One document matched: draft-fairhurst-tcpm-newcwv-01.xml
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<rfc category="std" docName="draft-fairhurst-tcpm-newcwv-01" 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="11" month="March" 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 includes periods where the transmission rate
is limited by the application. It evaluates TCP Congestion Window
Validation (TCP-CWV), an IETF experimental specification defined in RFC
2861, and concludes that TCP-CWV sought to address important issues, but
failed to deliver a widely used solution.</t>
<t>The document recommends that the IETF should consider moving RFC 2861
from Experimental to Historic status, and replacing this with the
current specification, which updates TCP to allow a TCP sender to
restart quickly following either an idle or data-limited period. The
method is expected to benefit variable-rate TCP applications, while also
providing an appropriate response if congestion is experienced.</t>
</abstract>
</front>
<middle>
<!-- text starts here -->
<section title="Introduction" toc="include">
<t>The TCP congestion window (cwnd) controls the number of packets a TCP
flow may have in the network at any time. A bulk application that always
sends as fast as possible, will continue to grow the cwnd, and increase
its transmission rate until it reaches the maximum permitted by the
receiver and congestion windows. In contrast, a variable-rate
application may experience long periods when the sender is either idle
or 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. Standard TCP does
not control growth of the cwnd when the TCP sender application-limited.
A application-limited sender may therefore grow a cwnd that does not
reflect any current information about the state of the network. Use of
an invalid cwnd may result in reduced application performance or could
significantly contribute to network congestion. These issues were noted
in [RFC2861].</t>
<t>CWV proposed a solution to help reduce the cases where TCP
experienced an invalid cwnd. The use and drawbacks of CWV are discussed
in Section 2.</t>
<t>Section 4 discusses 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 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” application-limited period, while using
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 did not modify the behaviour for an
application-limited session where a sender continues to transmit 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
CWV is able to use the available capacity after an idle period over a
shared path and that this can have benefit, especially over long delay
paths, when compared to slow-start restart specified by standard TCP,
but this behaviour can be too conservative to be attractive to many
common variable-rate applications. Experience from using applications
with CWV suggests that this mechanism does not therefore offer the
desirable increase in application performance for variable rate
applications and it is unclear that applications actually use the
mechanism.</t>
<t>CWV offers a benefit, compared to standard TCP, for an application
that exhibits regular 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 reset
cwnd to the RW. Although CWV benefits the network in an
application-limited scenario, the conservative approach of CWV does not
provide an incentive to application to use it. It is therefore suggested
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 also 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
allows 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
nonvalidation phase. This 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. If a
sender does not take advantage of the preserved cwnd within 6 minutes,
the value of cwnd is updated, ensuring the value then reflects the
capacity that was recently used.</t>
<t>The new method does not differentiate between times when the sender
has become idle or application-limited. It recognises that applications
can result in variable-rate transmission. This therefore reduces the
incentive for an application to send data, simply to keep transport
congestion state. The method requires SACK to be enabled. This allows a
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 proposed TCP update will satisfy the
requirements of many variable-rate applications and at the same time
provide an appropriate method for use in the Internet. This change may
also encourage applications to use standards-based congestion control
methods.</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 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 valid or nonvalidated value. The phases are
defined as:</t>
<t>
<list style="symbols">
<t>Valid 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>Nonvalidated phase: FlightSize <(2/3)*cwnd. This is the
nonvalidated phase, where the cwnd was based on a previous
approximation of the available capacity, and the usage of this
capacity has not been validated in the previous RTT. That is, the
transmission rate is not being constrained by the cwnd. The
methods to be used in this phase 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 is less than one SMSS), then the
sender MUST exit the nonvalidated phase. </t>
<t>If the Retransmission Time Out (RTO) expires during the
nonvalidated phase, the sender MUST exit the nonvalidated phase.
It then resumes using the Standard TCP RTO mechanism [RFC 5861].
(The resulting reduction of cwnd is appropriate, since any
accumulatd path history is considered unreliable).</t>
</list>The threshold value of cwnd required to enter the
nonvalidated 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 at up to the cwnd in size. This is no different to normal
TCP, however 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 nonvalidated phase for a
period greater than six minutes is required to adjust its congestion
control state.</t>
<t>At the end of the nonvalidated 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 applications-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 nonvalidated 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 nonvalidated phase,
i.e., it detects a packet-drop or receives an Explicit Congestion
Notification (ECN), indicates 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 cwnd
does not reflect a validated value, a new cwnd value must be
selected based on the utilised rate.</t>
<t>When congestion is detected, the sender MUST calculate a safe
cwnd:</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:</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 the
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. The period a sender
may safely preserve the cwnd, is a function of the period that a
network path is expected to sustain capacity reflected by cwnd. There
is no perfect choice for this time. The period of 6 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 an
Internet path may commonly be regarded as stable.</t>
<t>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 6 minutes. Examples could include
many 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 6 minutes is expected to be sufficient for most
current applications. Simulation studies also suggest that for most
practical applications, the performance using this value will not be
significantly different to that observed using a non-standard method
that does not reset 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 a algorithm that updates
one aspect of those 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 protocol proposed in this draft.</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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