One document matched: draft-fairhurst-tcpm-newcwv-04.txt
Differences from draft-fairhurst-tcpm-newcwv-03.txt
TCPM Working Group G. Fairhurst
Internet-Draft A. Sathiaseelan
Obsoletes: 2861 (if approved) University of Aberdeen
Updates: 5681 (if approved) August 06, 2012
Intended status: Standards Track
Expires: February 7, 2013
Updating TCP to support Application-Limited Traffic
draft-fairhurst-tcpm-newcwv-04
Abstract
This document addresses issues that arise when TCP is used to support
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
application-limited TCP applications, while also providing an
appropriate response if congestion is experienced.
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 proposes an update to
the status of RFC 2861 by recommending it is moved from Experimental
to Historic status, and that it is replaced by the current
specification.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 7, 2013.
Copyright Notice
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Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . 6
4.3. TCP congestion control during the nonvalidated phase . . . 6
4.3.1. Response to congestion in the nonvalidated phase . . . 7
4.3.2. Adjustment at the end of the nonvalidated phase . . . 7
5. Determining a safe period to preserve cwnd . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. Other related work - Author Notes . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
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 rate-limited 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.
Standard TCP [RFC5681] requires the cwnd to be reset to the restart
window (RW) when an application becomes idle. [RFC2861] noted that
this TCP 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 a 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.
[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.
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.
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 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
[RFC5681] for an idle session, intended to improve application
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performance. CWV also modified the behaviour for an application-
limited session where a sender transmitted 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 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 [RFC6298], since the
behaviour would otherwise be the same as for standard TCP, which
resets the cwnd to the RW after this period.
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 rate-limited applications. This mechanism does not therefore
offer the desirable increase in application performance for rate-
limited applications and it is unclear whether applications actually
use this mechanism in the general Internet.
It is therefore concluded that CWV is often a poor solution for many
rate-limited applications. It 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 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 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, see section 5). 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
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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.
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 [Liu07], as
a congestion response after more rapid opening of a TCP connection.
It is expected that this update will satisfy the requirements of many
rate-limited 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").
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 rate-
limited, 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).
The method is specified in following subsections.
4.1. A method for preserving cwnd in idle and application-limited
periods.
The method described in this document updates [RFC5681]. Use of the
method REQUIRES a TCP sender and the corresponding receiver to enable
the TCP SACK option [RFC3517].
[RFC5681] defines a variable FlightSize , that indicates the amount
of outstanding data in the network. This equal to the value of Pipe
calculated based on the pipe algorithm [RFC3517]. In RFC5681 this
value 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.
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4.2. The nonvalidated phase
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:
o Validated phase: FlightSize >=(3/4)*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 to increase cwnd (currently
[RFC5681]).
o Non-validated phase: FlightSize <(1/4)*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.
The values 1/4 and 3/4 were selected to reduce the effects of
variations in the measured FlightSize.
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
(five minutes, as explained in section 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 in bytes is less than one Sender
Maximum Segment Size, SMSS), then the sender MUST exit the non-
validated phase. 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.
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o 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 [RFC3168]), the sender MUST exit the non-
validated phase (reducing the cwnd as defined in section 4.3.1).
o 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 [RFC5681]. (The
resulting reduction of cwnd describe din section 4.3.2 is
appropriate, since any accumulated path history is considered
unreliable).
4.3.1. Response to congestion in the nonvalidated phase
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.
A sender that detects a packet-drop or receives an ECN marked packet
MUST calculate a safe cwnd, by setting it to the value specified in
Section 3.2 of [RFC5681].
At the end of the recovery phase, the TCP sender MUST reset the cwnd
using the method below:
cwnd = ((FlightSize - R)/2).
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].
The inclusion of the term R makes this adjustment is more
conservative than standard TCP. This is required, since the sender
may have sent more segments than Standard TCP would have done.
If the sender implements a method that allows it to identify the
number of ECN-marked segments within a windowthat were observed by
the receiver, the sender SHOULD use the method above, further
reducing R by the number of marked segments.
4.3.2. Adjustment at the end of the nonvalidated phase
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
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method [Hug01].
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 the ssthresh:
sthresh = 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
application-limited flows that encounter occasional congestion, and
could otherwise suffer an unwanted additional delay in recovering the
transmission rate.)
The sender MUST then update cwnd:
cwnd = max(FlightSize*2, IW).
Where IW is the TCP inital window [RFC5681].
(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.)
After completing this adjustment, the sender MAY re-enter the non-
validated phase, if required (see section 4.2).
5. Determining a safe period to preserve cwnd
This section documents the rationale for selecting the maximum period
that cwnd may be preserved.
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.
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.
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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.
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 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.
Finally, other TCP sender mechanisms have used a 5 minute timer, and
there could be simplifications in some implementations by reusing the
same interval. TCP defines a default user timeout of 5 minutes
[RFC0793] i.e. how long transmitted data may remain unacknowledged
before a connection is forcefully closed.
6. Security Considerations
General security considerations concerning TCP congestion control are
discussed in [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.
7. IANA Considerations
There are no IANA considerations.
8. Acknowledgments
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
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Research Group, in particular Yuchung Cheng, Mirja Kuehlewind, and
Joe Touch.
9. Other related work - Author Notes
There are several issues to be discussed more widely:
o Should the method explicitly state a procedure for limiting
burstiness or pacing?
This is often regarded as good practice, but isn't a formal
part of TCP. draft-hughes-restart-00.txt provides some
discussion of this topic.
o There are potential interaction with the proposal to raise the
TCP initial Window to ten segments, do these cases need to be
elaborated?
This relates to draft-ietf-tcpm-initcwnd.
The two methods have different functions and different response
to loss/congestion.
IW=10 proposes an experimental update to TCP that would allow
faster opening of the cwnd, and also a large (same size)
restart window. This approach is based on the assumption that
many forward paths can sustain bursts of up to ten segments
without (appreciable) loss. Such a significant increase in
cwnd must be matched with an equally large reduction of cwnd if
loss/congestion is detected, and such a congestion indication
is likely to require future use of IW=10 to be disabled for
this path for some time. This guards against the unwanted
behaviour of a series of short flows continuously flooding a
network path without network congestion feedback.
In contrast, new-CWV proposes a standards-track update with a
rationale that relies on recent previous path history to select
an appropriate cwnd after restart.
The behaviour differs in three ways:
1) For applications that send little initially, new-cwv may
constrain more than IW=10, but would not require the connection
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to reset any path information when a restart incurred loss. In
contrast, new-cwv would allow the TCP connection to preserve
the cached cwnd, any loss, would impact cwnd, but not impact
other flows.
2) For applications that utilise more capacity than provided by
a cwnd=10, this method would permit a larger restart window
compared to a restart using IW=10. This is justified by the
recent path history.
3) new-CWV is attended to also be used for application-limited
use, where the application sends, but does not seek to fully
utilise the cwnd. In this case, new-cwv constrains the cwnd to
that justified by the recent path history. The performance
trade-offs are hence different, and it would be possible to
enable new-cwv when also using IW=10, and yield the benefits of
this.
o There is potential overlap with the Laminar proposal
(draft-mathis-tcpm-tcp-laminar)
The current draft was intended as a standards-track update to
TCP, rather than a new transport variant. At least, it would
be good to understand how the two interact and whether there is
a possibility of a single method.
10. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[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.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
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Recovery Algorithm for TCP", RFC 3517, April 2003.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
June 2011.
10.2. 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", June 2008.
[Bis10] Biswas, Sathiaseelan, Secchi, and Fairhurst, "Analysing
TCP for Bursty Traffic, Int'l J. of Communications,
Network and System Sciences, 7(3)", June 2010.
[Hug01] Hughes, Touch, and Heidemann, "Issues in TCP Slow-Start
Restart After Idle (Work-in-Progress)", December 2001.
[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", February 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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Arjuna Sathiaseelan
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, Scotland AB24 3UE
UK
Email: arjuna@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
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