Internet Engineering Task Force Gonzalo Camarillo Internet draft Jan Holler Goran AP Eriksson Ericsson April 2001 Expires October 2001 The SDP fid attribute 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 an SDP media attribute. The use of this attribute allows 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 1 The SDP fid attribute TABLE OF CONTENTS 1 Motivation...................................................2 1.1 SIP and cellular access......................................2 1.2 DTMF tones...................................................3 2 Media flow definition........................................3 3 Flow identification attribute................................3 4 Semantics of the fid attribute...............................4 4.1 Interactions with other media level attributes...............4 5 Usage of the fid attribute in SIP............................5 5.1 Backward compatibility.......................................5 5.2 Caller does not support fid..................................5 5.3 Callee does not support fid..................................5 6 Acknoledgements..............................................6 7 References...................................................6 8 Authors³ Addresses...........................................6 1. Motivation The RTSP RFC [1] 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 [2] 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)". However, 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. 1.1 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 [4]. 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 Camarillo/Holler/Eriksson 2 The SDP fid attribute 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). 1.2 DTMF tones Some voice sessions include DTMF tones. Sometimes the voice handling is performed by a different host than the DTMF handling. [5] contains several examples of how application servers in the network gather 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. 2. Media flow definition The previous examples show that the definition of a media stream in [1] has to be updated. 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. For instance, in a two party call where the voice exchanged can be encoded using GSM or PCM, the receiver wants to receive GSM on a port number and PCM on a different port number. Two RTP sessions will be established, one carrying GSM and the other carrying PCM. At any particular moment just one codec is in use. Therefore, at any moment one of the RTP sessions will not transport any voice. Here the systems are dealing with a single media flow, but two RTP sessions. 3. Flow identification attribute An RTP session is described in SDP [6] using an "m" line. When a media flow comprises more than one RTP session, we need a way to associate several "m" lines together into a media flow. A new "flow identification" media attribute is defined. It is used for identifying media flows within a session. Its formatting in SDP is described by the following BNF: Camarillo/Holler/Eriksson 3 The SDP fid attribute fid-attribute = "a=fid:" identification-tag identification-tag = token The identification tag is unique within the SDP session description. Syntactically fid is a media-level attribute. It provides information about a media stream defined by an "m" line. Semantically fid would be defined as a session-level attribute since it provides flow hierarchy inside a session description. 4. Semantics of the fid attribute A media agent handling a media flow that comprises several "m" lines sends media to different destinations (IP address/port number) depending on the codec used at any moment. If several "m" lines contain the codec used media is sent to different destinations in parallel. For instance, a SIP user agent receives an INVITE with the following body: v=0 o=Laura 289083124 289083124 IN IP4 second.example.com t=0 0 c=IN IP4 131.160.1.112 m=audio 30000 RTP/AVP 0 a=fid:1 m=audio 30002 RTP/AVP 8 a=fid:1 m=audio 30004 RTP/AVP 0 8 a=fid:1 At a particular point of time, if the media agent is sending PCM u- law (payload 0) it sends RTP packets to ports 30000 and 30004 (first and third "m" lines). If it is sending PCM A-law (payload 8) it sends RTP packets to ports 30002 and 30004 (second and third "m" lines). Note that if several "m" lines with the same fid value contain the same codec the media agent MUST send media over several RTP sessions at the same time. 4.1 Interactions with other media level attributes Media level attributes affect a media stream defined by an "m" line. The presence of fid does not modify this behavior. For instance, a SIP user agent receives an INVITE with the following body: v=0 o=Laura 289083124 289083124 IN IP4 second.example.com t=0 0 Camarillo/Holler/Eriksson 4 The SDP fid attribute c=IN IP4 131.160.1.112 m=audio 30000 RTP/AVP 0 a=fid:1 m=audio 30002 RTP/AVP 8 a=recvonly a=fid:1 The media agent 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). Note that the fid attribute allows to express uni-directional codecs for a bi-directional media flow, as it is shown in the example above. 5. Usage of the fid attribute in SIP 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 [6] is one of the protocols that can be used for this purpose. Appendix B of [3] 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 fid attribute in an SDP session description does not modify this behavior. 5.1 Backward compatibility This document does not define any SIP "Require" header. Therefore, if one of the SIP user agents does not understand the fid attribute the standard SDP fall back mechanism is used. A system that understands the fid attribute MUST add it to any SDP session description that it generates. 5.2 Caller does not support fid This situation does not represent a problem. The SDP in the INVITE will not contain any fid attribute. The callee knows that the caller does not support fid. 5.3 Callee does not support fid The callee will ignore the fid attribute, since it does not understand it. It will consider that the session comprises several media streams. Camarillo/Holler/Eriksson 5 The SDP fid attribute 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 callee does not support the type of session that the caller wanted to establish. In case the caller is willing to establish a simpler session anyway, he should re-try the request without the fid attribute and only one "m" line per flow. 6. Acknowledgments The authors would like to thank Jonathan Rosenberg and Adam Roach for their feedback on this document. 7. References [1] H. Schulzrinne/A. Rao/R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, IETF; April 1998. [2] H. Schulzrinne/S. Casner/R. Frederick/V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, IETF; January 1996. [3] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, IETF; Mach 1999. [4] L. Westberg/M. Lindqvist, "Realtime Traffic over Cellular Access Networks", draft-westberg-realtime-cellular-03.txt, IETF; November 2000. Work in progress. [5] J. Rosemberg/P.Mataga/H.Schulzrinne, "An Applcation Server Component Architecture for SIP", draft-rosenberg-sip-app-components- 00.txt, IETF; November 2000. Work in progress. [6] M. Handley/V. Jacobson, "SDP: Session Description Protocol", RFC 2327, IETF; April 1998. 8. 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 Camarillo/Holler/Eriksson 6 The SDP fid attribute 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 7