Internet DRAFT - draft-dt-rmcat-feedback-message
draft-dt-rmcat-feedback-message
IETF RMCAT Working Group Z. Sarker
Internet-Draft Ericsson AB
Intended status: Standards Track C. Perkins
Expires: April 30, 2018 University of Glasgow
V. Singh
callstats.io
M. Ramalho
Cisco Systems
October 27, 2017
RTP Control Protocol (RTCP) Feedback for Congestion Control
draft-dt-rmcat-feedback-message-04
Abstract
This document describes a feedback message intended to enable
congestion control for interactive real-time traffic. The RTP Media
Congestion Avoidance Techniques (RMCAT) Working Group formed a design
team to analyze feedback requirements from various congestion control
algorithms and to design a generic feedback message to help ensure
interoperability across those algorithms. The feedback message is
designed for a sender-based congestion control, which means the
receiver of the media will send necessary feedback to the sender of
the media to perform the congestion control at the sender.
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 April 30, 2018.
Copyright Notice
Copyright (c) 2017 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. Feedback Message . . . . . . . . . . . . . . . . . . . . . . 3
3.1. RTCP Congestion Control Feedback Report . . . . . . . . . 4
4. Feedback Frequency and Overhead . . . . . . . . . . . . . . . 6
5. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
For interactive real-time traffic the typical protocol choice is
Realtime Transport Protocol (RTP) over User Datagram Protocol (UDP).
RTP does not provide any guarantee of Quality of Service (QoS),
reliable or timely delivery and expects the underlying transport
protocol to do so. UDP alone certainly does not meet that
expectation. However, RTP Control Protocol (RTCP) provides a
mechanism to periodically send transport and media metrics to the
media sender which can be utilized and extended for the purposes of
RMCAT congestion control. For a congestion control algorithm which
operates at the media sender, RTCP messages can be transmitted from
the media receiver back to the media sender to enable congestion
control. In the absence of standardized messages for this purpose,
the congestion control algorithm designers have designed proprietary
RTCP messages that convey only those parameters required for their
respective designs. As a direct result, the different congestion
control (a.k.a. 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.
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To help achieve interoperability for unicast RTP congestion control,
this memo proposes a common RTCP feedback format that can be used by
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], and hopefully future RTP congestion
control algorithms as well.
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. Feedback Message
The design team analyzed the feedback requirements from the different
proposed candidate in RMCAT WG. The analysis showed some
commonalities between the proposed solution candidate and some can be
derived from other information. The design team has agreed to have
following packet information block in the feedback message to satisfy
different requirement analyzed.
o Packet Identifier : RTP sequence number. The RTP packet header
includes an incremental packet sequence number that the sender
needs to correlate packets sent at the sender with packets
received at the receiver.
o Packet Arrival Time : Arrival time stamp at the receiver of the
media. The sender requires the arrival time stamp of the
respective packet to determine delay and jitter the packet had
experienced during transmission. In a sender based congestion
control solution the sender requires to keep track of the sent
packets - usually packet sequence number, packet size and packet
send time. With the packet arrival time the sender can detect the
delay and jitter information. Along with packet loss and delay
information the sender can estimate the available bandwidth and
thus adapt to the situation.
o Packet Explicit Congestion Notification (ECN) Marking : If ECN
[RFC3168] is used, it is necessary to report on the 2-bit ECN mark
in received packets, indicating for each packet whether it is
marked not-ECT, ECT(0), ECT(1), or ECN-CE. If the path on which
the media traffic traversing is ECN capable then the sender can
use the Congestion Experienced (ECN-CE) marking information for
congestion control. It is important that the receiver sends the
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ECN-CE marking information of the packet back to the sender to
take the advantages of ECN marking. Note that how the receiver
gets the ECN marking information at application layer is out of
the scope of this design team. Additional information for ECN use
with RTP can be found at [RFC6679].
The feedback messages can have one or more of the above information
blocks. For RTCP based feedback message the packet information block
will be grouped by Synchronization Source (SSRC) identifier.
As a practical matter, we note that host Operating System (OS)
process interruptions can occur at inopportune times. Thus, the
recording of the sent times at the sender and arrival times at the
receiver should be made with 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 should be recorded at the latest opportunity prior to
outputting the media packet at the sender (e.g., socket or RTP API)
and the arrival time at the receiver (e.g., socket or RTP API) should
be recorded at the earliest opportunity available to the receiver.
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 as follows:
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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 packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of 1st media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L|ECN| Arrival time offset | ... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of nth media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L|ECN| Arrival time offset | ... |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Report Timestamp (32bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first 8 octets are the RTCP header, with PT=205 and FMT=CCFB
specifying the remainder is a congestion control feedback packet, and
including the SSRC of the packet sender. (NOTE TO RFC EDITOR: please
replace CCFB here and in the above diagram with the IANA assigned
RTCP feedback packet type)
Section 6.1 of [RFC4585] requires the RTCP header to be followed by
the SSRC of the media source being reported upon. Accordingly, the
RTCP header is followed by a report for each SSRC received, followed
by the Report Timestamp.
The report for each SSRC received starts with the SSRC of that media
source. Then, each sequence number between the begin_seq and end_seq
(both inclusive) is represented by a packet metric block of 16-bits
that contains the L, ECN, and ATO fields. If an odd number of
reports are included, i.e., end_seq - begin_seq is odd then 16 bits
of zero padding MUST be added after the last report, to align the
RTCP packet to a four (4) bytes boundary. The L, ECN, and ATO fields
are as follows:
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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): it the relative arrival time
of the RTP packets at the receiver before this feedback report was
generated measured in milliseconds. It is calculated by
subtracting the reception timestamp of the RTP packet denoted by
this 16bit block and the timestamp (RTS) of this report. If the
measured value is greater than 8.189 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.
Report Timestamp (RTS, 32 bits): represents the timestamp when this
report was generated. The sender of the feedback message decides on
the wall-clock. Usually, it should be derived from the same wall-
clock that is used for timestamping RTP packets arrival . Consistency
in the unit and resolution (10th of millisecond should be good enough
) is important here. In addition, the media sender can ask for a
specific resolution it wants.
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, and the possible rates of feedback.
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
media receiver to the media 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. The design team also have noted that
even if a higher frequency of feedback is desired it is not viable if
the feedback messages starts to compete against the media 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.
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5. Design Rationale
The primary function of RTCP Sender Report (SR) / Receiver Report
(RR) is to report the reception quality of media. The regular SR /
RR reports contain information about observed jitter, fractional
packet loss and cumulative packet loss. The original intent of this
information was to assist flow and congestion control mechanisms.
Even though it is possible to do congestion control based on
information provided in the SR/RR reports it is not sufficient to
design an efficient congestion control algorithm for interactive
real-time communication. An efficient congestion control algorithm
requires more fine grain information on per packet (see Section 3) to
react to the congestion or to avoid funder congestion on the path.
Codec Control Message for AVPF [RFC5104] defines Temporary Maximum
Media Bit Rate (TMMBR) message which conveys a temporary maximum
bitrate limitation from the receiver of the media to the sender of
the media. Even though it is 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 with reduced sized reports
[RFC5506]. This requires the receiver of the media to analyze the
data reception, detect congestion level and recommend a maximum
bitrate suitable for current available bandwidth on the path with an
assumption that the sender of the media always honors the TMMBR
message. This requirement is completely opposite of the sender based
congestion control approach. Hence, TMMBR cannot be as a signaling
means for a sender based congestion control mechanism. However,
TMMBR should be viewed a complimentary signaling mechanism to
establish receiver's current view of acceptable maximum bitrate which
a sender based congestion control should honor.
There are number of RTCP eXtended Report (XR) blocks have been
defined for reporting of delay, loss and ECN marking. It is possible
to combine several XR blocks to report the loss and ECN marking at
the cost of overhead and complexity. However, there is no existing
RTCP XR block to report packet arrival time.
Considering the issues discussed here it is rational to design a new
congestion control feedback signaling mechanism for sender based
congestion control algorithm.
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 and Randell Jesup for their valuable
contribution to the discussions and to the document.
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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
There is a risk of causing congestion if an on-path attacker modifies
the feedback messages in such a manner to make available bandwidth
greater than it is in reality. [More on security consideration TBD.]
9. References
9.1. Normative References
[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>.
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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,
<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>.
[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-05
(work in progress), September 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.
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[I-D.ietf-rmcat-scream-cc]
Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", draft-ietf-rmcat-scream-cc-13 (work in
progress), October 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>.
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
Nemu Dialogue Systems Oy
Runeberginkatu 4c A 4
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
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