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Internet Engineering Task Force                                E. Kohler
INTERNET-DRAFT                                                      UCLA
Intended status: Experimental                                   S. Floyd
Expires: March 2008                                                 ICIR
                                                         A. Sathiaseelan
                                                  University of Aberdeen
                                                       27 September 2007


          Faster Restart for TCP Friendly Rate Control (TFRC)
             draft-ietf-dccp-tfrc-faster-restart-04.txt


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Copyright Notice

   Copyright (C) The IETF Trust (2007).







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Abstract

   TCP-Friendly Rate Control (TFRC) is a congestion control mechanism
   for unicast flows operating in a best-effort Internet environment.
   This document introduces Faster Restart, an optional mechanism for
   safely improving the behavior of interactive flows that use TFRC.
   Faster Restart is proposed for use with TFRC and with TFRC-SP, the
   Small Packet variant of TFRC.  We present Faster Restart in general
   terms as a congestion control mechanism, and further describe how to
   implement Faster Restart in Datagram Congestion Control Protocol
   (DCCP) Congestion Control IDs 3 and 4.








































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Table of Contents

   1. Introduction ....................................................5
   2. Conventions .....................................................8
   3. Faster Restart: Changes to TFRC ................................10
      3.1. Feedback Packets ..........................................10
      3.2. Nofeedback Timer ..........................................12
   4. Faster Restart: DCCP-specific Specifications ...................13
      4.1. DCCP: Receive Rate Adjustment .............................13
      4.2. DCCP: The Receive Rate Length .............................14
   5. Faster Restart Discussion ......................................15
      5.1. Worst-Case Scenarios ......................................15
      5.2. Incentives for applications to send unnecessary packets
      during idle or data-limited periods? ...........................16
      5.3. Faster Restart for TFRC-SP ................................17
   6. Simulations of Faster Restart ..................................17
   7. Implementation Issues ..........................................17
   8. Security Considerations ........................................18
   9. IANA Considerations ............................................18
   10. Thanks ........................................................18
   Normative References ..............................................18
   Informative References ............................................18
   A. Appendix: Simulations ..........................................19
   Authors' Addresses ................................................21
   Full Copyright Statement ..........................................23
   Intellectual Property .............................................23

























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

   Changes from draft-ietf-dccp-tfrc-faster-restart-03.txt:

   * Deleted ping packets, and the section about the implementation
     of ping packets in DCCP.

   * In Section 3.2, calls to
     "Update X_active_recv and X_fast_max;" and
     "Interpolate X_fast_max;"
     had been reversed accidentally.  Put them back in the right order.

   * Changed Intended Status back to Experimental (where it started
     out).

   * General editing is response to feedback from Gorry.

   * Added simulation tests to the list in Section 6: (1) simulations
     with a worst-case scenario of high congestion, all flows using
     TFRC, all flows having various idle times, all flows using Faster
     Restart, and variable arrival rates for the TFRC flows (to create
     variable levels of congestion).  And compare this to the same
     scenario with no flows using Faster Restart.  (2) scenarios with
     transient changes from routing changes and from variable traffic.
     The goal is to explore worse-case scenarios showing off the worst
     aspects of Faster Restart.

   * Targeted an idle period of at most six minutes, not thirty
     minutes.  Feedback from Gorry and Ian McDonald.

   * Added a section of whether Faster Restart encourages flows to
     pad their sending rate during idle periods.

   * Didn't implement suggestion from Lachlan Andrew to decay from
     qradupling to doubling the sending rate gradually.  The last
     more-than-doubling of the sending rate is probably not a
     quadrupling in any case, since the allowed sending rate is
     not increased due to quadrupling to more than X_fast_max.

   Changes from draft-ietf-dccp-tfrc-faster-restart-02.txt:

   * Deleted proposed response to dealing with X_recv for idle or
     data-limited periods;  RFC3448bis now deals with this instead.

   * Deleted the Receive Rate Length option.  Also
     removed all text about using the inflation factor to
     reduce X_recv_in based on the sender's idle time.




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   * Moved TFRC changes and DCCP-specific changes to separate sections.

   * Revised draft to refer to RFC3448bis instead of to RFC3448.
     This included modifying sections on "Feedback Packets" and
     "Nofeedback Timer".

   * Said that CCID 3 could calculate the receive rate only
     for one RTT, rather than for longer, after an idle period.
     (When used with RFC3448bis, it shouldn't affect performance
     one way or another).

   Changes from draft-ietf-dccp-tfrc-faster-restart-01.txt:

   * Added a sentence to Abstract about DCCP.

   * Added some text to the Introduction,

   * Added sections on "Minimum Sending Rate", "Send Receive
     Rate Length Feature", "Nofeedback Timer", and "Simulations
     of Faster Restart".

   * Added an Appendix on "Simulations".

   Changes from draft-ietf-dccp-tfrc-faster-restart-00.txt:

   * Added mechanisms for dealing with a more general problem with
     idle periods.  This includes a section of "Receive Rate
     Adjustment".

   END OF NOTE TO RFC EDITOR.

1.  Introduction

   This document defines congestion control mechanisms that improve the
   performance of occasionally idle flows using TCP-Friendly Rate
   Control (TFRC) [RFC3448] [RFC3448bis].  A data-limited or idle flow
   uses less than its allowed sending rate for application-specific
   reasons, such as lack of data to send.  The responses of Standard
   TFRC [RFC3448], and Revised TFRC [RFC3448bis] to long idle or data-
   limited periods are summarized in Table 1 below, and the responses of
   Standard TCP [RFC2581] and TCP with Congestion Window Validation
   [RFC2861] are described in Appendix C of [RFC3448bis].  All of these
   mechanisms allow a flow to recover from a long idle period by ramping
   up to allowed sending rate or window.  This document specifies
   mechanisms that allow TFRC to start at a higher sending rate after an
   idle period, and to ramp up faster to the old sending rate after an
   idle period.




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   As this draft is being written, Standard TFRC is specified in
   [RFC3448].  and TFRC is in the process of being revised, as Revised
   TFRC, in [RFC3448bis].  When [RFC3448bis] is approved as a Proposed
   Standard document, this draft will be revised, with the phrase
   "Standard TFRC" replaced by "Old TFRC", and other language changes as
   appropriate.

   For Standard TFRC as specified in [RFC3448], a TFRC flow may not send
   more than twice X_recv, the rate at which data was received at the
   receiver over the previous RTT.  Thus in Standard TFRC the previous
   receive rate limits the sending rate of applications with highly
   variable sending rates, forcing the applications to ramp up, by
   doubling their sending rate each round-trip time, from the earlier
   application-limited rate to the sending rate allowed by the
   throughput equation.  TFRC's nofeedback timer halves the allowed
   sending rate after each nofeedback timer interval (at least four
   round-trip times) in which no feedback is received.  One result is
   that applications must slow-start after going idle for any
   significant length of time, in the absence of mechanisms such as
   Quick-Start [RFC4782].

   For Revised TFRC as specified in [RFC3448bis], the previous receive
   rate is not used to limit the sending rate during data-limited
   periods.  Thus, unlike [RFC3448], in [RFC3448bis] applications with
   highly variable sending rates are not limited by the previous receive
   rates.  However, [RFC3448bis] is like [RFC3448] in that the
   nofeedback timer is used to halve the allowed sending rate after each
   nofeedback timer interval in which no feedback is received.  With
   [RFC3448] the allowed sending rate is not reduced below two packets
   per RTT during idle periods, and with [RFC3448bis] the allowed
   sending rate is not reduced below the allowed initial sending rate
   during idle periods.

   This behavior is safe, though conservative, for best-effort traffic
   in the network.  A silent application stops receiving feedback about
   the condition of the current network path, and thus should not be
   able to send at an arbitrary rate.  A data-limited application stops
   receiving feedback about whether current network conditions would
   support higher rates.  However, this behavior also affects the
   perceived performance of interactive applications such as voice.
   Connections for interactive telephony and conference applications,
   for example, will usually have one party active at a time, with
   seamless switching between active parties.  TFRC's reduction of the
   allowed sending rate, and slow-starting back up, after every switch
   between parties may seriously degrade perceived performance.  Some of
   the strategies suggested for coping with this problem, such as
   sending padding data during application idle periods, might have
   worse effects on the network than simply switching onto the desired



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   rate with no slow-start.

   There is some justification for somewhat accelerating the slow start
   process after idle periods, as opposed to at the beginning of a
   connection.  A flow that fairly achieves a sending rate of X has
   proved, at least, that some path between the endpoints can support
   that rate.  The path might change, due to endpoint reset or routing
   adjustments; or many new connections might start up, significantly
   reducing the application's fair rate.  However, it seems reasonable
   to allow an application to possibly contribute to limited transient
   congestion in times of change, in return for improving application
   responsiveness.

   This document suggests a relatively simple approach to this problem.
   Standard TFRC [RFC3448] specifies that the allowed sending rate is
   never reduced below two packets per RTT as the result of a nofeedback
   timer after an idle period.  Following [RFC3390], CCID-3 [RFC4342]
   and Revised TFRC [RFC3448bis] specify that the allowed sending rate
   is never reduced below the TCP initial sending rate of two or four
   packets per RTT, depending on packet size, as the result of a
   nofeedback timer after an idle period.  Faster Restart doubles this
   allowed sending rate after idle periods.  Thus, the sending rate
   after an idle period is not reduced below a rate Y between four and
   eight packets per RTT, depending on the packet size.  The rate Y is
   restricted to at most 8760 bytes per RTT.

   In addition, because flows already have some (possibly old)
   information about the path, Faster Restart allows flows to quadruple
   their sending rate in every congestion-free RTT, instead of doubling,
   upwards towards the previously achieved rate.  When the TFRC sender
   detects congestion, the sender leaves Faster Restart and changes into
   congestion avoidance.  These changes are summarized in the table
   below.


















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   ------------------------------------------------------------------
   - Standard TFRC -
   ------------------------------------------------------------------
   Idle period:
     Halve allowed sending rate each RTO, not below two packets per RTT.
     After sending again, double the sending rate each RTT.
   Data-limited period:
     Send at most twice X_recv.
     As a result, at most double the sending rate each RTT.
   ------------------------------------------------------------------

   ------------------------------------------------------------------
   - Revised TFRC -
   ------------------------------------------------------------------
   Idle period:
     Halve allowed sending rate each RTO, not below initial sending rate.
     After sending again, double the sending rate each RTT.
   Data-limited period:
     Sending rate not limited by X_recv.
   ------------------------------------------------------------------

   ------------------------------------------------------------------
   - Revised TFRC with Faster Restart -
   ------------------------------------------------------------------
   Idle period:
     Halve allowed sending rate each RTO, not below twice initial rate.
       (Specified in Section 3.2.)
     After sending again, quadruple the sending rate towards old rate.
       (Specified in Section 3.1.)
   Data-limited period:
     Sending rate not limited by X_recv.
   ------------------------------------------------------------------

   Table 1: Behavior of TFRC, with and without Faster Restart.

   The congestion control mechanisms here are intended to apply to any
   implementations of TFRC, including that in DCCP's CCID 3 and CCID 4
   [RFC4342], [CCID4].  While we also believe that TCP could safely use
   a similar Faster Restart mechanism, we do not specify it here.  Our
   assumption is that flows that are sensitive to restrictions to the
   sending rate after idle periods are more likely to use TFRC than to
   use TCP or TCP-like congestion control.

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].



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   The Faster Restart mechanism refers to several existing TFRC state
   variables, including the following:

   R: The RTT estimate.

   X: The current allowed sending rate in bytes per second.

   p: The recent loss event rate.

   X_recv:
      The rate at which the receiver estimates that data was received
      since the last feedback report was sent.

   s: The packet size in bytes.

   Faster Restart used the following variable from [RFC3448bis]:

   recv_limit:
      The limit on the sending rate that is computed from the receive
      rate.

   Faster Restart also introduces new state variables to TFRC, as
   follows.

   X_active_recv:
      The receiver's estimated receive rate reported during a recent
      active sending period.  An active sending period is a period in
      which the sender has not experienced a loss event.  X_active_recv
      is initialized to 0 until there has been an active sending period,
      and X_active_recv is reduced after a loss event.

   T_active_recv:
      The time at which X_active_recv was measured.  T_active_recv is
      initialized to the connection's start time.

   recover_rate:
      The minimum restart rate allowed by Faster Restart after an idle
      period.  Note that Faster Restart flows can drop below this rate
      as the result of actual loss feedback.  Recover_rate is defined as
      follows:

      recover_rate = min(8*s, max(4*s, 8760 bytes))/R.

   Faster Restart also uses the following, which could be implemented as
   a temporary variable:

   X_fast_max:
      The rate at which the sender should stop quadrupling its sending



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      rate, and return to at most doubling its sending rate.

   Other variables have values as described in [RFC3448] and
   [RFC3448bis].

3.  Faster Restart: Changes to TFRC

3.1.  Feedback Packets

   The Faster Restart algorithm replaces the line

       recv_limit = 2 * max (X_recv_set);

   in step (4) of Section 4.3, "Sender Behavior When a Feedback Packet
   is Received", of [RFC3448bis].  This line specifies the limitation on
   the sending rate from the recent receive rate, and in [RFC3448bis]
   allows the sender to slow-start back up after an idle period,
   doubling its sending rate after each round-trip time.

   This document replaces the line above, so that during recovery from
   an idle period, the TFRC sender can quadruple its sending rate,
   instead of just doubling it, up towards its old sending rate before
   the idle period.  This modification uses three new variables,
   X_active_recv specifying the maximum receive rate achieved before the
   idle period, T_active_recv specifying the time of the last update of
   X_active_recv, and X_fast_max specifying the adjusted rate at which
   the sender should stop quadrupling its sending rate, and return to at
   most doubling its sending rate.

   The procedure `Update X_active_recv and X_fast_max" below increases
   the two variables in response to increases in the reported receive
   rate, and reduces them following a lost or marked packet.

       Update X_active_recv and X_fast_max:
           If (the feedback packet does not indicate a loss or mark,
                 and X_recv >= X_fast_max)
               X_active_recv := X_fast_max := X_recv,
               T_active_recv := current time.
           Else if (the feedback packet DOES indicate a loss or mark,
                 and X_recv < X_fast_max)
               X_active_recv := X_fast_max := X_recv/2,
               T_active_recv := current time.

   The parameter X_active_recv gives an upper bound on the rate
   achievable through Faster Restart, and is only modified by the
   `Update X_active_rate and X_fast_max' procedure.  This modification
   is based on the contents of the feedback packet and the value of
   X_fast_max.  X_active_recv is updated as the connection achieves



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   higher congestion-free transmit rates.  X_active_recv is reduced on
   congestion feedback, to prevent an inappropriate Faster Restart until
   a new stable active rate is achieved.  Specifically, on congestion
   feedback at low rates, the sender reduces X_active_recv to X_recv/2,
   allowing a limited Faster Restart up to a likely-safe rate.

   For some transport protocols using TFRC, the feedback packets might
   report the loss event rate, but not explicity report lost or marked
   packets.  For such protocols, the sender in the `Update X_active_rate
   and X_fast_max' procedure can infer that a feedback packet indicates
   a loss or mark by looking at the reported loss event rate.  If the
   current or previous feedback packet reported an increase in the loss
   event rate, then the current feedback packet is assumed to indicate a
   loss or mark.  (If the previous feedback packet reported an increase
   in the loss event rate, then a loss event began in the interval
   covered by that feedback packet.  However, the loss event can cover
   up to a round-trip time of data, so the second half of the loss
   event, including additional lost or marked packets, could be covered
   by the second feedback packet.)

   The `Interpolate X_fast_max' procedure determines X_fast_max, the
   adjusted rate at which Faster Restart should stop.  The procedure
   sets X_fast_max to something between zero and X_active_recv,
   depending on the time since X_active_recv was last updated.  The
   procedure allows full Faster Restart up to the old sending rate
   X_active_recv after a short idle period, but requires more
   conservative behavior after a longer idle period.  Thus, if two
   minutes or less have elapsed since the last update of X_active_recv,
   then X_fast_max is set to X_active_recv.  If six minutes or more have
   elapsed, X_fast_max is set to zero.  Linear interpolation is used
   between these extremes.

   The Faster Restart time interval of from two to six minutes is chosen
   to strike a balance between the needs of applications, and the time
   intervals over which connections might reasonably quadruple back up
   to their old sending rates after idle periods.  In terms of the needs
   of applications, models of voice traffic generally use average idle
   times between 0.5 and two seconds [JS00] (Section 3).  On the other
   side, in terms of changes in path characteristics, Faster Restart
   doesn't assume that the previous sending rate is valid after an idle
   period;  Faster Restart simply assumes that a connection may
   *quadruple* rather than *double* its sending rate up to the previous
   rate.  Path congestion levels can change over time scales of round-
   trip times, which are generally between 10 and 200 ms; more dramatic
   changes in path characteristics (e.g., routing changes, changes in
   link bandwidth) happen less frequently.





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       Interpolate X_fast_max:
          // If achieved X_active_recv <= 1 minute ago,
          //    set X_fast_max to X_active_recv;
          // If achieved X_active_recv >= 3 minutes ago,
          //    set X_fast_max to zero;
          // If in between, interpolate.
          delta_T := now - T_active_recv;
          F := (6 min - min(max(delta_T, 2 min), 6 min)) / (2 min);
          X_fast_max := F * X_active_recv;

   The pseudocode above uses the temporary variables delta_T and F.

   Faster Restart replaces the following line from step (4) of Section
   4.3 of [RFC3448bis]:

       recv_limit := 2 * max (X_recv_set);

   with the following:

       Interpolate X_fast_max;
       Update X_active_recv and X_fast_max;
       recv_limit :=  2 * max (X_recv_set);
       If (recv_limit < X_fast_max)
           recv_limit := min(2*recv_limit, X_fast_max);

   In summary, when a feedback packet is received, as specified in
   [RFC3448bis], then the sender updates the round-trip time estimate
   and the RTO (Retransmit Timeout interval), and updates X_recv_set,
   the set of recent X_recv values, and then executes the procedure
   above.  X_fast_max always represents the interpolated value from
   highest X_recv reported since the last loss event.  However, because
   X_recv_set contains only X_recv values from the most recent two
   round-trip times, the calculated recv_limit could be less than
   X_fast_max.  In this case, recv_limit is doubled, up to at most
   X_fast_max.  Faster Restart's doubling of recv_limit allows the TFRC
   sender to quadruple its sending rate each round-trip time after an
   idle period.

3.2.  Nofeedback Timer

   Section 4.4 of [RFC3448bis] specifies when the allowed sending rate
   is halved after the nofeedback timer expires.  In particular,
   [RFC3448bis] specifies that if the sender has been idle since the
   nofeedback timer was set, then the allowed sending rate is not
   reduced below recover_rate, which in [RFC3448bis] is set to the
   initial_rate of W_init/R, for





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

   for segment size s.  In contrast, this document sets recover_rate to
   twice the initial_rate, as follows:

        recover_rate = 2*W_init/R;


4.  Faster Restart: DCCP-specific Specifications

4.1.  DCCP: Receive Rate Adjustment

   Unlike [RFC3448] and [RFC3448bis], Section 8.3 of DCCP CCID 2
   [RFC4342] specifies that the Receive Rate option reports the receive
   rate since the last feedback packet was sent.  In contrast, Section
   6.2 of [RFC3448] and of [RFC3448bis] specify that the feedback packet
   reports the receive rate over the last round-trip time.  As a result,
   the receive rate reported by [RFC4342] differs from that of TFRC for
   a feedback packet after an idle period; the receive rate report
   specified in [RFC4342] reports the receive rate over the entire idle
   period.  The receive rate reported by [RFC4342] also differs from
   that of TFRC for an early feedback packet reporting a new loss event.
   In this document, [RFC4342] and [CCID4] are updated to use the
   definition of the receive rate as specified in [RFC3448] and
   [RFC3448bis].

   In particular, the fourth paragraph in Section 6 of [RFC4342] is
   changed from:

      2. A Receive Rate option, defined in Section 8.3, specifying the
         rate at which data was received since the last DCCP-Ack was
         sent.

   to:

      2. A Receive Rate option, defined in Section 8.3, specifying the
         rate at which data was received over the last round-trip time.

   Similarly, the first paragraph in Section 8.3 of [RFC4342] is changed
   from:

      This option MUST be sent by the data receiver on all required
      acknowledgements.  Its four data bytes indicate the rate at which
      the receiver has received data since it last sent an
      acknowledgement, in bytes per second.  To calculate this receive
      rate, the receiver sets t to the larger of the estimated round-
      trip time and the time since the last Receive Rate option was
      sent.  (Received data packets' window counters can be used to



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      produce a suitable RTT estimate, as described in Section 8.1.)
      The receive rate then equals the number of data bytes received in
      the most recent t seconds, divided by t.

   to:

      This option MUST be sent by the data receiver on all required
      acknowledgements.  Its four data bytes indicate the rate at which
      the receiver has received data over the last round-trip time, in
      bytes per second.  To calculate the time interval t for
      calculating this receive rate, the receiver follows Section 6.2 of
      [RFC3448bis], or roughly equivalently, Section 6.2 of [RFC3448].
      (Received data packets' window counters can be used to produce a
      suitable RTT estimate, as described in Section 8.1.)  The receive
      rate then equals the number of data bytes received in the most
      recent t seconds, divided by t.

   Thus, a feedback packet sent in response to the first packet received
   after an idle period reports a receive rate of one packet per round-
   trip time.  As a change from [RFC3448], [RFC3448bis] doesn't use the
   receive rate reported in such packets to reduce the allowed sending
   rate.  Because [RFC3448bis] doesn't use the receive rate to reduce
   the allowed sending rate when the data sender was data-limited over
   the entire interval covered by the receive rate, the DCCP sender that
   follows [RFC3448bis] generally would not use the receive rate from an
   interval in which the sender has not sent any data packets.

   We specify language for DCCP so that if the entire period covered by
   the last feedback packet doesn't include any data packets, then the
   sender doesn't use the reported receive rate to reduce the sending
   rate, even if the sender was not data-limited over that interval.  To
   do that, we add the following:

      Assume that the sender receives two feedback packets with
      Acknowledgement Numbers A1 and A2, respectively.  Further assume
      that the sender sent no data packets in between Sequence Numbers
      A1+1 and A2, inclusive.  (All those packets must have been pure
      acknowledgements, Sync and SyncAck packets, and so forth.)  Then
      the sender MAY, at its discretion, ignore the second feedback
      packet's Receive Rate option.  Note that when the sender decides
      to ignore such an option, it MUST NOT reset the nofeedback timer
      as it normally would; the nofeedback timer will expire as if the
      second feedback packet had not been received.

4.2.  DCCP: The Receive Rate Length

   [This section will be deleted prior to publication.]




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   [The Receive Rate Length option in earlier versions of this document
   has been deleted.  The Receive Rate Length option is not needed for
   feedback packets sent after an idle period, because of changes in
   [RFC3448bis].  The Receive Rate Length option should not be used for
   the sender to account for short idle periods within a feedback
   period.  The Receive Rate Length option is also not needed for the
   case discussed above when the sender is not data-limited, but the
   data sending rate is less than one packet per round-trip time, and
   the interval covered by the feedback packet doesn't include any data
   packets; this case is dealt with above without the use of the Receive
   Rate Length.]

5.  Faster Restart Discussion

   Standard TCP has historically dealt with idleness and data-limited
   flows either by keeping cwnd entirely open ("immediate start") or by
   entering slow-start, as recommended in RFC 2581 in response to an
   idle period.  The first option is too liberal, the second too
   conservative.  Clearly a short idle or data-limited period is not a
   new connection: recent evidence shows that the connection could
   fairly sustain some rate without adversely impacting other flows.
   However, longer idle periods are more problematic.  Idle periods of
   many minutes would seem to require slow-start.

   RFC 2861 [RFC2861] gives a moderate mechanism for TCP, where the
   congestion window is halved for every retransmit timeout interval
   that the sender has remained idle, down to the initial window, and
   the window is re-opened in slow-start when the idle period is over.
   TFRC in [RFC3448bis] roughly follows [RFC2861] for the response to an
   idle period.  Unlike [RFC2861], however, [RFC3448bis] follows
   Standard TCP in its responses to a data-limited period, and does not
   reduce the allowed sending rate in response to data-limited periods.

5.1.  Worst-Case Scenarios

   Faster Restart should be acceptable for TFRC if its worst-case
   scenarios are acceptable. Realistic worst-case scenarios might
   include the following scenarios:

   o  Path changes: The path changes and the old rate isn't acceptable
      on the new path.  RTTs are shorter on the new path too, so Faster
      Restart takes bandwidth from other connections for multiple RTTs,
      not just one.  (This can happen with TCP or with TFRC without
      Faster Restart, but Faster Restart could make this behavior more
      severe.)

   o  Synchronized flows: Several connections enter Faster Restart
      simultaneously.  If the path is congested, the extra load



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      resulting from Faster Restart could be twice as bad as the extra
      load if the connections had simply slow-started from their allowed
      initial sending rate.

   o  Many forms of burstiness: Variable-rate connections using Faster
      Restart share the congested link with short TCP or DCCP
      connections starting and stopping, with initial windows of three
      or four packets.  The aggregate traffic could also include TCP
      connections with short quiescent periods (e.g., web browsing
      sessions using HTTP 1.1), or bursty higher-priority traffic.  As a
      result of the bursty traffic, the aggregate arrival rate varies
      from one RTT to the next.  The transient congestion will be
      particularly severe if the congested link is an access link
      instead of a backbone link; the level of statistical multiplexing
      on an access link may not be sufficiently high to "smooth out" the
      burstiness.

   o  Wireless links: The network allocates capacity based on traffic
      conditions, as in some current wireless technologies, such as
      Bandwidth on Demand (BoD) links [RFC3819] where capacity is
      variable and dependent on several parameters other than network
      congestion.  In this case, the old sending rate might not be
      acceptable after a change in capacity for the wireless link during
      an idle period.

   Further analysis is required to analyze the effects of these
   scenarios.

5.2.  Incentives for applications to send unnecessary packets during
idle or data-limited periods?

   How does Faster Restart affect an application's incentive to pad its
   sending rate by sending unnecessary packets during idle or data-
   limited periods?  We would like to limit an application's incentive
   to pad its sending rate during idle or data-limited periods;  if all
   applications were to pad their sending rates, it could reduce the
   available bandwidth, and degrade the performance for all flows on the
   congested link.

   With Standard TFRC as specified in [RFC3448], a data-limited TFRC
   flow may not send more than twice X_recv, the rate at which data was
   received at the receiver over the previous RTT.  Thus, with Standard
   TFRC, one could argue that a variable-rate application over an
   uncongested path does have some incentive to pad its sending rate.

   With Revised TFRC as specified in [RFC3448bis], the allowed sending
   rate after an idle period is larger than the allowed sending rate
   with Standard TFRC.  Further, with Revised TFRC the receive rate



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   reported in feedback packets is not used to limit the sending rate
   during data-limited periods.  Thus, with Revised TFRC an application
   has less incentive to pad its sending rate than with Standard TFRC.
   However, with Revised TFRC an application could have some incentive
   to pad its sending rate just enough to maintain the status of "data-
   limited" instead of "idle", by sending at least one packet every four
   round-trip times.

   By allowing TFRC to revert to its old sending rate more quickly after
   an idle period, Faster Restart could reduce an application's
   incentive to pad its sending rate.

5.3.  Faster Restart for TFRC-SP

   We note that Faster Restart with TFRC-SP [RFC4828] is considerably
   more restrained than Faster Restart with TFRC.  In TFRC-SP, the
   sender is restricted to sending at most one packet every Min
   Interval.

6.  Simulations of Faster Restart

   Some test case scenarios based on simulation analysis are described
   in Appendix A.  These simulations follow the guidelines set in
   [RFC4828].  These are:

   1. Fairness to standard TCP and TFRC: The simulation tests examine
      whether flows that use Faster Restart allow TCP and TFRC flows can
      achieve their share of the path capacity.

   2. Fairness within FR: The simulation tests examine how multiple
      competing FR flows share the available capacity among them.

   3. Response to transient events: The simulation tests examine how a
      FR flow reacts to a sudden congestion event.

   4. Behaviour in a range of environments: Tests assess a range of
      bandwidths, RTTs, and varying idle periods.

   A later version of this draft will provide more discussion on these
   results in the appendix and implications will be noted here.

7.  Implementation Issues

   TBA







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8.  Security Considerations

   DCCP security considerations are discussed in [RFC4340].  Faster
   Restart adds no additional security considerations.

9.  IANA Considerations

   There are no IANA considerations.

10.  Thanks

   We thank the DCCP Working Group for feedback and discussions; we
   particularly thank Gorry Fairhurst.  We thank Vlad Balan for pointing
   out problems with the mechanisms discussed in previous versions of
   the draft.  We also thank Gerrit Renker for feedback.

Normative References

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

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

   [RFC3448bis]   Handley, M., Floyd, S., Padhye, J., and J. Widmer,
                  "TCP Friendly Rate Control (TFRC): Protocol
                  Specification", internet draft draft-ietf-dccp-
                  rfc3448bis-02.txt, work-in-progress, July 2007.

   [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.

Informative References

   [CCID4]        Floyd, S., and E. Kohler, "Profile for Datagram
                  Congestion Control Protocol (DCCP) Congestion ID 4:
                  TCP-Friendly Rate Control for Small Packets (TFRC-
                  SP)", Internet-Draft draft-floyd-dccp-ccid4-01.txt,
                  work in progress, June 2007.




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   [JCH84]        R.K. Jain, Dah-Ming Chiu, and Willian R. Hawe, A
                  Quantitative Measure of Fairness and Discrimination
                  for Resource Allocation in Shared Systems, DEC
                  Technical tleport TR-301, Digital Equipment
                  Corporation, September 1984.

   [JS00]         W. Jiang and H. Schulzrinne, Analysis of On-Off
                  Patterns in VoIP and Their Effect on Voice Traffic
                  Aggregation, Proceedings of the Ninth Conference on
                  Computer Communications and Networks (ICCCN), October
                  2000.

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

   [RFC2861]      Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
                  Window Validation", RFC 2861, June 2000.

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

   [RFC3819]      Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman,
                  D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch,
                  J., and L. Wood, "Advice for Internet Subnetwork
                  Designers", RFC 3819, July 2004.

   [RFC4782]      Floyd, S., Allman, M., Jain, A., and P. Sarolahti,
                  "Quick-Start for TCP and IP", RFC 4782, June 2006.

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

A.  Appendix: Simulations

   This appendix describes a set of initial test case scenarios for
   simulation analysis of Faster Restart. The topology will be the
   classic dumb-bell topology used in many simulations of TCP.

   Several types of flows are considered:

   o  Bulk TCP Flows.

   o  Interactive (short) TCP Flows.

   o  TFRC Flows with and without Faster Restart.





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   o  TFRC-SP Flows with and without Faster Restart.

   The implications on other flows (e.g. using UDP) may be extrapolated
   from this.

   For these simulations, we consider three application-limited rates.

   o  The first resembles Constant Bit Rate (CBR) voice over IP with a
      media bit rate of 64 Kbps (using packets of size 160 bytes and a
      nominal transmit rate of 8 Kbps).

   o  The second resembles Constant Bit Rate (CBR) medium quality video
      over IP with a media bit rate of 512 Kbps (using packets of size
      1000 bytes and a nominal transmit rate of 64 Kbps).

   o  The third class uses an unspecified upper limit on the sending
      rate, but experiences period of idleness.

   These are intended to be illustrative, rather than exact models of
   the application behaviour.

   The simulations will model the effect of an idle period in which the
   application does not attempt to send any data for a period of time,
   then resumes transmission.

   In the first case, we shall examine periods of idleness of 1s, 2s,
   and 4s with a path RTT of 50ms, 300ms.

   The scenarios to be examined are:

   o  Performance of a long-lived (bulk) TCP flow (e.g. FTP) with TFRC
      (with and without FR): The test scenario would involve a single
      large FTP flow with varying number of CBR flows.  Each CBR flow
      becomes idle for 1s and then restarts.  The FTP flow starts during
      the idle period.  The throughput performance of the single FTP
      flow would be plotted for varying number of CBR flows.
      Simulations would be performed by varying parameters such as CBR
      rate and number of silence periods.  Does the single FTP flow get
      at least 1/n share of the bandwidth, where 'n' is the total number
      of flows?  Does the use of Faster Restart by the TFRC flows
      decrease the bandwidth received by the TCP flow?

   o  Fairness test: The test scenario would involved 'n' CBR and long
      lived TCP flows.  The CBR flows become idle for 1s and then
      restart.  During the silence period, the FTP flows arrive.  Do all
      flows get at least a 1/n share of the bandwidth?  Jain's Fairness
      Index [JCH84] would be an appropriate measure.




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   o  Performance of small TCP flows (HTTP) with TFRC with and without
      Faster Restart: The test scenario would involve a single CBR flow
      which runs for 10s, becomes idle between 2s and 3s and then
      restarts.  At 3s, a number of HTTP flows are started.  The min,
      max and median of the request/response time of these HTTP flows
      would be plotted.  Simulations would be performed by varying
      several parameters such as CBR rate, bottleneck bandwidth, delay
      and queue size.  Do the request/response times of these HTTP flows
      differ? If so, by how much?

   o  High-congestion test: In a worst-case scenario with high
      congestion, all flows use TFRC, with a range of arrival times and
      idle times.  The simulations are run both with and without Faster
      Restart.  How does the use of Faster Restart affect the aggregate
      packet drop rate?

   o  Transient changes: The first worst-case scenario with transient
      changes includes a routing change, where the new path has less
      bandwidth than the old path.  The second scenario with transient
      changes includes transient congestion from a sudden increase in
      traffic.  This increase in traffic could be from long-lived TCP
      traffic, or from higher-priority traffic, or from many new TFRC
      sessions.  The transient congestion could be particularly severe
      if the congested link is an access link instead of a backbone
      link.  The third scenario with transient changes could include a
      wireless link with variable bandwidth, as discussed earlier in
      Section 5.  In all cases, the simulations are run both with and
      without Faster Restart.  How does the use of Faster Restart affect
      the aggregate packet drop rate?

   o  An ideal scenario showing the benefits of Faster Restart: A
      scenario with an uncongested network, just a few TFRC flows,
      comparing the per-packet delay distribution with and without
      Faster Restart.  Without Faster Restart, there should be a few
      packets in each flow with very large delay times, from waiting at
      the sender until they can be sent.

   o  A scenario showing the benefits (to the flow, not to competing
      traffic) of padding during idle periods: Are there any scenarios
      where Faster Restart *increases* a flow's incentives to pad its
      sending rate during idle or under-utilized periods?


Authors' Addresses

   Eddie Kohler <kohler@cs.ucla.edu>
   4531C Boelter Hall
   UCLA Computer Science Department



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   Los Angeles, CA 90095
   USA

   Sally Floyd <floyd@icir.org>
   ICSI Center for Internet Research
   1947 Center Street, Suite 600
   Berkeley, CA 94704
   USA

   Arjuna Sathiaseelan <arjuna@erg.abdn.ac.uk>
   Electronics Research Group
   University of Aberdeen
   Aberdeen
   UK





































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