MPEG-4 Design Team Report

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Proposals

• draft-ietf-avt-rtp-mpeg4-02.txt• draft-guillemot-genrtp-01.txt• draft-jnb-mpeg4av-rtp-00.txt• FlexMux packetization

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Issues

• Use of MPEG-4 systems vs elementary streams• Multiplexing multiple streams• Grouping• Fragmentation• Error protection• Timing model• Byte alignment

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Systems vs elementary streams

• MPEG-4 has a complete system model– some applications need the entire framework– others just desire to use the codecs

• We accept that we need to generate payload formats for both cases, even though this has the potential for interoperability problems later– compare issues with MPEG-2

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Systems vs elementary streams

• MPEG-4 encompasses codecs which may have an existing RTP payload format– This does not preclude the use of MPEG-4 specific

packetization in those cases• For error resilience, it is desirable to packetize in

a media aware manner– This does not imply a choice between systems or

elementary streams

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Multiplexing multiple streams

• Five multiplexing options were identified– GeRM– FlexMux– “PPP Mux” and CRTP– Don’t multiplex– Don’t multiplex, but compress headers

• We may need a FlexMux format, but nothing else requires special consideration

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Grouping within a stream

• Why group?– to amortize header overhead– to aid error resilience

• duplicate and group picture headers with each packet• group FEC with media data

• Disagreement over the importance of grouping, and the mechanism to be used

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Grouping within a stream

• draft-guillemot-genrtp-01.txt has support for grouping access units (ie: ADUs) and for sub-access units (eg: unequal FEC)– eg: group a repeated picture header with the next

frame• other proposals perform no additional grouping

– assuming anything needed will be done in the encoder• need more experimental testing

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Fragmentation

• It is necessary to fragment a codec bit-stream in a media aware manner to achieve error resilience

• We believe that the choice of fragmentation is a matter for the encoder, and that MPEG-4 should be able to fragment accordingly– a payload format should be able to transport oversize

fragments, but this need not be efficient

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Error protection

We considered two forms of error protection• within the payload• across packets

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Error protection - within payload

• draft-guillemot-genrtp-01.txt uses unequal FEC within the payload– uses a “typed segment” abstraction for genericity– this abstraction doesn’t exist in MPEG, although the

information is available in an ES specific manner• other proposals assume the MPEG bitstream is

robust enough “as is”• need more experimental testing

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Error protection - across packets

• May also apply any of the existing error protection mechanisms across packets– parity FEC, for example

• draft-guillemot-genrtp-01.txt also duplicates these functions as part of its unequal FEC scheme

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Error Protection

• Some MPEG-4 elementary streams MUST be reliably delivered– Control streams– Streaming media such as Java class files

• No conclusion on how we do this - have focused on audio and video to date

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Timing model

• MPEG-4 and RTP have different timing models• If it is desired to synchronize MPEG-4 data with

data using a native RTP packetization we must align the models– capture time vs composition time

• Believe text in draft-ietf-avt-rtp-mpeg4-02.txt is correct, but needs edits for clarity– see Andrea’s presentation...

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Byte alignment

• MPEG-4 systems has the potential to produce not byte aligned bit-streams– audio and video codecs do not– others may?

• May have to include padding bits to compensate– SL packets include these– mostly affects ES packetization

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Presentations of drafts...

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Future directions

• MPEG-4 codecs can be packetized as any other codec, with standards track payload formats– Adopt draft-jnb-mpeg4av-rtp-00.txt as an AVT work

item, for eventual submission on the standards track• Multiplexed MPEG-4 media is to be treated in a

similar manner to earlier bundled MPEG transport– We will consider a FlexMux payload format, if one is

submitted

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Future directions

• We do not believe we fully understand the issues involved in the transport of the complete MPEG-4 system over RTP

• Submit such payload formats for publication as experimental RFCs, whilst we gain implementation experience – draft-ietf-avt-rtp-mpeg4-02.txt– draft-guillemot-genrtp-01.txt