RTP Control Protocol
(RTCP) Feedback for Congestion ControlEricsson ABLuleaeSweden+46107173743zaheduzzaman.sarker@ericsson.comUniversity of GlasgowSchool of Computing ScienceGlasgowG12 8QQUnited Kingdomcsp@csperkins.orgNemu Dialogue Systems
OyRuneberginkatu 4c A 4Helsinki00100Finlandvarun.singh@iki.fihttp://www.callstats.io/Cisco Systems, Inc.6310 Watercrest Way Unit 203Lakewood RanchFL34202USA+1 919 476 2038mramalho@cisco.com
Transport
IETF RMCAT Working GroupCongestion control, feedback message, RTP, RTCPThis 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.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.To help achieve interoperability for unicast RTP congestion control,
this memo proposes a common RTCP feedback format that can be used by
NADA , SCReAM , Google Congestion Control
and Shared Bottleneck
Detection , and hopefully
future RTP congestion control algorithms as well.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 .In addition the terminology defined in , , , , and applies.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.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.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.Packet Explicit Congestion Notification (ECN) Marking : If ECN
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
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 .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.Congestion control feedback can be sent as part of a scheduled RTCP
report, or as RTP/AVPF early feedback. If sent as part of a scheduled
RTCP report, the feedback is sent as an XR block, as part of a regular
compound RTCP packet. The format of the RTCP XR report block is as
follows (this will be preceded in the compound RTCP packet by an RTCP
XR header, described in , that includes
the SSRC of the report sender):The XR Discard RLE report block uses the same format as specified
for the loss and duplicate report blocks in . The fields "block length", "begin_seq", and
"end_seq" have the same semantics and representation as defined in
Block Type (BT, 8 bits): The RMCAT congestion control feedback
Report Block is identified by the constant RC2F. [Note to RFC Editor:
Please replace RC2F with the IANA provided RTCP XR block type for this
block.]Report Count (8 bits): field describes the number of SSRCs reported
by this report block. The number should at least be 1.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.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 metrics. 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.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.ECN (2 bits): is the echoed ECN mark of the packet (00 if not
received or if ECN is not used).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.Congestion control feedback can also be sent in a non-compound RTCP
packet if the RTP/AVPF profile or the RTP/SAVPF profile is used. In this case, the congestion control
feedback is sent as a Transport Layer FB message (RTCP packet type
205), with FMT=2 (congestion control feedback). The format of this
RTCP packet is as follows:The first 8 octets are the RTCP header, with PT=205 and FMT=2
specifying the remainder is a congestion control feedback packet, and
including the SSRC of the packet sender.Section 6.1 of requires this is
followed by the SSRC of the media source being reported upon.
Accordingly, the format of the report is changed from that of the RTCP
XR report block, to move the timestamp to the end. The meaning of all
the fields is a described in .There is a trade-off between speed and accuracy of reporting, and the
overhead of the reports. 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 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 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.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 ) to react to the
congestion or to avoid funder congestion on the path.Codec Control Message for AVPF 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 . 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.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.TBDTBDThere 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.]RMCAT Feedback Requirements