Updating TCP to support Rate-Limited Traffic
draft-ietf-tcpm-newcwv-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7661.
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|---|---|---|---|
| Authors | Gorry Fairhurst , Arjuna Sathiaseelan , Raffaello Secchi | ||
| Last updated | 2013-06-19 | ||
| Replaces | draft-fairhurst-tcpm-newcwv | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-tcpm-newcwv-01
TCPM Working Group G. Fairhurst
Internet-Draft A. Sathiaseelan
Obsoletes: 2861 (if approved) R. Secchi
Updates: 5681 (if approved) University of Aberdeen
Intended status: Standards Track June 20, 2013
Expires: December 22, 2013
Updating TCP to support Rate-Limited Traffic
draft-ietf-tcpm-newcwv-01
Abstract
This document proposes an update to RFC 5681 to address issues that
arise when TCP is used to support traffic that exhibits periods where
the sending 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 rate-limited interval. This
method is expected to benefit applications that send rate-limited
traffic using TCP, while also providing an appropriate response if
congestion is experienced.
It also evaluates the Experimental specification of TCP Congestion
Window Validation, CWV, defined in RFC 2861, and concludes that RFC
2861 sought to address important issues, but failed to deliver a
widely used solution. This document therefore recommends that the
status of RFC 2861 is moved from Experimental to Historic, and that
it is replaced by the current specification.
NOTE: The standards status of this WG document is under review for
consideration as either Experimental (EXP) or Proposed Standard (PS).
This decision will be made later as the document is finalised.
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
working documents as Internet-Drafts. The list of current Internet-
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."
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This Internet-Draft will expire on December 22, 2013.
Copyright Notice
Copyright (c) 2013 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
Provisions Relating to IETF Documents
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Reviewing experience with TCP-CWV . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. An updated TCP response to idle and application-limited
periods . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. A method for preserving cwnd during the idle and
application-limited periods. . . . . . . . . . . . . . . . 7
4.2. Initialisation . . . . . . . . . . . . . . . . . . . . . . 8
4.3. The nonvalidated phase . . . . . . . . . . . . . . . . . . 8
4.4. TCP congestion control during the nonvalidated phase . . . 8
4.4.1. Response to congestion in the nonvalidated phase . . . 9
4.4.2. Adjustment at the end of the nonvalidated phase . . . 10
4.4.3. Examples of Implementation . . . . . . . . . . . . . . 11
5. Determining a safe period to preserve cwnd . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
9. Author Notes . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Other related work . . . . . . . . . . . . . . . . . . . . 14
9.2. Revision notes . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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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 unacknowledged
packets/bytes that a TCP flow may have in the network at any time, a
value known as the FlightSize [RFC5681]. 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
advertised window and the sender congestion window (cwnd). 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 rate-limited. The
focus of this document is on the operation of TCP in such an idle or
rate-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 impose additional restrictions on the growth of
the cwnd when a TCP sender is rate-limited. A rate-limited sender
may therefore grow a cwnd far 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 the CWV algorithm in RFC 2861 with an
application are discussed in Section 2.
Section 3 defines relevant terminology.
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 sending rate. The method
described applies to both a rate-limited and an idle condition.
Section 5 describes the rationale for selecting the safe period to
preserve the cwnd.
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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
performance. CWV also modified the behaviour for a rate-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] [Fai12]) 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 the
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 RTCP Restart Window (RW) after this period.
Experience with RFC 2861 suggests that although the CWV method
benefited the network in a rate-limited scenario (reducing the
probability of network congestion), the behaviour was too
conservative for many common rate-limited applications. This
mechanism did 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, as defined in RFC2681, was often
a poor solution for many rate-limited applications. It had the
correct motivation, but had 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].
The following new terminology is introduced:
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pipeACK: A variable that records the volume of data acknowledged by
the network within an RTT.
pipeACK Sampling Period: The maximum period that a measured sample of
the pipeACK may influence the pipeACK variable.
Non-validated phase: The phase where the cwnd reflects a previous
measurement of the available path capacity.
Non-validated period, NVP: The maximum period for which cwnd is
preserved in the non-validated phase.
Rate-limited: A TCP flow that does not consume more than one half of
cwnd, and hence operates in the non-validated phase.
Validated phase: The phase where the cwnd reflects a current estimate
of the available path capacity.
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 rate-limited period. The new method
permits a TCP sender to preserve the cwnd when an application becomes
idle for a period of time (the non-validated period, NVP, see section
5). The period where actual usage is less than allowed by cwnd, is
named as the non-validated phase. This 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 the NVP, the
value of cwnd is reduced, ensuring the value better reflects the
capacity that was recently actually used.
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 rate-limited. This is partly a response to
recognition that some applications wish to transmit at a rate less
than allowed by the sender cwnd, and that it can be hard to make a
distinction between rate-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
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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 during the idle and application-
limited periods.
[RFC5681] defines a variable, FlightSize, that indicates the amount
of outstanding data in the network. This is assumed to be 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 a new variable "pipeACK" is introduced to measure the
acknowledged size of the pipe, which is used to determine if the
sender has validated the cwnd.
A sender determines a value for pipeACK by measuring the volume of
data that was acknowledged by the network over the period of a
measured Round Trip Time (RTT). Using the variables defined in
[RFC3517], a value could be measured by caching the value of HighACK
and after one RTT measuring the difference between the cached HighACK
value and the current HighACK value. Other equivalent methods may be
used.
A sender is not required to continuously update the pipeACK variable
after each received ACK, but MUST make a measurement at least once
per RTT when it has sent unacknowledged segments. The pipeACK value
used by the algorithm MAY consider multiple pipeACK measurements over
the pipeACK Sampling Period. The calculated pipeACK value MUST NOT
exceed the maximum (highest value) within the sampling period. This
specification degiones the pipeACK Sampling Period as Max(3*RTT, 1
second). This period enables a sender to compensate for large
fluctuations in the sending rate, where there may be pauses in
transmission, and allows pipeACK to reflect the largest recently
measured size of "pipeACK".
When no measurements are available, the pipeACK variable is set to
the maximum (undefined) value. This value is used to inhibit
entering the nonvalidated phase until the first measurement of
pipeACK completes.
The method RECOMMENDS that the TCP SACK option [RFC3517] is enabled.
This allows the sender to more accurately determine the number of
missing bytes during the loss recovery phase, and using this method
will result in a higher cwnd following loss.
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4.2. Initialisation
A sender starts a TCP connection in the Validated phase and
initialises the pipeACK variable to the maximum (undefined) value.
4.3. 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: pipeACK >=(1/2)*cwnd. This is the normal phase,
where cwnd is expected to be an approximate indication of the
capacity currently available along the network path, and the
standard methods are used to increase cwnd (currently [RFC5681]).
The rule for transitioning to the non-validated phase is specified
in section 4.3.
o Non-validated phase: pipeACK <(1/2)*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 pipeACK Sampling Period. That is, when it is not
known whether the cwnd reflects the currently available capacity
along the network path. The mechanisms to be used in this phase
seek to determine a safe value for cwnd and an appropriate
reaction to congestion. These mechanisms are specified in section
4.3.
The value 1/2 was selected to reduce the effects of variations in the
measured pipeACK, and to allow the sender some flexibility in when it
sends data.
4.4. TCP congestion control during the nonvalidated phase
A TCP sender MUST enter the non-validated phase when the measured
pipeACK is less than (1/2)*cwnd.
A TCP sender that enters the non-validated phase will preserve the
cwnd (i.e., this neither grows nor reduces while the sender remains
in this phase). If the sender receives an indication of congestion
(loss or Explicit Congestion Notification, ECN, mark [RFC3168]) it
uses the method described below. The phase is concluded after a
fixed period of time (the NVP, as explained in section 4.3.2) or when
the sender transmits sufficient data so that pipeACK > (1/2)*cwnd
(i.e. it is no longer rate-limited).
The behaviour in the non-validated phase is specified as:
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o The cwnd is not increased when ACK packets are received in this
phase.
o If the sender receives an indication of congestion while in the
non-validated phase (i.e. detects loss, or an ECN mark), 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 described in section 4.3.2 is
appropriate, since any accumulated path history is considered
unreliable).
o A sender that measures a pipeACK greater than (1/2)*cwnd SHOULD
enter the validated phase. (A rate-limited sender will not
normally be impacted by whether it is in a validated or non-
validate phase, since it will normally not consume the entire
cwnd. However a change to the validated phase will release the
sender from constraints on the growth of cwnd, and restore the use
of the standard congestion response.)
4.4.1. 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 record the current FlightSize in the variable LossFlightSize and
calculate a safe cwnd, by setting it to the value specified in
Section 3.2 of [RFC5681].
A TCP sender MUST calculate a safe cwnd to use for loss recovery
using the method below:
cwnd = Min(cwnd/2,Max(pipeACK,LossFlightSize)).
This new cwnd is set to reflect that a nonvalidated cwnd may be much
larger than the actual flightsize, or recently used flightsize
(recorded in pipeACK). The updated cwnd therefore prevents overshoot
by a sender significantly increasing its transmission rate during the
recovery period.
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At the end of the recovery phase, the TCP sender MUST reset the cwnd
using the method below:
cwnd = ((LossFlightSize - R)/2).
Where, R is the volume of data that was retransmitted during the
recovery phase. This follows the method proposed for Jump Start
[Liu07]. The inclusion of the term R makes this adjustment more
conservative than standard TCP. (This is required, since the sender
may have sent more segments than a Standard TCP sender would have
done. The additional reduction is beneficial when the LossFlightSize
significantly overshoots the available path capacity incurring
significant loss, for instance an intense traffic burst following a
non-validated period.)
If the sender implements a method that allows it to identify the
number of ECN-marked segments within a window that were observed by
the receiver, the sender SHOULD use the method above, further
reducing R by the number of marked segments.
The sender MUST also re-initialise the pipeACK variable to the
maximum (undefined) value. This ensures that standard TCP methods
are used immediately after completing loss recovery.
4.4.2. Adjustment at the end of the nonvalidated phase
During the non-validated phase, a sender can 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 [Hug01].
An application that remains in the non-validated phase for a period
greater than the NVP is required to adjust its congestion control
state. If the sender exits the non-validated phase after this
period, it MUST update 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
rate-limited flows that encounter occasional congestion, and could
otherwise suffer an unwanted additional delay in recovering the
sending rate.)
The sender MUST then update cwnd to be not greater than:
cwnd = max(1/2*cwnd, IW).
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Where IW is the appropriate TCP initial window, used by the TCP
sender (e.g. [RFC5681]).
(This adjustment ensures that sender responds conservatively at the
end of the non-validated phase by reducing the cwnd to better reflect
the current rate of the sender. The cwnd update does not take into
account FlightSize or pipeACK because these values only reflect data
during the last RTT and do not reflect the average or maximum sending
rate.)
4.4.3. Examples of Implementation
This section is intended to provide informative examples of
implementation methods. Implementations may choose to use other
methods that comply with the normative requirements.
XXX This section is work in progress - discussion is welcome to help
complete this section XXX
The pipeACK value may be sampled once each RTT. This reduces the
sender processing burden for calculating after each acknowledgement
and also reduces storage requirements at the sender.
Since application behaviour can be bursty using CWV, it may be
desirable to implement a maximum filter to accumulate the measured
values so that the pipeACK variable records the largest value within
the pipeACK Sampling Period. One simple way to implement this is to
divide the pipeACK Sampling Period into several (e.g. 5) equal length
measurement periods. The sender then records the start time for each
measurement period and the highest measured pipeACK value. At the
end of the measurement period, any measurement(s) that are older than
the pipeACK Sampling Period are discarded. The pipeACK variable is
then assigned the largest of the set of the highest measured values.
+----------+----------+ +----------+---......
| Sample A | Sample B | No | Sample C | Sample D
| | | Sample | |
| |\ 5 | | | |
| | | | | | /\ 4 |
| | | | |\ 3 | | | \ |
| | \ | | \--- | | / \ | /| 2
|/ \------| - | | / \------/ \...
+----------+---------\---/ /-----//--------+-------------> Time
<------------------------------------------------|
Sampling Period Current Time
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Figure XX: Example of sampling pipeACK values
Figure XX shows an example of how measurement samples may be
collected. At the time represented by the figure new samples are
being accumulated into sample D. Three previous samples also fall
within the pipeACK Sampling Period: A, B, and C. There was also a
period of inactivity between samples B and C during which no
measurements were taken. The current value of the pipeACK variable
will be 5, the maximum across all samples.
After one further measurement period, Sample A will be discarded,
since it then is older than the pipeACK Sampling Period and the
pipeACK variable will be recalculated, Its value will be the larger
of Sample C or the final value accumulated in Sample D.
The NVP period does not necessarily require a new timer to be
implemented. An alternative is to record a timestamp when the sender
enters the NVP. Each time a sender transmits a new segment, this
timestamp may be used to determine if the NVP period has expired. If
the period expires, the sender may take into account how many units
of the NVP period have passed and make one reduction (as defined in
section 4.3.2) for each NVP period.
5. Determining a safe period to preserve cwnd
This section documents the rationale for selecting the maximum period
that cwnd may be preserved, known as the non-validated period, NVP.
Limiting the period that cwnd may be preserved avoids undesirable
side effects that would result if the cwnd were to be kept
unnecessarily high 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.
A period of five minutes was chosen for this NVP. This is 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.
There are cases where the TCP throughput exhibits significant
variability over a time less than five minutes. Examples could
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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 (e.g. [Bis11]) 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, Mr Ziaul
Hossain 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, in particular Yuchung Cheng, Mirja Kuehlewind, and
Joe Touch. trhis work was part-funded by the European Community under
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its Seventh Framework Programme through the Reducing Internet
Transport Latency (RITE) project (ICT-317700).
9. Author Notes
9.1. Other related work
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 is not presently a
formal part of TCP. draft-hughes-restart-00.txt provides some
discussion of this topic.
o There are potential interactions with the Experimental update in
[RFC6928] that raises the TCP initial Window to ten segments, do
these cases need to be elaborated?
This relates to the Experimental specification for increasing
the TCP IW defined in RFC 6928.
The two methods have different functions and different response
to loss/congestion.
RFC 6928 proposes an experimental update to TCP that would
increase the IW to ten segments. This 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, this document proposes an update with a rationale
that relies on recent previous path history to select an
appropriate cwnd after restart.
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The behaviour differs in three ways:
1) For applications that send little initially, new-cwv may
constrain more than RFC 6928, but would not require the
connection 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 of 10 segments, this method would permit a larger
restart window compared to a restart using the method in RFC
6928. This is justified by the recent path history.
3) new-CWV is attended to also be used for rate-limited
applications, 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 the method in RFC
6928, and yield benefits.
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.
o There is potential performance loss in loss of a short burst
(off list with M Allman)
A sender can transmit several segments then become idle. If
the first segments are all ACK'ed the ssthresh collapses to a
small value (no new data is sent by the idle sender). Loss of
the later data results in congestion (e.g. maybe a RED drop or
some other cause, rather than the maximum rate of this flow).
When performs loss recovery it may have an appreciable pipeACK
and cwnd, but a very low flight size - the Standard algorithm
results in an unusually low cwnd (1/2 Flight size).
A constant rate flow would have maintained a flight size
appropriate to pipeACK (cwnd if it is a bulk flow).
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This could be fixed by adding a new state variable? It could
also be argued this is a corner case (e.g. loss of only the
last segments would have resulted in RTO), the impact could be
significant.
o There is potential interaction with TCP Control Block Sharing(M
Welzl)
An application that is non-validated can accumulate a cwnd that
is larger than the actual capacity. Is this a fair value to
use in TCB sharing?
9.2. Revision notes
RFC-Editor note: please remove this section prior to publication.
Draft 03 was submitted to ICCRG to receive comments and feedback.
Draft 04 contained the first set of clarifications after feedback:
o Changed name to application limited and used the term rate-limited
in all places.
o Added justification and many minor changes suggested on the list.
o Added text to tie-in with more accurate ECN marking.
o Added ref to Hug01
Draft 05 contained various updates:
o New text to redefine how to measure the acknowledged pipe,
differentiating this from the FlightSize, and hence avoiding
previous issues with infrequent large bursts of data not being
validated. A key point new feature is that pipeACK only triggers
leaving the NVP after the size of the pipe has been acknowledged.
This removed the need for hysteresis.
o Reduction values were changed to 1/2, following analysis of
suggestions from ICCRG. This also sets the "target" cwnd as twice
the used rate for non-validated case.
o Introduced a symbolic name (NVP) to denote the 5 minute period.
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Draft 06 contained various updates:
o Required reset of pipeACK after congestion.
o Added comment on the effect of congestion after a short burst (M.
Allman).
o Correction of minor Typos.
WG draft 00 contained various updates:
o Updated initialisation of pipeACK to maximum value.
o Added note on intended status still to be determined.
WG draft 01 contained:
o Added corrections from Richard Scheffenegger.
o Raffaello Secchi added to the mechanism, based on implementation
experience.
o Removed that the requirement for the method to use TCP SACK option
[RFC3517] to be enabled - Although it may be desirable to use
SACK, this is not essential to the algorithm.
o Added the notion of the sampling period to accommodate large rate
variations and ensure that the method is stable. This algorithm
to be validated through implementation.
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.
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[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.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
June 2011.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928, April 2013.
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.
[Bis11] Biswas, "PhD Thesis, Internet congestion control for
variable rate TCP traffic, School of Engineering,
University of Aberdeen", June 2011.
[Fai12] Fairhurst, Biswas, Biswas, and Biswas, "Enhancing TCP
Performance to support Variable-Rate Traffic, 2nd Capacity
Sharing Workshop, ACM CoNEXT, Nice, France, 10th December
2012.", June 2008.
[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.
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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
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
Raffaello Secchi
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, Scotland AB24 3UE
UK
Email: raffaello@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
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