RFC 5898

Internet Engineering Task Force (IETF)                      F. Andreasen
Request for Comments: 5898                                 Cisco Systems
Category: Standards Track                                   G. Camarillo
ISSN: 2070-1721                                                 Ericsson
                                                                 D. Oran
                                                                 D. Wing
                                                           Cisco Systems
                                                               July 2010

   Connectivity Preconditions for Session Description Protocol (SDP)
                             Media Streams


   This document defines a new connectivity precondition for the Session
   Description Protocol (SDP) precondition framework.  A connectivity
   precondition can be used to delay session establishment or
   modification until media stream connectivity has been successfully
   verified.  The method of verification may vary depending on the type
   of transport used for the media.  For unreliable datagram transports
   such as UDP, verification involves probing the stream with data or
   control packets.  For reliable connection-oriented transports such as
   TCP, verification can be achieved simply by successful connection
   establishment or by probing the connection with data or control
   packets, depending on the situation.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Andreasen, et al.            Standards Track                    [Page 1]

RFC 5898                Connectivity Precondition              July 2010

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Andreasen, et al.            Standards Track                    [Page 2]

RFC 5898                Connectivity Precondition              July 2010

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Connectivity Precondition Definition . . . . . . . . . . . . .  4
     3.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Operational Semantics  . . . . . . . . . . . . . . . . . .  4
     3.3.  Status Type  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.4.  Direction Tag  . . . . . . . . . . . . . . . . . . . . . .  5
     3.5.  Precondition Strength  . . . . . . . . . . . . . . . . . .  5
   4.  Verifying Connectivity . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Correlation of Dialog to Media Stream  . . . . . . . . . .  7
     4.2.  Explicit Connectivity Verification Mechanisms  . . . . . .  7
     4.3.  Verifying Connectivity for Connection-Oriented
           Transports . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Connectivity and Other Precondition Types  . . . . . . . . . .  9
   6.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   The concept of a Session Description Protocol (SDP) [RFC4566]
   precondition in the Session Initiation Protocol (SIP) [RFC3261] is
   defined in RFC 3312 [RFC3312] (updated by RFC 4032 [RFC4032]).  A
   precondition is a condition that has to be satisfied for a given
   media stream in order for session establishment or modification to
   proceed.  When the precondition is not met, session progress is
   delayed until the precondition is satisfied or the session
   establishment fails.  For example, RFC 3312 [RFC3312] defines the
   Quality of Service precondition, which is used to ensure availability
   of network resources prior to establishing a session (i.e., prior to
   starting to alert the callee).

   SIP sessions are typically established in order to set up one or more
   media streams.  Even though a media stream may be negotiated
   successfully through an SDP offer-answer exchange, the actual media
   stream itself may fail.  For example, when there is one or more
   Network Address Translators (NATs) or firewalls in the media path,
   the media stream may not be received by the far end.  In cases where
   the media is carried over a connection-oriented transport such as TCP
   [RFC0793], the connection-establishment procedures may fail.  The
   connectivity precondition defined in this document ensures that
   session progress is delayed until media stream connectivity has been

Andreasen, et al.            Standards Track                    [Page 3]

RFC 5898                Connectivity Precondition              July 2010

   The connectivity precondition type defined in this document follows
   the guidelines provided in RFC 4032 [RFC4032] to extend the SIP
   preconditions framework.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Connectivity Precondition Definition

3.1.  Syntax

   The connectivity precondition type is defined by the string "conn",
   and hence we modify the grammar found in RFC 3312 [RFC3312] and RFC
   5027 [RFC5027] as follows:

      precondition-type = "conn" / "sec" / "qos" / token

   This precondition tag is registered with the IANA in Section 8.

3.2.  Operational Semantics

   According to RFC 4032 [RFC4032], documents defining new precondition
   types need to describe the behavior of UAs (User Agents) from the
   moment session establishment is suspended due to a set of
   preconditions, until it is resumed when these preconditions are met.
   An entity that wishes to delay session establishment or modification
   until media stream connectivity has been established uses this
   precondition-type in an offer.  When a mandatory connectivity
   precondition is received in an offer, session establishment or
   modification is delayed until the connectivity precondition has been
   met (i.e., until media stream connectivity has been established in
   the desired direction or directions).  The delay of session
   establishment defined here implies that alerting of the called party
   does not occur until the precondition has been satisfied.

   Packets may be both sent and received on the media streams in
   question.  However, such packets SHOULD be limited to packets that
   are necessary to verify connectivity between the two endpoints
   involved on the media stream.  That is, the underlying media stream
   SHOULD NOT be cut through.  For example, Interactive Connectivity
   Establishment (ICE) connectivity checks [RFC5245] and TCP SYN, SYN-
   ACK, and ACK packets can be exchanged on media streams that support
   them as a way of verifying connectivity.

Andreasen, et al.            Standards Track                    [Page 4]

RFC 5898                Connectivity Precondition              July 2010

   Some media streams are described by a single 'm' line but,
   nevertheless, involve multiple addresses.  For example, RFC 5109
   [RFC5109] specifies how to send FEC (Forward Error Correction)
   information as a separate stream (the address for the FEC stream is
   provided in an 'a=fmtp' line).  When a media stream consists of
   multiple destination addresses, connectivity to all of them MUST be
   verified in order for the precondition to be met.  In the case of RTP
   media streams [RFC3550] that use RTCP, connectivity MUST be verified
   for both RTP and RTCP; the RTCP transmission interval rules MUST
   still be adhered to.

3.3.  Status Type

   RFC 3312 [RFC3312] defines support for two kinds of status types --
   namely, segmented and end-to-end.  The connectivity precondition-type
   defined here MUST be used with the end-to-end status type; use of the
   segmented status type is undefined.

3.4.  Direction Tag

   The direction attributes defined in RFC 3312 [RFC3312] are
   interpreted as follows:

   o  send: the party that generated the session description is sending
      packets on the media stream to the other party, and the other
      party has received at least one of those packets.  That is, there
      is connectivity in the forward (sending) direction.

   o  recv: the other party is sending packets on the media stream to
      the party that generated the session description, and this party
      has received at least one of those packets.  That is, there is
      connectivity in the backwards (receiving) direction.

   o  sendrecv: both the send and recv conditions hold.

   Note that a "send" connectivity precondition from the offerer's point
   of view corresponds to a "recv" connectivity precondition from the
   answerer's point of view, and vice versa.  If media stream
   connectivity in both directions is required before session
   establishment or modification continues, the desired status needs to
   be set to "sendrecv".

3.5.  Precondition Strength

   Connectivity preconditions may have a strength-tag of either
   "mandatory" or "optional".

Andreasen, et al.            Standards Track                    [Page 5]

RFC 5898                Connectivity Precondition              July 2010

   When a mandatory connectivity precondition is offered and the
   answerer cannot satisfy the connectivity precondition (e.g., because
   the offer does not include parameters that enable connectivity to be
   verified without media cut through) the offer MUST be rejected as
   described in RFC 3312 [RFC3312].

   When an optional connectivity precondition is offered, the answerer
   MUST generate its answer SDP as soon as possible.  Since session
   progress is not delayed in this case, it is not known whether the
   associated media streams will have connectivity.  If the answerer
   wants to delay session progress until connectivity has been verified,
   the answerer MUST increase the strength of the connectivity
   precondition by using a strength-tag of "mandatory" in the answer.

   Note that use of a mandatory precondition requires the presence of a
   SIP "Require" header with the option tag "precondition".  Any SIP UA
   that does not support a mandatory precondition will reject such
   requests.  To get around this issue, an optional connectivity
   precondition and the SIP "Supported" header with the option tag
   "precondition" can be used instead.

   Offers with connectivity preconditions in re-INVITEs or UPDATEs
   follow the rules given in Section 6 of RFC 3312 [RFC3312].  That is:

      Both user agents SHOULD continue using the old session parameters
      until all the mandatory preconditions are met.  At that moment,
      the user agents can begin using the new session parameters.

4.  Verifying Connectivity

   Media stream connectivity is ascertained by use of a connectivity
   verification mechanism between the media endpoints.  A connectivity
   verification mechanism may be an explicit mechanism, such as ICE
   [RFC5245] or ICE TCP [ICE-TCP], or it may be an implicit mechanism,
   such as TCP.  Explicit mechanisms provide specifications for when
   connectivity between two endpoints using an offer/answer exchange is
   ascertained, whereas implicit mechanisms do not.  The verification
   mechanism is negotiated as part of the normal offer/answer exchange;
   however, it is not identified explicitly.  More than one mechanism
   may be negotiated, but the offerer and answerer need not use the
   same.  The following rules guide which connectivity verification
   mechanism to use:

   o  If an explicit connectivity verification mechanism (e.g., ICE) is
      negotiated, the precondition is met when the mechanism verifies
      connectivity successfully.

Andreasen, et al.            Standards Track                    [Page 6]

RFC 5898                Connectivity Precondition              July 2010

   o  Otherwise, if a connection-oriented transport (e.g., TCP) is
      negotiated, the precondition is met when the connection is

   o  Otherwise, if an implicit verification mechanism is provided by
      the transport itself or the media stream data using the transport,
      the precondition is met when the mechanism verifies connectivity

   o  Otherwise, connectivity cannot be verified reliably, and the
      connectivity precondition will never be satisfied if requested.

   This document does not mandate any particular connectivity
   verification mechanism; however, in the following, we provide
   additional considerations for verification mechanisms.

4.1.  Correlation of Dialog to Media Stream

   SIP and SDP do not provide any inherent capabilities for associating
   an incoming media stream packet with a particular dialog.  Thus, when
   an offerer is trying to ascertain connectivity, and an incoming media
   stream packet is received, the offerer may not know which dialog had
   its "recv" connectivity verified.  Explicit connectivity verification
   mechanisms therefore typically provide a means to correlate the media
   stream, whose connectivity is being verified, with a particular SIP
   dialog.  However, some connectivity verification mechanisms may not
   provide such a correlation.  In the absence of a mechanism for the
   correlation of dialog to media stream (e.g., ICE), a UAS (User Agent
   Server) MUST NOT require the offerer to confirm a connectivity

4.2.  Explicit Connectivity Verification Mechanisms

   Explicit connectivity verification mechanisms typically use probe
   traffic with some sort of feedback to inform the sender whether
   reception was successful.  Below we provide two examples of such
   mechanisms, and how they are used with connectivity preconditions:

   Interactive Connectivity Establishment (ICE) [RFC5245] provides one
   or more candidate addresses in signaling between the offerer and the
   answerer and then uses STUN (Simple Traversal of the UDP Protocol
   through NAT) Binding Requests to determine which pairs of candidate
   addresses have connectivity.  Each STUN Binding Request contains a
   password that is communicated in the SDP as well; this enables
   correlation between STUN Binding Requests and candidate addresses for
   a particular media stream.  It also provides correlation with a
   particular SIP dialog.

Andreasen, et al.            Standards Track                    [Page 7]

RFC 5898                Connectivity Precondition              July 2010

   ICE implementations may be either full or lite (see [RFC5245]).  Full
   implementations generate and respond to STUN Binding Requests,
   whereas lite implementations only respond to them.  With ICE, one
   side is a controlling agent, and the other side is a controlled
   agent.  A full implementation can take on either role, whereas a lite
   implementation can only be a controlled agent.  The controlling agent
   decides which valid candidate to use and informs the controlled agent
   of it by identifying the pair as the nominated pair.  This leads to
   the following connectivity precondition rules:

   o  A full implementation ascertains both "send" and "recv"
      connectivity when it operates as a STUN client and has sent a STUN
      Binding Request that resulted in a successful check for all the
      components of the media stream (as defined further in ICE).

   o  A full or a lite implementation ascertains "recv" connectivity
      when it operates as a STUN server and has received a STUN Binding
      Request that resulted in a successful response for all the
      components of the media stream (as defined further in ICE).

   o  A lite implementation ascertains "send" and "recv" connectivity
      when the controlling agent has informed it of the nominated pair
      for all the components of the media stream.

   A simpler and slightly more delay-prone alternative to the above
   rules is for all ICE implementations to ascertain "send" and "recv"
   connectivity for a media stream when the ICE state for that media
   stream has moved to Completed.

   Note that there is never a need for the answerer to request
   confirmation of the connectivity precondition when using ICE: the
   answerer can determine the status locally.  Also note, that when ICE
   is used to verify connectivity preconditions, the precondition is not
   satisfied until connectivity has been verified for all the component
   transport addresses used by the media stream.  For example, with an
   RTP-based media stream where RTCP is not suppressed, connectivity
   MUST be ascertained for both RTP and RTCP.  Finally, it should be
   noted, that although connectivity has been ascertained, a new offer/
   answer exchange may be required before media can flow (per ICE).

   The above are merely examples of explicit connectivity verification
   mechanisms.  Other techniques can be used as well.  It is however
   RECOMMENDED that ICE be supported by entities that support
   connectivity preconditions.  Use of ICE has the benefit of working
   for all media streams (not just RTP) as well as facilitating NAT and
   firewall traversal, which may otherwise interfere with connectivity.
   Furthermore, the ICE recommendation provides a baseline to ensure

Andreasen, et al.            Standards Track                    [Page 8]

RFC 5898                Connectivity Precondition              July 2010

   that all entities that require probe traffic to support the
   connectivity preconditions have a common way of ascertaining

4.3.  Verifying Connectivity for Connection-Oriented Transports

   Connection-oriented transport protocols generally provide an implicit
   connectivity verification mechanism.  Connection establishment
   involves sending traffic in both directions thereby verifying
   connectivity at the transport-protocol level.  When a three-way (or
   more) handshake for connection establishment succeeds, bi-directional
   communication is confirmed and both the "send" and "recv"
   preconditions are satisfied whether requested or not.  In the case of
   TCP for example, once the TCP three-way handshake has completed (SYN,
   SYN-ACK, ACK), the TCP connection is established and data can be sent
   and received by either party (i.e., both a send and a receive
   connectivity precondition has been satisfied).  SCTP (Stream Control
   Transmission Protocol) [RFC4960] connections have similar semantics
   as TCP and SHOULD be treated the same.

   When a connection-oriented transport is part of an offer, it may be
   passive, active, or active/passive [RFC4145].  When it is passive,
   the offerer expects the answerer to initiate the connection
   establishment, and when it is active, the offerer wants to initiate
   the connection establishment.  When it is active/passive, the
   answerer decides.  As noted earlier, lack of a media-stream-to-dialog
   correlation mechanism can make it difficult to guarantee with whom
   connectivity has been ascertained.  When the offerer takes on the
   passive role, the offerer will not necessarily know which SIP dialog
   originated an incoming connection request.  If the offerer instead is
   active, this problem is avoided.

5.  Connectivity and Other Precondition Types

   The role of a connectivity precondition is to ascertain media stream
   connectivity before establishing or modifying a session.  The
   underlying intent is for the two parties to be able to exchange media
   packets successfully.  However, connectivity by itself may not fully
   satisfy this.  Quality of Service, for example, may be required for
   the media stream; this can be addressed by use of the "qos"
   precondition defined in RFC 3312 [RFC3312].  Similarly, successful
   security parameter negotiation may be another prerequisite; this can
   be addressed by use of the "sec" precondition defined in RFC 5027

Andreasen, et al.            Standards Track                    [Page 9]

RFC 5898                Connectivity Precondition              July 2010

6.  Examples

   The first example uses the connectivity precondition with TCP in the
   context of a session involving a wireless access medium.  Both UAs
   use a radio access network that does not allow them to send any data
   (not even a TCP SYN) until a radio bearer has been set up for the
   connection.  Figure 1 shows the message flow of this example (the
   required PRACK transaction has been omitted for clarity -- see
   [RFC3312] for details):

               A                                    B
               |  INVITE                            |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
               |  a=setup:holdconn                  |
               |                                    |
               |  183 Session Progress              |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
               |  a=setup:holdconn                  |
               |                                    |
               |  UPDATE                            |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
     A's radio |  a=setup:actpass                   |
     bearer is +----------------------------------->|
     up        |                                    |
               |  200 OK                            |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
               |  a=setup:active                    |
               |                                    |
               |                                    |
               |                                    |
               |                                    | B's radio
               |<---TCP Connection Establishment--->+ bearer is up
               |                                    | B sends TCP SYN
               |                                    |
               |                                    |
               |  180 Ringing                       | TCP connection
               |<-----------------------------------+ is up
               |                                    | B alerts the user
               |                                    |

          Figure 1: Message Flow with Two Types of Preconditions

Andreasen, et al.            Standards Track                   [Page 10]

RFC 5898                Connectivity Precondition              July 2010

   A sends an INVITE requesting connection-establishment preconditions.
   The setup attribute in the offer is set to holdconn [RFC4145] because
   A cannot send or receive any data before setting up a radio bearer
   for the connection.

   B agrees to use the connectivity precondition by sending a 183
   (Session Progress) response.  The setup attribute in the answer is
   also set to holdconn because B, like A, cannot send or receive any
   data before setting up a radio bearer for the connection.

   When A's radio bearer is ready, A sends an UPDATE to B with a setup
   attribute with a value of actpass.  This attribute indicates that A
   can perform an active or a passive TCP open.  A is letting B choose
   which endpoint will initiate the connection.

   Since B's radio bearer is not ready yet, B chooses to be the one
   initiating the connection and indicates this with a setup attribute
   with a value of active.  At a later point, when B's radio bearer is
   ready, B initiates the TCP connection towards A.

   Once the TCP connection is established successfully, B knows the
   "sendrecv" precondition is satisfied, and B proceeds with the session
   (i.e., alerts the Callee), and sends a 180 (Ringing) response.

   The second example shows a basic SIP session establishment using SDP
   connectivity preconditions and ICE (the required PRACK transaction
   and some SDP details have been omitted for clarity).  The offerer (A)
   is a full ICE implementation whereas the answerer (B) is a lite ICE
   implementation.  The message flow for this scenario is shown in
   Figure 2 below.

Andreasen, et al.            Standards Track                   [Page 11]

RFC 5898                Connectivity Precondition              July 2010

                  A                                            B

                  |                                            |
                  |-------------(1) INVITE SDP1--------------->|
                  |                                            |
                  |<------(2) 183 Session Progress SDP2--------|
                  |                                            |
                  |~~~~~ Connectivity check to B ~~~~~~~~~~~~~>|
                  |<~~~~ Connectivity to B OK ~~~~~~~~~~~~~~~~~|
                  |                                            |
                  |-------------(3) UPDATE SDP3--------------->|
                  |                                            |
                  |<--------(4) 200 OK (UPDATE) SDP4-----------|
                  |                                            |
                  |<-------------(5) 180 Ringing---------------|
                  |                                            |
                  |                                            |

     Figure 2: Connectivity Precondition with ICE Connectivity Checks

   SDP1: A includes a mandatory end-to-end connectivity precondition
   with a desired status of "sendrecv"; this will ensure media stream
   connectivity in both directions before continuing with the session
   setup.  Since media stream connectivity in either direction is
   unknown at this point, the current status is set to "none".  A's
   local status table (see [RFC3312]) for the connectivity precondition
   is as follows:

       Direction |  Current | Desired Strength |  Confirm
         send    |    no    |   mandatory      |    no
         recv    |    no    |   mandatory      |    no

   and the resulting offer SDP is:

     m=audio 20000 RTP/AVP 0
     c=IN IP4
     a=curr:conn e2e none
     a=des:conn mandatory e2e sendrecv
     a=candidate:1 1 UDP 2130706431 20000 typ host

   SDP2: When B receives the offer, B sees the mandatory sendrecv
   connectivity precondition.  B is a lite ICE implementation and hence
   B can only ascertain "recv" connectivity (from B's point of view)

Andreasen, et al.            Standards Track                   [Page 12]

RFC 5898                Connectivity Precondition              July 2010

   from A; thus, B wants A to inform it about connectivity in the other
   direction ("send" from B's point of view).  B's local status table
   therefore looks as follows:

       Direction |  Current | Desired Strength |  Confirm
         send    |    no    |   mandatory      |    no
         recv    |    no    |   mandatory      |    no

   Since B is a lite ICE implementation and B wants to ask A for
   confirmation about the "send" (from B's point of view) connectivity
   precondition, the resulting answer SDP becomes:

     m=audio 30000 RTP/AVP 0
     c=IN IP4
     a=curr:conn e2e none
     a=des:conn mandatory e2e sendrecv
     a=conf:conn e2e send
     a=candidate:1 1 UDP 2130706431 30000 typ host

   Since the "send" and the "recv" connectivity precondition (from B's
   point of view) are still not satisfied, session establishment remains

   SDP3: When A receives the answer SDP, A notes that B is a lite ICE
   implementation and that confirmation was requested for B's "send"
   connectivity precondition, which is the "recv" precondition from A's
   point of view.  A performs a successful send and recv connectivity
   check to B by sending an ICE connectivity check to B and receiving
   the corresponding response.  A's local status table becomes:

       Direction |  Current | Desired Strength |  Confirm
         send    |    yes   |   mandatory      |    no
         recv    |    yes   |   mandatory      |    yes

   whereas B's local status table becomes:

       Direction | Current  | Desired Strength | Confirm
         send    |    no    |   mandatory      |   no
         recv    |    yes   |   mandatory      |   no

Andreasen, et al.            Standards Track                   [Page 13]

RFC 5898                Connectivity Precondition              July 2010

   Since B asked for confirmation about the "recv" connectivity (from
   A's point of view), A now sends an UPDATE (5) to B to confirm the
   connectivity from A to B:

     m=audio 20000 RTP/AVP 0
     c=IN IP4
     a=curr:conn e2e sendrecv
     a=des:conn mandatory e2e sendrecv
     a=candidate:1 1 UDP 2130706431 20000 typ host

   B knows it has recv connectivity (verified by ICE as well as A's
   UPDATE) and send connectivity (confirmed by A's UPDATE) at this
   point.  B's local status table becomes:

       Direction | Current  | Desired Strength | Confirm
         send    |    yes   |   mandatory      |   no
         recv    |    yes   |   mandatory      |   no

   and the session can continue.

7.  Security Considerations

   General security considerations for preconditions are discussed in
   RFC 3312 [RFC3312] and RFC 4032 [RFC4032].  As discussed in RFC 4032
   [RFC4032], it is strongly RECOMMENDED that S/MIME [RFC3853] integrity
   protection be applied to the SDP session descriptions.  When the user
   agent provides identity services (rather than the proxy server), the
   SIP identity mechanism specified in RFC 4474 [RFC4474] provides an
   alternative end-to-end integrity protection.  Additionally, the
   following security issues relate specifically to connectivity

   Connectivity preconditions rely on mechanisms beyond SDP, such as TCP
   [RFC0793] connection establishment or ICE connectivity checks
   [RFC5245], to establish and verify connectivity between an offerer
   and an answerer.  An attacker that prevents those mechanisms from
   succeeding (e.g., by keeping ICE connectivity checks from arriving at
   their destination) can prevent media sessions from being established.
   While this attack relates to connectivity preconditions, it is
   actually an attack against the connection-establishment mechanisms
   used by the endpoints.  This attack can be performed in the presence
   or in the absence of connectivity preconditions.  In their presence,
   the whole session setup will be disrupted.  In their absence, only
   the establishment of the particular stream under attack will be

Andreasen, et al.            Standards Track                   [Page 14]

RFC 5898                Connectivity Precondition              July 2010

   disrupted.  This specification does not provide any mechanism against
   attackers able to block traffic between the endpoints involved in the
   session because such an attacker will always be able to launch DoS
   (Denial-of-Service) attacks.

   Instead of blocking the connectivity checks, the attacker can
   generate forged connectivity checks that would cause the endpoints to
   assume that there was connectivity when there was actually no
   connectivity.  This attack would result in the user experience being
   poor because the session would be established without all the media
   streams being ready.  The same attack can be used, regardless of
   whether or not connectivity preconditions are used, to attempt to
   hijack a connection.  The forged connectivity checks would trick the
   endpoints into sending media to the wrong direction.  To prevent
   these attacks, it is RECOMMENDED that the mechanisms used to check
   connectivity are adequately secured by message authentication and
   integrity protection.  For example, Section 2.5 of [RFC5245]
   discusses how message integrity and data origin authentication are
   implemented in ICE connectivity checks.

8.  IANA Considerations

   IANA has registered a new precondition type under the Precondition
   Types used with SIP subregistry, which is located under the Session
   Initiation Protocol (SIP) Parameters registry.

   Precondition-Type  Description                          Reference
   -----------------  -----------------------------------  ---------
   conn               Connectivity precondition            [RFC5898]

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3312]  Camarillo, G., Marshall, W., and J. Rosenberg,
              "Integration of Resource Management and Session Initiation
              Protocol (SIP)", RFC 3312, October 2002.

Andreasen, et al.            Standards Track                   [Page 15]

RFC 5898                Connectivity Precondition              July 2010

   [RFC3853]  Peterson, J., "S/MIME Advanced Encryption Standard (AES)
              Requirement for the Session Initiation Protocol (SIP)",
              RFC 3853, July 2004.

   [RFC4032]  Camarillo, G. and P. Kyzivat, "Update to the Session
              Initiation Protocol (SIP) Preconditions Framework",
              RFC 4032, March 2005.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5027]  Andreasen, F. and D. Wing, "Security Preconditions for
              Session Description Protocol (SDP) Media Streams",
              RFC 5027, October 2007.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

9.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5109]  Li, A., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, December 2007.

   [ICE-TCP]  Perreault, S., Ed. and J. Rosenberg, "TCP Candidates with
              Interactive Connectivity Establishment (ICE)", Work
              in Progress, October 2009.

Andreasen, et al.            Standards Track                   [Page 16]

RFC 5898                Connectivity Precondition              July 2010

Authors' Addresses

   Flemming Andreasen
   Cisco Systems, Inc.
   499 Thornall Street, 8th Floor
   Edison, NJ  08837

   EMail: fandreas@cisco.com

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420

   EMail: Gonzalo.Camarillo@ericsson.com

   David Oran
   Cisco Systems, Inc.
   7 Ladyslipper Lane
   Acton, MA  01720

   EMail: oran@cisco.com

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134

   EMail: dwing@cisco.com

Andreasen, et al.            Standards Track                   [Page 17]