Congestion Control Using FEC for Conversational Media
draft-singh-rmcat-adaptive-fec-00
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| Authors | Varun Singh , Marcin Nagy , Joerg Ott , Lars Eggert | ||
| Last updated | 2014-07-04 | ||
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draft-singh-rmcat-adaptive-fec-00
RMCAT WG V. Singh
Internet-Draft M. Nagy
Intended status: Experimental J. Ott
Expires: January 5, 2015 Aalto University
L. Eggert
NetApp
July 4, 2014
Congestion Control Using FEC for Conversational Media
draft-singh-rmcat-adaptive-fec-00
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
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This Internet-Draft will expire on January 5, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Concept: FEC for Congestion Control . . . . . . . . . . . . . 4
3.1. States . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Framework . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. FEC Scheme . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Applicability to other RMCAT Schemes . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Simulations . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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)
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
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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],
Forward Error Correction (FEC) Framework [RFC6363], and Support for
Reduced-Size RTCP [RFC5506] apply.
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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).
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. Therefore, the
congestion control algorithm is always able to probe for available
capacity, as improved reliability compensates for possible errors
resulting from overuse (i.e., increase in observed latency and/or
losses).
+------------+ (B) Good conditions +-----------+
| |------------------------------------>| |
| STEADY | | PROBE |
| |<------------------------------------| |
+------------+ Probed, but Loss recovered +-----------+
/\ | | /\ |
| |(A) | | |
| |_______________________________________________| | |(C)
(B) | | (A) | |
| \/ (B) | \/
+------------+ +------------+
| | (A) Unstable conditions | |
| REDUCE |<------------------------------------| INCREASE |
| | | |
+------------+ +------------+
Figure 2: State machine of a Congestion Control enabling FEC.
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3.1. States
The Figure 2 illustrates the the state machine of a congestion
control algorithm incorporating FEC for probing. The state-machine
includes 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
attempts to increase 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 (i.e.,
the transmission rate should be increased). The endpoint
maintains the same target media bit rate, and instead increases
the amount of FEC.
o INCREASE state: Depending on the congestion feedback, i.e., if no
congestion is observed, the media transmission rate can be
increased while maintaining minimal amount of FEC for error
protection. If packets are lost, but the lost packets are
recovered by the FEC packets, the congestion control can keep the
same media bit rate and reduce the amount of FEC (compared to the
previous PROBE state). If congestion is observed, the congestion
control can transition to the REDUCE state and decrease the
transmission rate.
3.2. Framework
The Figure 3 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
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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.
+ - - - - - - - - - - - - - - - - - - - - - - - -+
| +--------------------------------------------+ |
| Media Encoder/Decoder |
| +--------------------------------------------+ |
| |
| +- -- -- -- -- -- -- -+ +- -- -- -- -+ |
| Rate Control | | RTP |
| | Module | | Queue | |
+- -- -- -- -- -- -- -+ +- -- -- -- -+
| ^ | | |
| | | Source
| | R +--------------------+ | RTP |
| T | |
| | C | | |
| P | |
| | +----------+ +----------------+ |
| F | FEC Code |<--->| FEC Module |
| | B +----------+ +----------------+ |
| | | |
| |------------------------+ | | |
| RTCP FB Repair | | Source
| | RTP | | RTP |
| | |
| +--------------------------------------------+ |
| Transport Layer (UDP) |
| +--------------------------------------------+ |
|
| +--------------------------------------------+ |
| IP |
| +--------------------------------------------+ |
| Endpoint |
+ - - - - - - - - - - - - - - - - - - - - - - - +
Figure 3: Interaction of Congestion Control and FEC Module.
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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 [RFC5109].
The endpoint MAY use a single-frame FEC or a multi-frame FEC 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
[I-D.ietf-xrblock-rtcp-xr-post-repair-loss-count] (packet count of
repaired packets).
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.alvestrand-rmcat-congestion],
SCReaM [I-D.johansson-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.
5. IANA Considerations
There are no IANA impacts in this memo.
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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. 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.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November
2003.
[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, July
2006.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
[I-D.ietf-avtcore-rtp-circuit-breakers]
Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", draft-ietf-
avtcore-rtp-circuit-breakers-05 (work in progress),
February 2014.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
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[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, February 2010.
[I-D.ietf-xrblock-rtcp-xr-post-repair-loss-count]
Huang, R. and V. Singh, "RTP Control Protocol (RTCP)
Extended Report (XR) for Post-Repair Loss Count Metrics",
draft-ietf-xrblock-rtcp-xr-post-repair-loss-count-05 (work
in progress), June 2014.
7.2. Informative References
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363, October 2011.
[I-D.ietf-rmcat-cc-requirements]
Jesup, R., "Congestion Control Requirements For RMCAT",
draft-ietf-rmcat-cc-requirements-02 (work in progress),
February 2014.
[I-D.alvestrand-rmcat-congestion]
Holmer, S., Cicco, L., Mascolo, S., and H. Alvestrand, "A
Google Congestion Control Algorithm for Real-Time
Communication", draft-alvestrand-rmcat-congestion-02 (work
in progress), February 2014.
[I-D.johansson-rmcat-scream-cc]
Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", draft-johansson-rmcat-scream-cc-02 (work
in progress), June 2014.
[I-D.sarker-rmcat-eval-test]
Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test
Cases for Evaluating RMCAT Proposals", draft-sarker-rmcat-
eval-test-00 (work in progress), February 2014.
[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.
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[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.
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.sarker-rmcat-eval-test] will be published
shorty.
Authors' Addresses
Varun Singh
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: varun@comnet.tkk.fi
URI: http://www.netlab.tkk.fi/~varun/
Marcin Nagy
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: marcin.nagy@aalto.fi
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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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