[Docs] [txt|pdf|xml] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-uberti-rtcweb-fec) 00 01 02 03 04 05 06 07 08 Draft is active
In: Approved-announcement_to_be_sent
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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.






Uberti                   Expires April 20, 2016                 [Page 1]


Internet-Draft                 WebRTC FEC                   October 2015


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.




Uberti                   Expires April 20, 2016                 [Page 2]


Internet-Draft                 WebRTC FEC                   October 2015


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



Uberti                   Expires April 20, 2016                 [Page 3]


Internet-Draft                 WebRTC FEC                   October 2015


   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.







Uberti                   Expires April 20, 2016                 [Page 4]


Internet-Draft                 WebRTC FEC                   October 2015


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



Uberti                   Expires April 20, 2016                 [Page 5]


Internet-Draft                 WebRTC FEC                   October 2015


   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>.




Uberti                   Expires April 20, 2016                 [Page 6]


Internet-Draft                 WebRTC FEC                   October 2015


   [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.







Uberti                   Expires April 20, 2016                 [Page 7]


Internet-Draft                 WebRTC FEC                   October 2015


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]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/