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Versions: (draft-floyd-rfc3448bis) 00 01 02 03 04 05 06 RFC 5348

Internet Engineering Task Force                                 S. Floyd
INTERNET-DRAFT                                                      ICIR
Intended status: Proposed Standard                            M. Handley
Expires: January 2008                          University College London
                                                               J. Padhye
                                                               Microsoft
                                                               J. Widmer
                                                  University of Mannheim
                                                             8 July 2007


        TCP Friendly Rate Control (TFRC): Protocol Specification
                   draft-ietf-dccp-rfc3448bis-02.txt


Status of this Memo

    By submitting this Internet-Draft, each author represents that any
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    This Internet-Draft will expire on January 2008.

Copyright Notice

    Copyright (C) The IETF Trust (2007).





Handley/Floyd/Padhye/Widmer                                     [Page 1]

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Abstract

    This document specifies TCP-Friendly Rate Control (TFRC).  TFRC is a
    congestion control mechanism for unicast flows operating in a best-
    effort Internet environment.  It is reasonably fair when competing
    for bandwidth with TCP flows, but has a much lower variation of
    throughput over time compared with TCP, making it more suitable for
    applications such as streaming media where a relatively smooth
    sending rate is of importance.










































Handley/Floyd/Padhye/Widmer                                     [Page 2]

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Table of Contents

    1. Introduction ...................................................9
    2. Conventions ...................................................10
    3. Protocol Mechanism ............................................10
       3.1. TCP Throughput Equation ..................................11
       3.2. Packet Contents ..........................................12
            3.2.1. Data Packets ......................................13
            3.2.2. Feedback Packets ..................................13
    4. Data Sender Protocol ..........................................14
       4.1. Measuring the Segment Size ...............................14
       4.2. Sender Initialization ....................................15
       4.3. Sender Behavior When a Feedback Packet is Received .......16
       4.4. Expiration of Nofeedback Timer ...........................18
       4.5. Reducing Oscillations ....................................21
       4.6. Scheduling of Packet Transmissions .......................22
            4.6.1. Sending Packets Before their Nominal Send Time ....23
    5. Calculation of the Loss Event Rate (p) ........................24
       5.1. Detection of Lost or Marked Packets ......................24
       5.2. Translation from Loss History to Loss Events .............25
       5.3. Inter-loss Event Interval ................................27
       5.4. Average Loss Interval ....................................27
       5.5. History Discounting ......................................28
    6. Data Receiver Protocol ........................................30
       6.1. Receiver Behavior When a Data Packet is Received .........31
       6.2. Expiration of Feedback Timer .............................32
       6.3. Receiver Initialization ..................................33
            6.3.1. Initializing the Loss History after the First Loss
            Event ....................................................33
    7. Sender-based Variants .........................................34
    8. Implementation Issues .........................................35
    9. Changes from RFC 3448 .........................................36
    10. Security Considerations ......................................39
    11. IANA Considerations ..........................................40
    12. Acknowledgments ..............................................40
    A. Terminology ...................................................40
    B. The Initial Value of the Nofeedback Timer .....................42
    C. Response to Idle or Data-limited Periods ......................42
       C.1. Long Idle or Data-limited Periods ........................43
       C.2. Short Idle or Data-limited Periods .......................45
       C.3. Moderate Idle or Data-limited Periods ....................46
       C.4. Other Patterns ...........................................46
    Normative References .............................................46
    Informational References .........................................47
    Authors' Addresses ...............................................48
    Full Copyright Statement .........................................49
    Intellectual Property ............................................49




Handley/Floyd/Padhye/Widmer                                     [Page 3]

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    NOTE TO RFC EDITOR: PLEASE DELETE THIS NOTE UPON PUBLICATION.

     Changes from draft-ietf-dccp-rfc3448bis-01.txt:

     * Specified that the sender is not limited by the receive rate
       if the sender has been data-limited for an entire feedback
       interval.

     * Added variables "initial_rate" and "recover_rate, for the
       initial transmit rate and the rate for resuming after an idle
       period, for easier specification of Faster Restart (in a separate
       document).  Also added the variable "recv_limit" to specify
       the limit on the sending rate that is computed from the receive
       rate, and the variable "timer_limit" to specify the
       limit on the sending rate from the expiration of the nofeedback
       timer.
       Explained why recover_rate is not used as lower bound
       for nofeedback timer expirations after a data-limited period.

     * Added Appendix C on "Response to Idle or Data-limited Periods".

     * Revised the section on "Scheduling of Packet Transmissions"
       to make clear what is specification, and what is
       implementation.  From Gerrit.  Also stated that the
       accumulation of sending credits should be limited
       to a round-trip time's worth of packets.

     * For measuring the receive rate, added that after a loss event,
       the receive rate SHOULD be measured over the most recent RTT,
       but for simplicity of implementation, MAY be measured over
       a slightly longer time interval.

     * Clarified that RTT measurements don't necessarily come from
       feedback packets; they could also come from other places,
       e.g., from the SYN exchange.

     * Specified that the sender may maintain unused sent credits
       up to one RTT.  This gives behavior similar to TCP.
       Also specified that the sender should not sent packets more
       that rtt/2 seconds before their nominal send time.

     * Reinserted the last paragraph of Section 4.4 from RFC 3448.
       It must have been deleted accidently.

      * TODO in ns-2
        - Add a variable to ns-2 to allow either TFRC or CCID3.

     * Feedback from Arjuna Sathiaseelan:



Handley/Floyd/Padhye/Widmer                                     [Page 4]

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       - Changing W_init to be in terms of segment size s, not MSS.

     * Changed THRESHOLD, the lower bound on the history
       discounting parameter DF, from 0.5 to 0.25, for more
       history discounting when the current interval is long.

     * Relying on the sender not to use X_recv from data-limited
       periods.  This gives behavior similar to TCP, when
       ACK-clocking is not in effect in data-limited periods.
       The largest X_recv over the most recent two round-trip
       times is used to limit the sending rate.  This is
       maintained using X_recv_set.  Taken together, these avoid
       problems with the first feedback packet after an idle
       period, and this avoids problems with limitations
       from X_recv during data-limited periods.

     * Clarified that when the receiver receives a data packet,
       and didn't send a feedback packet when the feedback timer
       last expired (because no data packets were received),
       then the receiver sends a feedback packet immediately.

     * Clarified that the feedback packet reports the rate over
       the last RTT, not necessarily the rate since the
       last feedback packet was sent (if no feedback packet was
       sent when the feedback timer last expired).

     * Corrected earlier code designed to prevent the receive
       rate from limiting the sending rate when the first feedback
       packet received, or for the first feedback packet received
       after an idle period.

     * Clarified that we have p=0 only until the first loss event.
       After the first loss event, p>0, and it is not possible to go
       back to p=0.  In response to old email.

     * Clarified in Section 6.1 that the loss event rate does not
       have to be recalculated with the arrival of each new data
       packet.

     * Clarified the section on Reducing Oscillations.  Feedback from
       Gerrit Renker.

     Changes from draft-ietf-dccp-rfc3448bis-00.txt:

     * When initializing the loss history after the first
       data packet sent is lost or ECN-marked, TFRC uses
       a minimum receive rate of 0.5 packets per second.




Handley/Floyd/Padhye/Widmer                                     [Page 5]

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     * For initializing the estimated packet drop rate
       for the first loss interval when coming out of slow-start,
       it is ok to use the maximum receive rate so far, not just
       the receive rate in the last round-trip time.
       Feedback from Ladan Gharai.

     * General feedback from Gorry Fairhurst:
       - Added a reference for RFC4828.
       - Clarified that R_m is sender's estimate of RTT, as reported
         in Section 3.2.1.
       - Added a definition of terms.
       - Added a discussion of why the initial value of the nofeedback
         timer is two seconds, instead of three seconds for the
         recommended initial value for TCP's retransmit timer.

     * General feedback from Arjuna Sathiaseelan:
       - Added more details about sending multiple feedback
          packets per RTT.
       - Added change to Section 4.3 to use the first feedback
          packet, or the first feedback packet after a
          nofeedback timer during slow-start, *if min_rate > X*.

     * General feedback from Gerrit Renker:
       - Changed "delta" to "t_delta".
       - Changed X_calc to X_Bps, clarified X.
       - Clarified send times in "Scheduling of Packet Transmissions".
       - Changed so that tld can be initialized to either 0 or -1.
       - Fixed Section 5.5 to say that the most recent lost
         interval has weight 1/(0.75*n) *when there have been
         at least eight loss intervals*.
       - Clarified introduction about fixed-size and variable-size
         packets.

     * Added more about sender-based variants.
       Feedback from Guillaume Jourjon.

     * Corrected that the loss interval I_0 includes all transmitted
       packets, including lost and marked packets (as defined in Section
       5.3 in the general definition.)  Email from Eddie Kohler and
       Gerrit Renker.

     * Not done:  I didn't add a minimum value for the nofeedback
       timer.  (Why would a nofeedback timer need to be bigger
       than max(4*R, 2*s/X)?  Email discussing pros and cons from
       Arjuna.

     Changes from draft-floyd-rfc3448bis-00.txt:




Handley/Floyd/Padhye/Widmer                                     [Page 6]

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     * Name change to draft-ietf-dccp-rfc3448bis-00.txt.

     * Specified the receiver's initialization of the feedback timer
       when the first data packet doesn't have an estimate of the
       RTT.  From feedback from Dado Colussi.

     * Added the procedure for sending receiver
       feedback packets when a coarse-grained
       timestamp is used. From RFC 4243.

     Changes from RFC 3448:

     * Incorporated changes in the RFC 3448 errata:

       -  "If the sender does not receive a feedback report for
          four round trip times, it cuts its sending rate in half."
          ("Two" changed to "four", for consistency with the rest
          of the document.  Reported by Joerg Widmer).

       - "If the nofeedback timer expires when the sender does not
         yet have an RTT sample, and has not yet received any
         feedback from the receiver, or when p == 0,..."
         (Added "or when p == 0,", reported by Wim Heirman).

       - In Section 5.5, changed:
           for (i = 1 to n) { DF_i = 1; }
         to:
           for (i = 0 to n) { DF_i = 1; }
         Reported by Michele R.

     * Changed RFC 3448 to correspond to the larger initial windows
       specified in RFC 3390.  This includes the following:

       - Incorporated Section 5.1 from [RFC4342], saying that
         when reducing the sending rate after an idle period, don't
         reduce the sending rate below the initial sending rate.

       - Change for a datalimited sender:
         When the sender has been datalimited, the sender doesn't
         let the receive rate limit it to a sending rate less than
         the initial rate.

       - Small change to slow-start:
         Changed so that for the first feedback packet received,
         or for the first feedback packet received after an idle
         period, the receive rate is not used to limit the
         sending rate.  This is because the receiver might not yet
         have seen an entire window of data.



Handley/Floyd/Padhye/Widmer                                     [Page 7]

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     * Clarified how the average loss interval is calculated when
       the receiver has not yet seen eight loss intervals.

     * Discussed more about estimating the average segment size:

       - For initializing the loss history after the first loss event,
         either the receiver knows the sender's value for s, or
         the receiver uses the throughput equation for X_pps and does
         not need to know an estimate for s.

       - Added a discussion about estimating the average segment size
         s in Section 4.1 on "Measuring the Segment Size".

       - Changed "packet size" to "segment size".

    END OF NOTE TO RFC EDITOR.



































Handley/Floyd/Padhye/Widmer                                     [Page 8]

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1.  Introduction

    This document specifies TCP-Friendly Rate Control (TFRC).  TFRC is a
    congestion control mechanism designed for unicast flows operating in
    an Internet environment and competing with TCP traffic [FHPW00].
    Instead of specifying a complete protocol, this document simply
    specifies a congestion control mechanism that could be used in a
    transport protocol such as DCCP (Datagram Congestion Control
    Protocol) [RFC4340], in an application incorporating end-to-end
    congestion control at the application level, or in the context of
    endpoint congestion management [BRS99]. This document does not
    discuss packet formats or reliability.  Implementation-related
    issues are discussed only briefly, in Section 8.

    TFRC is designed to be reasonably fair when competing for bandwidth
    with TCP flows, where a flow is "reasonably fair" if its sending
    rate is generally within a factor of two of the sending rate of a
    TCP flow under the same conditions.  However, TFRC has a much lower
    variation of throughput over time compared with TCP, which makes it
    more suitable for applications such as telephony or streaming media
    where a relatively smooth sending rate is of importance.

    The penalty of having smoother throughput than TCP while competing
    fairly for bandwidth is that TFRC responds slower than TCP to
    changes in available bandwidth.  Thus TFRC should only be used when
    the application has a requirement for smooth throughput, in
    particular, avoiding TCP's halving of the sending rate in response
    to a single packet drop.  For applications that simply need to
    transfer as much data as possible in as short a time as possible we
    recommend using TCP, or if reliability is not required, using an
    Additive-Increase, Multiplicative-Decrease (AIMD) congestion control
    scheme with similar parameters to those used by TCP.

    TFRC is designed for best performance with applications that use a
    fixed segment size, and vary their sending rate in packets per
    second in response to congestion.  TFRC can also be used, perhaps
    with less optimal performance, with applications that don't have a
    fixed segment size, but where the segment size varies according to
    the needs of the application (e.g., video applications).

    Some applications (e.g., some audio applications) require a fixed
    interval of time between packets and vary their segment size instead
    of their packet rate in response to congestion.  The congestion
    control mechanism in this document is not designed for those
    applications; TFRC-SP (Small-Packet TFRC) is a variant of TFRC for
    applications that have a fixed sending rate in packets per second
    but either use small packets, or vary their packet size in response
    to congestion.  TFRC-SP is specified in a separate document



Handley/Floyd/Padhye/Widmer                         Section 1.  [Page 9]

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    [RFC4828].

    This document specifies TFRC as a receiver-based mechanism, with the
    calculation of the congestion control information (i.e., the loss
    event rate) in the data receiver rather in the data sender.  This is
    well-suited to an application where the sender is a large server
    handling many concurrent connections, and the receiver has more
    memory and CPU cycles available for computation.  In addition, a
    receiver-based mechanism is more suitable as a building block for
    multicast congestion control.  However, it is also possible to
    implement TFRC in sender-based variants, as allowed in DCCP's
    Congestion Control ID 3 (CCID 3) [RFC4342].

2.  Conventions

    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].

    Appendix A gives a list of technical terms used in this document.

3.  Protocol Mechanism

    For its congestion control mechanism, TFRC directly uses a
    throughput equation for the allowed sending rate as a function of
    the loss event rate and round-trip time.  In order to compete fairly
    with TCP, TFRC uses the TCP throughput equation, which roughly
    describes TCP's sending rate as a function of the loss event rate,
    round-trip time, and segment size.  We define a loss event as one or
    more lost or marked packets from a window of data, where a marked
    packet refers to a congestion indication from Explicit Congestion
    Notification (ECN) [RFC3168].

    Generally speaking, TFRC's congestion control mechanism works as
    follows:

    o   The receiver measures the loss event rate and feeds this
        information back to the sender.

    o   The sender also uses these feedback messages to measure the
        round-trip time (RTT).

    o   The loss event rate and RTT are then fed into TFRC's throughput
        equation, and the resulting sending rate is limited to at most
        twice the receive rate to give the allowed transmit rate X.

    o   The sender then adjusts its transmit rate to match the allowed
        transmit rate X.



Handley/Floyd/Padhye/Widmer                        Section 3.  [Page 10]

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    The dynamics of TFRC are sensitive to how the measurements are
    performed and applied.  We recommend specific mechanisms below to
    perform and apply these measurements.  Other mechanisms are
    possible, but it is important to understand how the interactions
    between mechanisms affect the dynamics of TFRC.

3.1.  TCP Throughput Equation

    Any realistic equation giving TCP throughput as a function of loss
    event rate and RTT should be suitable for use in TFRC.  However, we
    note that the TCP throughput equation used must reflect TCP's
    retransmit timeout behavior, as this dominates TCP throughput at
    higher loss rates.  We also note that the assumptions implicit in
    the throughput equation about the loss event rate parameter have to
    be a reasonable match to how the loss rate or loss event rate is
    actually measured.  While this match is not perfect for the
    throughput equation and loss rate measurement mechanisms given
    below, in practice the assumptions turn out to be close enough.

    The throughput equation we currently recommend for TFRC is a
    slightly simplified version of the throughput equation for Reno TCP
    from [PFTK98]. Ideally we'd prefer a throughput equation based on
    SACK TCP, but no one has yet derived the throughput equation for
    SACK TCP, and from both simulations and experiments, the differences
    between the two equations are relatively minor.

    The throughput equation is:

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


    Where:

        X_Bps is the transmit rate in bytes/second.  (X_Bps is the same
        as X_calc in RFC 3448.)

        s is the segment size in bytes.

        R is the round trip time in seconds.

        p is the loss event rate, between 0 and 1.0, of the number of
        loss events as a fraction of the number of packets transmitted.

        t_RTO is the TCP retransmission timeout value in seconds.





Handley/Floyd/Padhye/Widmer                      Section 3.1.  [Page 11]

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        b is the maximum number of packets acknowledged by a single TCP
        acknowledgement.

    We further simplify this by setting t_RTO = 4*R.  A more accurate
    calculation of t_RTO is possible, but experiments with the current
    setting have resulted in reasonable fairness with existing TCP
    implementations [W00].  Another possibility would be to set t_RTO =
    max(4R, one second), to match the recommended minimum of one second
    on the RTO [RFC2988].

    Many current TCP connections use delayed acknowledgements, sending
    an acknowledgement for every two data packets received, and thus
    have a sending rate modeled by b = 2.  However, TCP is also allowed
    to send an acknowledgement for every data packet, and this would be
    modeled by b = 1.  Because many TCP implementations do not use
    delayed acknowledgements, we recommend b = 1.

    In future, different TCP equations may be substituted for this
    equation.  The requirement is that the throughput equation be a
    reasonable approximation of the sending rate of TCP for conformant
    TCP congestion control.

    The throughput equation can also be expressed as

    X_Bps =  X_pps * s ,

    with X_pps, the sending rate in packets per second, given as

                                      1
    X_pps =  --------------------------------------------------------
            R*sqrt(2*b*p/3) + (t_RTO*(3*sqrt(3*b*p/8)*p*(1+32*p^2)))


    The parameters s (segment size), p (loss event rate) and R (RTT)
    need to be measured or calculated by a TFRC implementation.  The
    measurement of s is specified in Section 4.1, measurement of R is
    specified in Section 4.3, and measurement of p is specified in
    Section 5. In the rest of this document data rates are measured in
    bytes/second unless otherwise specified.

3.2.  Packet Contents

    Before specifying the sender and receiver functionality, we describe
    the contents of the data packets sent by the sender and feedback
    packets sent by the receiver.  As TFRC will be used along with a
    transport protocol, we do not specify packet formats, as these
    depend on the details of the transport protocol used.




Handley/Floyd/Padhye/Widmer                      Section 3.2.  [Page 12]

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3.2.1.  Data Packets

    Each data packet sent by the data sender contains the following
    information:

    o   A sequence number. This number is incremented by one for each
        data packet transmitted.  The field must be sufficiently large
        that it does not wrap causing two different packets with the
        same sequence number to be in the receiver's recent packet
        history at the same time.

    o   A timestamp indicating when the packet is sent. We denote by
        ts_i the timestamp of the packet with sequence number i.  The
        resolution of the timestamp should typically be measured in
        milliseconds.

        This timestamp is used by the receiver to determine which losses
        belong to the same loss event.  The timestamp is also echoed by
        the receiver to enable the sender to estimate the round-trip
        time, for senders that do not save timestamps of transmitted
        data packets.

        We note that as an alternative to a timestamp incremented in
        milliseconds, a "timestamp" that increments every quarter of a
        round-trip time would be sufficient for determining when losses
        belong to the same loss event, in the context of a protocol
        where this is understood by both sender and receiver, and where
        the sender saves the timestamps of transmitted data packets.

    o   The sender's current estimate of the round trip time. The
        estimate reported in packet i is denoted by R_i.  The round-trip
        time estimate is used by the receiver, along with the timestamp,
        to determine when multiple losses belong to the same loss event.
        The round-trip time estimate is also used by the receiver to
        determine the interval to use for calculating the receive rate,
        and to determine when to send feedback packets.

        If the sender sends a coarse-grained "timestamp" that increments
        every quarter of a round-trip time, as discussed above, then the
        sender does not need to send its current estimate of the round
        trip time.


3.2.2.  Feedback Packets

    Each feedback packet sent by the data receiver contains the
    following information:




Handley/Floyd/Padhye/Widmer                    Section 3.2.2.  [Page 13]

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    o   The timestamp of the last data packet received. We denote this
        by t_recvdata.  If the last packet received at the receiver has
        sequence number i, then t_recvdata = ts_i.
        This timestamp is used by the sender to estimate the round-trip
        time, and is only needed if the sender does not save timestamps
        of transmitted data packets.

    o   The amount of time elapsed between the receipt of the last data
        packet at the receiver, and the generation of this feedback
        report. We denote this by t_delay.

    o   The rate at which the receiver estimates that data was received
        in the previous round-trip time.  We denote this by X_recv.

    o   The receiver's current estimate of the loss event rate p.


4.  Data Sender Protocol

    The data sender sends a stream of data packets to the data receiver
    at a controlled rate. When a feedback packet is received from the
    data receiver, the data sender changes its sending rate, based on
    the information contained in the feedback report. If the sender does
    not receive a feedback report for four round trip times, it cuts its
    sending rate in half.  This is achieved by means of a timer called
    the nofeedback timer.

    We specify the sender-side protocol in the following steps:

    o   Measurement of the mean segment size being sent.

    o   Sender initialization.

    o   The sender behavior when a feedback packet is received.

    o   The sender behavior when the nofeedback timer expires.

    o   Oscillation prevention (optional)

    o   Scheduling of transmission on non-realtime operating systems.


4.1.  Measuring the Segment Size

    The parameter s (segment size) is normally known to an application.
    This may not be so in two cases:





Handley/Floyd/Padhye/Widmer                      Section 4.1.  [Page 14]

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    o   (1) The segment size naturally varies depending on the data.  In
        this case, although the segment size varies, that variation is
        not coupled to the transmit rate.  The TFRC sender can either
        compute the average segment size or use the maximum segment size
        for the segment size s.

    o   (2) The application needs to change the segment size rather than
        the number of segments per second to perform congestion control.
        This would normally be the case with packet audio applications
        where a fixed interval of time needs to be represented by each
        packet.  Such applications need to have a completely different
        way of measuring parameters.

    For the first class of applications where the segment size varies
    depending on the data, the sender MAY estimate the segment size s as
    the average segment size over the last four loss intervals.  The
    sender MAY also estimate the average segment size over longer time
    intervals, if so desired.  The TFRC sender uses the segment size s
    in the throughput equation, in the setting of the maximum receive
    rate and the minimum and initial sending rates, and in the setting
    of the nofeedback timer.

    The TFRC receiver may use the average segment size s in initializing
    the loss history after the first loss event, but Section 6.3.1 also
    gives an alternate procedure that does not use the average segment
    size s.

    The second class of applications are discussed separately in a
    separate document on TFRC-SP.  For the remainder of this section we
    assume the sender can estimate the segment size, and that congestion
    control is performed by adjusting the number of packets sent per
    second.


4.2.  Sender Initialization

    The initial values for X (the allowed sending rate in bytes per
    second) and tld (the Time Last Doubled during slow-start) are
    undefined until they are set as described below.  If the sender is
    ready to send data when it does not yet have a round trip sample,
    the value of X is set to s bytes per second, for segment size s, the
    nofeedback timer is set to expire after two seconds, and tld is set
    either to 0 or to -1.  Upon receiving a round trip time measurement
    (e.g., after the first feedback packet or the SYN exchange, or from
    a previous connection [RFC2140]), tld is set to the current time,
    and the allowed transmit rate X is set to the initial_rate, specifed
    as W_init/R, for W_init based on [RFC3390]:




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         W_init = min(4*s, max(2*s, 4380)).

    For responding to the initial feedback packet, this replaces step
    (4) of Section 4.3 below.

    Appendix B explains why the initial value of TFRC's nofeedback timer
    is set to two seconds, instead of the recommended initial value of
    three seconds for TCP's retransmit timer from [RFC2988].

4.3.  Sender Behavior When a Feedback Packet is Received

    The sender knows its current allowed sending rate X, and maintains
    an estimate of the current round trip time R.  The sender also
    maintains X_recv_set as a small set of recent X_recv values.
    X_recv_set is first initialized to contain the value Infinity (or a
    suitably large number).  The variable recv_limit is defined as the
    limit on the sending rate that is computed from the receive rate.
    In this document, in step (4) below, recv_limit is specified as
    twice the maximum value in X_recv_set.  Future documents might
    specify alternate values for recv_limit.

    When a feedback packet is received by the sender at time t_now, the
    following actions should be performed.


    1)  Calculate a new round trip sample.
        R_sample = (t_now - t_recvdata) - t_delay.

    2)  Update the round trip time estimate:

             If no feedback has been received before
                 R = R_sample;
             Else
                 R = q*R + (1-q)*R_sample;

        TFRC is not sensitive to the precise value for the filter
        constant q, but we recommend a default value of 0.9.

    3)  Update the timeout interval:

             RTO = max(4*R, 2*s/X)


    4)  Update the allowed sending rate as follows:







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             If (the entire interval covered by the feedback packet
                   was a data-limited interval) {
                 Replace X_recv_set contents by Infinity;
             } Else                        // typical behavior
                 Update X_recv_set;
             recv_limit = 2 * max (X_recv_set);
             If (p > 0)            // congestion avoidance phase
                 Calculate X_Bps using the TCP throughput equation.
                 X = max(min(X_Bps, recv_limit), s/t_mbi);
             Else if (t_now - tld >= R)    // initial slow-start
                 X = max(min(2*X, recv_limit), initial_rate);
                 tld = t_now;


    5)  If oscillation reduction is used, calculate the instantaneous
        transmit rate X_inst, following Section 4.5.

    6)  Reset the nofeedback timer to expire after RTO seconds.

    The subroutine for updating X_recv_set below keeps a set of X_recv
    values received for non-data-limited periods over the most recent
    two round-trip times.

         Update X_recv_set:
             Add X_recv to X_recv_set;
             Delete from X_recv_set values older than
                 two round-trip times.


    We define a sender a data-limited any time it is not sending as much
    as it is allowed to send (including unused send credits discussed in
    Section 4.6).  We define an interval as a `data-limited interval' if
    the sender was data-limited over the *entire* interval.  The first
    ``if'' condition in step (4) prevents a sender from having to reduce
    the sending rate as a result of a feedback packet reporting the
    receive rate from a data-limited period.

    Thus, consider a sender that is sending at its full allowed rate,
    except that it is sending packets in pairs, rather than sending each
    packet as soon as it can.  Such a sender is considered data-limited
    part of the time, because it is not always sending packets as soon
    as it can.  However, any interval that covers the transmission of at
    least two data packets is not a data-limited interval for this
    sender.

    Because X_recv_set is initialized with the value Infinity,
    recv_limit is set to Infinity for the first two round-trip times of
    the connection.  As a result, the sending rate is not limited by the



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    receive rate during that period.  This avoids the problem of the
    sending rate being limited by the value of X_recv from the first
    feedback packet, which reports only one segment received in the last
    round-trip time,

    How does the sender determine the period covered by a feedback
    packet?  In general, the receiver will be sending a feedback packet
    once per round-trip time, so typically the sender will be able to
    determine exactly the period covered by the current feedback packet
    from the previous feedback packet.  However, in cases when the
    previous feedback packet was lost, or when the receiver sends a
    feedback packet early because it detected a lost or ECN-marked
    packet, the sender will have to estimate the interval covered by the
    feedback packet.  As specified in Section 6.2, each feedback packet
    sent by the receiver covers a round-trip time, for the round-trip
    time estimate R_m maintained by the receiver R_m seconds before the
    feedback packet was sent.

    Note that when p=0, the sender has not yet learned of any loss
    events, and the sender is in the initial slow-start phase.  In this
    initial slow-start phase, the sender can approximately double the
    sending rate each round-trip time until a loss occurs. The
    initial_rate term in step (4) gives a minimum allowed sending rate
    during slow-start of the initial allowed sending rate.  We note that
    if the sender is data-limited during slow-start, or if the
    connection is limited by the path bandwidth, then the sender isn't
    necessarily able to double its sending rate each round-trip time;
    the sender's sending rate is limited to at most twice the receive
    rate, or at most initial_rate, whichever is larger.

    This is similar to TCP's behavior, where the sending rate is
    limiting by the rate of incoming acknowledgement packets, along with
    the modification of the window increase algorithm.  Thus in TCP's
    Slow-Start, for the most aggressive case of the TCP receiver
    acknowledging every data packet, the TCP sender's sending rate is
    limited to at most twice the rate of these incoming acknowledgment
    packets.

    The parameter t_mbi is 64 seconds, and represents the maximum inter-
    packet backoff interval in the persistent absence of feedback.
    Thus, when p > 0 the sender sends at least one packet every 64
    seconds.

4.4.  Expiration of Nofeedback Timer

    This section specifies the sender's response to a nofeedback timer.
    The nofeedback timer could expire because of an idle period, or
    because of data or feedback packets dropped in the network.



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    This section uses the variable recover_rate.  When the TFRC sender
    reduces the allowed sending rate in response to a nofeedback timer,
    and the sender has been idle ever since the nofeedback timer was
    set, the allowed sending rate is not reduced below the recover_rate.
    For this document, the recover_rate is set to the initial_rate.
    Future documents may explore other possible values for the
    recover_rate.

    If the nofeedback timer expires, the sender should perform the
    following actions:

    1)  Cut the allowed sending rate in half.  If the sender has an RTT
        measurement, the allowed sending rate is reduced by setting
        X_recv; the sending rate is limited to at most twice X_recv.
        Modifying X_recv limits the sending rate, but also allows the
        sender to slow-start, doubling its sending rate each RTT, if
        feedback messages resume reporting no losses.

        If the nofeedback timer expires when the sender does not yet
        have an RTT sample and has not yet received any feedback from
        the receiver, or when p == 0, then X_recv is not halved, and the
        sending rate is cut in half directly.

        If the sender has been idle since this nofeedback timer was set
        and X_recv is less than the recover_rate, then X_recv should not
        be halved in response to the timer expiration.  This ensures
        that the allowed sending rate is never reduced to less than half
        the recover_rate as a result of an idle period.

    X_recv_set is the set of recent X_recv values.  We use the variable
    timer_limit for the limit on the sending rate computed from the
    expiration of the nofeedback timer.



















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        X_recv = max (X_recv_set);
        If (sender does not have an RTT sample and has not received
                  any feedback from receiver)
            // We don't have X_Bps or recover_rate yet.
            X = max(X/2, s/t_mbi);
        Else if (X_recv < recover_rate, and
               sender has been idle ever
               since nofeedback timer was set)
            Timer_limit is not updated;
        Else if (p==0)
            // We don't have X_Bps yet.
            X = max(X/2, s/t_mbi);
        Else if (X_Bps > 2*X_recv))
            // 2*X_recv was already limiting the sending rate.
            timer_limit = X_recv;
        Else
            // The sending rate was limited by X_Bps, not by X_recv.
            timer_limit = X_Bps/2;
        If (timer_limit < s/t_mbi)
            timer_limit = s/t_mbi;


        The term s/t_mbi limits the backoff to one packet every 64
        seconds.

    2)  If timer_limit has been changed, then do the following:

            If (timer_limit has been updated)
              Replace X_recv_set contents with timer_limit/2.
              Recalculate X as in step (4) of Section 4.3.


    3)  Restart the nofeedback timer to expire after max(4*R, 2*s/X)
        seconds.

    Note that when the allowed sending rate is limited after an idle
    period, it is never reduced below half the recover_rate.

    If the sender has been data-limited but not idle since the
    nofeedback timer was set, it is possible that the nofeedback timer
    expired because data or feedback packets were dropped in the
    network.  In this case, the nofeedback timer is the backup mechanism
    for the sender to detect these losses, similar to the retransmit
    timer in TCP.

    Note that when the sender stops sending, the receiver will stop
    sending feedback.  When the sender's nofeedback timer expires, the
    sender could use the procedure above to limit the sending rate.  If



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    the sender subsequently starts to send again, X_recv_set will be
    used to limit the transmit rate, and slowstart behavior will occur
    until the transmit rate reaches X_Bps.

    The TFRC sender's reduction of the allowed sending rate after the
    nofeedback timer expires is similar to TCP's reduction of the
    congestion window cwnd after each RTO seconds of an idle period, for
    TCP with Congestion Window Validation [RFC2861].

4.5.  Reducing Oscillations

    To reduce oscillations in queueing delay and sending rate in
    environments with a low degree of statistical multiplexing at the
    congested link, it can be useful for the sender to reduce the
    transmit rate as the queuing delay (and hence RTT) increases.  To do
    this the sender maintains R_sqmean, a long-term estimate of the
    square root of the RTT, and modifies its sending rate depending on
    how the square root of R_sample, the most recent sample of the RTT,
    differs from the long-term estimate.  The long-term estimate
    R_sqmean is set as follows:

         If no feedback has been received before
             R_sqmean = sqrt(R_sample);
         Else
             R_sqmean = q2*R_sqmean + (1-q2)*sqrt(R_sample);

    Thus R_sqmean gives the exponentially weighted moving average of the
    square root of the RTT samples.  The constant q2 should be set
    similarly to q, the constant used in the round trip time estimate R.
    We recommend a value of 0.9 as the default for q2.

    When sqrt(R_sample) is greater than R_sqmean then the current round-
    trip time is greater than the long-term average, implying that
    queueing delay is probably increasing.  In this case, the transmit
    rate is decreased to minimize oscillations in queueing delay.

    The sender obtains the base allowed transmit rate, X, as described
    in step (4) of Section 4.3 above.  It then calculates a modified
    instantaneous transmit rate X_inst, as follows:

         X_inst = X * R_sqmean / sqrt(R_sample);
         If (p > 0)                // congestion avoidance phase
             X_inst = max(X_inst, s/t_mbi)
         Else if (t_now - tld >= R)        // initial slow-start
             X_inst = max(X_inst, s/R)

    Because we are using square roots, there is generally only a
    moderate difference between the instantaneous transmit rate X_inst



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    and the allowed transmit rate X.  For example, in a somewhat extreme
    case when the current RTT sample R_sample is twice as large as the
    long-term average, then sqrt(R_sample) will be roughly 1.44 times
    R_sqmean, and the allowed transmit rate will be reduced by a factor
    of roughly 0.7.

    Note: This modification for reducing oscillatory behavior is not
    always needed, especially if the degree of statistical multiplexing
    in the network is high.  However, it SHOULD be implemented because
    it makes TFRC behave better in environments with a low level of
    statistical multiplexing.  The performance of this modification is
    illustrated in Section 3.1.3 of [FHPW00].  If it is not implemented,
    we recommend using a very low value of the weight q for the average
    round-trip time.

4.6.  Scheduling of Packet Transmissions

    As TFRC is rate-based, and as operating systems typically cannot
    schedule events precisely, it is necessary to be opportunistic about
    sending data packets so that the correct average rate is maintained
    despite the coarse-grain or irregular scheduling of the operating
    system.  To help maintain the correct average sending rate, the TFRC
    sender may send some packets before their nominal send time.

    In addition, the scheduling of packet transmissions controls the
    allowed burstiness of senders after an idle or data-limited period.
    Allowing the TFRC sender to accumulate sending `credits' for past
    unused send times allows the TFRC sender to send a burst or data
    after an idle period.  As a comparison with TCP, TCP may send up to
    a round-trip time's worth of packets in a single burst, but never
    more.  As examples, for TCP bursts can be sent when an ACK arrives
    acknowledging a window of data, or when a data-limited sender, after
    a delay of nearly a round-trip time, suddenly has a window of data
    to send.

    To limit burstiness, a TFRC implementation MUST prevent bursts of
    arbitrary size.  This limit MUST be less than or equal to one round-
    trip time's worth of packets.  A TFRC implementation MAY limit
    bursts to less than a round-trip time's worth of packets, if so
    desired.  However, we note that such limits also constrain TFRC's
    performance beyond the case for the current TCP.

    A typical sending loop will calculate the correct inter-packet
    interval, t_ipi, as follows:

         t_ipi = s/X_inst;

    Let t_now be the current time and i be a natural number, i = 0, 1,



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    ..., with t_i the nominal send time for the i-th packet.  Then the
    nominal send time t_(i+1) derives recursively as

           t_0 = t_now,
           t_(i+1) = t_i + t_ipi.

    For TFRC senders allowed to accumulate sending `credits' for unused
    sent time over the last T seconds, the sender would be allowed to
    use unused nominal sent times t_j for t_j < now - T.  We recommend T
    set to the round-trip time.

4.6.1.  Sending Packets Before their Nominal Send Time

    Let t_gran be the scheduling timer granularity of the operating
    system.  If the operating system has a coarse timer granularity or
    otherwise cannot support short t_ipi intervals, then either the TFRC
    sender will be restricted to a sending rate of at most 1 packet
    every t_gran seconds, or the TFRC sender must be allowed to send
    short bursts of packets.  In addition to allowing the sender to
    accumulate sending `credits' for past unused send times, it can be
    useful to allow the sender to send a packet before its scheduled
    send time, as described in the section below.

    A parameter t_delta MAY be used to allow a packet to be sent before
    its nominal send time.  Consider an application that becomes idle
    and requests re-scheduling for time t_i = t_(i-1) + t_ipi, for
    t_(i-1) the send time for the previous packet.  When the application
    is re-scheduled, it checks the current time, t_now.  If (t_now > t_i
    - t_delta) then packet i is sent.  When the nominal send time, t_i,
    of the next packet is calculated, it may already be the case that
    t_now > t_i - t_delta.  In such a case the packet would be sent
    immediately.

    In order to send at most one packet before its nominal send time,
    and never to send a packet more than a round-trip time before its
    nominal send time the parameter t_delta would be set as follows:

         t_delta = min(t_ipi, t_gran, rtt)/2;

    The scheduling granularity t_gran is 10ms on many Unix systems.  If
    t_gran is not known, a value of 10ms could be assumed.

    As an example, consider a TFRC flow with an allowed sending rate X
    of 10 packets per round-trip time, a round-trip time of 100 ms, a
    system with a scheduling granularity t_gran of 10 ms, and the
    ability to accumulate unused sending credits for a round-trip time.
    In this case, t_ipi is 1 ms.  The TFRC sender would be allowed to
    send packets 0.5 ms before their nominal sending time, and would be



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    allowed to save unused sending credits for 100 ms.  The scheduling
    granularity of 10 ms would not significantly affect the performance
    of the connection.

    As a different example, consider a TFRC flow with a scheduling
    granularity less than the round-trip time, for example, with a
    round-trip time of 0.1 ms and a system with a scheduling granularity
    of 1 ms, and with the ability to accumulate unused sending credits
    for a round-trip time.  The TFRC sender would be allowed to save
    unused sending credits for 0.1 ms.  If the scheduling granularity
    *did not* affect the sender's response to an incoming feedback
    packet, then the TFRC sender would be able to send an RTT of data
    (as determined by the allowed sending rate) each RTT, in response to
    incoming feedback packets.  In this case, the coarse scheduling
    granularity would not significantly reduce the sending rate, but the
    sending rate would be bursty, with a round-trip time of data sent in
    response to each feedback packet.

    However, performance would be different in this case if the
    operating system scheduling granularity affected the sender's
    response to feedback packets as well as the general scheduling of
    the sender, In this case the sender's performance would be severely
    limited by the scheduling granularity being less than the round-trip
    time, with the sender able to send an RTT of data, at the allowed
    sending rate, at most once every 1 ms.  This restriction of the
    sending rate is an unavoidable consequence of allowing burstiness of
    at most a round-trip time of data.

5.  Calculation of the Loss Event Rate (p)

    Obtaining an accurate and stable measurement of the loss event rate
    is of primary importance for TFRC. Loss rate measurement is
    performed at the receiver, based on the detection of lost or marked
    packets from the sequence numbers of arriving packets. We describe
    this process before describing the rest of the receiver protocol.
    If the receiver has not yet detected a lost or marked packet, then
    the receiver doesn't calculate the loss event rate, but reports a
    loss event rate of zero.

5.1.  Detection of Lost or Marked Packets

    TFRC assumes that all packets contain a sequence number that is
    incremented by one for each packet that is sent.  For the purposes
    of this specification, we require that if a lost packet is
    retransmitted, the retransmission is given a new sequence number
    that is the latest in the transmission sequence, and not the same
    sequence number as the packet that was lost.  If a transport
    protocol has the requirement that it must retransmit with the



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    original sequence number, then the transport protocol designer must
    figure out how to distinguish delayed from retransmitted packets and
    how to detect lost retransmissions.

    The receiver maintains a data structure that keeps track of which
    packets have arrived and which are missing.  For the purposes of
    specification, we assume that the data structure consists of a list
    of packets that have arrived along with the receiver timestamp when
    each packet was received.  In practice this data structure will
    normally be stored in a more compact representation, but this is
    implementation-specific.

    The loss of a packet is detected by the arrival of at least NDUPACK
    packets with a higher sequence number than the lost packet, for
    NDUPACK set to 3.  The requirement for NDUPACK subsequent packets is
    the same as with TCP, and is to make TFRC more robust in the
    presence of reordering.  In contrast to TCP, if a packet arrives
    late (after NDUPACK subsequent packets arrived) in TFRC, the late
    packet can fill the hole in TFRC's reception record, and the
    receiver can recalculate the loss event rate.  Future versions of
    TFRC might make the requirement for NDUPACK subsequent packets
    adaptive based on experienced packet reordering, but we do not
    specify such a mechanism here.

    For an ECN-capable connection, a marked packet is detected as a
    congestion event as soon as it arrives, without having to wait for
    the arrival of subsequent packets.


5.2.  Translation from Loss History to Loss Events

    TFRC requires that the loss fraction be robust to several
    consecutive packets lost or marked where those packets are part of
    the same loss event.  This is similar to TCP, which (typically) only
    performs one halving of the congestion window during any single RTT.
    Thus the receiver needs to map the packet loss history into a loss
    event record, where a loss event is one or more packets lost or
    marked in an RTT.  To perform this mapping, the receiver needs to
    know the RTT to use, and this is supplied periodically by the
    sender, typically as control information piggy-backed onto a data
    packet.  TFRC is not sensitive to how the RTT measurement sent to
    the receiver is made, but we recommend using the sender's calculated
    RTT, R, (see Section 4.3) for this purpose.

    To determine whether a lost or marked packet should start a new loss
    event, or be counted as part of an existing loss event, we need to
    compare the sequence numbers and timestamps of the packets that
    arrived at the receiver.  For a marked packet S_new, its reception



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    time T_new can be noted directly.  For a lost packet, we can
    interpolate to infer the nominal "arrival time".  Assume:

        S_loss is the sequence number of a lost packet.

        S_before is the sequence number of the last packet to arrive
        with sequence number before S_loss.

        S_after is the sequence number of the first packet to arrive
        with sequence number after S_loss.

        S_max is the largest sequence number.

    Therefore, S_before < S_loss < S_after <= S_max.

        T_loss is the nominal estimated arrival time for the lost
        packet.

        T_before is the reception time of S_before.

        T_after is the reception time of S_after.

    Note that T_before can either be before or after T_after due to
    reordering.

    For a lost packet S_loss, we can interpolate its nominal "arrival
    time" at the receiver from the arrival times of S_before and
    S_after. Thus:

         T_loss = T_before + ( (T_after - T_before)
                     * (S_loss - S_before)/(S_after - S_before) );


    Note that if the sequence space wrapped between S_before and
    S_after, then the sequence numbers must be modified to take this
    into account before performing this calculation.  If the largest
    possible sequence number is S_max, and S_before > S_after, then
    modifying each sequence number S by S' = (S + (S_max + 1)/2) mod
    (S_max + 1) would normally be sufficient.

    If the lost packet S_old was determined to have started the previous
    loss event, and we have just determined that S_new has been lost,
    then we interpolate the nominal arrival times of S_old and S_new,
    called T_old and T_new respectively.

    If T_old + R >= T_new, then S_new is part of the existing loss
    event. Otherwise S_new is the first packet in a new loss event.




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5.3.  Inter-loss Event Interval

    If a loss interval, A, is determined to have started with packet
    sequence number S_A and the next loss interval, B, started with
    packet sequence number S_B, then the number of packets in loss
    interval A is given by (S_B - S_A).  Thus, loss interval A contains
    all of the packets transmitted by the sender starting with the first
    packet transmitted in loss interval A, and ending with but not
    including the first packet transmitted in loss interval B.


5.4.  Average Loss Interval

    To calculate the loss event rate p, we first calculate the average
    loss interval.  This is done using a filter that weights the n most
    recent loss event intervals in such a way that the measured loss
    event rate changes smoothly.  If the receiver has not yet seen a
    lost or marked packet, then the receiver doesn't calculate the
    average loss interval.

    Weights w_0 to w_(n-1) are calculated as:

         If (i < n/2)
            w_i = 1;
         Else
            w_i = 1 - (i - (n/2 - 1))/(n/2 + 1);


    Thus if n=8, the values of w_0 to w_7 are:

        1.0, 1.0, 1.0, 1.0, 0.8, 0.6, 0.4, 0.2

    The value n for the number of loss intervals used in calculating the
    loss event rate determines TFRC's speed in responding to changes in
    the level of congestion.  As currently specified, TFRC SHOULD NOT
    use values of n significantly greater than 8, for traffic that might
    compete in the global Internet with TCP.  At the very least, safe
    operation with values of n greater than 8 would require a slight
    change to TFRC's mechanisms to include a more severe response to two
    or more round-trip times with heavy packet loss.

    When calculating the average loss interval we need to decide whether
    to include the current loss interval, defined as the loss interval
    containing the most recent loss event.  We only include the current
    loss interval if it is sufficiently large to increase the average
    loss interval.





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    Let the most recent loss intervals be I_0 to I_k, where I_0 is the
    current loss interval.  If there have been at least n loss
    intervals, then k is set to n; otherwise k is the maximum number of
    loss intervals seen so far.  We calculate the average loss interval
    I_mean as follows:

         I_tot0 = 0;
         I_tot1 = 0;
         W_tot = 0;
         for (i = 0 to k-1) {
           I_tot0 = I_tot0 + (I_i * w_i);
           W_tot = W_tot + w_i;
         }
         for (i = 1 to k) {
           I_tot1 = I_tot1 + (I_i * w_(i-1));
         }
         I_tot = max(I_tot0, I_tot1);
         I_mean = I_tot/W_tot;

    The loss event rate, p is simply:

         p = 1 / I_mean;


5.5.  History Discounting

    As described in Section 5.4, when there have been at least eight
    loss intervals, the most recent loss interval is only assigned
    1/(0.75*n) of the total weight in calculating the average loss
    interval, regardless of the size of the most recent loss interval.
    This section describes an optional history discounting mechanism,
    discussed further in [FHPW00a] and [W00], that allows the TFRC
    receiver to adjust the weights, concentrating more of the relative
    weight on the most recent loss interval, when the most recent loss
    interval is more than twice as large as the computed average loss
    interval.

    To carry out history discounting, we associate a discount factor
    DF_i with each loss interval L_i, for i > 0, where each discount
    factor is a floating point number.  The discount array maintains the
    cumulative history of discounting for each loss interval.  At the
    beginning, the values of DF_i in the discount array are initialized
    to 1:

         for (i = 0 to n) {
           DF_i = 1;
         }




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    History discounting also uses a general discount factor DF, also a
    floating point number, that is also initialized to 1.  First we show
    how the discount factors are used in calculating the average loss
    interval, and then we describe later in this section how the
    discount factors are modified over time.

    As described in Section 5.4 the average loss interval is calculated
    using the n previous loss intervals I_1, ..., I_n and the current
    loss interval I_0.  The computation of the average loss interval
    using the discount factors is a simple modification of the procedure
    in Section 5.4, as follows:

         I_tot0 = I_0 * w_0
         I_tot1 = 0;
         W_tot0 = w_0
         W_tot1 = 0;
         for (i = 1 to n-1) {
           I_tot0 = I_tot0 + (I_i * w_i * DF_i * DF);
           W_tot0 = W_tot0 + w_i * DF_i * DF;
         }
         for (i = 1 to n) {
           I_tot1 = I_tot1 + (I_i * w_(i-1) * DF_i);
           W_tot1 = W_tot1 + w_(i-1) * DF_i;
         }
         p = min(W_tot0/I_tot0, W_tot1/I_tot1);

    The general discounting factor DF is updated on every packet arrival
    as follows. First, the receiver computes the weighted average I_mean
    of the loss intervals I_1, ..., I_n:

         I_tot = 0;
         W_tot = 0;
         for (i = 1 to n) {
           W_tot = W_tot + w_(i-1) * DF_i;
           I_tot = I_tot + (I_i * w_(i-1) * DF_i);
         }
         I_mean = I_tot / W_tot;

    This weighted average I_mean is compared to I_0, the size of current
    loss interval.  If I_0 is greater than twice I_mean, then the new
    loss interval is considerably larger than the old ones, and the
    general discount factor DF is updated to decrease the relative
    weight on the older intervals, as follows:








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         if (I_0 > 2 * I_mean) {
           DF = 2 * I_mean/I_0;
           if (DF < THRESHOLD)
             DF = THRESHOLD;
         } else
           DF = 1;

    A nonzero value for THRESHOLD ensures that older loss intervals from
    an earlier time of high congestion are not discounted entirely.  We
    recommend a THRESHOLD of 0.25.  Note that with each new packet
    arrival, I_0 will increase further, and the discount factor DF will
    be updated.

    When a new loss event occurs, the current interval shifts from I_0
    to I_1, loss interval I_i shifts to interval I_(i+1), and the loss
    interval I_n is forgotten.  The previous discount factor DF has to
    be incorporated into the discount array.  Because DF_i carries the
    discount factor associated with loss interval I_i, the DF_i array
    has to be shifted as well. This is done as follows:

         for (i = 1 to n) {
           DF_i = DF * DF_i;
         }
         for (i = n-1 to 0 step -1) {
           DF_(i+1) = DF_i;
         }
         I_0 = 1;
         DF_0 = 1;
         DF = 1;


    This completes the description of the optional history discounting
    mechanism. We emphasize that this is an optional mechanism whose
    sole purpose is to allow TFRC to response somewhat more quickly to
    the sudden absence of congestion, as represented by a long current
    loss interval.


6.  Data Receiver Protocol

    The receiver periodically sends feedback messages to the sender.
    Feedback packets should normally be sent at least once per RTT,
    unless the sender is sending at a rate of less than one packet per
    RTT, in which case a feedback packet should be send for every data
    packet received.  A feedback packet should also be sent whenever a
    new loss event is detected without waiting for the end of an RTT,
    and whenever an out-of-order data packet is received that removes a
    loss event from the history.



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    If the sender is transmitting at a high rate (many packets per RTT)
    there may be some advantages to sending periodic feedback messages
    more than once per RTT as this allows faster response to changing
    RTT measurements, and more resilience to feedback packet loss.  If
    the receiver was sending k feedback packets per RTT, step (4) of
    Section 6.2 would be modified to set the feedback timer to expire
    after R_m/k seconds.  However, each feedback packet would still
    report the receiver rate over the last RTT, not over a fraction of
    an RTT.  We note that there is little gain from sending a large
    number of feedback messages per RTT.


6.1.  Receiver Behavior When a Data Packet is Received

    When a data packet is received, the receiver performs the following
    steps:

    1)  Add the packet to the packet history.

    2)  Check if done: If the new packet results in the detection of a
        new loss event, or if no feedback packet was sent when the
        feedback timer last expired, go to step 3).  Otherwise, no
        action need be performed (unless the optimization in the next
        paragraph is used), so exit the procedure.

        An optimization might check to see if the arrival of the packet
        caused a hole in the packet history to be filled and
        consequently two loss intervals were merged into one.  If this
        is the case, the receiver might also send feedback immediately.
        The effects of such an optimization are normally expected to be
        small.

    3)  Calculate p: Let the previous value of p be p_prev.  Calculate
        the new value of p as described in Section 5.

    4)  Expire feedback timer?: If p > p_prev, cause the feedback timer
        to expire, and perform the actions described in Section 6.2

        If p <= p_prev and no feedback packet was sent when the feedback
        timer last expired, cause the feedback timer to expire, and
        perform the actions described in Section 6.2 If p <= p_prev and
        a feedback packet was sent when the feedback timer last expired,
        no action need be performed.








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6.2.  Expiration of Feedback Timer

    When the feedback timer at the receiver expires, the action to be
    taken depends on whether data packets have been received since the
    last feedback was sent.

    For the m-th expiration of the feedback timer, let the maximum
    sequence number of a packet at the receiver so far be S_m, and the
    value of the RTT measurement included in packet S_m be R_m.  As
    described in Section 3.2.1, R_m is the sender's current estimate of
    the round trip time, reported in data packets.  If data packets have
    been received since the previous feedback was sent, the receiver
    performs the following steps:

    1)  Calculate the average loss event rate using the algorithm
        described above.

    2a) If the feedback timer expired at its normal time, or expired
        early due to a new lost or marked packet (i.e., step (3) in
        Section 6.1), calculate the measured receive rate, X_recv, based
        on the packets received within the previous R_(m-1) seconds.  In
        the typical case, when the receiver is sending only one feedback
        packet per round-trip time and the feedback timer did not expire
        early due to an idle period, then R_(m-1) would be the time
        interval since the feedback timer last expired.

    2b) If the feedback timer expired early due to a new lost or marked
        packet (i.e., step (3) in Section 6.1), the receive rate X_recv
        SHOULD be calculated based on the packets received within the
        previous R_(m-1) seconds.  For ease of implementation, the
        receive rate MAY be calculated over a longer time interval, the
        time interval going back to the most recent feedback timer
        expiration that was at least R_(m-1) seconds ago.

    3)  Prepare and send a feedback packet containing the information
        described in Section 3.2.2.

    4)  Restart the feedback timer to expire after R_m seconds.

    Note that rule 2) above gives a minimum value for the measured
    receive rate X_recv of one packet per round-trip time.  If the
    sender is limited to a sending rate of less than one packet per
    round-trip time, this will be due to the loss event rate, not from a
    limit imposed by the measured receive rate at the receiver.

    If no data packets have been received since the last feedback was
    sent, no feedback packet is sent, and the feedback timer is
    restarted to expire after R_m seconds.



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6.3.  Receiver Initialization

    The receiver is initialized by the first data packet that arrives at
    the receiver. Let the sequence number of this packet be i.

    When the first packet is received:

    o   Set p=0

    o   Set  X_recv = 0.

    o   Prepare and send a feedback packet.

    o   Set the feedback timer to expire after R_i seconds.

    If the first data packet doesn't contain an estimate R_i of the
    round-trip time, then the receiver sends a feedback packet for every
    arriving data packet, until a data packet arrives containing an
    estimate of the round-trip time.

    If the sender is using a coarse-grained timestamp that increments
    every quarter of a round-trip time, then a feedback timer is not
    needed, and the following procedure from RFC 4342 is used to
    determine when to send feedback messages.

    o   Whenever the receiver sends a feedback message, the receiver
        sets a local variable last_counter to the greatest received
        value of the window counter since the last feedback message was
        sent, if any data packets have been received since the last
        feedback message was sent.

    o   If the receiver receives a data packet with a window counter
        value greater than or equal to last_counter + 4, then the
        receiver sends a new feedback packet.  ("Greater" and "greatest"
        are measured in circular window counter space.)

6.3.1.  Initializing the Loss History after the First Loss Event

    The number of packets until the first loss can not be used to
    compute the allowed sending rate directly, as the sending rate
    changes rapidly during this time.  TFRC assumes that the correct
    data rate after the first loss is half of the maximum sending rate
    before the loss occurred.  TFRC approximates this target rate
    X_target by the maximum X_rec so far, for X_recv the receive rate
    over a single round-trip time.  (For a TFRC sender that always has
    data to send, it is sufficient to approximate the target rate by the
    most recent X_recv.  However, for a TFRC sender that is sometimes
    data-limited or idle, it is best to use the maximum X_recv so far.)



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    After the first loss, instead of initializing the first loss
    interval to the number of packets sent until the first loss, the
    TFRC receiver calculates the loss interval that would be required to
    produce the data rate X_target, and uses this synthetic loss
    interval to seed the loss history mechanism.

    TFRC does this by finding some value p for which the throughput
    equation in Section 3.1 gives a sending rate within 5% of X_target,
    given the round-trip time R, and the first loss interval is then set
    to 1/p.  If the receiver knows the segment size s used by the
    sender, then the receiver can use the throughput equation for X;
    otherwise, the receiver can measure the receive rate in packets per
    second instead of bytes per second for this purpose, and use the
    throughput equation for X_pps.  (The 5% tolerance is introduced
    simply because the throughput equation is difficult to invert, and
    we want to reduce the costs of calculating p numerically.)

    Special care is needed for initializing the first loss interval when
    the first data packet is lost or marked.  When the first data packet
    is lost in TCP, the TCP sender retransmits the packet after the
    retransmit timer expires.  If TCP's first data packet is ECN-marked,
    the TCP sender resets the retransmit timer, and sends a new data
    packet only when the retransmit timer expires [RFC3168] (Section
    6.1.2).  For TFRC, if the first data packet is lost or ECN-marked,
    then the first loss interval consists of the null interval with no
    data packets.  In this case, the loss interval length for this
    (null) loss interval should be set to give a similar sending rate to
    that of TCP.

    When the first TFRC loss interval is null, meaning that the first
    data packet is lost or ECN-marked, in order to follow the behavior
    of TCP, TFRC wants the allowed sending rate to be 1 packet every two
    round-trip times, or equivalently, 0.5 packets per RTT.  Thus, the
    TFRC receiver calculates the loss interval that would be required to
    produce the target rate X_target of 0.5/R packets per second, for
    the round-trip time R, and uses this synthetic loss interval for the
    first loss interval.  The TFRC receiver uses 0.5/R packets per
    second as the minimum value for X_target when initializing the first
    loss interval.


7.  Sender-based Variants

    In a sender-based variant of TFRC, the receiver uses reliable
    delivery to send information about packet losses to the sender, and
    the sender computes the packet loss rate and the acceptable transmit
    rate.




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    The main advantage of a sender-based variant of TFRC is that the
    sender does not have to trust the receiver's calculation of the
    packet loss rate.  However, with the requirement of reliable
    delivery of loss information from the receiver to the sender, a
    sender-based TFRC would have much tighter constraints on the
    transport protocol in which it is embedded.

    In contrast, the receiver-based variant of TFRC specified in this
    document is robust to the loss of feedback packets, and therefore
    does not require the reliable delivery of feedback packets.  It is
    also better suited for applications where it is desirable to offload
    work from the server to the client as much as possible.

    RFC 4340 and RFC 4342 together specify CCID 3, which can be used as
    a sender-based variant of TFRC.  In CCID 3, each feedback packet
    from the receiver contains a Loss Intervals option, reporting the
    lengths of the most recent loss intervals.  Feedback packets may
    also include the Ack Vector option, allowing the sender to determine
    exactly which packets were dropped or marked and to check the
    information reported in the Loss Intervals options.  The Ack Vector
    option can also include ECN Nonce Echoes, allowing the sender to
    verify the receiver's report of having received an unmarked data
    packet.  The Ack Vector option allows the sender to see for itself
    which data packets were lost or ECN-marked, to determine loss
    intervals, and to calculate the loss event rate.  Section 9.2 of RFC
    4342 discusses issues in the sender verifying information reported
    by the receiver.


8.  Implementation Issues

    This document has specified the TFRC congestion control mechanism,
    for use by applications and transport protocols.  This section
    mentions briefly some of the implementation issues.

    Computing the throughput equation (Section 3.1):  For t_RTO = 4*R
    and b = 1, the throughput equation in Section 3.1 can be expressed
    as follows:

                     s
         X_Bps =  --------
                  R * f(p)

    for

         f(p) =  sqrt(2*p/3) + (12*sqrt(3*p/8) * p * (1+32*p^2)).

    A table lookup could be used for the function f(p).



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    Many of the multiplications (e.g., q and 1-q for the round-trip time
    average, a factor of 4 for the timeout interval) are or could be by
    powers of two, and therefore could be implemented as simple shift
    operations.

    The sender mechanism for preventing oscillations (Section 4.5):  We
    note that the optional sender mechanism for preventing oscillations
    described in Section 4.5 uses a square-root computation.

    Calculating the nominal packet arrival time (Section 5.2).  For the
    calculation of the nominal arrival time T_loss for a lost packet
    from Section 5.2, one way to implement this that would avoid
    concerns about wrapped sequence space would be to use the following:

         T_loss = T_before +  (T_after - T_before)
              * Dist(S_loss, S_before)/Dist(S_after, S_before)

    where

         Dist(Seqno_A, Seqno_B) = (Seqno_A + 2^48 - Seqno_B) % 2^48


    The calculation of the average loss interval (Section 5.4):  The
    calculation of the average loss interval in Section 5.4 involves
    multiplications by the weights w_0 to w_(n-1), which for n=8 are:

        1.0, 1.0, 1.0, 1.0, 0.8, 0.6, 0.4, 0.2.

    With a minor loss of smoothness, it would be possible to use weights
    that were powers of two or sums of powers of two, e.g.,

        1.0, 1.0, 1.0, 1.0, 0.75, 0.5, 0.25, 0.25.

    The optional history discounting mechanism (Section 5.5):  The
    optional history discounting mechanism described in Section 5.5 is
    used in the calculation of the average loss rate.  The history
    discounting mechanism is invoked only when there has been an
    unusually long interval with no packet losses.  For a more efficient
    operation, the discount factor DF_i could be restricted to be a
    power of two.


9.  Changes from RFC 3448

    This section summarizes the changes from RFC 3448.

    Section 4.1, estimating the average segment size: Section 4.1 was
    modified to give a specific algorithm that could be used for



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    estimating the average segment size.

    Section 4.2, update to the initial sending rate: In RFC 3448, the
    initial sending rate was two packets per round trip time.  In this
    document, the initial sending rate can be as high as four packets
    per round trip time, following RFC 3390.  The initial sending rate
    was changed to be in terms of the segment size s, not in terms of
    the MSS.

    Section 4.2 now says that tld, the Time Last Doubled during slow-
    start, can be initialized to either 0 or to -1.  Section 4.2 was
    also clarified to say that RTT measurements don't only come from
    feedback packets; they could also come from other places, such as
    the SYN exchange.

    Section 4.3, response to feedback packets: Section 4.3 was modified
    to change the way that the receive rate is used in limiting the
    sender's allowed sending rate, by using the set of receive rate
    values of the last two round-trip times, and initializing the set of
    receive rate values by a large value.

    The larger initial sending rate in Section 4.2 is of little use if
    the receiver sends a feedback packet after the first packet is
    received, and the sender in response reduces the allowed sending
    rate to at most two packets per RTT, which would be twice the
    receive rate.  Because of the change in the sender's processing of
    the receive rate, the sender now does not reduce the allowed sending
    rate to twice the reported receive rate in response to the first
    feedback packet.

    The sender never uses the receive rate from a data-limited period to
    restrict the allowed sending rate.  Appendix C discusses this
    response further.

    Section 4.4, response to an idle period: Following Section 5.1 from
    [RFC4342], this document specifies that when the sending rate is
    reduced after an idle period that covers the period since the
    nofeedback timer was set, the allowed sending rate is not reduced
    below the initial sending rate.

    Section 4.4, correction from [RFC3448Err].  RFC 3448 had
    contradictory text about whether the sender halved its sending rate
    after *two* round-trip times without receiving a feedback report, or
    after *four* round-trip times.  This document clarifies that the
    sender halves its sending rate after four round-trip times without
    receiving a feedback report [RFC3448Err].

    Section 4.4, clarification for Slow-Start: Section 4.4 was clarified



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    to specify that on the expiration of the nofeedback timer, if p = 0,
    X_Bps can't be used, because the sender doesn't yet have a value for
    X_Bps.

    Section 4.7: credits for unused send time: Section 4.7 has been
    clarified to say that the TFRC sender gets to accumulate up to an
    RTT of `credits' for unused send time.  Section 4.7 was also
    rewritten to clarity what is specification and what is
    implementation.

    Section 5.4, clarification: Section 5.4 was modified to clarify the
    receiver's calculation of the average loss interval when the
    receiver has not yet seen eight loss intervals.

    Section 5.5, correction: Section 5.5 was corrected to say that the
    loss interval I_0 includes all transmitted packets, including lost
    and marked packets (as defined in Section 5.3 in the general
    definition of loss intervals.)

    Section 5.5, correction from [RFC3448Err]: A line in Section 5.5 was
    changed from

         ``for (i = 1 to n) { DF_i = 1; }''

    to

         ``for (i = 0 to n) { DF_i = 1; }''

    [RFC3448Err].

    Section 5.5, history discounting: THRESHOLD, the lower bound on the
    history discounting parameter DF, has been changed from 0.5 to 0.25,
    to allow more history discounting when the current interval is long.

    Section 6, multipe feedback packets: Section 6 now contains more
    discussion of procedures if the receiver sends multiple feedback
    packets each round-trip time.

    Section 6.3, initialization of the feedback timer: Section 6.3 now
    specifies the receiver's initialization of the feedback timer if the
    first data packet received doesn't have an estimate of the round-
    trip time.

    Section 6.3, a coarse-grained timestamp: Section 6.3 was modified to
    incorporate, as an option, a coarse-grained timestamp from the
    sender that increments every quarter of a round-trip time, instead
    of a more fine-grained timestamp.  This follows RFC 4243.




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    Section 6.3.1, after the first loss event: Section 6.3.1 now says
    that for initializing the loss history after the first loss event,
    the receiver uses the maximum receive rate so far, instead of the
    receive rate in the last round-trip time.

    Section 6.3.1, if the first data packet is dropped: Section 6.3.1
    now contains a specification for initializing the loss history if
    the first data packet sent is lost or ECN-marked.

    Section 7, sender-based variants: Section 7's discussion of sender-
    based variants has been expanded, with reference to RFC 4342.


10.  Security Considerations

    TFRC is not a transport protocol in its own right, but a congestion
    control mechanism that is intended to be used in conjunction with a
    transport protocol.  Therefore security primarily needs to be
    considered in the context of a specific transport protocol and its
    authentication mechanisms.

    Congestion control mechanisms can potentially be exploited to create
    denial of service.  This may occur through spoofed feedback.  Thus
    any transport protocol that uses TFRC should take care to ensure
    that feedback is only accepted from the receiver of the data.  The
    precise mechanism to achieve this will however depend on the
    transport protocol itself.

    In addition, congestion control mechanisms may potentially be
    manipulated by a greedy receiver that wishes to receive more than
    its fair share of network bandwidth.  A receiver might do this by
    claiming to have received packets that in fact were lost due to
    congestion.  Possible defenses against such a receiver would
    normally include some form of nonce that the receiver must feed back
    to the sender to prove receipt.  However, the details of such a
    nonce would depend on the transport protocol, and in particular on
    whether the transport protocol is reliable or unreliable.

    We expect that protocols incorporating ECN with TFRC will also want
    to incorporate feedback from the receiver to the sender using the
    ECN nonce [RFC3540].   The ECN nonce is a modification to ECN that
    protects the sender from the accidental or malicious concealment of
    marked packets.  Again, the details of such a nonce would depend on
    the transport protocol, and are not addressed in this document.







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11.  IANA Considerations

    There are no IANA actions required for this document.


12.  Acknowledgments

    We would like to acknowledge feedback and discussions on equation-
    based congestion control with a wide range of people, including
    members of the Reliable Multicast Research Group, the Reliable
    Multicast Transport Working Group, and the End-to-End Research
    Group.   We would like to thank Dado Colussi, Gorry Fairhurst, Ladan
    Gharai, Wim Heirman, Eddie Kohler, Ken Lofgren, Mike Luby, Ian
    McDonald, Michele R., Gerrit Renker, Arjuna Sathiaseelan, Vladica
    Stanisic, Randall Stewart, Eduardo Urzaiz, Shushan Wen, and Wendy
    Lee (lhh@zsu.edu.cn) for feedback on earlier versions of this
    document, and to thank Mark Allman for his extensive feedback from
    using [RFC3448] to produce a working implementation.

A.  Terminology

    This document uses the following terms.

    DF: Discount factor for a loss interval (Section 5.5).

    initial_rate:
        Allowed initial sending rate.

    last_counter:
        Greatest received value of the window counter (Section 6.3).

    min_rate:
        Minimum transmit rate (Section 4.3).

    n:  Number of loss intervals.

    NDUPACK:
        Number of dupacks for inferring loss (constant) (Section 5.1).

    nofeedback timer:
        Sender-side timer (Section 4).

    p:  Estimated Loss Event Rate.

    p_prev:
        Previous value of p (Section 6.1).





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    q:  Filter constant for RTT (constant) (Section 4.3).

    q2: Filter constant for long-term RTT (constant) (Section 4.6).

    R:  Estimated path round-trip time.

    R_sample:
        Measured path RTT (Section 4.3).

    R_sqmean:
        Long-term estimate of the square root of the RTT (Section 4.6).

    recover_rate:
        Allowed rate for resuming after an idle period.

    recv_limit;
        Limit on sending rate computed from the receive rate.

    s:  Nominal packet size in bytes.

    S:  Sequence number.

    t_delay:
        Reported time delay between receipt of the last packet at the
        receiver and the generation of the feedback packet (Section
        3.2.2).

    t_delta:
        Parameter for flexibility in send time (Section 4.7).

    t_gran:
        Schedular granularity (constant) (Section 4.7).

    t_ipi:
        Inter-packet interval for sending packets (Section 4.7).

    t_mbi:
        Maximum RTO value of TCP (constant) (Section 4.3).

    tld:
        Time Last Doubled (Section 4.2).

    t_now:
        Current time (Section 4.3).

    t_RTO:
        Estimated RTO of TCP (Section 4.3).




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    X:  Allowed transmit rate, as limited by the receive rate.

    X_Bps:
        Calculated sending rate in bytes per second (Section 3.1).

    X_pps:
        Calculated sending rate in packets per second (Section 3.1).

    X_recv:
        Estimated receive rate at the receiver (Section 3.2.2).

    X_inst:
        Instantaneous allowed transmit rate (Section 4.6).

    W_init:
        TCP initial window (constant) (Section 4.2).

B.  The Initial Value of the Nofeedback Timer

    Why is the initial value of TFRC's nofeedback timer set to two
    seconds, instead of the recommended initial value of three seconds
    for TCP's retransmit timer, from [RFC2988]?  There isn't any
    particular reason why TFRC's nofeedback timer should have the same
    initial value as TCP's retransmit timer.  TCP's retransmit timer is
    used not only to reduce the sending rate in response to congestion,
    but also to retransmit a packet that is assumed to have been dropped
    in the network.  In contrast, TFRC's nofeedback timer is only used
    to reduce the allowed sending rate, not to trigger the sending of a
    new packet.  As a result, there is no danger to the network for the
    initial value of TFRC's nofeedback timer to be smaller than the
    recommended initial value for TCP's retransmit timer.

    Further, when the nofeedback timer has not yet expired, TFRC has a
    more slowly-responding congestion control mechanism that TCP, and
    TFRC's use of the receive rate for limiting the sending rate is
    somewhat less precise than TCP's use of windows and ack-clocking, so
    the nofeedback timer is a particularly important safety mechanism
    for TFRC.  For all of these reasons, it is perfectly reasonable for
    TFRC's nofeedback timer to have a smaller initial value than that of
    TCP's retransmit timer.

C.  Response to Idle or Data-limited Periods

    Future work could explore alternate responses to using the receive
    rate during a data-limited period.






Handley/Floyd/Padhye/Widmer                        Section C.  [Page 42]

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C.1.  Long Idle or Data-limited Periods

    Table 1 summarizes the response of Standard TCP [RFC2581], TCP with
    Congestion Window Validation [RFC2861], Standard TFRC [RFC3448], and
    Revised TFRC (this document) in response to long idle or data-
    limited periods.  For the purposes of this section, we define a long
    period as a period of at least an RTO.


      Protocol         Long idle periods      Long data-limited periods
    --------------   --------------------     ----------------------
    Standard TCP:       Window -> initial.     No change in window.

    TCP with CWV:         Halve window         Reduce window half way
                    (not below initial cwnd).     to used window.

    Standard TFRC:        Halve rate              Rate limited to
                     (not below 1 pkt/64 sec).   twice receive rate.
                     One RTT after sending pkt,
                     rate is limited by X_recv.

    Revised TFRC:         Halve rate             Rate not limited to
                     (not below initial rate).   twice receive rate.

      Table 1: Response to long idle or data-limited periods.

    Standard TCP after long idle periods: For Standard TCP, [RFC2581]
    specifies that TCP SHOULD set the congestion window to no more than
    the initial window after an idle period of at least an RTO.

    Standard TCP after long data-limited periods: Standard TCP [RFC2581]
    does not reduce TCP's congestion window after a data-limited period,
    when the congestion window is not fully used.  Standard TCP in
    [RFC2581] uses the FlightSize, the amount of outstanding data in the
    network, only in setting the slow-start threshold after a retransmit
    timeout.  Standard TCP is not limited by TCP's ack-clocking
    mechanism during a data-limited period.

    Standard TCP's lax response to a data-limited period is quite
    different from its stringent response to an idle period.

    TCP with Congestion Window Validation (CWV) after long idle periods:
    As an experimental alternative, [RFC2861] specifies a more moderate
    response to an idle period than that of Standard TCP, where during
    an idle period the TCP sender halves cwnd after each RTO, down to
    the initial cwnd.

    TCP with Congestion Window Validation after long data-limited



Handley/Floyd/Padhye/Widmer                      Section C.1.  [Page 43]

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    periods: As an experimental alternative, [RFC2861] specifies a more
    stringent response to a data-limited period than that of Standard
    TCP, where after each RTO seconds of a data-limited period, the
    congestion window is reduced half way down to the window that is
    actually used.

    The response of TCP with CWV to an idle period is similar to its
    response to a data-limited period.  TCP with CWV is less restrictive
    than Standard TCP in response to an idle period, and more
    restrictive than Standard TCP in response to a data-limited period.

    Standard TFRC after long idle periods: For Standard TFRC, [RFC3448]
    specifies that the allowed sending rate is halved after each RTO
    seconds of an idle period.  The allowed sending rate is not reduced
    below one packet in 64 seconds.  After an idle period, the first
    feedback packet receives reports a receive rate of one packet per
    round-trip time, and this receive rate is used to limit the sending
    rate.  Standard TFRC effectively slow-starts up from this allowed
    sending rate.

    Standard TFRC after long data-limited periods: [RFC3448] does not
    distinguish between data-limited and not-data-limited periods.  As a
    consequence, the allowed sending rate is limited to at most twice
    the receive rate during and after a data-limited period.  This is a
    very restrictive response, more restrictive than that of either
    Standard TCP or of TCP with CWV.

    Revised TFRC after long idle periods: For Revised TFRC, this
    document specifies that the allowed sending rate is halved after
    each RTO seconds of an idle period.  The allowed sending rate is not
    reduced below the initial sending rate as the result of an idle
    period.  The first feedback packet received after the idle period
    reports a receive rate of one packet per round-trip time.  However,
    the Revised TFRC sender does not use this receive rate for limiting
    the sending rate.  Thus, Revised TFRC differs from Standard TFRC in
    the lower limit used in the reduction of the sending rate, and in
    the better response to the first feedback packet received after the
    idle period.

    Revised TFRC after long data-limited periods: For Revised TFRC, this
    document distinguishs between data-limited and not-data-limited
    periods.  As specified in Section 4.3, Revised TFRC does not reduce
    the allowed sending rate in response to the receive rate during a
    data-limited period.  This is perhaps an overly-lax response, but it
    is similar to the response of Standard TCP, and is quite different
    from the very restrictive response of Standard TFRC to a data-
    limited period.




Handley/Floyd/Padhye/Widmer                      Section C.1.  [Page 44]

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    Recovery after idle or data-limited periods: When TCP reduces the
    congestion window after an idle or data-utilized period, TCP can set
    the slow-start threshold ssthresh to allow the TCP sender to slow-
    start back up towards its old sending rate when the idle or data-
    limited period is over.  However in TFRC, even when the TFRC
    sender's sending rate is restricted by twice the previous receive
    rate, this results in the sender being able to double the sending
    rate from one round-trip time to the next, if permitted by the
    throughput equation.  Thus, TFRC doesn't need a mechanism such as
    TCP's setting of ssthresh to allow a slow-start after an idle or
    data-limited period.

    For future work, one avenue to explore would be the addition of
    Congestion Window Validation mechanisms for TFRC's response to data-
    limited periods.  Currently, following Standard TCP, during data-
    limited periods Revised TFRC does not limit its allowed sending rate
    as a function of the receive rate.

C.2.  Short Idle or Data-limited Periods

    Table 2 summarizes the response of Standard TCP [RFC2581], TCP with
    Congestion Window Validation [RFC2861], Standard TFRC [RFC3448], and
    Revised TFRC (this document) in response to short idle or data-
    limited periods.  For the purposes of this section, we define a
    short period as a period of less than an RTT.


      Protocol         Short idle periods   Short data-limited periods
    --------------   --------------------     ----------------------
    Standard TCP:    Send a burst up to cwnd.  Send a burst up to cwnd.

    TCP with CWV:    Send a burst up to cwnd.  Send a burst up to cwnd.

    Standard TFRC:             ?                         ?

    Revised TFRC:         Send a burst               Send a burst
                         (up to an RTT of           (up to an RTT of
                       unused send credits).      unused send credits).

      Table 2: Response to short idle or data-limited periods.

    Table 2 shows that Revised TFRC has a similar response to that of
    Standard TCP and of TCP with CWV to a short idle or data-limited
    period.  For a short idle or data-limited period, TCP is limited
    only by the size of the unused congestion window, and Revised TFRC
    is limited only by the number of unused send credits (up to an RTT's
    worth).  For Standard TFRC, [RFC3448] did not explicitly specify the
    behavior with respect to unused send credits.



Handley/Floyd/Padhye/Widmer                      Section C.2.  [Page 45]

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C.3.  Moderate Idle or Data-limited Periods

    Table 3 summarizes the response of Standard TCP [RFC2581], TCP with
    Congestion Window Validation [RFC2861], Standard TFRC [RFC3448], and
    Revised TFRC (this document) in response to moderate idle or data-
    limited periods.  For the purposes of this section, we define a
    moderate period as a period greater than an RTT, but less than an
    RTO.


      Protocol      Moderate idle periods  Moderate data-limited periods
    -------------   ---------------------      -------------------------
    Standard TCP:    Send a burst up to cwnd.  Send a burst up to cwnd.

    TCP with CWV:    Send a burst up to cwnd.  Send a burst up to cwnd.

    Standard TFRC:             ?                   Limited by X_recv.

    Revised TFRC:         Send a burst               Send a burst
                         (up to an RTT of           (up to an RTT of
                       unused send credits).      unused send credits).

      Table 3: Response to moderate idle or data-limited periods.

    Table 3 shows that Revised TFRC has a similar response to that of
    Standard TCP and of TCP with CWV to a moderate idle or data-limited
    period.  For a moderate idle or data-limited period, TCP is limited
    only by the size of the unused congestion window.  For a moderate
    idle period, Revised TFRC is limited only by the number of unused
    send credits (up to an RTT's worth).  For a moderate data-limited
    period, Standard TCP would be limited by X_recv from the most recent
    feedback packet.  In contrast, Revised TFRC isn't limited by the
    receive rate from data-limited periods that cover an entire feedback
    period of a round-trip time.  For Standard TFRC, [RFC3448] did not
    explicitly specify the behavior with respect to unused send credits.

C.4.  Other Patterns

    Other possible patterns to consider in evaluting Revised TFRC would
    be to compare the behavior of TCP, Standard TFRC, and Revised TFRC
    for connections with alternating busy and idle periods, alternating
    idle and data-limited periods, or with idle or data-limited periods
    during Slow-Start,

Normative References

     [RFC3448]      M. Handley, S. Floyd, J. Padhye, and J. Widmer, TCP
                    Friendly Rate Control (TFRC): Protocol



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                    Specification, RFC 3448, January 2003.

Informational References

     [BRS99]        Balakrishnan, H., Rahul, H., and Seshan, S., "An
                    Integrated Congestion Management Architecture for
                    Internet Hosts," Proc. ACM SIGCOMM, Cambridge, MA,
                    September 1999.

     [FHPW00]       S. Floyd, M. Handley, J. Padhye, and J. Widmer,
                    "Equation-Based Congestion Control for Unicast
                    Applications", August 2000, Proc SIGCOMM 2000.

     [FHPW00a]      S. Floyd, M. Handley, J. Padhye, and J. Widmer,
                    "Equation-Based Congestion Control for Unicast
                    Applications: the Extended Version", ICSI tech
                    report TR-00-03, March 2000.

     [PFTK98]       Padhye, J. and  Firoiu, V. and Towsley, D. and
                    Kurose, J., "Modeling TCP Throughput: A Simple Model
                    and its Empirical Validation", Proc ACM SIGCOMM
                    1998.

     [RFC2119]      S. Bradner, Key Words For Use in RFCs to Indicate
                    Requirement Levels, RFC 2119.

     [RFC2140]      J. Touch, "TCP Control Block Interdependence", RFC
                    2140, April 1997.

     [RFC2581]      Allman, M., Paxson, V., and W. Stevens, "TCP
                    Congestion Control", RFC 2581, April 1999.

     [RFC2861]      M. Handley, J. Padhye, and S. Floyd, TCP Congestion
                    Window Validation, RFC2861, June 2000.

     [RFC2988]      V. Paxson and M. Allman, "Computing TCP's
                    Retransmission Timer", RFC 2988, November 2000.

     [RFC3168]      K. Ramakrishnan and S. Floyd, "The Addition of
                    Explicit Congestion Notification (ECN) to IP", RFC
                    3168, September 2001.

     [RFC3390]      Allman, M., Floyd, S., and C. Partridge, "Increasing
                    TCP's Initial Window", RFC 3390, October 2002.

     [RFC3448Err]   RFC 3448 Errata, URL
                    ``http://www.icir.org/tfrc/rfc3448.errata''.




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     [RFC3540]      Wetherall, D., Ely, D., and Spring, N., "Robust ECN
                    Signaling with Nonces", RFC 3540, Experimental, June
                    2003

     [RFC4340]      Kohler, E., Handley, M., and S. Floyd, "Datagram
                    Congestion Control Protocol (DCCP)", RFC 4340, March
                    2006.

     [RFC4342]      Floyd, S., Kohler, E., and J. Padhye, "Profile for
                    Datagram Congestion Control Protocol (DCCP)
                    Congestion Control ID 3: TCP-Friendly Rate Control
                    (TFRC)", RFC 4342, March 2006.

     [RFC4828]      Floyd, S., and E. Kohler, TCP Friendly Rate Control
                    (TFRC): the Small-Packet (SP) Variant, RFC 4828,
                    Experimental, April 2007.

     [W00]          Widmer, J., "Equation-Based Congestion Control",
                    Diploma Thesis, University of Mannheim, February
                    2000.  URL "http://www.icir.org/tfrc/".


Authors' Addresses

         Mark Handley,
         Department of Computer Science
         University College London
         Gower Street
         London WC1E 6BT
         UK
         EMail: M.Handley@cs.ucl.ac.uk


         Sally Floyd
         ICSI
         1947 Center St, Suite 600
         Berkeley, CA 94708
         floyd@icir.org


         Jitendra Padhye
         Microsoft Research
         padhye@microsoft.com








Handley/Floyd/Padhye/Widmer                                    [Page 48]

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         Joerg Widmer
         DoCoMo Euro-Labs
         Landsberger Strasse 312
         80687 Munich
         Germany
         widmer@acm.org



Full Copyright Statement

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