[AVT] Draft minutes from Vancouver meeting
Colin Perkins <csp@csperkins.org> Sat, 26 November 2005 15:37 UTC
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Subject: [AVT] Draft minutes from Vancouver meeting
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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. - + - _______________________________________________ Audio/Video Transport Working Group avt@ietf.org https://www1.ietf.org/mailman/listinfo/avt
- [AVT] Draft minutes from Vancouver meeting Colin Perkins