draft-ietf-rtcweb-fec-02.txt   draft-ietf-rtcweb-fec-03.txt 
Network Working Group J. Uberti Network Working Group J. Uberti
Internet-Draft Google Internet-Draft Google
Intended status: Standards Track October 18, 2015 Intended status: Standards Track March 20, 2016
Expires: April 20, 2016 Expires: September 21, 2016
WebRTC Forward Error Correction Requirements WebRTC Forward Error Correction Requirements
draft-ietf-rtcweb-fec-02 draft-ietf-rtcweb-fec-03
Abstract Abstract
This document provides information and requirements for how Forward This document provides information and requirements for how Forward
Error Correction (FEC) should be used by WebRTC applications. Error Correction (FEC) should be used by WebRTC applications.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2016. This Internet-Draft will expire on September 21, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Types of FEC . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Types of FEC . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. Separate FEC Stream . . . . . . . . . . . . . . . . . . . 3 3.1. Separate FEC Stream . . . . . . . . . . . . . . . . . . . 3
3.2. Redundant Encoding . . . . . . . . . . . . . . . . . . . 3 3.2. Redundant Encoding . . . . . . . . . . . . . . . . . . . 3
3.3. Codec-Specific In-band FEC . . . . . . . . . . . . . . . 3 3.3. Codec-Specific In-band FEC . . . . . . . . . . . . . . . 3
4. FEC for Audio Content . . . . . . . . . . . . . . . . . . . . 3 4. FEC for Audio Content . . . . . . . . . . . . . . . . . . . . 4
4.1. Recommended Mechanism . . . . . . . . . . . . . . . . . . 3 4.1. Recommended Mechanism . . . . . . . . . . . . . . . . . . 4
4.2. Negotiating Support . . . . . . . . . . . . . . . . . . . 4 4.2. Negotiating Support . . . . . . . . . . . . . . . . . . . 4
5. FEC for Video Content . . . . . . . . . . . . . . . . . . . . 4 5. FEC for Video Content . . . . . . . . . . . . . . . . . . . . 5
5.1. Recommended Mechanism . . . . . . . . . . . . . . . . . . 4 5.1. Recommended Mechanism . . . . . . . . . . . . . . . . . . 5
5.2. Negotiating Support . . . . . . . . . . . . . . . . . . . 5 5.2. Negotiating Support . . . . . . . . . . . . . . . . . . . 5
6. FEC for Application Content . . . . . . . . . . . . . . . . . 5 6. FEC for Application Content . . . . . . . . . . . . . . . . . 6
7. Implementation Requirements . . . . . . . . . . . . . . . . . 5 7. Implementation Requirements . . . . . . . . . . . . . . . . . 6
8. Adaptive Use of FEC . . . . . . . . . . . . . . . . . . . . . 5 8. Adaptive Use of FEC . . . . . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6 9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
12.1. Normative References . . . . . . . . . . . . . . . . . . 6 12.1. Normative References . . . . . . . . . . . . . . . . . . 7
12.2. Informative References . . . . . . . . . . . . . . . . . 7 12.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
In situations where packet loss is high, or perfect media quality is In situations where packet loss is high, or perfect media quality is
essential, Forward Error Correction (FEC) can be used to proactively essential, Forward Error Correction (FEC) can be used to proactively
recover from packet losses. This specification provides guidance on recover from packet losses. This specification provides guidance on
which FEC mechanisms to use, and how to use them, for WebRTC client which FEC mechanisms to use, and how to use them, for WebRTC client
implementations. implementations.
2. Terminology 2. Terminology
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the redundant data could include multiple prior packets. the redundant data could include multiple prior packets.
Since there is only a single set of packet headers, this approach Since there is only a single set of packet headers, this approach
allows for a very efficient representation of primary + redundant allows for a very efficient representation of primary + redundant
data. However, this savings is only realized when the data all fits 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 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. result, this approach is generally not useful for video content.
3.3. Codec-Specific In-band FEC 3.3. Codec-Specific In-band FEC
Some audio codecs, notably Opus [RFC6716], support their own in-band Some audio codecs, notably Opus [RFC6716] and AMR [RFC4867] support
FEC mechanism, where FEC data is included in the codec payload. In their own in-band FEC mechanism, where redundant data is included in
the case of Opus specifically, packets deemed as important are re- the codec payload.
encoded at a lower bitrate and added to the subsequent packet,
allowing partial recovery of a lost packet. See [RFC6716], For Opus, packets deemed as important are re-encoded at a lower
Section 2.1.7 for details. bitrate and added to the subsequent packet, allowing partial recovery
of a lost packet. This scheme is fairly efficient; experiments
performed indicate that when Opus FEC is used, the overhead imposed
is about 20-30%, depending on the amount of protection needed. Note
that this mechanism can only carry redundancy information for the
immediately preceding packet; as such the decoder cannot fully
recover multiple consecutive lost packets. See [RFC6716],
Section 2.1.7 for complete details.
For AMR/AMR-WB, packets can contain copies or lower-quality encodings
of multiple prior audio frames. This mechanism is similar to the
[RFC2198] mechanism described above, but as it adds no additional
framing, it can be slightly more efficient. See [RFC4867],
Section 3.7.1 for details on this mechanism.
4. FEC for Audio Content 4. FEC for Audio Content
The following section provides guidance on how to best use FEC for The following section provides guidance on how to best use FEC for
transmitting audio data. As indicated in Section 8 below, FEC should transmitting audio data. As indicated in Section 8 below, FEC should
only be activated if network conditions warrant it, or upon explicit only be activated if network conditions warrant it, or upon explicit
application request. application request.
4.1. Recommended Mechanism 4.1. Recommended Mechanism
When using the Opus codec in its default (hybrid) mode, use of the When using the Opus codec, use of the built-in Opus FEC mechanism is
built-in Opus FEC mechanism is RECOMMENDED. This provides reasonable RECOMMENDED. This provides reasonable protection of the audio stream
protection of the audio stream against typical losses, with minimal against typical losses, with modest overhead. Note that as indicated
overhead. [TODO: add stats] above the built-in Opus FEC only provides single-frame redundancy; if
multi-packet protection is needed, the built-in FEC should be
combined with [RFC2198] redundancy to protect the N-2th, N-3rd, etc.
packets.
When using variable-bitrate codecs without an internal FEC, use of When using the AMR/AMR-WB codecs, use of their built-in FEC mechanism
[RFC2198] redundant encoding with a lower-fidelity version of is RECOMMENDED. This provides slightly more efficient protection of
previous packet(s) is RECOMMENDED. This provides reasonable the audio stream than [RFC2198].
protection of the payload with moderate overhead.
When using variable-bitrate codecs without an internal FEC, [RFC2198]
redundant encoding with lower-fidelity version(s) of 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] When using constant-bitrate codecs, e.g. PCMU, use of [RFC2198]
redundant encoding MAY be used, but note that this will result in a redundant encoding MAY be used, but note that this will result in a
potentially significant bitrate increase, and that suddenly potentially significant bitrate increase, and that suddenly
increasing bitrate to deal with losses from congestion may actually increasing bitrate to deal with losses from congestion may actually
make things worse. make things worse.
Because of the lower packet rate of audio encodings, usually a single 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 packet per frame, use of a separate FEC stream comes with a higher
overhead than other mechanisms, and therefore is NOT RECOMMENDED. overhead than other mechanisms, and therefore is NOT RECOMMENDED.
4.2. Negotiating Support 4.2. Negotiating Support
Support for redundant encoding can be indicated by offering "red" as Support for redundant encoding can be indicated by offering "red" as
a supported payload type in the offer. Answerers can reject the use a supported payload type in the offer. Answerers can reject the use
of redundant encoding by not including "red" as a supported payload of redundant encoding by not including "red" as a supported payload
type in the answer. type in the answer.
Support for codec-specific FEC mechanisms are typically indicated via Support for codec-specific FEC mechanisms are typically indicated via
"a=fmtp" parameters. For Opus specifically, this is controlled by "a=fmtp" parameters.
the "useinbandfec=1" parameter, as specified in
[I-D.ietf-payload-rtp-opus]. These parameters are declarative and For Opus, support for FEC at the received side is controlled by the
can be negotiated separately for either media direction. "useinbandfec=1" parameter, as specified in
[I-D.ietf-payload-rtp-opus]. This parameter is declarative and can
be negotiated separately for either media direction.
For AMR/AMR-WB, support for redundant encoding, and the maximum
supported depth, are controlled by the 'max-red' parameter, as
specified in [RFC4867], Section 8.1. [TODO: figure out any
additional recommendations are needed.]
5. FEC for Video Content 5. FEC for Video Content
The following section provides guidance on how to best use FEC for The following section provides guidance on how to best use FEC for
transmitting video data. As indicated in Section 8 below, FEC should transmitting video data. As indicated in Section 8 below, FEC should
only be activated if network conditions warrant it, or upon explicit only be activated if network conditions warrant it, or upon explicit
application request. application request.
5.1. Recommended Mechanism 5.1. Recommended Mechanism
For video content, use of a separate FEC stream with the RTP payload For video content, use of a separate FEC stream with the RTP payload
format described in [I-D.ietf-payload-flexible-fec-scheme] is format described in [I-D.ietf-payload-flexible-fec-scheme] is
RECOMMENDED. The receiver can demultiplex the incoming FEC stream by RECOMMENDED. The receiver can demultiplex the incoming FEC stream by
SSRC and correlate it with the primary stream via the ssrc-group SSRC and correlate it with the primary stream via the SSRC field
mechanism. present in the FEC header.
Support for protecting multiple primary streams with a single FEC Support for protecting multiple primary streams with a single FEC
stream is complicated by WebRTC's 1-m-line-per-stream policy, which stream is complicated by WebRTC's 1-m-line-per-stream policy, which
does not allow for a m-line dedicated specifically to FEC. does not allow for a m-line dedicated specifically to FEC.
5.2. Negotiating Support 5.2. Negotiating Support
To offer support for a separate SSRC-multiplexed FEC stream, the To offer support for a SSRC-multiplexed FEC stream that is associated
offerer MUST offer one of the formats described in with a given primary stream, the offerer MUST offer the formats
[I-D.ietf-payload-flexible-fec-scheme], Section 5.1, as well as a supported for the primary stream, as well as one of the formats
ssrc-group with "FEC-FR" semantics as described in [RFC5956], described in [I-D.ietf-payload-flexible-fec-scheme], Section 5.1.
Section 4.3.
Use of FEC-only m-lines, and grouping using the SDP group mechanism, Use of FEC-only m-lines, and grouping using the SDP group mechanism
is not currently defined for WebRTC, and SHOULD NOT be offered. as described in [RFC5956], Section 4.1 is not currently defined for
WebRTC, and SHOULD NOT be offered.
Answerers can reject the use of SSRC-multiplexed FEC, by not Answerers can reject the use of SSRC-multiplexed FEC, by not
including FEC payload types in the answer. including FEC formats in the answer.
Answerers SHOULD reject any FEC-only m-lines, unless they Answerers SHOULD reject any FEC-only m-lines, unless they
specifically know how to handle such a thing in a WebRTC context specifically know how to handle such a thing in a WebRTC context
(perhaps defined by a future version of the WebRTC specifications). (perhaps defined by a future version of the WebRTC specifications).
This ensures that implementations will not malfunction when said This ensures that implementations will not malfunction when said
future version of WebRTC enables offers of FEC-only m-lines. future version of WebRTC enables offers of FEC-only m-lines.
6. FEC for Application Content 6. FEC for Application Content
WebRTC also supports the ability to send generic application data, WebRTC also supports the ability to send generic application data,
and provides transport-level retransmission mechanisms that the and provides transport-level retransmission mechanisms to support
application can use to ensure that its data is delivered reliably. full and partial (e.g. timed) reliability. See
[I-D.ietf-rtcweb-data-channel] for details.
Because the application can control exactly what data to send, it has Because the application can control exactly what data to send, it has
the ability to monitor packet statistics and perform its own the ability to monitor packet statistics and perform its own
application-level FEC, if necessary. application-level FEC, if necessary.
As a result, this document makes no recommendations regarding FEC for As a result, this document makes no recommendations regarding FEC for
the underlying data transport. the underlying data transport.
7. Implementation Requirements 7. Implementation Requirements
To support the functionality recommended above, implementations MUST To support the functionality recommended above, implementations MUST
support the redundant encoding mechanism described in [RFC2198] and support the relevant mechanisms for their supported audio codecs, as
the FEC mechanism described in [RFC5956] and described in Section 4, and the general FEC mechanism described in
[I-D.ietf-payload-flexible-fec-scheme]. [I-D.ietf-payload-flexible-fec-scheme].
Implementations MAY support additional FEC mechanisms if desired, Implementations MAY support additional FEC mechanisms if desired,
e.g. [RFC5109]. e.g. [RFC5109].
8. Adaptive Use of FEC 8. Adaptive Use of FEC
Since use of FEC causes redundant data to be transmitted, this will Since use of FEC causes redundant data to be transmitted, this will
lead to less bandwidth available for the primary encoding, when in a lead to less bandwidth available for the primary encoding, when in a
bandwidth-constrained environment. Given this, WebRTC bandwidth-constrained environment. Given this, WebRTC
implementations SHOULD only transmit FEC data when network conditions implementations SHOULD only transmit the amount of FEC needed to
indicate that this is advisable (e.g. by monitoring transmit packet protect against the observed packet loss (which can be determined,
loss data from RTCP Receiver Reports), or the application indicates e.g., by monitoring transmit packet loss data from RTCP Receiver
it is willing to pay a quality penalty to proactively avoid losses. Reports [RFC3550]), or the application indicates it is willing to pay
a quality penalty to proactively avoid losses.
9. Security Considerations 9. Security Considerations
This document makes recommendations regarding the use of FEC. This document makes recommendations regarding the use of FEC.
Generally, it should be noted that although applying redundancy is Generally, it should be noted that although applying redundancy is
often useful in protecting a stream against packet loss, if the loss often useful in protecting a stream against packet loss, if the loss
is caused by network congestion, the additional bandwidth used by the is caused by network congestion, the additional bandwidth used by the
redundant data may actually make the situation worse, and can lead to redundant data may actually make the situation worse, and can lead to
significant degradation of the network. significant degradation of the network.
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Several people provided significant input into this document, Several people provided significant input into this document,
including Jonathan Lennox, Giri Mandyam, Varun Singh, Tim Terriberry, including Jonathan Lennox, Giri Mandyam, Varun Singh, Tim Terriberry,
and Mo Zanaty. and Mo Zanaty.
12. References 12. References
12.1. Normative References 12.1. Normative References
[I-D.ietf-payload-flexible-fec-scheme] [I-D.ietf-payload-flexible-fec-scheme]
Singh, V., Begen, A., and M. Zanaty, "RTP Payload Format Singh, V., Begen, A., Zanaty, M., and G. Mandyam, "RTP
for Non-Interleaved and Interleaved Parity Forward Error Payload Format for Flexible Forward Error Correction
Correction (FEC)", draft-ietf-payload-flexible-fec- (FEC)", draft-ietf-payload-flexible-fec-scheme-01 (work in
scheme-00 (work in progress), February 2015. progress), October 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
DOI 10.17487/RFC2198, September 1997, DOI 10.17487/RFC2198, September 1997,
<http://www.rfc-editor.org/info/rfc2198>. <http://www.rfc-editor.org/info/rfc2198>.
[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956, DOI 10.17487/ the Session Description Protocol", RFC 5956,
RFC5956, September 2010, DOI 10.17487/RFC5956, September 2010,
<http://www.rfc-editor.org/info/rfc5956>. <http://www.rfc-editor.org/info/rfc5956>.
12.2. Informative References 12.2. Informative References
[I-D.ietf-payload-rtp-opus] [I-D.ietf-payload-rtp-opus]
Spittka, J., Vos, K., and J. Valin, "RTP Payload Format Spittka, J., Vos, K., and J. Valin, "RTP Payload Format
for the Opus Speech and Audio Codec", draft-ietf-payload- for the Opus Speech and Audio Codec", draft-ietf-payload-
rtp-opus-11 (work in progress), April 2015. rtp-opus-11 (work in progress), April 2015.
[I-D.ietf-rtcweb-data-channel]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
Channels", draft-ietf-rtcweb-data-channel-13 (work in
progress), January 2015.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
"RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 4867, DOI 10.17487/RFC4867,
April 2007, <http://www.rfc-editor.org/info/rfc4867>.
[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <http://www.rfc-editor.org/info/rfc5109>. 2007, <http://www.rfc-editor.org/info/rfc5109>.
[RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the [RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the
Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716, Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
September 2012, <http://www.rfc-editor.org/info/rfc6716>. September 2012, <http://www.rfc-editor.org/info/rfc6716>.
Appendix A. Change log Appendix A. Change log
Changes in draft -03:
o Added overhead stats for Opus.
o Expanded discussion of multi-packet FEC for Opus.
o Added discussion of AMR/AMR-WB.
o Removed discussion of ssrc-group.
o Referenced the data channel doc.
o Referenced the RTP/RTCP RFC.
o Several small edits based on feedback from Magnus.
Changes in draft -02: Changes in draft -02:
o Expanded discussion of FEC-only m-lines, and how they should be o Expanded discussion of FEC-only m-lines, and how they should be
handled in offers and answers. handled in offers and answers.
Changes in draft -01: Changes in draft -01:
o Tweaked abstract/intro text that was ambiguously normative. o Tweaked abstract/intro text that was ambiguously normative.
o Removed text on FEC for Opus in CELT mode. o Removed text on FEC for Opus in CELT mode.
skipping to change at page 8, line 9 skipping to change at page 9, line 21
o Updated flexible-fec reference. o Updated flexible-fec reference.
Changes in draft -00: Changes in draft -00:
o Initial version, from sidebar conversation at IETF 90. o Initial version, from sidebar conversation at IETF 90.
Author's Address Author's Address
Justin Uberti Justin Uberti
Google Google
747 6th Ave S 747 6th St S
Kirkland, WA 98033 Kirkland, WA 98033
USA USA
Email: justin@uberti.name Email: justin@uberti.name
 End of changes. 26 change blocks. 
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