Network Working Group S. Wenger
Internet-Draft J. Lennox
Updates: 5104 (if approved) Vidyo, Inc.
Intended status: Standards Track B. Burman
Expires: November 18, 2016 M. Westerlund
Ericsson
May 17, 2016
Using Codec Control Messages in the RTP Audio-Visual Profile with
Feedback with Layered Codecs
draft-ietf-avtext-avpf-ccm-layered-01
Abstract
This document fixes a shortcoming in the specification language of
the Codec Control Message Full Intra Request (FIR) as defined in
RFC5104 when using with layered codecs. In particular, a Decoder
Refresh Point needs to be sent by a media sender when a FIR is
received on any layer of the layered bitstream, regardless on whether
those layers are being sent in a single or in multiple RTP flows.
The other payload-specific feedback messages defined in RFC 5104 and
RFC 4585 as updated by RFC 5506 have also been analyzed, and no
corresponding shortcomings have been found.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 18, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction and Problem Statement . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Updated definition of Decoder Refresh Point . . . . . . . . . 4
4. Full Intra Request for Layered Codecs . . . . . . . . . . . . 4
5. Identifying the use of Layered Codecs (Informative) . . . . . 5
6. Layered Codecs and non-FIR codec control messages
(Informative) . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Picture Loss Indication (PLI) . . . . . . . . . . . . . . 6
6.2. Slice Loss Indication (SLI) . . . . . . . . . . . . . . . 6
6.3. Reference Picture Selection Indication (RPSI) . . . . . . 6
6.4. Temporal-Spatial Trade-off Request and Notification
(TSTR/TSTN) . . . . . . . . . . . . . . . . . . . . . . . 7
6.5. H.271 Video Back Channel Message (VBCM) . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction and Problem Statement
RFC 4585 [RFC4585] and RFC 5104 [RFC5104] specify a number of
payload-specific feedback messages which a media receiver can use to
inform a media sender of certain conditions, or make certain
requests. The feedback messages are being sent as RTCP receiver
reports, and RFC 4585 specifies timing rules that make the use of
those messages practical for time-sensitive codec control.
Since the time those RFCs were developed, layered codecs have gained
in popularity and deployment. Layered codecs use multiple sub-
bitstreams called layers to represent the content in different
fidelities. Depending on the media codec and its RTP payload format
in use, single layers or groups of layers may be sent in their own
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RTP streams (in MRST or MRMT mode as defined in RFC 7656 [RFC7656]),
or multiplexed (using media-codec specific multiplexing mechanisms)
in a single RTP stream (SRST mode as defined in RFC 7656 [RFC7656]).
The dependency relationship between layers forms a directed graph,
with the base layer at the root. Enhancement layers depend on the
base layer and potentially on other enhancement layers, and the
target layer and all layers it depends on have to be decoded jointly
in order to re-create the uncompressed media signal at the fidelity
of the target layer.
Implementation experience has shown that the Full Intra Request
command as defined in RFC 5104 [RFC5104] is underspecified when used
with layered codecs and when more than one RTP stream is used to
transport the layers of a layered bitstream at a given fidelity. In
particular, from the RFC 5104 [RFC5104] specification language it is
not clear whether an FIR received for only a single RTP stream of
multiple RTP streams covering the same layered bitstream necessarily
triggers the sending of a Decoder Refresh Point (as defined in
RFC 5104 [RFC5104] section 2.2) for all layers, or only for the layer
which is transported in the RTP stream which the FIR request is
associated with.
This document fixes this shortcoming by:
a. Updating the definition of the Decoder Refresh Point (as defined
in RFC 5104 [RFC5104] section 2.2) to cover layered codecs, in
line with the corresponding definitions used in a popular layered
codec format, namely H.264/SVC [H.264]. Specifically, a decoder
refresh point, in conjunction with layered codecs, resets the
state of the whole decoder, which implies that it includes hard
or gradual single-layer decoder refresh for all layers;
b. Requiring that, when a media sender receives a Full Intra Request
over the RTCP stream associated with any of the RTP streams over
which a part of the layered bitstream is transported, to send a
Decoder Refresh Point;
c. Require that a media receiver sends the FIR on the RTCP stream
associated with the base layer (the option of receiving FIR on
enhancement layer-associated RTCP stream as specified in point b)
above is kept for backward compatibility); and
d. Providing guidance on how to detect that a layered codec is in
use for which the above rules apply.
While, clearly, the reaction to FIR for layered codecs in RFC 5104
[RFC5104] and companion documents is underspecified, it appears that
this is not the case for any of the other payload-specific codec
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control messages defined in any of RFC 4585 [RFC4585], RFC 5104
[RFC5104], or RFC 5506 [RFC5506]. A brief summary of the analysis
that led to this conclusion is also included in this document.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Updated definition of Decoder Refresh Point
The text below updates the definition of Decoder Refresh Point in
section 2.2 of RFC 5104 [RFC5104].
Decoder Refresh Point: A bit string, packetized in one or more RTP
packets, that completely resets the decoder to a known state.
Examples for "hard" single layer decoder refresh points are Intra
pictures in H.261 [H.261], H.263 [H.263], MPEG-1 [MPEG-1], MPEG-2
[MPEG-2], and MPEG-4 [MPEG-4]; Instantaneous Decoder Refresh (IDR)
pictures in H.264 [H.264], and H.265 [H.265]; and Keyframes in VP8
[RFC6386] and VP9 [I-D.grange-vp9-bitstream]. "Gradual" decoder
refresh points may also be used; see for example H.264 [H.264].
While both "hard" and "gradual" decoder refresh points are acceptable
in the scope of this specification, in most cases the user experience
will benefit from using a "hard" decoder refresh point.
A decoder refresh point also contains all header information above
the syntactical level of the picture layer (or equivalent, depending
on the video compression standard) that is conveyed in-band. In
H.264 [H.264], for example, a decoder refresh point contains
parameter set Network Adaptation Layer (NAL) units that generate
parameter sets necessary for the decoding of the following slice/data
partition NAL units (and that are not conveyed out of band).
When a layered codec is in use, the above definition (and, in
particular, the requirement to COMPLETELY reset the decoder to a
known state) implies that the decoder refresh point includes hard or
gradual single layer decoder refresh points for all layers.
4. Full Intra Request for Layered Codecs
When a media receiver or middlebox has decided to send a FIR command
(based on the guidance provided in Section 4.3.1 of RFC 5104
[RFC5104], it MUST do so in the RTCP stream related to the forward
RTP stream that carries the base layer of the layered bitstream, and
the Feedback Control Information (FCI, and in particular the SSRC
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field therein) MUST also refer to the forward RTP stream that carries
the base layer.
When a Full Intra Request Command is received by the designated media
sender in the RTCP stream associated with any of the RTP streams in
which any layer of a layered bitstream are sent, the designated media
sender MUST send a Decoder Refresh Point (Section 3) as defined above
at its earliest opportunity. The requirements related to congestion
control on the forward RTP streams as specified in sections 3.5.1.5
of RFC 5104 [RFC5104] apply for the RTP streams both in isolation and
combined.
Note: the requirement to react to FIR commands associated with
enhancement layers is included for robustness and backward
compatibility reasons.
5. Identifying the use of Layered Codecs (Informative)
The above modifications to RFC 5104 unambiguously define how to deal
with FIR when layered bitstreams are in use. However, it is
surprisingly difficult to identify this situation. In general, it is
expected that implementers know when layered coding (in its commonly
understood sense: with inter-layer prediction between pyramided-
arranged layers) is in use and when not, and can therefore implement
the above updates to RFC 5104 correctly. However, there are use
cases of the use of layered codecs that may be viewed as somewhat
exotic today but clearly are supported by the video coding syntax, in
which the above rules would lead to suboptimal system behavior.
Nothing would break, and there would not be an interop failure, but
the user experience may suffer through the sending or receiving of
Decoder Refresh Points at times or on parts of the bitstream that are
unnecessary from a user experience viewpoint. Therefore, this
informative section is included that provides the current
understanding of when a layered codec is in use and when not.
The key observation made here is that the RTP payload format
negotiated for the RTP streams, in isolation, is not necessarily an
indicator for the use of layering. Some layered codecs (including
H.264/SVC) can form decodable bitstreams including only (one or more)
enhancement layers, without the base layer, effectively creating
simulcastable sub-bitstreams in a scalable bitstream that does not
take advantage of inter-layer prediction. In such a scenario, it is
potentially (though not necessarily) unnecessary--or even counter-
productive--to send a decoder refresh point on all RTP streams using
that payload format and SSRC.
One good indication of the likely use of layering with interlayer
prediction is when the various RTP streams are "bound" together on
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the signaling level. In an SDP environment, this would be the case
if they are marked as being dependent from each other using the
grouping framework RFC 4588 [RFC4588] and the layer dependency
RFC 5583 [RFC5583].
6. Layered Codecs and non-FIR codec control messages (Informative)
Between them, RFC 4585 [RFC4585] (as updated by RFC 5506 [RFC5506])
and RFC 5104 [RFC5104] define a total of seven Payload-specific
Feedback messages. For the FIR command message, guidance has been
provided above. In this section, some information is provided with
respect to the remaining six codec control messages.
6.1. Picture Loss Indication (PLI)
PLI is defined in RFC 4585 [RFC4585] section 6.3.1. The prudent
response to a PLI message received for an enhancement layer is to
"repair" (through whatever source-coding specific means) that
enhancement layer and all dependent enhancement layers, but not the
reference layer(s) used by the enhancement layer for which the PLI
was received. The encoder can figure out by itself what constitutes
a dependent enhancement layer and does not need help from the system
stack in doing so. Insofar, there is nothing that needs to be
specified herein.
6.2. Slice Loss Indication (SLI)
SLI is defined in RFC 4585 [RFC4585] section 6.3.2. The authors'
current understanding is that the prudent response to a SLI message
received for an enhancement layer is to "repair" (through whatever
source-coding specific means) the affected spatial area of that
enhancement layer and all dependent enhancement layers, but not the
reference layers used by the enhancement layer for which the SLI was
received. The encoder can figure out by itself what constitutes a
dependent enhancement layer and does not need help from the system
stack in doing so. Insofar, there is nothing that needs to be
specified herein. SLI has seen very little implementation and, as
far as it is known, none in conjunction with layered systems.
6.3. Reference Picture Selection Indication (RPSI)
RPSI is defined in RFC 4585 [RFC4585] section 6.3.3. While a
technical equivalent of RPSI has been in use with non-layered systems
for many years, no implementations are known in conjunction of
layered codecs. The authors' current understanding is that the
reception of an RPSI message on any layer forces the encoder to
"repair" the bitstream on that layer and all dependent layers without
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the need of any system-provided guidance. Insofar, RPSI should work
without further need for specification language.
6.4. Temporal-Spatial Trade-off Request and Notification (TSTR/TSTN)
TSTN/TSTR are defined in RFC 5104 [RFC5104] section 4.3.2 and 4.3.3,
respectively. The TSTR request allows to communicate (typically
user-interface-obtained) guidance of the preferred trade-off between
spatial quality and frame rate. A technical equivalent of TSTN/TSTR
has seen deployment for many years in non-scalable systems.
The Temporal-Spatial Trade-off request and notification messages
include an SSRC target, which (similarly to FIR) may refer to an RTP
stream carrying a base layer, an enhancement layer, or multiple
layers. Therefore, the authors' current understanding is that the
semantics of the message applies to the layers present in the
targeted RTP stream.
It is noted that per-layer TSTR/TSTN is a mechanism that is, in some
ways, counterproductive in a system using layered codecs. Given a
sufficiently complex layered bitstream layout, a sending system has
flexibility in adjusting the spatio/temporal quality balance by
adding and removing temporal, spatial, or quality enhancement layers.
At present it is unclear whether an allowed (or even recommended)
option to the reception of a TSTR is to adjust the bit allocation
within the layer(s) present in the addressed RTP stream, or to adjust
the layering structure accordingly--which can involve more than just
the addressed RTP stream.
Until there is a sufficient critical mass of implementation practice,
it is probably prudent for an implementer not to assume either of the
two options (or any middleground that may exist between the two), be
liberal in accepting TSTR messages, perhaps responding in TSTN
indicating "no change," not sending TSTR messages except when
operating in SRST mode as defined in RFC 7656 [RFC7656], and
contribute to the IETF documentation of any implementation
requirements that make per-layer TSTR/TSTN useful.
6.5. H.271 Video Back Channel Message (VBCM)
VBCM is defined in RFC 5104 [RFC5104] section 4.3.4. What was said
above for RPSI (Section 6.3) applies here as well.
7. Acknowledgements
The authors want to thank Mo Zanaty for useful discussions.
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8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
The security considerations of RFC 4585 [RFC4585] (as updated by
RFC 5506 [RFC5506]) and RFC 5104 [RFC5104] apply. The clarified
response to FIR does not require any updates.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <http://www.rfc-editor.org/info/rfc5104>.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
2009, <http://www.rfc-editor.org/info/rfc5506>.
10.2. Informative References
[H.261] ITU-T, "ITU-T Rec. H.261: Video codec for audiovisual
services at p x 64 kbit/s", 1993,
<http://handle.itu.int/11.1002/1000/1088>.
[H.263] ITU-T, "ITU-T Rec. H.263: Video coding for low bit rate
communication", 2005,
<http://handle.itu.int/11.1002/1000/7497>.
[H.264] ITU-T, "ITU-T Rec. H.264: Advanced video coding for
generic audiovisual services", 2014,
<http://handle.itu.int/11.1002/1000/12063>.
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[H.265] ITU-T, "ITU-T Rec. H.265: High efficiency video coding",
2015, <http://handle.itu.int/11.1002/1000/12455>.
[I-D.grange-vp9-bitstream]
Grange, A. and H. Alvestrand, "A VP9 Bitstream Overview",
draft-grange-vp9-bitstream-00 (work in progress), February
2013.
[I-D.ietf-mmusic-rid]
Thatcher, P., Zanaty, M., Nandakumar, S., Burman, B.,
Roach, A., and B. Campen, "RTP Payload Format
Constraints", draft-ietf-mmusic-rid-05 (work in progress),
March 2016.
[I-D.ietf-mmusic-sdp-simulcast]
Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
"Using Simulcast in SDP and RTP Sessions", draft-ietf-
mmusic-sdp-simulcast-04 (work in progress), February 2016.
[MPEG-1] ISO/IEC, "ISO/IEC 11172-2:1993 Information technology --
Coding of moving pictures and associated audio for digital
storage media at up to about 1,5 Mbit/s -- Part 2: Video",
1993.
[MPEG-2] ISO/IEC, "ISO/IEC 13818-2:2013 Information technology --
Generic coding of moving pictures and associated audio
information -- Part 2: Video", 2013.
[MPEG-4] ISO/IEC, "ISO/IEC 14496-2:2004 Information technology --
Coding of audio-visual objects -- Part 2: Visual", 2004.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>.
[RFC5583] Schierl, T. and S. Wenger, "Signaling Media Decoding
Dependency in the Session Description Protocol (SDP)",
RFC 5583, DOI 10.17487/RFC5583, July 2009,
<http://www.rfc-editor.org/info/rfc5583>.
[RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
Guide", RFC 6386, DOI 10.17487/RFC6386, November 2011,
<http://www.rfc-editor.org/info/rfc6386>.
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[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
Appendix A. Change Log
NOTE TO RFC EDITOR: Please remove this section prior to publication.
draft-wenger-avtext-avpf-ccm-layered-00-00: initial version
draft-ietf-avtext-avpf-ccm-layered-00: resubmit as avtext WG draft
per IETF95 and list confirmation by Rachel 4/25/2016
draft-ietf-avtext-avpf-ccm-layered-00: In section "Identifying the
use of Layered Codecs (Informative)", removed last sentence that
could be misread that the explicit signaling of simulcasting in
conjunction with payload formats supporting layered coding implies no
layering.
Authors' Addresses
Stephan Wenger
Vidyo, Inc.
Email: stewe@stewe.org
Jonathan Lennox
Vidyo, Inc.
Email: jonathan@vidyo.com
Bo Burman
Ericsson
Kistavagen 25
SE - 164 80 Kista
Sweden
Email: bo.burman@ericsson.com
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Magnus Westerlund
Ericsson
Farogatan 6
SE- 164 80 Kista
Sweden
Phone: +46107148287
Email: magnus.westerlund@ericsson.com
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