Internet Engineering Task Force Harri Honko INTERNET-DRAFT Petri Koskelainen Jouni Salonen Expires: July 1998 Nokia February 1998 SDP syntax for H.263 options STATUS OF THIS MEMO This document is an Internet-Draft. 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." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). ABSTRACT This document defines the SDP syntax for H.263 codec options and parameters for IETF multimedia conferencing architecture. It is often useful to know beforehand (in call setup phase) what features of H.263 the other end supports. This document specifies the format spefic parameters for H.263, which exists in a=fmtp: as defined in the SDP document. Moreover, the usage of this feature with SIP and SAP is specified. 1. Introduction Internet multimedia conferencing is becaming a reality and new protocols have been defined to provide co-operation between different applications. Current IETF protocols SIP [3] and SAP [4] are quite widely used to simple MBone conferencing and Internet unicast conferencing is also taking its first steps. Today, the most widely used video codec in this environment is the ITU-T H.261 video standard, but a better and more efficient low-bit rate video standard, ITU-T H.263 [1] is becoming more and more popular. However, the usage of H.263 with current IETF multimedia conferencing architecture is difficult, since improved efficiency of H.263 depends on use of coding options and parameters which must be told somehow to the other end. SIP and SAP protocols can deliver this information, but current SDP [2] format does not specify standardised way of representing these options and parameters. This document specifies the SDP syntax which describes H.263 options and parameters to be delivered by some signaling protocol (e.g. SIP or SAP). The syntax presented in this document uses SDP "a=fmtp" attribute which is meant for carrying codec specific parameters. These options are especially useful in SIP. In SAP it is not clear whether they have any use. In SIP, these format specific parameters are decoder properties, and in SAP they are encoder capabilities. The SAP rules are applied also if a multicast session is advertised in a web page in SDP format. 2. H.263 Codec Parameters All these parameters must be presented in SDP a=fmtp line exactly in this order if they are present. Different parameter types are separated by single slash ("/") and parameters with space. For further description about these parameters, please refer to ITU documents [1]. 2.1 Picture sizes and MPI H.263 bitstream supports many picture sizes and different frame rates. Supported picture sizes and their corresponding minimum picture interval (MPI) values are combined with "=" symbol. MPI is an integer value (1..32) and it means that maximum picture (frame) rate is (29.97/MPI) frames/sec. MPI value may be bigger, meaning that picture rate is slower. Word: SQCIF=MPI(1..32) Explanation: SQCIF picture size and its MPI value. Word: QCIF=MPI(1..32) Explanation: QCIF picture size and its MPI value. Word: CIF=MPI(1..32) Explanation: CIF picture size and its MPI value. Word: CIF4=MPI(1..32) Explanation: CIF4 picture size and its MPI value. Word: CIF16=MPI(1..32) Explanation: CIF16 picture size and its MPI value. Word: XMAX=maximum_x_size YMAX=maximum_y_size MPI=MPI(1..32) Explanation: This parameter means that terminal is capable and/or willing to support arbitrary picture sizes. Both picture sizes must be divisible by 4. These X and Y values are the maximum of allowed picture sizes with corresponding MPI value. All three words must exist together in this order, or none is allowed to exist. More than one mode can exist in the same line (see example later). There is a space between picture modes and slash ("/") symbol after all of them. At least one picture mode must be present. 2.2 MaxBitRate and BitsPerPictureMaxKb MaxBitRate=INTEGER(1..19200) Explanation: Maximum video stream bitrate, presented with units of 100 bits/s. BitsPerPictureMaxKb=INTEGER(0..65536) Explanation: Maximum amount of kilobits allowed to represent a single picture frame, value is specified by largest supported picture resolution, see [1]. If this parameter is not present, then default value, that is based on the maximum supported resolution, is used. These parameters are separated by space but before/after these parameter(s) there must be a slash symbol ("/"). 3. H.263 Codec Options Following words describe 4 options defined for H.263. Later versions of H.263 (e.g. so called H.263+, currently a ITU-T draft) defines more options and they should be merged later into this document. These options are separated by a space between them. This group of options (words) is separated by slash ("/"). All these options must be presented in SDP a=fmtp line exactly in this order if they are present. Word: URV Explanation: UnRestricted motion Vector option. Word: SAC Explanation: Syntax based Arithmetic Coding. Word: AP Explanation: Advanced Prediction. Word: PB Explanation: PB Frames. The actual interpretation of these words is defined differently for one-way and two-way negotiations (see chapters 4 and 5). 4. Usage of SDP H.263 options and parameters with SIP This document specifies how these options and parameters can be represented in SDP format. Current SIP protocol does not support any negotiation about codecs or codec parameters. However, this negotiation is quite needless since it is enough to tell the preferred decoder options and parameters and let the other end to decide actual options and parameters as long as the other party sends only such options and parameters which are advertized. This scheme keeps the IETF multimedia terminal simple. Codec options: (URV,SAC,AP,PB) These words exist only if the sender of this SDP message is able or willing to decode those. E.g. If a terminal is capable of decoding SAC and AP options, it can put SAC AP in the end of . Then the other party knows it, and can use those options in its encoder. Picture sizes and MPI: Supported picture sizes and their corresponding minimum picture interval (MPI) information can be combined. All picture sizes can be advertised to other party, or only some subset of it. Terminal announces only those picture sizes (with their MPIs) which it is willing to receive. MPI is an integer value (1..32) and it means that maximum picture (frame) rate is (29.97/MPI) frames/s. MPI value may be bigger, meaning that picture rate is slower. For example, MPI=2 means that maximum (decodeable) picture rate per sec is about 15. Parameter occurring first is the most preferred picture mode to be received, and last is the least preferred (but still supported) one. Parameter words are presented with no slash ("/") symbol between them, and are separated by space (see example later in this chapter). These words are present in SDP line only if terminal is willing to decode the picture size with corresponding MPI. If terminal is not willing to receive some mode, it is not present in the list. Example of the usage of these words: CIF=4 QCIF=3 SQCIF=2 XMAX=360 YMAX=240 MPI=2 This means that sender hopes to receive CIF picture size, which it can decode at MPI=4. If that is not possible, then QCIF with MPI value 3, if that is neither possible, then SQCIF with MPI value =2. It is also allowed (but least preferred) to send arbitrary picture sizes (max 360x240) with MPI=2. Note that most encoders support at least QCIF and CIF fixed resolutions and they are expected to be available almost in every H.263-based video application. MaxBitRate and BitsPerPictureMaxKb parameters: Both these parameters are useful in SIP. MaxBitRate is video decoder property, hence it differs from SDP b: attribute which refers more to application's total bandwidth (an application consists often of both audio and video). Use of MaxBitRate parameter is mandatory. BitsPerPictureMaxKb is needed especially for decoder buffer size estimation to reduce the propability of video buffer overflow. Below is an example of H.263 SDP syntax in SIP message. a=fmtp 34 CIF=4 QCIF=2/MaxBitRate=1000/SAC AP This means that the sender of this message can decode H.263 (RTP payload type 34) bitstream with following options and parameters: Preferred resolution is CIF (its MPI is 4), but if that is not possible then QCIF size is ok. Maximum receivable bitrate is 100 kbit/s (1000*100 bit/s) and SAC and AP options can be used. 5. Use of SDP H.263 options and parameters with SAP SAP announcements are one-way only. H.263 options (like SAC) mean that sending terminal (host) is going to use these options in its transmitted H.263 stream. It is just an informal message. Usually only one picture size (with its MPI) exists. However, since it is possible for a video source (terminal) to change its picture size during session, several picture sizes can exist in the parameter list. First one is the original picture size to be used in the beginning of the session. Other H.263 parameters are not used with SAP (nor when announced in web-page). Example with SAP: a=fmtp 34 CIF=2/URV SAC The video source is sending an H.263 bitstream and picture size is CIF, MPI=2 and URV and SAC options are used. This kind of announcement can be used e.g. in the MBone. Authors' Addresses Harri Honko, Petri Koskelainen Nokia Research Center P.O.Box 100 FIN-33721 Tampere Finland e-mail: harri.honko@research.nokia.com, petri koskelainen@research.nokia.com, jouni.salonen@ntc.nokia.com References: [1] International Telecommunication Union. Video Coding for Low Bitrate Communication, ITU-T Recommendation H.263, ITU 1996 [2] Handley et al, ``SDP - Session Description Protocol'', INTERNET-DRAFT, IETF 1997. [3] Handley et al, ``SIP - Session Initiation Protocol'', INTERNET-DRAFT, IETF 1997. [4] ``SAP - Session Announcement Protocol'', INTERNET-DRAFT, IETF 1997.