[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: 00 01 02 03 draft-alvestrand-rmcat-congestion

Network Working Group                                          H. Lundin
Internet-Draft                                                 S. Holmer
Intended status: Informational                        H. Alvestrand, Ed.
Expires: April 25, 2013                                           Google
                                                        October 22, 2012


A Google Congestion Control Algorithm for Real-Time Communication on the
                             World Wide Web
                 draft-alvestrand-rtcweb-congestion-03

Abstract

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

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

Requirements Language

   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 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 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 April 25, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



Lundin, et al.           Expires April 25, 2013                 [Page 1]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   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
   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.  Mathemathical notation conventions . . . . . . . . . . . .  3
   2.  System model . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Receiver side control  . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Procsesing multiple streams using RTP timestamp to NTP
           time conversion  . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Arrival-time model . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Arrival-time filter  . . . . . . . . . . . . . . . . . . .  7
     3.4.  Over-use detector  . . . . . . . . . . . . . . . . . . . .  8
     3.5.  Rate control . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Sender side control  . . . . . . . . . . . . . . . . . . . . . 11
   5.  Interoperability Considerations  . . . . . . . . . . . . . . . 13
   6.  Implementation Experience  . . . . . . . . . . . . . . . . . . 13
   7.  Further Work . . . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     11.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Change log  . . . . . . . . . . . . . . . . . . . . . 16
     A.1.  Version -00 to -01 . . . . . . . . . . . . . . . . . . . . 16
     A.2.  Version -01 to -02 . . . . . . . . . . . . . . . . . . . . 17
     A.3.  Version -02 to -03 . . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17













Lundin, et al.           Expires April 25, 2013                 [Page 2]


Internet-Draft        Congestion Control for RTCWEB         October 2012


1.  Introduction

   Congestion control is a requirement for all applications that wish to
   share the Internet [RFC2914].

   The problem of doing congestion control for real-time media is made
   difficult for a number of reasons:

   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
      steps

   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
      retransmission

   This memo describes two congestion control algorithms that together
   are seen to give reasonable performance and reasonable (not perfect)
   bandwidth sharing with other conferences and with TCP-using
   applications that share the same links.

   The signalling used consists of standard RTP timestamps [RFC3550]
   possibly augmented with RTP transmission time offsets [RFC5450],
   standard RTCP feedback reports and Temporary Maximum Media Stream Bit
   Rate Requests (TMMBR) as defined in [RFC5104] section 3.5.4, or by
   using the REMB feedback report defined in [I-D.alvestrand-rmcat-remb]

1.1.  Mathemathical 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.







Lundin, et al.           Expires April 25, 2013                 [Page 3]


Internet-Draft        Congestion Control for RTCWEB         October 2012


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

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

   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  Frame - a set of RTP packets transmitted from the sender at the
      same time instant.  This could be a video frame, an audio frame,
      or a mix of audio and video packets.  A frame can be defined by
      the RTP packet send time (RTP timestamp + transmission time
      offset), or by the RTP timestamp if the transmission time offset
      field is not present.

   o  Incoming media streams - a stream of frames consisting of RTP
      packets.

   o  Media codec - has a bandwidth control, and encodes the incoming
      media stream into an RTP stream.

   o  RTP sender - sends the RTP stream over the network to the RTP
      receiver.  Generates the RTP timestamp.

   o  RTP receiver - receives the RTP stream, notes the time of arrival.
      Regenerates the media stream for the recipient.

   o  RTCP sender at RTP sender - sends sender reports with mappings
      between RTP timestamps and NTP time.

   o  RTCP sender at RTP receiver - sends receiver reports and TMMBR/
      REMB messages.

   o  RTCP receiver at RTP sender - receives receiver reports and TMMBR/
      REMB messages, reports these to sender side control.

   o  RTCP receiver at RTP receiver.

   o  Sender side control - takes loss rate info, round trip time info,
      and TMMBR/REMB messages and computes a sending bitrate.




Lundin, et al.           Expires April 25, 2013                 [Page 4]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   o  Receiver side control - takes the packet arrival info at the RTP
      receiver and decides when to send TMMBR/REMB messages.

   Together, sender side control and receiver side control implement the
   congestion control algorithm.


3.  Receiver side control

   The receive-side algorithm can be further decomposed into four parts:
   an RTP timestamp to NTP time conversion, arrival-time filter, an
   over-use detector, and a remote rate-control.

3.1.  Procsesing multiple streams using RTP timestamp to NTP time
      conversion

   It is common that multiple RTP streams are sent from the sender to
   the receiver.  In such a situation the RTP timestamps of incoming can
   first be converted to a common time base using the RTP timestamp and
   NTP time pairs in RTCP SR reports[RFC3550].  The converted timestamps
   can then be used instead of RTP timestamps in the arrival-time
   filtering, and since all streams from the same sender have timestamps
   in the same time base they can all be processed by the same filter.
   This has the advantage of quicker reactions and reduces problems of
   noisy measurements due to self-inflicted cross-traffic.

   In the time interval from the start of the call until a stream from
   the same sender has received an RTCP SR report, the receiver-side
   control operates in single-stream mode.  In that mode only one RTP
   stream can be processed by the over-use detector.  As soon as a
   stream has received one or more RTCP SR reports the receiver-side
   control can change to a multi-stream mode, where all RTP streams from
   the same sender which have received one or more RTCP SR reports can
   be processed by the over-use detector.  When switching to the multi-
   stream mode the state of the over-use detector must be modified to
   avoid a time base mismatch.  This can either be done by resetting the
   stored RTP timestamp values or by converting them using the newly
   received RTCP SR report.

3.2.  Arrival-time model

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

   At the receiving side we are observing groups of incoming packets,
   where each group of packets corresponding to the same frame having
   timestamp T(i).



Lundin, et al.           Expires April 25, 2013                 [Page 5]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   Each frame is assigned a receive time t(i), which corresponds to the
   time at which the whole frame has been received (ignoring any packet
   losses).  A frame is delayed relative to its predecessor if t(i)-t(i-
   1)>T(i)-T(i-1), i.e., if the arrival time difference is larger than
   the timestamp difference.

   We define the (relative) inter-arrival time, d(i) as

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


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

     ts = L/C


   we can model the inter-arrival time as

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


   Here, w(i) is a sample from a stochastic process W, which is a
   function of the capacity C, the current cross traffic X(i), and the
   current send bit rate R(i).  We model W as a white Gaussian process.
   If we are over-using the channel we expect w(i) to increase, and if a
   queue on the network path is being emptied, 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 5

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


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

   When graphing the values for d(i) versus dL(i) on a scatterplot, we
   find that most samples cluster around the center, and the outliers
   are clustered along a line with average slope 1/C and zero offset.




Lundin, et al.           Expires April 25, 2013                 [Page 6]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   For instance, when using a regular video codec, most frames are
   roughly the same size after encoding (the central "cloud"); the
   exceptions are I-frames (or key frames) which are typically much
   larger than the average causing positive outliers (the I-frame
   itself) and negative outliers (the frame after an I-frame) on the dL
   axis.  Audio frames on the other hand often consist of single packets
   of equal size, and an audio-only media stream would have its frames
   scattered at dL = 0.

3.3.  Arrival-time filter

   The parameters d(i) and dL(i) are readily available for each frame i
   > 1, and we want to estimate C(i) and m(i) and use those estimates to
   detect whether or not we are over-using the bandwidth currently
   available.  These parameters are easily estimated by any adaptive
   filter - we are using the Kalman filter.

   Let

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

   and call it the state of 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 zero mean white Gaussian process noise with
   covariance

   Equation 7

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


   Given equation 5 we get

   Equation 8

     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



Lundin, et al.           Expires April 25, 2013                 [Page 7]


Internet-Draft        Congestion Control for RTCWEB         October 2012


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

   as

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

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

   I is the 2-by-2 identity matrix.

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

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

     beta = (1-alpha)^(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 frames have been captured by the camera the
   last K frames and alpha 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 var_v_hat the filter is updated with 3
   sqrt(var_v_hat) rather 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.  In a similar way, Q(i) is chosen as a diagonal matrix with
   main diagonal elements given by

     diag(Q(i)) = 30/(1000 * f_max)[10^-10 10^-2]^T

   It is necessary to scale these filter parameters with the frame rate
   to make the detector respond as quickly at low frame rates as at high
   frame rates.

3.4.  Over-use detector

   The offset estimate m(i) is compared with a threshold gamma_1.  An
   estimate above the threshold is considered as an indication of over-
   use.  Such an indication is not enough for the detector to signal



Lundin, et al.           Expires April 25, 2013                 [Page 8]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   over-use to the rate control subsystem.  Not until over-use has been
   detected for at least gamma_2 milliseconds and at least gamma_3
   frames, a definitive over-use will be signaled.  However, if the
   offset estimate m(i) was decreased in the last update, 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.  If
   neither over-use nor under-use is detected, the detector will be in
   the normal state.

3.5.  Rate control

   The rate control at the receiving side is designed to increase the
   receive-side estimate of the available bandwidth A_hat as long as the
   detected state is normal.  Doing that assures that we, sooner or
   later, will reach the available bandwidth of the channel and detect
   an over-use.

   As soon as over-use has been detected the receive-side estimate of
   the available bandwidth 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"
   state.

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

   State ---->  | Hold      |Increase    |Decrease
   Signal-----------------------------------------
     v          |           |            |
   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.



Lundin, et al.           Expires April 25, 2013                 [Page 9]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   On every update the receive-side estimate of the available bandwidth
   is increased with a factor which is a function of the global system
   response time and the estimated measurement noise variance var_v_hat.
   The global system response time is the time from an increase that
   causes over-use until that over-use can be detected by the over-use
   detector.  The variance var_v_hat affects how responsive the Kalman
   filter is, and is thus used as an indicator of the delay inflicted by
   the Kalman filter.

     A_hat(i) = eta*A_hat(i-1)
                                    1.001+B
     eta(RTT, var_v_hat) = ------------------------------------------
                              1+e^(b(d*RTT - (c1 * var_v_hat + c2)))

   Here, B, b, d, c1 and c2 are design parameters.

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

     A_hat(i) < 1.5 * R_hat(i)

   where R_hat(i) is the incoming bit rate measured 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 frames received the past T seconds and L(j) is
   the payload size of frame j.  Ideally T should be chosen to match the
   rate controller at the sender.  A window between 0.5 and 1 second is
   recommended.

   When an over-use is detected the system transitions to the decrease
   state, where the receive-side available bandwidth estimate is
   decreased to a factor times the currently incoming bit rate.

     A_hat(i) = alpha*R_hat(i)

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

   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 is lower than the actual
   available bandwidth.  Upon that signal the rate control subsystem



Lundin, et al.           Expires April 25, 2013                [Page 10]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   will enter the hold state, where the receive-side available bandwidth
   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.  In
   either case we want to measure the highest incoming rate during the
   under-use interval:

     R_max = max{R_hat(i)} for i in 1..K


   where K is the number of frames of under-use before returning to the
   normal state.  R_max is a measure of the actual bandwidth available
   and is a good guess of what bit rate the sender should be able to
   transmit at.  Therefore the receive-side available bandwidth estimate
   will be set to R_max when we transition from the hold state to the
   increase state.

   One design decision is when to send rate control messages.  The time
   from a change in congestion to the sending of the feedback message is
   a limitation on how fast the sender can react.  Sending too many
   messages giving no new information is a waste of bandwidth - but in
   the case of severe congestion, feedback messages can be lost,
   resulting in a failure to react in a timely manner.

   The conclusion is that feedback messages should be sent on a
   "heartbeat" schedule, allowing the sender side control to react to
   missing feedback messages by reducing its send rate, but they should
   also be sent whenever the estimated bandwidth value has changed
   significantly, without waiting for the heartbeat time, up to some
   limiting upper bound on the send rate.

   The minimum interval is named t_min_fb_interval.

   The maximum interval is named t_max_fb_interval.

   The permissible values of these intervals will be bounded by the RTP
   session's RTCP bandwidth and its rtcp_frr setting.

   [TODO: Get some example values for these timers]


4.  Sender side control

   An additional congestion controller resides at the sending side.  It
   bases its decisions on the round-trip time, packet loss and available
   bandwidth estimates transmitted from the receiving side.



Lundin, et al.           Expires April 25, 2013                [Page 11]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   The available bandwidth estimates produced by the receiving side are
   only reliable when the size of the queues along the channel are large
   enough.  If the queues are very short, over-use will only be visible
   through packet losses, which aren't used by the receiving side
   algorithm.

   This algorithm is run every time a receive report arrives at the
   sender, which will happen no more often than t_min_fb_interval, and
   no less often than t_max_fb_interval.  If no receive report is
   received within 2x t_max_fb_interval (indicating at least 2 lost
   feedback reports), the algorithm will take action as if all packets
   in the interval have been lost, resulting in a halving of the send
   rate.

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

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

   o  As long as less than 2% of the packets have been lost As(i) will
      be increased as As(i)=1.05(As(i-1)+1000)

   The new send-side estimate is limited by the TCP Friendly Rate
   Control formula [RFC3448] and the receive-side estimate of the
   available bandwidth A(i):
                                  8 s
   As(i) >= ----------------------------------------------------------
            R*sqrt(2*b*p/3) + (t_RTO*(3*sqrt(3*b*p/8) * p * (1+32*p^2)))

   As(i) <= A(i)


   where b is the number of packets acknowledged by a single TCP
   acknowledgement (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 sender-side estimate will never be larger than the
   receiver-side estimate, and will never be lower than the estimate
   from the TFRC formula.

   We motivate the packet loss thresholds by noting that if the
   transmission channel has a small amount of packet loss due to over-



Lundin, et al.           Expires April 25, 2013                [Page 12]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   use, that amount will soon increase if the sender does not adjust his
   bit rate.  Therefore we will soon enough reach above the 10 %
   threshold and adjust As(i).  However if the packet loss rate does not
   increase, the losses are probably not related to self-induced channel
   over-use and therefore we should not react on them.


5.  Interoperability Considerations

   There are three scenarios of interest, and one included for reference

   o  Both parties implement the algorithms described here

   o  Sender implements the algorithm described in section Section 4,
      recipient does not implement Section 3

   o  Recipient implements the algorithm in section Section 3, sender
      does not implement Section 4.

   In the case where both parties implement the algorithms, we expect to
   see most of the congestion control response to slowly varying
   conditions happen by TMMBR/REMB messages from recipient to sender.
   At most times, the sender will send less than the congestion-inducing
   bandwidth limit C, and when he sends more, congestion will be
   detected before packets are lost.

   If sudden changes happen, packets will be lost, and the sender side
   control will trigger, limiting traffic until the congestion becomes
   low enough that the system switches back to the receiver-controlled
   state.

   In the case where sender only implements, we expect to see somewhat
   higher loss rates and delays, but the system will still be overall
   TCP friendly and self-adjusting; the governing term in the
   calculation will be the TFRC formula.

   In the case where recipient implements this algorithm and sender does
   not, congestion will be avoided for slow changes as long as the
   sender understands and obeys TMMBR/REMB; there will be no backoff for
   packet-loss-inducing changes in capacity.  Given that some kind of
   congestion control is mandatory for the sender according to the TMMBR
   spec, this case has to be reevaluated against the specific congestion
   control implemented by the sender.


6.  Implementation Experience

   This algorithm has been implemented in the open-source WebRTC



Lundin, et al.           Expires April 25, 2013                [Page 13]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   project.


7.  Further Work

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

   Work that can be done on this basis includes:

   o  Consideration of timing info: It may be sensible to use the
      proposed TFRC RTP header extensions [I-D.gharai-avtcore-rtp-tfrc]
      to carry per-packet timing information, which would both give more
      data points and a timestamp applied closer to the network
      interface.  This draft includes consideration of using the
      transmission time offset defined in [RFC5450]

   o  Considerations of cross-channel calculation: If all packets in
      multiple streams follow the same path over the network, congestion
      or queueing information should be considered across all packets
      between two parties, not just per media stream.  A feedback
      message (REMB) that may be suitable for such a purpose is given in
      [I-D.alvestrand-rmcat-remb].

   o  Considerations of cross-channel balancing: The decision to slow
      down sending in a situation with multiple media streams should be
      taken across all media streams, not per stream.

   o  Considerations of additional input: How and where packet loss
      detected at the recipient can be added to the algorithm.

   o  Considerations of locus of control: Whether the sender or the
      recipient is in the best position to figure out which media
      streams it makes sense to slow down, and therefore whether one
      should use TMMBR to slow down one channel, signal an overall
      bandwidth change and let the sender make the decision, or signal
      the (possibly processed) delay info and let the sender run the
      algorithm.

   o  Considerations of over-bandwidth estimation: Whether we can use
      the estimate of how much we're over bandwidth in section 3 to
      influence how much we reduce the bandwidth, rather than using a
      fixed factor.

   o  Startup considerations.  It's unreasonable to assume that just
      starting at full rate is always the best strategy.

   o  Dealing with sender traffic shaping, which delays sending of
      packets.  Using send-time timestamps rather than RTP timestamps



Lundin, et al.           Expires April 25, 2013                [Page 14]


Internet-Draft        Congestion Control for RTCWEB         October 2012


      may be useful here, but as long as the sender's traffic shaping
      does not spread out packets more than the bottleneck link, it
      should not matter.

   o  Stability considerations.  It is not clear how to show that the
      algorithm cannot provide an oscillating state, either alone or
      when competing with other algorithms / flows.

   These are matters for further work; since some of them involve
   extensions that have not yet been standardized, this could take some
   time.


8.  IANA Considerations

   This document makes no request of IANA.

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


9.  Security Considerations

   An attacker with the ability to insert or remove messages on the
   connection will, of course, have the ability to mess up rate control,
   causing people to send either too fast or too slow, and causing
   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.


10.  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
   draft.


11.  References






Lundin, et al.           Expires April 25, 2013                [Page 15]


Internet-Draft        Congestion Control for RTCWEB         October 2012


11.1.  Normative References

   [I-D.alvestrand-rmcat-remb]
              Alvestrand, H., "RTCP message for Receiver Estimated
              Maximum Bitrate", draft-alvestrand-rmcat-remb-01 (work in
              progress), July 2012.

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

   [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.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC5450]  Singer, D. and H. Desineni, "Transmission Time Offsets in
              RTP Streams", RFC 5450, March 2009.

11.2.  Informative References

   [I-D.gharai-avtcore-rtp-tfrc]
              Gharai, L. and C. Perkins, "RTP with TCP Friendly Rate
              Control", draft-gharai-avtcore-rtp-tfrc-01 (work in
              progress), September 2011.

   [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.





Lundin, et al.           Expires April 25, 2013                [Page 16]


Internet-Draft        Congestion Control for RTCWEB         October 2012


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

   o  Added startup considerations to "further work" section.

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

   o  Filled in acknowledgements 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
      frame".

   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.


Authors' Addresses

   Henrik Lundin
   Google
   Kungsbron 2
   Stockholm  11122
   Sweden












Lundin, et al.           Expires April 25, 2013                [Page 17]


Internet-Draft        Congestion Control for RTCWEB         October 2012


   Stefan Holmer
   Google
   Kungsbron 2
   Stockholm  11122
   Sweden

   Email: holmer@google.com


   Harald Alvestrand (editor)
   Google
   Kungsbron 2
   Stockholm  11122
   Sweden

   Email: harald@alvestrand.no



































Lundin, et al.           Expires April 25, 2013                [Page 18]


Html markup produced by rfcmarkup 1.121, available from https://tools.ietf.org/tools/rfcmarkup/