MPEG-4 RTP transport Philippe Gentric Philips Digital Networks 49th IETF Conference San Diego, 14 December 2000

Context: draft-ietf-avt-rtp-mpeg4-03.txt uses MPEG-4 Synchronization Layer (SL) What is new ? draft-gentric-avt-rtp-mpeg4-00.txt Various small changes One new thing: SDP signaling One big change: support multiple SL packets Overview

Media aware Delivery unaware Media unaware Delivery aware Media unaware Delivery unaware COMPRESSION LAYER DELIVERY LAYER SYNCHRONIZATION LAYER MPEG-4 Codecs MPEG-4 “SL” MPEG-4 video MPEG-4 audio MPEG-4 system (scene, etc) RTP MPEG-2 TS More … Access Units SL packets RTP packets The MPEG-4 “system” transport framework Composition Units

SL provides (media unaware !) ALF There are many different stream types in MPEG-4 Having a single framework (SL) to describe all of them: Simplifies implementation Simplifies conformance definition Simplifies conformance testing Simplifies interoperability Why is SL so important for MPEG-4 ?

Same payload can be used for all MPEG-4 stream types Video: all sizes, modes, frame rates, scalable schemes Audio: CELP, HVXE, HILN, AAC, TTS, scalable schemes System: OD (Object Descriptors) BIFS (BInary Format for Scene) IPMP (Intellectual Property Management and Protection) MPEGJ (Java) OCI (Object Content Information) etc... Defining a “generic” RTP MPEG-4 payload Use SL

RTP packet 1 SL packet = 1 Access Unit (or fragment) ALF OK: Each SL packet is a codec-semantic fragment RTP packet SL packet Access Unit KISS: 1 RTP packet = 1 SL packet SL packet payloadheader payloadreduced header RTP header Remove redundancy CTS is mapped to RTP TS AU end is mapped to M bit

The original draft (draft-ietf-avt-rtp-mpeg4-03.txt) Has been implemented and tested Has a problem: low bit rate audio overhead. Low delay: use header compression Media delivery: have to align with competition... Once dissected other issues popped up... There is a clear degree of urgency to advance the status... because there are products in the field now. What is new ?

Some SL streams may have to undergo SL-SL translation before being usable. For IETF this is out of scope: the payload format just says “such streams cannot be used with this payload format”. Random RTP-TS offset prevents receivers from absolute value of CTS/DTS/OCR reconstruction: this is now a documented issue. OCR can be mapped to RTP-TS RTCP uses NTP *format*, the nature of that clock is “an implementation issue”. (e.g. MPEG-2 TS synchronization) RTP packets do not transport SL packets but mapped SL packets SLConfigDescriptor is transported unchanged You need additional (mapping) information in SDP What is new ? Clarifications

Minor changes: MPEG-4 seq. num. *not* correlated anymore to RTP seq. num. OCR/DTS/CTS encoded as difference to first CTS = RTPTS New thing: Configuration via SDP (a=fmtp:...) Required for mapping parameters e.g. DecodingTimeStampDeltaLength... Some flags can be removed (SDP signaled) useCTS (never, flagged, always) useDTS (never, flagged, always) SLConfigDescriptor can also be transported in SDP Major change: transport of multiple SL packets Interleaving (optional), using packetSequenceNumber What is new ? Changes

aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) { if (SL.useAccessUnitStartFlag) bit(1) accessUnitStartFlag; if (SL.useAccessUnitEndFlag) bit(1) accessUnitEndFlag; if (SL.OCRLength>0) bit(1) OCRflag; if (SL.useIdleFlag) bit(1) idleFlag; if (SL.usePaddingFlag) bit(1) paddingFlag; if (paddingFlag) bit(3) paddingBits; if (!idleFlag && (!paddingFlag || paddingBits!=0)) { if (SL.packetSeqNumLength>0) bit(SL.packetSeqNumLength) packetSequenceNumber; if (SL.degradationPriorityLength>0) bit(1) DegPrioflag; if (DegPrioflag) bit(SL.degradationPriorityLength) degradationPriority; if (OCRflag) bit(SL.OCRLength) objectClockReference; if (accessUnitStartFlag) { if (SL.useRandomAccessPointFlag) bit(1) randomAccessPointFlag; if (SL.AU_seqNumLength >0) bit(SL.AU_seqNumLength) AU_sequenceNumber; if (SL.useTimeStampsFlag) { bit(1) decodingTimeStampFlag; bit(1) compositionTimeStampFlag; } if (SL.instantBitrateLength>0) bit(1) instantBitrateFlag; if (decodingTimeStampFlag) bit(SL.timeStampLength) decodingTimeStamp; if (compositionTimeStampFlag) bit(SL.timeStampLength) compositionTimeStamp; if (SL.AU_Length > 0) bit(SL.AU_Length) accessUnitLength; if (instantBitrateFlag) bit(SL.instantBitrateLength) instantBitrate; }

1 RTP packet = N * SL packets 1 SL packet = 1 Access Unit (or fragment) ALF OK The only required reduced header field is the payload size RTP packet SL packet payload New: Transport of multiple (small) Access Units header SL packet payloadheader SL packet payloadheader reduced header payload RTP header

More consistent with other RTP payloads Better payload encryption (longer encryption block) Better overhead (headers are bit-wise concatenated) Faster processing (AU are packed together just like in.mp4) (?) Why concatenated headers first ? non-first headers use differences for: packetSequenceNumber AuNumber compositionTimeStamp all headers use differences for decodingTimeStamp New: Additional overhead reduction

Using optional packetSequenceNumber *Not* AUNumber (because is already used by BIFS carousel) Very useful for CELP and AAC (and maybe more) New: Interleaving Pack only same degradationPriority (maybe) Pack with randomAccessPoint first (maybe) Only one OCR per RTP packet New: Packing Rules

Generic MPEG-4 payload format is IETF draft draft-ietf-avt-rtp-mpeg4-03.txt rapidly evolving: draft-gentric-avt-rtp-mpeg4-00.txt (already obsolete) Joint work by: MPEG ad hoc group (4onIP) IETF AVT experts Technical work to be approved at next MPEG meeting (Pisa, Italy, January 2001) Aim: proceed to IETF Standard Track ASAP Conclusion