Subject: [AVT] Draft minutes from Vancouver meeting

From: Colin Perkins <[email-masked]>
Date: Sat, 26 Nov 2005 15:37:06 +0000

Enclosed are draft minutes from the AVT working group session in  
Vancouver. Comments, corrections or additions to the chairs please.

Colin

Audio/Video Transport (AVT) Working Group Minutes
=================================================

Reported by Colin Perkins

    The AVT working group met once at the 64th IETF meeting (November  
2005,
    Vancouver).  Subjects under discussion included media type  
registration
    rules, RTP and RTCP XR MIBs, codec control messages, the scalable  
video
    codec, a framework for multiple description encoding, and RTP  
payloads
    for Speex, Vorbis, Theora, VC-1 and EVRC.  The meeting was  
chaired by
    Colin Perkins and Magnus Westerlund.

Introduction and Status Update

    The chairs introduced the meeting with the usual status review. The
    group has had three RFCs published since the last meeting (RFCs  
4175,
    4184 and 4247), has a further 11 drafts in the RFC editor queue, and
    three more at various points in the IESG review process. The revised
    H.263+ and telephony tones/events formats have recently completed WG
    last call and need minor changes as a result of comments received.
    The following draft are believed almost ready for WG last call: MIME
    type registration for H.224, RTP/SAVPF profile, RTCP SSM extensions,
    revised H.261 payload format, reclassification of RFC 2190 as  
historic,
    the ULP payload format, and MIDI payload format and implementers  
guide.
    Review of these draft is solicited.

    The chairs reviewed the milestones, noting we are slightly behind
    schedule. The chairs also noted that there is no draft that defines
    RTP over TLS, and apparently no interest in the work item.  If there
    is no volunteer to proceed with this, we will consider removing that
    item from the working group's charter.  On a related note, Steve
    Casner asked is there was interest in RTP over DTLS?  Colin Perkins
    noted that this subject has been mentioned informally, but there is
    no active work at present (although it can be expected to fit into
    the charter should there be interest).

MIME Type Registration of RTP Payload Formats

    draft-ietf-avt-rfc3555bis-01.txt

    Steve Casner asked for input regarding the text in section 2.1 of  
the
    draft.  Ralph Giles and Roni Even asked if payload formats exist  
that
    don't follow these rules, and if these rules are intended to  
apply to
    all formats, not just those defined in RFC 3555. Steve clarified  
that
    the rules in section 2.1 of the draft document existing practice,  
and
    are believed consistent with existing media type registrations of  
RTP
    payload formats. There were no other comments or objections.

    Steve noted that several people have questioned if the draft should
    include updated media type registrations, and asked for input on  
this
    subject. Roni Even noted that we're moving the registrations into  
new
    drafts as we take the payload formats to draft standard, and that  
this
    is easier if we separate out the registrations from 3555bis.   
Stephan
    Wenger agreed, but suggested that trivial registrations might remain
    in the 3555bis draft. Conclusion: most media type registrations will
    be removed from the 3555bis draft.

RTCP XR MIB and RTP MIB Updates

    draft-ietf-avt-rtcp-xr-mib-03.txt

    Alan Clark discussed the RTCP XR MIB, summarising the changes  
since the
    last version, and describing the proposed alignment between the  
RTCP XR
    MIB and the newly updated RTP MIB. The proposed alignment uses a  
common
    Session Table between the two MIBs, and there was some discussion  
about
    how this is to be achieved, with Dan Romascanu offering  
suggestions for
    the mechanics of aligning the two MIBs.

    Magnus Westerlund expressed concern that the new Session Table  
includes
    RTP flows as its basic object, rather than RTP sessions. This led  
Steve
    Casner and Colin Perkins to wonder if the new structure would  
limit the
    monitoring of multicast sessions? The existing RTP MIB allows  
monitoring
    of the full range of RTP sessions, losing that ability is not  
desirable.

    Alan is expecting to have a revised RTCP XR MIB and an initial  
draft of
    the updated RTP MIB by mid-December, and will circulate on the  
list for
    comments. Dan Romascanu asked if the aim is to progress these  
together,
    and Alan clarified that is the intent.

Codec Control Messages

    draft-wenger-avt-avpf-ccm-01.txt

    Magnus Westerlund discussed codec control messages, outlining  
changes
    since the previous version of the draft and the open issues.

    The temporal-spatial trade-off message is now acknowledged,  
informing
    receivers that the sender has chosen a specific trade-off. This  
is more
    feedback than provided by existing systems, but we might chose to  
add a
    reason code to the response, to inform receivers why the sender  
chose a
    trade off that might not match their preference. Stephan Wenger  
noted
    that he doesn't see a need for this. Colin Perkins noted that he  
would
    prefer a model where the sender announces the trade-off it has  
chosen,
    rather than acknowledging specific requests, since this allows  
senders
    to make unsolicited changes, and better conforms to a "hint"  
model for
    the control messages.

    Regarding the TMMA feedback message, there is presently no signal  
back
    to the receivers of the current maximum bit rate in force -  
should one
    be added? Stephan Wenger and Roni Even discussed use of this  
feature,
    with Stephan outlining possible use cases and Roni noting that he  
did
    not find them persuasive; there was no real conclusion.

    Colin Perkins noted that the draft assumes a particular design of  
MCU,
    which makes each leg of the call a separate RTP session, and that  
this
    doesn't match the RTP translator/mixer model described in RFC  
3550. To
    avoid confusion, Colin requested that the next version of the  
draft be
    updated to include some discussion of the MCU design model  
chosen, and
    how it differs from RFC 3550.

    The consensus of the room was to accept this as a working group  
draft,
    although some push-back was noted.

Scalable Video Codec (SVC) Payload Format

    draft-wenger-avt-rtp-svc-00.txt

    Stephan Wenger discussed signalling layered coding structures and  
the
    SVC payload format. He began with a discussion of scalable and  
layered
    coding, explaining possible topologies and requirements, and  
outlining
    how layered codec streams have previously been handled in AVT and
    MMUSIC.

    Leaving aside the details of how scalable coding works, Stephan  
focussed
    on the options for signalling layer dependencies: 1) layer  
dependencies
    are signalled using a media-level SDP attribute that is  
independent of
    the codec; 2) layer dependencies are signalled using codec  
specific SDP
    attributes, in the native syntax of the payload format; or 3)  
dependency
    information is sent in-band within the RTP payload format. It is  
desired
    to keep signalling and media separate, so the third option is  
discounted.
    The group should consider if we want a generic signalling method,  
perhaps
    with limited capabilities, or a codec specific signalling method  
that can
    express complex dependencies.

    Stephan also briefly outlined some other properties of the SVC  
extension
    to H.264, and the initial proposal for an RTP payload format.  
There is a
    desire to keep this format aligned as much as possible with RFC  
3984, in
    particular to ensure the base layer can decoded by an unmodified  
RFC 3984
    decoder. Colin Perkins expressed his support for the aim of  
aligning the
    format with RFC 3984, emphasising the desire for backwards  
compatibility.

    Stephan noted that the draft is presented to AVT very early, to  
allow
    coordinated standardisation, and asked the group to consider it as a
    work item. Colin Perkins noted that this is, in principle,  
appropriate
    for AVT, but deferred the decision to accept the draft after a  
poll of
    the room showed that only a few people had read it.

Multiple Description Standard-Compatible Framework

    draft-vitali-ietf-avt-mdc-lc-00.txt

    Andrea Vitali described the concept of multiple description  
coding for
    error resilience and scalability, and outlined proposed SDP  
attributes
    that can be used to signal various multiple description coding  
schemes.
    The proposed attributes are an example of general SDP signalling,  
much
    as described earlier in the session by Stephan Wenger, and  
demonstrate
    the complexity of defining codec independent signalling for multiple
    description coding.

    Magnus Westerlund asked if the transforms described are standardised
    anywhere? No, this is a proposal for standard SDP signalling, not an
    attempt to standardise the set of transforms.  Magnus expressed some
    concern that the flexibility was inappropriate, and that it might be
    better to define a standard set of transforms elsewhere, which could
    be signalled within SDP.

    Stephan Wenger noted that there were useful parts to the signalling,
    but agreed with Magnus that signalling filter coefficients in SDP is
    not appropriate. Colin Perkins and Steve Casner agreed: this is at a
    level below that where AVT and MMUSIC usually work.

    There is clearly some interest in multiple description and scalable
    coding, but no consensus on this particular approach. Discussion of
    the alternatives and trade-offs on the mailing list is encouraged.

RTP Payload Formats for Speex, Vorbis and Theora

    draft-ietf-avt-rtp-speex-00.txt
    draft-kerr-avt-vorbis-rtp-05.txt

    Ralph Giles discussed the RTP payload formats for Speex, Vorbis and
    Theora. The Speex format is straight-forward: Ralph requested review
    of the draft, but didn't devote extensive time to it.

    The aim is to closely align the Vorbis and Theora formats, and to  
this
    end the Theora format is being held while details of the Vorbis  
format
    are resolved. The issue with both formats is that there is no  
standard
    codebook, and the codec requires a stream-specific codebook to be  
sent
    before decoding can start. Discussion focussed on the need for  
separate
    codebooks, and options for transferring them.

    Colin Perkins suggested Ralph consider the IMG work in MMUSIC as an
    alternative for distributing code books, in addition to using HTTP.

    Ross Finlayson asked how many codebooks are being used with  
Vorbis, and
    if it is possible to standardise a set for streaming. There are  
only a
    small number, but the set of codebooks used changes over time, as  
the
    codec is improved, so flexibility is desired. Stephan Wenger  
seconded
    the desire for flexibility.

    Magnus Westerlund and Steve Casner noted that well-known  
codebooks can
    be identified by their URI, or that a longer hash can be used a  
unique
    identifier, avoiding the need for a registry of codebooks. Colin  
Perkins
    suggested the Vorbis community could manage a portion of their  
URL space
    in a manner that allows persistent caching of codebooks, or for  
off-line
    distribution of well-known codebooks.

    The Vorbis payload format was previously accepted as an AVT work  
item,
    although it was never submitted as a working group draft; there  
are no
    objections to taking it now. The Theora draft is appropriate as a  
work
    item, but we cannot formally accept it until a draft is available  
for
    review.

RTP Payload Format for VC-1

    draft-ietf-avt-rtp-vc1-01.txt

    Anders Klemets discussed the RTP payload format for VC-1. He  
noted that
    an IPR disclosure for the payload format has been submitted to  
the IETF.
    The codec specification is available as SMPTE-421M, and DVB-H  
expressed
    interest in referencing this format.

    Anders outlined the changes since the previous draft, and  
discussed the
    open issues. The main issue is the need for offer/answer  
considerations
    in the draft, but Colin Perkins, Roni Even and Magnus Westerlund  
noted
    that this was essential, and other drafts missing offer/answer  
section
    need to be updated.

    Stephan Wenger, Ross Finlayson and Ralph Giles noted concern  
about IPR
    on the payload format, and asked for clarification of licensing  
terms.
    There was some considerable discussion on this subject, with the  
group
    expressing a general preference for royalty free licensing, but  
noting
    that there are existing AVT RFCs with RAND terms.

RTP Support for Enhancements to EVRC Family Codecs

    draft-xie-avt-compact-bundled-evrc-01.txt

    Qiaobing Xie discussed enhancements to the payload formats for EVRC
    family codecs, noting that 3GGP2 is working on extensions to the  
codec.
    The draft defines a payload format for EVRC-B, a complementary  
bundled
    format, and an update to RFC 3558 with DTX support. Steve Casner  
asked
    if there are interoperability issues with the extra parameters  
defined
    for this format? Perhaps - need to investigate.

    The consensus of the room was that this is appropriate as an AVT  
work
    item.

				   - + -

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