One document matched: draft-fairhurst-tcpm-newcwv-03.xml
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<rfc category="std" docName="draft-fairhurst-tcpm-newcwv-03" ipr="trust200902" obsoletes ="2861" updates="5681" >
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<front>
<title abbrev="">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="Arjuna Sathiaseelan" initials="A."
surname="Sathiaseelan">
<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>arjuna@erg.abdn.ac.uk</email>
<uri>http://www.erg.abdn.ac.uk</uri>
</address>
</author>
<date day="06" month="June" year="2012" />
<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 exhibits 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 application-limited interval. The method is expected to benefit
variable-rate TCP applications, while also providing an appropriate
response if congestion is experienced.</t>
<t>It 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 therefore 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>
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<section title="Introduction" toc="include">
<t>TCP is used to support a range of application behaviours. 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 at which it sends is therefore
limited by the maximum permitted by the receiver and congestion windows.
In contrast, a variable-rate application will experience periods when
the sender is either idle or is unable to send at the maximum rate
permitted by the cwnd. This latter case is called application-limited.
The focus of this document is on the operation of TCP in such an idle or
application-limited case.</t>
<t>Standard TCP <xref
target="RFC5681" /> requires the cwnd to be reset to the restart
window (RW) when an application becomes idle. <xref
target="RFC2861" /> noted that this
TCP behaviour was not always observed in current implementations. Recent
experiments <xref
target="Bis08" /> confirm this to still be the case.</t>
<t>Standard TCP does not control growth of the cwnd when a variable-rate
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 the flow is using. Use of such an
invalid cwnd may result in reduced application performance and/or could
significantly contribute to network congestion.</t>
<t><xref
target="RFC2861" /> proposed a solution to these issues in an experimental
method known as Congestion Window Validation (CWV). CWV was intended to
help reduce cases where TCP accumulated an invalid cwnd. The use and
drawbacks of using CWV with an application 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 to a
“sufficiently-long” idle period. This used the slow-start
threshold (ssthresh) to save information about the previous value of the
congestion window. The approach relaxed the standard TCP behaviour <xref
target="RFC5681" /> for an idle session, intended to improve application
performance. CWV also modified the behaviour for an application-limited
session where a sender transmitted at a rate less than allowed by
cwnd.</t>
<t>RFC 2861 has been implemented in some mainstream operating systems as
the default behaviour <xref target="Bis08" />. Analysis (e.g. <xref
target="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
benefit some applications, especially over long delay paths, when
compared to slow-start restart specified by standard TCP. However, CWV
would only benefit an application if the idle period were less than
several Retransmission Time Out (RTO) intervals <xref
target="RFC2988" />, 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. It 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 <xref target="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 5 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 five minutes, the value of cwnd
is reduced, ensuring the value then reflects the capacity that was
recently actually used.</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, better reflecting the fair-share. A
similar approach was proposed by TCP Jump Start <xref target="Liu07" />,
as a congestion response after more rapid opening of a TCP
connection.</t>
<t>It is expected that this update will satisfy the requirements of many
variable-rate applications and at the same time provide an appropriate
method for use in the Internet. It also reduces the incentive for an
application to send data simply to keep transport congestion state.
(This is sometimes known as "padding").</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. This 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>
<t>The method is specified in following subsections.</t>
<section title="A method for preserving cwnd in idle and application-limited periods."
toc="include">
<t>The method described in this document updates <xref
target="RFC5681" />. Use of the
method REQUIRES a TCP sender and the corresponding receiver to enable
the TCP SACK option <xref target="RFC3517" />.</t>
<t><xref target="RFC5681" /> defines a variable FlightSize , that
indicates the amount of outstanding data in the network. In RFC5681
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 sender 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 expected to be an approximate indication of
the available capacity currently available along the network path,
and the standard methods are used (currently <xref
target="RFC5681" />).</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, when it is not known
whether the cwnd reflects the currently available capacity
available along the network path. The mechanisms 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 is known to induce congestion. These mechanisms are specified
in section 4.3.</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 (five
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 Sender
Maximum Segment Size, 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 <xref target="RFC3168" />), the sender
MUST exit the non-validated phase (reducing the cwnd as defined in
section 4.3.1).</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 <xref
target="RFC5681" />. (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 a change between phases does not
significantly impact an application-limited sender, but serves to
determine its behaviour if it substantially increases its transmission
rate.</t>
<section title="Adjustment at the end of the nonvalidated phase "
toc="default">
<t>During the non-validated phase, the sender may produce bursts of
data of up to the cwnd in size. While this is no different to
standard 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 five minutes is required to adjust its
congestion control state. 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 <xref target="RFC5681" />.</t>
<t>(This allows 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>After completing this adjustment, 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
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 must be
selected based on the utilised rate.</t>
<t>A sender that detects a packet-drop or receives an ECN marked
packet MUST 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 [<xref target="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 - R)/2).]]></artwork>
</figure>
<t />
</section>
</section>
<section title="Determining a safe period to preserve cwnd"
toc="include">
<t>This section documents the rationale for selecting the maximum
period that cwnd may be preserved.</t>
<t>Preserving cwnd 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.
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. </t>
<t>The period of five 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 five minutes. Examples could include
wireless topologies, where TCP rate variations may fluctuate on the
order of a few seconds as a consequence of medium access protocol
instabilities. Mobility changes may also impact TCP 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 five 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>
<t>Finally, other TCP sender mechanisms have used a 5 minute timer,
and there could be simplifications in some implementations by reusing
the same interval.</t>
</section>
</section>
<section title="Security Considerations" toc="include">
<t>General security considerations concerning TCP congestion control are
discussed in <xref target="RFC5681" />. This document describes an
algorithm that updates one aspect of the congestion control procedures,
and so the considerations described in RFC 5681 also apply to this
algorithm.</t>
</section>
<section title="IANA Considerations" toc="include">
<t>There are no IANA considerations.</t>
</section>
<section title="Acknowledgments">
<t>The authors acknowledge the contributions of Dr I Biswas and Dr R
Secchi in supporting the evaluation of CWV and for their help in
developing the mechanisms proposed in this draft. We also acknowledge
comments received from the Internet Congestion Control Research
Group.</t>
<t>
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</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.2988.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3168.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</title>
<author fullname="I." surname="Biswas">
<organization />
</author>
<author fullname="G." surname="Fairhurst">
<organization />
</author>
<date day="01" month="June" year="2008" />
</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</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>
<date day="01" month="February" year="2007" />
</front>
</reference>
<reference anchor="Bis10">
<front>
<title>Analysing TCP for Bursty Traffic, Int'l J. of Communications,
Network and System Sciences, 7(3)</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>
<date day="01" month="June" year="2010" />
</front>
</reference>
</references>
</back>
</rfc>
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