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Network Working Group                                          J. LennoxRequest for Comments: 5576                                         VidyoCategory: Standards Track                                         J. Ott                                       Helsinki University of Technology                                                              T. Schierl                                                          Fraunhofer HHI                                                               June 2009Source-Specific Media Attributes in theSession 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 toBCP 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.Lennox, et al.              Standards Track                     [Page 1]

RFC 5576             Source-Specific SDP Attributes            June 2009Abstract   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 Contents1. Introduction ....................................................22. Terminology .....................................................33. Overview ........................................................34. Media Attributes ................................................44.1. The "ssrc" Media Attribute .................................54.2. The "ssrc-group" Media Attribute ...........................65. Usage of Identified Source Identifiers in RTP ...................76. Source Attributes ...............................................86.1. The "cname" Source Attribute ...............................86.2. The "previous-ssrc" Source Attribute .......................96.3. The "fmtp" Source Attribute ................................96.4. Other Source Attributes ...................................107. Examples .......................................................108. Usage With the Offer/Answer Model ..............................119. Backward Compatibility .........................................1110. Formal Grammar ................................................1211. Security Considerations .......................................1312. IANA Considerations ...........................................1412.1. New SDP Media-Level Attributes ...........................1412.2. Registry for Source-Level Attributes .....................1412.3. Registry for Source Grouping Semantics ...................1513. References ....................................................1613.1. Normative References .....................................1613.2. Informative References ...................................161.  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.Lennox, et al.              Standards Track                     [Page 2]

RFC 5576             Source-Specific SDP Attributes            June 2009   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 inRFC 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 sourceLennox, et al.              Standards Track                     [Page 3]

RFC 5576             Source-Specific SDP Attributes            June 2009   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.Lennox, et al.              Standards Track                     [Page 4]

RFC 5576             Source-Specific SDP Attributes            June 20094.  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 inSection 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 inSection 12.2.Lennox, et al.              Standards Track                     [Page 5]

RFC 5576             Source-Specific SDP Attributes            June 2009   Figure 4 inSection 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 inSection 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.Lennox, et al.              Standards Track                     [Page 6]

RFC 5576             Source-Specific SDP Attributes            June 2009   Figure 5 inSection 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 inSection 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.Lennox, et al.              Standards Track                     [Page 7]

RFC 5576             Source-Specific SDP Attributes            June 2009   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 inSection 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 (seeSection 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.  SeeSection 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" attributeLennox, et al.              Standards Track                     [Page 8]

RFC 5576             Source-Specific SDP Attributes            June 2009   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 inSection 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 inSection 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 inSection 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.Lennox, et al.              Standards Track                     [Page 9]

RFC 5576             Source-Specific SDP Attributes            June 20096.4.  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 sourcesLennox, et al.              Standards Track                    [Page 10]

RFC 5576             Source-Specific SDP Attributes            June 2009   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 inSection 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.Lennox, et al.              Standards Track                    [Page 11]

RFC 5576             Source-Specific SDP Attributes            June 200910.  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 (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.Lennox, et al.              Standards Track                    [Page 12]

RFC 5576             Source-Specific SDP Attributes            June 2009   ssrc-attr = "ssrc:" ssrc-id SP attribute   ; The base definition of "attribute" is inRFC 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                    ; MatchesRFC 3388 definition and                    ; IANA registration rules in this doc.   token           = <as defined inRFC 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 inRFC 3550.   ; The definition of "byte-string" is inRFC 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 attribute11.  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.Lennox, et al.              Standards Track                    [Page 13]

RFC 5576             Source-Specific SDP Attributes            June 200912.  IANA Considerations12.1.  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 inSection 4.1.   The "ssrc-group" attribute is used to identify relationships among   media sources within a media stream.  Its format is defined inSection 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:   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.Lennox, et al.              Standards Track                    [Page 14]

RFC 5576             Source-Specific SDP Attributes            June 2009   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 inSection 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 Registry12.3.  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:   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.Lennox, et al.              Standards Track                    [Page 15]

RFC 5576             Source-Specific SDP Attributes            June 2009   The initial set of source grouping semantic values, for the semantics   specified inSection 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 Registry13.  References13.1.  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.Lennox, et al.              Standards Track                    [Page 16]

RFC 5576             Source-Specific SDP Attributes            June 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.Lennox, et al.              Standards Track                    [Page 17]

RFC 5576             Source-Specific SDP Attributes            June 2009Authors' 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.deLennox, et al.              Standards Track                    [Page 18]