Internet DRAFT - draft-uberti-rtcweb-fec
draft-uberti-rtcweb-fec
Network Working Group J. Uberti
Internet-Draft Google
Intended status: Standards Track October 27, 2014
Expires: April 30, 2015
WebRTC Forward Error Correction Requirements
draft-uberti-rtcweb-fec-00
Abstract
This document makes recommendations for how Forward Error Correction
(FEC) should be used by WebRTC applications.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 30, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Types of FEC . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. Separate FEC Stream . . . . . . . . . . . . . . . . . . . 3
3.2. Redundant Encoding . . . . . . . . . . . . . . . . . . . 3
3.3. Codec-Specific In-band FEC . . . . . . . . . . . . . . . 3
4. FEC for Audio Content . . . . . . . . . . . . . . . . . . . . 3
4.1. Recommended Mechanism . . . . . . . . . . . . . . . . . . 3
4.2. Negotiating Support . . . . . . . . . . . . . . . . . . . 4
5. FEC for Video Content . . . . . . . . . . . . . . . . . . . . 4
5.1. Recommended Mechanism . . . . . . . . . . . . . . . . . . 4
5.2. Negotiating Support . . . . . . . . . . . . . . . . . . . 5
6. Implementation Requirements . . . . . . . . . . . . . . . . . 5
7. Adaptive Use of FEC . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
11.1. Normative References . . . . . . . . . . . . . . . . . . 6
11.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
In situations where packet loss is high, or media quality must be
perfect, Forward Error Correction (FEC) can be used to proactively
recover from packet losses. This document describes what FEC
mechanisms should be used by WebRTC client implementations.
2. Terminology
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 [RFC2119].
3. Types of FEC
By its name, FEC describes the sending of redundant information in an
outgoing packet stream so that information can still be recovered
even in the face of packet loss. There are multiple ways in which
this can be accomplished; this section enumerates the various
mechanisms and describes their tradeoffs.
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3.1. Separate FEC Stream
This approach, as described in [RFC5956], Section 4.3, sends FEC
packets as an independent SSRC-multiplexed stream, with its own SSRC
and payload type. While by far the most flexible, each FEC packet
will have its own IP+UDP+RTP+FEC header, leading to additional
overhead of the FEC stream.
3.2. Redundant Encoding
This approach, as descibed in [RFC2198], allows for redundant data to
be piggybacked on an existing primary encoding in a single packet.
This redundant data may be an exact copy of a previous packet, or for
codecs that support variable-bitrate encodings, possibly a smaller,
lower-quality representation. Since there is only a single set of
packet headers, this allows for a very efficient representation of
primary + redundant data. However, this savings is only realized
when the two encodings both fit into a single packet (i.e. less than
a MTU). This approach is also only applicable to audio content.
3.3. Codec-Specific In-band FEC
Some audio codecs, notably Opus [RFC6716], support their own in-band
FEC mechanism, where FEC data is included in the codec payload. In
the case of Opus specifically, packets deemed as important are re-
encoded at a lower bitrate and added to the subsequent packet,
allowing partial recovery of a lost packet. See [RFC6716],
Section 2.1.7 for details.
4. FEC for Audio Content
The following section provides guidance on how to best use FEC for
transmitting audio data. As indicated in Section 7 below, FEC should
only be activated if network conditions warrant it, or upon explicit
application request.
4.1. Recommended Mechanism
When using the Opus codec in its default (hybrid) mode, use of the
built-in Opus FEC mechanism is RECOMMENDED. This provides reasonable
protection of the audio stream against typical losses, with moderate
overhead. [TODO: add stats] Note though that this mechanism only
protects the SILK layer of the Opus codec; the CELT portion is not
protected. This is not an issue when Opus is running in hybrid mode,
as the lower frequencies will still be able to be recovered, with
minimal quality impact.
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When using Opus in CELT mode, or other variable-bitrate codecs, use
of [RFC2198] redundant encoding with a lower-fidelity version of the
previous packet is RECOMMENDED. When using Opus specifically, the
lower-fidelity version can simply be a truncated version of the
previous Opus packet. [TODO: decide exact truncated size] This
provides reasonable protection of the payload with minimal overhead.
When using constant-bitrate codecs, e.g. PCMU, use of [RFC2198]
redundant encoding is NOT RECOMMENDED, as this will result in a
potentially significant bitrate increase. Furthermore, suddenly
increasing the bitrate to deal with packet losses may actually make
things worse.
Because of the lower packet rate of audio encodings, usually a single
packet per frame, use of a separate FEC stream comes with a higher
overhead than other mechanisms, and therefore is NOT RECOMMENDED.
4.2. Negotiating Support
Support for redundant encoding can be indicated by offering "red" as
a supported payload type in the offer. Answerers can reject the use
of redundant encoding by not including "red" as a supported payload
type in the answer.
Support for codec-specific FEC mechanisms are typically indicated via
"a=fmtp" parameters. For Opus specifically, this is controlled by
the "useinbandfec=1" parameter, as specified in
[I-D.ietf-payload-rtp-opus]. These parameters are declarative and
can be negotiated separately for either media direction.
5. FEC for Video Content
The following section provides guidance on how to best use FEC for
transmitting video data. As indicated in Section 7 below, FEC should
only be activated if network conditions warrant it, or upon explicit
application request.
5.1. Recommended Mechanism
For video content, use of a separate FEC stream with the RTP payload
format described in [I-D.singh-payload-rtp-1d2d-parity-scheme] is
RECOMMENDED. The receiver can demultiplex the incoming FEC stream by
SSRC and correlate it with the primary stream via the ssrc-group
mechanism.
Note that this only allows the FEC stream to protect a single primary
stream. Support for protecting multiple primary streams with a
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single FEC stream is complicated by WebRTC's 1-m-line-per-stream
policy and requires further study.
5.2. Negotiating Support
To offer support for a separate FEC stream, the offerer MUST offer
one of the formats described in
[I-D.singh-payload-rtp-1d2d-parity-scheme], Section 5.1, as well as a
ssrc-group with "FEC-FR" semantics as described in [RFC5956],
Section 4.3.
Answerers can reject the use of FEC by not including FEC payloads in
the answer.
6. Implementation Requirements
To support the functionality recommended above, implementations MUST
support the redundant encoding mechanism described in [RFC2198] and
the FEC mechanism described in [RFC5956] and
[I-D.singh-payload-rtp-1d2d-parity-scheme].
Implementations MAY support additional FEC mechanisms if desired,
e.g. [RFC5109].
7. Adaptive Use of FEC
Since use of FEC causes redundant data to be transmitted, this will
lead to less bandwidth available for the primary encoding, when in a
bandwidth-constrained environment. Given this, WebRTC
implementations SHOULD only transmit FEC data when network conditions
indicate that this is advisable (e.g. by monitoring transmit packet
loss data from RTCP Receiver Reports), or the application indicates
it is willing to pay a quality penalty to proactively avoid losses.
8. Security Considerations
TODO
9. IANA Considerations
This document requires no actions from IANA.
10. Acknowledgements
Several people provided significant input into this document,
including Jonathan Lennox, Giri Mandyam, Varun Singh, Tim Terriberry,
and Mo Zanaty.
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11. References
11.1. Normative References
[I-D.singh-payload-rtp-1d2d-parity-scheme]
Singh, V., Begen, A., and M. Zanaty, "RTP Payload Format
for Non-Interleaved and Interleaved Parity Forward Error
Correction (FEC)", draft-singh-payload-rtp-1d2d-parity-
scheme-00 (work in progress), October 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997.
[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956, September
2010.
11.2. Informative References
[I-D.ietf-payload-rtp-opus]
Spittka, J., Vos, K., and J. Valin, "RTP Payload Format
for Opus Speech and Audio Codec", draft-ietf-payload-rtp-
opus-03 (work in progress), July 2014.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the
Opus Audio Codec", RFC 6716, September 2012.
Appendix A. Change log
Changes in draft -00:
o Initial version, from sidebar conversation at IETF 90.
Author's Address
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Justin Uberti
Google
747 6th Ave S
Kirkland, WA 98033
USA
Email: justin@uberti.name
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