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Internet Engineering Task Force (IETF)                      G. FairhurstRequest for Comments: 7661                               A. SathiaseelanObsoletes:2861                                                R. SecchiCategory: Experimental                            University of AberdeenISSN: 2070-1721                                             October 2015Updating TCP to Support Rate-Limited TrafficAbstract   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 inRFC 2861 and concludes   thatRFC 2861 sought to address important issues but failed to   deliver a widely used solution.  This document therefore reclassifies   the status ofRFC 2861 from Experimental to Historic.  This document   obsoletesRFC 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7661.Fairhurst, et al.             Experimental                      [Page 1]

RFC 7661                         New CWV                    October 2015Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................31.1. Implementation of New CWV ..................................51.2. Standards Status of This Document ..........................52. Reviewing Experience with TCP-CWV ...............................53. Terminology .....................................................74. A New Congestion Window Validation Method .......................84.1. Initialisation .............................................84.2. Estimating the Validated Capacity Supported by a Path ......84.3. Preserving cwnd during a Rate-Limited Period ..............104.4. TCP Congestion Control during the Non-validated Phase .....114.4.1. 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) .......................................144.5. Examples of Implementation ................................154.5.1. Implementing the pipeACK Measurement ...............154.5.2. Measurement of the NVP and pipeACK Samples .........16           4.5.3. Implementing Detection of the cwnd-Limited                  Condition ..........................................175. Determining a Safe Period to Preserve cwnd .....................176. Security Considerations ........................................187. References .....................................................187.1. Normative References ......................................187.2. Informative References ....................................19   Acknowledgments ...................................................21   Authors' Addresses ................................................21Fairhurst, et al.             Experimental                      [Page 2]

RFC 7661                         New CWV                    October 20151.  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 inRFC 2861 with an   application are discussed inSection 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 2015Section 5 describes the rationale for selecting the safe period to   preserve the cwnd.1.1.  Implementation of New CWV   The method specified inSection 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 (seeSection 4.3).  It defines how a   TCP sender manages the growth of the cwnd using the set of rules   defined inSection 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 inSection 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 (seeSection 4.5.3).   A sender is also recommended to provide a method that controls the   maximum burst size (seeSection 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 behaviourFairhurst, 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 (seeSection 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 (seeSection 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 usedFairhurst, 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 ofFairhurst, 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 inSection 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.  ThisFairhurst, 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 ofSection 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 (seeSection 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 inSection 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 (seeSection 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 inSection 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 20154.4.2.  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 inSection 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 (seeSection 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 TCPFairhurst, 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.  References7.1.  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 2015Acknowledgments   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.ukFairhurst, et al.             Experimental                     [Page 21]