TCPM Working Group                                          G. Fairhurst
Internet-Draft                                           A. Sathiaseelan
Updates: 5681 (if approved)                       University of Aberdeen
Obsoletes: 2861 (if approved)                              June 06, 2012
Intended status: Standards Track
Expires: December 06, 2012

             Updating TCP to support Variable-Rate Traffic


   This document addresses issues that arise when TCP is used to support
   variable-rate traffic that exhibits periods where the transmission
   rate is limited by the application rather than the congestion window.
   It updates TCP to allow a TCP sender to restart quickly following
   either an idle or application-limited interval.  The method is
   expected to benefit variable-rate TCP applications, while also
   providing an appropriate response if congestion is experienced.

   It also evaluates TCP Congestion Window Validation, CWV, an IETF
   experimental specification defined in RFC 2861, and concludes that
   CWV sought to address important issues, but failed to deliver a
   widely used solution.  This document therefore recommends that the
   IETF should consider moving RFC 2861 from Experimental to Historic
   status, and that this is replaced by the current specification.

Status of this Memo

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   This Internet-Draft will expire on December 06, 2012.

Copyright Notice

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

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   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

1.  Introduction

   TCP is used to support a range of application behaviours.  The TCP
   congestion window (cwnd) controls the number of packets/bytes that a
   TCP flow may have in the network at any time.  A bulk application
   will always have data available to transmit.  The rate at which it
   sends is therefore limited by the maximum permitted by the receiver
   and congestion windows.  In contrast, a variable-rate application
   will experience periods when the sender is either idle or is unable
   to send at the maximum rate permitted by the cwnd.  This latter case
   is called application-limited.  The focus of this document is on the
   operation of TCP in such an idle or application-limited case.

   Standard TCP [RFC5681] requires the cwnd to be reset to the restart
   window (RW) when an application becomes idle.  [RFC2861] noted that
   this TCP behaviour was not always observed in current
   implementations.  Recent experiments [Bis08] confirm this to still be

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   the case.

   Standard TCP does not control growth of the cwnd when a variable-rate
   TCP sender is application-limited.  An application-limited sender may
   therefore grow a cwnd beyond that corresponding to the current
   transmit rate, resulting in a value that does not reflect current
   information about the state of the network path the flow is using.
   Use of such an invalid cwnd may result in reduced application
   performance and/or could significantly contribute to network

   [RFC2861] proposed a solution to these issues in an experimental
   method known as Congestion Window Validation (CWV). CWV was intended
   to help reduce cases where TCP accumulated an invalid cwnd.  The use
   and drawbacks of using CWV with an application are discussed in
   Section 2.

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

2.  Reviewing experience with TCP-CWV

   RFC 2861 described a simple modification to the TCP congestion
   control algorithm that decayed the cwnd after the transition to a
   "sufficiently-long" idle period.  This used the slow-start threshold
   (ssthresh) to save information about the previous value of the
   congestion window.  The approach relaxed the standard TCP behaviour
   [RFC5681] for an idle session, intended to improve application
   performance.  CWV also modified the behaviour for an application-
   limited session where a sender transmitted at a rate less than
   allowed by cwnd.

   RFC 2861 has been implemented in some mainstream operating systems as
   the default behaviour [Bis08].  Analysis (e.g.  [Bis10]) has shown
   that a TCP sender using CWV is able to use available capacity on a
   shared path after an idle period.  This can benefit some
   applications, especially over long delay paths, when compared to
   slow-start restart specified by standard TCP. However, CWV would only
   benefit an application if the idle period were less than several
   Retransmission Time Out (RTO) intervals [RFC2988], since the
   behaviour would otherwise be the same as for standard TCP, which
   resets the cwnd to the RW after this period.

   Experience with CWV suggests that although CWV benefits the network
   in an application-limited scenario (reducing the probability of
   network congestion), the behaviour can be too conservative for many
   common variable-rate applications.  This mechanism does not therefore
   offer the desirable increase in application performance for variable-
   rate applications and it is unclear whether applications actually use
   this mechanism in the general Internet.

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   It is therefore concluded that CWV is often a poor solution for many
   variable rate applications.  It 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 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 application-limited period.  The new
   method permits a TCP sender to preserve the cwnd when an application
   becomes idle for a period of time (set in this specification to 5
   minutes). This period, where actual usage is less than allowed by
   cwnd, is named the non-validated phase.  The method 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 specified by the
   method.  If a sender does not take advantage of the preserved cwnd
   within five minutes, the value of cwnd is reduced, ensuring the value
   then reflects the capacity that was recently actually used.

   The method requires that the TCP SACK option is enabled.  This allows
   the sender to select a cwnd following a congestion event that is
   based on the measured path capacity, 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 TCP connection.

   It is expected that this update will satisfy the requirements of many
   variable-rate applications and at the same time provide an
   appropriate method for use in the Internet.  It also reduces the
   incentive for an application to send data simply to keep transport
   congestion state.  (This is sometimes known as "padding").

   The new method does not differentiate between times when the sender
   has become idle or application-limited.  This is partly a response to
   recognition that some applications wish to transmit at a variable-
   rate, and that it can be hard to make a distinction between
   application-limited and idle behaviour.  This is expected to
   encourage applications and TCP stacks to use standards-based
   congestion control methods.  It may also encourage the use of long-

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   lived connections where this offers benefit (such as persistent

   The method is specified in following subsections.

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

   The method described in this document updates [RFC5681].  Use of the
   method REQUIRES a TCP sender and the corresponding receiver to enable
   the TCP SACK option [RFC3517].

   [RFC5681] defines a variable FlightSize , that indicates the amount
   of outstanding data in the network.  In RFC5681 this is used during
   loss recovery, whereas in this method it is also used during normal
   data transfer.  A sender is not required to continuously track this
   value, but SHOULD measure the volume of data in the network with a
   sampling period of not less than one RTT period.

4.2.  The nonvalidated phase

   The updated method creates a new TCP sender phase that captures
   whether the cwnd reflects a validated or non-validated value.  The
   phases are defined as:

   o  Validated phase: FlightSize >=(2/3)*cwnd.  This is the normal
      phase, where cwnd is expected to be an approximate indication of
      the available capacity currently available along the network path,
      and the standard methods are used (currently [RFC5681]).

   o  Non-validated phase: FlightSize <(2/3)*cwnd.  This is the phase
      where the cwnd has a value based on a previous measurement of the
      available capacity, and the usage of this capacity has not been
      validated in the previous RTT. That is, when it is not known
      whether the cwnd reflects the currently available capacity
      available along the network path.  The mechanisms to be used in
      this phase seek to determine whether any resumed rate remains safe
      for the Internet path, i.e., it quickly reduces the rate if the
      flow is known to induce congestion.  These mechanisms are
      specified in section 4.3.

4.3.  TCP congestion control during the nonvalidated phase

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

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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 unused cwnd in bytes is less than one Sender
      Maximum Segment Size, SMSS), then the sender MUST exit the non-
      validated phase.

   o  If the sender receives an indication of congestion while in the
      non-validated phase (i.e.  detects loss, or an Explicit Congestion
      Notification, ECN, mark [RFC3168]), the sender MUST exit the non-
      validated phase (reducing the cwnd as defined in section 4.3.1).

   o  If the Retransmission Time Out (RTO) expires while in the non-
      validated phase, the sender MUST exit the non-validated phase.  It
      then resumes using the Standard TCP RTO mechanism [RFC5681].  (The
      resulting reduction of cwnd is appropriate, since any accumulated
      path history is considered unreliable).

   The threshold value of cwnd required for the sender to enter the non-
   validated phase is intentionally different to that required to leave
   the phase.  This introduces hysteresis to avoid rapid oscillation
   between the phases.  Note that a change between phases does not
   significantly impact an application-limited sender, but serves to
   determine its behaviour if it substantially increases its
   transmission rate.

4.3.1.  Adjustment at the end of the nonvalidated phase

   During the non-validated phase, the sender may produce bursts of data
   of up to the cwnd in size.  While this is no different to standard
   TCP, it is desirable to control the maximum burst size, e.g.  by
   setting a burst size limit, using a pacing algorithm, or some other

   An application that remains in the non-validated phase for a period
   greater than five minutes is required to adjust its congestion
   control state.  At the end of the non-validated phase, the sender
   MUST update cwnd:

           cwnd = max(FlightSize*2, IW).

   Where IW is the TCP initial window [RFC5681].

   (This allows an application to continue to send at the currently
   utilised rate, and not incur delay should it increase to twice the
   utilised rate.)

   The sender also MUST reset the ssthresh:

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

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

   After completing this adjustment, the sender MAY re-enter the non-
   validated phase, if required (see section 4.2).

4.3.2.  Response to congestion in the nonvalidated phase

   Reception of congestion feedback while in the non-validated phase is
   interpreted as an indication that it was inappropriate for the
   sender to use the preserved cwnd.  The sender is therefore required
   to quickly reduce the rate to avoid further congestion.  Since the
   cwnd does not have a validated value, a new cwnd value must be
   selected based on the utilised rate.

   A sender that detects a packet-drop or receives an ECN marked packet
   MUST calculate a safe cwnd, based on the volume of acknowledged data:

        cwnd = FlightSize - R.

   Where, R is the volume of data that was reported as unacknowledged by
   the SACK information.  This follows the method proposed for Jump
   Start [[Liu07].

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

        cwnd = ((FlightSize - R)/2).

4.4.  Determining a safe period to preserve cwnd

   This section documents the rationale for selecting the maximum period
   that cwnd may be preserved.

   Preserving cwnd avoids undesirable side effects that would result if
   the cwnd were to be preserved for an arbitrary long period, which was
   a part of the problem that 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 the capacity
   reflected by cwnd.  There is no ideal choice for this time.

   The period of five 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 the capacity of 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.

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   There are cases where the TCP throughput exhibits significant
   variability over a time less than five minutes.  Examples could
   include 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

   Routing algorithms may modify the network path, disrupting the RTT
   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 five minutes is therefore expected to be sufficient for
   most current applications.  Simulation studies also suggest that for
   many practical applications, the performance using this value will
   not be significantly different to that observed using a non-standard
   method that does not reset the cwnd after idle.

   Finally, other TCP sender mechanisms have used a 5 minute timer, and
   there could be simplifications in some implementations by reusing the
   same interval.

5.  Security Considerations

   General security considerations concerning TCP congestion control are
   discussed in [RFC5681].  This document describes an algorithm that
   updates one aspect of the congestion control procedures, and so the
   considerations described in RFC 5681 also apply to this algorithm.

6.  IANA Considerations

   There are no IANA considerations.

7.  Acknowledgments

   The authors acknowledge the contributions of Dr I Biswas and Dr R
   Secchi in supporting the evaluation of CWV and for their help in
   developing the mechanisms proposed in this draft.  We also
   acknowledge comments received from the Internet Congestion Control
   Research Group.

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.

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

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

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

   [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.

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", June 2008.

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

   [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", February 2007.

Authors' Addresses

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


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   Arjuna Sathiaseelan
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, Scotland AB24 3UE


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