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Network Working Group
Request for Comments: 5576
Category: Standards Track
J. Lennox
Vidyo
J. Ott
Helsinki University of Technology
T. Schierl
Fraunhofer HHI
June 2009

Source-Specific Media Attributes in the

Session Description Protocol (SDP)

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.

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Copyright © 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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Abstract

The Session Description Protocol (SDP) provides mechanisms to describe attributes of multimedia sessions and of individual media streams (e.g., Real-time Transport Protocol (RTP) sessions) within a multimedia session, but does not provide any mechanism to describe individual media sources within a media stream. This document defines a mechanism to describe RTP media sources, which are identified by their synchronization source (SSRC) identifiers, in SDP, to associate attributes with these sources, and to express relationships among sources. It also defines several source-level attributes that can be used to describe properties of media sources.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Overview ........................................................3
   4. Media Attributes ................................................4
      4.1. The "ssrc" Media Attribute .................................5
      4.2. The "ssrc-group" Media Attribute ...........................6
   5. Usage of Identified Source Identifiers in RTP ...................7
   6. Source Attributes ...............................................8
      6.1. The "cname" Source Attribute ...............................8
      6.2. The "previous-ssrc" Source Attribute .......................9
      6.3. The "fmtp" Source Attribute ................................9
      6.4. Other Source Attributes ...................................10
   7. Examples .......................................................10
   8. Usage With the Offer/Answer Model ..............................11
   9. Backward Compatibility .........................................11
   10. Formal Grammar ................................................12
   11. Security Considerations .......................................13
   12. IANA Considerations ...........................................14
      12.1. New SDP Media-Level Attributes ...........................14
      12.2. Registry for Source-Level Attributes .....................14
      12.3. Registry for Source Grouping Semantics ...................15
   13. References ....................................................16
      13.1. Normative References .....................................16
      13.2. Informative References ...................................16

1. Introduction

The Session Description Protocol (SDP) [RFC4566] provides mechanisms to describe attributes of multimedia sessions and of media streams (e.g., Real-time Transport Protocol (RTP) [RFC3550] sessions) within a multimedia session, but does not provide any mechanism to describe individual media sources within a media stream.

Several recently proposed protocols, notably RTP single-source multicast [EXT-SSM], have found it useful to describe specific media sources in SDP messages. Single-source multicast, in particular, needs to ensure that receivers' RTP synchronization source (SSRC) identifiers do not collide with those of media senders, as the RTP specification [RFC3550] requires that colliding sources change their SSRC values after a collision has been detected. Earlier work has used mechanisms specific to each protocol to describe the individual sources of an RTP session.

Moreover, whereas the Real-time Transport Protocol (RTP) [RFC3550] is defined as allowing multiple sources in an RTP session (for example, if a user has more than one camera), SDP has no existing mechanism for an endpoint to indicate that it will be using multiple sources or to describe their characteristics individually.

To address all these problems, this document defines a mechanism to describe RTP sources, identified by their synchronization source (SSRC) identifier, in SDP, to associate attributes with these sources, and to express relationships among individual sources. It also defines a number of new SDP attributes that apply to individual sources ("source-level" attributes), describes how a number of existing media stream ("media-level") attributes can also be applied at the source level, and establishes IANA registries for source-level attributes and source grouping semantics.

2. 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 [RFC2119] and indicate requirement levels for compliant implementations.

3. Overview

In the Real-time Transport Protocol (RTP) [RFC3550], an association among a group of communicating participants is known as an RTP Session. An RTP session is typically associated with a single transport address (in the case of multicast) or communication flow (in the case of unicast), though RTP translators and single-source multicast [EXT-SSM] can make the situation more complex. RTP topologies are discussed in more detail in [RFC5117].

Within an RTP session, the source of a single stream of RTP packets is known as a synchronization source (SSRC). Every synchronization source is identified by a 32-bit numeric identifier. In addition, receivers (who may never send RTP packets) also have source identifiers, which are used to identify their RTP Control Protocol (RTCP) receiver reports and other feedback messages.

Messages of the Session Description Protocol (SDP) [RFC4566], known as session descriptions, describe multimedia sessions. A multimedia session is a set of multimedia senders and receivers as well as the data streams flowing from senders to receivers. A multimedia session contains a number of media streams, which are the individual RTP sessions or other media paths over which one type of multimedia data is carried. Information that applies to an entire multimedia session is called session-level information, while information pertaining to one media stream is called media-level information. The collection of all the information describing a media stream is known as a media description. (Media descriptions are also sometimes known informally as SDP "m"-lines, after the SDP syntax that begins a media description.) Several standard information elements are defined at both the session level and the media level. Extended information can be included at both levels through the use of attributes.

(The term "media stream" does not appear in the SDP specification itself, but is used by a number of SDP extensions, for instance, Interactive Connectivity Establishment (ICE) [ICE], to denote the object described by an SDP media description. This term is unfortunately rather confusing, as the RTP specification [RFC3550] uses the term "media stream" to refer to an individual media source or RTP packet stream, identified by an SSRC, whereas an SDP media stream describes an entire RTP session, which can contain any number of RTP sources. In this document, the term "media stream" means an SDP media stream, i.e., the thing described by an SDP media description, whereas "media source" is used for a single source of media packets, i.e., an RTP media stream.)

The core SDP specification does not have any way of describing individual media sources, particularly RTP synchronization sources, within a media stream. To address this problem, in this document we introduce a third level of information, called source-level information. Syntactically, source-level information is described by a new SDP media-level attribute, "ssrc", which identifies specific synchronization sources within an RTP session and acts as a meta- attribute mapping source-level attribute information to these sources.

This document also defines an SDP media-level attribute, "ssrc- group", which can represent relationships among media sources within an RTP session in much the same way as the "group" attribute [RFC3388] represents relationships among media streams within a multimedia session.

4. Media Attributes

This section defines two media-level attributes, "ssrc" and "ssrc- group".

4.1. The "ssrc" Media Attribute

   a=ssrc:<ssrc-id> <attribute>
   a=ssrc:<ssrc-id> <attribute>:<value>

The SDP media attribute "ssrc" indicates a property (known as a "source-level attribute") of a media source (RTP stream) within an RTP session. <ssrc-id> is the synchronization source (SSRC) ID of the source being described, interpreted as a 32-bit unsigned integer in network byte order and represented in decimal. <attribute> or <attribute>:<value> represents the source-level attribute specific to the given media source. The source-level attribute follows the syntax of the SDP "a=" line. It thus consists of either a single attribute name (a flag) or an attribute name and value, e.g., "cname:user@example.com". No attributes of the former type are defined by this document.

Within a media stream, "ssrc" attributes with the same value of <ssrc-id> describe different attributes of the same media sources. Across media streams, <ssrc-id> values are not correlated (unless correlation is indicated by media-stream grouping or some other mechanism) and MAY be repeated.

Each "ssrc" media attribute specifies a single source-level attribute for the given <ssrc-id>. For each source mentioned in SDP, the source-level attribute "cname", defined in Section 6.1, MUST be provided. Any number of other source-level attributes for the source MAY also be provided.

The "ssrc" media attribute MAY be used for any RTP-based media transport. It is not defined for other transports.

If any other SDP attributes also mention RTP SSRC values (for example, Multimedia Internet KEYing (MIKEY) [RFC3830] [RFC4567]), the values used MUST be consistent. (These attributes MAY provide additional information about a source described by an "ssrc" attribute or MAY describe additional sources.)

Though the source-level attributes specified by the ssrc property follow the same syntax as session-level and media-level attributes, they are defined independently. All source-level attributes MUST be registered with IANA, using the registry defined in Section 12.2.

Figure 4 in Section 10 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for the "ssrc" attribute.

The "ssrc" media attribute is not dependent on charset.

4.2. The "ssrc-group" Media Attribute

a=ssrc-group:<semantics> <ssrc-id> ...

The SDP media attribute "ssrc-group" expresses a relationship among several sources of an RTP session. It is analogous to the "group" session-level attribute [RFC3388], which expresses a relationship among media streams in an SDP multimedia session (i.e., a relationship among several logically related RTP sessions). As sources are already identified by their SSRC IDs, no analogous property to the "mid" attribute is necessary; groups of sources are identified by their SSRC IDs directly.

The <semantics> parameter is taken from the specification of the "group" attribute [RFC3388]. The initial semantic values defined for the "ssrc-group" attribute are FID (Flow Identification) [RFC3388] and FEC (Forward Error Correction) [RFC4756]. In each case, the relationship among the grouped sources is the same as the relationship among corresponding sources in media streams grouped using the SDP "group" attribute.

Though the "ssrc-group" semantic values follow the same syntax as "group" semantic values, they are defined independently. All "ssrc- group" semantic values MUST be registered with IANA, using the registry defined in Section 12.3.

(The other "group" semantics registered with IANA as of this writing are not useful for source grouping. LS (Lip Synchronization) [RFC3388] is redundant for sources within a media stream as RTP sources with the same CNAME are implicitly synchronized in RTP. SRF (Single Reservation Flow) [RFC3524] and ANAT (Alternative Network Address Types) [RFC4091] refer specifically to the media stream's transport characteristics. CS (Composite Session) [FLUTE] is used to group FLUTE sessions, and so is not applicable to RTP.)

The "ssrc-group" attribute indicates the sources in a group by listing the <ssrc-id>s of the sources in the group. It MUST list at least one <ssrc-id> for a group and MAY list any number of additional ones. Every <ssrc-id> listed in an "ssrc-group" attribute MUST be defined by a corresponding "ssrc:" line in the same media description.

The "ssrc-group" media attribute is not dependent on charset.

Figure 5 in Section 10 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for the "ssrc-group" attribute.

5. Usage of Identified Source Identifiers in RTP

The synchronization source identifiers used in an RTP session are chosen randomly and independently by endpoints. As such, it is possible for two RTP endpoints to choose the same SSRC identifier. Though the probability of this is low, the RTP specification [RFC3550] requires that all RTP endpoints MUST be prepared to detect and resolve collisions.

As a result, all endpoints MUST be prepared for the fact that information about specific sources identified in a media stream might be out of date. The actual binding between SSRCs and source CNAMEs can only be identified by the source description (SDES) RTCP packets transmitted on the RTP session.

When endpoints are choosing their own local SSRC values for media streams for which source-level attributes have been specified, they MUST NOT use for themselves any SSRC identifiers mentioned in media descriptions they have received for the media stream.

However, sources identified by SDP source-level attributes do not otherwise affect RTP transport logic. Specifically, sources that are only known through SDP, for which neither RTP nor RTCP packets have been received, MUST NOT be counted for RTP group size estimation, and report blocks MUST NOT be sent for them in SR or RR RTCP messages.

Endpoints MUST NOT assume that only the sources mentioned in SDP will be present in an RTP session; additional sources, with previously unmentioned SSRC IDs, can be added at any time, and endpoints MUST be prepared to receive packets from these sources. (How endpoints handle such packets is not specified here; they SHOULD be handled in the same manner as packets from additional sources would be handled had the endpoint not received any a=ssrc: attributes at all.)

An endpoint that observes an SSRC collision between its explicitly signaled source and another entity that has not explicitly signaled an SSRC MAY delay its RTP collision-resolution actions [RFC3550] by 5*1.5*Td, where Td is the deterministic, calculated, reporting interval for receivers defined in Section 6.3.1 of the RTP specification [RFC3550], to see whether the conflict still exists. (This gives precedence to explicitly signaled sources and places the burden of collision resolution on non-signaled sources.) SSRC collisions between multiple explicitly-signaled sources, however, MUST be acted upon immediately.

If, following RTP's collision-resolution procedures [RFC3550], a source identified by source-level attributes has been forced to change its SSRC identifier, the author of the SDP containing the source-level attributes for these sources SHOULD send out an updated SDP session description with the new SSRC if the mechanism by which SDP is being distributed for the multimedia session has a mechanism to distribute updated SDP. This updated SDP MUST include a "previous-ssrc" source-level attribute, described in Section 6.2, listing the source's previous SSRC ID. (If only a single source with a given CNAME has collided, the other RTP session members can infer a correspondence between the source's old and new SSRC IDs without requiring an updated session description. However, if more than one source collides at once, or if sources are leaving and re-joining, this inference is not possible. To avoid confusion, therefore, sending updated SDP messages is always RECOMMENDED.)

Endpoints MUST NOT reuse the same SSRC ID for identified sources with the same CNAME for at least the duration of the RTP session's participant timeout interval (see Section 6.3.5 of [RFC3550]). They SHOULD NOT reuse any SSRC ID ever mentioned in SDP (either by themselves or by other endpoints) for the entire lifetime of the RTP session.

Endpoints MUST be prepared for the possibility that other parties in the session do not understand SDP source-level attributes, unless some higher-level mechanism normatively requires them. See Section 9 for more discussion of this.

6. Source Attributes

This section describes specific source attributes that can be applied to RTP sources.

6.1. The "cname" Source Attribute

   a=ssrc:<ssrc-id> cname:<cname>

The "cname" source attribute associates a media source with its Canonical End-Point Identifier (CNAME) source description (SDES) item. This MUST be the CNAME value that the media sender will place in its RTCP SDES packets; it therefore MUST follow the syntax conventions of CNAME defined in the RTP specification [RFC3550]. If a session participant receives an RTCP SDES packet associating this SSRC with a different CNAME, it SHOULD assume there has been an SSRC collision and that the description of the source that was carried in the SDP description is not applicable to the actual source being received. This source attribute is REQUIRED to be present if any source attributes are present for a source. The "cname" attribute

MUST NOT occur more than once for the same ssrc-id within a given media stream.

The "cname" source attribute is not dependent on charset.

Figure 6 in Section 10 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for the "cname" attribute.

6.2. The "previous-ssrc" Source Attribute

a=ssrc:<ssrc-id> previous-ssrc:<ssrc-id> ...

The "previous-ssrc" source attribute associates a media source with previous source identifiers used for the same media source. Following an SSRC change due to an SSRC collision involving a media source described in SDP, the updated session description describing the source's new SSRC (described in Section 5) MUST include the "previous-ssrc" attribute associating the new SSRC with the old one. If further updated SDP descriptions are published describing the media source, the "previous-ssrc" attribute SHOULD be included if the session description was generated before the participant timeout of the old SSRC, and MAY be included after that point. This attribute, if present, MUST list at least one previous SSRC and MAY list any number of additional SSRCs for the source if the source has collided more than once. This attribute MUST be present only once for each source.

The "previous-ssrc" source attribute is not dependent on charset.

Figure 7 in Section 10 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for the previous-ssrc attribute.

6.3. The "fmtp" Source Attribute

   a=ssrc:<ssrc> fmtp:<format> <format specific parameters>

The "fmtp" source attribute allows format-specific parameters to be conveyed about a given source. The <format> parameter MUST be one of the media formats (i.e., RTP payload types) specified for the media stream. The meaning of the <format specific parameters> is unique for each media type. This parameter MUST only be used for media types for which source-level format parameters have explicitly been specified; media-level format parameters MUST NOT be carried over blindly.

The "fmtp" source attribute is not dependent on charset.

6.4. Other Source Attributes

This document only defines source attributes that are necessary or useful for an endpoint to decode and render the sources in a media stream. It does not include any attributes that would contribute to an endpoint's decision to accept or reject a stream, e.g., in an offer/answer exchange. Such attributes are for future consideration.

7. Examples

This section gives several examples of SDP descriptions of media sessions containing source attributes. For brevity, only the media sections of the descriptions are given. 

   m=audio 49168 RTP/AVP 0
   a=ssrc:314159 cname:user@example.com

Figure 1: Example of a declaration of a single synchronization source

The example in Figure 1 shows an audio stream advertising a single source. 

   m=video 49170 RTP/AVP 96
   a=rtpmap:96 H264/90000
   a=ssrc:12345 cname:another-user@example.com
   a=ssrc:67890 cname:another-user@example.com

Figure 2: Example of a media stream containing several independent

sources from a single session member

The example in Figure 2 shows a video stream where one participant (identified by a single CNAME) has several cameras. The sources could be further distinguished by RTCP Source Description (SDES) information. 

   m=video 49174 RTP/AVPF 96 98
   a=rtpmap:96 H.264/90000
   a=rtpmap:98 rtx/90000
   a=fmtp:98 apt=96;rtx-time=3000
   a=ssrc-group:FID 11111 22222
   a=ssrc:11111 cname:user3@example.com
   a=ssrc:22222 cname:user3@example.com
   a=ssrc-group:FID 33333 44444
   a=ssrc:33333 cname:user3@example.com
   a=ssrc:44444 cname:user3@example.com

Figure 3: Example of the relationships among

several retransmission sources

The example in Figure 3 shows how the relationships among sources used for RTP retransmission [RFC4588] can be explicitly signaled. This prevents the complexity of associating original sources with retransmission sources when SSRC multiplexing is used for RTP retransmission, as is described in Section 5.3 of [RFC4588].

8. Usage With the Offer/Answer Model

When used with the SDP Offer/Answer Model [RFC3264], SDP source- specific attributes describe only the sources that each party is willing to send (whether it is sending RTP data or RTCP report blocks). No mechanism is provided by which an answer can accept or reject individual sources within a media stream; if the set of sources in a media stream is unacceptable, the answerer's only option is to reject the media stream or the entire multimedia session.

The SSRC IDs for sources described by an SDP answer MUST be distinct from the SSRC IDs for sources of that media stream in the offer. Similarly, new SSRC IDs in an updated offer MUST be distinct from the SSRC IDs for that media stream established in the most recent offer/ answer exchange for the session and SHOULD be distinct from any SSRC ID ever used by either party within the multimedia session (whether or not it is still being used).

9. Backward Compatibility

According to the definition of SDP, interpreters of SDP session descriptions ignore unknown attributes. Thus, endpoints MUST be prepared that recipients of their RTP media session may not understand their explicit source descriptions, unless some external mechanism indicates that they were understood. In some cases (such as RTP Retransmission [RFC4588]), this may constrain some choices about the bitstreams that are transmitted.

Source descriptions are specified in this document such that RTP endpoints that are compliant with the RTP specification [RFC3550] will be able to decode the media streams they describe whether or not they support explicit source descriptions. However, some deployed RTP implementations may not actually support multiple media sources in a media stream. Media senders MAY wish to restrict themselves to a single source at a time unless they have some means of concluding that the receivers of the media stream support source multiplexing.

10. Formal Grammar

This section gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for each of the new media and source attributes defined in this document. Grammars for existing session or media attributes that have been extended to be source attributes are not included.

Copyright © 2009 IETF Trust and the persons identified as authors of the code. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

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  • Neither the name of Internet Society, IETF or IETF Trust, nor the names of specific contributors, may be used to endorse or promote products derived from this software without specific prior written permission.

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ssrc-attr = "ssrc:" ssrc-id SP attribute
; The base definition of "attribute" is in RFC 4566.
; (It is the content of "a=" lines.) 

   ssrc-id = integer ; 0 .. 2**32 - 1
   
   attribute =/ ssrc-attr

Figure 4: Syntax of the "ssrc" media attribute

   ssrc-group-attr = "ssrc-group:" semantics *(SP ssrc-id)
   
   semantics       = "FEC" / "FID" / token
                    ; Matches RFC 3388 definition and
                    ; IANA registration rules in this doc.
   
   token           = <as defined in RFC 4566>
   
   attribute       =/ ssrc-group-attr

Figure 5: Syntax of the "ssrc-group" media attribute

cname-attr = "cname:" cname

cname = byte-string
; Following the syntax conventions for CNAME as defined in RFC 3550. ; The definition of "byte-string" is in RFC 4566. 

   attribute =/ cname-attr

Figure 6: Syntax of the "cname" source attribute

   previous-ssrc-attr = "previous-ssrc:" ssrc-id *(SP ssrc-id)
   
   attribute =/ previous-ssrc-attr

Figure 7: Syntax of the "previous-ssrc" source attribute

11. Security Considerations

All the security implications of RTP [RFC3550] and of SDP [RFC4566] apply. Explicitly describing the multiplexed sources of an RTP media stream does not appear to add any further security issues.

12. IANA Considerations

12.1. New SDP Media-Level Attributes

This document defines two SDP media-level attributes: "ssrc" and "ssrc-group". These attributes have been registered by IANA under "Session Description Protocol (SDP) Parameters" under "att-field (media level only)".

The "ssrc" attribute is used to identify characteristics of media sources within a media stream. Its format is defined in Section 4.1.

The "ssrc-group" attribute is used to identify relationships among media sources within a media stream. Its format is defined in Section 4.2.

12.2. Registry for Source-Level Attributes

This specification creates a new IANA registry named "att-field (source level)" within the SDP parameters registry. Source attributes MUST be registered with IANA and documented under the same rules as for SDP session-level and media-level attributes as specified in [RFC4566].

New attribute registrations are accepted according to the "Specification Required" policy of [RFC5226], provided that the specification includes the following information:

  • contact name, email address, and telephone number
  • attribute name (as it will appear in SDP)
  • long-form attribute name in English
  • whether the attribute value is subject to the charset attribute
  • a one-paragraph explanation of the purpose of the attribute
  • a specification of appropriate attribute values for this attribute

The above is the minimum that IANA will accept. The Expert Reviewer will determine if the proposed attributes are expected to see widespread use and interoperability; in that case, the attributes MUST be specified in a Standards Track RFC.

Submitters of registrations should ensure that the specification is in the spirit of SDP attributes, most notably that the attribute is platform independent in the sense that it makes no implicit assumptions about operating systems and does not name specific pieces of software in a manner that might inhibit interoperability.

Source-level attributes that are substantially similar in semantics to existing session-level or media-level attributes SHOULD reuse the same attribute name as those session-level or media-level attributes. Source-level attributes SHOULD NOT reuse attribute names of session- level or media-level attributes that are unrelated or substantially different.

The initial set of source attribute names, with definitions in Section 6 of this document, is in Figure 8. 

   Type            SDP Name                     Reference
   ----            ------------------           ---------
   att-field (source level)
                   cname                        [RFC5576]
                   previous-ssrc                [RFC5576]
                   fmtp                         [RFC5576]

Figure 8: Initial contents of the IANA Source Attribute Registry

12.3. Registry for Source Grouping Semantics

This specification creates a new IANA registry named 'Semantics for the "ssrc-group" SDP Attribute' within the SDP parameters registry. Source group semantics MUST be defined in Standards Track RFCs, under the same rules as [RFC3388].

The IANA Considerations section of the RFC MUST include the following information, which appears in the IANA registry along with the RFC number of the publication:

  • A brief description of the semantics.
  • Token to be used within the group attribute. This token may be of any length, but SHOULD be no more than four characters long.
  • Reference to a Standards Track RFC.

Source grouping semantic values that are substantially similar to existing media grouping semantic values SHOULD reuse the same semantics name as those media grouping semantics. Source grouping semantics SHOULD NOT reuse source grouping semantic names that are unrelated or substantially different.

The initial set of source grouping semantic values, for the semantics specified in Section 4.2 of this document, is in Figure 9. 

   Semantics                           Token     Reference
   -------------------                 -----     ---------
   Flow Identification                 FID       [RFC5576]
   Forward Error Correction            FEC       [RFC5576]

Figure 9: Initial Contents of IANA Source Group Semantics Registry

13. References

13.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
   
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.
   
   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, December 2002.
   
   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.
   
   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.
   
   [RFC4756]  Li, A., "Forward Error Correction Grouping Semantics in
              Session Description Protocol", RFC 4756, November 2006.
   
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
   
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

13.2. Informative References

   [EXT-SSM]  Schooler, E., Ott, J., and J. Chesterfield, "RTCP
              Extensions for Single-Source Multicast Sessions with
              Unicast Feedback", Work in Progress, March 2009.
   
   [FLUTE]    Mehta, H., "SDP Descriptors for FLUTE", Work in Progress,
              January 2006.
   
   [ICE]      Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", Work in Progress,
              October 2007.
   
   [RFC3524]  Camarillo, G. and A. Monrad, "Mapping of Media Streams to
              Resource Reservation Flows", RFC 3524, April 2003.
   
   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.
   
   [RFC4091]  Camarillo, G. and J. Rosenberg, "The Alternative Network
              Address Types (ANAT) Semantics for the Session Description
              Protocol (SDP) Grouping Framework", RFC 4091, June 2005.
   
   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, July 2006.
   
   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.
   
   [RFC5117]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
              January 2008.

Authors' Addresses

   Jonathan Lennox
   Vidyo, Inc.
   433 Hackensack Avenue
   Sixth Floor
   Hackensack, NJ  07601
   US

EMail: 

          jonathan@vidyo.com

Joerg Ott
Helsinki University of Technology (TKK)
Department of Communications and Networking
PO Box 3000
FIN-02015 TKK
Finland

EMail: 

          jo@acm.org

Thomas Schierl
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany 

   Phone: +49-30-31002-227
   EMail: ts@thomas-schierl.de