TCPM Working Group G. Fairhurst
Internet-Draft I. Biswas
Intended status: Standards Track University of Aberdeen
Expires: September 12, 2011 March 11, 2011
Updating TCP to support Variable-Rate Traffic
draft-fairhurst-tcpm-newcwv-01
Abstract
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.
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.
Status of this Memo
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Reviewing experience with TCP-CWV . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. An updated TCP response to idle and application-limited
periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. A method for preserving cwnd in idle and
application-limited periods. . . . . . . . . . . . . . . . 5
4.2. The nonvalidated phase . . . . . . . . . . . . . . . . . . 5
4.3. TCP congestion control during the nonvalidated phase . . . 6
4.3.1. Adjustment at the end of the nonvalidated phase . . . . 6
4.3.2. Response to congestion in the nonvalidated phase . . . 7
4.4. Determining a safe period to preserve cwnd . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
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.
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].
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.
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.
2. Reviewing experience with TCP-CWV
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.
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
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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.
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.
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 also assumes familiarity with the terminology of TCP
congestion control [RFC5681].
4. An updated TCP response to idle and application-limited periods
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.
The new method does not differentiate between times when the sender
has become idle or application-limited. It recognises that
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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.
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.
4.1. A method for preserving cwnd in idle and application-limited
periods.
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].
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.
4.2. The nonvalidated phase
The updated method creates a new TCP phase that captures whether the
cwnd reflects a valid or nonvalidated value. The phases are defined
as:
o 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].
o 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.
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4.3. TCP congestion control during the nonvalidated phase
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).
The behaviour in the non-validated phase is specified as:
o 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.
o 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).
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.
4.3.1. Adjustment at the end of the nonvalidated phase
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.
An application that remains in the nonvalidated phase for a period
greater than six minutes is required to adjust its congestion control
state.
At the end of the nonvalidated phase, the sender MUST update cwnd:
cwnd = max(FlightSize*2, IW).
Where IW is the TCP initial window [RFC5681].
(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.)
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The sender also MUST reset 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
applications-limited flows that encounter occasional congestion, and
could otherwise suffer an unwanted additional delay in recovering the
transmission rate.
The sender MAY re-enter the nonvalidated phase, if required (see
section 4.2).
4.3.2. Response to congestion in the nonvalidated phase
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.
When congestion is detected, the sender MUST calculate a safe cwnd:
cwnd = FlightSize? R.
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].
At the end of the recovery phase, the TCP sender MUST reset the cwnd:
cwnd = (FlightSize/2).
4.4. Determining a safe period to preserve cwnd
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.
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
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few minutes.
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.
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.
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.
5. Security Considerations
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.
6. IANA Considerations
None.
7. Acknowledgments
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.
8. References
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8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2861] Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
Window Validation", RFC 2861, June 2000.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, April 2003.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
8.2. Informative References
[Bis08] Biswas and Fairhurst, "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.".
[Bis10] Biswas, Sathiaseelan, Secchi, and Fairhurst, "Analysing
TCP for Bursty Traffic, Int'l J. of Communications,
Network and System Sciences, 7(3), July 2010.".
[Liu07] Liu, Allman, Jiny, and Wang, "Congestion Control without a
Startup Phase, 5th International Workshop on Protocols for
Fast Long-Distance Networks (PFLDnet), Los Angeles,
California, USA, Feb. 2007.".
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, Scotland AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
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Israfil Biswas
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, Scotland AB24 3UE
UK
Email: israfil@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
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