Internet DRAFT - draft-singh-rmcat-adaptive-fec
draft-singh-rmcat-adaptive-fec
RMCAT WG V. Singh
Internet-Draft callstats.io
Intended status: Experimental M. Nagy
Expires: September 21, 2016 J. Ott
Aalto University
L. Eggert
NetApp
March 20, 2016
Congestion Control Using FEC for Conversational Media
draft-singh-rmcat-adaptive-fec-03
Abstract
This document describes a new mechanism for conversational multimedia
flows. The proposed mechanism uses Forward Error Correction (FEC)
encoded RTP packets (redundant packets) along side the media packets
to probe for available network capacity. A straightforward
interpretation is, the sending endpoint increases the transmission
rate by keeping the media rate constant but increases the amount of
FEC. If no losses and discards occur, the endpoint can then increase
the media rate. If losses occur, the redundant FEC packets help in
recovering the lost packets. Consequently, the endpoint can vary the
FEC bit rate to conservatively (by a small amount) or aggressively
(by a large amount) probe for available network capacity.
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 September 21, 2016.
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Copyright Notice
Copyright (c) 2016 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Concept: FEC for Congestion Control . . . . . . . . . . . . . 4
3.1. States . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Framework . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. FEC Scheme . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Applicability to other RMCAT Schemes . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Simulations . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The Real-time Transport Protocol (RTP) [RFC3550] is widely used in
voice telephony and video conferencing systems. Many of these
systems run over best-effort UDP/IP networks, and are required to
implement congestion to adapt the transmission rate of the RTP
streams to match the available network capacity, while maintaing the
user-experience [I-D.ietf-rmcat-cc-requirements]. The circuit
breakers [I-D.ietf-avtcore-rtp-circuit-breakers] describe a minimal
set of conditions when an RTP stream is causing severe congestion and
should cease transmission. Consequently, the congestion control
algorithm are expected to avoid triggering these conditions.
Conversational multimedia systems use Negative Acknowlegment (NACK),
Forward Error Correction (FEC), and Reference Picture Selection (RPS)
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to protect against packet loss. These are used in addition to the
codec-dependent resilience methods (for e.g., full intra-refresh and
error-concealment). In this way, the multimedia system is anyway
trading off part of the transmission rate for redundancy or
retransmissions to reduce the effects of packet loss. An endpoint
often prefers using FEC in high latency networks where
retransmissions may arrive later than the playout time of the packet
(due to the size of the dejitter buffer) [Holmer13]. Therefore, the
endpoint needs to adapt the transmission rate to best fit the
changing network capacity and the amount of redundancy based on the
observed/expected loss rate and network latency. Figure 1 shows the
applicatbility of different error-resilience schemes based on the
end-to-end latency and the observed packet loss [Devadoss08].
^
| .__________.
| | |
| | UEP/FEC |
l |____________|____. |
a | | | |
t | RPS | | |
e |_______. | | |
n | | | | |
c | | |____|_____|
y | NACK | |
| | |
+------------------------------->
Packet loss
Figure 1: Applicability of Error Resilience Schemes based on the
network delay and observed packet loss
In this document, we describe the use of FEC packets not only for
error-resilience but also as a probing mechanism for congestion
control (ramping up the transmission rate).
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 BCP 14, [RFC2119] and
indicate requirement levels for compliant implementations.
The terminology defined in RTP [RFC3550], RTP Profile for Audio and
Video Conferences with Minimal Control [RFC3551], RTCP Extended
Report (XR) [RFC3611], Extended RTP Profile for RTCP-based Feedback
(RTP/AVPF) [RFC4585], RTP Retransmission Payload Format [RFC4588],
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Forward Error Correction (FEC) Framework [RFC6363], and Support for
Reduced-Size RTCP [RFC5506] apply.
3. Concept: FEC for Congestion Control
FEC is one method for providing error-resilience, it improves
reliability by adding redundant data to the primary media flow, which
is used by received to recover packets that have been lost due to
congestion or bit-errors. The congestion control algorithm on the
other hand aims at maximizing the network path utilization, but risks
over-estimating the avaiable end-to-end network capacity leading to
congestion (and therefore losses).
Figure 2 shows the timeline of enabling and disabling FEC. The main
idea behind using FEC for congestion control is as follows: the
sending endpoint chooses a high FEC rate to aggressively probe for
available capacity and conversely chooses a low FEC rate to
conservatively probe for available capacity. During the ramp up, if
a packet is lost and the FEC packet arrives in time for decoding, the
receiver is be able to recover the lost packet; if no packet is lost,
the sender is able to increase the media encoding rate by swapping
out a part of the FEC rate. This method can be especially useful
when the transmission rate is close to the bottleneck link rate: by
choosing an appropriate FEC rate, the endpoint is able to probe for
available capacity without changing the target media rate and
therefore not affecting the user-experience.
Hence, the congestion control algorithm is always able to probe for
available capacity, as improved reliability compensates for possible
errors resulting from probing for additional capacity (i.e., increase
in observed latency and/or losses).
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^ .........
| / \ /
t |/ \ /
h | +===+===+=\=+ /
r | | F | | \| +===+ +==/+
o +===+---+ | \...........|.F.|...|./F|===+
u | | | | | +===+===+---+===+---+---+
g | | | | | | F | | | | | |
h | | | | |===+---+ | | | | |
p | | | | | | | | | | | |
u | | | | | | | | | | | |
t | | | | | | | | | | | |
| s | p | i | s | d | p | i | p | s | p | i |
+---+---+---+---+---+---+---+---+---+---+---+-->
time
Key:
+===+ Media with minimal FEC for error protection
+===+
| F | Media with FEC for probing and error protection
+---+
....
/ \ Available capacity
d,s,p,i: are the states: Decrease, Stay, Probe, Increase
Figure 2: Congestion Control enabling FEC.
+------------+ (B) Good conditions +-----------+
| |------------------------------------>| |
| STEADY | | PROBE |
| |<------------------------------------| |
+------------+ Probed, but Loss recovered +-----------+
/\ | | /\ |
| |(A) | | |
| |_______________________________________________| | |(C)
(B) | | (A) | |
| \/ (B) | \/
+------------+ +------------+
| | (A) Unstable conditions | |
| REDUCE |<------------------------------------| INCREASE |
| | | |
+------------+ +------------+
Figure 3: State machine of a Congestion Control enabling FEC.
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3.1. States
The Figure 3 illustrates the the state machine of a congestion
control algorithm incorporating FEC for probing. The state
transitions occur based on the information reported in the feedback
packet. In Figure 3 (A) indicates congestion, i.e., the congestion
control observes increasing delay and/or packet loss, or any other
congestion metric, and in response the congestion control reduces the
transmission rate. In Figure 3 (B) occurs when the congestion cues
report improvement in congestion metrics, and in response the
congestion cue increases the transmission rate. For probing using
FEC, the congestion control needs to map to the following 4 states:
STEADY, PROBE, INCREASE, and REDUCE.
o STEADY state: The congestion control keeps the same target media
rate and no additional FEC packets are generated for probing.
This is a transient state, after which the congestion control
either attempts to increase the transmission rate, or observes
congestion and reduces the transmission rate.
o REDUCE state: The congestion control reduces the transmission rate
based on the observed congestion cues, and generated no additional
FEC packets than the minimum required for error-resilience. If in
subsequent reports the conditions improve, the congestion control
can directly transition to the PROBE state.
o PROBE state: The congestion control observes no congestion for two
reporting intervals (i.e., the transmission rate should be
increased). The endpoint maintains the same target media bit
rate, and instead increases the amount of FEC packets, therby
increasing the transmission rate.
o INCREASE state: The endpoint is sending FEC packets and the
congestion control observes no congestion (as reported in RTCP
feedback), the media transmission rate is increased while
maintaining minimal amount of FEC for error protection. In this
case, the combined transmission rate (FEC+media) may remain the
same as in the PROBE state. If the feedback reports packet loss,
but some of these lost packets are recovered by the FEC packets,
the congestion control can keep the same media bit rate and adjust
the amount of FEC (compared to the previous PROBE state). If
congestion is observed (the target rate calculated by the
congestion control is much lower than the current media rate), the
congestion control can transition to the REDUCE state and decrease
the transmission rate.
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3.2. Framework
The Figure 4 shows the interaction between the rate control module,
the RTP and the FEC module.
At the sender, the rate control module calculates the new bit rate.
If the new bit rate is higher than the previous than the previous bit
rate indicates to the FEC module that the congestion control intends
to probe. The FEC module depending on its internal state machine
decides to add FEC for probing or not. Thereafter it indicates to
the rate control module the bit rate remaining for the RTP media
stream, which may be less than equal to the calculated bit rate.
At the reciver, the FEC module reconstructs lost packets in the
primary stream from the packets received in the repair stream. If
packets are repair it generates the post-repair loss report
(discussed in Section 3.3) for the corresponding RTP packets.
At the sender, The FEC module also receives the RTCP Feedback related
to the primary stream and any post-repair loss report. It uses the
information from these RTCP reports to calculate the effectiveness of
FEC for congestion control and is also the basis for changing its
internal state.
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+ - - - - - - - - - - - - - - - - - - - - - - - -+
| +--------------------------------------------+ |
| Media Encoder/Decoder |
| +--------------------------------------------+ |
| |
| +- -- -- -- -- -- -- -+ +- -- -- -- -+ |
| Rate Control | | RTP |
| | Module | | | |
+- -- -- -- -- -- -- -+ +- -- -- -- -+
| ^ | | |
| | | Source
| | R +--------------------+ | RTP |
| T | |
| | C | | |
| P | |
| | +----------+ +----------------+ |
| F | FEC Code |<--->| FEC Module |
| | B +----------+ +----------------+ |
| | | |
| |------------------------+ | | |
| RTCP FB Repair | | Source
| | RTP | | RTP |
| | |
| +--------------------------------------------+ |
| RTP Processing Layer |
| | (Queue) | |
+--------------------------------------------+
| | |
+--------------------------------------------+
| | Transport Layer (UDP) | |
+--------------------------------------------+
| | |
+--------------------------------------------+
| | IP | |
+--------------------------------------------+
| |
| Endpoint |
+ - - - - - - - - - - - - - - - - - - - - - - - +
Figure 4: Interaction of Congestion Control and FEC Module.
3.3. FEC Scheme
[RFC6363] describes a framework for using Forward Error Correction
(FEC) codes with RTP and allows any FEC code to be used with the
framework. For this proposal, the FEC packets are created by XORing
RTP media packets, the resulting redundant RTP packets are encoded
using the scheme defined in [I-D.ietf-payload-flexible-fec-scheme].
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The endpoint MAY use a single-frame FEC (1-dimensional) or a multi-
frame FEC (2-dimensional) for protecting the primary RTP stream. A
single-frame FEC protects against a single packet loss and fails when
burst loss occurs. Using multi-frame FEC helps mitigate these issues
at the cost of higher overhead and latency in recovering lost
packets. [Holmer13] shows examples of using a single- and multi-
frame FEC.
The receiving endpoint may report the post-repair loss (or residual
loss) using either the report block defined in [RFC5725] (Run-length
encoding of packets repaired) or in [RFC7509] (packet count of
repaired packets).
Additionally, the receiving may report the occurance of losses and
discards via a run-length encoding (RLE) of lost [RFC3611]
(Section 4.1), which enables the sender to detect the burst loss
length and apply appropriate FEC scheme.
Packet that arrive too late to be played out by the receiver are
discarded by the de-jitter buffer. Typically, the de-jitter buffer
adjust the playout delay based on the observed frame inter-arrival
delay, so that packets are played out smoothly. Reporting RLE of
discarded packets [RFC7097] may further enable a sender to detect
losses that occur after packet discards.
3.4. Applicability to other RMCAT Schemes
[Open issue: The current implementation is delay based and is
documented in [Nagy14]. However, we would like to generalize the
concept and apply it to different RMCAT algorithms for e.g., Google's
Congestion Control algorithm [I-D.ietf-rmcat-gcc], SCReaM
[I-D.ietf-rmcat-scream-cc], etc.]
4. Security Considerations
The security considerations of [RFC3550], RTP/AVPF profile for rapid
RTCP feedback [RFC4585], circuit breaker
[I-D.ietf-avtcore-rtp-circuit-breakers], and Generic Forward Error
Correction [RFC5109] apply.
If non-authenticated RTCP reports are used, an on-path attacker can
send forged RTCP feedback packets that can disrupt the operation of
the underlying congestion control. Additionally, the forged packets
can either indicate no packet loss causing the congestion control to
ramp-up quickly, or indicate high packet loss or RTT causing the
circuit breaker to trigger.
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5. IANA Considerations
There are no IANA impacts in this memo.
6. Acknowledgements
This document is based on the results published in [Nagy14].
The work of Varun Singh, and Joerg Ott has been partially supported
by the European Institute of Innovation and Technology (EIT) ICT Labs
activity RCLD 11882 (2012-2014). The views expressed here are those
of the author(s) only. Neither the European Commission nor the
EITICT labs is liable for any use that may be made of the information
in this document.
Lars Eggert has received funding from the European Union's Horizon
2020 research and innovation programme 2014-2018 under grant
agreement No. 644866. This document reflects only the authors' views
and the European Commission is not responsible for any use that may
be made of the information it contains.
7. References
7.1. Normative References
[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>.
[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>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<http://www.rfc-editor.org/info/rfc3551>.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<http://www.rfc-editor.org/info/rfc3611>.
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[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
2009, <http://www.rfc-editor.org/info/rfc5506>.
[I-D.ietf-avtcore-rtp-circuit-breakers]
Perkins, C. and V. Varun, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", draft-ietf-
avtcore-rtp-circuit-breakers-14 (work in progress), March
2016.
[I-D.ietf-payload-flexible-fec-scheme]
Singh, V., Begen, A., Zanaty, M., and G. Mandyam, "RTP
Payload Format for Flexible Forward Error Correction
(FEC)", draft-ietf-payload-flexible-fec-scheme-01 (work in
progress), October 2015.
[RFC7509] Huang, R. and V. Singh, "RTP Control Protocol (RTCP)
Extended Report (XR) for Post-Repair Loss Count Metrics",
RFC 7509, DOI 10.17487/RFC7509, May 2015,
<http://www.rfc-editor.org/info/rfc7509>.
7.2. Informative References
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<http://www.rfc-editor.org/info/rfc6363>.
[I-D.ietf-rmcat-cc-requirements]
Jesup, R. and Z. Sarker, "Congestion Control Requirements
for Interactive Real-Time Media", draft-ietf-rmcat-cc-
requirements-09 (work in progress), December 2014.
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[I-D.ietf-rmcat-gcc]
Holmer, S., Lundin, H., Carlucci, G., Cicco, L., and S.
Mascolo, "A Google Congestion Control Algorithm for Real-
Time Communication", draft-ietf-rmcat-gcc-01 (work in
progress), October 2015.
[I-D.ietf-rmcat-scream-cc]
Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", draft-ietf-rmcat-scream-cc-03 (work in
progress), February 2016.
[I-D.ietf-rmcat-eval-test]
Sarker, Z., Varun, V., Zhu, X., and M. Ramalho, "Test
Cases for Evaluating RMCAT Proposals", draft-ietf-rmcat-
eval-test-03 (work in progress), March 2016.
[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>.
[RFC5725] Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE
Report Block Type for RTP Control Protocol (RTCP) Extended
Reports (XRs)", RFC 5725, DOI 10.17487/RFC5725, February
2010, <http://www.rfc-editor.org/info/rfc5725>.
[RFC7097] Ott, J., Singh, V., Ed., and I. Curcio, "RTP Control
Protocol (RTCP) Extended Report (XR) for RLE of Discarded
Packets", RFC 7097, DOI 10.17487/RFC7097, January 2014,
<http://www.rfc-editor.org/info/rfc7097>.
[Nagy14] Nagy, M., Singh, V., Ott, J., and L. Eggert, "Congestion
Control using FEC for Conversational Multimedia
Communication", Proc. of 5th ACM Internation Conference on
Multimedia Systems (MMSys 2014) , 3 2014.
[Devadoss08]
Devadoss, J., Singh, V., Ott, J., Liu, C., Wang, Y-K., and
I. Curcio, "Evaluation of Error Resilience Mechanisms for
3G Conversational Video", Proc. of IEEE International
Symposium on Multimedia (ISM 2008) , 3 2014.
[Holmer13]
Holmer, S., Shemer, M., and M. Paniconi, "Handling Packet
Loss in WebRTC", Proc. of IEEE International Conference on
Image Processing (ICIP 2013) , 9 2013.
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Appendix A. Simulations
This document is based on the results published in [Nagy14]. See the
paper for ns-2 and testbed results; more results based on the
scenarios listed in [I-D.ietf-rmcat-eval-test] will be published
shorty.
Authors' Addresses
Varun Singh
Nemu Dialogue Systems Oy
Runeberginkatu 4c A 4
Helsinki 00100
Finland
Email: varun.singh@iki.fi
URI: http://www.callstats.io/
Marcin Nagy
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: marcin.nagy@aalto.fi
Joerg Ott
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: jo@comnet.tkk.fi
Lars Eggert
NetApp
Sonnenallee 1
Kirchheim 85551
Germany
Phone: +49 151 12055791
Email: lars@netapp.com
URI: http://eggert.org/
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