Internet Engineering Task Force Gonzalo Camarillo Internet draft Jan Holler Goran AP Eriksson Ericsson Henning Schulzrinne Columbia University September 2001 Expires March 2002 Grouping of media lines in SDP Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document defines two 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), encoded in different formats during a particular session, in different ports and host interfaces. Camarillo/Holler/Eriksson/Schulzrinne 1 Grouping of media lines in SDP TABLE OF CONTENTS 1 Introduction...............................................2 2 Terminology................................................3 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..............................................4 7 Flow Identification (FID)..................................5 7.1 SIP and cellular access....................................5 7.2 DTMF tones.................................................6 7.3 Media flow definition......................................6 7.4 FID semantics..............................................6 7.4.1 Examples of FID............................................6 7.5 Scenarios that FID does not cover..........................9 7.5.1 Parallel encoding using different codecs...................9 7.5.2 Layered encoding..........................................10 7.5.3 Same IP address and port number...........................10 8 Usage of the "group" attribute in SIP.....................11 8.1 Mid value in responses....................................11 8.1.1 Example...................................................12 8.2 Group value in responses..................................12 8.2.1 Example...................................................13 8.3 Capability negotiation....................................14 8.3.1 Example...................................................14 8.4 Backward compatibility....................................14 8.4.1 Client does not support "group"...........................15 8.4.2 Server does not support "group"...........................15 9 IANA considerations.......................................15 10 Acknowledgements..........................................16 11 References................................................16 12 Authors³ Addresses........................................16 1 Introduction An SDP session description typically contains a number (one or more) of 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 allowed to express how different media streams within a session description relate to each other. This document defines such extensions. Camarillo/Holler/Eriksson/Schulzrinne 2 Grouping of media lines in SDP 2 Terminology 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 RFC 2119 [1] and indicate requirement levels for compliant implementations. 3. Media stream identification attribute 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: mid-attribute = "a=mid:" identification-tag identification-tag = token The identification tag MUST be unique within an SDP session description. 4. Group attribute 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: group-attribute = "a=group:" semantics *(space identification-tag) semantics = "LS" | "FID" This document defines two standard semantics: LS (Lip Synchronization) and FID (Flow Identification). If in the future it was needed to standardize further semantics they would 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. 5. Use of "group" and "mid" All the "m" lines of a session description that uses "group" MUST be identified with an "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 simply 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. Camarillo/Holler/Eriksson/Schulzrinne 3 Grouping of media lines in SDP 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 wants to be compliant to this specification MUST support both "group" and "mid". An application that supported just one of them would not be compliant. 6. Lip Synchronization (LS) 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 perfectly 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. 6.1 Example of LS 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. v=0 o=Laura 289083124 289083124 IN IP4 one.example.com t=0 0 c=IN IP4 224.2.17.12/127 a=group:LS 1 2 m=audio 30000 RTP/AVP 0 a=mid:1 m=video 30002 RTP/AVP 31 a=mid:2 m=audio 30004 RTP/AVP 0 i=This media stream contains the Spanish translation a=mid:3 Camarillo/Holler/Eriksson/Schulzrinne 4 Grouping of media lines in SDP Note that although the third media stream is not present in the group line it still MUST contain an mid attribute (mid:3), as stated before. 7. Flow Identification (FID) 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 [3] 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 [4] 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 [5] and systems receiving DTMF tones on a different host than the voice. 7.1 SIP and cellular access 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). Camarillo/Holler/Eriksson/Schulzrinne 5 Grouping of media lines in SDP 7.2 DTMF tones 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. 7.3 Media flow definition The previous examples show that the definition of a media stream in [3] 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. 7.4 FID semantics 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 certain 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. 7.4.1 Examples of FID The session description below would be the SDP 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, Camarillo/Holler/Eriksson/Schulzrinne 6 Grouping of media lines in SDP 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. v=0 o=Laura 289083124 289083124 IN IP4 two.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 m=audio 30000 RTP/AVP 3 a=rtpmap:3 GSM/8000 a=mid:1 m=audio 30002 RTP/AVP 97 a=rtpmap:97 AMR/8000 a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change- neighbor; maxframes=1 a=mid:2 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. v=0 o=Laura 289083124 289083124 IN IP4 three.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 m=audio 20000 RTP/AVP 0 c=IN IP4 131.160.1.111 a=rtpmap:0 PCMU/8000 a=mid:1 m=audio 30002 RTP/AVP 97 a=rtpmap:97 AMR/8000 a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change- neighbor; maxframes=1 a=mid:2 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 that the "group" attribute used with FID semantics allows to express uni-directional codecs for a bi- directional media flow. That is, a codec that is only used in one direction within a sendrecv media stream. Camarillo/Holler/Eriksson/Schulzrinne 7 Grouping of media lines in SDP v=0 o=Laura 289083124 289083124 IN IP4 four.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 m=audio 30000 RTP/AVP 0 a=mid:1 m=audio 30002 RTP/AVP 8 a=recvonly a=mid:2 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). The following example shows a session description with different "m" lines grouped together using FID semantics that contain the same codec. v=0 o=Laura 289083124 289083124 IN IP4 five.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 3 m=audio 30000 RTP/AVP 0 a=mid:1 m=audio 30002 RTP/AVP 8 a=mid:2 m=audio 20000 RTP/AVP 0 8 c=IN IP4 131.160.1.111 a=recvonly a=mid:3 At a particular point of 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 all 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). Camarillo/Holler/Eriksson/Schulzrinne 8 Grouping of media lines in SDP 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. 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 [6]. Below there is an example of an SDP session description using FID semantics and this payload type. v=0 o=Laura 289083124 289083124 IN IP4 six.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 m=audio 30000 RTP/AVP 0 a=mid:1 m=audio 20000 RTP/AVP 97 c=IN IP4 131.160.1.111 a=rtpmap:97 telephone-events a=mid:2 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. 7.5 Scenarios that FID does not cover It is worthwhile mentioning some scenarios where the "group" attribute using existing semantics (particularly FID) might seem to be applicable but it is not. This section has been included because we have observed some confusion within the community regarding the three scenarios described below. This section helps clarify them. 7.5.1 Parallel encoding using different codecs 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, effectively 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 Camarillo/Holler/Eriksson/Schulzrinne 9 Grouping of media lines in SDP context to group both media streams since such a system is not using alternative codecs but rather different parallel encodings for the same information. 7.5.2 Layered encoding 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: c=IN IP4 224.2.1.1/127/3 is equivalent to the following three "c" lines: c=IN IP4 224.2.1.1/127 c=IN IP4 224.2.1.2/127 c=IN IP4 224.2.1.3/127 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. 7.5.3 Same IP address and port number 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. v=0 o=Laura 289083124 289083124 IN IP4 six.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 m=audio 30000 RTP/AVP 0 a=mid:1 m=audio 30000 RTP/AVP 8 a=mid:2 The correct SDP for the session above would be the following one: v=0 o=Laura 289083124 289083124 IN IP4 six.example.com t=0 0 c=IN IP4 131.160.1.112 m=audio 30000 RTP/AVP 0 8 Camarillo/Holler/Eriksson/Schulzrinne 10 Grouping of media lines in SDP If two "m" lines are grouped using FID they MUST differ in their transport addresses (i.e., IP address plus port). 8. Usage of the "group" attribute in SIP 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 [5] 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. SDPs MAY be present in INVITE and 200 OK or in 200 OK and ACK. The following sections assume that INVITE and 200 OK are the ones carrying SDP for the sake of clarity, but everything is also applicable to the other possible scenario (200 OK and ACK). 8.1 Mid value in responses The "mid" attribute is an identifier for a particular media stream. Therefore, the "mid" value in the response MUST be the same as the "mid" value in the request. Besides, subsequent requests such as re- INVITEs SHOULD use the same "mid" value for the already existing media streams. Appendix B of [5] describes the usage of SDP in relation to SIP. It states: "The caller and callee align their media description so that the nth media stream ("m=" line) in the caller³s session description corresponds to the nth media stream in the callee³s 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 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 request contains a different identifier in the response the application ignores all the "mid" and "group" lines that might appear in the session description. The following example illustrates this scenario: Camarillo/Holler/Eriksson/Schulzrinne 11 Grouping of media lines in SDP 8.1.1 Example Two SIP entities exchange SDPs during session establishment. The INVITE contained the SDP below: v=0 o=Laura 289083124 289083124 IN IP4 seven.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 m=audio 30000 RTP/AVP 0 8 a=mid:1 m=audio 30002 RTP/AVP 0 8 a=mid:2 The 200 OK response contains the following SDP: v=0 o=Bob 289083122 289083122 IN IP4 eigth.example.com t=0 0 c=IN IP4 131.160.1.113 a=group:FID 1 2 m=audio 25000 RTP/AVP 0 8 a=mid:2 m=audio 25002 RTP/AVP 0 8 a=mid:1 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. A well-behaved SIP user agent would have returned the SDP below in the 200 OK: v=0 o=Bob 289083122 289083122 IN IP4 nine.example.com t=0 0 c=IN IP4 131.160.1.113 a=group:FID 1 2 m=audio 25002 RTP/AVP 0 8 a=mid:1 m=audio 25000 RTP/AVP 0 8 a=mid:2 8.2 Group value in responses A SIP entity that receives a request that contains an "a=group" line with semantics that it does not understand MUST return a response without the "group" line. Note that, as it was described in the previous section, the "mid" lines MUST still be present in the response. Camarillo/Holler/Eriksson/Schulzrinne 12 Grouping of media lines in SDP A SIP entity that receives a request that contains an "a=group" line which semantics that are understood MUST return a response 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 request or a subset of them (zero identification-tags is a valid subset). When the identification-tags in the response are a subset the "group" value to be used in the session MUST be the one present in the response. 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 issuer of the request (the client), never by the issuer of the response (the server). Since SIP provides a two-way SDP exchange, a server that requested grouping in a response would not know whether the "group" attribute was accepted by the client or not. A server that wants to group media lines SHOULD issue another request after having responded to the first one (a re-INVITE for instance). Note that, as we mentioned previously, in this section we are assuming that the SDPs are present in the INVITE and in the 200 OK. Applying the statement above to the scenario where SDPs are present in the 200 OK and in the ACK, the entity requesting grouping would be the server. 8.2.1 Example 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 response. SDP in the INVITE from caller to callee: v=0 o=Laura 289083124 289083124 IN IP4 ten.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID 1 2 3 m=audio 30000 RTP/AVP 0 a=mid:1 m=audio 30002 RTP/AVP 8 a=mid:2 m=audio 30004 RTP/AVP 3 a=mid:3 SDP in the INVITE from callee to caller: v=0 o=Bob 289083125 289083125 IN IP4 eleven.example.com Camarillo/Holler/Eriksson/Schulzrinne 13 Grouping of media lines in SDP t=0 0 c=IN IP4 131.160.1.113 a=group:FID 1 3 m=audio 20000 RTP/AVP 0 a=mid:1 m=audio 0 RTP/AVP 8 a=mid:2 m=audio 20002 RTP/AVP 3 a=mid:3 8.3 Capability negotiation A client that understands "group" and "mid" but does not want to make use of them in a particular session MAY want indicate that it supports them. If a client decides to do that, it SHOULD add an "a=group" line with zero identification-tags for every semantics it understands. If a server receives a request that contains empty "a=group" lines it SHOULD add its capabilities also in the form of empty "a=group" lines to its response. 8.3.1 Example A system that supports both LS and FID semantics but does not want to group any media stream for this particular session generates the following SDP: v=0 o=Bob 289083125 289083125 IN IP4 twelve.example.com t=0 0 c=IN IP4 131.160.1.113 a=group:LS a=group:FID m=audio 20000 RTP/AVP 0 8 The server that receives that request supports FID but not LS. It responds with the SDP below: v=0 o=Laura 289083124 289083124 IN IP4 thirteen.example.com t=0 0 c=IN IP4 131.160.1.112 a=group:FID m=audio 30000 RTP/AVP 0 8.4 Backward compatibility This document does not define any SIP "Require" header. Therefore, if one of the SIP user agents does not understand the "group" Camarillo/Holler/Eriksson/Schulzrinne 14 Grouping of media lines in SDP attribute the standard SDP fall back mechanism MUST be used (attributes that are not understood are simply ignored). 8.4.1 Client does not support "group" This situation does not represent a problem because grouping requests is always performed by clients, not by servers. If the client does not support "group" this attribute will just not be used. 8.4.2 Server does not support "group" The server will ignore the "group" attribute, since it does not understand it (it will also ignore the "mid" attribute). For LS semantics, the server might decide to perform or to not perform synchronization between media streams. For FID semantics, the server will consider that the session comprises several media streams. Different implementations would behave in different ways. 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. 9. IANA considerations 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. Section 4 also defines two standard semantics related to the "group" attribute: LS (Lip Synchronization) and FID (Flow Identification). If in the future it was needed to standardize further semantics they would need to be defined in a standards track document. Camarillo/Holler/Eriksson/Schulzrinne 15 Grouping of media lines in SDP 10. Acknowledgments The authors would like to thank Jonathan Rosenberg, Adam Roach, Orit Levin and Joerg Ott for their feedback on this document. 11. References [1] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, IETF; March 1997. [2] M. Handley/V. Jacobson, "SDP: Session Description Protocol", RFC 2327, IETF; April 1998. [3] H. Schulzrinne/A. Rao/R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, IETF; April 1998. [4] H. Schulzrinne/S. Casner/R. Frederick/V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, IETF; January 1996. [5] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, IETF; Mach 1999. [6] H. Schulzrinne/S. Petrack, "RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals", RFC 2833, IETF; May 2000. 12. Authors³ Addresses Gonzalo Camarillo Ericsson Advanced Signalling Research Lab. FIN-02420 Jorvas Finland Phone: +358 9 299 3371 Fax: +358 9 299 3052 Email: Gonzalo.Camarillo@ericsson.com Jan Holler Ericsson Research S-16480 Stockholm Sweden Phone: +46 8 58532845 Fax: +46 8 4047020 Email: Jan.Holler@era.ericsson.se Goran AP Eriksson Ericsson Research S-16480 Stockholm Sweden Phone: +46 8 58531762 Fax: +46 8 4047020 Email: Goran.AP.Eriksson@era.ericsson.se Camarillo/Holler/Eriksson/Schulzrinne 16 Grouping of media lines in SDP Henning Schulzrinne Dept. of Computer Science Columbia University 1214 Amsterdam Avenue New York, NY 10027 USA Email: schulzrinne@cs.columbia.edu Camarillo/Holler/Eriksson/Schulzrinne 17