Network Working Group J. Lennox Request for Comments: 5576 Vidyo Category: Standards Track 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. Copyright Notice Copyright (c) 2009 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 in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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.
AbstractThe 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. 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 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. RFC2119] and indicate requirement levels for compliant implementations. 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.
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. 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. 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. 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. 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.
m=audio 49168 RTP/AVP 0 a=ssrc:314159 cname:email@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:firstname.lastname@example.org a=ssrc:67890 cname:email@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:firstname.lastname@example.org a=ssrc:22222 cname:email@example.com a=ssrc-group:FID 33333 44444 a=ssrc:33333 cname:firstname.lastname@example.org a=ssrc:44444 cname:email@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]. 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). 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.
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 (c) 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: o Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. o Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. o 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. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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 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.
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. RFC4566]. New attribute registrations are accepted according to the "Specification Required" policy of [RFC5226], provided that the specification includes the following information: o contact name, email address, and telephone number o attribute name (as it will appear in SDP) o long-form attribute name in English o whether the attribute value is subject to the charset attribute o a one-paragraph explanation of the purpose of the attribute o 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 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: o A brief description of the semantics. o Token to be used within the group attribute. This token may be of any length, but SHOULD be no more than four characters long. o 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 [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. [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.