MMUSIC Working Group G. Camarillo Internet-Draft Ericsson Obsoletes: 3388 (if approved)
July 7, 2008January 13, 2009 Intended status: Standards Track Expires: January 8,July 17, 2009 The SDP (Session Description Protocol) Grouping Framework draft-ietf-mmusic-rfc3388bis-01.txtdraft-ietf-mmusic-rfc3388bis-02.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or sheThis Internet-Draft is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed,submitted to IETF in accordancefull conformance with Section 6the provisions of BCP 78 and BCP 79. 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. This Internet-Draft will expire on January 8,July 17, 2009. Copyright Notice Copyright (C) The(c) 2009 IETF Trust (2008).and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract In this specification, we define a framework to group "m" lines in SDP (Session Description Protocol) for different purposes. This framework uses the "group" and "mid" SDP attributes, both of which are defined in this specification. Additionally, we specify how to use the framework for two different purposes: for lip synchronization and for receiving a media flow consisting of several media streams on different transport addresses. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 3 4. Media Stream Identification Attribute . . . . . . . . . . . . 4 5. Group Attribute . . . . . . . . . . . . . . . . . . . . . . . 4 6. Use of "group" and "mid" . . . . . . . . . . . . . . . . . . . 4 7. Lip Synchronization (LS) . . . . . . . . . . . . . . . . . . . 5 7.1. Example of LS . . . . . . . . . . . . . . . . . . . . . . 5 8. Flow Identification (FID) . . . . . . . . . . . . . . . . . . 6 8.1. SIP and Cellular Access . . . . . . . . . . . . . . . . . 6 8.2. DTMF Tones . . . . . . . . . . . . . . . . . . . . . . . . 7 8.3. Media Flow Definition . . . . . . . . . . . . . . . . . . 7 8.4. FID Semantics . . . . . . . . . . . . . . . . . . . . . . 7 8.4.1. Examples of FID . . . . . . . . . . . . . . . . . . . 8 8.5. Scenarios that FID does not Cover . . . . . . . . . . . . 11 8.5.1. Parallel Encoding Using Different Codecs . . . . . . . 11 8.5.2. Layered Encoding . . . . . . . . . . . . . . . . . . . 11 8.5.3. Same IP Address and Port Number . . . . . . . . . . . 12 9. Usage of the "group" Attribute in SIP . . . . . . . . . . . . 13 9.1. Mid Value in Answers . . . . . . . . . . . . . . . . . . . 13 9.1.1. Example . . . . . . . . . . . . . . . . . . . . . . . 14 9.2. Group Value in Answers . . . . . . . . . . . . . . . . . . 15 9.2.1. Example . . . . . . . . . . . . . . . . . . . . . . . 15 9.3. Capability Negotiation . . . . . . . . . . . . . . . . . . 16 9.3.1. Example . . . . . . . . . . . . . . . . . . . . . . . 16 9.4. Backward Compatibility . . . . . . . . . . . . . . . . . . 17 9.4.1. Offerer does not Support "group" . . . . . . . . . . . 17 9.4.2. Answerer does not Support "group" . . . . . . . . . . 17 10. Changes from RFC 3388 . . . . . . . . . . . . . . . . . . . . 18 11. Security Considerations . . . . . . . . . . . . . . . . . . . 18 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 13.1. Normative References . . . . . . . . . . . . . . . . . . . 19 13.2. Informational References . . . . . . . . . . . . . . . . . 19 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . . . 211. Introduction An SDP [RFC4566] session description typically contains one or more media lines, which 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 is necessary to have a mechanism in SDP to express how different media streams within a session description relate to each other. The framework defined in this specification is such a mechanism. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Overview of Operation This section provides a non-normative description on how the SDP Grouping Framework defined in this document works. In a given session description, each "m" line is identified by a token, which is carried in an "mid" attribute below the "m" line. The session description carries session-level "group" attributes that group different "m" lines (identified by their tokens) using different group semantics. The semantics of a group describe the purpose for which the "m" lines are grouped. For example, the "group" line in the session description below indicates that the "m" lines identified by tokens 1 and 2 (the audio and the video "m" lines respectively) and group for the purpose of lip synchronization (LS). v=0 o=Laura 289083124 289083124 IN IP4 one.example.com t=0 0 c=IN IP4 192.0.2.1 a=group:LS 1 2 m=audio 30000 RTP/AVP 0 a=mid:1 m=video 30002 RTP/AVP 31 a=mid:2 4. 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 [RFC4566] is described by the following BNF (Backus-Naur Form): mid-attribute = "a=mid:" identification-tag identification-tag = token The identification tag MUST be unique within an SDP session description. 5. 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" / semantics-extension semantics-extension = token This document defines two standard semantics: LS (Lip Synchronization) and FID (Flow Identification). Further semantics MUST be defined in a standards-track document. 6. Use of "group" and "mid" 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. 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. 7. 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 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. 7.1. 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 have to be synchronized. v=0 o=Laura 289083124 289083124 IN IP4 one.example.com t=0 0 c=IN IP4 184.108.40.206/127 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 Note that although the third media stream is not present in the group line, it still has to contain a mid attribute (mid:3), as stated before. 8. 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 [RFC2326] 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 [RFC1889] (at present obsoleted by [RFC3550]) used to define 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 (Session Initiation Protocol; [RFC3261]) and systems receiving DTMF (Dual-Tone Multi-Frequency) tones on a different host than the voice. 8.1. 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). 8.2. 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. 8.3. Media Flow Definition The previous examples show that the definition of a media stream in [RFC2326] 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. 8.4. 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 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. 8.4.1. Examples of FID The session description below might be sent by a SIP user agent using a cellular access. The user agent supports GSM (Global System for Mobile communications) on port 30000 and AMR (Adaptive Multi-Rate) 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. v=0 o=Laura 289083124 289083124 IN IP4 two.example.com t=0 0 c=IN IP4 192.0.2.1 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 (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. v=0 o=Laura 289083124 289083124 IN IP4 three.example.com t=0 0 c=IN IP4 192.0.2.1 a=group:FID 1 2 m=audio 20000 RTP/AVP 0 c=IN IP4 192.0.2.2 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 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 (Pulse-Code Modulation; payload 0). In order to be able to interoperate with them, the cellular terminal uses a transcoder whose IP address is 192.0.2.2. 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. v=0 o=Laura 289083124 289083124 IN IP4 four.example.com t=0 0 c=IN IP4 192.0.2.1 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 192.0.2.1 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 192.0.2.2 a=recvonly a=mid:3 At a particular point in time, if the media agent is sending PCM u- law (payload 0), it sends RTP packets to 192.0.2.1 on port 30000 and to 192.0.2.2 on port 20000 (first and third "m" lines). If it is sending PCM A-law (payload 8), it sends RTP packets to 192.0.2.1 on port 30002 and to 192.0.2.2 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 192.0.2.2 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. 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 [RFC2833]. 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 192.0.2.1 a=group:FID 1 2 m=audio 30000 RTP/AVP 0 a=mid:1 m=audio 20000 RTP/AVP 97 c=IN IP4 192.0.2.2 a=rtpmap:97 telephone-events a=mid:2 The remote party would send PCM encoded voice (payload 0) to 192.0.2.1 and DTMF tones encoded as telephony events to 192.0.2.2. 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. 8.5. 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 is not. 8.5.1. 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, 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 alternative codecs but rather different parallel encodings for the same information. 8.5.2. 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 220.127.116.11/127/3 is equivalent to the following three "c" lines: c=IN IP4 18.104.22.168/127 c=IN IP4 22.214.171.124/127 c=IN IP4 126.96.36.199/127 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. 8.5.3. 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 192.0.2.1 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 192.0.2.1 m=audio 30000 RTP/AVP 0 8 If two "m" lines are grouped using FID they MUST differ in their transport addresses (i.e., IP address plus port). 9. 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 [RFC3261] 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 [RFC4566] 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 [RFC3264]. 9.1. Mid Value in Answers 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. [RFC3264] 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 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. 9.1.1. Example Two SIP entities exchange SDPs during session establishment. The INVITE contains the SDP below: v=0 o=Laura 289083124 289083124 IN IP4 seven.example.com t=0 0 c=IN IP4 192.0.2.1 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 192.0.2.3 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 192.0.2.3 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 9.2. Group Value in Answers 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). 9.2.1. 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 answer. 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 192.0.2.1 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 t=0 0 c=IN IP4 192.0.2.3 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 9.3. Capability Negotiation A client that understands "group" and "mid" but does not want to make use of them in a particular session MAY want 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. 9.3.1. 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 192.0.2.3 a=group:LS a=group:FID m=audio 20000 RTP/AVP 0 8 The server that receives that offer 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 192.0.2.1 a=group:FID m=audio 30000 RTP/AVP 0 9.4. Backward Compatibility This document does not define any SIP "Require" header. Therefore, if one of the SIP user agents does not understand the "group" attribute the standard SDP fall back mechanism MUST be used (attributes that are not understood are simply ignored). 9.4.1. Offerer does not Support "group" 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. 9.4.2. Answerer does not Support "group" 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. 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. 10. Changes from RFC 3388 The grouping mechanism is now defined as an extendible framework. Earlier, [RFC3388] used to discourage extensions to this mechanism in favor of using new session description protocols. Given a semantics value, [RFC3388] used to restrict "m" line identifiers to only appear in a single group using that semantics. That restriction has been lifted. From conversations with implementers, it seems that the lifting of this restriction is unlikely to cause backwards compatibility problems. 11. Security Considerations 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. In SIP, S/MIME [RFC3850] and TLS [RFC4346] can be used to protect session description exchanges in an end-to-end and a hop-by-hop fashion respectively. 12. IANA Considerations [RFC3388] already registered the "mid" and "group" SDP attributes with the IANA and created a registry for semantics. This document does not contain any additional actions for the IANA. 13. References 13.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC3261] 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. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3850] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Certificate Handling", RFC 3850, July 2004. [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. 13.2. Informational References [RFC1889] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. [RFC2833] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals", RFC 2833, May 2000. [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. Author's Address Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: Gonzalo.Camarillo@ericsson.com Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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