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Versions: (draft-fairhurst-tcpm-newcwv) 00 01 02 03 04 05 06

TCPM Working Group                                          G. Fairhurst
Internet-Draft                                           A. Sathiaseelan
Obsoletes: 2861 (if approved)                                  R. Secchi
Updates: 5681 (if approved)                       University of Aberdeen
Intended status: Standards Track                           June 20, 2013
Expires: December 22, 2013


              Updating TCP to support Rate-Limited Traffic
                       draft-ietf-tcpm-newcwv-01

Abstract

   This document proposes an update to RFC 5681 to address issues that
   arise when TCP is used to support traffic that exhibits periods where
   the sending 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 rate-limited interval.  This
   method is expected to benefit applications that send rate-limited
   traffic using TCP, while also providing an appropriate response if
   congestion is experienced.

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

   NOTE: The standards status of this WG document is under review for
   consideration as either Experimental (EXP) or Proposed Standard (PS).
   This decision will be made later as the document is finalised.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on December 22, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Reviewing experience with TCP-CWV  . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  An updated TCP response to idle and application-limited
       periods  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  A method for preserving cwnd during the idle and
           application-limited periods. . . . . . . . . . . . . . . .  7
     4.2.  Initialisation . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  The nonvalidated phase . . . . . . . . . . . . . . . . . .  8
     4.4.  TCP congestion control during the nonvalidated phase . . .  8
       4.4.1.  Response to congestion in the nonvalidated phase . . .  9
       4.4.2.  Adjustment at the end of the nonvalidated phase  . . . 10
       4.4.3.  Examples of Implementation . . . . . . . . . . . . . . 11
   5.  Determining a safe period to preserve cwnd . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Author Notes . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Other related work . . . . . . . . . . . . . . . . . . . . 14
     9.2.  Revision notes . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

























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1.  Introduction

   TCP is used to support a range of application behaviours.  The TCP
   congestion window (cwnd) controls the number of unacknowledged
   packets/bytes that a TCP flow may have in the network at any time, a
   value known as the FlightSize [RFC5681].  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
   advertised window and the sender congestion window (cwnd).  In
   contrast, a rate-limited 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 rate-limited.  The
   focus of this document is on the operation of TCP in such an idle or
   rate-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
   the case.

   Standard TCP does not impose additional restrictions on the growth of
   the cwnd when a TCP sender is rate-limited.  A rate-limited sender
   may therefore grow a cwnd far 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
   congestion.

   [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 the CWV algorithm in RFC 2861 with an
   application are discussed in Section 2.

   Section 3 defines relevant terminology.

   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 sending rate.  The method
   described applies to both a rate-limited and an idle condition.
   Section 5 describes the rationale for selecting the safe period to
   preserve the cwnd.







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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 a rate-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] [Fai12]) 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 the
   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 [RFC6298], since the
   behaviour would otherwise be the same as for standard TCP, which
   resets the cwnd to the RTCP Restart Window (RW) after this period.

   Experience with RFC 2861 suggests that although the CWV method
   benefited the network in a rate-limited scenario (reducing the
   probability of network congestion), the behaviour was too
   conservative for many common rate-limited applications.  This
   mechanism did not therefore offer the desirable increase in
   application performance for rate-limited applications and it is
   unclear whether applications actually use this mechanism in the
   general Internet.

   It is therefore concluded that CWV, as defined in RFC2681, was often
   a poor solution for many rate-limited applications.  It had the
   correct motivation, but had 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].

   The following new terminology is introduced:



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   pipeACK: A variable that records the volume of data acknowledged by
   the network within an RTT.

   pipeACK Sampling Period: The maximum period that a measured sample of
   the pipeACK may influence the pipeACK variable.

   Non-validated phase: The phase where the cwnd reflects a previous
   measurement of the available path capacity.

   Non-validated period, NVP: The maximum period for which cwnd is
   preserved in the non-validated phase.

   Rate-limited: A TCP flow that does not consume more than one half of
   cwnd, and hence operates in the non-validated phase.

   Validated phase: The phase where the cwnd reflects a current estimate
   of the available path capacity.


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 rate-limited period.  The new method
   permits a TCP sender to preserve the cwnd when an application becomes
   idle for a period of time (the non-validated period, NVP, see section
   5).  The period where actual usage is less than allowed by cwnd, is
   named as the non-validated phase.  This 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 the NVP, the
   value of cwnd is reduced, ensuring the value better reflects the
   capacity that was recently actually used.

   It is expected that this update will satisfy the requirements of many
   rate-limited 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 rate-limited.  This is partly a response to
   recognition that some applications wish to transmit at a rate less
   than allowed by the sender cwnd, and that it can be hard to make a
   distinction between rate-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



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   long-lived connections where this offers benefit (such as persistent
   http).

   The method is specified in following subsections.

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

   [RFC5681] defines a variable, FlightSize, that indicates the amount
   of outstanding data in the network.  This is assumed to be equal to
   the value of Pipe calculated based on the pipe algorithm [RFC3517].
   In RFC5681 this value is used during loss recovery, whereas in this
   method a new variable "pipeACK" is introduced to measure the
   acknowledged size of the pipe, which is used to determine if the
   sender has validated the cwnd.

   A sender determines a value for pipeACK by measuring the volume of
   data that was acknowledged by the network over the period of a
   measured Round Trip Time (RTT).  Using the variables defined in
   [RFC3517], a value could be measured by caching the value of HighACK
   and after one RTT measuring the difference between the cached HighACK
   value and the current HighACK value.  Other equivalent methods may be
   used.

   A sender is not required to continuously update the pipeACK variable
   after each received ACK, but MUST make a measurement at least once
   per RTT when it has sent unacknowledged segments.  The pipeACK value
   used by the algorithm MAY consider multiple pipeACK measurements over
   the pipeACK Sampling Period.  The calculated pipeACK value MUST NOT
   exceed the maximum (highest value) within the sampling period.  This
   specification degiones the pipeACK Sampling Period as Max(3*RTT, 1
   second).  This period enables a sender to compensate for large
   fluctuations in the sending rate, where there may be pauses in
   transmission, and allows pipeACK to reflect the largest recently
   measured size of "pipeACK".

   When no measurements are available, the pipeACK variable is set to
   the maximum (undefined) value.  This value is used to inhibit
   entering the nonvalidated phase until the first measurement of
   pipeACK completes.

   The method RECOMMENDS that the TCP SACK option [RFC3517] is enabled.
   This allows the sender to more accurately determine the number of
   missing bytes during the loss recovery phase, and using this method
   will result in a higher cwnd following loss.






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4.2.  Initialisation

   A sender starts a TCP connection in the Validated phase and
   initialises the pipeACK variable to the maximum (undefined) value.

4.3.  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: pipeACK >=(1/2)*cwnd.  This is the normal phase,
      where cwnd is expected to be an approximate indication of the
      capacity currently available along the network path, and the
      standard methods are used to increase cwnd (currently [RFC5681]).
      The rule for transitioning to the non-validated phase is specified
      in section 4.3.

   o  Non-validated phase: pipeACK <(1/2)*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 pipeACK Sampling Period.  That is, when it is not
      known whether the cwnd reflects the currently available capacity
      along the network path.  The mechanisms to be used in this phase
      seek to determine a safe value for cwnd and an appropriate
      reaction to congestion.  These mechanisms are specified in section
      4.3.

   The value 1/2 was selected to reduce the effects of variations in the
   measured pipeACK, and to allow the sender some flexibility in when it
   sends data.

4.4.  TCP congestion control during the nonvalidated phase

   A TCP sender MUST enter the non-validated phase when the measured
   pipeACK is less than (1/2)*cwnd.

   A TCP sender that enters the non-validated phase will preserve the
   cwnd (i.e., this neither grows nor reduces while the sender remains
   in this phase).  If the sender receives an indication of congestion
   (loss or Explicit Congestion Notification, ECN, mark [RFC3168]) it
   uses the method described below.  The phase is concluded after a
   fixed period of time (the NVP, as explained in section 4.3.2) or when
   the sender transmits sufficient data so that pipeACK > (1/2)*cwnd
   (i.e. it is no longer rate-limited).

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




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   o  The cwnd is not increased when ACK packets are received in this
      phase.

   o  If the sender receives an indication of congestion while in the
      non-validated phase (i.e. detects loss, or an ECN mark), 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 described in section 4.3.2 is
      appropriate, since any accumulated path history is considered
      unreliable).

   o  A sender that measures a pipeACK greater than (1/2)*cwnd SHOULD
      enter the validated phase.  (A rate-limited sender will not
      normally be impacted by whether it is in a validated or non-
      validate phase, since it will normally not consume the entire
      cwnd.  However a change to the validated phase will release the
      sender from constraints on the growth of cwnd, and restore the use
      of the standard congestion response.)

4.4.1.  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 record the current FlightSize in the variable LossFlightSize and
   calculate a safe cwnd, by setting it to the value specified in
   Section 3.2 of [RFC5681].

   A TCP sender MUST calculate a safe cwnd to use for loss recovery
   using the method below:
           cwnd = Min(cwnd/2,Max(pipeACK,LossFlightSize)).


   This new cwnd is set to reflect that a nonvalidated cwnd may be much
   larger than the actual flightsize, or recently used flightsize
   (recorded in pipeACK).  The updated cwnd therefore prevents overshoot
   by a sender significantly increasing its transmission rate during the
   recovery period.




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   At the end of the recovery phase, the TCP sender MUST reset the cwnd
   using the method below:
           cwnd = ((LossFlightSize - R)/2).

   Where, R is the volume of data that was retransmitted during the
   recovery phase.  This follows the method proposed for Jump Start
   [Liu07].  The inclusion of the term R makes this adjustment more
   conservative than standard TCP.  (This is required, since the sender
   may have sent more segments than a Standard TCP sender would have
   done.  The additional reduction is beneficial when the LossFlightSize
   significantly overshoots the available path capacity incurring
   significant loss, for instance an intense traffic burst following a
   non-validated period.)

   If the sender implements a method that allows it to identify the
   number of ECN-marked segments within a window that were observed by
   the receiver, the sender SHOULD use the method above, further
   reducing R by the number of marked segments.

   The sender MUST also re-initialise the pipeACK variable to the
   maximum (undefined) value.  This ensures that standard TCP methods
   are used immediately after completing loss recovery.

4.4.2.  Adjustment at the end of the nonvalidated phase

   During the non-validated phase, a sender can 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
   method [Hug01].

   An application that remains in the non-validated phase for a period
   greater than the NVP is required to adjust its congestion control
   state.  If the sender exits the non-validated phase after this
   period, it MUST update 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
   rate-limited flows that encounter occasional congestion, and could
   otherwise suffer an unwanted additional delay in recovering the
   sending rate.)

   The sender MUST then update cwnd to be not greater than:

            cwnd = max(1/2*cwnd, IW).




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   Where IW is the appropriate TCP initial window, used by the TCP
   sender (e.g.  [RFC5681]).

   (This adjustment ensures that sender responds conservatively at the
   end of the non-validated phase by reducing the cwnd to better reflect
   the current rate of the sender.  The cwnd update does not take into
   account FlightSize or pipeACK because these values only reflect data
   during the last RTT and do not reflect the average or maximum sending
   rate.)

4.4.3.  Examples of Implementation

   This section is intended to provide informative examples of
   implementation methods.  Implementations may choose to use other
   methods that comply with the normative requirements.

   XXX This section is work in progress - discussion is welcome to help
   complete this section XXX

   The pipeACK value may be sampled once each RTT.  This reduces the
   sender processing burden for calculating after each acknowledgement
   and also reduces storage requirements at the sender.

   Since application behaviour can be bursty using CWV, it may be
   desirable to implement a maximum filter to accumulate the measured
   values so that the pipeACK variable records the largest value within
   the pipeACK Sampling Period.  One simple way to implement this is to
   divide the pipeACK Sampling Period into several (e.g. 5) equal length
   measurement periods.  The sender then records the start time for each
   measurement period and the highest measured pipeACK value.  At the
   end of the measurement period, any measurement(s) that are older than
   the pipeACK Sampling Period are discarded.  The pipeACK variable is
   then assigned the largest of the set of the highest measured values.

     +----------+----------+         +----------+---......
     | Sample A | Sample B | No      | Sample C | Sample D
     |          |          | Sample  |          |
     | |\ 5     |          |         |          |
     | | |      |          |         |  /\ 4    |
     | | |      |  |\ 3    |         |  | \     |
     | | \      | |  \---  |         |  /  \    |   /| 2
     |/   \------|       - |         | /    \------/ \...
     +----------+---------\---/ /-----//--------+-------------> Time

     <------------------------------------------------|
                         Sampling Period          Current Time





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   Figure XX: Example of sampling pipeACK values

   Figure XX shows an example of how measurement samples may be
   collected.  At the time represented by the figure new samples are
   being accumulated into sample D. Three previous samples also fall
   within the pipeACK Sampling Period: A, B, and C. There was also a
   period of inactivity between samples B and C during which no
   measurements were taken.  The current value of the pipeACK variable
   will be 5, the maximum across all samples.

   After one further measurement period, Sample A will be discarded,
   since it then is older than the pipeACK Sampling Period and the
   pipeACK variable will be recalculated, Its value will be the larger
   of Sample C or the final value accumulated in Sample D.

   The NVP period does not necessarily require a new timer to be
   implemented.  An alternative is to record a timestamp when the sender
   enters the NVP.  Each time a sender transmits a new segment, this
   timestamp may be used to determine if the NVP period has expired.  If
   the period expires, the sender may take into account how many units
   of the NVP period have passed and make one reduction (as defined in
   section 4.3.2) for each NVP period.


5.  Determining a safe period to preserve cwnd

   This section documents the rationale for selecting the maximum period
   that cwnd may be preserved, known as the non-validated period, NVP.

   Limiting the period that cwnd may be preserved avoids undesirable
   side effects that would result if the cwnd were to be kept
   unnecessarily high 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.

   A period of five minutes was chosen for this NVP.  This is 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 five minutes.  Examples could



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   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 five minutes is therefore expected to be sufficient for
   most current applications.  Simulation studies (e.g.  [Bis11]) 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.  TCP defines a default user timeout of 5 minutes
   [RFC0793] i.e. how long transmitted data may remain unacknowledged
   before a connection is forcefully closed.


6.  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.


7.  IANA Considerations

   There are no IANA considerations.


8.  Acknowledgments

   The authors acknowledge the contributions of Dr I Biswas, Mr Ziaul
   Hossain 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, in particular Yuchung Cheng, Mirja Kuehlewind, and
   Joe Touch. trhis work was part-funded by the European Community under



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   its Seventh Framework Programme through the Reducing Internet
   Transport Latency (RITE) project (ICT-317700).


9.  Author Notes

9.1.  Other related work

   There are several issues to be discussed more widely:

      o Should the method explicitly state a procedure for limiting
      burstiness or pacing?



         This is often regarded as good practice, but is not presently a
         formal part of TCP. draft-hughes-restart-00.txt provides some
         discussion of this topic.

      o There are potential interactions with the Experimental update in
      [RFC6928] that raises the TCP initial Window to ten segments, do
      these cases need to be elaborated?



         This relates to the Experimental specification for increasing
         the TCP IW defined in RFC 6928.

         The two methods have different functions and different response
         to loss/congestion.

         RFC 6928 proposes an experimental update to TCP that would
         increase the IW to ten segments.  This would allow faster
         opening of the cwnd, and also a large (same size) restart
         window.  This approach is based on the assumption that many
         forward paths can sustain bursts of up to ten segments without
         (appreciable) loss.  Such a significant increase in cwnd must
         be matched with an equally large reduction of cwnd if loss/
         congestion is detected, and such a congestion indication is
         likely to require future use of IW=10 to be disabled for this
         path for some time.  This guards against the unwanted behaviour
         of a series of short flows continuously flooding a network path
         without network congestion feedback.

         In contrast, this document proposes an update with a rationale
         that relies on recent previous path history to select an
         appropriate cwnd after restart.




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         The behaviour differs in three ways:

         1) For applications that send little initially, new-cwv may
         constrain more than RFC 6928, but would not require the
         connection to reset any path information when a restart
         incurred loss.  In contrast, new-cwv would allow the TCP
         connection to preserve the cached cwnd, any loss, would impact
         cwnd, but not impact other flows.

         2) For applications that utilise more capacity than provided by
         a cwnd of 10 segments, this method would permit a larger
         restart window compared to a restart using the method in RFC
         6928.  This is justified by the recent path history.

         3) new-CWV is attended to also be used for rate-limited
         applications, where the application sends, but does not seek to
         fully utilise the cwnd.  In this case, new-cwv constrains the
         cwnd to that justified by the recent path history.  The
         performance trade-offs are hence different, and it would be
         possible to enable new-cwv when also using the method in RFC
         6928, and yield benefits.

      o There is potential overlap with the Laminar proposal
      (draft-mathis-tcpm-tcp-laminar)



         The current draft was intended as a standards-track update to
         TCP, rather than a new transport variant.  At least, it would
         be good to understand how the two interact and whether there is
         a possibility of a single method.

      o There is potential performance loss in loss of a short burst
      (off list with M Allman)



         A sender can transmit several segments then become idle.  If
         the first segments are all ACK'ed the ssthresh collapses to a
         small value (no new data is sent by the idle sender).  Loss of
         the later data results in congestion (e.g. maybe a RED drop or
         some other cause, rather than the maximum rate of this flow).
         When performs loss recovery it may have an appreciable pipeACK
         and cwnd, but a very low flight size - the Standard algorithm
         results in an unusually low cwnd (1/2 Flight size).

         A constant rate flow would have maintained a flight size
         appropriate to pipeACK (cwnd if it is a bulk flow).



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         This could be fixed by adding a new state variable?  It could
         also be argued this is a corner case (e.g. loss of only the
         last segments would have resulted in RTO), the impact could be
         significant.



      o There is potential interaction with TCP Control Block Sharing(M
      Welzl)



         An application that is non-validated can accumulate a cwnd that
         is larger than the actual capacity.  Is this a fair value to
         use in TCB sharing?

9.2.  Revision notes

   RFC-Editor note: please remove this section prior to publication.

   Draft 03 was submitted to ICCRG to receive comments and feedback.

   Draft 04 contained the first set of clarifications after feedback:

   o  Changed name to application limited and used the term rate-limited
      in all places.

   o  Added justification and many minor changes suggested on the list.

   o  Added text to tie-in with more accurate ECN marking.

   o  Added ref to Hug01

   Draft 05 contained various updates:

   o  New text to redefine how to measure the acknowledged pipe,
      differentiating this from the FlightSize, and hence avoiding
      previous issues with infrequent large bursts of data not being
      validated.  A key point new feature is that pipeACK only triggers
      leaving the NVP after the size of the pipe has been acknowledged.
      This removed the need for hysteresis.

   o  Reduction values were changed to 1/2, following analysis of
      suggestions from ICCRG.  This also sets the "target" cwnd as twice
      the used rate for non-validated case.

   o  Introduced a symbolic name (NVP) to denote the 5 minute period.




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   Draft 06 contained various updates:

   o  Required reset of pipeACK after congestion.

   o  Added comment on the effect of congestion after a short burst (M.
      Allman).

   o  Correction of minor Typos.

   WG draft 00 contained various updates:

   o  Updated initialisation of pipeACK to maximum value.

   o  Added note on intended status still to be determined.

   WG draft 01 contained:

   o  Added corrections from Richard Scheffenegger.

   o  Raffaello Secchi added to the mechanism, based on implementation
      experience.

   o  Removed that the requirement for the method to use TCP SACK option
      [RFC3517] to be enabled - Although it may be desirable to use
      SACK, this is not essential to the algorithm.

   o  Added the notion of the sampling period to accommodate large rate
      variations and ensure that the method is stable.  This algorithm
      to be validated through implementation.


10.  References

10.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [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.

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




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   [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.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              June 2011.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928, April 2013.

10.2.  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 2010.

   [Bis11]    Biswas, "PhD Thesis, Internet congestion control for
              variable rate TCP traffic, School of Engineering,
              University of Aberdeen", June 2011.

   [Fai12]    Fairhurst, Biswas, Biswas, and Biswas, "Enhancing TCP
              Performance to support Variable-Rate Traffic, 2nd Capacity
              Sharing Workshop, ACM CoNEXT, Nice, France, 10th December
              2012.", June 2008.

   [Hug01]    Hughes, Touch, and Heidemann, "&#8730;&#8730;Issues in TCP
              Slow-Start Restart After Idle (Work-in-Progress)",
              December 2001.

   [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.








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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


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

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


   Raffaello Secchi
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, Scotland  AB24 3UE
   UK

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


















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