One document matched: draft-fairhurst-tcpm-newcwv-02.xml
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<rfc category="std" docName="draft-fairhurst-tcpm-newcwv-02" ipr="trust200902">
updates="2861"
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<front>
<title abbrev="TCP support for Variable-Rate Traffic">Updating TCP to
support Variable-Rate Traffic</title>
<author fullname="Godred Fairhurst" initials="G." surname="Fairhurst">
<organization>University of Aberdeen</organization>
<address>
<postal>
<street>School of Engineering</street>
<street>Fraser Noble Building</street>
<city>Aberdeen</city>
<region>Scotland</region>
<code>AB24 3UE</code>
<country>UK</country>
</postal>
<email>gorry@erg.abdn.ac.uk</email>
<uri>http://www.erg.abdn.ac.uk</uri>
</address>
</author>
<author fullname="Israfil Biswas" initials="I." surname="Biswas">
<organization>University of Aberdeen</organization>
<address>
<postal>
<street>School of Engineering</street>
<street>Fraser Noble Building</street>
<city>Aberdeen</city>
<region>Scotland</region>
<code>AB24 3UE</code>
<country>UK</country>
</postal>
<email>israfil@erg.abdn.ac.uk</email>
<uri>http://www.erg.abdn.ac.uk</uri>
</address>
</author>
<date day="24" month="December" year="2011" />
<area>Transport</area>
<workgroup>TCPM Working Group</workgroup>
<keyword>CWV</keyword>
<keyword>TCP</keyword>
<abstract>
<t>This document addresses issues that arise when TCP is used to support
variable-rate traffic that exhibts periods where the transmission rate
is limited by the application rather than the congestion window. It
updates TCP to allow a TCP sender to restart quickly following either an
idle or applications-limited interval. The method is expected to benefit
variable-rate TCP applications, while also providing an appropriate
response if congestion is experienced.</t>
<t>The document also evaluates TCP Congestion Window Validation (CWV),
an IETF experimental specification defined in RFC 2861, and concludes
that CWV sought to address important issues, but failed to deliver a
widely used solution. This document recommends that the IETF should
consider moving RFC 2861 from Experimental to Historic status, and that
this is replaced by the current specification.</t>
</abstract>
</front>
<middle>
<!-- text starts here -->
<section title="Introduction" toc="include">
<t>The TCP congestion window (cwnd) controls the number of packets/bytes
that a TCP flow may have in the network at any time. A bulk application
will always have data available to transmit. The rate it sends is
therefore limited by the maximum permitted by the receiver and
congestion windows. In contrast, a variable-rate application may
experience periods when the sender is either idle or is unable to send
at the maximum permitted rate. This latter case is called
application-limited. The focus of this document is on the operation of
TCP with such an idle or application-limited case.</t>
<t>Standard TCP [RFC5681] requires the cwnd to be reset to the restart
window (RW) when an application becomes idle. RFC 2861 noted that this
behaviour was not always observed in current implementations. Recent
experiments [Bis08] confirm this to still be the case. However, standard
TCP does not control growth of the cwnd when the TCP sender is
application-limited. An application-limited sender may therefore grow a
cwnd beyond that corresponding to the current transmit rate, resulting
in a value that does not reflect current information about the state of
the network path. Use of such an invalid cwnd may result in reduced
application performance and/or could significantly contribute to network
congestion.</t>
<t>These issues were noted in [RFC2861], which proposed a solution,
known as Congestion Window Validation (CWV). CWV was intended to help
reduce the cases where TCP accumulated an invalid cwnd. The use and
drawbacks of CWV are discussed in Section 2.</t>
<t>Section 4 specifies an alternative to CWV that seeks to address the
same issues, but does this in a way that is expected to mitigate the
impact on an application that varies its transmission rate. The method
described applies to both an application-limited and an idle
condition.</t>
</section>
<section title="Reviewing experience with TCP-CWV">
<t>RFC 2861 described a simple modification to the TCP congestion
control algorithm that decayed the cwnd after the transition from a
“sufficiently-long” idle period. It used the slow-start
threshold (ssthresh) to save information about the previous value of the
congestion window. This approach relaxed the standard TCP behaviour
[RFC5681] for an idle session, intended to improve application
performance. CWV also modified the behaviour for an application-limited
session where a sender transmits at a rate less than allowed by
cwnd.</t>
<t>RFC 2861 has been implemented in some mainstream operating systems as
the default behaviour [Bis08]. Analysis (e.g. [Bis10]) has shown that a
TCP sender using CWV is able to use available capacity on a shared path
after an idle period. This can have benefit, especially over long delay
paths, when compared to slow-start restart specified by standard TCP.
CWV offers a benefit compared to standard TCP for an application that
has periods of idleness. However, CWV would only benefit the application
if the idle period were less than several RTOs, since the behaviour
would otherwise be the same as for standard TCP, which resets the cwnd
to the RW after this period.</t>
<t>Experience with CWV suggests that although CWV benefits the network
in an application-limited scenario (reducing the probability of network
congestion), the behaviour can be too conservative for many common
variable-rate applications. This mechanism does not therefore offer the
desirable increase in application performance for variable rate
applications and it is unclear whether applications actually use this
mechanism in the general Internet.</t>
<t>It is therefore concluded that CWV is often a poor solution for many
variable rate applications. In summary, CWV has the correct motivation,
but has the wrong approach to solving this problem.</t>
</section>
<section title="Terminology" toc="include">
<t>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 <xref
target="RFC2119" />.</t>
<t>The document assumes familiarity with the terminology of TCP
congestion control [RFC5681].</t>
</section>
<section title="An updated TCP response to idle and application-limited periods "
toc="include">
<t>This section proposes an update to the TCP congestion control
behaviour during an idle or application-limited period. The new method
permits a TCP sender to preserve the cwnd when an application becomes
idle for a period of time (set in this specification to 6 minutes). This
period, where actual usage is less than allowed by cwnd, is named the
non-validated phase. The method allows an application to resume
transmission at a previous rate without incurring the delay of
slow-start. However, if the TCP sender experiences congestion using the
preserved cwnd, it is required to immediately reset the cwnd to an
appropriate value specified by the method. If a sender does not take
advantage of the preserved cwnd within 6 minutes, the value of cwnd is
reduced, ensuring the value then reflects the capacity that was recently
actually used.</t>
<t>The new method does not differentiate between times when the sender
has become idle or application-limited. This is partly a response to
recognition that some applications wish to transmit at a variable-rate,
and that it can be hard to make a distinction between
application-limited and idle behaviour.</t>
<t>The method requires that the TCP SACK option is enabled. This allows
the sender to select a cwnd following a congestion event that is based
on the measured path capacity path, better reflecting the fair-share. A
similar approach was proposed by TCP Jump Start [Liu07], as a congestion
response after more rapid opening of a TCP connection.</t>
<t>It is expected that the update will satisfy the requirements of many
variable-rate applications and at the same time provide an appropriate
method for use in the Internet. The method reduces the incentive for an
application to send data simply to keep transport congestion state.
(This is sometimes known as padding). It does not differentiate between
times when the sender has become idle or application-limited. This is
partly a response to recognition that some applications wish to transmit
at a variable-rate, and that it can be hard to make a distinction
between application-limited and idle behaviour. This update is expected
to encourage applications and TCP stacks to use standards-based
congestion control methods. It may also encourage the use of long-lived
connections where this offers benefit (such as persistent http).</t>
<section title="A method for preserving cwnd in idle and application-limited periods."
toc="include">
<t>The method described in this document updates RFC 5681. Use of the
method REQUIRES a TCP sender and the corresponding receiver to enable
the TCP SACK option [RFC3517].</t>
<t>RFC 5681 defines a variable FlightSize, that indicates the amount
of outstanding data in the network. In RFC 5681 this is used during
loss recovery, whereas in this method it is also used during normal
data transfer. A sender is not required to continuously track this
value, but SHOULD measure the volume of data in the network with a
sampling period of not less than one RTT period.</t>
</section>
<section title="The nonvalidated phase" toc="include">
<t>The updated method creates a new TCP phase that captures whether
the cwnd reflects a validated or non-validated value. The phases are
defined as:</t>
<t>
<list style="symbols">
<t>Validated phase: FlightSize >=(2/3)*cwnd. This is the normal
phase, where cwnd is an approximate indication of available
capacity currently available along the network path, and standard
mechanisms are used [RFC5861].</t>
<t>Non-validated phase: FlightSize <(2/3)*cwnd. This is the
phase where the cwnd has a value based on a previous measurement
of the available capacity, and the usage of this capacity has not
been validated in the previous RTT. That is, the transmission rate
was not being constrained by the cwnd. The methods to be used in
this phase seek to determine whether any resumed rate remains safe
for the Internet path, i.e., it quickly reduces the rate if the
flow induces congestion. The mechanisms are specified in the
following sections.</t>
</list>
</t>
</section>
<section title="TCP congestion control during the nonvalidated phase"
toc="include">
<t>A TCP sender that enters the non-validated phase MUST preserve the
cwnd (i.e., this neither grows nor reduces while the sender remains in
this phase). The phase is concluded after a fixed period of time (6
minutes, as explained in section 4.4) or when the sender transmits
using the full cwnd (i.e. it is no longer application-limited).</t>
<t>The behaviour in the non-validated phase is specified as:</t>
<t><list style="symbols">
<t>If the sender consumes all the available space within the cwnd
(i.e., the remaining unused cwnd in bytes is less than one SMSS),
then the sender MUST exit the non-validated phase.</t>
<t>If the sender receives an indication of congestion while in the
non-validated phase (i.e. detects loss, or an Explicit Congestion
Notification (ECN) mark), the sender MUST exit the non-validated
phase (reducing the cwnd).</t>
<t>If the Retransmission Time Out (RTO) expires while in the
non-validated phase, the sender MUST exit the non-validated phase.
It then resumes using the Standard TCP RTO mechanism [RFC 5861].
(The resulting reduction of cwnd is appropriate, since any
accumulated path history is considered unreliable).</t>
</list>The threshold value of cwnd required for the sender to enter
the non-validated phase is intentionally different to that required to
leave the phase. This introduces hysteresis to avoid rapid oscillation
between the phases. Note that the change between phases does not
significantly impact an application-limited sender.</t>
<section title="Adjustment at the end of the nonvalidated phase "
toc="default">
<t>During the non-validated phase, an application may produce bursts
of data of up to the cwnd in size. This is no different to normal
TCP, however, as for TCP, it is desirable to control the maximum
burst size, e.g. by setting a burst size limit, using a pacing
algorithm, or some other method.</t>
<t>An application that remains in the non-validated phase for a
period greater than six minutes is required to adjust its congestion
control state.</t>
<t>At the end of the non-validated phase, the sender MUST update
cwnd:</t>
<figure>
<artwork><![CDATA[ cwnd = max(FlightSize*2, IW).
]]></artwork>
</figure>
<t>Where IW is the TCP initial window [RFC5681].</t>
<t>(The value for cwnd was chosen to allow an application to
continue to send at the currently utilised rate, and not incur delay
should it increase to twice the utilised rate.)</t>
<t>The sender also MUST reset the ssthresh:</t>
<figure>
<artwork><![CDATA[ ssthresh = max(ssthresh, 3*cwnd/4).]]></artwork>
</figure>
<t />
<t>This adjustment of ssthresh ensures that the sender records that
it has safely sustained the present rate. The change is beneficial
to application-limited flows that encounter occasional congestion,
and could otherwise suffer an unwanted additional delay in
recovering the transmission rate.</t>
<t>The sender MAY re-enter the non-validated phase, if required (see
section 4.2).</t>
</section>
<section title="Response to congestion in the nonvalidated phase"
toc="include">
<t>Reception of congestion feedback while in the non-validated
phase, i.e., a sender that detects a packet-drop or receives an
Explicit Congestion Notification (ECN), indicating it was
inappropriate for the sender to use the preserved cwnd. The sender
is therefore required to quickly reduce the rate to avoid further
congestion. Since the cwnd does not have a validated value, a new
cwnd value must be selected based on the utilised rate.</t>
<t>When congestion is detected, the sender MUST therefore calculate
a safe cwnd, based on the volume of acknowledged data:</t>
<figure>
<artwork><![CDATA[ cwnd = FlightSize - R.]]></artwork>
</figure>
<t />
<t>Where, R is the volume of data that was reported as
unacknowledged by the SACK information. This follows the method
proposed for Jump Start [Liu07].</t>
<t>At the end of the recovery phase, the TCP sender MUST reset the
cwnd using the method below:</t>
<figure>
<artwork><![CDATA[ cwnd = (FlightSize/2).]]></artwork>
</figure>
<t />
</section>
</section>
<section title="Determining a safe period to preserve cwnd"
toc="include">
<t>Setting a limit to the period that cwnd is preserved avoids
undesirable side effects that would result if the cwnd were to be
preserved for an arbitrary long period, which was a part of the
problem that CWV originally attempted to address.</t>
<t>The period a sender may safely preserve the cwnd, is a function of
the period that a network path is expected to sustain the capacity
reflected by cwnd. There is no perfect choice for this time. The
period of six minutes was chosen as a compromise that was larger than
the idle intervals of common applications, but not sufficiently larger
than the period for which the capacity of an Internet path may
commonly be regarded as stable. The capacity of wired networks is
usually relatively stable for periods of several minutes and that load
stability increases with the capacity. This suggests that cwnd may be
preserved for at least a few minutes.</t>
<t>There are cases where the TCP throughput exhibits significant
variability over a time less than six minutes. Examples could include
wireless topologies, where TCP rate variations may fluctuate on the
order of a few seconds as a consequence of medium access protocol
instabilities. Mobility changes may also impact TCP performance over
short time scales. Senders that observe such rapid changes in the path
characteristic may also experience increased congestion with the new
method, however such variation would likely also impact TCP’s
behaviour when supporting interactive and bulk applications.</t>
<t>Routing algorithms may modify the network path, disrupting the RTT
measurement and changing the capacity available to a TCP connection,
however such changes do not often occur within a time frame of a few
minutes.</t>
<t>The value of six minutes is therefore expected to be sufficient for
most current applications. Simulation studies also suggest that for
many practical applications, the performance using this value will not
be significantly different to that observed using a non-standard
method that does not reset the cwnd after idle.</t>
</section>
</section>
<section title="Security Considerations" toc="include">
<t>General security considerations concerning TCP congestion control are
discussed in RFC 5681. This document describes an algorithm that updates
one aspect of the congestion control procedures, and so the
considerations described in RFC 5681 apply to this algorithm also.</t>
</section>
<section title="IANA Considerations" toc="include">
<t>None.</t>
</section>
<section title="Acknowledgments">
<t>The authors acknowledge the contributions of Dr A Sathiaseelan and Dr
R Secchi in supporting the evaluation of CWV and for their help in
developing the mechanisms proposed in this draft.</t>
<t>Israfil Biswas was partially supported by the School of Engineering,
University of Aberdeen, Scotland, UK.</t>
<t>
<!-- -->
</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc sortrefs="yes"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2861.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3517.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5681.xml"?>
</references>
<references title="Informative References">
<reference anchor="Bis08">
<front>
<title>A Practical Evaluation of Congestion Window Validation
Behaviour, 9th Annual Postgraduate Symposium in the Convergence of
Telecommunications, Networking and Broadcasting (PGNet), Liverpool,
UK, Jun. 2008.</title>
<author fullname="I." surname="Biswas">
<organization />
</author>
<author fullname="G." surname="Fairhurst">
<organization />
</author>
</front>
</reference>
<reference anchor="Liu07">
<front>
<title>Congestion Control without a Startup Phase, 5th International
Workshop on Protocols for Fast Long-Distance Networks (PFLDnet), Los
Angeles, California, USA, Feb. 2007.</title>
<author fullname="D" surname="Liu">
<organization />
</author>
<author fullname="M." surname="Allman">
<organization />
</author>
<author fullname="S" surname="Jiny">
<organization />
</author>
<author fullname="L." surname="Wang">
<organization />
</author>
</front>
</reference>
<reference anchor="Bis10">
<front>
<title>Analysing TCP for Bursty Traffic, Int'l J. of Communications,
Network and System Sciences, 7(3), July 2010.</title>
<author fullname="I." surname="Biswas">
<organization />
</author>
<author fullname="A." surname="Sathiaseelan">
<organization />
</author>
<author fullname="R." surname="Secchi">
<organization />
</author>
<author fullname="G." surname="Fairhurst">
<organization />
</author>
</front>
</reference>
</references>
</back>
</rfc>
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