Internet Engineering Task Force Gonzalo Camarillo Internet draft Jan H÷ller G÷ran AP Eriksson Ericsson June 2000 Expires March 2000 SDP media alignment in SIP 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. This attribute is intended to be used in conjunction with SIP in order to align different media streams belonging to a session. The use of this attribute allows sending media from a single media stream, encoded in different formats during the session, to different ports and host interfaces. 1. Introduction SIP [1] 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. Appendix B of [1] 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 callerĘs session description corresponds to the nth media stream in the calleeĘs description." Camarillo/H÷ller/Eriksson 1 SDP media alignment in SIP This way of performing the media alignment implies that all the media from one single media stream has to be sent to the same port number. This does not suit fine systems that handle different codecs in different port numbers. Nor does it suit hosts supporting multiple interfaces that handle different codecs on different interface types. 2. SIP and cellular access Systems using a cellular access (such as UMTS or EDGE) 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 [3]. 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. In UMTS, for instance, when the RTP packets shall be delivered over the air interface, a packet filtering function routes the packets to the proper radio bearer towards the UMTS/SIP terminal. The packet filtering function operates using a Traffic Flow Template (TFT) [4], which is established when configuring the radio bearer. The TFT hence specifies the profile of the data that should be carried by the radio bearer. A TFT can contain the following data: -Source Address and Subnet Mask. -Protocol Number (IPv4) / Next Header (IPv6). -Destination Port Range. -Source Port Range. -IPSec Security Parameter Index (SPI). -Type of Service (TOS) (IPv4) / Traffic class (IPv6) and Mask. -Flow Label (IPv6). It is worth noticing that just certain combinations of these parameters are allowed. The media has to have different destination port numbers for the different possible codecs in order to be filtered and routed properly to the correct radio bearer. Examining the payload of every packet entering the UMTS network would require too much processing power. It would not a viable solution. Demultiplexing of media streams should be done at the port level. So, when a session is negotiated using SIP, a means for providing different port numbers for different codecs is needed. This procedure has to be backwards compatible with appendix B of [1]. Camarillo/H÷ller/Eriksson 2 SDP media alignment in SIP Systems using certain architectures have also this requirement in order to be able to use different audio libraries. Discussions about this can be found in [5]. 3. SIP, SDP and multiple host interfaces Two typical scenarios apply where a single media stream may be received or sent on different host interfaces. The first is when the cellular device is equipped with several access technologies. The second is the case where media format transcoding may be needed. Cellular terminals are traditionally equipped with only one access type, the cellular access. Cellular terminals may also be further equipped with other higher speed wireless access technologies of short range nature such as Wireless LAN and Bluetooth. Multiple accesses on the terminal implies that multiple IP addresses will be in use for a single terminal. Applications based on SIP may rely on the fact that these additional interfaces may also be available for use, but of a more sporadic nature. The availability and applicability may for instance be based on coverage or simply by the userĘs preferences. In a session invitation, the caller (on a mobile SIP terminal) may be willing to receive a single media stream that could be delivered on one of the available interfaces. A typical case could be where a stream of high quality audio can be supported over a high bit rate interface such as WLAN, and a low rate codec is used over the cellular access. Depending on the particular situation, the caller may offer both codec types, whereas the callee may select only one of the codec types, e.g. depending on what he supports. If the caller moves out of coverage of the high bit rate access, an ordinary re-invitation may move the media stream to be transported over the low bit rate cellular access. The second scenario concerns the need for a transcoding service. Speech codecs have been developed basically to be robust against the high bit error and frame error rate associated with wireless accesses. One such codec is GSM. Other encoding formats developed for wired accesses with negligible bit errors do not operate properly over a lossy link. As fixed terminals may not support the wireless codecs, while cellular terminals can generally not support wired codecs, there is a need for a transcoding function between the two. The transcoding function is typically placed in another host connected to the internet. This host is placed on the other side of the cellular access as viewed from the cellular terminal. Camarillo/H÷ller/Eriksson 3 SDP media alignment in SIP To make use of the transcoding function, the cellular terminal may send an INVITE with an SDP part displaying support for the cellular codec as well as support for the codecs supported by the transcoder. In this situation, there is one media stream that depending on which codec is selected, should be directed to different host interfaces. The callee then makes a selection of codec and sends the media to the appropriate host interface. The transcoder will also be configured to forward the transcoded media stream to the cellular terminal. 4. Alignment of SDP in SIP to support multiple port numbers A new "flow identification" media attribute is defined. It is used for identifying media streams within a session. It provides a means for aligning a number of flows within a session between members participating in the session. Its formatting in SDP is described by the following BNF: fid-attribute = "a=fid:" identification-tag identification-tag = token The identification tag is unique within the SDP session description. The following example illustrates its usage. 5. Example of flow identification attribute In the following example John uses a traditional access such as an ethernet while Laura has a UMTS/SIP terminal. The caller John sends the following session description to the callee Laura. v=0 o=John 289085535 289085535 IN IP4 first.example.com t=0 0 c=IN IP4 111.111.111.111 m=audio 20000 RTP/AVP 0 8 a=fid:1 m=audio 20002 RTP/AVP 0 8 a=fid:2 The callee Laura is on a UMTS/SIP terminal. She configures the necessary radio bearers and implements the TFTs: For the first media stream with flow identification equal to 1 (fid:1): All the incoming IP packets with destination port UDP 30000 will be carried by the radio access bearer configured for G-711 u-law (payload type 0). All the incoming IP packets with destination port UDP 30002 will be carried by the radio access bearer configured for G-711 A-law (payload type 8). Camarillo/H÷ller/Eriksson 4 SDP media alignment in SIP For the second media stream with flow identification equal to 2 (fid:2): All the incoming IP packets with destination port UDP 30004 will be carried by the radio access bearer configured for G-711 u-law (payload type 0). All the incoming IP packets with destination port UDP 30006 will be carried by the radio access bearer configured for G-711 A-law (payload type 8). Accordingly, the following SDP is returned to the caller: v=0 o=Laura 289083124 289083124 IN IP4 second.example.com t=0 0 c=IN IP4 222.222.222.222 m=audio 30000 RTP/AVP 0 a=fid:1 m=audio 30002 RTP/AVP 8 a=fid:1 m=audio 30004 RTP/AVP 0 a=fid:2 m=audio 30006 RTP/AVP 8 a=fid:2 In another example, the callee Laura might want to receive single flows in different IP addresses depending on the codec. It would return an SDP description as follows. v=0 o=Laura 289083124 289083124 IN IP4 second.example.com t=0 0 c=IN IP4 222.222.222.222 m=audio 30000 RTP/AVP 0 a=fid:1 m=audio 30002 RTP/AVP 8 c=IN IP4 123.123.123.123 a=fid:1 m=audio 30004 RTP/AVP 0 a=fid:2 m=audio 30006 RTP/AVP 8 c=IN IP4 123.123.123.123 a=fid:2 6. Media-level versus session-level attribute 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. Camarillo/H÷ller/Eriksson 5 SDP media alignment in SIP 7. Backward compatibility Upon the reception of an SDP description with the fid attribute, the callee must add also this attribute to the response. If the callee does not understand the fid attribute it will not include fid in the response. In this case the callee will proceed as it is described in appendix B of [1]. Thus, a flow will be created for every "m" line in the session description. The caller will have to issue a re- INVITE in order to remove the undesirable media streams. RFC2543bis prevents the caller to send an updated session description in an ACK request since a complete session description was sent already in the INVITE request. Until the re-INVITE is issued, the communication between the terminals is possible because they have enough information for receiving and sending media. It would be possible to use different approaches that would resolve these issues (demultiplexing of media stream on port level) and provide better backward compatibility. One solution might be to add additional codecs supported in different IP addresses or port number in a new a attribute. This way, a callee that did not understand the attribute would ignore it but still create the correct number of media streams. v=0 o=Laura 289083124 289083124 IN IP4 second.example.com t=0 0 c=IN IP4 222.222.222.222 m=audio 30000 RTP/AVP 0 a=addcodec:8 222.222.222.222 30002 m=audio 30004 RTP/AVP 0 a=addcodec:8 222.222.222.222 30006 However, we believe that the fid attribute provides a more general mechanism to bundle different media lines into a single flow. This mechanism can be used by other applications different than the one described in this draft to overcome different problems. We have chosen generality over a specific solution. The backward compatibility problem might not exist in a future depending on the format chosen for SDPng. This draft is also intended to serve as input for the SDPng work. Not on the syntax level but on the semantics of the fid attribute. 8. Open issues How should RTCP be handled? 9. Acronyms BNF Backus-Naur Form EDGE Enhanced Data rates for GSM and TDMA/136 Evolution GSM Global System for Mobile communication Camarillo/H÷ller/Eriksson 6 SDP media alignment in SIP IP Internet Protocol SDP Session Description Protocol SIP Session Initiation Protocol TFT Traffic Flow Template UMTS Universal Mobile Telecommunication System WLAN Wireless Local Area Network 10. References [1] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, IETF; Mach 1999. [2] M. Handley/V. Jacobson, "SDP: Session Description Protocol", RFC 2327, IETF; April 1998. [3] L. Westberg/M. Lindqvist, "Realtime Traffic over Cellular Access Networks", draft-westberg-realtime-cellular-02.txt, IETF; May 2000. Work in progress. [4] 3G TS 23.060 v3.2.1 General Packet Radio Service Description. [5] ftp://ftp.isi.edu/confctrl/confctrl.mail ,thread with subject "SDP media alignment in SIP". 11. Authors' Addresses Gonzalo Camarillo Ericsson Advanced Signalling Research Lab. FIN-02420 Jorvas Finland Phone: +358 9 299 3371 Fax: +358 9 299 3118 Email: Gonzalo.Camarillo@ericsson.com Jan H÷ller Ericsson Research S-16480 Stockholm Sweden Phone: +46 8 58532845 Fax: +46 8 4047020 Email: Jan.Holler@era.ericsson.se G÷ran AP Eriksson Ericsson Research S-16480 Stockholm Sweden Phone: +46 8 58531762 Fax: +46 8 4047020 Email: Goran.AP.Eriksson@era.ericsson.se Camarillo/H÷ller/Eriksson 7