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rfc:rfc7661

Internet Engineering Task Force (IETF) G. Fairhurst Request for Comments: 7661 A. Sathiaseelan Obsoletes: 2861 R. Secchi Category: Experimental University of Aberdeen ISSN: 2070-1721 October 2015

            Updating TCP to Support Rate-Limited Traffic

Abstract

 This document provides a mechanism to address issues that arise when
 TCP is used for traffic that exhibits periods where the sending rate
 is limited by the application rather than the congestion window.  It
 provides an experimental update to TCP that allows a TCP sender to
 restart quickly following a 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.
 This document 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 reclassifies
 the status of RFC 2861 from Experimental to Historic.  This document
 obsoletes RFC 2861.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7661.

Fairhurst, et al. Experimental [Page 1] RFC 7661 New CWV October 2015

Copyright Notice

 Copyright (c) 2015 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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
    1.1. Implementation of New CWV ..................................5
    1.2. Standards Status of This Document ..........................5
 2. Reviewing Experience with TCP-CWV ...............................5
 3. Terminology .....................................................7
 4. A New Congestion Window Validation Method .......................8
    4.1. Initialisation .............................................8
    4.2. Estimating the Validated Capacity Supported by a Path ......8
    4.3. Preserving cwnd during a Rate-Limited Period ..............10
    4.4. TCP Congestion Control during the Non-validated Phase .....11
         4.4.1. Response to Congestion in the Non-validated Phase ..12
         4.4.2. Sender Burst Control during the
                Non-validated Phase ................................14
         4.4.3. Adjustment at the End of the Non-validated
                Period (NVP) .......................................14
    4.5. Examples of Implementation ................................15
         4.5.1. Implementing the pipeACK Measurement ...............15
         4.5.2. Measurement of the NVP and pipeACK Samples .........16
         4.5.3. Implementing Detection of the cwnd-Limited
                Condition ..........................................17
 5. Determining a Safe Period to Preserve cwnd .....................17
 6. Security Considerations ........................................18
 7. References .....................................................18
    7.1. Normative References ......................................18
    7.2. Informative References ....................................19
 Acknowledgments ...................................................21
 Authors' Addresses ................................................21

Fairhurst, et al. Experimental [Page 2] RFC 7661 New CWV October 2015

1. Introduction

 TCP is used for traffic with a range of application behaviours.  The
 TCP congestion window (cwnd) controls the maximum number of
 unacknowledged packets/bytes that a TCP flow may have in the network
 at any time, a value known as the FlightSize [RFC5681].  FlightSize
 is a measure of the volume of data that is unacknowledged at a
 specific time.  A bulk application will always have data available to
 transmit.  The rate at which it sends is therefore limited by the
 maximum permitted by the receiver advertised window and the sender
 congestion window (cwnd).  The FlightSize of a bulk flow increases
 with the cwnd and tracks the volume of data acknowledged in the last
 Round-Trip Time (RTT).
 In contrast, a rate-limited application will experience periods when
 the sender is either idle or unable to send at the maximum rate
 permitted by the cwnd.  In this case, the volume of data sent
 (FlightSize) can change significantly from one RTT to another and can
 be much less than the cwnd.  Hence, it is possible that the
 FlightSize could significantly exceed the recently used capacity.
 The update in this document targets the operation of TCP in such
 rate-limited cases.
 Standard TCP states that a TCP sender SHOULD set cwnd to no more than
 the Restart Window (RW) before beginning transmission if the TCP
 sender has not sent data in an interval exceeding the retransmission
 timeout, i.e., when an application becomes idle [RFC5681].  [RFC2861]
 notes that this TCP behaviour was not always observed in current
 implementations.  Experiments confirm this to still be the case (see
 [Bis08]).
 Congestion Window Validation (CWV) [RFC2861] introduced the term
 "application-limited period" for the time when the sender sends less
 than is allowed by the congestion or receiver windows.  [RFC2861]
 described a method that improved support for applications that vary
 their transmission rate, i.e., applications that either have (short)
 idle periods between transmissions or change the rate at which they
 send.  These applications are characterised by the TCP FlightSize
 often being less than the cwnd.  Many Internet applications exhibit
 this behaviour, including web browsing, HTTP-based adaptive
 streaming, applications that support query/response type protocols,
 network file sharing, and live video transmission.  Many such
 applications currently avoid using long-lived (persistent) TCP
 connections (e.g., servers that use HTTP/1.1 [RFC7230] typically
 support persistent HTTP connections but do not enable this by
 default).  Instead, such applications often either use a succession
 of short TCP transfers or use UDP.

Fairhurst, et al. Experimental [Page 3] RFC 7661 New CWV October 2015

 Standard TCP does not impose additional restrictions on the growth of
 the congestion window when a TCP sender is unable to send at the
 maximum rate allowed by the cwnd.  In this case, the rate-limited
 sender may 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 CWV.  CWV was intended to help reduce cases where TCP
 accumulated an invalid (inappropriately large) cwnd.  The use and
 drawbacks of using the CWV algorithm described 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 so in a way that is expected to mitigate the
 impact on an application that varies its sending rate.  The updated
 method applies to the rate-limited conditions (including both
 application-limited and idle senders).
 The goals of this update are:
 o  To not change the behaviour of a TCP sender that performs bulk
    transfers that fully use the cwnd.
 o  To provide a method that co-exists with standard TCP and other
    flows that use this updated method.
 o  To reduce transfer latency for applications that change their rate
    over short intervals of time.
 o  To avoid a TCP sender growing a large "non-validated" cwnd, when
    it has not recently sent using this cwnd.
 o  To remove the incentive for ad hoc application or network stack
    methods (such as "padding") solely to maintain a large cwnd for
    future transmission.
 o  To provide an incentive for the use of long-lived connections
    rather than a succession of short-lived flows, benefiting both the
    long-lived flows and other flows sharing capacity with these flows
    when congestion is encountered.

Fairhurst, et al. Experimental [Page 4] RFC 7661 New CWV October 2015

 Section 5 describes the rationale for selecting the safe period to
 preserve the cwnd.

1.1. Implementation of New CWV

 The method specified in Section 4 of this document is a sender-side-
 only change to the TCP congestion control behaviour of TCP.
 The method creates a new protocol state and requires a sender to
 determine when the cwnd is validated or non-validated to control the
 entry and exit from this state (see Section 4.3).  It defines how a
 TCP sender manages the growth of the cwnd using the set of rules
 defined in Section 4.
 Implementation of this specification requires an implementor to
 define a method to measure the available capacity using a set of
 pipeACK samples.  The details of this measurement are implementation-
 specific.  An example is provided in Section 4.5.1, but other methods
 are permitted.  A sender also needs to provide a method to determine
 when it becomes cwnd-limited.  Implementation of this may require
 consideration of other TCP methods (see Section 4.5.3).
 A sender is also recommended to provide a method that controls the
 maximum burst size (see Section 4.4.2).  However, implementors are
 allowed flexibility in how this method is implemented, and the choice
 of an appropriate method is expected to depend on the way in which
 the sender stack implements other TCP methods (such as TCP Segment
 Offload (TSO)).

1.2. Standards Status of This Document

 The document obsoletes the methods described in [RFC2861].  It
 recommends a set of mechanisms, including the use of pacing during a
 non-validated period.  The updated mechanisms are intended to have a
 less aggressive congestion impact than would be exhibited by a
 standard TCP sender.
 The specification in this document is classified as "Experimental"
 pending experience with deployed implementations of the methods.

2. Reviewing Experience with TCP-CWV

 [RFC2861] 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

Fairhurst, et al. Experimental [Page 5] RFC 7661 New CWV October 2015

 for an idle session [RFC5681], which was intended to improve
 application performance.  CWV also modified the behaviour when a
 sender transmitted at a rate less than allowed by cwnd.
 [RFC2861] proposed two sets of responses: one after an "application-
 limited period" and one after an "idle period".  Although this
 distinction was argued, in practice, differentiating the two
 conditions was found problematic in actual networks (see, e.g.,
 [Bis10]).  While this offered predictable performance for long on-off
 periods (>>1 RTT) or slowly varying rate-based traffic, the
 performance could be unpredictable for variable-rate traffic and
 depended both upon whether an accurate RTT had been obtained and the
 pattern of application traffic relative to the measured RTT.
 Many applications can and often do vary their transmission over a
 wide range of rates.  Using [RFC2861], such applications often
 experienced varying performance, which made it hard for application
 developers to predict the TCP latency even when using a path with
 stable network characteristics.  We argue that an attempt to classify
 application behaviour as application-limited or idle is problematic
 and also inappropriate.  This document therefore explicitly avoids
 trying to differentiate these two cases, instead treating all rate-
 limited traffic uniformly.
 [RFC2861] has been implemented in some mainstream operating systems
 as the default behaviour [Bis08].  Analysis (e.g., [Bis10] and
 [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 variable-rate 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 Timeout (RTO) intervals
 [RFC6298], since the behaviour would otherwise be the same as for
 standard TCP, which resets the cwnd to the TCP Restart Window after
 this period.
 To enable better performance for variable-rate applications with TCP,
 some operating systems have chosen to support non-standard methods,
 or applications have resorted to "padding" streams by sending dummy
 data to maintain their sending rate when they have no data to
 transmit.  Although transmitting redundant data across a network path
 provides good evidence that the path can sustain data at the offered
 rate, padding also consumes network capacity and reduces the
 opportunity for congestion-free statistical multiplexing.  For
 variable-rate flows, the benefits of statistical multiplexing can be
 significant, and it is therefore a goal to find a viable alternative
 to padding streams.

Fairhurst, et al. Experimental [Page 6] RFC 7661 New CWV October 2015

 Experience with [RFC2861] 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.
 Therefore, it was concluded that CWV, as defined in [RFC2861], was
 often a poor solution for many rate-limited applications.  It had the
 correct motivation but 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 additional terminology is introduced in this document:
 o  cwnd-limited: A TCP flow that has sent the maximum number of
    segments permitted by the cwnd, where the application utilises the
    allowed sending rate (see Section 4.5.3).
 o  pipeACK sample: A measure of the volume of data acknowledged by
    the network within an RTT.
 o  pipeACK variable: A variable that measures the available capacity
    using the set of pipeACK samples (see Section 4.2).
 o  pipeACK Sampling Period: The maximum period that a measured
    pipeACK sample may influence the pipeACK variable.
 o  Non-validated phase: The phase where the cwnd reflects a previous
    measurement of the available path capacity.
 o  Non-validated period (NVP): The maximum period for which cwnd is
    preserved in the non-validated phase.
 o  Rate-limited: A TCP flow that does not consume more than one half
    of cwnd and hence operates in the non-validated phase.  This
    includes periods when an application is either idle or chooses to
    send at a rate less than the maximum permitted by the cwnd.

Fairhurst, et al. Experimental [Page 7] RFC 7661 New CWV October 2015

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

4. A New Congestion Window Validation Method

 This section proposes an update to the TCP congestion control
 behaviour during a rate-limited interval.  This new method
 intentionally does not differentiate between times when the sender
 has become idle or chooses to send at a rate less than the maximum
 allowed by the cwnd.
 In the non-validated phase, the capacity used by an application can
 be less than that allowed by the TCP cwnd.  This update allows an
 application to preserve a recently used cwnd while in the non-
 validated phase and then 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 non-validated period (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.  New CWV reduces this incentive for
 an application to send "padding" data simply to keep transport
 congestion state.
 The method is specified in the following subsections and 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. 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 cwnd calculations.

4.2. Estimating the Validated Capacity Supported by a Path

 [RFC6675] defines "FlightSize", a variable that indicates the
 instantaneous amount of data that has been sent but not cumulatively
 acknowledged.  In this method, a new variable "pipeACK" is introduced
 to measure the acknowledged size of the network pipe.  This is used

Fairhurst, et al. Experimental [Page 8] RFC 7661 New CWV October 2015

 to determine if the sender has validated the cwnd. pipeACK differs
 from FlightSize in that it is evaluated over a window of acknowledged
 data, rather than reflecting the amount of data outstanding.
 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 [RFC6675], 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.  A sender MAY count TCP DupACKs that
 acknowledge new data when collecting the pipeACK sample.  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 pipeACK 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 (e.g., a sender that has just
 started transmission or immediately after loss recovery), the pipeACK
 variable is set to the "undefined value".  This value is used to
 inhibit entering the non-validated phase until the first new
 measurement of a pipeACK sample.  (Section 4.5 provides examples of
 implementation.)
 The pipeACK variable MUST NOT be updated during TCP Fast Recovery.
 That is, the sender stops collecting pipeACK samples during loss
 recovery.  The method RECOMMENDS enabling the TCP SACK option
 [RFC2018] and RECOMMENDS the method defined in [RFC6675] to recover
 missing segments.  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 more appropriate cwnd
 following loss.
 Note: The use of pipeACK rather than FlightSize can change the
 behaviour of a TCP flow when a sender does not always have data
 available to send.  One example arises when there is a pause in
 transmission after sending a sequence of many packets, and the sender
 experiences loss at or near the end of its transmission sequence.  In
 this case, the TCP flow may have used a significant amount of

Fairhurst, et al. Experimental [Page 9] RFC 7661 New CWV October 2015

 capacity just prior to the loss (which would be reflected in the
 volume of data acknowledged, recorded in the pipeACK variable), but
 at the actual time of loss, the number of unacknowledged packets in
 flight (at the end of the sequence) may be small, i.e., there is a
 small FlightSize.  After loss recovery, the sender resets its
 congestion control state.
 [Fai12] explored the benefits of different responses to congestion
 for application-limited streams.  If the response is based only on
 the Loss FlightSize, the sender would assign a small cwnd and
 ssthresh, based only on the volume of data sent after the loss.  When
 the sender next starts to transmit, it can incur many RTTs of delay
 in slow-start before it reacquires its previous rate.  When the
 pipeACK value is also used to calculate the cwnd and ssthresh (as
 specified in Section 4.4.1), the sender can use a value that also
 reflects the recently used capacity before the loss.  This prevents a
 variable-rate application from being unduly penalised.  When the
 sender resumes, it starts at one-half its previous rate, similar to
 the behaviour of a bulk TCP flow [Hos15].  To ensure an appropriate
 reaction to ongoing congestion, this method requires that the pipeACK
 variable is reset after it is used in this way.

4.3. Preserving cwnd during a Rate-Limited Period

 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
    (i.e., at the start or directly after loss recovery).  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, the standard methods are described in [RFC5681]).
 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.
 Note: A threshold is needed to determine whether a sender is in the
 validated or non-validated phase.  A standard TCP sender in slow-
 start is permitted to double its FlightSize from one RTT to the next.
 This motivated the choice of a threshold value of 1/2.  This

Fairhurst, et al. Experimental [Page 10] RFC 7661 New CWV October 2015

 threshold ensures a sender does not further increase the cwnd as long
 as the FlightSize is less than (1/2*cwnd).  Furthermore, a sender
 with a FlightSize less than (1/2*cwnd) may, in the next RTT, be
 permitted by the cwnd to send at a rate that more than doubles the
 FlightSize; hence, this case needs to be regarded as non-validated,
 and a sender therefore needs to employ additional mechanisms while in
 this phase.

4.4. TCP Congestion Control during the Non-validated Phase

 A TCP sender implementing this specification MUST enter the non-
 validated phase when the pipeACK is less than (1/2)*cwnd.  (The note
 at the end of Section 4.4.1 describes why pipeACK<=(1/2)*cwnd is
 expected to be a safe value.)
 A TCP sender that enters the non-validated phase preserves the cwnd
 (i.e., the cwnd only increases after a sender fully uses the cwnd in
 this phase; otherwise, the cwnd neither grows nor reduces).  The
 phase is concluded when the sender transmits sufficient data so that
 pipeACK > (1/2)*cwnd (i.e., the sender is no longer rate-limited) or
 when the sender receives an indication of congestion.
 After a fixed period of time (the non-validated period (NVP)), the
 sender adjusts the cwnd (Section 4.4.3).  The NVP SHOULD NOT exceed
 five minutes.  Section 5 discusses the rationale for choosing a safe
 value for this period.
 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 4.5.3):
  • A sender that is cwnd-limited MAY use the standard TCP method

to increase cwnd (i.e., the standard method permits a TCP

       sender that fully utilises the cwnd to increase the cwnd each
       time it receives an ACK).
  • A sender that is not cwnd-limited MUST NOT increase the cwnd

when ACK packets are received in this phase (i.e., needs to

       avoid growing the cwnd when it has not recently sent using the
       current size of cwnd).
 o  If the sender receives an indication of congestion while in the
    non-validated phase (i.e., detects loss), the sender MUST exit the
    non-validated phase (reducing the cwnd as defined in
    Section 4.4.1).

Fairhurst, et al. Experimental [Page 11] RFC 7661 New CWV October 2015

 o  If the Retransmission Timeout (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].
 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 increase FlightSize to
    use the entire cwnd.  However, a change to the validated phase
    will release the sender from constraints on the growth of cwnd and
    result in using 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 MAY then use the standard method
 to increase the cwnd.  (Note that if the sender succeeds 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 Non-validated 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 needs to be
 selected based on the utilised rate.
 A sender that detects a packet drop 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.
 The pipeACK value is not updated during loss recovery (see
 Section 4.2).  If there is a valid pipeACK value, the new cwnd is
 adjusted to reflect that a non-validated 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.

Fairhurst, et al. Experimental [Page 12] RFC 7661 New CWV October 2015

 Where R is the volume of data that was successfully retransmitted
 during the recovery phase.  This corresponds to segments
 retransmitted and considered lost by the pipe estimation algorithm at
 the end of recovery.  It does not include the additional cost of
 multiple retransmission of the same data.  The loss of segments
 indicates that the path capacity was exceeded by at least R; hence,
 the calculated cwnd is reduced by at least R before the window is
 halved.
 The calculated cwnd value MUST NOT be reduced below 1 TCP Maximum
 Segment Size (MSS).
 After completing the loss recovery phase, the sender MUST
 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.
 The ssthresh is adjusted using the standard TCP method (Step 6 in
 Section 3.2 of RFC 5681 assigns the ssthresh a value equal to cwnd at
 the end of the loss recovery).
 Note: The adjustment by reducing cwnd by the volume of data not sent
 (R) follows the method proposed for Jump Start [Liu07].  The
 inclusion of the term R makes the adjustment more conservative than
 standard TCP.  This is required, since a sender in the non-validated
 phase is allowed a rate higher than a standard TCP sender would have
 achieved in the last RTT (i.e., to have more than doubled the number
 of segments in flight relative to what was sent in the previous RTT).
 The additional reduction after congestion is beneficial when the
 LossFlightSize has significantly overshot the available path
 capacity, incurring significant loss (e.g., following a change of
 path characteristics or when additional traffic has taken a larger
 share of the network bottleneck during a period when the sender
 transmits less).
 Note: The pipeACK value is only valid during a non-validated phase;
 therefore, this does not exceed cwnd/2.  If LossFlightSize and R were
 small, then this can result in the final cwnd after loss recovery
 being at most one-quarter of the cwnd on detection of congestion.
 This reduction is conservative, and pipeACK is then reset to
 undefined; hence, cwnd updates after a congestion event do not depend
 upon the pipeACK history before congestion was detected.

Fairhurst, et al. Experimental [Page 13] RFC 7661 New CWV October 2015

4.4.2. Sender Burst Control during the Non-validated Phase

 TCP congestion control allows a sender to accumulate a cwnd that
 would allow it to send a burst of segments with a total size up to
 the difference between the FlightSize 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 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 cannot be used to control a sender that has not
 sent appreciable data in the previous RTT [All05].
 This document recommends using a method to avoid line-rate bursts
 after an idle or rate-limited interval 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 TSO) is used.  This document does not recommend any specific
 method.

4.4.3. Adjustment at the End of the Non-validated Period (NVP)

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

Fairhurst, et al. Experimental [Page 14] RFC 7661 New CWV October 2015

 Note: These cwnd and ssthresh adjustments cause the sender to enter
 slow-start (since ssthresh > cwnd).  This adjustment ensures that the
 sender responds conservatively after remaining in the non-validated
 phase for more than the non-validated period.  In this case, it
 reduces the cwnd by a factor of two from the preserved value.  This
 adjustment is helpful when flows accumulate but do not use a large
 cwnd; this adjustment seeks to mitigate the impact when these flows
 later resume transmission.  This could, for instance, mitigate the
 impact if multiple high-rate application flows were to become idle
 over an extended period of time and then were simultaneously awakened
 by an external event.

4.5. Examples of Implementation

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

4.5.1. Implementing the pipeACK Measurement

 A pipeACK sample may be measured once each RTT.  This reduces the
 sender processing burden for calculating after each acknowledgment
 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., five) 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 is older than the pipeACK Sampling Period is
 discarded.  The pipeACK variable is then assigned the largest of the
 set of the highest measured values.

Fairhurst, et al. Experimental [Page 15] RFC 7661 New CWV October 2015

 pipeACK sample (Bytes)
 ^
 |   +----------+----------+           +----------+---......
 |   | 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 (because no new data segments were
 acknowledged).  The current value of the pipeACK variable will be 5,
 the maximum across all samples.  During this period, the pipeACK
 samples may be regarded as zero and hence do not contribute to the
 calculated pipeACK value.
 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.

4.5.2. Measurement of the NVP and pipeACK Samples

 The mechanism requires a number of measurements of time.  These
 measurements could be implemented using protocol timers but do not
 necessarily require a new timer to be implemented.  Avoiding the use
 of dedicated timers can save operating system resources, especially
 when there may be large numbers of TCP flows.
 The NVP could be measured by recording a timestamp when the sender
 enters the non-validated phase.  Each time a sender transmits a new
 segment, this timestamp can be used to determine if the NVP has
 expired.  If the measured period exceeds the NVP, the sender can then
 take into account how many units of the NVP have passed and make one
 reduction (defined in Section 4.4.3) for each NVP.

Fairhurst, et al. Experimental [Page 16] RFC 7661 New CWV October 2015

 Similarly, the time measurements for collecting pipeACK samples and
 determining the pipeACK Sampling Period could be derived by using a
 timestamp to record when each sample was measured and using this to
 calculate how much time has passed when each new ACK is received.

4.5.3. Implementing Detection of the cwnd-Limited Condition

 A sender needs to implement a method that detects the cwnd-limited
 condition (see Section 4.4).  This detects 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 condition is true only when the FlightSize of a
 TCP sender is equal to or larger than the current cwnd.  However, an
 implementation also needs to consider constraints on the way in which
 the cwnd variable can be used; for instance, implementations need to
 support other TCP methods such as the Nagle Algorithm and TCP Segment
 Offload (TSO) that also use cwnd to control transmission.  These
 other methods 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 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 arbitrarily 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

Fairhurst, et al. Experimental [Page 17] RFC 7661 New CWV October 2015

 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 change the network path that is used by a
 transport.  Although a change of path can in turn disrupt the RTT
 measurement and may result in a change of the capacity available to a
 TCP connection, we assume these path changes do not usually occur
 frequently (compared to 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 from that observed
 using a non-standard method that does not reset the cwnd after idle.
 Finally, other TCP sender mechanisms have used a five-minute timer,
 and there could be simplifications in some implementations by reusing
 the same interval.  TCP defines a default user timeout of five
 minutes [RFC793], which is 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, so the
 considerations described in [RFC5681] also apply to this algorithm.

7. References

7.1. Normative References

 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, DOI 10.17487/RFC0793, September 1981,
            <http://www.rfc-editor.org/info/rfc793>.
 [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
            Selective Acknowledgment Options", RFC 2018,
            DOI 10.17487/RFC2018, October 1996,
            <http://www.rfc-editor.org/info/rfc2018>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.

Fairhurst, et al. Experimental [Page 18] RFC 7661 New CWV October 2015

 [RFC2861]  Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
            Window Validation", RFC 2861, DOI 10.17487/RFC2861, June
            2000, <http://www.rfc-editor.org/info/rfc2861>.
 [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
            Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
            <http://www.rfc-editor.org/info/rfc5681>.
 [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
            "Computing TCP's Retransmission Timer", RFC 6298,
            DOI 10.17487/RFC6298, June 2011,
            <http://www.rfc-editor.org/info/rfc6298>.
 [RFC6675]  Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
            and Y. Nishida, "A Conservative Loss Recovery Algorithm
            Based on Selective Acknowledgment (SACK) for TCP",
            RFC 6675, DOI 10.17487/RFC6675, August 2012,
            <http://www.rfc-editor.org/info/rfc6675>.

7.2. Informative References

 [All05]    Allman, M. and E. Blanton, "Notes on Burst Mitigation for
            Transport Protocols", ACM SIGCOMM Computer Communication
            Review, Volume 35, Issue 2, DOI 10.1145/1064413.1064419,
            April 2005.
 [Bis08]    Biswas, I. and G. Fairhurst, "A Practical Evaluation of
            Congestion Window Validation Behaviour", 9th Annual
            Postgraduate Symposium in the Convergence of
            Telecommunications, Networking and Broadcasting
            (PGNet), Liverpool, UK, 2008.
 [Bis10]    Biswas, I., Sathiaseelan, A., Secchi, R., and G.
            Fairhurst, "Analysing TCP for Bursty Traffic", Int'l J. of
            Communications, Network and System Sciences,
            DOI 10.4236/ijcns.2010.37078, July 2010.
 [Bis11]    Biswas, I., "Internet Congestion Control for Variable-Rate
            TCP Traffic", PhD Thesis, School of Engineering,
            University of Aberdeen, 2011.
 [Fai12]    Sathiaseelan, A., Secchi, R., Fairhurst, G., and I.
            Biswas, "Enhancing TCP Performance to support Variable-
            Rate Traffic", 2nd Capacity Sharing Workshop, ACM
            CoNEXT, Nice, France, December 2012.

Fairhurst, et al. Experimental [Page 19] RFC 7661 New CWV October 2015

 [Hos15]    Hossain, Z., "A Study of Mechanisms to Support Variable-
            Rate Internet Applications over a Multi-service Satellite
            Platform", PhD Thesis, School of Engineering, University
            of Aberdeen, January 2015.
 [Hug01]    Hughes, A., Touch, J., and J. Heidemann, "Issues in TCP
            Slow-Start Restart After Idle", Work in Progress,
            draft-hughes-restart-00, December 2001.
 [Liu07]    Liu, D., Allman, M., Jin, S., and L. Wang, "Congestion
            Control without a Startup Phase", 5th International
            Workshop on Protocols for Fast Long-Distance Networks
            (PFLDnet), Los Angeles, California, February 2007.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, DOI 10.17487/RFC7230, June 2014,
            <http://www.rfc-editor.org/info/rfc7230>.

Fairhurst, et al. Experimental [Page 20] RFC 7661 New CWV October 2015

Acknowledgments

 This document was produced by the TCP Maintenance and Minor
 Extensions (tcpm) working group.
 The authors acknowledge the contributions of Dr. I. Biswas and Dr.
 Ziaul Hossain in supporting the evaluation of CWV and for their help
 in developing the mechanisms proposed in this document.  We also
 acknowledge comments received from the Internet Congestion Control
 Research Group, in particular Yuchung Cheng, Mirja Kuehlewind, Joe
 Touch, and Mark Allman.  This work was partly funded by the European
 Community under its Seventh Framework Programme through the Reducing
 Internet Transport Latency (RITE) project (ICT-317700).

Authors' Addresses

 Godred Fairhurst
 University of Aberdeen
 School of Engineering
 Fraser Noble Building
 Aberdeen, Scotland  AB24 3UE
 United Kingdom
 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
 United Kingdom
 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
 United Kingdom
 Email: raffaello@erg.abdn.ac.uk
 URI:   http://www.erg.abdn.ac.uk

Fairhurst, et al. Experimental [Page 21]

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