Updating TCP to support Rate-Limited Traffic
draft-ietf-tcpm-newcwv-06
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (tcpm WG) | |
|---|---|---|---|
| Authors | Gorry Fairhurst , Arjuna Sathiaseelan , Raffaello Secchi | ||
| Last updated | 2014-07-05 (Latest revision 2014-03-21) | ||
| Replaces | draft-fairhurst-tcpm-newcwv | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
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| IESG | IESG state | AD is watching | |
| Consensus boilerplate | Unknown | ||
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| Responsible AD | Martin Stiemerling | ||
| Send notices to | tcpm-chairs@tools.ietf.org, draft-ietf-tcpm-newcwv@tools.ietf.org |
draft-ietf-tcpm-newcwv-06
TCPM Working Group G. Fairhurst
Internet-Draft A. Sathiaseelan
Obsoletes: 2861 (if approved) R. Secchi
Updates: 5681 (if approved) University of Aberdeen
Intended status: Experimental March 23, 2014
Expires: September 24, 2014
Updating TCP to support Rate-Limited Traffic
draft-ietf-tcpm-newcwv-06
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 provides an experimental update to TCP that
allows a TCP sender to restart quickly following either 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.
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.
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."
This Internet-Draft will expire on September 24, 2014.
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Copyright Notice
Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Standards Status of this Document . . . . . . . . . . . . 4
2. Reviewing experience with TCP-CWV . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Initialisation . . . . . . . . . . . . . . . . . . . . . 8
4.2. Estimating the validated capacity supported by a path . . 8
4.3. Preserving cwnd during a rate-limited period. . . . . . . 9
4.4. TCP congestion control during the non-validated phase . . 9
4.4.1. Response to congestion in the non-validated phase . . 11
4.4.2. Sender burst control during the non-validated phase . 12
4.4.3. Adjustment at the end of the non-validated phase . . 13
4.5. Examples of Implementation . . . . . . . . . . . . . . . 13
4.5.1. Implementing the pipeACK measurement . . . . . . . . 13
4.5.2. Implementing detection of the cwnd-limited condition 15
5. Determining a safe period to preserve cwnd . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9. Author Notes . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Other related work . . . . . . . . . . . . . . . . . . . 16
9.2. Revision notes . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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
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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. The update in this document targets the
operation of TCP in such rate-limited cases.
Standard TCP [RFC5681] 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.
[RFC2861] noted that this TCP behaviour was not always observed in
current implementations. Experiments [Bis08] confirm this to still
be the case.
CWV introduced the terminology of "application limited periods".
This document describes any time that an application limits the
sending rate, rather than being limited by the transport, as "rate-
limited". This update improves support for applications that vary
their transmission rate, either with (short) idle periods between
transmission or by changing the rate the application sends. These
applications are characterised by the TCP FlightSize often being less
than 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. [RFC2616] servers
typically support persistent HTTP connections, but short server
timeouts often prevent using it). Such applications often instead
either use a succession of short TCP transfers or use UDP.
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 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.
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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 updated
method applies to the rate-limited conditions (including both an
application-limited and idle sender).
The goals of this update are:
o To not change the behaviour of a TCP sender that performs bulk
transfers that consume 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 incentivise the use of long-lived connections, rather than a
succession of short-lived flows, benefiting both flows and network
when actual congestion is encountered.
Section 5 describes the rationale for selecting the safe period to
preserve the cwnd.
1.1. Standards Status of this Document
This document was produced by the TCP Maintenance and Minor
Extensions (tcpm) working group.
The document updates and 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 draft is classified as "Experimental"
pending experience with deployed implementations of the methods.
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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
[RFC5681] for an idle session, intended to improve application
performance. CWV also modified the behaviour where a sender
transmitted at a rate less than allowed by cwnd.
[RFC2861] proposed two set of responses, one after an "application-
limited" and one after an "idle period". Although this distinction
was argued, in practice differentiating the two conditions was found
problematic in actual networks (e.g.[Bis10]). This offers
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 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] [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 Time Out (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 to maintain their
sending rate when they have no data to transmit. Although
transmitting redundant data across a network path provides good
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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.
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.
It is therefore concluded that CWV, as defined in [RFC2861], 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 terminology is used in this document:
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 4.5.2).
pipeACK sample: A measure of the volume of data acknowledged by the
network within an RTT.
pipeACK variable: A variable that measures the available capacity
using the set of pipeACK samples.
pipeACK Sampling Period: The maximum period that a measured pipeACK
sample may influence the pipeACK variable.
Non-validated phase: The phase where the cwnd reflects a previous
measurement of the available path capacity.
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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. This includes
periods when an application is either idle or chooses to send at a
rate less than the maximum permitted by the cwnd.
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.
The period where actual usage is less than allowed by cwnd, is named
as the non-validated phase. The update allows an application in the
non-validated phase 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. Some applications use dummy packets
(aka "padding") to maintain a sending rate when an application has
now data to send. Although this ensures the path continues to
support the rate permitted by the cwnd, it wastes network capacity
sending useless data. New-CWV reduces this incentive for an
application to send data simply to keep transport congestion state.
The method is specified in 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).
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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 a variable, FlightSize, 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
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. 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 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, 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.
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 that the TCP SACK option [RFC2018]
is enabled and the method defined on [RFC6675]is used to recover
missing segments. This allows the sender to more accurately
determine the number of missing bytes during the loss recovery phase,
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and using this method will result in a more appropriate cwnd
following loss.
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.
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]).
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. We start by noting that 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 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, and 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 MUST enter the non-validated phase when the pipeACK is
less than (1/2)*cwnd.
A TCP sender that enters the non-validated phase SHOULD 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 4.4.3) or when
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the sender transmits sufficient data so that pipeACK > (1/2)*cwnd
(i.e. the sender is no longer rate-limited).
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 4.5.2):
o
* A sender that is cwnd-limited MAY use the standard TCP method
to increase cwnd (i.e. a TCP sender that fully utilises the
cwnd is permitted to increase cwnd each received ACK using
standard methods).
* A sender that is not cwnd-limited MUST NOT increase the cwnd
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 4.4.1).
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].
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 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.)
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 is then allowed to use the
standard method to increase the cwnd. (Note, 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.)
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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 must be selected
based on the utilised rate.
A sender that detects a packet-drop, or receives an indication of an
ECN marked packet, 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 recoverySection 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.
Where R is the volume of data that was retransmitted during the
recovery phase.
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.
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.
ssthresh is adjusted using the standard TCP method.
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
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state may increase the rate more than a standard TCP would have done
relative to what was sent in the last RTT (i.e., more than doubled
the number of segments in flight relative to what it sent in the last
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,
and therefore does not exceed cwnd/2. If LossFlightSize and R were
small, then this can result in the final cwnd after loss recovery
being not more than 1/4 of the cwnd on detection of congestion. This
reduction is conservative compared to standard TCP. pipeACK is reset
to undefined after completing loss recovery. Subsequent updates to
cwnd do not therefore reflect pipeACK history before any congestion
event.
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 FlightsSize 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 can not be used to control a sender that has
not send 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 TCP Segment Offload, TSO) is used. The present document does not
recommend any specific method.
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4.4.3. Adjustment at the end of the non-validated phase
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 (e.g. [RFC5681]).
Note: 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, and 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 some 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 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 pipeACK
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sample 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 sample. 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
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. 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.
Note that the pipeACK Sampling Period and the NVP period do 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 4.4.3) for each
NVP period.
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4.5.2. Implementing detection of the cwnd-limited condition
A method is required to detect the cwnd-limited condition (see
Section 4.4. This is used to detect 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 TCP sender's
FlightSize is equal to or larger than the cwnd. However, an
implementation must consider other constraints on the way in which
cwnd variable is used, for instance the need to support methods such
as the Nagle Algorithm and TCP Segment Offload (TSO). This 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 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
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.
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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, Joe
Touch, and Mark Allman. This work was part-funded by the European
Community under its Seventh Framework Programme through the Reducing
Internet Transport Latency (RITE) project (ICT-317700).
9. Author Notes
RFC-Editor note: please remove this section prior to publication.
9.1. Other related work
RFC-Editor note: please remove this section prior to publication.
There are several issues to be discussed more widely:
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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.
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
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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 the sender performs loss recovery it may have an
appreciable pipeACK and cwnd, but a very low FlightSize - the
Standard algorithm results in an unusually low cwnd ((1/2)*
FlightSize).
A constant rate flow would have maintained a FlightSize
appropriate to pipeACK (cwnd if it is a bulk flow).
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?
We propose that TCB sharing should use the pipeACK in place of
cwnd when a TCP sender is in the Non-validated phase. This
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value better reflects the capacity that the flow has utilised
in the network path.
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.
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.
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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
o 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.
WG draft 02 contained:
o Clarified language around pipeACK variable and pipeACK sample -
Feedback from Aris Angelogiannopoulos.
WG draft 03 contained:
o Editorial corrections - Feedback from Anna Brunstrom.
o An adjustment to the procedure at the start and end of Reoloss
recovery to align the two equations.
o Further clarification of the "undefined" value of the pipeACK
variable.
WG draft 04 contained:
o Editorial corrections.
o Introduced the "cwnd-limited" term.
o An adjustment to the procedure at the start of a cwnd-limited
phase - the new text is intended to ensure that new-cwv is not
unnecessarily more conservative than standard TCP when the flow is
cwnd-limited. This resolves two issues: first it prevents
pathologies in which pipeACK increases slowly and erratically. It
also ensures that performance of bulk applications is not
significantly impacted when using the method.
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o Clearly identifies that pacing (or equivalent) is requiring during
the NVP to control burstiness. New section added.
WG draft 05 contained:
o Clarification to first two bullets in Section 4.4 describing cwnd-
limited, to explain these are really alternates to the same case.
o Section giving implementation examples was restructured to clarify
there are two methods described.
o Cross References to sections updated - thanks to comments from
Martin Winbjoerk and Tim Wicinski.
WG draft 06 contained:
o The section giving implementation examples was restructured to
clarify there are two methods described.
o Justification of design decisions.
o Re-organised text to improve clarity of argument.
10. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[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.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
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[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, August 2012.
10.2. Informative References
[All05] Allman, M. and E. Blanton, "Notes on burst mitigation for
transport protocols", March 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", June 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, 7(3)", June
2010.
[Bis11] Biswas, I., "PhD Thesis, Internet congestion control for
variable rate TCP traffic, School of Engineering,
University of Aberdeen", June 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, 10th December 2012.", June 2008.
[Hug01] Hughes, A., Touch, J., and J. Heidemann, "Issues in TCP
Slow-Start Restart After Idle (Work-in-Progress)",
December 2001.
[Liu07] Liu, D., Allman, M., Jiny, S., and L. Wang, "Congestion
Control without a Startup Phase, 5th International
Workshop on Protocols for Fast Long-Distance Networks
(PFLDnet), Los Angeles, California, USA", February 2007.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, June
2011.
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[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928, April 2013.
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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