TCPM Working Group G. Fairhurst
Internet-Draft I. Biswas
Intended status: Standards Track University of Aberdeen
Expires: September 8, 2011 March 07, 2011
Updating TCP to support Variable-Rate Traffic
draft-fairhurst-tcpm-newcwv-00
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 2581, 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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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 . . . 5
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
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.
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].
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.
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. .
2. Reviewing experience with TCP-CWV
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.
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
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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.
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
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 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.
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,
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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.
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
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 [RFC 3517].
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.
4.2. The nonvalidated phase
The updated method creates a new TCP phase that captures where the
cwnd reflects a valid or nonvalidated value. The phases are defined
as:
o 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.
o 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.
4.3. TCP congestion control during the nonvalidated phase
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
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data-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 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).
4.3.1. Adjustment at the end of the nonvalidated phase
An application that remains in the nonvalidated phase for a period
greater than six minutes is required to adjust its congestion control
state.
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.
At the end of the nonvalidated phase, the sender MUST update cwnd:
cwnd = max(FlightSize*2, IW).
Where IW is the TCP initial window.
The sender also MUST reset the ssthresh:
ssthresh = max(ssthresh, 3*cwnd/4).
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.
The sender MAY re-enter the nonvalidated phase if required (see
section 4.2).
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4.3.2. Response to congestion in the nonvalidated phase
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.
When loss is detected, the sender MUST calculate a safe cwnd:
cwnd = FlightSize- R.
Where, R is the volume of data reported as unacknowledged by the SACK
information. Following the method proposed for JumpStart {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 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.
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.
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
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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 for 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 TCP-CWV and for their help
in developing the protocol proposed in this draft.
8. References
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.
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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
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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