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Obsoletes:
Keywords: CWV, TCP
Internet Engineering Task Force (IETF) G. Fairhurst
Request for Comments: 7661 A. Sathiaseelan
Obsoletes: 2861 R. Secchi
Category: Experimental University of Aberdeen
ISSN: 2070-1721 October 2015
Updating TCP to Support Rate-Limited Traffic
Abstract
This document provides a mechanism to address issues that arise when
TCP is used for traffic that exhibits periods where the sending rate
is limited by the application rather than the congestion window. It
provides an experimental update to TCP that allows a TCP sender to
restart quickly following a 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.
This document 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 reclassifies
the status of RFC 2861 from Experimental to Historic. This document
obsoletes RFC 2861.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7661.
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Copyright Notice
Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Implementation of New CWV ..................................5
1.2. Standards Status of This Document ..........................5
2. Reviewing Experience with TCP-CWV ...............................5
3. Terminology .....................................................7
4. A New Congestion Window Validation Method .......................8
4.1. Initialisation .............................................8
4.2. Estimating the Validated Capacity Supported by a Path ......8
4.3. Preserving cwnd during a Rate-Limited Period ..............10
4.4. TCP Congestion Control during the Non-validated Phase .....11
4.4.1. Response to Congestion in the Non-validated Phase ..12
4.4.2. Sender Burst Control during the
Non-validated Phase ................................14
4.4.3. Adjustment at the End of the Non-validated
Period (NVP) .......................................14
4.5. Examples of Implementation ................................15
4.5.1. Implementing the pipeACK Measurement ...............15
4.5.2. Measurement of the NVP and pipeACK Samples .........16
4.5.3. Implementing Detection of the cwnd-Limited
Condition ..........................................17
5. Determining a Safe Period to Preserve cwnd .....................17
6. Security Considerations ........................................18
7. References .....................................................18
7.1. Normative References ......................................18
7.2. Informative References ....................................19
Acknowledgments ...................................................21
Authors' Addresses ................................................21
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1. Introduction
TCP is used for traffic with a range of application behaviours. The
TCP congestion window (cwnd) controls the maximum number of
unacknowledged packets/bytes that a TCP flow may have in the network
at any time, a value known as the FlightSize [RFC5681]. FlightSize
is a measure of the volume of data that is unacknowledged at a
specific 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 advertised window and the sender
congestion window (cwnd). The FlightSize of a bulk flow increases
with the cwnd and tracks the volume of data acknowledged in the last
Round-Trip Time (RTT).
In contrast, a rate-limited application will experience periods when
the sender is either idle or unable to send at the maximum rate
permitted by the cwnd. In this case, the volume of data sent
(FlightSize) can change significantly from one RTT to another and can
be much less than the cwnd. Hence, it is possible that the
FlightSize could significantly exceed the recently used capacity.
The update in this document targets the operation of TCP in such
rate-limited cases.
Standard TCP states that a TCP sender SHOULD set cwnd to no more than
the Restart Window (RW) before beginning transmission if the TCP
sender has not sent data in an interval exceeding the retransmission
timeout, i.e., when an application becomes idle [RFC5681]. [RFC2861]
notes that this TCP behaviour was not always observed in current
implementations. Experiments confirm this to still be the case (see
[Bis08]).
Congestion Window Validation (CWV) [RFC2861] introduced the term
"application-limited period" for the time when the sender sends less
than is allowed by the congestion or receiver windows. [RFC2861]
described a method that improved support for applications that vary
their transmission rate, i.e., applications that either have (short)
idle periods between transmissions or change the rate at which they
send. These applications are characterised by the TCP FlightSize
often being less than the cwnd. Many Internet applications exhibit
this behaviour, including web browsing, HTTP-based adaptive
streaming, applications that support query/response type protocols,
network file sharing, and live video transmission. Many such
applications currently avoid using long-lived (persistent) TCP
connections (e.g., servers that use HTTP/1.1 [RFC7230] typically
support persistent HTTP connections but do not enable this by
default). Instead, such applications often either use a succession
of short TCP transfers or use UDP.
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Standard TCP does not impose additional restrictions on the growth of
the congestion window when a TCP sender is unable to send at the
maximum rate allowed by the cwnd. In this case, the rate-limited
sender may 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 CWV. CWV was intended to help reduce cases where TCP
accumulated an invalid (inappropriately large) cwnd. The use and
drawbacks of using the CWV algorithm described 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 so in a way that is expected to mitigate the
impact on an application that varies its sending rate. The updated
method applies to the rate-limited conditions (including both
application-limited and idle senders).
The goals of this update are:
o To not change the behaviour of a TCP sender that performs bulk
transfers that fully use the cwnd.
o To provide a method that co-exists with standard TCP and other
flows that use this updated method.
o To reduce transfer latency for applications that change their rate
over short intervals of time.
o To avoid a TCP sender growing a large "non-validated" cwnd, when
it has not recently sent using this cwnd.
o To remove the incentive for ad hoc application or network stack
methods (such as "padding") solely to maintain a large cwnd for
future transmission.
o To provide an incentive for the use of long-lived connections
rather than a succession of short-lived flows, benefiting both the
long-lived flows and other flows sharing capacity with these flows
when congestion is encountered.
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Section 5 describes the rationale for selecting the safe period to
preserve the cwnd.
1.1. Implementation of New CWV
The method specified in Section 4 of this document is a sender-side-
only change to the TCP congestion control behaviour of TCP.
The method creates a new protocol state and requires a sender to
determine when the cwnd is validated or non-validated to control the
entry and exit from this state (see Section 4.3). It defines how a
TCP sender manages the growth of the cwnd using the set of rules
defined in Section 4.
Implementation of this specification requires an implementor to
define a method to measure the available capacity using a set of
pipeACK samples. The details of this measurement are implementation-
specific. An example is provided in Section 4.5.1, but other methods
are permitted. A sender also needs to provide a method to determine
when it becomes cwnd-limited. Implementation of this may require
consideration of other TCP methods (see Section 4.5.3).
A sender is also recommended to provide a method that controls the
maximum burst size (see Section 4.4.2). However, implementors are
allowed flexibility in how this method is implemented, and the choice
of an appropriate method is expected to depend on the way in which
the sender stack implements other TCP methods (such as TCP Segment
Offload (TSO)).
1.2. Standards Status of This Document
The document obsoletes the methods described in [RFC2861]. It
recommends a set of mechanisms, including the use of pacing during a
non-validated period. The updated mechanisms are intended to have a
less aggressive congestion impact than would be exhibited by a
standard TCP sender.
The specification in this document is classified as "Experimental"
pending experience with deployed implementations of the methods.
2. Reviewing Experience with TCP-CWV
[RFC2861] 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
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for an idle session [RFC5681], which was intended to improve
application performance. CWV also modified the behaviour when a
sender transmitted at a rate less than allowed by cwnd.
[RFC2861] proposed two sets of responses: one after an "application-
limited period" and one after an "idle period". Although this
distinction was argued, in practice, differentiating the two
conditions was found problematic in actual networks (see, e.g.,
[Bis10]). While this offered predictable performance for long on-off
periods (>>1 RTT) or slowly varying rate-based traffic, the
performance could be unpredictable for variable-rate traffic and
depended both upon whether an accurate RTT had been obtained and the
pattern of application traffic relative to the measured RTT.
Many applications can and often do vary their transmission over a
wide range of rates. Using [RFC2861], such applications often
experienced varying performance, which made it hard for application
developers to predict the TCP latency even when using a path with
stable network characteristics. We argue that an attempt to classify
application behaviour as application-limited or idle is problematic
and also inappropriate. This document therefore explicitly avoids
trying to differentiate these two cases, instead treating all rate-
limited traffic uniformly.
[RFC2861] has been implemented in some mainstream operating systems
as the default behaviour [Bis08]. Analysis (e.g., [Bis10] and
[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 variable-rate 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 Timeout (RTO) intervals
[RFC6298], since the behaviour would otherwise be the same as for
standard TCP, which resets the cwnd to the TCP Restart Window after
this period.
To enable better performance for variable-rate applications with TCP,
some operating systems have chosen to support non-standard methods,
or applications have resorted to "padding" streams by sending dummy
data to maintain their sending rate when they have no data to
transmit. Although transmitting redundant data across a network path
provides good evidence that the path can sustain data at the offered
rate, padding also consumes network capacity and reduces the
opportunity for congestion-free statistical multiplexing. For
variable-rate flows, the benefits of statistical multiplexing can be
significant, and it is therefore a goal to find a viable alternative
to padding streams.
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Experience with [RFC2861] 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.
Therefore, it was concluded that CWV, as defined in [RFC2861], was
often a poor solution for many rate-limited applications. It had the
correct motivation but 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 additional terminology is introduced in this document:
o cwnd-limited: A TCP flow that has sent the maximum number of
segments permitted by the cwnd, where the application utilises the
allowed sending rate (see Section 4.5.3).
o pipeACK sample: A measure of the volume of data acknowledged by
the network within an RTT.
o pipeACK variable: A variable that measures the available capacity
using the set of pipeACK samples (see Section 4.2).
o pipeACK Sampling Period: The maximum period that a measured
pipeACK sample may influence the pipeACK variable.
o Non-validated phase: The phase where the cwnd reflects a previous
measurement of the available path capacity.
o Non-validated period (NVP): The maximum period for which cwnd is
preserved in the non-validated phase.
o Rate-limited: A TCP flow that does not consume more than one half
of cwnd and hence operates in the non-validated phase. This
includes periods when an application is either idle or chooses to
send at a rate less than the maximum permitted by the cwnd.
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o Validated phase: The phase where the cwnd reflects a current
estimate of the available path capacity.
4. A New Congestion Window Validation Method
This section proposes an update to the TCP congestion control
behaviour during a rate-limited interval. This new method
intentionally does not differentiate between times when the sender
has become idle or chooses to send at a rate less than the maximum
allowed by the cwnd.
In the non-validated phase, the capacity used by an application can
be less than that allowed by the TCP cwnd. This update allows an
application to preserve a recently used cwnd while in the non-
validated phase and then 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 non-validated period (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. New CWV reduces this incentive for
an application to send "padding" data simply to keep transport
congestion state.
The method is specified in the following subsections and 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).
4.1. Initialisation
A sender starts a TCP connection in the validated phase and
initialises the pipeACK variable to the "undefined" value. This
value inhibits use of the value in cwnd calculations.
4.2. Estimating the Validated Capacity Supported by a Path
[RFC6675] defines "FlightSize", a variable that indicates the
instantaneous amount of data that has been sent but not cumulatively
acknowledged. In this method, a new variable "pipeACK" is introduced
to measure the acknowledged size of the network pipe. This is used
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to determine if the sender has validated the cwnd. pipeACK differs
from FlightSize in that it is evaluated over a window of acknowledged
data, rather than reflecting the amount of data outstanding.
A sender determines a pipeACK sample 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 [RFC6675], 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. A sender MAY count TCP DupACKs that
acknowledge new data when collecting the pipeACK sample. Other
equivalent methods may be used.
A sender is not required to continuously update the pipeACK variable
after each received ACK but SHOULD perform a pipeACK sample at least
once per RTT when it has sent unacknowledged segments.
The pipeACK variable MAY consider multiple pipeACK samples over the
pipeACK Sampling Period. The value of the pipeACK variable MUST NOT
exceed the maximum (highest value) within the pipeACK Sampling
Period. This specification defines 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 the pipeACK variable to reflect the largest
recently measured pipeACK sample.
When no measurements are available (e.g., a sender that has just
started transmission or immediately after loss recovery), the pipeACK
variable is set to the "undefined value". This value is used to
inhibit entering the non-validated phase until the first new
measurement of a pipeACK sample. (Section 4.5 provides examples of
implementation.)
The pipeACK variable MUST NOT be updated during TCP Fast Recovery.
That is, the sender stops collecting pipeACK samples during loss
recovery. The method RECOMMENDS enabling the TCP SACK option
[RFC2018] and RECOMMENDS the method defined in [RFC6675] to recover
missing segments. 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 more appropriate cwnd
following loss.
Note: The use of pipeACK rather than FlightSize can change the
behaviour of a TCP flow when a sender does not always have data
available to send. One example arises when there is a pause in
transmission after sending a sequence of many packets, and the sender
experiences loss at or near the end of its transmission sequence. In
this case, the TCP flow may have used a significant amount of
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capacity just prior to the loss (which would be reflected in the
volume of data acknowledged, recorded in the pipeACK variable), but
at the actual time of loss, the number of unacknowledged packets in
flight (at the end of the sequence) may be small, i.e., there is a
small FlightSize. After loss recovery, the sender resets its
congestion control state.
[Fai12] explored the benefits of different responses to congestion
for application-limited streams. If the response is based only on
the Loss FlightSize, the sender would assign a small cwnd and
ssthresh, based only on the volume of data sent after the loss. When
the sender next starts to transmit, it can incur many RTTs of delay
in slow-start before it reacquires its previous rate. When the
pipeACK value is also used to calculate the cwnd and ssthresh (as
specified in Section 4.4.1), the sender can use a value that also
reflects the recently used capacity before the loss. This prevents a
variable-rate application from being unduly penalised. When the
sender resumes, it starts at one-half its previous rate, similar to
the behaviour of a bulk TCP flow [Hos15]. To ensure an appropriate
reaction to ongoing congestion, this method requires that the pipeACK
variable is reset after it is used in this way.
4.3. Preserving cwnd during a Rate-Limited Period
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, or pipeACK is undefined
(i.e., at the start or directly after loss recovery). 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, the standard methods are described in [RFC5681]).
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.
Note: A threshold is needed to determine whether a sender is in the
validated or non-validated phase. A standard TCP sender in slow-
start is permitted to double its FlightSize from one RTT to the next.
This motivated the choice of a threshold value of 1/2. This
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threshold ensures a sender does not further increase the cwnd as long
as the FlightSize is less than (1/2*cwnd). Furthermore, a sender
with a FlightSize less than (1/2*cwnd) may, in the next RTT, be
permitted by the cwnd to send at a rate that more than doubles the
FlightSize; hence, this case needs to be regarded as non-validated,
and a sender therefore needs to employ additional mechanisms while in
this phase.
4.4. TCP Congestion Control during the Non-validated Phase
A TCP sender implementing this specification MUST enter the non-
validated phase when the pipeACK is less than (1/2)*cwnd. (The note
at the end of Section 4.4.1 describes why pipeACK<=(1/2)*cwnd is
expected to be a safe value.)
A TCP sender that enters the non-validated phase preserves the cwnd
(i.e., the cwnd only increases after a sender fully uses the cwnd in
this phase; otherwise, the cwnd neither grows nor reduces). The
phase is concluded when the sender transmits sufficient data so that
pipeACK > (1/2)*cwnd (i.e., the sender is no longer rate-limited) or
when the sender receives an indication of congestion.
After a fixed period of time (the non-validated period (NVP)), the
sender adjusts the cwnd (Section 4.4.3). The NVP SHOULD NOT exceed
five minutes. Section 5 discusses the rationale for choosing a safe
value for this period.
The behaviour in the non-validated phase is specified as:
o A sender determines whether to increase the cwnd based upon
whether it is cwnd-limited (see Section 4.5.3):
* A sender that is cwnd-limited MAY use the standard TCP method
to increase cwnd (i.e., the standard method permits a TCP
sender that fully utilises the cwnd to increase the cwnd each
time it receives an ACK).
* A sender that is not cwnd-limited MUST NOT increase the cwnd
when ACK packets are received in this phase (i.e., needs to
avoid growing the cwnd when it has not recently sent using the
current size of cwnd).
o If the sender receives an indication of congestion while in the
non-validated phase (i.e., detects loss), the sender MUST exit the
non-validated phase (reducing the cwnd as defined in
Section 4.4.1).
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o If the Retransmission Timeout (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].
o A sender with a pipeACK variable 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-
validated phase, since it will normally not increase FlightSize to
use the entire cwnd. However, a change to the validated phase
will release the sender from constraints on the growth of cwnd and
result in using the standard congestion response.)
The cwnd-limited behaviour may be triggered during a transient
condition that occurs when a sender is in the non-validated phase and
receives an ACK that acknowledges received data, the cwnd was fully
utilised, and more data is awaiting transmission than may be sent
with the current cwnd. The sender MAY then use the standard method
to increase the cwnd. (Note that if the sender succeeds in sending
these new segments, the updated cwnd and pipeACK variables will
eventually result in a transition to the validated phase.)
4.4.1. Response to Congestion in the Non-validated 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 needs to be
selected based on the utilised rate.
A sender that detects a packet drop MUST record the current
FlightSize in the variable LossFlightSize and MUST calculate a safe
cwnd for loss recovery using the method below:
cwnd = (Max(pipeACK,LossFlightSize))/2.
The pipeACK value is not updated during loss recovery (see
Section 4.2). If there is a valid pipeACK value, the new cwnd is
adjusted to reflect that a non-validated cwnd may be 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.
At the end of the recovery phase, the TCP sender MUST reset the cwnd
using the method below:
cwnd = (Max(pipeACK,LossFlightSize) - R)/2.
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Where R is the volume of data that was successfully retransmitted
during the recovery phase. This corresponds to segments
retransmitted and considered lost by the pipe estimation algorithm at
the end of recovery. It does not include the additional cost of
multiple retransmission of the same data. The loss of segments
indicates that the path capacity was exceeded by at least R; hence,
the calculated cwnd is reduced by at least R before the window is
halved.
The calculated cwnd value MUST NOT be reduced below 1 TCP Maximum
Segment Size (MSS).
After completing the loss recovery phase, the sender MUST
re-initialise the pipeACK variable to the "undefined" value. This
ensures that standard TCP methods are used immediately after
completing loss recovery until a new pipeACK value can be determined.
The ssthresh is adjusted using the standard TCP method (Step 6 in
Section 3.2 of RFC 5681 assigns the ssthresh a value equal to cwnd at
the end of the loss recovery).
Note: The adjustment by reducing cwnd by the volume of data not sent
(R) follows the method proposed for Jump Start [Liu07]. The
inclusion of the term R makes the adjustment more conservative than
standard TCP. This is required, since a sender in the non-validated
phase is allowed a rate higher than a standard TCP sender would have
achieved in the last RTT (i.e., to have more than doubled the number
of segments in flight relative to what was sent in the previous RTT).
The additional reduction after congestion is beneficial when the
LossFlightSize has significantly overshot the available path
capacity, incurring significant loss (e.g., following a change of
path characteristics or when additional traffic has taken a larger
share of the network bottleneck during a period when the sender
transmits less).
Note: The pipeACK value is only valid during a non-validated phase;
therefore, this does not exceed cwnd/2. If LossFlightSize and R were
small, then this can result in the final cwnd after loss recovery
being at most one-quarter of the cwnd on detection of congestion.
This reduction is conservative, and pipeACK is then reset to
undefined; hence, cwnd updates after a congestion event do not depend
upon the pipeACK history before congestion was detected.
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4.4.2. Sender Burst Control during the Non-validated Phase
TCP congestion control allows a sender to accumulate a cwnd that
would allow it to send a burst of segments with a total size up to
the difference between the FlightSize and cwnd. Such bursts can
impact other flows that share a network bottleneck and/or may induce
congestion when buffering is limited.
Various methods have been proposed to control the sender burstiness
[Hug01] [All05]. For example, TCP can limit the number of new
segments it sends per received ACK. This is effective when a flow of
ACKs is received but cannot be used to control a sender that has not
sent appreciable data in the previous RTT [All05].
This document recommends using a method to avoid line-rate bursts
after an idle or rate-limited interval when there is less reliable
information about the capacity of the network path. A TCP sender in
the non-validated phase SHOULD control the maximum burst size, e.g.,
using a rate-based pacing algorithm in which a sender paces out the
cwnd over its estimate of the RTT, or some other method, to prevent
many segments being transmitted contiguously at line-rate. The most
appropriate method(s) to implement pacing depend on the design of the
TCP/IP stack, speed of interface, and whether hardware support (such
as TSO) is used. This document does not recommend any specific
method.
4.4.3. Adjustment at the End of the Non-validated Period (NVP)
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).
Where IW is the appropriate TCP initial window used by the TCP sender
(see, e.g., [RFC5681]).
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Note: These cwnd and ssthresh adjustments cause the sender to enter
slow-start (since ssthresh > cwnd). This adjustment ensures that the
sender responds conservatively after remaining in the non-validated
phase for more than the non-validated period. In this case, it
reduces the cwnd by a factor of two from the preserved value. This
adjustment is helpful when flows accumulate but do not use a large
cwnd; this adjustment seeks to mitigate the impact when these flows
later resume transmission. This could, for instance, mitigate the
impact if multiple high-rate application flows were to become idle
over an extended period of time and then were simultaneously awakened
by an external event.
4.5. Examples of Implementation
This section provides informative examples of implementation methods.
Implementations may choose to use other methods that comply with the
normative requirements.
4.5.1. Implementing the pipeACK Measurement
A pipeACK sample may be measured once each RTT. This reduces the
sender processing burden for calculating after each acknowledgment
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 pipeACK
sample within the pipeACK Sampling Period. One simple way to
implement this is to divide the pipeACK Sampling Period into several
(e.g., five) equal-length measurement periods. The sender then
records the start time for each measurement period and the highest
measured pipeACK sample. At the end of the measurement period, any
measurement(s) that is older than the pipeACK Sampling Period is
discarded. The pipeACK variable is then assigned the largest of the
set of the highest measured values.
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pipeACK sample (Bytes)
^
| +----------+----------+ +----------+---......
| | Sample A | Sample B | No | Sample C | Sample D
| | | | Sample | |
| | |\ 5 | | | |
| | | | | | | /\ 4 |
| | | | | |\ 3 | | | \ |
| | | \ | | \--- | | / \ | /| 2
| |/ \------| - | | / \------/ \...
+//-+----------+---------\+----/ /----+/---------+-------------> Time
<------------------------------------------------|
Sampling Period Current Time
Figure 1: Example of Measuring pipeACK Samples
Figure 1 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 (because no new data segments were
acknowledged). The current value of the pipeACK variable will be 5,
the maximum across all samples. During this period, the pipeACK
samples may be regarded as zero and hence do not contribute to the
calculated pipeACK value.
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.
4.5.2. Measurement of the NVP and pipeACK Samples
The mechanism requires a number of measurements of time. These
measurements could be implemented using protocol timers but do not
necessarily require a new timer to be implemented. Avoiding the use
of dedicated timers can save operating system resources, especially
when there may be large numbers of TCP flows.
The NVP could be measured by recording a timestamp when the sender
enters the non-validated phase. Each time a sender transmits a new
segment, this timestamp can be used to determine if the NVP has
expired. If the measured period exceeds the NVP, the sender can then
take into account how many units of the NVP have passed and make one
reduction (defined in Section 4.4.3) for each NVP.
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Similarly, the time measurements for collecting pipeACK samples and
determining the pipeACK Sampling Period could be derived by using a
timestamp to record when each sample was measured and using this to
calculate how much time has passed when each new ACK is received.
4.5.3. Implementing Detection of the cwnd-Limited Condition
A sender needs to implement a method that detects the cwnd-limited
condition (see Section 4.4). This detects a condition where a sender
in the non-validated phase receives an ACK, but the size of cwnd
prevents sending more new data.
In simple terms, this condition is true only when the FlightSize of a
TCP sender is equal to or larger than the current cwnd. However, an
implementation also needs to consider constraints on the way in which
the cwnd variable can be used; for instance, implementations need to
support other TCP methods such as the Nagle Algorithm and TCP Segment
Offload (TSO) that also use cwnd to control transmission. These
other methods can result in a sender becoming cwnd-limited when the
cwnd is nearly, rather than completely, equal to the FlightSize.
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 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 arbitrarily 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
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
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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 change the network path that is used by a
transport. Although a change of path can in turn disrupt the RTT
measurement and may result in a change of the capacity available to a
TCP connection, we assume these path changes do not usually occur
frequently (compared to 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 from that observed
using a non-standard method that does not reset the cwnd after idle.
Finally, other TCP sender mechanisms have used a five-minute timer,
and there could be simplifications in some implementations by reusing
the same interval. TCP defines a default user timeout of five
minutes [RFC793], which is 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, so the
considerations described in [RFC5681] also apply to this algorithm.
7. References
7.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<http://www.rfc-editor.org/info/rfc2018>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC2861] Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
Window Validation", RFC 2861, DOI 10.17487/RFC2861, June
2000, <http://www.rfc-editor.org/info/rfc2861>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<http://www.rfc-editor.org/info/rfc6298>.
[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP",
RFC 6675, DOI 10.17487/RFC6675, August 2012,
<http://www.rfc-editor.org/info/rfc6675>.
7.2. Informative References
[All05] Allman, M. and E. Blanton, "Notes on Burst Mitigation for
Transport Protocols", ACM SIGCOMM Computer Communication
Review, Volume 35, Issue 2, DOI 10.1145/1064413.1064419,
April 2005.
[Bis08] Biswas, I. and G. Fairhurst, "A Practical Evaluation of
Congestion Window Validation Behaviour", 9th Annual
Postgraduate Symposium in the Convergence of
Telecommunications, Networking and Broadcasting
(PGNet), Liverpool, UK, 2008.
[Bis10] Biswas, I., Sathiaseelan, A., Secchi, R., and G.
Fairhurst, "Analysing TCP for Bursty Traffic", Int'l J. of
Communications, Network and System Sciences,
DOI 10.4236/ijcns.2010.37078, July 2010.
[Bis11] Biswas, I., "Internet Congestion Control for Variable-Rate
TCP Traffic", PhD Thesis, School of Engineering,
University of Aberdeen, 2011.
[Fai12] Sathiaseelan, A., Secchi, R., Fairhurst, G., and I.
Biswas, "Enhancing TCP Performance to support Variable-
Rate Traffic", 2nd Capacity Sharing Workshop, ACM
CoNEXT, Nice, France, December 2012.
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[Hos15] Hossain, Z., "A Study of Mechanisms to Support Variable-
Rate Internet Applications over a Multi-service Satellite
Platform", PhD Thesis, School of Engineering, University
of Aberdeen, January 2015.
[Hug01] Hughes, A., Touch, J., and J. Heidemann, "Issues in TCP
Slow-Start Restart After Idle", Work in Progress,
draft-hughes-restart-00, December 2001.
[Liu07] Liu, D., Allman, M., Jin, S., and L. Wang, "Congestion
Control without a Startup Phase", 5th International
Workshop on Protocols for Fast Long-Distance Networks
(PFLDnet), Los Angeles, California, February 2007.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
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Acknowledgments
This document was produced by the TCP Maintenance and Minor
Extensions (tcpm) working group.
The authors acknowledge the contributions of Dr. I. Biswas and Dr.
Ziaul Hossain in supporting the evaluation of CWV and for their help
in developing the mechanisms proposed in this document. We also
acknowledge comments received from the Internet Congestion Control
Research Group, in particular Yuchung Cheng, Mirja Kuehlewind, Joe
Touch, and Mark Allman. This work was partly funded by the European
Community under its Seventh Framework Programme through the Reducing
Internet Transport Latency (RITE) project (ICT-317700).
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, Scotland AB24 3UE
United Kingdom
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
United Kingdom
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
United Kingdom
Email: raffaello@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
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