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                       December 16, 2013                        February 4, 2014
Expires: June 19, August 8, 2014

              Updating TCP to support Rate-Limited Traffic
                       draft-ietf-tcpm-newcwv-04
                       draft-ietf-tcpm-newcwv-05

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 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
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on June 19, August 8, 2014.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4   3
   2.  Reviewing experience with TCP-CWV . . . . . . . . . . . . . .  5   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  5   4
   4.  An updated TCP response to idle and application-limited
       periods . . . . . . . . . . . . . . . . . . . . . . . . . . .  6   5
     4.1.  A method for preserving cwnd during the idle and
           application-limited periods.  . . . . . . . . . . . . . . .  7   6
     4.2.  Initialisation  . . . . . . . . . . . . . . . . . . . . . .  8   7
     4.3.  The nonvalidated phase  . . . . . . . . . . . . . . . . . .  8   7
     4.4.  TCP congestion control during the nonvalidated phase  . . .  8   7
       4.4.1.  Response to congestion in the nonvalidated phase  . . .   9
       4.4.2.  Sender burst control during the nonvalidated phase  . .  10
       4.4.3.  Adjustment at the end of the nonvalidated phase . . . 11
       4.4.4.  10
     4.5.  Examples of Implementation  . . . . . . . . . . . . . . .  11
       4.5.1.  Implementing pipeACK  . . . . . . . . . . . . . . . .  11
       4.5.2.  Implementing detection of the cwnd-limited condition   12
   5.  Determining a safe period to preserve cwnd  . . . . . . . . . . 13  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . 14  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Author Notes  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Other related work  . . . . . . . . . . . . . . . . . . . .  14
     9.2.  Revision notes  . . . . . . . . . . . . . . . . . . . . . . 17  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 19  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . . . 19  18
     10.2.  Informative References . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . .  20

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.

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:

   cwnd-limited: A TCP flow that sends has sent the maximum number of segments
   permitted by the cwnd, where the application utilises the allowed
   sending rate. rate (see section 4.5.2).

   pipeACK sample: A meaure measure of the volume of data acknowledged by the
   network within an RTT.

   pipeACK variable: A variable that measures the available capacity
   using the set of pipeACK samples.

   pipeACK Sampling Period: The maximum period that a measured pipeACK
   sample 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 a rate-limited period.  The new method permits a TCP
   sender to preserve the cwnd when an application becomes idle or when
   the sender is unable to send at the maximum rate permitted by the
   cwnd (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
   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 pipeACK sample 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 SHOULD perform a pipeACK sample at least
   once per RTT when it has sent unacknowledged segments.

   The pipeACK variable MAY consider multiple pipeACK samples over the
   pipeACK Sampling Period.  The value of the pipeACK variable MUST NOT
   exceed the maximum (highest value) within the sampling period.  This
   specification defines 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 the pipeACK variable to reflect the largest
   recently measured pipeACK sample.

   When no measurements are available, the pipeACK variable is set to
   the "undefined value".  This value is used to inhibit entering the
   nonvalidated phase until the first new measurement of a pipeACK
   sample.

   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.

4.2.  Initialisation

   A sender starts a TCP connection in the Validated phase and
   initialises the pipeACK variable to the "undefined" value.  This
   value inhibits use of the value in cwv calculations.

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, or pipeACK is undefined.
      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 4.4.

   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
      4.4.

   The value 1/2 was selected to reduce the effects of variations in the
   pipeACK variable, 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 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 4.4.2) 4.4.3) 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:

   o  A sender determines whether to increase the cwnd based upon
      whether it is cwnd-limited (see section 4.5.2):

   o

      *  A sender uses that is cwnd-limited MAY use the standard TCP method
         to increase cwnd (i.e. a TCP sender that fully utilises the
         cwnd is permitted to increase cwnd each received ACK).

   o ACK using
         standard methods).

      *  A sender that is not cwnd-limited MUST NOT increase the cwnd
         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. (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). 4.4.1).

   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 4.4.3 is
      appropriate, since any accumulated path history is considered
      unreliable).

   o  A sender with a pipeACK variable 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-
      validated 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.)

   The cwnd-limited behaviour may be triggered during a transient
   condition that occurs when a sender is in the non-validated phase and
   receives an ACK that acknowledges received data, the cwnd was fully
   utilised, and more data is awaiting transmission than may be sent
   with the current cwnd.  The sender is then allowed to use the
   standard method to increase the cwnd.  (Note, if the sender suceeds
   in sending these new segments, the updated cwnd and pipeACK variables
   will eventually result in a transition to the validated phase.)

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 indication of an
   ECN marked packet, MUST record the current FlightSize in the variable
   LossFlightSize and MUST calculate a safe cwnd for loss recovery using
   the method below:

           cwnd = (Max(pipeACK,LossFlightSize))/2.

   If there is a valid pipeACK value, the new cwnd is adjusted to
   reflect that a nonvalidated cwnd may be 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.

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

           cwnd = (Max(pipeACK,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 an adjustment more
   conservative than standard TCP.  (This is required, since the sender
   may have sent more segments than a Standard 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
   "undefined" value.  This ensures that standard TCP methods are used
   immediately after completing loss recovery until a new pipeACK value
   can be determined.

   ssthresh is adjusted using the standard TCP method.

4.4.2.  Sender burst control during the nonvalidated phase

   TCP congestion control allows a sender to accumulate a cwnd that
   would allow it to send a bursts burst of segments with a total size up to
   the difference between the FlightsSize and cwnd.  Such bursts can
   impact other flows that share a network bottleneck and/or may induce
   congestion when buffering is limited.

   Various methods have been proposed to control the sender bustiness burstiness
   [Hug01], [All05].  For example, TCP can limit the number of new
   segments it sends per received ACK . This is effective when a flow of
   ACKs is received, but can not be used to control a sender that has
   not send appreciable data in the previous RTT [All05].

   This document recommends using a method to avoid line-rate bursts
   after an idle or rate-limited period when there is less reliable
   information about the capacity of the network path: A TCP sender in
   the non-validated phase SHOULD control the maximum burst size, e.g.
   using a rate-based pacing algorithm in which a sender paces out the
   cwnd over its estimate of the RTT, or some other method, to prevent
   many segments being transmitted contiguously at line-rate.  The most
   appropriate method(s) to implement pacing depend on the design of the
   TCP/IP stack, speed of interface and whether hardware support (such
   as TCP Segment Offload, TSO) is used.  The present document does not
   recommend any specific method.

4.4.3.  Adjustment at the end of the nonvalidated phase

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

   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 value because these values only reflect
   historical data and do not reflect the current sending rate.)

4.4.4.

4.5.  Examples of Implementation

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

4.5.1.  Implementing pipeACK

   A pipeACK sample may be measured 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 pipeACK
   sample 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 sample.  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

   Figure 1: Example of measuring pipeACK samples
   Figure 1 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.

   Note that the pipeACK Sampling Period and the NVP period does do 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) 4.4.3) for each
   NVP period.

4.5.2.  Implementing detection of the cwnd-limited condition

   A method is required to detect the cwnd-limited condition. condition (see
   section 4.4).  This is used to detect a condition where a sender in
   the non-validated phase receives an ACK, but the size of cwnd
   prevents sending more new data.

   In simple terms this method condition is true only when the TCP sender's
   FlightSize is equal to or larger than the cwnd.  However, an
   implementation must consider other constraints on the way in which
   cwnd variable is used, for instance the need to support methods such
   as the Nagle Algorithm and TCP Segment Offload (TSO).  This can
   result in a sender becoming cwnd-limited when the cwnd is nearly,
   rather than completely, equal to the FlightSize.

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
   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.  This work was part-funded by the European Community under
   its Seventh Framework Programme through the Reducing Internet
   Transport Latency (RITE) project (ICT-317700).

9.  Author Notes

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

9.1.  Other related work

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

   There are several issues to be discussed more widely:

      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.

         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) (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 the sender performs loss recovery it may have an
         appreciable pipeACK and cwnd, but a very low FlightSize - the
         Standard algorithm results in an unusually low cwnd (1/2 ((1/2)*
         FlightSize).

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

         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?

         We propose that TCB sharing should use the pipeACK in place of
         cwnd when a TCP sender is in the Nonvalidated phase.  This
         value better reflects the capacity that the flow has utilised
         in the network path.

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.

   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.

   WG draft 02 contained:

   o  Clarified language around pipeACK variable and pipeACK sample -
      Feedback from Aris Angelogiannopoulos.

   WG draft 03 contained:

   o  Editorial corrections - Feedback from Anna Brunstrom.

   o  An adjustment to the procedure at the start and end of loss
      recovery to align the two equations.

   o  Further clarification of the "undefined" value of the pipeACK
      variable.

   WG draft 04 contained:

   o  Editorial corrections.

   o  Introduced the "cwnd-limited" term.

   o  An adjustment to the procedure at the start of a cwnd-limited
      phase - the new text is intended to ensure that new-cwv is not
      unnecessarily more conservative than standard TCP when the flow is
      cwnd-limited.  This resolves two issues: first it prevents
      pathologies in which pipeACK increases slowly and eraticaly. eratically.  It
      also ensures that performance of bulk applications is not
      significantly impacted when using the method.

   o  Clearly identifies that pacing (or equivalent) is requiring during
      the NVP to control bustiness. burstiness.  New section added.

   WG draft 05 contained:

   o  Clarification to first two bullets in section 4.4 describing cwnd-
      limited, to explain these are really alternates to the same case.

   o  Section giving implementation examples was restructured to clarify
      there are two methods described.

   o  Cross References to sections updated - thanks to comments from
      Martin Winbjoerk and Tim Wicinski.

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.

   [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

   [All05]    and , "Notes on burst mitigation for transport protocols",
              March 2005.

   [Bis08]    Biswas    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]    Sathiaseelan, , Secchi, , Fairhurst, , 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, "Issues in TCP Slow-Start 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.

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