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Versions: 00 draft-ietf-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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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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
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   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.  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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