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A Google Congestion Control Algorithm for Real-Time Communication

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Stefan Holmer , (Unknown), Gaetano Carlucci , Luca De Cicco, Saverio Mascolo
Last updated 2015-09-08
Replaces draft-alvestrand-rmcat-congestion
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Network Working Group                                          S. Holmer
Internet-Draft                                                 H. Lundin
Intended status: Informational                                    Google
Expires: March 11, 2016                                      G. Carlucci
                                                             L. De Cicco
                                                              S. Mascolo
                                                     Politecnico di Bari
                                                       September 8, 2015

   A Google Congestion Control Algorithm for Real-Time Communication


   This document describes two methods of congestion control when using
   real-time communications on the World Wide Web (RTCWEB); one delay-
   based and one loss-based.

   It is published as an input document to the RMCAT working group on
   congestion control for media streams.  The mailing list of that
   working group is

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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

   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 March 11, 2016.

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Copyright Notice

   Copyright (c) 2015 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
   ( in effect on the date of
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Mathematical notation conventions . . . . . . . . . . . .   3
   2.  System model  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Feedback and extensions . . . . . . . . . . . . . . . . . . .   5
   4.  Delay-based control . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Arrival-time model  . . . . . . . . . . . . . . . . . . .   5
     4.2.  Arrival-time filter . . . . . . . . . . . . . . . . . . .   7
     4.3.  Over-use detector . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Rate control  . . . . . . . . . . . . . . . . . . . . . .  10
     4.5.  Parameters settings . . . . . . . . . . . . . . . . . . .  13
   5.  Loss-based control  . . . . . . . . . . . . . . . . . . . . .  13
   6.  Interoperability Considerations . . . . . . . . . . . . . . .  15
   7.  Implementation Experience . . . . . . . . . . . . . . . . . .  15
   8.  Further Work  . . . . . . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .  17
     A.1.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  17
     A.2.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  17
     A.3.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  18
     A.4.  rtcweb-03 to rmcat-00 . . . . . . . . . . . . . . . . . .  18
     A.5.  rmcat -00 to -01  . . . . . . . . . . . . . . . . . . . .  18
     A.6.  rmcat -01 to -02  . . . . . . . . . . . . . . . . . . . .  18
     A.7.  rmcat -02 to -03  . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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1.  Introduction

   Congestion control is a requirement for all applications sharing the
   Internet resources [RFC2914].

   Congestion control for real-time media is challenging for a number of

   o  The media is usually encoded in forms that cannot be quickly
      changed to accommodate varying bandwidth, and bandwidth
      requirements can often be changed only in discrete, rather large

   o  The participants may have certain specific wishes on how to
      respond - which may not be reducing the bandwidth required by the
      flow on which congestion is discovered

   o  The encodings are usually sensitive to packet loss, while the
      real-time requirement precludes the repair of packet loss by

   This memo describes two congestion control algorithms that together
   are able to provide good performance and reasonable bandwidth sharing
   with other video flows using the same congestion control and with TCP
   flows that share the same links.

   The signaling used consists of experimental RTP header extensions and
   RTCP messages RFC 3550 [RFC3550] as defined in [abs-send-time],
   [I-D.alvestrand-rmcat-remb] and

1.1.  Mathematical notation conventions

   The mathematics of this document have been transcribed from a more
   formula-friendly format.

   The following notational conventions are used:

   X_bar  The variable X, where X is a vector - conventionally marked by
      a bar on top of the variable name.

   X_hat  An estimate of the true value of variable X - conventionally
      marked by a circumflex accent on top of the variable name.

   X(i)  The "i"th value of vector X - conventionally marked by a
      subscript i.

   [x y z]  A row vector consisting of elements x, y and z.

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   X_bar^T  The transpose of vector X_bar.

   E{X}  The expected value of the stochastic variable X

2.  System model

   The following elements are in the system:

   o  RTP packet - an RTP packet containing media data.

   o  Packet group - a set of RTP packets transmitted from the sender
      uniquely identified by the group departure and group arrival time
      (absolute send time) [abs-send-time].  These could be video
      packets, audio packets, or a mix of audio and video packets.

   o  Incoming media stream - a stream of frames consisting of RTP

   o  RTP sender - sends the RTP stream over the network to the RTP
      receiver.  It generates the RTP timestamp and the abs-send-time
      header extension

   o  RTP receiver - receives the RTP stream, marks the time of arrival.

   o  RTCP sender at RTP receiver - sends receiver reports, REMB
      messages and transport-wide RTCP feedback messages.

   o  RTCP receiver at RTP sender - receives receiver reports and REMB
      messages and transport-wide RTCP feedback messages, reports these
      to the sender side controller.

   o  RTCP receiver at RTP receiver, receives sender reports from the

   o  Loss-based controller - takes loss rate measurement, round trip
      time measurement and REMB messages, and computes a target sending

   o  Delay-based controller - takes the packet arrival info, either at
      the RTP receiver, or from the feedback received by the RTP sender,
      and computes a maximum bitrate which it passes to the loss-based

   Together, loss-based controller and delay-based controller implement
   the congestion control algorithm.

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3.  Feedback and extensions

   There are two ways to implement the proposed algorithm.  One where
   both the controllers are running at the send-side, and one where the
   delay-based controller runs on the receive-side and the loss-based
   controller runs on the send-side.

   The first version can be realized by using a per-packet feedback
   protocol as described in
   [I-D.holmer-rmcat-transport-wide-cc-extensions].  Here, the RTP
   receiver will record the arrival time and the transport-wide sequence
   number of each received packet, which will be sent back to the sender
   periodically using the transport-wide feedback message.  The
   RECOMMENDED feedback interval is once per received video frame or at
   least once every 30 ms if audio-only or multi-stream.  If the
   feedback overhead needs to be limited this interval can be increased
   to 100 ms.

   The sender will map the received {sequence number, arrival time}
   pairs to the send-time of each packet covered by the feedback report,
   and feed those timestamps to the delay-based controller.  It will
   also compute a loss ratio based on the sequence numbers in the
   feedback message.

   The second version can be realized by having a delay-based controller
   at the receive-side, monitoring and processing the arrival time and
   size of incoming packets.  The sender SHOULD use the abs-send-time
   RTP header extension [abs-send-time] to enable the receiver to
   compute the inter-group delay variation.  The output from the delay-
   based controller will be a bitrate, which will be sent back to the
   sender using the REMB feedback message [I-D.alvestrand-rmcat-remb].
   The packet loss ratio is sent back via RTCP receiver reports.  At the
   sender the bitrate in the REMB message and the fraction of packets
   lost are fed into the loss-based controller, which outputs a final
   target bitrate.  It is RECOMMENDED to send the REMB message as soon
   as congestion is detected, and otherwise at least once every second.

4.  Delay-based control

   The delay-based control algorithm can be further decomposed into
   three parts: an arrival-time filter, an over-use detector, and a rate

4.1.  Arrival-time model

   This section describes an adaptive filter that continuously updates
   estimates of network parameters based on the timing of the received

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   We define the inter-arrival time, t(i) - t(i-1), as the difference in
   arrival time of two packets or two groups of packets.
   Correspondingly, the inter-departure time, T(i) - T(i-1), is defined
   as the difference in departure-time of two packets or two groups of
   packets.  Finally, the inter-group delay variation, d(i), is defined
   as the difference between the inter-arrival time and the inter-
   departure time.  Or interpreted differently, as the difference
   between the delay of group i and group i-1.

     d(i) = t(i) - t(i-1) - (T(i) - T(i-1))

   At the receiving side we are observing groups of incoming packets,
   where a group of packets is defined as follows:

   o  A sequence of packets which are sent within a burst_time interval
      constitute a group.  RECOMMENDED value for burst_time is 5 ms.

   o  In addition, any packet which has an inter-arrival time less than
      burst_time and an inter-group delay variation d(i) less than 0 is
      also considered being part of the current group of packets.  The
      reasoning behind including these packets in the group is to better
      handle delay transients, caused by packets being queued up for
      reasons unrelated to congestion.  As an example this has been
      observed to happen on many Wi-Fi and wireless networks.

   An inter-departure time is computed between consecutive groups as
   T(i) - T(i-1), where T(i) is the departure timestamp of the last
   packet in the current packet group being processed.  Any packets
   received out of order are ignored by the arrival-time model.

   Each group is assigned a receive time t(i), which corresponds to the
   time at which the last packet of the group was received.  A group is
   delayed relative to its predecessor if t(i) - t(i-1) > T(i) - T(i-1),
   i.e., if the inter-arrival time is larger than the inter-departure

   Since the time ts to send a group of packets of size L over a path
   with a capacity of C is roughly

     ts = L/C

   we can model the inter-group delay variation as:

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     d(i) = L(i)/C(i) - L(i-1)/C(i-1) + w(i) =

          = -------------- + w(i) = dL(i)/C(i) + w(i)

   Here, w(i) is a sample from a stochastic process W, which is a
   function of the capacity C(i), the current cross traffic, and the
   current sent bitrate.  C is modeled as being constant as we expect it
   to vary more slowly than other parameters of this model.  We model W
   as a white Gaussian process.  If we are over-using the channel we
   expect the mean of w(i) to increase, and if a queue on the network
   path is being emptied, the mean of w(i) will decrease; otherwise the
   mean of w(i) will be zero.

   Breaking out the mean, m(i), from w(i) to make the process zero mean,
   we get

   Equation 1

     d(i) = dL(i)/C(i) + m(i) + v(i)

   This is our fundamental model, where we take into account that a
   large group of packets need more time to traverse the link than a
   small group, thus arriving with higher relative delay.  The noise
   term represents network jitter and other delay effects not captured
   by the model.

4.2.  Arrival-time filter

   The parameters d(i) and dL(i) are readily available for each group of
   packets, i > 1, and we want to estimate C(i) and m(i) and use those
   estimates to detect whether or not the bottleneck link is over-used.
   These parameters can be estimated by any adaptive filter - we are
   using the Kalman filter.


     theta_bar(i) = [1/C(i)  m(i)]^T

   and call it the state at time i.  We model the state evolution from
   time i to time i+1 as

     theta_bar(i+1) = theta_bar(i) + u_bar(i)

   where u_bar(i) is the state noise that we model as a stationary
   process with Gaussian statistic with zero mean and covariance

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     Q(i) = E{u_bar(i) * u_bar(i)^T}

   Q(i) is RECOMMENDED as a diagonal matrix with main diagonal elements

     diag(Q(i)) = [10^-13 10^-3]^T

   Given equation 1 we get

     d(i) = h_bar(i)^T * theta_bar(i) + v(i)

     h_bar(i) = [dL(i)  1]^T

   where v(i) is zero mean white Gaussian measurement noise with
   variance var_v = sigma(v,i)^2

   The Kalman filter recursively updates our estimate

     theta_hat(i) = [1/C_hat(i) m_hat(i)]^T


     z(i) = d(i) - h_bar(i)^T * theta_hat(i-1)

     theta_hat(i) = theta_hat(i-1) + z(i) * k_bar(i)

                       ( E(i-1) + Q(i) ) * h_bar(i)
     k_bar(i) = ------------------------------------------------------
                var_v_hat(i) + h_bar(i)^T * (E(i-1) + Q(i)) * h_bar(i)

     E(i) = (I - k_bar(i) * h_bar(i)^T) * (E(i-1) + Q(i))

   where I is the 2-by-2 identity matrix.

   The variance var_v(i) = sigma_v(i)^2 is estimated using an
   exponential averaging filter, modified for variable sampling rate

     var_v_hat(i) = max(beta * var_v_hat(i-1) + (1-beta) * z(i)^2, 1)

     beta = (1-chi)^(30/(1000 * f_max))

   where f_max = max {1/(T(j) - T(j-1))} for j in i-K+1,...,i is the
   highest rate at which the last K packet groups have been received and
   chi is a filter coefficient typically chosen as a number in the
   interval [0.1, 0.001].  Since our assumption that v(i) should be zero
   mean WGN is less accurate in some cases, we have introduced an
   additional outlier filter around the updates of var_v_hat.  If z(i) >
   3*sqrt(var_v_hat) the filter is updated with 3*sqrt(var_v_hat) rather

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   than z(i).  For instance v(i) will not be white in situations where
   packets are sent at a higher rate than the channel capacity, in which
   case they will be queued behind each other.

4.3.  Over-use detector

   The offset estimate m(i), obtained as the output of the arrival-time
   filter, is compared with a threshold gamma_1(i).  An estimate above
   the threshold is considered as an indication of over-use.  Such an
   indication is not enough for the detector to signal over-use to the
   rate control subsystem.  A definitive over-use will be signaled only
   if over-use has been detected for at least gamma_2 milliseconds.
   However, if m(i) < m(i-1), over-use will not be signaled even if all
   the above conditions are met.  Similarly, the opposite state, under-
   use, is detected when m(i) < -gamma_1(i).  If neither over-use nor
   under-use is detected, the detector will be in the normal state.

   The threshold gamma_1 has a remarkable impact on the overall dynamics
   and performance of the algorithm.  In particular, it has been shown
   that using a static threshold gamma_1, a flow controlled by the
   proposed algorithm can be starved by a concurrent TCP flow [Pv13].
   This starvation can be avoided by increasing the threshold gamma_1 to
   a sufficiently large value.

   The reason is that, by using a larger value of gamma_1, a larger
   queuing delay can be tolerated, whereas with a small gamma_1, the
   over-use detector quickly reacts to a small increase in the offset
   estimate m(i) by generating an over-use signal that reduces the
   delay-based estimate of the available bandwidth A_hat (see
   Section 4.4).  Thus, it is necessary to dynamically tune the
   threshold gamma_1 to get good performance in the most common
   scenarios, such as when competing with loss-based flows.

   For this reason, we propose to vary the threshold gamma_1(i)
   according to the following dynamic equation:

gamma_1(i) = gamma_1(i-1) + (t(i)-t(i-1)) * K(i) * (|m(i)|-gamma_1(i-1))

   with K(i)=K_d if |m(i)| < gamma_1(i-1) or K(i)=K_u otherwise.  The
   rationale is to increase gamma_1(i) when m(i) is outside of the range
   [-gamma_1(i-1),gamma_1(i-1)], whereas, when the offset estimate m(i)
   falls back into the range, gamma_1 is decreased.  In this way when
   m(i) increases, for instance due to a TCP flow entering the same
   bottleneck, gamma_1(i) increases and avoids the uncontrolled
   generation of over-use signals which may lead to starvation of the
   flow controlled by the proposed algorithm [Pv13].  Moreover,
   gamma_1(i) SHOULD NOT be updated if this condition holds:

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     |m(i)| - gamma_1(i) > 15

   It is also RECOMMENDED to clamp gamma_1(i) to the range [6, 600],
   since a too small gamma_1(i) can cause the detector to become overly

   On the other hand, when m(i) falls back into the range
   [-gamma_1(i-1),gamma_1(i-1)] the threshold gamma_1(i) is decreased so
   that a lower queuing delay can be achieved.

   It is RECOMMENDED to choose K_u > K_d so that the rate at which
   gamma_1 is increased is higher than the rate at which it is
   decreased.  With this setting it is possible to increase the
   threshold in the case of a concurrent TCP flow and prevent starvation
   as well as enforcing intra-protocol fairness.  RECOMMENDED values for
   gamma_1(0), gamma_2, K_u and K_d are respectively 12.5 ms, 10 ms,
   0.01 and 0.00018.

4.4.  Rate control

   The rate control is split in two parts, one controlling the bandwidth
   estimate based on delay, and one controlling the bandwidth estimate
   based on loss.  Both are designed to increase the estimate of the
   available bandwidth A_hat as long as there is no detected congestion
   and to ensure that we will eventually match the available bandwidth
   of the channel and detect an over-use.

   As soon as over-use has been detected, the available bandwidth
   estimated by the delay-based controller is decreased.  In this way we
   get a recursive and adaptive estimate of the available bandwidth.

   In this document we make the assumption that the rate control
   subsystem is executed periodically and that this period is constant.

   The rate control subsystem has 3 states: Increase, Decrease and Hold.
   "Increase" is the state when no congestion is detected; "Decrease" is
   the state where congestion is detected, and "Hold" is a state that
   waits until built-up queues have drained before going to "increase"

   The state transitions (with blank fields meaning "remain in state")

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   |     \ State |   Hold    |  Increase  |Decrease|
   |      \      |           |            |        |
   | Signal\     |           |            |        |
   |  Over-use   | Decrease  |  Decrease  |        |
   |  Normal     | Increase  |            |  Hold  |
   |  Under-use  |           |   Hold     |  Hold  |

   The subsystem starts in the increase state, where it will stay until
   over-use or under-use has been detected by the detector subsystem.
   On every update the delay-based estimate of the available bandwidth
   is increased, either multiplicatively or additively, depending on its
   current state.

   The system does a multiplicative increase if the current bandwidth
   estimate appears to be far from convergence, while it does an
   additive increase if it appears to be closer to convergence.  We
   assume that we are close to convergence if the currently incoming
   bitrate, R_hat(i), is close to an average of the incoming bitrates at
   the time when we previously have been in the Decrease state.  "Close"
   is defined as three standard deviations around this average.  It is
   RECOMMENDED to measure this average and standard deviation with an
   exponential moving average with the smoothing factor 0.95, as it is
   expected that this average covers multiple occasions at which we are
   in the Decrease state.  Whenever valid estimates of these statistics
   are not available, we assume that we have not yet come close to
   convergence and therefore remain in the multiplicative increase

   If R_hat(i) increases above three standard deviations of the average
   max bitrate, we assume that the current congestion level has changed,
   at which point we reset the average max bitrate and go back to the
   multiplicative increase state.

   R_hat(i) is the incoming bitrate measured by the delay-based
   controller over a T seconds window:

     R_hat(i) = 1/T * sum(L(j)) for j from 1 to N(i)

   N(i) is the number of packets received the past T seconds and L(j) is
   the payload size of packet j.  A window between 0.5 and 1 second is

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   During multiplicative increase, the estimate is increased by at most
   8% per second.

     eta = 1.08^min(time_since_last_update_ms / 1000, 1.0)
     A_hat(i) = eta * A_hat(i-1)

   During the additive increase the estimate is increased with at most
   half a packet per response_time interval.  The response_time interval
   is estimated as the round-trip time plus 100 ms as an estimate of
   over-use estimator and detector reaction time.

     response_time_ms = 100 + rtt_ms
     beta = 0.5 * min(time_since_last_update_ms / response_time_ms, 1.0)
     A_hat(i) = A_hat(i-1) + max(1000, beta * expected_packet_size_bits)

   expected_packet_size_bits is used to get a slightly slower slope for
   the additive increase at lower bitrates.  It can for instance be
   computed from the current bitrate by assuming a frame rate of 30
   frames per second:

     bits_per_frame = A_hat(i-1) / 30
     packets_per_frame = ceil(bits_per_frame / (1200 * 8))
     avg_packet_size_bits = bits_per_frame / packets_per_frame

   Since the system depends on over-using the channel to verify the
   current available bandwidth estimate, we must make sure that our
   estimate does not diverge from the rate at which the sender is
   actually sending.  Thus, if the sender is unable to produce a bit
   stream with the bitrate the congestion controller is asking for, the
   available bandwidth estimate should stay within a given bound.
   Therefore we introduce a threshold

     A_hat(i) < 1.5 * R_hat(i)

   When an over-use is detected the system transitions to the decrease
   state, where the delay-based available bandwidth estimate is
   decreased to a factor times the currently incoming bitrate.

     A_hat(i) = alpha * R_hat(i)

   alpha is typically chosen to be in the interval [0.8, 0.95], 0.85 is
   the RECOMMENDED value.

   When the detector signals under-use to the rate control subsystem, we
   know that queues in the network path are being emptied, indicating
   that our available bandwidth estimate A_hat is lower than the actual
   available bandwidth.  Upon that signal the rate control subsystem
   will enter the hold state, where the receive-side available bandwidth

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   estimate will be held constant while waiting for the queues to
   stabilize at a lower level - a way of keeping the delay as low as
   possible.  This decrease of delay is wanted, and expected,
   immediately after the estimate has been reduced due to over-use, but
   can also happen if the cross traffic over some links is reduced.

   It is RECOMMENDED that the routine to update A_hat(i) is run at least
   once every response_time interval.

4.5.  Parameters settings

   | Parameter  | Description                         | RECOMMENDED    |
   |            |                                     | Value          |
   | burst_time | Time limit in milliseconds between  | 5 ms           |
   |            | packet bursts which identifies a    |                |
   |            | group                               |                |
   | Q          | State noise covariance matrix       | diag(Q(i)) =   |
   |            |                                     | [10^-13        |
   |            |                                     | 10^-3]^T       |
   | E(0)       | Initial value of the  system error  | diag(E(0)) =   |
   |            | covariance                          | [100 0.1]^T    |
   | chi        | Coefficient used  for the measured  | [0.1, 0.001]   |
   |            | noise variance                      |                |
   | gamma_1(0) | Initial value for the adaptive      | 12.5 ms        |
   |            | threshold                           |                |
   | gamma_2    | Time required to trigger an overuse | 10 ms          |
   |            | signal                              |                |
   | K_u        | Coefficient for the adaptive        | 0.01           |
   |            | threshold                           |                |
   | K_d        | Coefficient for the adaptive        | 0.00018        |
   |            | threshold                           |                |
   | T          | Time window for measuring the       | [0.5, 1] s     |
   |            | received bitrate                    |                |
   | alpha      | Decrease rate factor                | 0.85           |

          Table 1: RECOMMENDED values for delay based controller

                                  Table 1

5.  Loss-based control

   A second part of the congestion controller bases its decisions on the
   round-trip time, packet loss and available bandwidth estimates A_hat
   received from the delay-based controller.  The available bandwidth

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   estimates computed by the loss-based controller are denoted with

   The available bandwidth estimates A_hat produced by the delay-based
   controller are only reliable when the size of the queues along the
   path sufficiently large.  If the queues are very short, over-use will
   only be visible through packet losses, which are not used by the
   delay-based controller.

   The loss-based controller SHOULD run every time feedback from the
   receiver is received.

   o  If 2-10% of the packets have been lost since the previous report
      from the receiver, the sender available bandwidth estimate
      As_hat(i) will be kept unchanged.

   o  If more than 10% of the packets have been lost a new estimate is
      calculated as As_hat(i) = As_hat(i-1)(1-0.5p), where p is the loss

   o  As long as less than 2% of the packets have been lost As_hat(i)
      will be increased as As_hat(i) = 1.05(As_hat(i-1))

   The new bandwidth estimate is lower-bounded by the TCP Friendly Rate
   Control formula [RFC3448] and upper-bounded by the delay-based
   estimate of the available bandwidth A_hat(i), where the delay-based
   estimate has precedence:

                                    8 s
  As_hat(i) >= ---------------------------------------------------------
               R*sqrt(2*b*p/3) + (t_RTO*(3*sqrt(3*b*p/8)*p*(1+32*p^2)))

  As_hat(i) <= A_hat(i)

   where b is the number of packets acknowledged by a single TCP
   acknowledgment (set to 1 per TFRC recommendations), t_RTO is the TCP
   retransmission timeout value in seconds (set to 4*R) and s is the
   average packet size in bytes.  R is the round-trip time in seconds.

   (The multiplication by 8 comes because TFRC is computing bandwidth in
   bytes, while this document computes bandwidth in bits.)

   In words: The loss-based estimate will never be larger than the
   delay-based estimate, and will never be lower than the estimate from
   the TFRC formula except if the delay-based estimate is lower than the
   TFRC estimate.

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   We motivate the packet loss thresholds by noting that if the
   transmission channel has a small amount of packet loss due to over-
   use, that amount will soon increase if the sender does not adjust his
   bitrate.  Therefore we will soon enough reach above the 10% threshold
   and adjust As_hat(i).  However, if the packet loss ratio does not
   increase, the losses are probably not related to self-inflicted
   congestion and therefore we should not react on them.

6.  Interoperability Considerations

   In case a sender implementing these algorithms talks to a receiver
   which do not implement any of the proposed RTCP messages and RTP
   header extensions, it is suggested that the sender monitors RTCP
   receiver reports and uses the fraction of lost packets and the round-
   trip time as input to the loss-based controller.  The delay-based
   controller should be left disabled.

7.  Implementation Experience

   This algorithm has been implemented in the open-source WebRTC
   project, has been in use in Chrome since M23, and is being used by
   Google Hangouts.

   Deployment of the algorithm have revealed problems related to, e.g,
   congested or otherwise problematic WiFi networks, which have led to
   algorithm improvements.  The algorithm has also been tested in a
   multi-party conference scenario with a conference server which
   terminates the congestion control between endpoints.  This ensures
   that no assumptions are being made by the congestion control about
   maximum send and receive bitrates, etc., which typically is out of
   control for a conference server.

8.  Further Work

   This draft is offered as input to the congestion control discussion.

   Work that can be done on this basis includes:

   o  Considerations of integrated loss control: How loss and delay
      control can be better integrated, and the loss control improved.

   o  Considerations of locus of control: evaluate the performance of
      having all congestion control logic at the sender, compared to
      splitting logic between sender and receiver.

   o  Considerations of utilizing ECN as a signal for congestion
      estimation and link over-use detection.

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9.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

10.  Security Considerations

   An attacker with the ability to insert or remove messages on the
   connection would have the ability to disrupt rate control.  This
   could make the algorithm to produce either a sending rate under-
   utilizing the bottleneck link capacity, or a too high sending rate
   causing network congestion.

   In this case, the control information is carried inside RTP, and can
   be protected against modification or message insertion using SRTP,
   just as for the media.  Given that timestamps are carried in the RTP
   header, which is not encrypted, this is not protected against
   disclosure, but it seems hard to mount an attack based on timing
   information only.

11.  Acknowledgements

   Thanks to Randell Jesup, Magnus Westerlund, Varun Singh, Tim Panton,
   Soo-Hyun Choo, Jim Gettys, Ingemar Johansson, Michael Welzl and
   others for providing valuable feedback on earlier versions of this

12.  References

12.1.  Normative References

              Alvestrand, H., "RTCP message for Receiver Estimated
              Maximum Bitrate", draft-alvestrand-rmcat-remb-03 (work in
              progress), October 2013.

              Holmer, S., Flodman, M., and E. Sprang, "RTP Extensions
              for Transport-wide Congestion Control", draft-holmer-
              rmcat-transport-wide-cc-extensions-00 (work in progress),
              March 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [RFC3448]  Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              3448, January 2003.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

              "RTP Header Extension for Absolute Sender Time",

12.2.  Informative References

   [Pv13]     De Cicco, L., Carlucci, G., and S. Mascolo, "Understanding
              the Dynamic Behaviour of the Google Congestion Control",
              Packet Video Workshop , December 2013.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
              2914, September 2000.

Appendix A.  Change log

A.1.  Version -00 to -01

   o  Added change log

   o  Added appendix outlining new extensions

   o  Added a section on when to send feedback to the end of section 3.3
      "Rate control", and defined min/max FB intervals.

   o  Added size of over-bandwidth estimate usage to "further work"

   o  Added startup considerations to "further work" section.

   o  Added sender-delay considerations to "further work" section.

   o  Filled in acknowledgments section from mailing list discussion.

A.2.  Version -01 to -02

   o  Defined the term "frame", incorporating the transmission time
      offset into its definition, and removed references to "video

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   o  Referred to "m(i)" from the text to make the derivation clearer.

   o  Made it clearer that we modify our estimates of available
      bandwidth, and not the true available bandwidth.

   o  Removed the appendixes outlining new extensions, added pointers to
      REMB draft and RFC 5450.

A.3.  Version -02 to -03

   o  Added a section on how to process multiple streams in a single
      estimator using RTP timestamps to NTP time conversion.

   o  Stated in introduction that the draft is aimed at the RMCAT
      working group.

A.4.  rtcweb-03 to rmcat-00

   Renamed draft to link the draft name to the RMCAT WG.

A.5.  rmcat -00 to -01

   Spellcheck.  Otherwise no changes, this is a "keepalive" release.

A.6.  rmcat -01 to -02

   o  Added Luca De Cicco and Saverio Mascolo as authors.

   o  Extended the "Over-use detector" section with new technical
      details on how to dynamically tune the offset gamma_1 for improved
      fairness properties.

   o  Added reference to a paper analyzing the behavior of the proposed

A.7.  rmcat -02 to -03

   o  Swapped receiver-side/sender-side controller with delay-based/
      loss-based controller as there is no longer a requirement to run
      the delay-based controller on the receiver-side.

   o  Removed the discussion about multiple streams and transmission
      time offsets.

   o  Introduced a new section about "Feedback and extensions".

   o  Improvements to the threshold adaptation in the "Over-use
      detector" section.

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   o  Swapped the previous MIMD rate control algorithm for a new AIMD
      rate control algorithm.

Authors' Addresses

   Stefan Holmer
   Kungsbron 2
   Stockholm  11122


   Henrik Lundin
   Kungsbron 2
   Stockholm  11122

   Gaetano Carlucci
   Politecnico di Bari
   Via Orabona, 4
   Bari  70125


   Luca De Cicco
   Politecnico di Bari
   Via Orabona, 4
   Bari  70125


   Saverio Mascolo
   Politecnico di Bari
   Via Orabona, 4
   Bari  70125


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