2.7.1 Audio/Video Transport (avt)

NOTE: This charter is a snapshot of the 63rd IETF Meeting in Paris, France. It may now be out-of-date.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: 2005-06-29


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 advance the main RTP specification and RTP Profile
  for Audio and Video Conferences with Minimal Control
  for publication as full Internet Standards

- 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 in the ULP and
  UXP payload formats

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

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

- to review the applicability of Compressed RTP operation on
  MPLS networks, developing extensions as necessary

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

The group will also coordinate with the DCCP working group to
ensure that RTP can be efficiently transported over DCCP.

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 05  Submit RTP/SAVPF profile for Proposed Standard
Sep 05  Submit RTCP/SSM draft for Proposed Standard
Nov 05  Submit ULP Payload Format for Proposed Standard
Nov 05  Finished investigation of advanced FEC codes for RTP, update plan
Nov 05  Submit update of RTP MIB for Proposed or Draft Standard
Nov 05  Submit Framing of RTP for TLS for Proposed Standard
Nov 05  Submit RTP Profile for TFRC for Proposed Standard
Nov 05  Submit RTCP XR MIB for Proposed Standard
Dec 05  Submit any extensions for RTP HC on MPLS networks for Proposed Standard
Mar 06  Submit SRTP for Draft Standard
Sep 06  Submit RTP/AVPF for Draft Standard


  • draft-ietf-avt-tcrtp-08.txt
  • draft-ietf-avt-ulp-11.txt
  • draft-ietf-avt-rtcp-feedback-11.txt
  • draft-ietf-avt-rtcpssm-09.txt
  • draft-ietf-avt-rtp-retransmission-11.txt
  • draft-ietf-avt-rtp-jpeg2000-08.txt
  • draft-ietf-avt-rfc2833bis-09.txt
  • draft-ietf-avt-rtp-ac3-07.txt
  • draft-ietf-avt-uncomp-video-06.txt
  • draft-ietf-avt-profile-savpf-02.txt
  • draft-ietf-avt-rtp-midi-format-10.txt
  • draft-ietf-avt-rtp-midi-guidelines-10.txt
  • draft-ietf-avt-rtp-framing-contrans-05.txt
  • draft-ietf-avt-rfc2032-bis-06.txt
  • draft-ietf-avt-rfc2429-bis-05.txt
  • draft-ietf-avt-rtp-3gpp-timed-text-15.txt
  • draft-ietf-avt-hc-mpls-reqs-03.txt
  • draft-ietf-avt-rtp-vmr-wb-10.txt
  • draft-ietf-avt-rtp-amrwbplus-06.txt
  • draft-ietf-avt-tfrc-profile-04.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-02.txt
  • draft-ietf-avt-rtp-amr-bis-01.txt
  • draft-ietf-avt-mime-h224-02.txt
  • draft-ietf-avt-rfc2833bisdata-02.txt
  • draft-ietf-avt-rfc2833biscas-00.txt
  • draft-ietf-avt-rtp-jpeg2000-beam-01.txt
  • draft-ietf-avt-rtp-no-op-00.txt
  • draft-ietf-avt-rtp-vmr-wb-extension-00.txt
  • draft-ietf-avt-rfc3555bis-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

    Current Meeting Report

    Audio/Video Transport (AVT) Working Group Minutes

    Reported by Colin Perkins
    The AVT working group met twice at the 63rd IETF meeting (August 2005, Paris). Subjects under discussion includes using SMPTE timestamps with RTP, new extensions to RTCP XR, an RTP payload for ECN probing, packet reordering in ECRTP, RTP header compression over MPLS paths, using RTP with TFRC, a FEC streaming framework, RTCP extensions for SSM sessions with unicast feedback, RTP payload formats for EVRC, G.722.1 and VC-1, and codec control messages. The meeting was chaired by Colin Perkins and Magnus Westerlund. There were approximately 80 attendees at each session.

    Introduction and Status Update
    The chairs introduced the meeting, with the usual status review. The group has had four RFCs published since the last meeting, six others are with the RFC editor, and we have eight drafts under IESG review. The new working group charter and milestones have been approved; see the web page for details.

    Discussion regarding the use of the media types registry to name RTP payload formats is ongoing, although progress since the last meeting has been limited. Roni Even expressed concern that the discussion is delaying progress of several payload formats; the chairs agreed that this is a significant problem.

    RTP extensions for SMPTE time codes
    draft-singer-rtp-hdrext-00.txt draft-singer-smpte-rtp-01.txt

    Dave Singer briefly outlined the contents of the drafts, and requested input from the working group. The main open issue is the registration rules for header extensions, which need to include a date as well as a reverse DNS name. The group accepted both drafts as work items.

    RTCP XR VoIP Metrics Management Information Base

    Amy Pendleton discussed the RTCP XR VoIP metrics MIB, outlining changes since the previous version (see slides). Several implementations exist, and it is believed there are no significant open issues. One item still to do is to incorporate the Session Table into the RTP MIB, otherwise the draft is believed complete.

    Colin Perkins asked if incorporating the Session Table into the RTP MIB would cause any backwards compatibility issues? Dan Romascanu commented that the semantics of the MIB change (due to the extra table), but it's otherwise an extension that won't break anything.

    Dan Romascanu noted that there are still some places in the draft which refer to the old MIB structure, and need updating. He will propose text to correct these.

    Magnus Westerlund and Dan Romascanu noted that it's unclear how the source and destination fields are handled in multiparty conferences. The entries in the Session Table should be clarified, perhaps using the terms "local" and "remote"?

    RTCP XR - New Parameter Extensions

    Amy Pendleton discussed new parameter extensions to the RTCP XR block. The feedback from the previous meeting was to define a separate draft for video metrics - this hasn't been done yet, but will occur shortly. The present draft includes basic video metrics, and the more developed high resolution audio metrics (see slides for a summary of the changes).

    Colin Perkins and Magnus Westerlund expressed concern about the number of options and possibilities for vendor extensions in the new HR Voice Metrics block. Rajesh Kumar commented that there are various standards for jitter measurement that exist and must be reflected, hence the use of so many options. Colin agreed, stating that he didn't ask to remove all the options, just to use caution in adding only those which fulfil a real need.

    Colin Perkins and Dave Oran expressed concern about partial duplication of information between the RTCP XR block and the SDP signalling. Colin was of the opinion that this information can be retrieved from the SDP and doesn't need to be included in RTCP XR. Dave accepted the potential usefulness of this information, but was opposed to duplicating it in a different format: either include the complete SDP data, or rely on the signalling protocol.

    RTP Payload Format for ECN Probing
    draft-alexander-rtp-payload-for-ecn-probing-01.txt draft-babiarz-tsvwg-rtecn-04.txt draft-alexander-rtecn-use-cases-00.txt draft-alexander-congestion-status-preconditions-00.txt

    Corey Alexander presented a proposal for an RTP Payload Format for ECN probing, which can be used for congestion-based admission control. The SDP aspects of this were presented in MMUSIC; and the drafts were also discussed in TSVWG.

    Dan Wing questioned the use scenario: what is meant by admission control, since there is no guarantee of resource availability? The mechanism will allow the state of the network to be measured, but it's unclear what can be done with that information: a measurement of the congestion state for a particular point in time implies nothing about the future state of the network.

    Colin Perkins noted that this proposal uses RTP in a manner that is far from the usual semantics: no media data is conveyed or played out. This implies that it's not an appropriate use of the RTP protocol. Dave Oran agreed, but noted that there is a desire to make probing packets appear similar to data packets. Joerg Ott wondered why appropriately sized and spaced UDP packets can't be used to probe the path, before the RTP flow starts.

    Michael Ramalho noted that while this format may not be suitable for admission control, it can be used to provide an indication that some requested level of QoS is not available. Colin Perkins agreed with the concept, but noted that ECN could be applied to standard media streams for that purpose, without needing a new payload format.

    Finally, Colin Perkins noted that, for this to be useful, it must be applied to all media types, including text. Colin thought it likely that a MIME registration for this format under the "text" top-level would receive considerable push back during the media types review.

    Packet reordering in Extended CRTP (ECRTP)

    Andrew Johnson discussed packet reordering and ECRTP, explaining how a decompressor can handle various jumps in sequence number that might be caused by packet reordering. The group was asked to consider the draft as a work item: the chairs agreed that it covers appropriate material, but Magnus Westerlund requested that the draft include more details on the protocol changes, use cases, and limitations before it be accepted.

    Protocol Extensions for Header Compression over MPLS

    Jerry Ash presented protocol extensions for header compression over MPLS paths, reviewing the changes and outlining the open issues (see slides). Colin Perkins commented that the protocol constants needed to implement this are scattered over several drafts, which makes this hard to follow. Otherwise, this is progressing well, and will become an AVT work item now the new charter is in place.

    RTP Profile for TCP Friendly Rate Control

    Ladan Gharai outlined the changes to the RTP profile for TFRC since the last draft (see slides).

    Dave Oran asked if there is a need to describe how to combine this with the AVPF and SAVPF profiles? Colin Perkins that the timing rules may be difficult; Anders Klemets agreed, but noted that feedback packets might be useful in conjunction the TFRC profile. Should consider defining the TFRC profile to allow AVPF packet types, with different timing rules.

    Stephan Wenger commented that he believed the introduction of a second timestamp to be long overdue in RTP, and asked why not add more timing info? e.g. decoding timestamp, presentation timestamp, etc. There was some discussion of the merits of this, but no conclusion was reached.

    Magnus Westerlund wondered if additional guidelines were needed for how applications should handle media in conjunction with congestion control. Ladan agreed that this is a difficult subject, but didn't include it to avoid overlap with Tom Phelan's draft on Strategies for Streaming Media Applications with TFRC.

    Strategies for Streaming Media Applications Using TFRC

    Tom Phelan outlined the draft on streaming media and TFRC, explaining some issues that arise when designing congestion controlled streaming media applications. This was discussed more in the DCCP working group.

    FEC Streaming Framework

    Mark Watson outlined a proposed FEC Streaming Framework, building on work done in RMT and 3GPP to provide an FEC layer that can be used, amongst other things, to provide protection for RTP flows. This was discussed in detail in TSVWG.

    RTCP Extensions for SSM sessions with unicast feedback

    Joerg Ott discussed the RTCP extensions for SSM with unicast feedback, the latest version of which has been reworked to separate the concepts of media source and distribution source. The remaining open issue is how to signal the link from media source to distribution source in SDP: the proposal is to make this a separate session at the SDP level, hiding the details from the receivers. Colin Perkins asked that the draft clarify that there is a single RTP session, despite it being separate at the SDP level, but otherwise there was general agreement on this choice.

    Dave Oran asked if there are any exceptions for RTCP packets that don't need to be reflected down the tree (e.g. NACK feedback packets?) and if the document needs to say something about this? Joerg replied that there is some wording related to this about summary reports, but that in the reflection mode you'll get compound packets, and it may not be possible to split out the subpacket when reflecting (e.g. due to authentication). Need to think about it, and include some words.

    Update of ITU G.722.1 payload format

    Roni Even discussed the updated payload format for G.722.1 (adding 14kHz support to the RFC 3047 format). Magnus Westerlund noted that there is a need for clarifications on how the RTP marker and sequence number are to be handled, but there was otherwise little discussion because the format is not complex. The sense of the room was to accept this draft as an AVT work item.

    RTP Payload Format for Video Codec 1 (VC-1)

    Anders Klemets outlining key features of the VC-1 codec, and described the payload format specification. Discussion related primarily to the coupling of RTP timestamp to decode time, rather than the presentation time. Dave Singer noted that RTP payload formats generally set the RTP timestamp equal to the presentation time and send in decode time order, and there are good consistency and backwards compatibility reasons to stick with this if possible. Dave also had some concern about how one could calculate the presentation time from potentially missing packets, wondering if it's always possible to calculate? Including an explicit presentation time would avoid this problem.

    Stephen Wenger also spoke in favour of using the presentation time as the RTP timestamp. He noted that this codec seems similar to MPEG in its timing model, and wondered if the decode time was actually a time or just a sequencing concept. If the only requirement is sequencing, then the RTP sequence number is sufficient, rather than including the decoding timestamp.

    Finally, Magnus Westerlund noted that using the decoding time for the RTP timestamp breaks the RTP synchronisation model, since there is no longer have a common timebase for synchronisation between media (but, if the presentation time is used, this is equivalent to the sampling instant used for audio codecs).

    The conclusion was to change to using the presentation timestamp.

    The sense of the room was to adopt this draft as an AVT work item.

    Open issues with Codec Control Messages in AVPF

    The final agenda item was a discussion of open issues with the codec control messages draft, led by Stephan Wenger.

    Colin Perkins, Stephan Wenger and Magnus Westerlund discussed the need for the Temporary Max Media Bit Rate (TMMBR) indication, to understand whether the usual RTCP mechanisms can be used to measure the bit rate, rather than requiring the sender to acknowledge the TMMBR message. The conclusion was that the acknowledgement is needed, but the rationale should be clarified.

    Dave Oran asked if the TemporalSpatialTradeOff (TSTR) message is not already in SDP? Yes, but there is a need for faster signalling than in a SIP exchange. Again, rationale to be clarified.

    Colin Perkins queried why FreezePicture Indication was needed, since it can be derived from the media stream (e.g. SSRC/CSRC will change, to indicate a change of source, and the decoder then parses but doesn't display the stream until it gets to a refresh point). Stephan replied that this is because some gateways don't correctly set the SSRC/ CSRC. Dave Singer also noted that some people don't like freeze picture, so the requirement for this is not universal. Stephan noted that he does not think this feature vital, and may remove it in the next revision.

    Dave Singer expressed concern that repeating requests does not always work; might want a method to stop a receiver making repeated requests to a sender that cannot satisfy them.

    Colin Perkins concluded by noting that these messages can potentially turn RTCP into a full scale control protocol, rather than a channel to convey presence, reception quality statistics and hints on the desired media coding. The draft needs to clearly set bounds on the appropriate codec control messages, and to understand the limits of what is being enabled.

    - + -


    Introduction and status update
    RTCP XR: VoIP Metrics MIB
    RTCP XR: New Parameter Extensions
    RTP Payload Format for ECN Probing
    Packet reordering in Extended CRTP
    Protocol Extensions for Header Compression over MPLS
    RTP Profile for TCP Friendly Rate Control
    Strategies for Streaming Media Applications Using TFRC
    FEC streaming framework
    RTCP Extensions for SSM sessions with unicast feedback
    Update of ITU G.722.1 payload format
    RTP Payload Format for Video Codec 1 (VC-1)
    Open issues with Codec Control Messages in AVPF