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IETF RFC 5681
TCP Congestion Control
Last modified on Saturday, September 5th, 2009
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Network Working Group M. Allman
Request for Comments: 5681 V. Paxson
Obsoletes: 2581 ICSI
Category: Standards Track E. Blanton
Purdue University
September 2009
TCP Congestion Control
 Abstract
This document defines TCP's four intertwined congestion control
algorithms: slow start, congestion avoidance, fast retransmit, and
fast recovery. In addition, the document specifies how TCP should
begin transmission after a relatively long idle period, as well as
discussing various acknowledgment generation methods. This document
obsoletes RFC 2581.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
Allman, et al. Standards Track PAGE 1 
RFC 5681 TCP Congestion Control September 2009
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table Of Contents
1. Introduction ....................................................2
2. Definitions .....................................................3
3. Congestion Control Algorithms ...................................4
3.1. Slow Start and Congestion Avoidance ........................4
3.2. Fast Retransmit/Fast Recovery ..............................8
4. Additional Considerations ......................................10
4.1. Restarting Idle Connections ...............................10
4.2. Generating Acknowledgments ................................11
4.3. Loss Recovery Mechanisms ..................................12
5. Security Considerations ........................................13
6. Changes between RFC 2001 and RFC 2581 ..........................13
7. Changes Relative to RFC 2581 ...................................14
8. Acknowledgments ................................................15
9. References .....................................................15
9.1. Normative References ......................................15
9.2. Informative References ....................................16
1. Introduction
This document specifies four TCP [RFC 793] congestion control
algorithms: slow start, congestion avoidance, fast retransmit and
fast recovery. These algorithms were devised in [Jac88] and [Jac90].
Their use with TCP is standardized in [RFC 1122]. Additional early
work in additive-increase, multiplicative-decrease congestion control
is given in [CJ89].
Note that [Ste94] provides examples of these algorithms in action and
[WS95] provides an explanation of the source code for the BSD
implementation of these algorithms.
In addition to specifying these congestion control algorithms, this
document specifies what TCP connections should do after a relatively
long idle period, as well as specifying and clarifying some of the
issues pertaining to TCP ACK generation.
This document obsoletes [RFC 2581], which in turn obsoleted [RFC 2001].
This document is organized as follows. Section 2 provides various
definitions that will be used throughout the document. Section 3
provides a specification of the congestion control algorithms.
Section 4 outlines concerns related to the congestion control
algorithms and finally, section 5 outlines security considerations.
Allman, et al. Standards Track PAGE 2
RFC 5681 TCP Congestion Control September 2009
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 [RFC 2119].
2. Definitions
This section provides the definition of several terms that will be
used throughout the remainder of this document.
SEGMENT: A segment is ANY TCP/IP data or acknowledgment packet (or
both).
SENDER MAXIMUM SEGMENT SIZE (SMSS): The SMSS is the size of the
largest segment that the sender can transmit. This value can be
based on the maximum transmission unit of the network, the path
MTU discovery [RFC 1191, RFC 4821] algorithm, RMSS (see next item),
or other factors. The size does not include the TCP/IP headers
and options.
RECEIVER MAXIMUM SEGMENT SIZE (RMSS): The RMSS is the size of the
largest segment the receiver is willing to accept. This is the
value specified in the MSS option sent by the receiver during
connection startup. Or, if the MSS option is not used, it is 536
bytes [RFC 1122]. The size does not include the TCP/IP headers and
options.
FULL-SIZED SEGMENT: A segment that contains the maximum number of
data bytes permitted (i.e., a segment containing SMSS bytes of
data).
RECEIVER WINDOW (rwnd): The most recently advertised receiver window.
CONGESTION WINDOW (cwnd): A TCP state variable that limits the amount
of data a TCP can send. At any given time, a TCP MUST NOT send
data with a sequence number higher than the sum of the highest
acknowledged sequence number and the minimum of cwnd and rwnd.
INITIAL WINDOW (IW): The initial window is the size of the sender's
congestion window after the three-way handshake is completed.
LOSS WINDOW (LW): The loss window is the size of the congestion
window after a TCP sender detects loss using its retransmission
timer.
RESTART WINDOW (RW): The restart window is the size of the congestion
window after a TCP restarts transmission after an idle period (if
the slow start algorithm is used; see section 4.1 for more
discussion).
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RFC 5681 TCP Congestion Control September 2009
FLIGHT SIZE: The amount of data that has been sent but not yet
cumulatively acknowledged.
DUPLICATE ACKNOWLEDGMENT: An acknowledgment is considered a
"duplicate" in the following algorithms when (a) the receiver of
the ACK has outstanding data, (b) the incoming acknowledgment
carries no data, (c) the SYN and FIN bits are both off, (d) the
acknowledgment number is equal to the greatest acknowledgment
received on the given connection (TCP.UNA from [RFC 793]) and (e)
the advertised window in the incoming acknowledgment equals the
advertised window in the last incoming acknowledgment.
Alternatively, a TCP that utilizes selective acknowledgments
(SACKs) [RFC 2018, RFC 2883] can leverage the SACK information to
determine when an incoming ACK is a "duplicate" (e.g., if the ACK
contains previously unknown SACK information).
3. Congestion Control Algorithms
This section defines the four congestion control algorithms: slow
start, congestion avoidance, fast retransmit, and fast recovery,
developed in [Jac88] and [Jac90]. In some situations, it may be
beneficial for a TCP sender to be more conservative than the
algorithms allow; however, a TCP MUST NOT be more aggressive than the
following algorithms allow (that is, MUST NOT send data when the
value of cwnd computed by the following algorithms would not allow
the data to be sent).
Also, note that the algorithms specified in this document work in
terms of using loss as the signal of congestion. Explicit Congestion
Notification (ECN) could also be used as specified in [RFC 3168].
3.1. Slow Start and Congestion Avoidance
The slow start and congestion avoidance algorithms MUST be used by a
TCP sender to control the amount of outstanding data being injected
into the network. To implement these algorithms, two variables are
added to the TCP per-connection state. The congestion window (cwnd)
is a sender-side limit on the amount of data the sender can transmit
into the network before receiving an acknowledgment (ACK), while the
receiver's advertised window (rwnd) is a receiver-side limit on the
amount of outstanding data. The minimum of cwnd and rwnd governs
data transmission.
Another state variable, the slow start threshold (ssthresh), is used
to determine whether the slow start or congestion avoidance algorithm
is used to control data transmission, as discussed below.
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RFC 5681 TCP Congestion Control September 2009
Beginning transmission into a network with unknown conditions
requires TCP to slowly probe the network to determine the available
capacity, in order to avoid congesting the network with an
inappropriately large burst of data. The slow start algorithm is
used for this purpose at the beginning of a transfer, or after
repairing loss detected by the retransmission timer. Slow start
additionally serves to start the "ACK clock" used by the TCP sender
to release data into the network in the slow start, congestion
avoidance, and loss recovery algorithms.
IW, the initial value of cwnd, MUST be set using the following
guidelines as an upper bound.
If SMSS > 2190 bytes:
IW = 2 * SMSS bytes and MUST NOT be more than 2 segments
If (SMSS > 1095 bytes) and (SMSS <= 2190 bytes):
IW = 3 * SMSS bytes and MUST NOT be more than 3 segments
if SMSS <= 1095 bytes:
IW = 4 * SMSS bytes and MUST NOT be more than 4 segments
As specified in [RFC 3390], the SYN/ACK and the acknowledgment of the
SYN/ACK MUST NOT increase the size of the congestion window.
Further, if the SYN or SYN/ACK is lost, the initial window used by a
sender after a correctly transmitted SYN MUST be one segment
consisting of at most SMSS bytes.
A detailed rationale and discussion of the IW setting is provided in
[RFC 3390].
When initial congestion windows of more than one segment are
implemented along with Path MTU Discovery [RFC 1191], and the MSS
being used is found to be too large, the congestion window cwnd
SHOULD be reduced to prevent large bursts of smaller segments.
Specifically, cwnd SHOULD be reduced by the ratio of the old segment
size to the new segment size.
The initial value of ssthresh SHOULD be set arbitrarily high (e.g.,
to the size of the largest possible advertised window), but ssthresh
MUST be reduced in response to congestion. Setting ssthresh as high
as possible allows the network conditions, rather than some arbitrary
host limit, to dictate the sending rate. In cases where the end
systems have a solid understanding of the network path, more
carefully setting the initial ssthresh value may have merit (e.g.,
such that the end host does not create congestion along the path).
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The slow start algorithm is used when cwnd < ssthresh, while the
congestion avoidance algorithm is used when cwnd > ssthresh. When
cwnd and ssthresh are equal, the sender may use either slow start or
congestion avoidance.
During slow start, a TCP increments cwnd by at most SMSS bytes for
each ACK received that cumulatively acknowledges new data. Slow
start ends when cwnd exceeds ssthresh (or, optionally, when it
reaches it, as noted above) or when congestion is observed. While
traditionally TCP implementations have increased cwnd by precisely
SMSS bytes upon receipt of an ACK covering new data, we RECOMMEND
that TCP implementations increase cwnd, per:
cwnd += min (N, SMSS) (2)
where N is the number of previously unacknowledged bytes acknowledged
in the incoming ACK. This adjustment is part of Appropriate Byte
Counting [RFC 3465] and provides robustness against misbehaving
receivers that may attempt to induce a sender to artificially inflate
cwnd using a mechanism known as "ACK Division" [SCWA99]. ACK
Division consists of a receiver sending multiple ACKs for a single
TCP data segment, each acknowledging only a portion of its data. A
TCP that increments cwnd by SMSS for each such ACK will
inappropriately inflate the amount of data injected into the network.
During congestion avoidance, cwnd is incremented by roughly 1 full-
sized segment per round-trip time (RTT). Congestion avoidance
continues until congestion is detected. The basic guidelines for
incrementing cwnd during congestion avoidance are:
* MAY increment cwnd by SMSS bytes
* SHOULD increment cwnd per equation (2) once per RTT
* MUST NOT increment cwnd by more than SMSS bytes
We note that [RFC 3465] allows for cwnd increases of more than SMSS
bytes for incoming acknowledgments during slow start on an
experimental basis; however, such behavior is not allowed as part of
the standard.
The RECOMMENDED way to increase cwnd during congestion avoidance is
to count the number of bytes that have been acknowledged by ACKs for
new data. (A drawback of this implementation is that it requires
maintaining an additional state variable.) When the number of bytes
acknowledged reaches cwnd, then cwnd can be incremented by up to SMSS
bytes. Note that during congestion avoidance, cwnd MUST NOT be
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RFC 5681 TCP Congestion Control September 2009
increased by more than SMSS bytes per RTT. This method both allows
TCPs to increase cwnd by one segment per RTT in the face of delayed
ACKs and provides robustness against ACK Division attacks.
Another common formula that a TCP MAY use to update cwnd during
congestion avoidance is given in equation (3):
cwnd += SMSS*SMSS/cwnd (3)
This adjustment is executed on every incoming ACK that acknowledges
new data. Equation (3) provides an acceptable approximation to the
underlying principle of increasing cwnd by 1 full-sized segment per
RTT. (Note that for a connection in which the receiver is
acknowledging every-other packet, (3) is less aggressive than allowed
-- roughly increasing cwnd every second RTT.)
Implementation Note: Since integer arithmetic is usually used in TCP
implementations, the formula given in equation (3) can fail to
increase cwnd when the congestion window is larger than SMSS*SMSS.
If the above formula yields 0, the result SHOULD be rounded up to 1
byte.
Implementation Note: Older implementations have an additional
additive constant on the right-hand side of equation (3). This is
incorrect and can actually lead to diminished performance [RFC 2525].
Implementation Note: Some implementations maintain cwnd in units of
bytes, while others in units of full-sized segments. The latter will
find equation (3) difficult to use, and may prefer to use the
counting approach discussed in the previous paragraph.
When a TCP sender detects segment loss using the retransmission timer
and the given segment has not yet been resent by way of the
retransmission timer, the value of ssthresh MUST be set to no more
than the value given in equation (4):
ssthresh = max (FlightSize / 2, 2*SMSS) (4)
where, as discussed above, FlightSize is the amount of outstanding
data in the network.
On the other hand, when a TCP sender detects segment loss using the
retransmission timer and the given segment has already been
retransmitted by way of the retransmission timer at least once, the
value of ssthresh is held constant.
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RFC 5681 TCP Congestion Control September 2009
Implementation Note: An easy mistake to make is to simply use cwnd,
rather than FlightSize, which in some implementations may
incidentally increase well beyond rwnd.
Furthermore, upon a timeout (as specified in [RFC 2988]) cwnd MUST be
set to no more than the loss window, LW, which equals 1 full-sized
segment (regardless of the value of IW). Therefore, after
retransmitting the dropped segment the TCP sender uses the slow start
algorithm to increase the window from 1 full-sized segment to the new
value of ssthresh, at which point congestion avoidance again takes
over.
As shown in [FF96] and [RFC 3782], slow-start-based loss recovery
after a timeout can cause spurious retransmissions that trigger
duplicate acknowledgments. The reaction to the arrival of these
duplicate ACKs in TCP implementations varies widely. This document
does not specify how to treat such acknowledgments, but does note
this as an area that may benefit from additional attention,
experimentation and specification.
3.2. Fast Retransmit/Fast Recovery
A TCP receiver SHOULD send an immediate duplicate ACK when an out-
of-order segment arrives. The purpose of this ACK is to inform the
sender that a segment was received out-of-order and which sequence
number is expected. From the sender's perspective, duplicate ACKs
can be caused by a number of network problems. First, they can be
caused by dropped segments. In this case, all segments after the
dropped segment will trigger duplicate ACKs until the loss is
repaired. Second, duplicate ACKs can be caused by the re-ordering of
data segments by the network (not a rare event along some network
paths [Pax97]). Finally, duplicate ACKs can be caused by replication
of ACK or data segments by the network. In addition, a TCP receiver
SHOULD send an immediate ACK when the incoming segment fills in all
or part of a gap in the sequence space. This will generate more
timely information for a sender recovering from a loss through a
retransmission timeout, a fast retransmit, or an advanced loss
recovery algorithm, as outlined in section 4.3.
The TCP sender SHOULD use the "fast retransmit" algorithm to detect
and repair loss, based on incoming duplicate ACKs. The fast
retransmit algorithm uses the arrival of 3 duplicate ACKs (as defined
in section 2, without any intervening ACKs which move SND.UNA) as an
indication that a segment has been lost. After receiving 3 duplicate
ACKs, TCP performs a retransmission of what appears to be the missing
segment, without waiting for the retransmission timer to expire.
Allman, et al. Standards Track PAGE 8
RFC 5681 TCP Congestion Control September 2009
After the fast retransmit algorithm sends what appears to be the
missing segment, the "fast recovery" algorithm governs the
transmission of new data until a non-duplicate ACK arrives. The
reason for not performing slow start is that the receipt of the
duplicate ACKs not only indicates that a segment has been lost, but
also that segments are most likely leaving the network (although a
massive segment duplication by the network can invalidate this
conclusion). In other words, since the receiver can only generate a
duplicate ACK when a segment has arrived, that segment has left the
network and is in the receiver's buffer, so we know it is no longer
consuming network resources. Furthermore, since the ACK "clock"
[Jac88] is preserved, the TCP sender can continue to transmit new
segments (although transmission must continue using a reduced cwnd,
since loss is an indication of congestion).
The fast retransmit and fast recovery algorithms are implemented
together as follows.
1. On the first and second duplicate ACKs received at a sender, a
TCP SHOULD send a segment of previously unsent data per [RFC 3042]
provided that the receiver's advertised window allows, the total
FlightSize would remain less than or equal to cwnd plus 2*SMSS,
and that new data is available for transmission. Further, the
TCP sender MUST NOT change cwnd to reflect these two segments
[RFC 3042]. Note that a sender using SACK [RFC 2018] MUST NOT send
new data unless the incoming duplicate acknowledgment contains
new SACK information.
2. When the third duplicate ACK is received, a TCP MUST set ssthresh
to no more than the value given in equation (4). When [RFC 3042]
is in use, additional data sent in limited transmit MUST NOT be
included in this calculation.
3. The lost segment starting at SND.UNA MUST be retransmitted and
cwnd set to ssthresh plus 3*SMSS. This artificially "inflates"
the congestion window by the number of segments (three) that have
left the network and which the receiver has buffered.
4. For each additional duplicate ACK received (after the third),
cwnd MUST be incremented by SMSS. This artificially inflates the
congestion window in order to reflect the additional segment that
has left the network.
Note: [SCWA99] discusses a receiver-based attack whereby many
bogus duplicate ACKs are sent to the data sender in order to
artificially inflate cwnd and cause a higher than appropriate
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RFC 5681 TCP Congestion Control September 2009
sending rate to be used. A TCP MAY therefore limit the number of
times cwnd is artificially inflated during loss recovery to the
number of outstanding segments (or, an approximation thereof).
Note: When an advanced loss recovery mechanism (such as outlined
in section 4.3) is not in use, this increase in FlightSize can
cause equation (4) to slightly inflate cwnd and ssthresh, as some
of the segments between SND.UNA and SND.NXT are assumed to have
left the network but are still reflected in FlightSize.
5. When previously unsent data is available and the new value of
cwnd and the receiver's advertised window allow, a TCP SHOULD
send 1*SMSS bytes of previously unsent data.
6. When the next ACK arrives that acknowledges previously
unacknowledged data, a TCP MUST set cwnd to ssthresh (the value
set in step 2). This is termed "deflating" the window.
This ACK should be the acknowledgment elicited by the
retransmission from step 3, one RTT after the retransmission
(though it may arrive sooner in the presence of significant out-
of-order delivery of data segments at the receiver).
Additionally, this ACK should acknowledge all the intermediate
segments sent between the lost segment and the receipt of the
third duplicate ACK, if none of these were lost.
Note: This algorithm is known to generally not recover efficiently
from multiple losses in a single flight of packets [FF96]. Section
4.3 below addresses such cases.
4. Additional Considerations
4.1. Restarting Idle Connections
A known problem with the TCP congestion control algorithms described
above is that they allow a potentially inappropriate burst of traffic
to be transmitted after TCP has been idle for a relatively long
period of time. After an idle period, TCP cannot use the ACK clock
to strobe new segments into the network, as all the ACKs have drained
from the network. Therefore, as specified above, TCP can potentially
send a cwnd-size line-rate burst into the network after an idle
period. In addition, changing network conditions may have rendered
TCP's notion of the available end-to-end network capacity between two
endpoints, as estimated by cwnd, inaccurate during the course of a
long idle period.
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RFC 5681 TCP Congestion Control September 2009
[Jac88] recommends that a TCP use slow start to restart transmission
after a relatively long idle period. Slow start serves to restart
the ACK clock, just as it does at the beginning of a transfer. This
mechanism has been widely deployed in the following manner. When TCP
has not received a segment for more than one retransmission timeout,
cwnd is reduced to the value of the restart window (RW) before
transmission begins.
For the purposes of this standard, we define RW = min(IW,cwnd).
Using the last time a segment was received to determine whether or
not to decrease cwnd can fail to deflate cwnd in the common case of
persistent HTTP connections [HTH98]. In this case, a Web server
receives a request before transmitting data to the Web client. The
reception of the request makes the test for an idle connection fail,
and allows the TCP to begin transmission with a possibly
inappropriately large cwnd.
Therefore, a TCP SHOULD set cwnd to no more than RW before beginning
transmission if the TCP has not sent data in an interval exceeding
the retransmission timeout.
4.2. Generating Acknowledgments
The delayed ACK algorithm specified in [RFC 1122] SHOULD be used by a
TCP receiver. When using delayed ACKs, a TCP receiver MUST NOT
excessively delay acknowledgments. Specifically, an ACK SHOULD be
generated for at least every second full-sized segment, and MUST be
generated within 500 ms of the arrival of the first unacknowledged
packet.
The requirement that an ACK "SHOULD" be generated for at least every
second full-sized segment is listed in [RFC 1122] in one place as a
SHOULD and another as a MUST. Here we unambiguously state it is a
SHOULD. We also emphasize that this is a SHOULD, meaning that an
implementor should indeed only deviate from this requirement after
careful consideration of the implications. See the discussion of
"Stretch ACK violation" in [RFC 2525] and the references therein for a
discussion of the possible performance problems with generating ACKs
less frequently than every second full-sized segment.
In some cases, the sender and receiver may not agree on what
constitutes a full-sized segment. An implementation is deemed to
comply with this requirement if it sends at least one acknowledgment
every time it receives 2*RMSS bytes of new data from the sender,
where RMSS is the Maximum Segment Size specified by the receiver to
the sender (or the default value of 536 bytes, per [RFC 1122], if the
receiver does not specify an MSS option during connection
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RFC 5681 TCP Congestion Control September 2009
establishment). The sender may be forced to use a segment size less
than RMSS due to the maximum transmission unit (MTU), the path MTU
discovery algorithm or other factors. For instance, consider the
case when the receiver announces an RMSS of X bytes but the sender
ends up using a segment size of Y bytes (Y < X) due to path MTU
discovery (or the sender's MTU size). The receiver will generate
stretch ACKs if it waits for 2*X bytes to arrive before an ACK is
sent. Clearly this will take more than 2 segments of size Y bytes.
Therefore, while a specific algorithm is not defined, it is desirable
for receivers to attempt to prevent this situation, for example, by
acknowledging at least every second segment, regardless of size.
Finally, we repeat that an ACK MUST NOT be delayed for more than 500
ms waiting on a second full-sized segment to arrive.
Out-of-order data segments SHOULD be acknowledged immediately, in
order to accelerate loss recovery. To trigger the fast retransmit
algorithm, the receiver SHOULD send an immediate duplicate ACK when
it receives a data segment above a gap in the sequence space. To
provide feedback to senders recovering from losses, the receiver
SHOULD send an immediate ACK when it receives a data segment that
fills in all or part of a gap in the sequence space.
A TCP receiver MUST NOT generate more than one ACK for every incoming
segment, other than to update the offered window as the receiving
application consumes new data (see [RFC 813] and page 42 of [RFC 793]).
4.3. Loss Recovery Mechanisms
A number of loss recovery algorithms that augment fast retransmit and
fast recovery have been suggested by TCP researchers and specified in
the RFC series. While some of these algorithms are based on the TCP
selective acknowledgment (SACK) option [RFC 2018], such as [FF96],
[MM96a], [MM96b], and [RFC 3517], others do not require SACKs, such as
[Hoe96], [FF96], and [RFC 3782]. The non-SACK algorithms use "partial
acknowledgments" (ACKs that cover previously unacknowledged data, but
not all the data outstanding when loss was detected) to trigger
retransmissions. While this document does not standardize any of the
specific algorithms that may improve fast retransmit/fast recovery,
these enhanced algorithms are implicitly allowed, as long as they
follow the general principles of the basic four algorithms outlined
above.
That is, when the first loss in a window of data is detected,
ssthresh MUST be set to no more than the value given by equation (4).
Second, until all lost segments in the window of data in question are
repaired, the number of segments transmitted in each RTT MUST be no
more than half the number of outstanding segments when the loss was
detected. Finally, after all loss in the given window of segments
Allman, et al. Standards Track PAGE 12
RFC 5681 TCP Congestion Control September 2009
has been successfully retransmitted, cwnd MUST be set to no more than
ssthresh and congestion avoidance MUST be used to further increase
cwnd. Loss in two successive windows of data, or the loss of a
retransmission, should be taken as two indications of congestion and,
therefore, cwnd (and ssthresh) MUST be lowered twice in this case.
We RECOMMEND that TCP implementors employ some form of advanced loss
recovery that can cope with multiple losses in a window of data. The
algorithms detailed in [RFC 3782] and [RFC 3517] conform to the general
principles outlined above. We note that while these are not the only
two algorithms that conform to the above general principles these two
algorithms have been vetted by the community and are currently on the
Standards Track.
5. Security Considerations
This document requires a TCP to diminish its sending rate in the
presence of retransmission timeouts and the arrival of duplicate
acknowledgments. An attacker can therefore impair the performance of
a TCP connection by either causing data packets or their
acknowledgments to be lost, or by forging excessive duplicate
acknowledgments.
In response to the ACK division attack outlined in [SCWA99], this
document RECOMMENDS increasing the congestion window based on the
number of bytes newly acknowledged in each arriving ACK rather than
by a particular constant on each arriving ACK (as outlined in section
3.1).
The Internet, to a considerable degree, relies on the correct
implementation of these algorithms in order to preserve network
stability and avoid congestion collapse. An attacker could cause TCP
endpoints to respond more aggressively in the face of congestion by
forging excessive duplicate acknowledgments or excessive
acknowledgments for new data. Conceivably, such an attack could
drive a portion of the network into congestion collapse.
6. Changes between RFC 2001 and RFC 2581
[RFC 2001] was extensively rewritten editorially and it is not
feasible to itemize the list of changes between [RFC 2001] and
[RFC 2581]. The intention of [RFC 2581] was to not change any of the
recommendations given in [RFC 2001], but to further clarify cases that
were not discussed in detail in [RFC 2001]. Specifically, [RFC 2581]
suggested what TCP connections should do after a relatively long idle
period, as well as specified and clarified some of the issues
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RFC 5681 TCP Congestion Control September 2009
pertaining to TCP ACK generation. Finally, the allowable upper bound
for the initial congestion window was raised from one to two
segments.
7. Changes Relative to RFC 2581
A specific definition for "duplicate acknowledgment" has been added,
based on the definition used by BSD TCP.
The document now notes that what to do with duplicate ACKs after the
retransmission timer has fired is future work and explicitly
unspecified in this document.
The initial window requirements were changed to allow Larger Initial
Windows as standardized in [RFC 3390]. Additionally, the steps to
take when an initial window is discovered to be too large due to Path
MTU Discovery [RFC 1191] are detailed.
The recommended initial value for ssthresh has been changed to say
that it SHOULD be arbitrarily high, where it was previously MAY.
This is to provide additional guidance to implementors on the matter.
During slow start, the usage of Appropriate Byte Counting [RFC 3465]
with L=1*SMSS is explicitly recommended. The method of increasing
cwnd given in [RFC 2581] is still explicitly allowed. Byte counting
during congestion avoidance is also recommended, while the method
from [RFC 2581] and other safe methods are still allowed.
The treatment of ssthresh on retransmission timeout was clarified.
In particular, ssthresh must be set to half the FlightSize on the
first retransmission of a given segment and then is held constant on
subsequent retransmissions of the same segment.
The description of fast retransmit and fast recovery has been
clarified, and the use of Limited Transmit [RFC 3042] is now
recommended.
TCPs now MAY limit the number of duplicate ACKs that artificially
inflate cwnd during loss recovery to the number of segments
outstanding to avoid the duplicate ACK spoofing attack described in
[SCWA99].
The restart window has been changed to min(IW,cwnd) from IW. This
behavior was described as "experimental" in [RFC 2581].
It is now recommended that TCP implementors implement an advanced
loss recovery algorithm conforming to the principles outlined in this
document.
Allman, et al. Standards Track PAGE 14
RFC 5681 TCP Congestion Control September 2009
The security considerations have been updated to discuss ACK division
and recommend byte counting as a counter to this attack.
8. Acknowledgments
The core algorithms we describe were developed by Van Jacobson
[Jac88, Jac90]. In addition, Limited Transmit [RFC 3042] was
developed in conjunction with Hari Balakrishnan and Sally Floyd. The
initial congestion window size specified in this document is a result
of work with Sally Floyd and Craig Partridge [RFC 2414, RFC 3390].
W. Richard ("Rich") Stevens wrote the first version of this document
[RFC 2001] and co-authored the second version [RFC 2581]. This present
version much benefits from his clarity and thoughtfulness of
description, and we are grateful for Rich's contributions in
elucidating TCP congestion control, as well as in more broadly
helping us understand numerous issues relating to networking.
We wish to emphasize that the shortcomings and mistakes of this
document are solely the responsibility of the current authors.
Some of the text from this document is taken from "TCP/IP
Illustrated, Volume 1: The Protocols" by W. Richard Stevens
(Addison-Wesley, 1994) and "TCP/IP Illustrated, Volume 2: The
Implementation" by Gary R. Wright and W. Richard Stevens (Addison-
Wesley, 1995). This material is used with the permission of
Addison-Wesley.
Anil Agarwal, Steve Arden, Neal Cardwell, Noritoshi Demizu, Gorry
Fairhurst, Kevin Fall, John Heffner, Alfred Hoenes, Sally Floyd,
Reiner Ludwig, Matt Mathis, Craig Partridge, and Joe Touch
contributed a number of helpful suggestions.
9. References
9.1. Normative References
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC 1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC 1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Allman, et al. Standards Track PAGE 15
RFC 5681 TCP Congestion Control September 2009
9.2. Informative References
[CJ89] Chiu, D. and R. Jain, "Analysis of the Increase/Decrease
Algorithms for Congestion Avoidance in Computer Networks",
Journal of Computer Networks and ISDN Systems, vol. 17, no.
1, pp. 1-14, June 1989.
[FF96] Fall, K. and S. Floyd, "Simulation-based Comparisons of
Tahoe, Reno and SACK TCP", Computer Communication Review,
July 1996, ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z.
[Hoe96] Hoe, J., "Improving the Start-up Behavior of a Congestion
Control Scheme for TCP", In ACM SIGCOMM, August 1996.
[HTH98] Hughes, A., Touch, J., and J. Heidemann, "Issues in TCP
Slow-Start Restart After Idle", Work in Progress, March
1998.
[Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer
Communication Review, vol. 18, no. 4, pp. 314-329, Aug.
1988. ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.
[Jac90] Jacobson, V., "Modified TCP Congestion Avoidance
Algorithm", end2end-interest mailing list, April 30, 1990.
ftp://ftp.isi.edu/end2end/end2end-interest-1990.mail.
[MM96a] Mathis, M. and J. Mahdavi, "Forward Acknowledgment:
Refining TCP Congestion Control", Proceedings of
SIGCOMM'96, August, 1996, Stanford, CA. Available from
http://www.psc.edu/networking/papers/papers.html
[MM96b] Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding
Parameters", Technical report. Available from
http://www.psc.edu/networking/papers/FACKnotes/current.
[Pax97] Paxson, V., "End-to-End Internet Packet Dynamics",
Proceedings of SIGCOMM '97, Cannes, France, Sep. 1997.
[RFC 813] Clark, D., "Window and Acknowledgement Strategy in TCP",
RFC 813, July 1982.
[RFC 2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001,
January 1997.
[RFC 2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
Allman, et al. Standards Track PAGE 16
RFC 5681 TCP Congestion Control September 2009
[RFC 2414] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 2414, September 1998.
[RFC 2525] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner, J.,
Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known TCP
Implementation Problems", RFC 2525, March 1999.
[RFC 2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC 2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, July 2000.
[RFC 2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC 3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
TCP's Loss Recovery Using Limited Transmit", RFC 3042,
January 2001.
[RFC 3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[RFC 3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.
[RFC 3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, February 2003.
[RFC 3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, April 2003.
[RFC 3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782,
April 2004.
[RFC 4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[SCWA99] Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
"TCP Congestion Control With a Misbehaving Receiver", ACM
Computer Communication Review, 29(5), October 1999.
[Ste94] Stevens, W., "TCP/IP Illustrated, Volume 1: The Protocols",
Addison-Wesley, 1994.
Allman, et al. Standards Track PAGE 17
RFC 5681 TCP Congestion Control September 2009
[WS95] Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
The Implementation", Addison-Wesley, 1995.
Authors' Addresses
Mark Allman
International Computer Science Institute (ICSI)
1947 Center Street
Suite 600
Berkeley, CA 94704-1198
Phone: +1 440 235 1792
EMail: mallman@icir.org
http://www.icir.org/mallman/
Vern Paxson
International Computer Science Institute (ICSI)
1947 Center Street
Suite 600
Berkeley, CA 94704-1198
Phone: +1 510/642-4274 x302
EMail: vern@icir.org
http://www.icir.org/vern/
Ethan Blanton
Purdue University Computer Sciences
305 North University Street
West Lafayette, IN 47907
EMail: eblanton@cs.purdue.edu
http://www.cs.purdue.edu/homes/eblanton/
Allman, et al. Standards Track PAGE 18
TCP Congestion Control
RFC TOTAL SIZE: 44339 bytes
PUBLICATION DATE: Saturday, September 5th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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