One document matched: draft-fairhurst-tcpm-newcwv-00.xml
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<rfc category="std" docName="draft-fairhurst-tcpm-newcwv" 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="07" 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
2581, 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>
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<section title="Introduction" toc="include">
<t>TCP's 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. In
contrast, a variable-rate application may experience long periods when
the sender is either idle or application-limited. </t>
<t>Standard TCP 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 an application is data-limited. A data-limited
application 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 [RFC 2861].
</t>
<t>TCP-CWV proposed a solution to help reduce the cases where TCP
experienced an invalid cwnd. The use of TCP-CWV is discussed in Section
2. </t>
<t>Section 4 discusses an alternative to TCP-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 proposal
described applies to both a data 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 algorithms 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. It 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]. Experience from using applications with
TCP-CWV suggests that this mechanism does not offer the desirable
increase in application performance for “bursty”
applications and it is unclear that applications actually use the
mechanism. Analysis (e.g. [Bis10]) has shown that TCP-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. </t>
<t>TCP-CWV offer a benefit, compared to standard TCP, for an application
that exhibits regular idleness. However TCP-CWV would only benefit the
application if the idle period was greater than several RTOs, since the
behaviour would be the same as for standard TCP. Although TCP-CWV
benefits the network in an application-limited scenario, the
conservative approach of TCP-CWV does not provide an incentive to
application to use this. It is therefore suggested that TCP-CWV is often
a poor solution for many variable rate applications. In summary, TCP-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 data 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 called 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 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 connection.</t>
<t>It is expected that the proposed TCP modification 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 serve to encourage application </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 [RFC 3517]. </t>
<t>RFC 5681 define 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 in normal data
transfer. A sender is not required to accurately record this value,
but must be able to measure the volume of data in the network at least
each RTT period. </t>
</section>
<section title="The nonvalidated phase" toc="include">
<t>The updated method creates a new TCP phase that captures where 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 - In this phase the
sender is in the normal phase, where cwnd is an approximate
indication of available capacity currently available along the
network path. </t>
<t>Nonvalidated phase: FlightSize <(2/3)*cwnd - In this phase
the sender is in 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.</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 preserves the cwnd
(i.e., this neither grows nor reduces). 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
data-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 cwnd is less than one SMSS), then the sender
MUST exit the nonvalidated phase. (The conditions for entering and
leaving this phase are intentionally different to introduce
hysteresis, although the change between phases is not impacted to
impact the application.) If the Retransmission Time Out (RTO)
expires during the nonvalidated phase, the sender MUST exit the
nonvalidated phase. It then uses the Standard TCP mechanism (in
this case the path history is considered unreliable). </t>
</list>
</t>
<t />
<section title="Adjustment at the end of the nonvalidated phase "
toc="default">
<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>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>At the end of the nonvalidated phase, the sender MUST update
cwnd:</t>
<t> cwnd = max(FlightSize*2, IW).</t>
<t>Where IW is the TCP initial window.</t>
<t>The sender also MUST reset the ssthresh:</t>
<t> ssthresh = max(ssthresh, 3*cwnd/4).</t>
<t>The adjustment of ssthresh ensures that the sender records that
it has safely sustained the present rate. This 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>If the sender receives congestion feedback while in the
nonvalidated phase, i.e. it detects a packet-drop or receives an
Explicit Congestion Notification (ECN), this indicates that it was
unsafe to start with the preserved cwnd, and TCP is required to
quickly reduce the rate to avoid further congestion.</t>
<t>When loss is detected, the sender MUST calculate a safe cwnd:</t>
<t> cwnd = FlightSize– R.</t>
<t>Where, R is the volume of data reported as unacknowledged by the
SACK information. Following the method proposed for JumpStart
{Liu07].</t>
<t>At the end of the recovery phase, the TCP sender MUST reset the
cwnd:</t>
<t> cwnd = (FlightSize/2).</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 cwnd were preserved for
an arbitrary long period, which was a part of the problem that TCP-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 for 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 TCP-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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