This document is obsolete. Please
refer to RFC 5888.
Network Working Group G. Camarillo Request for Comments: 3388 G. Eriksson Category: Standards Track J. Holler Ericsson H. Schulzrinne Columbia University December 2002
Grouping of Media Lines 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) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines two Session Description Protocol (SDP) attributes: "group" and "mid". They allow to group together several "m" lines for two different purposes: for lip synchronization and for receiving media from a single flow (several media streams) that are encoded in different formats during a particular session, on different ports and host interfaces.
Table of Contents
1. Introduction.................................................. 2 2. Terminology................................................... 2 3. Media Stream Identification Attribute......................... 3 4. Group Attribute............................................... 3 5. Use of "group" and "mid"...................................... 3 6. Lip Synchronization (LS)...................................... 4 6.1 Example of LS............................................. 5 7. Flow Identification (FID)..................................... 5 7.1 SIP and Cellular Access................................... 6 7.2 DTMF Tones................................................ 6 7.3 Media Flow Definition..................................... 6 7.4 FID Semantics............................................. 7 7.4.1 Examples of FID..................................... 8 7.5 Scenarios that FID does not Cover........................ 11
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7.5.1 Parallel Encoding Using Different Codecs........... 11 7.5.2 Layered Encoding................................... 12 7.5.3 Same IP Address and Port Number.................... 12 8. Usage of the "group" Attribute in SIP........................ 13 8.1 Mid Value in Answers..................................... 13 8.1.1 Example............................................ 14 8.2 Group Value in Answers................................... 15 8.2.1 Example............................................ 15 8.3 Capability Negotiation................................... 16 8.3.1 Example............................................ 17 8.4 Backward Compatibility................................... 17 8.4.1 Offerer does not Support "group"................... 17 8.4.2 Answerer does not Support "group".................. 17 9. Security Considerations................................... 18 10. IANA Considerations....................................... 18 11. Acknowledgements.......................................... 19 12. References................................................ 19 13. Authors' Addresses........................................ 20 14. Full Copyright Statement.................................. 21
An SDP session description typically contains one or more media lines - they are commonly known as "m" lines. When a session description contains more than one "m" line, SDP does not provide any means to express a particular relationship between two or more of them. When an application receives an SDP session description with more than one "m" line, it is up to the application what to do with them. SDP does not carry any information about grouping media streams.
While in some environments this information can be carried out of band, it would be desirable to have extensions to SDP that allow the expression of how different media streams within a session description relate to each other. This document defines such extensions.
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 [1] and indicate requirement levels for compliant implementations.
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A new "media stream identification" media attribute is defined. It is used for identifying media streams within a session description. Its formatting in SDP [2] is described by the following BNF:
A new "group" session-level attribute is defined. It is used for grouping together different media streams. Its formatting in SDP is described by the following BNF:
This document defines two standard semantics: LS (Lip Synchronization) and FID (Flow Identification). Further semantics need to be defined in a standards-track document. However, defining new semantics apart from LS and FID is discouraged. Instead, it is RECOMMENDED to use other session description mechanisms such as SDPng.
All the "m" lines of a session description that uses "group" MUST be identified with a "mid" attribute whether they appear in the group line(s) or not. If a session description contains at least one "m" line that has no "mid" identification the application MUST NOT perform any grouping of media lines.
"a=group" lines are used to group together several "m" lines that are identified by their "mid" attribute. "a=group" lines that contain identification-tags that do not correspond to any "m" line within the session description MUST be ignored. The application acts as if the "a=group" line did not exist. The behavior of an application receiving an SDP with grouped "m" lines is defined by the semantics field in the "a=group" line.
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There MAY be several "a=group" lines in a session description. All the "a=group" lines of a session description MAY or MAY NOT use the same semantics. An "m" line identified by its "mid" attribute MAY appear in more than one "a=group" line as long as the "a=group" lines use different semantics. An "m" line identified by its "mid" attribute MUST NOT appear in more than one "a=group" line using the same semantics.
An application that receives a session description that contains "m" lines that are grouped together using LS semantics MUST synchronize the playout of the corresponding media streams. Note that LS semantics not only apply to a video stream that has to be synchronized with an audio stream. The playout of two streams of the same type can be synchronized as well.
For RTP streams synchronization is typically performed using RTCP, which provides enough information to map time stamps from the different streams into a wall clock. However, the concept of media stream synchronization MAY also apply to media streams that do not make use of RTP. If this is the case, the application MUST recover the original timing relationship between the streams using whatever available mechanism.
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The following example shows a session description of a conference that is being multicast. The first media stream (mid:1) contains the voice of the speaker who speaks in English. The second media stream (mid:2) contains the video component and the third (mid:3) media stream carries the translation to Spanish of what he is saying. The first and the second media streams MUST be synchronized.
An "m" line in an SDP session description defines a media stream. However, SDP does not define what a media stream is. This definition can be found in the RTSP specification. The RTSP RFC [5] defines a media stream as "a single media instance, e.g., an audio stream or a video stream as well as a single whiteboard or shared application group. When using RTP, a stream consists of all RTP and RTCP packets created by a source within an RTP session".
This definition assumes that a single audio (or video) stream maps into an RTP session. The RTP RFC [6] defines an RTP session as follows: "For each participant, the session is defined by a particular pair of destination transport addresses (one network address plus a port pair for RTP and RTCP)".
While the previous definitions cover the most common cases, there are situations where a single media instance, (e.g., an audio stream or a video stream) is sent using more than one RTP session. Two examples (among many others) of this kind of situation are cellular systems using SIP [3] and systems receiving DTMF tones on a different host than the voice.
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Systems using a cellular access and SIP as a signalling protocol need to receive media over the air. During a session the media can be encoded using different codecs. The encoded media has to traverse the radio interface. The radio interface is generally characterized by being bit error prone and associated with relatively high packet transfer delays. In addition, radio interface resources in a cellular environment are scarce and thus expensive, which calls for special measures in providing a highly efficient transport. In order to get an appropriate speech quality in combination with an efficient transport, precise knowledge of codec properties are required so that a proper radio bearer for the RTP session can be configured before transferring the media. These radio bearers are dedicated bearers per media type, i.e., codec.
Cellular systems typically configure different radio bearers on different port numbers. Therefore, incoming media has to have different destination port numbers for the different possible codecs in order to be routed properly to the correct radio bearer. Thus, this is an example in which several RTP sessions are used to carry a single media instance (the encoded speech from the sender).
Some voice sessions include DTMF tones. Sometimes the voice handling is performed by a different host than the DTMF handling. It is common to have an application server in the network gathering DTMF tones for the user while the user receives the encoded speech on his user agent. In this situations it is necessary to establish two RTP sessions: one for the voice and the other for the DTMF tones. Both RTP sessions are logically part of the same media instance.
The previous examples show that the definition of a media stream in [5] do not cover some scenarios. It cannot be assumed that a single media instance maps into a single RTP session. Therefore, we introduce the definition of a media flow:
Media flow consists of a single media instance, e.g., an audio stream or a video stream as well as a single whiteboard or shared application group. When using RTP, a media flow comprises one or more RTP sessions.
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Several "m" lines grouped together using FID semantics form a media flow. A media agent handling a media flow that comprises several "m" lines MUST send a copy of the media to every "m" line part of the flow as long as the codecs and the direction attribute present in a particular "m" line allow it.
It is assumed that the application uses only one codec at a time to encode the media produced. This codec MAY change dynamically during the session, but at any particular moment only one codec is in use.
The application encodes the media using the current codec and checks one by one all the "m" lines that are part of the flow. If a particular "m" line contains the codec being used and the direction attribute is "sendonly" or "sendrecv", a copy of the encoded media is sent to the address/port specified in that particular media stream. If either the "m" line does not contain the codec being used or the direction attribute is neither "sendonly" nor "sendrecv", nothing is sent over this media stream.
The application typically ends up sending media to different destinations (IP address/port number) depending on the codec used at any moment.
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The session description below might be sent by a SIP user agent using a cellular access. The user agent supports GSM on port 30000 and AMR on port 30002. When the remote party sends GSM, it will send RTP packets to port number 30000. When AMR is the codec chosen, packets will be sent to port 30002. Note that the remote party can switch between both codecs dynamically in the middle of the session. However, in this example, only one media stream at a time carries voice. The other remains "muted" while its corresponding codec is not in use.
(The linebreak in the fmtp line accommodates RFC formatting restrictions; SDP does not have continuation lines.)
In the previous example, a system receives media on the same IP address on different port numbers. The following example shows how a system can receive different codecs on different IP addresses.
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(The linebreak in the fmtp line accomodates RFC formatting restrictions; SDP does not have continuation lines.)
The cellular terminal of this example only supports the AMR codec. However, many current IP phones only support PCM (payload 0). In order to be able to interoperate with them, the cellular terminal uses a transcoder whose IP address is 131.160.1.111. The cellular terminal includes in its SDP support for PCM at that IP address. Remote systems will send AMR directly to the terminal but PCM will be sent to the transcoder. The transcoder will be configured (using whatever method) to convert the incoming PCM audio to AMR and send it to the terminal.
The next example shows how the "group" attribute used with FID semantics can indicate the use of two different codecs in the two directions of a bidirectional media stream.
A user agent that receives the SDP above knows that at a certain moment it can send either PCM u-law to port number 30000 or PCM A-law to port number 30002. However, the media agent also knows that the other end will only send PCM u-law (payload 0).
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The following example shows a session description with different "m" lines grouped together using FID semantics that contain the same codec.
At a particular point in time, if the media agent is sending PCM u- law (payload 0), it sends RTP packets to 131.160.1.112 on port 30000 and to 131.160.1.111 on port 20000 (first and third "m" lines). If it is sending PCM A-law (payload 8), it sends RTP packets to 131.160.1.112 on port 30002 and to 131.160.1.111 on port 20000 (second and third "m" lines).
The system that generated the SDP above supports PCM u-law on port 30000 and PCM A-law on port 30002. Besides, it uses an application server whose IP address is 131.160.1.111 that records the conversation. That is why the application server always receives a copy of the audio stream regardless of the codec being used at any given moment (it actually performs an RTP dump, so it can effectively receive any codec).
Remember that if several "m" lines grouped together using FID semantics contain the same codec the media agent MUST send media over several RTP sessions at the same time.
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The last example of this section deals with DTMF tones. DTMF tones can be transmitted using a regular voice codec or can be transmitted as telephony events. The RTP payload for DTMF tones treated as telephone events is described in RFC 2833 [7]. Below, there is an example of an SDP session description using FID semantics and this payload type.
The remote party would send PCM encoded voice (payload 0) to 131.160.1.112 and DTMF tones encoded as telephony events to 131.160.1.111. Note that only voice or DTMF is sent at a particular point of time. When DTMF tones are sent, the first media stream does not carry any data and, when voice is sent, there is no data in the second media stream. FID semantics provide different destinations for alternative codecs.
It is worthwhile mentioning some scenarios where the "group" attribute using existing semantics (particularly FID) might seem to be applicable but is not.
FID semantics are useful when the application only uses one codec at a time. An application that encodes the same media using different codecs simultaneously MUST NOT use FID to group those media lines. Some systems that handle DTMF tones are a typical example of parallel encoding using different codecs.
Some systems implement the RTP payload defined in RFC 2833, but when they send DTMF tones they do not mute the voice channel. Therefore, in effect they are sending two copies of the same DTMF tone: encoded as voice and encoded as a telephony event. When the receiver gets both copies, it typically uses the telephony event rather than the tone encoded as voice. FID semantics MUST NOT be used in this context to group both media streams since such a system is not using
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alternative codecs but rather different parallel encodings for the same information.
Layered encoding schemes encode media in different layers. Quality at the receiver varies depending on the number of layers received. SDP provides a means to group together contiguous multicast addresses that transport different layers. The "c" line below:
FID MUST NOT be used to group "m" lines that do not represent the same information. Therefore, FID MUST NOT be used to group "m" lines that contain the different layers of layered encoding scheme. Besides, we do not define new group semantics to provide a more flexible way of grouping different layers because the already existing SDP mechanism covers the most useful scenarios.
If several codecs have to be sent to the same IP address and port, the traditional SDP syntax of listing several codecs in the same "m" line MUST be used. FID MUST NOT be used to group "m" lines with the same IP address/port. Therefore, an SDP like the one below MUST NOT be generated.
SDP descriptions are used by several different protocols, SIP among them. We include a section about SIP because the "group" attribute will most likely be used mainly by SIP systems.
SIP [3] is an application layer protocol for establishing, terminating and modifying multimedia sessions. SIP carries session descriptions in the bodies of the SIP messages but is independent from the protocol used for describing sessions. SDP [2] is one of the protocols that can be used for this purpose.
At session establishment SIP provides a three-way handshake (INVITE- 200 OK-ACK) between end systems. However, just two of these three messages carry SDP, as described in [4].
The "mid" attribute is an identifier for a particular media stream. Therefore, the "mid" value in the offer MUST be the same as the "mid" value in the answer. Besides, subsequent offers (e.g., in a re- INVITE) SHOULD use the same "mid" value for the already existing media streams.
RFC 3264 [4] describes the usage of SDP in relation to SIP. The offerer and the answerer align their media description so that the nth media stream ("m=" line) in the offerer's session description corresponds to the nth media stream in the answerer's description.
The presence of the "group" attribute in an SDP session description does not modify this behavior.
Since the "mid" attribute provides a means to label "m" lines, it would be possible to perform media alignment using "mid" labels rather than matching nth "m" lines. However this would not bring any
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gain and would add complexity to implementations. Therefore SIP systems MUST perform media alignment matching nth lines regardless of the presence of the "group" or "mid" attributes.
If a media stream that contained a particular "mid" identifier in the offer contains a different identifier in the answer the application ignores all the "mid" and "group" lines that might appear in the session description. The following example illustrates this scenario.
Since alignment of "m" lines is performed based on matching of nth lines, the first stream had "mid:1" in the INVITE and "mid:2" in the 200 OK. Therefore, the application MUST ignore every "mid" and "group" lines contained in the SDP.
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A well-behaved SIP user agent would have returned the SDP below in the 200 OK:
A SIP entity that receives an offer that contains an "a=group" line with semantics that it does not understand MUST return an answer without the "group" line. Note that, as it was described in the previous section, the "mid" lines MUST still be present in the answer.
A SIP entity that receives an offer that contains an "a=group" line with semantics that are understood MUST return an answer that contains an "a=group" line with the same semantics. The identification-tags contained in this "a=group" lines MUST be the same that were received in the offer or a subset of them (zero identification-tags is a valid subset). When the identification-tags in the answer are a subset, the "group" value to be used in the session MUST be the one present in the answer.
SIP entities refuse media streams by setting the port to zero in the corresponding "m" line. "a=group" lines MUST NOT contain identification-tags that correspond to "m" lines with port zero.
Note that grouping of m lines MUST always be requested by the offerer, never by the answerer. Since SIP provides a two-way SDP exchange, an answerer that requested grouping would not know whether the "group" attribute was accepted by the offerer or not. An answerer that wants to group media lines SHOULD issue another offer after having responded to the first one (in a re-INVITE for instance).
The example below shows how the callee refuses a media stream offered by the caller by setting its port number to zero. The "mid" value corresponding to that media stream is removed from the "group" value in the answer.
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A client that understands "group" and "mid" but does not want to make use of them in a particular session MAY want to indicate that it supports them. If a client decides to do that, it SHOULD add an "a=group" line with no identification-tags for every semantics it understands.
If a server receives an offer that contains empty "a=group" lines, it SHOULD add its capabilities also in the form of empty "a=group" lines to its answer.
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This document does not define any SIP "Require" header. Therefore, if one of the SIP user agents does not understand the "group" attribute the standard SDP fall back mechanism MUST be used (attributes that are not understood are simply ignored).
This situation does not represent a problem because grouping requests are always performed by offerers, not by answerers. If the offerer does not support "group" this attribute will just not be used.
The answerer will ignore the "group" attribute, since it does not understand it (it will also ignore the "mid" attribute). For LS semantics, the answerer might decide to perform or to not perform synchronization between media streams.
For FID semantics, the answerer will consider that the session comprises several media streams.
Different implementations would behave in different ways.
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In the case of audio and different "m" lines for different codecs an implementation might decide to act as a mixer with the different incoming RTP sessions, which is the correct behavior.
An implementation might also decide to refuse the request (e.g., 488 Not acceptable here or 606 Not Acceptable) because it contains several "m" lines. In this case, the server does not support the type of session that the caller wanted to establish. In case the client is willing to establish a simpler session anyway, he SHOULD re-try the request without "group" attribute and only one "m" line per flow.
Using the "group" parameter with FID semantics, an entity that managed to modify the session descriptions exchanged between the participants to establish a multimedia session could force the participants to send a copy of the media to any particular destination.
Integrity mechanism provided by protocols used to exchange session descriptions and media encryption can be used to prevent this attack.
This document defines two SDP attributes: "mid" and "group".
The "mid" attribute is used to identify media streams within a session description and its format is defined in Section 3.
The "group" attribute is used for grouping together different media streams and its format is defined in Section 4.
This document defines a framework to group media lines in SDP using different semantics. Semantics to be used with this framework are registered by the IANA when they are published in standards track RFCs.
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 an standards track RFC.
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The only entries in the registry for the time being are:
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with the Session Description Protocol (SDP)", RFC 3264, June 2002.
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