SDP中Video属性fmtp的含义，比如a=fmtp:34 QCIF=1;CIF=1

 
SDP syntax for H.263 optionsSTATUS 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).                ABSTRACTThis document defines the SDP syntax for H.263 codec options andparameters for IETF multimedia conferencing architecture. It is oftenuseful to know beforehand (in call setup phase) what features of H.263the other end supports. This document specifies the format speficparameters for H.263, which exists in a=fmtp:<format> <format specificparameters> as defined in the SDP document.Moreover, the usage of this feature with SIP and SAP is specified.
1. IntroductionInternet multimedia conferencing is becaming a reality and new protocolshave been defined to provide co-operation between different applications.Current IETF protocols SIP [3] and SAP [4] are quite widely used tosimple MBone conferencing and Internet unicast conferencing is alsotaking its first steps. Today, the most widely used video codec in thisenvironment is the ITU-T H.261 video standard, but a better and moreefficient low-bit rate video standard, ITU-T H.263 [1] is becoming moreand more popular.However, the usage of H.263 with current IETF multimedia conferencingarchitecture is difficult, since improved efficiency of H.263 depends onuse of coding options and parameters which must be told somehow to theother end. SIP and SAP protocols can deliver this information, butcurrent SDP [2] format does not specify standardised way of representingthese options and parameters.This document specifies the SDP syntax which describes H.263 options andparameters to be delivered by some signaling protocol (e.g. SIP or SAP).The syntax presented in this document uses SDP "a=fmtp" attribute whichis meant for carrying codec specific parameters.These options are especially useful in SIP. In SAP it is not clear whetherthey have any use. In SIP, these format specific parameters are decoderproperties, and in SAP they are encoder capabilities.  The SAP rules areapplied also if a multicast session is advertised in a web page in SDPformat.
2. H.263 Codec ParametersAll these parameters must be presented in SDP a=fmtp line exactly in thisorder if they are present. Different parameter types are separated bysingle slash ("/") and parameters with space.For further description about these parameters, please refer toITU 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 thatpicture 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 willingto support arbitrary picture sizes. Both picture sizes must be divisibleby 4. These X and Y values are the maximum of allowed picture sizes withcorresponding MPI value. All three words must exist together in thisorder, 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 allof them. At least one picture mode must be present.
2.2 MaxBitRate and BitsPerPictureMaxKbMaxBitRate=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 singlepicture frame, value is specified by largest supported picture resolution,see [1]. If this parameter is not present, then default value, that is basedon the maximum supported resolution, is used.These parameters are separated by space but before/afterthese parameter(s) there must be a slash symbol ("/").
3. H.263 Codec OptionsFollowing words describe 4 options defined for H.263. Later versionsof H.263 (e.g. so called H.263+, currently a ITU-T draft) defines moreoptions and they should be merged later into this document. These optionsare separated by a space between them. This group of options (words) isseparated by slash ("/").  All these options must be presented in SDPa=fmtp line exactly in this order if they are present.Word: URVExplanation: UnRestricted motion Vector option.Word: SACExplanation: Syntax based Arithmetic Coding.Word: APExplanation: Advanced Prediction.Word: PBExplanation: PB Frames.The actual interpretation of these words is defined differently forone-way and two-way negotiations (see chapters 4 and 5).
4. Usage of SDP H.263 options and parameters with SIPThis document specifies how these options and parameters can berepresented in SDP format. Current SIP protocol does not support anynegotiation about codecs or codec parameters. However, this negotiationis quite needless since it is enough to tell the preferred decoderoptions and parameters and let the other end to decide actual optionsand parameters as long as the other party sends only such options andparameters which are advertized.  This scheme keeps the IETF multimediaterminal simple.Codec options: (URV,SAC,AP,PB)These words exist only if the sender of this SDP message is able orwilling to decode those. E.g. If a terminal is capable of decodingSAC and AP options, it can put SAC AP in the end of <format specificparameters>. Then the other party knows it, and can use those optionsin 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 toother party, or only some subset of it.  Terminal announces only thosepicture 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 thatpicture 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 bereceived, 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 todecode the picture size with corresponding MPI. If terminal is notwilling 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=2This means that sender hopes to receive CIF picture size, which it candecode 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 alsoallowed (but least preferred) to send arbitrary picture sizes(max 360x240) with MPI=2.Note that most encoders support at least QCIF and CIF fixed resolutionsand they are expected to be available almost in every H.263-based videoapplication.MaxBitRate and BitsPerPictureMaxKb parameters:Both these parameters are useful in SIP. MaxBitRate is video decoderproperty, hence it differs from SDP b:<modifier> <bandwidth-value>attribute which refers more to application's total bandwidth (anapplication consists often of both audio and video). Use of MaxBitRateparameter is mandatory.BitsPerPictureMaxKb is needed especially for decoder buffer sizeestimation 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 APThis means that the sender of this message can decode H.263 (RTPpayload type 34) bitstream with following options and parameters:Preferred resolution is CIF (its MPI is 4), but if that is notpossible 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 SAPSAP announcements are one-way only. H.263 options (like SAC) mean thatsending terminal (host) is going to use these options in itstransmitted H.263 stream. It is just an informal message.Usually only one picture size (with its MPI) exists. However, since itis possible for a video source (terminal) to change its picture sizeduring session, several picture sizes can exist in the parameter list.First one is the original picture size to be used in the beginning ofthe session.Other H.263 parameters are not used with SAP (nor when announced inweb-page).Example with SAP:a=fmtp 34 CIF=2/URV SACThe 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 beused e.g.  in the MBone.