TCPM Working Group                                          G. Fairhurst
Internet-Draft                                                 I. Biswas
Intended status: Standards Track                  University of Aberdeen
Expires: September 8, 2011                                March 07, 2011

             Updating TCP to support Variable-Rate Traffic


   This document addresses issues that arise when TCP is used to support
   variable-rate traffic that includes periods where the transmission
   rate is limited by the application.  It evaluates TCP Congestion
   Window Validation (TCP-CWV), an IETF experimental specification
   defined in RFC 2581, and concludes that TCP-CWV sought to address
   important issues, but failed to deliver a widely used solution.

   The document recommends that the IETF should consider moving RFC 2861
   from Experimental to Historic status, and replacing this with the
   current specification, which updates TCP to allow a TCP sender to
   restart quickly following either an idle or data-limited period.  The
   method is expected to benefit variable-rate TCP applications, while
   also providing an appropriate response if congestion is experienced.

Status of this Memo

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   This Internet-Draft will expire on September 8, 2011.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Reviewing experience with TCP-CWV . . . . . . . . . . . . . . . 3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  An updated TCP response to idle and application-limited
       periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  A method for preserving cwnd in idle and
           application-limited periods.  . . . . . . . . . . . . . . . 5
     4.2.  The nonvalidated phase  . . . . . . . . . . . . . . . . . . 5
     4.3.  TCP congestion control during the nonvalidated phase  . . . 5
       4.3.1.  Adjustment at the end of the nonvalidated phase . . . . 6
       4.3.2.  Response to congestion in the nonvalidated phase  . . . 7
     4.4.  Determining a safe period to preserve cwnd  . . . . . . . . 7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   TCP's congestion window (cwnd) controls the number of packets a TCP
   flow may have in the network at any time.  A bulk application that
   always sends as fast as possible, will continue to grow the cwnd, and
   increase its transmission rate until it reaches the maximum
   permitted.  In contrast, a variable-rate application may experience
   long periods when the sender is either idle or application-limited.

   Standard TCP requires the cwnd to be reset to the restart window (rw)
   when an application becomes idle.  RFC 2861 noted that this behaviour
   was not always observed in current implementations.  Recent
   experiments [Bis08] confirm this to still be the case.  Standard TCP
   does not control growth of the cwnd when an application is data-
   limited.  A data-limited application may therefore grow a cwnd that
   does not reflect any current information about the state of the
   network.  Use of an invalid cwnd may result in reduced application
   performance or could significantly contribute to network congestion.
   These issues were noted in [RFC 2861].

   TCP-CWV proposed a solution to help reduce the cases where TCP
   experienced an invalid cwnd.  The use of TCP-CWV is discussed in
   Section 2.

   Section 4 discusses an alternative to TCP-CWV that seeks to address
   the same issues, but does so in a way that is expected to mitigate
   the impact on an application that varies its transmission rate.  The
   proposal described applies to both a data limited and an idle
   condition. .

2.  Reviewing experience with TCP-CWV

   RFC 2861 described a simple modification to the TCP congestion
   control algorithms that decayed the cwnd after the transition from a
   "sufficiently-long" application-limited period, while using the slow-
   start threshold ssthresh to save information about the previous value
   of the congestion window.  This approach relaxed the standard TCP
   behaviour [RFC5681] for an idle session, intended to improve
   application performance.  It did not modify the behaviour for an
   application-limited session where a sender continues to transmit at a
   rate less than allowed by cwnd.

   RFC 2861 has been implemented in some mainstream operating systems as
   the default behaviour [Bis08].  Experience from using applications
   with TCP-CWV suggests that this mechanism does not offer the
   desirable increase in application performance for "bursty"
   applications and it is unclear that applications actually use the

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   mechanism.  Analysis (e.g.  [Bis10]) has shown that TCP-CWV is able
   to use the available capacity after an idle period over a shared path
   and that this can have benefit, especially over long delay paths,
   when compared to slow-start restart specified by standard TCP, but
   this behaviour can be too conservative to be attractive to many
   common variable-rate applications.

   TCP-CWV offer a benefit, compared to standard TCP, for an application
   that exhibits regular idleness.  However TCP-CWV would only benefit
   the application if the idle period was greater than several RTOs,
   since the behaviour would be the same as for standard TCP.  Although
   TCP-CWV benefits the network in an application-limited scenario, the
   conservative approach of TCP-CWV does not provide an incentive to
   application to use this.  It is therefore suggested that TCP-CWV is
   often a poor solution for many variable rate applications.  In
   summary, TCP-CWV has the correct motivation, but has the wrong
   approach to solving this problem

3.  Terminology

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

   The document also assumes familiarity with the terminology of TCP
   congestion control [RFC5681].

4.  An updated TCP response to idle and application-limited periods

   This section proposes an update to the TCP congestion control
   behaviour during an idle or data limited period.  The new method
   allows a TCP sender to preserve the cwnd when an application becomes
   idle for a period of time (set in this specification to 6 minutes).
   This period where actual usage is less than allowed by cwnd is called
   the nonvalidation phase.  This allows an application to resume
   transmission at a previous rate without incurring the delay of slow-
   start.  However, if the TCP sender experiences congestion using the
   preserved cwnd, it is required to immediately reset the cwnd to an
   appropriate value.  If a sender does not take advantage of the
   preserved cwnd within 6 minutes, the value of cwnd is updated,
   ensuring the value then reflects the capacity was recently used.

   The new method does not differentiate between times when the sender
   has become idle or application-limited.  It recognises that
   applications can result in variable-rate transmission.  This
   therefore reduces the incentive for an application to send data,

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   simply to keep transport congestion state.  The method requires SACK
   to be enabled.  This allows a sender to select a cwnd following a
   congestion event that is based on the measured path capacity path,
   better reflecting the fair-share.  A similar approach was proposed by
   TCP Jump Start [Liu07], as a congestion response after more rapid
   opening of a connection.

   It is expected that the proposed TCP modification will satisfy the
   requirements of many variable rate applications and at the same time
   provide an appropriate method for use in the Internet.  This change
   may also serve to encourage application

4.1.  A method for preserving cwnd in idle and application-limited

   The method described in this document updates RFC 5681.  Use of the
   method REQUIRES a TCP sender and the corresponding receiver to enable
   the SACK option [RFC 3517].

   RFC 5681 define a variable FlightSize, that indicates the amount of
   outstanding data in the network.  In RFC 5681 this is used during
   loss recovery, whereas in this method it is also used in normal data
   transfer.  A sender is not required to accurately record this value,
   but must be able to measure the volume of data in the network at
   least each RTT period.

4.2.  The nonvalidated phase

   The updated method creates a new TCP phase that captures where the
   cwnd reflects a valid or nonvalidated value.  The phases are defined

   o  Valid phase: FlightSize >=(2/3)*cwnd - In this phase the sender is
      in the normal phase, where cwnd is an approximate indication of
      available capacity currently available along the network path.

   o  Nonvalidated phase: FlightSize <(2/3)*cwnd - In this phase the
      sender is in the nonvalidated phase, where the cwnd was based on a
      previous approximation of the available capacity, and the usage of
      this capacity has not been validated in the previous RTT.  That
      is, the transmission rate is not being constrained by the cwnd.

4.3.  TCP congestion control during the nonvalidated phase

   A TCP sender that enters the non-validated phase preserves the cwnd
   (i.e., this neither grows nor reduces).  The phase is concluded after
   a fixed period of time (6 minutes, as explained in section 4.4) or
   when the sender transmits using the full cwnd (i.e. it is no longer

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   The behaviour in the non-validated phase is specified as:

   o  If the sender consumes all the available space within the cwnd
      (i.e. the remaining cwnd is less than one SMSS), then the sender
      MUST exit the nonvalidated phase.  (The conditions for entering
      and leaving this phase are intentionally different to introduce
      hysteresis, although the change between phases is not impacted to
      impact the application.)  If the Retransmission Time Out (RTO)
      expires during the nonvalidated phase, the sender MUST exit the
      nonvalidated phase.  It then uses the Standard TCP mechanism (in
      this case the path history is considered unreliable).

4.3.1.  Adjustment at the end of the nonvalidated phase

   An application that remains in the nonvalidated phase for a period
   greater than six minutes is required to adjust its congestion control

   During the non-validated phase, an application may produce bursts of
   data at up to the cwnd in size.  This is no different to normal TCP,
   however it is desirable to control the maximum burst size, e.g. by
   setting a burst size limit, using a pacing algorithm, or some other

   At the end of the nonvalidated phase, the sender MUST update cwnd:

   cwnd = max(FlightSize*2, IW).

   Where IW is the TCP initial window.

   The sender also MUST reset the ssthresh:

   ssthresh = max(ssthresh, 3*cwnd/4).

   The adjustment of ssthresh ensures that the sender records that it
   has safely sustained the present rate.  This change is beneficial to
   applications-limited flows that encounter occasional congestion, and
   could otherwise suffer an unwanted additional delay in recovering the
   transmission rate.

   The sender MAY re-enter the nonvalidated phase if required (see
   section 4.2).

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4.3.2.  Response to congestion in the nonvalidated phase

   If the sender receives congestion feedback while in the nonvalidated
   phase, i.e. it detects a packet-drop or receives an Explicit
   Congestion Notification (ECN), this indicates that it was unsafe to
   start with the preserved cwnd, and TCP is required to quickly reduce
   the rate to avoid further congestion.

   When loss is detected, the sender MUST calculate a safe cwnd:

   cwnd = FlightSize- R.

   Where, R is the volume of data reported as unacknowledged by the SACK
   information.  Following the method proposed for JumpStart {Liu07].

   At the end of the recovery phase, the TCP sender MUST reset the cwnd:

   cwnd = (FlightSize/2).

4.4.  Determining a safe period to preserve cwnd

   Setting a limit to the period that cwnd is preserved avoids the
   undesirable side effects that would result if cwnd were preserved for
   an arbitrary long period, which was a part of the problem that TCP-
   CWV originally attempted to address.  The period a sender may safely
   preserve the cwnd, is a function of the period that a network path is
   expected to sustain capacity reflected by cwnd.  There is no perfect
   choice for this time.  The period of 6 minutes was chosen as a
   compromise that was larger than the idle intervals of common
   applications, but not sufficiently larger than the period for which
   an Internet path may commonly be regarded as stable.

   The capacity of wired networks is usually relatively stable for
   periods of several minutes and that load stability increases with the
   capacity.  This suggests that cwnd may be preserved for at least a
   few minutes.

   There are cases where the TCP throughput exhibits significant
   variability over a time less than 6 minutes.  Examples could include
   many wireless topologies, where TCP rate variations may fluctuate on
   the order of a few seconds as a consequence of medium access protocol
   instabilities.  Mobility changes may also impact TCP performance over
   short time scales.  Senders that observe such rapid changes in the
   path characteristic may also experience increased congestion with the
   new method, however such variation would likely also impact TCP's
   behaviour when supporting interactive and bulk applications.

   Routing algorithms may modify the network path, disrupting the RTT

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   measurement and changing the capacity available to a TCP connection,
   however such changes do not often occur within a time frame of a few

   The value of 6 minutes is expected to be sufficient for most current
   applications.  Simulation studies also suggest that for most
   practical applications, the performance using this value will not be
   significantly different to that observed using a non-standard method
   that does not reset cwnd after idle.

5.  Security Considerations

   General security considerations concerning TCP congestion control are
   discussed in RFC 5681.  This document describes a algorithm for one
   aspect of those congestion control procedures, and so the
   considerations described in RFC 5681 apply to this algorithm also.

6.  IANA Considerations


7.  Acknowledgments

   The authors acknowledge the contributions of Dr A Sathiaseelan and Dr
   R Secchi in supporting the evaluation of TCP-CWV and for their help
   in developing the protocol proposed in this draft.

8.  References

8.1.  Normative References

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

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

   [RFC3517]  Blanton, E., Allman, M., Fall, K., and L. Wang, "A
              Conservative Selective Acknowledgment (SACK)-based Loss
              Recovery Algorithm for TCP", RFC 3517, April 2003.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

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8.2.  Informative References

   [Bis08]    Biswas and Fairhurst, "A Practical Evaluation of
              Congestion Window Validation Behaviour, 9th Annual
              Postgraduate Symposium in the Convergence of
              Telecommunications, Networking and Broadcasting (PGNet),
              Liverpool, UK, Jun. 2008.".

   [Bis10]    Biswas, Sathiaseelan, Secchi, and Fairhurst, "Analysing
              TCP for Bursty Traffic, Int'l J. of Communications,
              Network and System Sciences, 7(3), July 2010.".

   [Liu07]    Liu, Allman, Jiny, and Wang, "Congestion Control without a
              Startup Phase, 5th International Workshop on Protocols for
              Fast Long-Distance Networks (PFLDnet), Los Angeles,
              California, USA, Feb. 2007.".

Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, Scotland  AB24 3UE


   Israfil Biswas
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, Scotland  AB24 3UE


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