2.8.1 Audio/Video Transport (avt)

NOTE: This charter is a snapshot of the 64th IETF Meeting in Vancouver, British Columbia Canada. It may now be out-of-date.
In addition to this official charter maintained by the IETF Secretariat, there is additional information about this working group on the Web at:

       RTP FAQ Page

Last Modified: 2005-10-03


Colin Perkins <csp@csperkins.org>
Magnus Westerlund <magnus.westerlund@ericsson.com>

Transport Area Director(s):

Allison Mankin <mankin@psg.com>
Jon Peterson <jon.peterson@neustar.biz>

Transport Area Advisor:

Allison Mankin <mankin@psg.com>

Mailing Lists:

General Discussion: avt@ietf.org
To Subscribe: https://www1.ietf.org/mailman//listinfo/avt
Archive: http://www.ietf.org/mail-archive/web/avt/index.html

Description of Working Group:

The Audio/Video Transport Working Group was formed to specify a
protocol for real-time transmission of audio and video over unicast
and multicast UDP/IP. This is the Real-time Transport Protocol, RTP,
together with its associated profiles and payload formats. The
current aims of the working group are: 

- to review and revise existing payload formats to advance those
  which are useful to Draft Standard, and to declare others
  as Historic. Milestones will be established as a champion for
  each payload format is identified.

- to develop payload formats for new media codecs, and to
  document best-current practices in payload format design.
  The group continues to be precluded from work on codecs
  themselves because of overlap with the other standards
  bodies, and because the IETF does not have the ability
  to effectively review new codecs. An exception was made
  for the freeware iLBC codec on a highly experimental basis,
  but acceptance of new codec work is unexpected and subject
  to rechartering.

- to complete the forward error correction work to update
  RFC 2733 in the form of the ULP payload format

- to investigate and if suitable develop a framework for advanced
  FEC codes and their usage for RTP, possibly with alignment to
  the RMT WG's FEC building block.

- to extend RTP to work with Source-Specific Multicast sessions
  with unicast feedback

- to provide a framing mechanism for RTP over TCP and TLS

- in collaboration with the MPLS and ROHC WGs, to develop a solution
  for header compression of RTP across MPLS networks that avoid
  decompression and compression at each MPLS node.

- to develop a new RTP profile as the combination of the SRTP
  profile and the Extended RTP Profile for RTCP-based Feedback

- to develop a new RTP profile for usage of TFRC (RFC 3448) with
  RTP over UDP to allow application developers to gain experience
  with TCP friendly congestion control.
- to develop a MIB for RTCP XR (RFC 3611).

- to update the RTP MIB, including aligning it with RFC 3550.

The longer term goals of the working group are to advance the
SRTP Profile, the Extended RTP Profile for RTCP-based Feedback,
the Compressed RTP framework, and the RTP MIB to Draft Standard.

The group has no plans to develop new RTP profiles beyond those
listed above, but will consider rechartering to produce profile
level extensions if appropriate.

Goals and Milestones:

Done  Review DCCP including prototypes and API; feedback to DCCP WG
Done  Initial draft requirements for ECRTP over MPLS; discuss with MPLS WG
Done  Submit iLBC payload format for Proposed Standard
Done  Submit iLBC codec specification for Experimental
Done  Advance RTP specification and A/V profile to Full Standard
Sep 2005  Submit RTP/SAVPF profile for Proposed Standard
Sep 2005  Submit RTCP/SSM draft for Proposed Standard
Nov 2005  Submit ULP Payload Format for Proposed Standard
Nov 2005  Finished investigation of advanced FEC codes for RTP, update plan
Nov 2005  Submit update of RTP MIB for Proposed or Draft Standard
Nov 2005  Submit Framing of RTP for TLS for Proposed Standard
Nov 2005  Submit RTP Profile for TFRC for Proposed Standard
Nov 2005  Submit RTCP XR MIB for Proposed Standard
Dec 2005  Submit any extensions for RTP HC on MPLS networks for Proposed Standard
Mar 2006  Submit SRTP for Draft Standard
Sep 2006  Submit RTP/AVPF for Draft Standard


  • draft-ietf-avt-tcrtp-08.txt
  • draft-ietf-avt-ulp-12.txt
  • draft-ietf-avt-rtcp-feedback-11.txt
  • draft-ietf-avt-rtcpssm-10.txt
  • draft-ietf-avt-rtp-retransmission-12.txt
  • draft-ietf-avt-rtp-jpeg2000-08.txt
  • draft-ietf-avt-rfc2833bis-12.txt
  • draft-ietf-avt-profile-savpf-03.txt
  • draft-ietf-avt-rtp-midi-format-13.txt
  • draft-ietf-avt-rtp-midi-guidelines-13.txt
  • draft-ietf-avt-rtp-framing-contrans-06.txt
  • draft-ietf-avt-rfc2032-bis-08.txt
  • draft-ietf-avt-rfc2429-bis-06.txt
  • draft-ietf-avt-rtp-3gpp-timed-text-15.txt
  • draft-ietf-avt-rtp-vmr-wb-11.txt
  • draft-ietf-avt-rtp-amrwbplus-07.txt
  • draft-ietf-avt-tfrc-profile-05.txt
  • draft-ietf-avt-rtp-bv-04.txt
  • draft-ietf-avt-rtp-atrac-family-04.txt
  • draft-ietf-avt-audio-t140c-00.txt
  • draft-ietf-avt-rfc2190-to-historic-03.txt
  • draft-ietf-avt-rtcp-xr-mib-03.txt
  • draft-ietf-avt-rtp-amr-bis-02.txt
  • draft-ietf-avt-mime-h224-03.txt
  • draft-ietf-avt-rfc2833bisdata-05.txt
  • draft-ietf-avt-rtp-jpeg2000-beam-01.txt
  • draft-ietf-avt-rtp-no-op-00.txt
  • draft-ietf-avt-rtp-vmr-wb-extension-02.txt
  • draft-ietf-avt-rfc3555bis-01.txt
  • draft-ietf-avt-hc-over-mpls-protocol-01.txt
  • draft-ietf-avt-rtp-hdrext-00.txt
  • draft-ietf-avt-smpte-rtp-00.txt
  • draft-ietf-avt-rfc3047-bis-00.txt
  • draft-ietf-avt-rtp-vc1-01.txt
  • draft-ietf-avt-uncomp-video-ext-01.txt
  • draft-ietf-avt-rtp-speex-00.txt

    Request For Comments:

    RFC1889 PS RTP: A Transport Protocol for Real-Time Applications
    RFC1890 PS RTP Profile for Audio and Video Conferences with Minimal Control
    RFC2029 PS RTP Payload Format of Sun's CellB Video Encoding
    RFC2032 PS RTP payload format for H.261 video streams
    RFC2035 PS RTP Payload Format for JPEG-compressed Video
    RFC2038 PS RTP Payload Format for MPEG1/MPEG2 Video
    RFC2190 PS RTP Payload Format for H.263 Video Streams
    RFC2198 PS RTP Payload for Redundant Audio Data
    RFC2250 PS RTP Payload Format for MPEG1/MPEG2 Video
    RFC2343 E RTP Payload Format for Bundled MPEG
    RFC2354 I Options for Repair of Streaming Media
    RFC2429 PS RTP Payload Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+)
    RFC2431 PS RTP Payload Format for BT.656 Video Encoding
    RFC2435 PS RTP Payload Format for JPEG-compressed Video
    RFC2508 PS Compressing IP/UDP/RTP Headers for Low-Speed Serial Links
    RFC2733 PS An RTP Payload Format for Generic Forward Error Correction
    RFC2736 BCP Guidelines for Writers of RTP Payload Format Specifications
    RFC2762 E Sampling of the Group Membership in RTP
    RFC2793 PS RTP Payload for Text Conversation
    RFC2833 PS RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals
    RFC2862 PS RTP Payload Format for Real-Time Pointers
    RFC2959 PS Real-Time Transport Protocol Management Information Base
    RFC3009 PS Registration of parityfec MIME types
    RFC3016 PS RTP payload format for MPEG-4 Audio/Visual streams
    RFC3047 PS RTP Payload Format for ITU-T Recommendation G.722.1
    RFC3119 PS A More Loss-Tolerant RTP Payload Format for MP3 Audio
    RFC3158 I RTP Testing Strategies
    RFC3189 PS RTP Payload Format for DV Format Video
    RFC3190 PS RTP Payload Format for 12-bit DAT, 20- and 24-bit Linear Sampled Audio
    RFC3267 PS RTP payload format and file storage format for the Adoptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) audio codecs
    RFC3389 PS RTP Payload for Comfort Noise
    RFC3497 PS RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) 292M Video
    RFC3545 PS Enhanced Compressed RTP (CRTP) for links with High Delay,Packet Loss and Reordering
    RFC3550 DS RTP: A Transport Protocol for Real-Time Applications
    RFC3551 DS RTP Profile for Audio and Video Conferences with Minimal Control
    RFC3555 PS MIME Type Registration of RTP Payload Formats
    RFC3556 PS Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth
    RFC3557 PS RTP Payload Format for European Telecommunications Standards Institute (ETSI) European Standard ES 201 108 Distributed Speech Recognition Encoding
    RFC3558 PS RTP Payload Format for Enhanced Variable Rate Codecs (EVRC) and Selectable Mode Vocoders SMV
    RFC3611 Standard RTP Control Protocol Extended Reports (RTCP XR)
    RFC3640 Standard RTP Payload Format for Transport of MPEG-4 Elementary Streams
    RFC3711 Standard The Secure Real-time Transport Protocol
    RFC3951 E Internet Low Bit Rate Codec
    RFC3952 E RTP Payload Format for iLBC Speech
    RFC3984 Standard RTP payload Format for H.264 Video
    RFC4040 Standard RTP payload format for a 64 kbit/s transparent call
    RFC4060 Standard RTP Payload Formats for European Telecommunications Standardsv Institute (ETSI) European Standard ES 202 050, ES 202 211, and ES 202 212 Distributed Speech Recognition Encoding
    RFC4102 Standard Registration of the text/red MIME Sub-Type
    RFC4103 Standard RTP Payload for Text Conversation
    RFC4175 Standard RTP Payload Format for Uncompressed Video
    RFC4184 Standard RTP Payload Format for AC-3 Audio
    RFC4247 I Requirements for Header Compression over MPLS

    Current Meeting Report

    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
       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
       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
       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
       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
       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
       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
       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
    RTP Payload Format for VC-1
       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
       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
    				   - + -


    RFC 3555bis
    Codec Control Messages
    Scalable Video Codec (SVC) payload format
    Multiple Description standard-compatible framework
    RTP Payload Formats for Speex, Vorbis and Theora
    RTP Payload Format for VC-1
    RTP Support for Enhancements to EVRC Family Codecs