IETF RMCAT Working Group Z. Sarker
Internet-Draft Ericsson AB
Intended status: Standards Track C. Perkins
Expires: September 3, 2018 University of Glasgow
V. Singh
callstats.io
M. Ramalho
Cisco Systems
March 2, 2018
RTP Control Protocol (RTCP) Feedback for Congestion Control
draft-ietf-avtcore-cc-feedback-message-01
Abstract
This document describes an RTCP feedback message intended to enable
congestion control for interactive real-time traffic using RTP. The
feedback message is designed for use with a sender-based congestion
control algorithm, in which the receiver of an RTP flow sends RTCP
feedback packets to the sender containing the information the sender
needs to perform congestion control.
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 September 3, 2018.
Copyright Notice
Copyright (c) 2018 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
Sarker, et al. Expires September 3, 2018 [Page 1]
Internet-Draft Congestion Control Feedback in RTCP March 2018
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. RTCP Feedback for Congestion Control . . . . . . . . . . . . 3
3.1. RTCP Congestion Control Feedback Report . . . . . . . . . 4
4. Feedback Frequency and Overhead . . . . . . . . . . . . . . . 6
5. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
For interactive real-time traffic, such as video conferencing flows,
the typical protocol choice is the Real-time Transport Protocol (RTP)
running over the User Datagram Protocol (UDP). RTP does not provide
any guarantee of Quality of Service (QoS), reliability, or timely
delivery, and expects the underlying transport protocol to do so.
UDP alone certainly does not meet that expectation. However, the RTP
Control Protocol (RTCP) provides a mechanism by which the receiver of
an RTP flow can periodically send transport and media quality metrics
to the sender of that RTP flow. This information can be used by the
sender to perform congestion control. In the absence of standardized
messages for this purpose, designers of congestion control algorithms
have developed proprietary RTCP messages that convey only those
parameters needed for their respective designs. As a direct result,
the different congestion control (i.e., rate adaptation) designs are
not interoperable. To enable algorithm evolution as well as
interoperability across designs (e.g., different rate adaptation
algorithms), it is highly desirable to have generic congestion
control feedback format.
To help achieve interoperability for unicast RTP congestion control,
this memo proposes a common RTCP feedback packet format that can be
used by NADA [I-D.ietf-rmcat-nada], SCReAM
[I-D.ietf-rmcat-scream-cc], Google Congestion Control
Sarker, et al. Expires September 3, 2018 [Page 2]
Internet-Draft Congestion Control Feedback in RTCP March 2018
[I-D.ietf-rmcat-gcc] and Shared Bottleneck Detection
[I-D.ietf-rmcat-sbd], and hopefully also by future RTP congestion
control algorithms.
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].
In addition the terminology defined in [RFC3550], [RFC3551],
[RFC3611], [RFC4585], and [RFC5506] applies.
3. RTCP Feedback for Congestion Control
Based on an analysis of NADA [I-D.ietf-rmcat-nada], SCReAM
[I-D.ietf-rmcat-scream-cc], Google Congestion Control
[I-D.ietf-rmcat-gcc] and Shared Bottleneck Detection
[I-D.ietf-rmcat-sbd], the following per-RTP packet congestion control
feedback information has been determined to be necessary:
o RTP sequence number: The receiver of an RTP flow needs to feedback
the sequence numbers of the received RTP packets to the sender, so
the sender can determine which packets were received and which
were lost. Packet loss is used as an indication of congestion by
many congestion control algorithms.
o Packet Arrival Time: The receiver of an RTP flow needs to feedback
the arrival time of each RTP packet to the sender. Packet delay
and/or delay variation (jitter) is used as a congestion signal by
some congestion control algorithms.
o Packet Explicit Congestion Notification (ECN) Marking: If ECN
[RFC3168], [RFC6679] is used, it is necessary to feedback the
2-bit ECN mark in received RTP packets, indicating for each RTP
packet whether it is marked not-ECT, ECT(0), ECT(1), or ECN-CE.
If the path used by the RTP traffic is ECN capable the sender can
use Congestion Experienced (ECN-CE) marking information as a
congestion control signal.
Every RTP flow is identified by its Synchronization Source (SSRC)
identifier. Accordingly, the RTCP feedback format needs to group its
reports by SSRC, sending one report block per received SSRC.
As a practical matter, we note that host operating system (OS)
process interruptions can occur at inopportune times. Accordingly,
recording RTP packet send times at the sender, and the corresponding
RTP packet arrival times at the receiver, needs to be done with
Sarker, et al. Expires September 3, 2018 [Page 3]
Internet-Draft Congestion Control Feedback in RTCP March 2018
deliberate care. This is because the time duration of host OS
interruptions can be significant relative to the precision desired in
the one-way delay estimates. Specifically, the send time needs to be
recorded at the last opportunity prior to transmitting the RTP packet
at the sender, and the arrival time at the receiver needs to be
recorded at the earliest available opportunity.
3.1. RTCP Congestion Control Feedback Report
Congestion control feedback can be sent as part of a regular
scheduled RTCP report, or in an RTP/AVPF early feedback packet. If
sent as early feedback, congestion control feedback MAY be sent in a
non-compound RTCP packet [RFC5506] if the RTP/AVPF profile [RFC4585]
or the RTP/SAVPF profile [RFC5124] is used.
Irrespective of how it is transported, the congestion control
feedback is sent as a Transport Layer Feedback Message (RTCP packet
type 205). The format of this RTCP packet is shown in Figure 1:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT=CCFB | PT = 205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of RTCP packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of 1st RTP Stream |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L|ECN| Arrival time offset | ... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of nth RTP Stream |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L|ECN| Arrival time offset | ... |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Report Timestamp (32bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTCP Congestion Control Feedback Packet Format
Sarker, et al. Expires September 3, 2018 [Page 4]
Internet-Draft Congestion Control Feedback in RTCP March 2018
The first eight octets comprise a standard RTCP header, with PT=205
and FMT=CCFB indicating that this is a congestion control feedback
packet, and with the SSRC set to that of the sender of the RTCP
packet. (NOTE TO RFC EDITOR: please replace CCFB here and in the
above diagram with the IANA assigned RTCP feedback packet type, and
remove this note)
Section 6.1 of [RFC4585] requires the RTCP header to be followed by
the SSRC of the RTP flow being reported upon. Accordingly, the RTCP
header is followed by a report block for each SSRC from which RTP
packets have been received, followed by a Report Timestamp.
Each report block begins with the SSRC of the received RTP Stream on
which it is reporting. Following this, each sequence number between
the begin_seq and end_seq (both inclusive; modulo 65535 to account
for possible sequence number wrap-around) is represented by a 16-bit
packet metric block that contains the L, ECN, and ATO fields. If the
number of 16-bit packet metric blocks included in the report block is
not a multiple of two, then 16 bits of zero padding MUST be added
after the last packet metric block, to align the end of the packet
metric blocks with the next 32 bit boundary. In each packet metric
block, the L, ECN, and ATO fields are as follows:
o L (1 bit): is a boolean to indicate if the packet was received. 0
represents that the packet was not yet received and all the
subsequent bits (ECN and ATO) are also set to 0. 1 represent the
packet was received and the subsequent bits in the block need to
be parsed.
o ECN (2 bits): is the echoed ECN mark of the packet. These are set
to 00 if not received, or if ECN is not used.
o Arrival time offset (ATO, 13 bits): is the arrival time of the RTP
packet at the receiver. It is measured as an offset from the time
at which the RTCP congestion control feedback report packet is
sent. The arrival time offset is calculated by subtracting the
reception time of the RTP packet denoted by this 16 bit packet
metric block from the Report Timestamp (RTS) field of the RTCP
congestion control feedback report packet in which the packet
metric report block is contained. The arrival time offset is
measured in units of 1/1024 seconds (this unit is chosen to give
exact offsets from the RTS field). If the measured value is
greater than 8189/1024 seconds (the value that would be coded as
0x1FFD), the value 0x1FFE MUST be reported to indicate an over-
range positive measurement. If the measurement is unavailable,
the value 0x1FFF MUST be reported.
Sarker, et al. Expires September 3, 2018 [Page 5]
Internet-Draft Congestion Control Feedback in RTCP March 2018
The RTCP congestion control feedback report packet concludes with the
Report Timestamp field (RTS, 32 bits). This represents the time
instant when the report packet was generated. The value of RTS field
is derived from the same wallclock used to generate the NTP timestamp
field in RTCP Sender Report (SR) and Receiver Report (RR) packets.
It is formatted as the middle 32 bits of an NTP format timestamp, as
described in Section 4 of [RFC3550].
RTCP congestion control feedback packets SHOULD include a report
block for each SSRC that is being congestion controlled. The
sequence number ranges reported on in consecutive reports for an SSRC
SHOULD be consecutive and SHOULD NOT overlap (i.e., begin_seq for a
report is expected to be one greater, modulo 65535, than end_seq of
the previous report for that SSRC). If overlapping reports are sent,
the information in the later report updates that in any previous
reports for packets included in both reports (although note that such
updated information will likely arrive too late to affect congestion
control decisions at the sender). Reports that cover RTP sequence
number ranges that are more than 16384 (i.e., one quarter of the
sequence number space) ahead of the last end_seq received from an
SSRC, or behind the last begin_seq received from an SSRC, modulo
65535 to account for wrap-around, MUST be ignored.
If no packets are received from an SSRC in a reporting interval, then
no report block is sent for that SSRC. A regular SR/RR packet SHOULD
be sent instead, since the non-increased extended highest sequence
number received field of that SR/RR packet will inform the sender
that no packets have been received.
4. Feedback Frequency and Overhead
There is a trade-off between speed and accuracy of reporting, and the
overhead of the reports. [I-D.ietf-rmcat-rtp-cc-feedback] discusses
this trade-off, suggests desirable RTCP feedback rates, and provides
guidance on how to configure the RTCP bandwidth fraction, etc., to
make appropriate use of the reporting block described in this memo.
Specifications for RTP congestion control algorithms can also provide
guidance.
It is a general understanding that the congestion control algorithms
will work better with more frequent feedback - per packet feedback.
However, RTCP bandwidth and transmission rules put some upper limits
on how frequently the RTCP feedback messages can be send from the RTP
receiver to the RTP sender. It has been shown
[I-D.ietf-rmcat-rtp-cc-feedback] that in most cases a per frame
feedback is a reasonable assumption on how frequent the RTCP feedback
messages can be transmitted. It has also been noted that even if a
higher frequency of feedback is desired it is not viable if the
Sarker, et al. Expires September 3, 2018 [Page 6]
Internet-Draft Congestion Control Feedback in RTCP March 2018
feedback messages starts to compete against the RTP traffic on the
feedback path during congestion period. Analyzing the feedback
interval requirement [feedback-requirements] it can be seen that the
candidate algorithms can perform with a feedback interval range of
50-200ms. A value within this range need to be negotiated at session
setup.
5. Design Rationale
The primary function of RTCP SR/RR packets is to report statistics on
the reception of RTP packets. The reception report blocks sent in
these packets contain information about observed jitter, fractional
packet loss, and cumulative packet loss. It was intended that this
information could be used to support congestion control algorithms,
but experience has shown that it is not sufficient for that purpose.
An efficient congestion control algorithm requires more fine grained
information on per packet reception quality than is provided by SR/RR
packets to react effectively.
The Codec Control Messages for the RTP/AVPF profile [RFC5104] include
a Temporary Maximum Media Bit Rate (TMMBR) message. This is used to
convey a temporary maximum bit rate limitation from a receiver of RTP
packets to their sender. Even though it was not designed to replace
congestion control, TMMBR has been used as a means to do receiver
based congestion control where the session bandwidth is high enough
to send frequent TMMBR messages, especially when used with non-
compound RTCP packets [RFC5506]. This approach requires the receiver
of the RTP packets to monitor their reception, determine the level of
congestion, and recommend a maximum bit rate suitable for current
available bandwidth on the path; it also assumes that the RTP sender
can/will respect that bit rate. This is the opposite of the sender
based congestion control approach suggested in this memo, so TMMBR
cannot be used to convey the information needed for a sender based
congestion control. TMMBR could, however, be viewed a complementary
mechanism that can inform the sender of the receiver's current view
of acceptable maximum bit rate.
A number of RTCP eXtended Report (XR) blocks have previously been
defined to report details of packet loss, arrival times [RFC3611],
delay [RFC6843], and ECN marking [RFC6679]. It is possible to
combine several such XR blocks to report the detailed loss, arrival
time, and ECN marking marking information needed for effective
sender-based congestion control. However, the result has high
overhead both in terms of bandwidth and complexity, due to the need
to stack multiple reports.
Considering these issues, we believe it appropriate to design a new
RTCP feedback mechanism to convey information for sender based
Sarker, et al. Expires September 3, 2018 [Page 7]
Internet-Draft Congestion Control Feedback in RTCP March 2018
congestion control algorithms. The new congestion control feedback
RTCP packet described in Section 3 provides such a mechanism.
6. Acknowledgements
This document is an outcome of RMCAT design team discussion. We
would like to thank all participants specially Xiaoquing Zhu, Stefan
Holmer, David, Ingemar Johansson, Randell Jesup, Ingemar Johansson,
and Magnus Westerlund for their valuable contribution to the
discussions and to the document.
7. IANA Considerations
IANA is requested to assign a new value in the "FMT Values for RTPFB
Payload Types" registry for the CCFB transport layer feedback packet
described in Section 3.1.
8. Security Considerations
The security considerations of the RTP specification [RFC3550], the
applicable RTP profile (e.g., [RFC3551], [RFC3711], or [RFC4585]),
and the RTP congestion control algorithm that is in use (e.g.,
[I-D.ietf-rmcat-nada], [I-D.ietf-rmcat-scream-cc],
[I-D.ietf-rmcat-gcc], or [I-D.ietf-rmcat-sbd]) apply.
A receiver that intentionally generates inaccurate RTCP congestion
control feedback reports might be able trick the sender into sending
at a greater rate than the path can support, thereby congesting the
path. This will negatively impact the quality of experience of that
receiver. Since RTP is an unreliable transport, a sender can
intentionally leave a gap in the RTP sequence number space without
causing harm, to check that the receiver is correctly reporting
losses.
An on-path attacker that can modify RTCP congestion control feedback
packets can change the reports to trick the sender into sending at
either an excessively high or excessively low rate, leading to denial
of service. The secure RTCP profile [RFC3711] can be used to
authenticate RTCP packets to protect against this attack.
9. References
9.1. Normative References
Sarker, et al. Expires September 3, 2018 [Page 8]
Internet-Draft Congestion Control Feedback in RTCP March 2018
[I-D.ietf-rmcat-rtp-cc-feedback]
Perkins, C., "RTP Control Protocol (RTCP) Feedback for
Congestion Control in Interactive Multimedia Conferences",
draft-ietf-rmcat-rtp-cc-feedback-03 (work in progress),
November 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[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, <https://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, <https://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,
<https://www.rfc-editor.org/info/rfc3611>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[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, <https://www.rfc-
editor.org/info/rfc4585>.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
2008, <https://www.rfc-editor.org/info/rfc5124>.
Sarker, et al. Expires September 3, 2018 [Page 9]
Internet-Draft Congestion Control Feedback in RTCP March 2018
[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, <https://www.rfc-editor.org/info/rfc5506>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <https://www.rfc-editor.org/info/rfc6679>.
9.2. Informative References
[feedback-requirements]
"RMCAT Feedback Requirements",
<://www.ietf.org/proceedings/95/slides/slides-95-rmcat-
1.pdf>.
[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-02 (work in
progress), July 2016.
[I-D.ietf-rmcat-nada]
Zhu, X., Pan, R., Ramalho, M., Cruz, S., Jones, P., Fu,
J., and S. D'Aronco, "NADA: A Unified Congestion Control
Scheme for Real-Time Media", draft-ietf-rmcat-nada-04
(work in progress), March 2017.
[I-D.ietf-rmcat-sbd]
Hayes, D., Ferlin, S., Welzl, M., and K. Hiorth, "Shared
Bottleneck Detection for Coupled Congestion Control for
RTP Media.", draft-ietf-rmcat-sbd-08 (work in progress),
July 2017.
[I-D.ietf-rmcat-scream-cc]
Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", draft-ietf-rmcat-scream-cc-10 (work in
progress), July 2017.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <https://www.rfc-editor.org/info/rfc5104>.
Sarker, et al. Expires September 3, 2018 [Page 10]
Internet-Draft Congestion Control Feedback in RTCP March 2018
[RFC6843] Clark, A., Gross, K., and Q. Wu, "RTP Control Protocol
(RTCP) Extended Report (XR) Block for Delay Metric
Reporting", RFC 6843, DOI 10.17487/RFC6843, January 2013,
<https://www.rfc-editor.org/info/rfc6843>.
Authors' Addresses
Zaheduzzaman Sarker
Ericsson AB
Luleae
Sweden
Phone: +46107173743
Email: zaheduzzaman.sarker@ericsson.com
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
Varun Singh
CALLSTATS I/O Oy
Annankatu 31-33 C 42
Helsinki 00100
Finland
Email: varun.singh@iki.fi
URI: http://www.callstats.io/
Michael A. Ramalho
Cisco Systems, Inc.
6310 Watercrest Way Unit 203
Lakewood Ranch, FL 34202
USA
Phone: +1 919 476 2038
Email: mramalho@cisco.com
Sarker, et al. Expires September 3, 2018 [Page 11]
