WebRTC Forward Error Correction Requirements
draft-ietf-rtcweb-fec-02
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (rtcweb WG) | |
|---|---|---|---|
| Author | Justin Uberti | ||
| Last updated | 2015-10-18 | ||
| Replaces | draft-uberti-rtcweb-fec | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
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| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
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draft-ietf-rtcweb-fec-02
Network Working Group J. Uberti
Internet-Draft Google
Intended status: Standards Track October 18, 2015
Expires: April 20, 2016
WebRTC Forward Error Correction Requirements
draft-ietf-rtcweb-fec-02
Abstract
This document provides information and requirements 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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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 April 20, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
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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. FEC for Application Content . . . . . . . . . . . . . . . . . 5
7. Implementation Requirements . . . . . . . . . . . . . . . . . 5
8. Adaptive Use of FEC . . . . . . . . . . . . . . . . . . . . . 5
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
12.1. Normative References . . . . . . . . . . . . . . . . . . 6
12.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
In situations where packet loss is high, or perfect media quality is
essential, Forward Error Correction (FEC) can be used to proactively
recover from packet losses. This specification provides guidance on
which FEC mechanisms to use, and how to use them, for 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, all 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. In certain cases,
the redundant data could include multiple prior packets.
Since there is only a single set of packet headers, this approach
allows for a very efficient representation of primary + redundant
data. However, this savings is only realized when the data all fits
into a single packet (i.e. the size is less than a MTU). As a
result, this approach is generally not useful for video 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 8 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 minimal
overhead. [TODO: add stats]
When using variable-bitrate codecs without an internal FEC, use of
[RFC2198] redundant encoding with a lower-fidelity version of
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previous packet(s) is RECOMMENDED. This provides reasonable
protection of the payload with moderate overhead.
When using constant-bitrate codecs, e.g. PCMU, use of [RFC2198]
redundant encoding MAY be used, but note that this will result in a
potentially significant bitrate increase, and that suddenly
increasing bitrate to deal with losses from congestion 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 8 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.ietf-payload-flexible-fec-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.
Support for protecting multiple primary streams with a single FEC
stream is complicated by WebRTC's 1-m-line-per-stream policy, which
does not allow for a m-line dedicated specifically to FEC.
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5.2. Negotiating Support
To offer support for a separate SSRC-multiplexed FEC stream, the
offerer MUST offer one of the formats described in
[I-D.ietf-payload-flexible-fec-scheme], Section 5.1, as well as a
ssrc-group with "FEC-FR" semantics as described in [RFC5956],
Section 4.3.
Use of FEC-only m-lines, and grouping using the SDP group mechanism,
is not currently defined for WebRTC, and SHOULD NOT be offered.
Answerers can reject the use of SSRC-multiplexed FEC, by not
including FEC payload types in the answer.
Answerers SHOULD reject any FEC-only m-lines, unless they
specifically know how to handle such a thing in a WebRTC context
(perhaps defined by a future version of the WebRTC specifications).
This ensures that implementations will not malfunction when said
future version of WebRTC enables offers of FEC-only m-lines.
6. FEC for Application Content
WebRTC also supports the ability to send generic application data,
and provides transport-level retransmission mechanisms that the
application can use to ensure that its data is delivered reliably.
Because the application can control exactly what data to send, it has
the ability to monitor packet statistics and perform its own
application-level FEC, if necessary.
As a result, this document makes no recommendations regarding FEC for
the underlying data transport.
7. 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.ietf-payload-flexible-fec-scheme].
Implementations MAY support additional FEC mechanisms if desired,
e.g. [RFC5109].
8. 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
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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.
9. Security Considerations
This document makes recommendations regarding the use of FEC.
Generally, it should be noted that although applying redundancy is
often useful in protecting a stream against packet loss, if the loss
is caused by network congestion, the additional bandwidth used by the
redundant data may actually make the situation worse, and can lead to
significant degradation of the network.
Additional security considerations for each individual FEC mechanism
are enumerated in their respective documents.
10. IANA Considerations
This document requires no actions from IANA.
11. Acknowledgements
Several people provided significant input into this document,
including Jonathan Lennox, Giri Mandyam, Varun Singh, Tim Terriberry,
and Mo Zanaty.
12. References
12.1. Normative References
[I-D.ietf-payload-flexible-fec-scheme]
Singh, V., Begen, A., and M. Zanaty, "RTP Payload Format
for Non-Interleaved and Interleaved Parity Forward Error
Correction (FEC)", draft-ietf-payload-flexible-fec-
scheme-00 (work in progress), February 2015.
[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>.
[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,
DOI 10.17487/RFC2198, September 1997,
<http://www.rfc-editor.org/info/rfc2198>.
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[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956, DOI 10.17487/
RFC5956, September 2010,
<http://www.rfc-editor.org/info/rfc5956>.
12.2. Informative References
[I-D.ietf-payload-rtp-opus]
Spittka, J., Vos, K., and J. Valin, "RTP Payload Format
for the Opus Speech and Audio Codec", draft-ietf-payload-
rtp-opus-11 (work in progress), April 2015.
[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <http://www.rfc-editor.org/info/rfc5109>.
[RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the
Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
September 2012, <http://www.rfc-editor.org/info/rfc6716>.
Appendix A. Change log
Changes in draft -02:
o Expanded discussion of FEC-only m-lines, and how they should be
handled in offers and answers.
Changes in draft -01:
o Tweaked abstract/intro text that was ambiguously normative.
o Removed text on FEC for Opus in CELT mode.
o Changed RFC 2198 recommendation for PCMU to be MAY instead of NOT
RECOMMENDED, based on list feedback.
o Explicitly called out application data as something not addressed
in this document.
o Updated flexible-fec reference.
Changes in draft -00:
o Initial version, from sidebar conversation at IETF 90.
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Author's Address
Justin Uberti
Google
747 6th Ave S
Kirkland, WA 98033
USA
Email: justin@uberti.name
Uberti Expires April 20, 2016 [Page 8]