Network Working Group S. Wenger
Internet-Draft J. Lennox
Updates: 5104 (if approved) Vidyo, Inc.
Intended status: Standards Track B. Burman
Expires: May 21, 2017 M. Westerlund
Ericsson
November 17, 2016
Using Codec Control Messages in the RTP Audio-Visual Profile with
Feedback with Layered Codecs
draft-ietf-avtext-avpf-ccm-layered-03
Abstract
This document updates RFC5104 by fixing a shortcoming in the
specification language of the Codec Control Message Full Intra
Request (FIR) as defined in RFC5104 when using it 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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This Internet-Draft will expire on May 21, 2017.
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 . . . . . . . . . . . . 5
5. Identifying the use of layered bitstreams (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) . . . . . . 7
6.4. Temporal-Spatial Trade-off Request and Notification
(TSTR/TSTN) . . . . . . . . . . . . . . . . . . . . . . . 7
6.5. H.271 Video Back Channel Message (VBCM) . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction and Problem Statement
The Extended RTP Profile for Real-time Transport Control Protocol
(RTCP)-Based Feedback (RTP/AVPF) [RFC4585] and Codec Control Messages
in the RTP Audio-Visual Profile with Feedback (AVPF) [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
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fidelities. Depending on the media codec and its RTP payload format
in use, a number of options exist how to transport those layers in
RTP. With reference to A Taxonomy of Semantics and Mechanisms for
Real-Time Transport Protocol (RTP) Sources [RFC7656]):
single layers or groups of layers may be sent in their own RTP
streams in MRST or MRMT mode;
using media-codec specific multiplexing mechanisms, multiple
layers may be sent in a single RTP stream in SRST mode.
The dependency relationship between layers in a truly layered,
pyramid-shaped bitstream 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.
Such a layering structure is assumed henceforth; for more exotic
layering structures please see Section 5.
Implementation experience has shown that the Full Intra Request
command as defined in [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 [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 [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 [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. Require a media sender to send a Decoder Refresh Point after the
media sender has received a Full Intra Request over an RTCP
stream associated with any of the RTP streams over which a part
of the layered bitstream is transported;
c. Require that a media receiver sends the FIR on the RTCP stream
associated with the base layer. The option of receiving FIR on
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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 bitstream is
in use for which the above rules apply.
While, clearly, the reaction to FIR for layered codecs in [RFC5104]
and companion documents is underspecified, it appears that this is
not the case for any of the other payload-specific codec control
messages defined in any of [RFC4585], [RFC5104]. 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 remainder of this section replaces the definition of Decoder
Refresh Point in section 2.2 of [RFC5104] in its entirety.
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 that is conveyed in-band.
In [H.264], for example, a decoder refresh point contains those
parameter set Network Adaptation Layer (NAL) units that generate
parameter sets necessary for the decoding of the following slice/data
partition NAL units. (That is assuming the parameter sets have not
been conveyed out of band.)
When a layered codec is in use, the above definition--in particular,
the requirement to completely reset the decoder to a known state--
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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
A media receiver or middlebox may decide to send a FIR command based
on the guidance provided in Section 4.3.1 of [RFC5104]. When sending
the FIR command, it MUST target the RTP stream that carries the base
layer of the layered bitstream, and this is done by setting the
Feedback Control Information (FCI, and in particular the SSRC field
therein) to refer to the SSRC of 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.
and 5. of [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 bitstreams (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 the use of a layered bitstream.
In general, it is expected that implementers know when layered
bitstreams (in its commonly understood sense: with inter-layer
prediction between pyramided-arranged layers) are in use and when
not, and can therefore implement the above updates to RFC 5104
correctly. However, there are scenarios in which layered codecs are
employed creating non-pyramid shaped bitstreams. Those scenarios may
be viewed as somewhat exotic today but clearly are supported by
certain video coding syntaxes, such as H.264/SVC. When blindly
applying the above rules to those non-pyramid-arranged layering
structures, suboptimal system behavior would result. Nothing would
break, and there would not be an interoperability 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 bitstream is in use and when not.
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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 a layered bitstream. 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 within a single scalable
bitstream (as defined in the video coding standard), but without
inter-layer prediction. In such a scenario, it is potentially,
though not necessarily, counter-productive to send a decoder refresh
point on all RTP streams using that payload format and SSRC. It is
beyond the scope of this document to discuss optimized reactions to
FIRs received on RTP streams carrying such exotic bitstreams.
One good indication of the likely use of pyramid-shaped layering with
interlayer prediction is when the various RTP streams are "bound"
together on the signaling level. In an SDP environment, this would
be the case if they are marked as being dependent from each other
using The Session Description Protocol (SDP) Grouping Framework
[RFC5888] and the layer dependency RFC 5583 [RFC5583].
6. Layered Codecs and non-FIR codec control messages (Informative)
Between them, AVPF [RFC4585] and Codec Control Messages [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 section 6.3.1 of [RFC4585]. The prudent response
to a PLI message received for an enhancement layer is to "repair"
that enhancement layer and all dependent enhancement layers through
appropriate source-coding specific means. However, the reference
layer(s) used by the enhancement layer for which the PLI was received
does not require repair. 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. Thus, there is nothing that needs to
be specified herein.
6.2. Slice Loss Indication (SLI)
SLI is defined in section 6.3.2 of [RFC4585]. The authors' current
understanding is that the prudent response to a SLI message received
for an enhancement layer is to "repair" the affected spatial area of
that enhancement layer and all dependent enhancement layers through
appropriate source-coding specific means. As in PLI, the reference
layers used by the enhancement layer for which the SLI was received
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do not need to be repaired. Again, as in PLI, the encoder can
determine by itself what constitutes a dependent enhancement layer
and does not need help from the system stack in doing so. Thus,
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 section 6.3.3 of [RFC4585]. 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 indicating a missing reference picture forces
the encoder to appropriately handle that missing reference picture in
the layer indicated, and all dependent layers. Thus, 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 section 4.3.2 and 4.3.3 of [RFC5104],
respectively. The TSTR request communicates 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.
Instead, it is suggested that an implementation be liberal in
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accepting TSTR messages, and upon receipt responding in TSTN
indicating "no change". Further, it is suggested that new
implementations do not send TSTR messages except when operating in
SRST mode as defined in [RFC7656]. Finally implementers are
encouraged to 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 section 4.3.4 of [RFC5104]. 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.
8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
The security considerations of AVPF [RFC4585] (as updated by Support
for Reduced-Size Real-Time Transport Control Protocol (RTCP):
Opportunities and Consequences [RFC5506]) and Codec Control Messages
[RFC5104] apply. The clarified response to FIR does not introduce
additional security considerations.
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>.
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[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>.
[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.
[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.
[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>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>.
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[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>.
[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.
draft-ietf-avtext-avpf-ccm-layered-01: clarifications in section 5.
draft-ietf-avtext-avpf-ccm-layered-02: addressing WGLC comments,
mostly editorial; see reflector discussions 09/2016
draft-ietf-avtext-avpf-ccm-layered-03: addressing AD writeup
comments, editorial
Authors' Addresses
Stephan Wenger
Vidyo, Inc.
Email: stewe@stewe.org
Jonathan Lennox
Vidyo, Inc.
Email: jonathan@vidyo.com
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Bo Burman
Ericsson
Kistavagen 25
SE - 164 80 Kista
Sweden
Email: bo.burman@ericsson.com
Magnus Westerlund
Ericsson
Farogatan 2
SE- 164 80 Kista
Sweden
Phone: +46107148287
Email: magnus.westerlund@ericsson.com
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