Updating TCP to support Variable-Rate Traffic
draft-fairhurst-tcpm-newcwv-02

TCPM Working Group                                          G. Fairhurst
Internet-Draft                                                 I. Biswas
Intended status: Standards Track                  University of Aberdeen
Expires: June 26, 2012                                 December 24, 2011

             Updating TCP to support Variable-Rate Traffic
                     draft-fairhurst-tcpm-newcwv-02

Abstract

   This document addresses issues that arise when TCP is used to support
   variable-rate traffic that exhibts 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 applications-limited interval.  The method is
   expected to benefit variable-rate TCP applications, while also
   providing an appropriate response if congestion is experienced.

   The document 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 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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Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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 . . .  6
       4.3.1.  Adjustment at the end of the nonvalidated phase  . . .  7
       4.3.2.  Response to congestion in the nonvalidated phase . . .  7
     4.4.  Determining a safe period to preserve cwnd . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   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 it
   sends is therefore limited by the maximum permitted by the receiver
   and congestion windows.  In contrast, a variable-rate application may
   experience periods when the sender is either idle or is unable to
   send at the maximum permitted rate.  This latter case is called
   application-limited.  The focus of this document is on the operation
   of TCP with 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.  RFC 2861 noted that
   this behaviour was not always observed in current implementations.
   Recent experiments [Bis08] confirm this to still be the case.
   However, standard TCP does not control growth of the cwnd when the
   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.  Use of such an
   invalid cwnd may result in reduced application performance and/or
   could significantly contribute to network congestion.

   These issues were noted in [RFC2861], which proposed a solution,
   known as Congestion Window Validation (CWV).  CWV was intended to
   help reduce the cases where TCP accumulated an invalid cwnd.  The use
   and drawbacks of CWV 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
   condition.

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 from a
   "sufficiently-long" idle period.  It used 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.  CWV also modified the behaviour for an application-
   limited session where a sender transmits at a rate less than allowed
   by cwnd.

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   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 have benefit, especially
   over long delay paths, when compared to slow-start restart specified
   by standard TCP.  CWV offers a benefit compared to standard TCP for
   an application that has periods of idleness.  However, CWV would only
   benefit the application if the idle period were less than several
   RTOs, 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.

   It is therefore concluded that CWV is often a poor solution for many
   variable rate applications.  In summary, 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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 6
   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 6 minutes, the value of cwnd is reduced, ensuring the value

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   then reflects the capacity that was recently actually used.

   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.

   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 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 TCP connection.

   It is expected that the update will satisfy the requirements of many
   variable-rate applications and at the same time provide an
   appropriate method for use in the Internet.  The method reduces the
   incentive for an application to send data simply to keep transport
   congestion state.  (This is sometimes known as padding).  It 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 update is expected to encourage applications
   and TCP stacks to use standards-based congestion control methods.  It
   may also encourage the use of long-lived connections where this
   offers benefit (such as persistent http).

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

   The method described in this document updates RFC 5681.  Use of the
   method REQUIRES a TCP sender and the corresponding receiver to enable
   the TCP SACK option [RFC3517].

   RFC 5681 defines 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 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 phase that captures whether the
   cwnd reflects a validated or non-validated value.  The phases are
   defined as:

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   o  Validated phase: FlightSize >=(2/3)*cwnd.  This is the normal
      phase, where cwnd is an approximate indication of available
      capacity currently available along the network path, and standard
      mechanisms are used [RFC5861].

   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, the transmission rate was
      not being constrained by the cwnd.  The methods 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
      induces congestion.  The mechanisms are specified in the following
      sections.

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
   (6 minutes, as explained in section 4.4) or when the sender transmits
   using the full cwnd (i.e. it is no longer application-limited).

   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 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), the sender MUST exit the non-validated
      phase (reducing the cwnd).

   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 [RFC 5861].
      (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 the change between phases does not
   significantly impact an application-limited sender.

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4.3.1.  Adjustment at the end of the nonvalidated phase

   During the non-validated phase, an application may produce bursts of
   data of up to the cwnd in size.  This is no different to normal TCP,
   however, as for 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 method.

   An application that remains in the non-validated phase for a period
   greater than six 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].

   (The value for cwnd was chosen to allow 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).

   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.

   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,
   i.e., a sender that detects a packet-drop or receives an Explicit
   Congestion Notification (ECN), indicating 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.

   When congestion is detected, the sender MUST therefore calculate a
   safe cwnd, based on the volume of acknowledged data:
           cwnd = FlightSize - R.

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   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/2).

4.4.  Determining a safe period to preserve cwnd

   Setting a limit to the period that cwnd is preserved 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 perfect choice for this time.  The
   period of six 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.

   There are cases where the TCP throughput exhibits significant
   variability over a time less than six 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
   applications.

   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
   minutes.

   The value of six 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.

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5.  Security Considerations

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

6.  IANA Considerations

   None.

7.  Acknowledgments

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

   Israfil Biswas was partially supported by the School of Engineering,
   University of Aberdeen, Scotland, UK.

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.

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.".

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   [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
   UK

   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk

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

   Email: israfil@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk

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