If a server cannot process a message because it is
   invalid for its context, the server MUST generate and process an
   Invalid Request error, as described in section 2.6.  A transaction
   can be an attempt to allocate a multicast address (consisting of
   DISCOVER, OFFER, REQUEST, ACK, and NAK messages), an attempt to renew
   a lease (consisting of RENEW, ACK, and NAK messages), an attempt to
   release a previously allocated multicast address (consisting of
   RELEASE, ACK, and NAK messages), or an attempt to acquire
   configuration parameters (consisting of GETINFO, ACK, and NAK
   messages).

2.6. Processing Errors

   If a MADCAP server encounters an error while processing a message,
   there are two different ways to process this error. If it is clear
   that the message is not a NAK, the server SHOULD respond with a NAK
   containing the appropriate Error option. However, the server MAY
   decide to completely ignore chronic offenders. If the message is a
   NAK or it is not clear whether the message is a NAK (for instance,
   the message is garbled or has an incorrect version number), the
   server SHOULD ignore the message. This avoids NAK loops.

   If a MADCAP client encounters an error while processing a message, it
   MUST ignore the message.

2.7. Multicast Scopes

   RFC 2365 [3] provides for dividing the multicast address space into a
   number of administrative scopes. Routers should be configured so that
   each scope corresponds to a particular partition of the network into
   disjoint regions. Messages sent to a multicast address that falls
   within a certain administrative scope should only be delivered to
   hosts that have joined that multicast group *and* fall within the
   same region as the sender. For instance, packets sent to an address
   in the organization-local scope should only be delivered to hosts
   that have joined that group and fall within the same organization as
   the sender.

   Different sets of scopes may be in effect at different places in the
   network and at different times. Before attempting to allocate an
   address from an administrative scope (other than global or link-level
   scope, which are always in effect), a MADCAP client SHOULD determine
   that the scope is in effect at its location at this time.  Several
   techniques that a MADCAP client may use to determine the set of
   administrative scopes in effect (the scope list) are: manual
   configuration or configuration via MADCAP (using the Multicast Scope
   List option).

   If a MADCAP client is unable to determine its scope list using one of
   these techniques, it MAY temporarily assume a scope list consisting
   of a single scope. If it is using IPv4, it SHOULD use IPv4 Local
   Scope (239.255.0.0/16), with a maximum TTL of 16.  If it is using
   IPv6, it SHOULD use SCOP 3, with a maximum hop count of 16. Using
   this temporary scope list, it MAY attempt to contact a MADCAP server
   that can provide a scope list for it.

   When a MADCAP client requests an address with a DISCOVER or REQUEST
   message, it MUST specify the administrative scope from which the
   address should be allocated. This scope is indicated with the
   Multicast Scope option. Likewise, the server MUST include the
   Multicast Scope option in all OFFER messages and all ACK messages
   sent in response to REQUEST messages.

2.8. Multicast TTL

   Another way to limit propagation of multicast messages is by using
   TTL scoping. This technique has several disadvantages in comparison
   to administratively scoped multicast addresses (as described in [3]),
   but it is currently in widespread usage.

   With TTL scoping, areas of the network are designated as scopes.
   Routers on the edges of these areas are configured with TTL
   thresholds so that multicast packets are not forwarded unless their

   remaining TTL exceeds this threshold. A packet which should be
   restricted to a given TTL scope should have an initial TTL less than
   that scope's TTL threshold. Similar techniques may be used with IPv6,
   using the Hop Count field instead of the TTL field.

   MADCAP may be used in an environment where administrative scoping is
   not in use and TTL scoping is. Under these circumstances, a MADCAP
   server MAY return a scope list that includes scopes with TTLs less
   than 255. The MADCAP client MAY then allocate addresses from these
   scopes, but MUST NOT set the TTL field of any packet sent to such an
   address to a value greater than the maximum TTL indicated in the
   scope list. In such an environment, it is recommended that the MADCAP
   Server Multicast Addresses associated with the IPv4 Local Scope (or
   SCOP 3 for IPv6) be configured using TTL thresholds so that packets
   sent to these addresses with TTL of 16 are not sent outside an
   appropriate boundary.  This will allow MADCAP clients to use their
   default behavior for finding MADCAP servers.

   In an environment where administrative scoping is in use, the maximum
   TTLs in the scope list SHOULD be 255. The admin scope zone boundary
   routers will prevent leakage of MADCAP packets beyond appropriate
   limits.

2.9. Locating MADCAP Servers

   There are several ways for a MADCAP client to locate a MADCAP server.
   For instance, the client may be configured with an IP address.

   The RECOMMENDED technique is for the client to send an GETINFO
   message to a MADCAP Server Multicast Address and wait for ACK
   responses. This technique is described in more detail in the next
   section.

2.10. MADCAP Server Multicast Address

   Each multicast scope has an associated MADCAP Server Multicast
   Address. This address has been reserved by the IANA as the address
   with a relative offset of -1 from the last address of a multicast
   scope.

   A MADCAP client looking for servers that can provide multicast
   allocation services MAY send an GETINFO message to a MADCAP Server
   Multicast Address. Any MADCAP servers listening to this address
   SHOULD respond with a unicast ACK message to the client if they wish
   to offer a response.

   The MADCAP Server Multicast Address used by a client MAY be
   established by configuration. If a client has no such configuration,
   it SHOULD use the MADCAP Server Multicast Address associated with
   IPv4 Local Scope (or SCOP 3 for IPv6) with maximum TTL of 16, unless
   otherwise configured.

2.11. Going Beyond the Local Scope

   If a client receives no response to a message sent to a MADCAP Server
   Multicast Address (after retransmission), it MAY send the message to
   a larger scope and repeat this process as necessary. However, the
   client MUST NOT send a MADCAP message to the MADCAP Server Multicast
   Address associated with the global scope.

   This technique allows MADCAP servers to provide services for scopes
   in which they do not reside. However, this is a dangerous and
   complicated technique and is NOT RECOMMENDED at this time.
   Therefore, MADCAP clients SHOULD only send multicast messages to the
   MADCAP Server Multicast Address corresponding to the IPv4 Local Scope
   (or SCOP 3, if using IPv6), unless configured otherwise.

   MADCAP servers that wish to provide services for scopes in which they
   do not reside MUST make special efforts to ensure that their services
   meet clients' needs for largely conflict-free allocation and accurate
   scope list information.  In particular, coordinating with other
   servers that provide services for this scope may be difficult. Also,
   establishing which scope the client is in may be difficult. If a
   MADCAP server is not prepared to provide services for scopes in which
   it does not reside, it SHOULD ignore DISCOVER and REQUEST messages
   whose scope does not match or enclose the scope of the MADCAP Server
   Multicast Address on which the request was received. It SHOULD also
   ignore GETINFO messages that are not received on the MADCAP Server
   Multicast Address for IPv4 Local Scope.

2.12. Clock Skew

   The Current Time option is used to detect and handle clock skew
   between MADCAP clients and servers. This option MUST be included in
   any MADCAP message that includes an absolute time (such as the Start
   Time option). It MAY be included in any DISCOVER, OFFER, REQUEST,
   RENEW, or ACK message.

   Clock skew is a situation where two systems have clocks that are not
   synchronized. Many protocols (such as DHCP) ignore clock skew by
   using relative times. MADCAP could use a similar technique, but this
   leads to nasty situations due to the way multicast addresses are
   used.

   For example, assume that at 1 PM UTC a client whose clock is one hour
   fast requests a lease for one hour starting in one hour. If we were
   using relative times for MADCAP, the server, whose clock is set
   correctly, would reserve a multicast address for 2 to 3 PM UTC and
   grant the request. If the client was the only one using the lease,
   everything would be OK. The client would start using the lease in one
   hour and continue for one hour. This would coincide with the time the
   server had reserved (although the client would think it was 3 to 4 PM
   UTC).

   However, multicast addresses are usually used by several parties at
   once.  The client would probably use SAP (or some other mechanism for
   conveying SDP) to advertise a session using the multicast address
   just leased. SDP uses absolute times, since it may be sent via email,
   web, or other store-and-forward mechanisms. So the client would
   advertise the session as running from 3 to 4 PM UTC. Any clients
   whose clocks are set correctly would use the address during this
   interval. Since the server only reserved the address from 2 to 3 PM
   UTC, this might cause the address to be used for multiple sessions
   simultaneously.

   MADCAP cannot solve all clock skew problems. That is the domain of
   NTP [4].  However, it does attempt to detect substantial clock skew
   between MADCAP clients and servers so that this clock skew does not
   cause massive collisions in multicast address usage later on.

   The Current Time option contains the sender's opinion of the current
   time in UTC at or about the time the message was assembled. Because
   of delays in transmission and processing, this value will rarely
   match the receiver's opinion of the current time at the time the
   option is processed by the receiver. However, difference greater than
   a minute or two probably indicate clock skew between the sender and
   the receiver.

   MADCAP servers SHOULD expect and tolerate a small amount of clock
   skew with their clients by ensuring that multicast addresses are
   allocated for an extra period of time [EXTRA-ALLOCATION-TIME] on
   either side of the lease given to the client. However, large amounts
   of clock skew require special handling. The value of [EXTRA-
   ALLOCATION-TIME] MUST be a configurable parameter, since local
   circumstances may vary.  The RECOMMENDED default is one hour.

   However, large amounts of clock skew will cause problems later when
   sessions are advertised. If a MADCAP server detects clock skew
   greater than [CLOCK-SKEW-ALLOWANCE], it MUST generate and process an
   Excessive Clock Skew error, as described in section 2.6. The server
   MAY also log a message. The value of [CLOCK-SKEW-ALLOWANCE] MUST be a
   configurable parameter, since local circumstances may vary.  The

RECOMMENDED default is 30 minutes.

2.13. Optional Features

   Each MADCAP client or server MAY implement one or more optional
   features.  Optional features of MADCAP are identified with a two
   octet feature code.

   A MADCAP client MAY request, require, or indicate support for an
   optional feature by including a Feature List option in a message. For
   more information about optional features, see the description of the
   Feature List option.

   Table 4 lists the feature codes defined at this time and sections
   2.13.1 and 2.13.2 describe how these features work.

   New MADCAP feature codes may only be defined by IETF Consensus, as
   described in section 5.

           Feature Code   Feature Name
           ------------   ------------
                0         Server Mobility
                1         Retry After
                2         Shared Lease Identifier

      Table 4:  MADCAP Feature Codes

2.13.1. Server Mobility

   The Server Mobility feature allows an address allocated on one MADCAP
   server to be renewed or released on a different MADCAP server. This
   requires communication and coordination among MADCAP servers. The
   primary benefits are immunity to the failure of a single MADCAP
   server and perhaps greater performance through load balancing.

   In order to take advantage of the Server Mobility feature, a MADCAP
   client must ensure that the feature is implemented by both the server
   that is used for the original allocation and the server that is used
   for the renewal or release. The best way to ensure this is to include
   the Server Mobility feature in the required list of a Feature List
   option in the REQUEST message used to allocate the address (and the
   DISCOVER message, if one is used). When the time comes to renew or
   release the address, the client SHOULD send a unicast RENEW or
   RELEASE message to the server from which it allocated the address.
   However, if this server is unavailable, the client MAY send the RENEW
   or RELEASE message to any other server that includes the Server
   Mobility feature in its list of supported features. The client can
   find such a server by (for instance) sending an GETINFO message with

   an Option Request List option that includes the Feature List option
   code.

   If the MADCAP client does not want to require this feature when
   allocating addresses, it may include the feature in the requested
   list of a Feature List option and see if the server includes the
   feature in the required list of a Feature List option in the ACK
   message.

   Even if the Server Mobility feature is used, there is no guarantee
   that a server will be available to perform the renewal or release or
   that the renewal or release will succeed. Server connectivity may
   have failed, for instance.

2.13.2. Retry After

   The Retry After feature allows a MADCAP server to ask the MADCAP
   client to retry its request later, as may be required when allocating
   large numbers of addresses or allocating addresses for a long period
   of time.

   For instance, if a MADCAP client requests 1000 addresses,
   administrative approval may be required or allocation of more
   addresses from another MASC domain may be necessary. This may take
   several hours or several days.  If the MADCAP client and server both
   support the Retry After feature, the MADCAP server can send back an
   ACK message with a Retry Time option indicating when the addresses
   may be ready. The client can retry its request after the Retry Time
   to get the addresses.

   If a MADCAP client includes the Retry After feature in the supported
   list of a Feature List option in a REQUEST message, a MADCAP server
   that supports the Retry After feature MAY decide to begin a lengthy
   allocation process. In this case, the MADCAP server will include an
   empty List of Address Ranges option in its ACK message, a Feature
   List option that includes the Retry After feature in the required
   list, and a Retry Time option with a time after which the client
   should retry the REQUEST.

   The client MUST NOT include the Retry After feature in the requested
   or required list of a Feature List option, since the decision about
   whether Retry After is desirable should be left to the MADCAP server.

   At some later time (preferably after the time indicated in the Retry
   Time option), the client SHOULD send a REQUEST message with all the
   same options as the original REQUEST message (especially the Lease
   Identifier option), but with a new xid value.  The server MAY return
   a normal ACK or NAK message at this point or it MAY continue the
   transaction to a later time by including an empty List of Address
   Ranges option in its ACK message, a Feature List option that includes
   the Retry After feature in the required list, and a Retry Time option
   with a later time after which the client should retry the REQUEST.

   At any point after receiving the initial ACK message with the Retry
   Time option, the client MAY terminate the allocation process and any
   accompanying lease by sending to the server performing the allocation
   (or another server if the Server Mobility feature is also in effect)
   a RELEASE message with the Lease Identifier included in the original
   REQUEST message.

   The Retry After feature may also be used when renewing a lease.  In
   this case, the description above applies except that the client sends
   a RENEW message instead of a REQUEST message.

   If a client sends a RENEW message with a Lease Identifier that
   matches a lease which is currently undergoing allocation with the
   Retry After feature in response to a REQUEST message, the server MUST
   generate and process an Invalid Request error in the manner described
   in section 2.6.  Also, if a client sends a RENEW message with a Lease
   Identifier that matches a lease which is currently undergoing
   allocation with the Retry After feature in response to a RENEW
   message, but the options supplied with the two RENEW messages do not
   match, the server MUST generate and process an Invalid Request error
   in the manner described in section 2.6.

   Note that the Retry After feature may complicate the application API.
   For this reason, a MADCAP client may request the Retry After feature
   for some messages and not for others. This should not cause problems
   for a robust MADCAP server. In general, servers should not expect
   consistent behavior from clients except as required by this
   specification. This also applies to clients' expectations.

2.13.3. Shared Lease Identifier

   For conferencing applications, it may be desirable to allow
   conference participants to modify a lease used for the conference.
   The Shared Lease Identifier feature code is used to support this
   requirement.

   If this feature code was requested by the client and implemented by
   the server when the lease was allocated, the server SHOULD disable
   any authentication requirements pertaining to this lease, allowing
   any client that knows the Lease Identifier to modify the lease.

   A MADCAP client wishing to use the Shared Lease Identifier feature
   should include this feature in the requested or required lists of the
   Feature List option of a REQUEST message when first allocating the
   lease. If the feature was required, the server SHOULD try to
   implement it for this request and include the feature in the required
   list of the response. If the server can not implement the feature for
   this request, it MUST generate and process a Required Feature Not
   Supported error in the manner described in section 2.6. If the
   feature was requested, the server SHOULD try to implement the feature
   and include the feature in the required list of the response.
   However, if the server cannot implement the feature, it may simply
   skip it.

   Subsequent requests pertaining to a lease for which the Shared Lease
   Identifier feature was implemented at allocation time MAY include the
   Shared Lease Identifier feature in the requested or required lists of
   the Feature List option. In this case, the server SHOULD try to
   implement the feature by disabling any authentication requirements
   pertaining to this lease, allowing any client that knows the Lease
   Identifier to modify the lease, and including the feature in the
   required list of the response.  If the server cannot implement the
   feature, it SHOULD skip it if the feature was requested. But if the
   feature was required and the server cannot implement it, the server
   MUST generate and process a Required Feature Not Supported error in
   the manner described in section 2.6.

3. MADCAP Options

   As described earlier, each MADCAP message includes an options field
   consisting of a list of tagged parameters that are called "options".
   All options consist of a two octet option code and a two octet option
   length, followed by the number of octets specified by the option
   length.

   This section defines a set of option codes for use in MADCAP
   messages.  New options may be defined using the process defined in
   section 5. The options are listed in numerical order.

   Table 5 summarizes which options are allowed with each message type.

   Option                  GETINFO        ACK (in response to GETINFO)
   ------                  ------         ---------------------------
   Lease Time              MUST NOT       MUST NOT
   Server Identifier       MUST NOT       MUST
   Lease Identifier        MUST           MUST
   Multicast Scope         MUST NOT       MUST NOT
   Option Request List     MUST           MUST NOT
   Start Time              MUST NOT       MUST NOT
   Number of Addresses
     Requested             MUST NOT       MUST NOT
   Requested Language      MAY            MUST NOT
   Multicast Scope List    MUST NOT       MAY
   List of Address Ranges  MUST NOT       MUST NOT
   Current Time            MUST NOT       MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MUST NOT
   Minimum Lease Time      MUST NOT       MUST NOT
   Maximum Start Time      MUST NOT       MUST NOT
   Error                   MUST NOT       MUST NOT

   Option                  DISCOVER       OFFER
   ------                  --------       -----
   Lease Time              MAY            MUST
   Server Identifier       MUST NOT       MUST
   Lease Identifier        MUST           MUST
   Multicast Scope         MUST           MUST
   Option Request List     MUST NOT       MUST NOT
   Start Time              MAY            MAY
   Number of Addresses
     Requested             MAY            MUST NOT
   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MAY            MAY
   Current Time            MAY            MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MUST NOT
   Minimum Lease Time      MAY            MUST NOT
   Maximum Start Time      MAY            MUST NOT
   Error                   MUST NOT       MUST NOT

   Option                  REQUEST        ACK (in response to REQUEST)
   ------                  -------        ----------------------------
   Lease Time              MAY            MUST
   Server Identifier       MUST (if       MUST
                             multicast)
   Lease Identifier        MUST           MUST
   Multicast Scope         MUST           MUST
   Option Request List     MUST NOT       MUST NOT
   Start Time              MAY            MAY
   Number of Addresses
     Requested             MAY            MUST NOT
   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MAY            MUST
   Current Time            MAY            MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MAY
   Minimum Lease Time      MAY            MUST NOT
   Maximum Start Time      MAY            MUST NOT
   Error                   MUST NOT       MUST NOT

   Option                  RENEW          ACK (in response to RENEW)
   ------                  -----          --------------------------
   Lease Time              MAY            MUST
   Server Identifier       MUST NOT       MUST
   Lease Identifier        MUST           MUST
   Multicast Scope         MUST NOT       MUST
   Option Request List     MUST NOT       MUST NOT
   Start Time              MAY            MAY
   Number of Addresses
     Requested             MUST NOT       MUST NOT
   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MUST NOT       MUST
   Current Time            MAY            MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MAY
   Minimum Lease Time      MAY            MUST NOT
   Maximum Start Time      MAY            MUST NOT
   Error                   MUST NOT       MUST NOT

   Option                  RELEASE        ACK (in response to RELEASE)
   ------                  -------        ----------------------------
   Lease Time              MUST NOT       MUST NOT
   Server Identifier       MUST NOT       MUST
   Lease Identifier        MUST           MUST
   Multicast Scope         MUST NOT       MUST NOT
   Option Request List     MUST NOT       MUST NOT
   Start Time              MUST NOT       MUST NOT
   Number of Addresses
     Requested             MUST NOT       MUST NOT
   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MUST NOT       MUST NOT
   Current Time            MUST NOT       MUST NOT
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MUST NOT
   Minimum Lease Time      MUST NOT       MUST NOT
   Maximum Start Time      MUST NOT       MUST NOT
   Error                   MUST NOT       MUST NOT

   Option                  NAK
   ------                  ---
   Lease Time              MUST NOT
   Server Identifier       MUST
   Lease Identifier        MUST
   Multicast Scope         MUST NOT
   Option Request List     MUST NOT
   Start Time              MUST NOT
   Number of Addresses
     Requested             MUST NOT
   Requested Language      MUST NOT
   Multicast Scope List    MUST NOT
   List of Address Ranges  MUST NOT
   Current Time            MUST NOT
   Feature List            MAY
   Retry Time              MUST NOT
   Minimum Lease Time      MUST NOT
   Maximum Start Time      MUST NOT
   Error                   MUST

             Table 5:  Options allowed in MADCAP messages

3.1. End

   The End option marks the end of valid information in the options
   field. This option MUST be included at the end of the options field
   in each MADCAP message.

   The code for this option is 0, and its length is 0.

        Code        Len
   +-----+-----+-----+-----+
   |     0     |     0     |
   +-----+-----+-----+-----+

3.2. Lease Time

   This option is used in a client request (DISCOVER, REQUEST, or RENEW)
   to allow the client to request a lease time for the multicast
   address. In a server reply (OFFER or ACK), a MADCAP server uses this
   option to specify the lease time it is willing to offer.

   The time is in units of seconds, and is specified as a 32-bit
   unsigned integer.

   The code for this option is 1, and its length is 4.

        Code        Len            Lease Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |     1     |     4     |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.3. Server Identifier

   This option contains the IP address of a MADCAP server. A two octet
   address family number (as defined by IANA, including those defined in
   [10]) is stored first, followed by the address.  The address family
   for this address is not determined by the addrfamily field in the
   fixed header so that addresses from one family may be allocated while
   communicating with a server via addresses of another family.

   All messages sent by a MADCAP server MUST include a Server Identifier
   option with the IP address of the server sending the message.

   MADCAP clients MUST include a Server Identifier option in multicast
   REQUEST messages in order to indicate which OFFER message has been
   accepted.

   The code for this option is 2, and its minimum length is 3.

           Code        Len    Address Family     Address
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |     2     |     n     |   family  |  a1 |  ...            |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

3.4. Lease Identifier

   This option is used by MADCAP clients to specify a unique lease
   identifier. For more information about this option and how it is
   used, see section 2.4.

   The code for this option is 3, and its minimum length is 1.

           Code        Len     Lease Identifier
   +-----+-----+-----+-----+-----+-----+---
   |     3     |     n     |  i1 |  i2 | ...
   +-----+-----+-----+-----+-----+-----+---

3.5. Multicast Scope

   The multicast scope option is used by the client to indicate the
   requested multicast scope in a DISCOVER or REQUEST message. It is
   also used by the MADCAP server to indicate the scope of an assigned
   address.

   The client may obtain the scope list through the Multicast Scope List
   option or using some other means. The Scope ID is the first multicast
   address in the scope. The address family of the Scope ID is
   determined by the addrfamily field in the fixed header.

   The code for this option is 4, and its minimum length is 1.

        Code        Len        Scope ID
   +-----+-----+-----+-----+-----+-----
   |     4     |     n     |  i1 |  ...
   +-----+-----+-----+-----+-----+-----

3.6. Option Request List

   This option is used by a MADCAP client in an GETINFO message to
   request that certain options be included in the server's ACK
   response. The server SHOULD try to include the specified options in
   its response, but is not required to do so.

   The format of this option is a list of option codes.

   The code for this option is 5 and the minimum length is 2.

        Code        Len      Requested Options
   +-----+-----+-----+-----+-----+-----+---...
   |     5     |     n     |  Option1  |
   +-----+-----+-----+-----+-----+-----+---...

3.7. Start Time

   The Start Time option specifies the starting time for a multicast
   address lease.

   A client may include this option in a DISCOVER, RENEW, or REQUEST
   message to request a multicast address for use at a future time. A
   server may include this option in an OFFER message or in an ACK in
   response to REQUEST or RENEW message to indicate that a lease has
   been granted which starts at a future time.

   If the Start Time option is present, the IP Address Lease Time option
   specifies the duration of the lease beginning at the Start Time
   option value.

   If the Start Time option is present, the Current Time option MUST
   also be present, as described in section 2.12.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP timestamp by adding decimal 2208988800.
   This time format will not wrap until the year 2106.

   The code for this option is 6 and the length is 4.

           Code        Len      Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |     6     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.8. Number of Addresses Requested

   This option specifies the minimum and desired number of addresses
   requested by the client. It is only used in DISCOVER and REQUEST
   messages and is only sent by the client.

   The minimum and desired number of addresses requested are unsigned 16
   bit integers in network byte order. The minimum MUST be less than or
   equal to the desired number. If a message is received where this is
   not the case, the MADCAP server MUST generate and process an Invalid
   Request error in the manner described in section 2.6.

   The client MAY obtain more than one address either by repeating the
   protocol for every address or by requesting several addresses at the
   same time via this option. When the client is requesting only one
   address, this option SHOULD NOT be included. A MADCAP server

   receiving a DISCOVER or REQUEST packet including this option MUST
   include between the minimum and desired number of addresses in any
   OFFER or ACK response.

   The code for this option is 7 and the length is 4.

           Code        Len      Minimum     Desired
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |     7     |     4     | min       | desired   |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.9. Requested Language

   This option specifies the language in which the MADCAP client would
   like strings such as zone names to be returned. It is only included
   in an GETINFO message sent by the client. It is an RFC 1766 [6]
   language tag. The proper way to handle this tag with respect to zone
   names is discussed further in the definition of the Multicast Scope
   List option.

   The code for this option is 8 and the minimum length is 0.

           Code        Len      Language Tag
   +-----+-----+-----+-----+-----+-...-+-----+
   |     8     |     n     | L1  |     | Ln  |
   +-----+-----+-----+-----+-----+-...-+-----+

3.10. Multicast Scope List

   This option is sent by the server in an ACK message in response to an
   GETINFO message sent by the client.

   If the client did not include a Requested Language option in its
   GETINFO message, the MADCAP server SHOULD return all zone names for
   each zone. If the client included a Requested Language option in its
   GETINFO message, the MADCAP server MUST return no more than one zone
   name for each zone. For each zone, the MADCAP server SHOULD first
   look for a zone name that matches the requested language tag (using a
   case-insensitive ASCII comparison). If any names match, one of them
   should be returned. Otherwise, the MADCAP server SHOULD choose
   another zone name to return (if any are defined). It SHOULD give
   preference to zone names that are marked to be used if no name is
   available in a desired language.

   The code for this option is 9 and the minimum length is 0.

   The format of the multicast scope list option is:

        Code        Len     Count  Scope List
   +-----+-----+-----+-----+-----+-----+-...-+-----+
   |     9     |     p     | m   | L1  |     | Lm  |
   +-----+-----+-----+-----+-----+-----+-...-+-----+

   The scope list is a list of m tuples, where each tuple is of the
   form:

       Scope ID      Last Address   TTL   Name  Encoded Name List
                                          Count
   +---+--...--+---+---+--...--+---+-----+-----+-----+-...-+-----+
   |  ... ID ...   | ... Last ...  | T   | n   | EN1 |     | ENn |
   +---+--...--+---+---+--...--+---+-----+-----+-----+-...-+-----+

   where Scope ID is the first multicast address in the scope, Last
   Address is the last multicast address in the scope, TTL is the
   multicast TTL value for the multicast addresses of the scope, and
   Name Count is the number of encoded names for this zone (which may be
   zero). The address family of the Scope ID and Last Address is
   determined by the addrfamily field in the fixed header.  Note that a
   particular MADCAP server may be allocating addresses out of some
   subset of the scope.  For instance, the addresses in the scope may be
   divided among several servers in some way.

   Each encoded name is of the form

    Name  Lang   Language Tag      Name   Name
    Flags Length                   Length
   +-----+-----+-----+-...-+-----+-----+-----+-...-+-----+
   | F   | q   | L1  |     | Lq  | r   | N1  |     | Nr  |
   +-----+-----+-----+-...-+-----+-----+-----+-...-+-----+

   where Name Flags is a flags field with flags defined below, Lang
   Length is the length of the Language Tag in octets (which MUST NOT be
   zero), Language Tag is a language tag indicating the language of the
   zone name (as described in [6]), Name Length is the length of the
   Name in octets (which MUST NOT be zero), and Name is a UTF-8 [5]
   string indicating the name given to the scope zone.

   The high bit of the Name Flags field is set if the following name
   should be used if no name is available in a desired language.
   Otherwise, this bit is cleared. All remaining bits in the octet
   SHOULD be set to zero and MUST be ignored.

   The Scope IDs of entries in the list MUST be unique and the scopes
   SHOULD be listed from smallest (topologically speaking) to largest.
   This makes it easier to display a consistent user interface, with
   scopes usually keeping the same order. However, scopes may not be
   strictly nested. In this circumstance, there is no strict ordering
   from smallest to largest and the server must use another technique
   for ordering the scope list.

   Example:

   There are two scopes supported by the multicast address allocation
   server: Inside abcd.com with addresses 239.192.0.0-239.195.255.255,
   and world with addresses 224.0.1.0-238.255.255.255. Then this option
   will be given as:

            Code        Len     Count
       +-----+-----+-----+-----+-----+...
       |     9     |     51    | 2   |
       +-----+-----+-----+-----+-----+...

           Scope ID     Last Address    TTL Name  Name  Lang   Language
                                            Count Flags Length Tag
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
       |239|192| 0 | 0 |239|195|255|255|10 | 1   | 128 |  2   | en  |
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...

        Name
        Length Name
       +------+--+--+-...-+--+--+...
       |  15  | Inside abcd.com |
       +------+--+--+-...-+--+--+...

           Scope ID     Last Address    TTL Name  Name  Lang   Language
                                            Count Flags Length Tag
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
       |224| 0 | 1 | 0 |238|255|255|255|16 | 1   | 128 |  2   | en  |
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...

        Name
        Length Name
       +------+--...--+
       |  5   | world |
       +------+--...--+

3.11. List of Address Ranges

   This option is used by the server to provide the list of all the
   address ranges allocated to the client.

   This option is also used by the client when requesting a lease for a
   specific set of addresses. This feature should be needed only rarely,
   such as when a lease is accidentally allowed to expire and it needs
   to be reallocated.

   The address family of the addresses is determined by the addrfamily
   field.

   The code for this option is 10 and the minimum length is 0.

        Code        Len       Address Range List
   +-----+-----+-----+-----+-----+-----+-...-+-----+
   |    10     |     n     | L1  | L2  |     | Ln  |
   +-----+-----+-----+-----+-----+-----+-...-+-----+

   where the Address Range List is of the following format.

           StartAddress1  BlockSize1 StartAddress2 BlockSize2 ...
           +---+---+---+---+---+---+---+---+---+---+---+---+--...--+
           |  ... S1 ...   |B11|B12|  ... S2 ...   |B21|B22|       |
           +---+---+---+---+---+---+---+---+---+---+---+---+--...--+

3.12. Current Time

   This option is used to express what the sender thinks the current
   time is. This is useful for detecting clock skew. This option MUST be
   included if the Start Time or Maximum Start Time options are used, as
   described in section 2.12.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP [4] timestamp by adding decimal
   2208988800. This time format will not wrap until the year 2106.

   The code for this option is 11 and the length is 4.

        Code        Len        Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    11     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.13. Feature List

   This option lists optional MADCAP features supported, requested, or
   required, by the sender. This option MAY be included in any message
   sent by a MADCAP server or client.

   Optional features of MADCAP are identified with a two octet feature
   code.  New MADCAP feature codes may only be defined by IETF
   Consensus, as described in section 5.

   The Feature List option consists of three separate lists: supported
   features, requested features, and required features. Each list
   consists of an unordered list of feature codes. The supported list is
   used by MADCAP clients or servers to indicate the features that the
   sender supports.  The requested and required lists are used by MADCAP
   clients to indicate which features are requested of or required from
   a MADCAP server.  The required list is used by MADCAP servers to
   indicate which features were implemented by the MADCAP server in
   processing this message. Messages sent by MADCAP servers MUST NOT
   include any feature codes in the requested list.

   If a MADCAP client includes the Feature List option in a message, it
   MAY include features in any of the lists: supported, requested, and
   required.  If a MADCAP server receives a message containing the
   Feature List option and it does not support all of the features in
   the required list, it MUST generate and process a Required Feature
   Not Supported error in the manner described in section 2.6. If the
   server supports all of the features in the required list, it MUST
   implement them as appropriate for this message.  It SHOULD try to
   implement the features in the requested list and it MAY implement any
   of the features in the supported list. If an optional feature (such
   as Retry After) is not included in any part of the Feature List
   option included in the client's message (or if the client does not
   include a Feature List option in its message), the server MUST NOT
   use that feature in its response.

   If a MADCAP server does respond to a client's message that includes a
   Feature List option, the server MUST include a Feature List option
   with a supported features list that lists the features that it
   supports, a required features list that lists the features that it
   implemented in responding to this message (which must be included in
   the supported features list of the client's Feature List option), and
   an empty requested features list.

   The code for this option is 12 and the minimum length is 6.

           Code        Len      Supported   Requested   Required
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |    12     |     n     |    FL1    |    FL2    |    FL3    |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

   where each of the Feature Lists is of the following format:

          Feature     Feature           Feature
           Count      Code 1            Code m
       +-----+-----+-----+-----+-...-+-----+-----+
       |     m     | FC1       |     |    FCm    |
       +-----+-----+-----+-----+-...-+-----+-----+

3.14. Retry Time

   The Retry Time option specifies the time at which a client should
   retry a REQUEST or RENEW message when using the Retry After feature.

   This option should only be sent by a MADCAP server in an ACK when
   responding to a REQUEST or RENEW message that includes the Retry
   After feature in the supported, requested, or required list. For more
   discussion of Retry After, see section 2.13.2.

   If the Retry Time option is present, the Current Time option MUST
   also be present.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP timestamp by adding decimal 2208988800.
   This time format will not wrap until the year 2106.

   The code for this option is 13 and the length is 4.

        Code        Len      Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    13     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.15. Minimum Lease Time

   This option is used in a client request (DISCOVER, REQUEST, or RENEW)
   to allow the client to specify a minimum lease time for the multicast
   address. If a server cannot meet this minimum lease time, it MUST
   generate and process a Valid Request Could Not Be Completed error in
   the manner described in section 2.6.

   The time is in units of seconds, and is specified as a 32-bit
   unsigned integer.

   The code for this option is 14, and its length is 4.

        Code        Len            Lease Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    14     |     4     |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.16. Maximum Start Time

   The Maximum Start Time option specifies the latest starting time that
   the client is willing to accept for a multicast address lease.

   A client may include this option in a DISCOVER, RENEW, or REQUEST
   message to specify that it does not want to receive a lease with a
   starting time later than the specified value. If a server cannot meet
   this maximum start time, it MUST generate and process a Valid Request
   Could Not Be Completed error in the manner described in section 2.6.

   If the Maximum Start Time option is present, the Current Time option
   MUST also be present, as described in section 2.12.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP timestamp by adding decimal 2208988800.
   This time format will not wrap until the year 2106.

   The code for this option is 15 and the length is 4.

        Code        Len      Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    15     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.17. Error

   A MADCAP server includes this option in a NAK message to indicate why
   the request failed. A MADCAP server MUST include an Error option in
   each NAK message.

   The first two octets of an Error option contain a MADCAP error code.
   Several MADCAP error codes are defined later in this section.  New
   MADCAP error codes may only be defined by IETF Consensus, as
   described in section 5.

   Any remaining octets in the Error option contain extra data about the
   error. The format of this data depends on the error code. The
   definition of a MADCAP error code must include a definition of the
   extra data to be included with that error code.

   A client that receives a NAK message containing an Error option MAY
   log or display a message indicating the error code and extra data
   received.  The client MUST NOT retransmit a message once a NAK
   response to that message has been received. The client MAY adjust the
   message to correct the error and send the corrected message or send a
   message to a different server.

   The code for this option is 16, and the minimum length is 2.

        Code        Len      Error Code  Extra Data
   +-----+-----+-----+-----+-----+-----+-----+-----+ ...
   |    16     |     n     |   ecode   |  d1    d2
   +-----+-----+-----+-----+-----+-----+-----+-----+ ...

3.17.1. Valid Request Could Not Be Completed

   MADCAP error code 0 indicates that the request was valid, but could
   not be completed with the available addresses and the current
   configuration.  The extra data is a two octet option code indicating
   which option caused the problem. A value of 0xFFFF indicates that the
   problem was not with a specific option.

3.17.2. Invalid Request

   MADCAP error code 1 indicates that the request was malformed or
   invalid in some other manner. The extra data is a two octet option
   code indicating which option caused the problem. A value of 0xFFFF
   indicates that the problem was not with a specific option.

3.17.3. Excessive Clock Skew

   MADCAP error code 2 indicates excessive clock skew (see section
   2.12).  The extra data consists of a four octet time value
   representing the server's idea of the current time, an unsigned 32
   bit integer in network byte order giving the number of seconds since
   00:00 UTC, 1st January 1970. This can be converted to an NTP
   timestamp by adding decimal 2208988800. This time format will not
   wrap until the year 2106.

3.17.4. Lease Identifier Not Recognized

   MADCAP error code 3 indicates that the Lease Identifier was not
   recognized (usually in response to a RENEW or RELEASE message). There
   is no extra data.

3.17.5. Required Feature Not Supported

   MADCAP error code 4 indicates that at least one feature included in
   the required list of the Feature List option is not supported. The
   extra data contains a list of the feature codes in the required list
   that are not supported.

3.17.6. Experimental Use

   MADCAP error codes 1024-2047 are reserved for experimental use. The
   format of the extra data included with these error codes is not
   defined.

4. Security Considerations

   MADCAP has relatively basic security requirements. At present there
   is no way of enforcing authorized use of multicast addresses in the
   multicast routing/management protocols.  Therefore, it is not
   possible to identify unauthorized use of multicast address by an
   adversary. Moreover, a multicast address allocated to a user/system
   can be used by other systems without violating terms of the multicast
   address allocation. For example, a system may reserve an address to
   be used for a work group session where each and every member of the
   work group is allowed to transmit packets using the allocated group
   address. In other words, the multicast address allocation protocol
   does not dictate how the address should be used, it only dictates the
   time period for which it can be used and who gets to release it or
   renew it. When an address is allocated to a system/user, it basically
   means that no other user/system (most likely) will be allocated that
   address for the time period, without any restrictions on its use.

   To protect against rogue MADCAP servers (mis-configured servers and
   intentional), clients in certain situations would like to
   authenticate the server. Similarly, for auditing or book-keeping
   purposes, the server may want to authenticate clients. Moreover, in
   some cases, the server may have certain policies in place to restrict
   the number of addresses that are allocated to a system or a user.
   This feature is of much value when a well behaved but naive user or
   client requests a large number of addresses, and therefore,
   inadvertently impacts other users or systems. Therefore, an
   administrator may want to exert a limited amount of control based on
   the client identification.  The client identification could be based

   on the system or user identity. In most practical situations, system
   identification will suffice, however, particularly in case of multi-
   user systems, at times, user identification will play an important
   role. Therefore, authentication capabilities based on user
   identification may be desirable. As usual, data integrity is a strong
   requirement and if not protected, can lead to many problems including
   denial of service attacks.

   In the case of MADCAP, confidentiality is not a strong requirement.
   In most of the cases, at least when a multicast address is in use, an
   adversary will be able to determine information that was contained in
   the MADCAP messages. In some cases, the users/systems may want to
   protect information in the MADCAP messages so that an adversary is
   not able to determine relevant information in advance and thus, plan
   an attack in advance. For example, if an adversary knows in advance
   (based on MADCAP messages) that a particular user has requested a
   large number of address for certain time period and scope, he may be
   able to guess the purpose behind such request and target an attack.
   When the Shared Lease Identifier feature is used, preserving the
   confidentiality of MADCAP messages becomes more important. Also,
   there may be features added to the protocol in the future that may
   have stronger confidentiality requirements.

   The IPSEC protocol [8] meets client/server identification and
   integrity protection requirements stated above, requires no
   modification to the MADCAP protocol, and leverages extensive work in
   IETF and industry. Therefore, when security is a strong requirement,
   IPSEC SHOULD be used for protecting all the unicast messages of
   MADCAP protocol. When IPSEC based security is in use, all the
   multicast packets except GETINFO MUST be dropped by the MADCAP
   server.  The prevalent implementations of IPSEC support client
   identification in form of system identification and do not support
   user identification. However, when desired, IPSEC with appropriate
   API's may be required to support user identification.

5. IANA Considerations

   This document defines several number spaces (MADCAP options, MADCAP
   message types, MADCAP Lease Identifier types, MADCAP features, and
   MADCAP error codes).  For all of these number spaces, certain values
   are defined in this specification. New values may only be defined by
   IETF Consensus, as described in [7]. Basically, this means that they
   are defined by RFCs approved by the IESG.

6. Acknowledgments

   The authors would like to thank Rajeev Byrisetty, Steve Deering,
   Peter Ford, Mark Handley, Van Jacobson, David Oran, Thomas Pfenning,
   Dave Thaler, Ramesh Vyaghrapuri and the participants of the IETF for
   their assistance with this protocol.

   Much of this document is based on [1] and [2]. The authors of this
   document would like to express their gratitude to the authors of
   these previous works. Any errors in this document are solely the
   fault of the authors of this document.

7. References

   [1]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [2]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, March 1997.

   [3]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
        2365, July 1998.

   [4]  Mills, D., "Network Time Protocol (Version 3) Specification,
        Implementation and Analysis", RFC 1305, March 1992.

   [5]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [6]  Alvestrand, H., "Tags for the Identification of Languages", RFC
        1766, March 1995.

   [7]  Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [8]  Atkinson, R. and S. Kent, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [9]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [10] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
        October 1994.

   [11] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
        1112, August 1989.

   [12] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 2460, December 1998.

   [13] Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.

   [14] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.

8. Authors' Addresses

   Stephen R. Hanna
   Sun Microsystems, Inc.
   One Network Drive
   Burlington, MA 01803

   Phone: +1.781.442.0166
   EMail: steve.hanna@sun.com

   Baiju V. Patel
   Intel Corp.
   Mail Stop: AG2-201
   5200 NE Elam Young Parkway
   Hillsboro, OR 97124

   Phone: 503 696 8192
   EMail: baiju.v.patel@intel.com

   Munil Shah
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: 425 703 3924
   EMail: munils@microsoft.com

APPENDIX A: Examples

   This appendix includes several examples of typical MADCAP protocol
   exchanges.

1. Multicast Scope List Discovery

   In this example, a MADCAP client wants to determine the scope list in
   effect. The client is using IPv4, so it starts by multicasting an
   GETINFO packet to the MADCAP Server Multicast Address corresponding
   to IPv4 Local Scope. This packet includes the Lease Identifier
   option, an Option Request List including the Multicast Scope List
   option code, and a Requested Language option containing the string
   "en", since the client is configured to prefer the English language.

   Two MADCAP servers respond by sending ACK messages. These ACK
   messages include the Lease Identifier option and xid supplied by the
   client, the server's Server Identifier, and the Multicast Scope List
   with one name per scope (the one that most closely matches the
   language tag "en").

   The following figure illustrates this exchange.

                    Server          Client          Server
                      v               v               v
                      |               |               |
                      |               |               |
                      | _____________/|\_____________ |
                      |/   GETINFO    |    GETINFO   \|
                      |               |               |
                      |               |               |
                      |\              |  ____________/|
                      | \_________    | /   ACK       |
                      |      ACK  \   |/              |
                      |            \  |               |
                      |               |               |
                      v               v               v

         Figure 2: Timeline diagram of messages exchanged
                   in Multicast Scope List Discovery example

2. Multicast Discovery and Address Allocation

   In this example, the MADCAP client wants to allocate a multicast
   address from the global scope for use during the next two hours.

   The client begins by multicasting a DISCOVER packet to the MADCAP
   Server Multicast Address associated with IPv4 Local Scope.  This
   packet includes the Lease Time, Lease Identifier, and Multicast Scope
   options.

   Any servers that receive the DISCOVER packet and can satisfy this
   request temporarily reserve an address for the client and unicast an
   OFFER packet to the client. These packets contain the Lease Time,
   Server Identifier, Lease Identifier, and Multicast Scope options.

   After an appropriate delay, the client multicasts a REQUEST packet to
   the MADCAP Server Multicast Address. This packet contains all of the
   options included in the DISCOVER packet, but also includes the Server
   Identifier option, indicating which server it has selected for the
   request.

   The server whose Server Identifier matches the one specified by the
   client responds with an ACK packet containing the options included in
   the OFFER packet, as well as a List of Address Ranges option listing
   the address allocated. All the other servers that had sent OFFER
   packets stop reserving an address for the client and forget about the
   whole exchange.

   The client now has a two hour "lease" on the multicast address.

   If the client had not received an ACK from the server, it would have
   retransmitted its REQUEST packet for a while. If it still received no
   response, it would start over with a new DISCOVER message.

   The following figure illustrates this exchange.

                    Server          Client          Server
                (not selected)                    (selected)
                      v               v               v
                      |               |               |
                      |Begin multicast address request|
                      |               |               |
                      | _____________/|\_____________ |
                      |/   DISCOVER   |   DISCOVER   \|
                      |               |               |
                  Reserves            |           Reserves
                  Address             |           Address
                      |               |               |
                      |\              |  ____________/|
                      | \_________    | /    OFFER    |
                      |     OFFER \   |/              |
                      |            \  |               |
                      |       Collects replies        |
                      |              \|               |
                      |     Selects Server            |
                      |               |               |
                      | _____________/|\_____________ |
                      |/   REQUEST    |    REQUEST   \|
                      |               |               |
                      |               |     Commits address
                      |               |               |
                      |               | _____________/|
                      |               |/    ACK       |
                      |               |               |
                      |     assignment complete       |
                      |               |               |
                      v               v               v

         Figure 3: Timeline diagram of messages exchanged
                   in Multicast Address Allocation example

3. Lease Extension

   This is a continuation of the previous example. The client has
   already allocated a multicast address from the global scope for use
   during the next two hours. Half way through this two hour period, it
   decides that it wants to extend its lease for another hour.

   The client unicasts a RENEW packet to the server from which it
   allocated the address. This packet includes the Lease Time and Lease
   Identifier options. The Lease Identifier matches the one used for the
   original allocation. The time included in the Lease Time is two

   hours, since the client wants the lease to expire two hours from the
   current time.

   The server responds with an ACK packet indicating that the lease
   extension has been granted. This packet includes the Lease Time,
   Server Identifier, Lease Identifier, Multicast Scope, and List of
   Address Ranges options.

   If the server did not want to grant the requested lease extension, it
   would have responded with a NAK packet with the Lease Identifier
   option.

   The following figure illustrates this exchange.

                    Client          Server
                      v               v
                      |               |
                      |\_____________ |
                      |    RENEW     \|
                      |               |
                      |        Extends lease
                      |               |
                      | _____________/|
                      |/    ACK       |
                      |               |
                      |               |
                      v               v

         Figure 4: Timeline diagram of messages exchanged
                   in Lease Extension example

4. Address Release

   This is a continuation of the previous example. The client has
   already allocated a multicast address and extended its lease for
   another two hours. Half an hour later, the client finishes its use of
   the multicast address and wants to release it so it can be reused.

   The client unicasts a RELEASE packet to the server from which it
   allocated the address. This packet includes the Lease Identifier
   option. The Lease Identifier matches the one used for the original
   allocation. When the server receives this packet, it cancels the
   client's lease on the address and sends an ACK packet to the client
   indicating that the lease has been released. This packet includes the
   Server Identifier and Lease Identifier options.

   The following figure illustrates this exchange.

                    Client          Server
                      v               v
                      |               |
                      |\_____________ |
                      |    RELEASE   \|
                      |               |
                      |        Cancels lease
                      |               |
                      | _____________/|
                      |/    ACK       |
                      |               |
                      v               v

         Figure 5: Timeline diagram of messages exchanged
                   in Address Release example

5. Unicast Address Allocation

   This is a continuation of the previous example. At some later time,
   the client decides to allocate another multicast address. Since it
   has recently worked with a server, it decides to try sending a
   unicast REQUEST to that server. If this doesn't work, it can always
   try a multicast DISCOVER, as illustrated in example 2.

   The client unicasts a REQUEST packet to the server from which it
   wants to allocate the address. This packet includes the Lease Time,
   Lease Identifier, and Multicast Scope options.

   The server responds with an ACK packet containing the Lease Time,
   Lease Identifier, and Multicast Scope options from the REQUEST
   packet, as well as the Server Identifier option and a List of Address
   Ranges option listing the address allocated.

   The client now has a lease on the multicast address.

   If the client had not received an ACK from the server, it would have
   retransmitted its REQUEST packet for a while. If it still received no
   response, it would start over with a multicast DISCOVER message.

   The following figure illustrates this exchange.

                    Client          Server
                      v               v
                      |               |
                      |\_____________ |
                      |    REQUEST   \|
                      |               |
                      |        Allocates address
                      |               |
                      | _____________/|
                      |/    ACK       |
                      |               |
                      v               v

         Figure 6: Timeline diagram of messages exchanged
                   in Unicast Address Allocation example

APPENDIX B: Recommended Constant Values

   Table 6 lists recommended values for constants defined in this
   specification.

       Constant Name             Recommended Value
       -------------             -----------------
       [CLOCK-SKEW-ALLOWANCE]    30 minutes
       [DISCOVER-DELAY]          current retransmit delay
       [EXTRA-ALLOCATION-TIME]   1 hour
       [NO-RESPONSE-DELAY]       60 seconds
       [OFFER-HOLD]              at least 60 seconds
       [RESPONSE-CACHE-INTERVAL] at least 60 seconds (5 minutes maximum)
       [XID-REUSE-INTERVAL]      10 minutes (required)

          Table 6:  Recommended Constant Values

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