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Versions: (RFC 4138) 00 01 02 03 04 RFC 5682
Internet Engineering Task Force P. Sarolahti
INTERNET-DRAFT Nokia Research Center
draft-ietf-tcpm-rfc4138bis-04.txt M. Kojo
Intended status: Proposed Standard University of Helsinki
Updates: 4138 K. Yamamoto
Expires: April 2009 M. Hata
NTT Docomo
30 October 2008
Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
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This Internet-Draft will expire on April 2009.
Abstract
The purpose of this document is to move the F-RTO functionality for
TCP in RFC 4138 from Experimental to Standards Track status. The F-
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RTO support for SCTP in RFC 4138 remains with Experimental status.
See Appendix B for the differences between this document and RFC
4138.
Spurious retransmission timeouts cause suboptimal TCP performance
because they often result in unnecessary retransmission of the last
window of data. This document describes the F-RTO detection
algorithm for detecting spurious TCP retransmission timeouts. F-RTO
is a TCP sender-only algorithm that does not require any TCP options
to operate. After retransmitting the first unacknowledged segment
triggered by a timeout, the F-RTO algorithm of the TCP sender
monitors the incoming acknowledgments to determine whether the
timeout was spurious. It then decides whether to send new segments
or retransmit unacknowledged segments. The algorithm effectively
helps to avoid additional unnecessary retransmissions and thereby
improves TCP performance in the case of a spurious timeout.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Terminology. . . . . . . . . . . . . . . 5
2. Basic F-RTO Algorithm . . . . . . . . . . . . . . . . . . . . 5
2.1. The Algorithm. . . . . . . . . . . . . . . . . . . . . . 6
2.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . 8
3. SACK-Enhanced Version of the F-RTO Algorithm. . . . . . . . . 10
3.1. The Algorithm. . . . . . . . . . . . . . . . . . . . . . 10
3.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . 12
4. Taking Actions after Detecting Spurious RTO . . . . . . . . . 12
5. Evaluation of RFC 4138. . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 15
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
A. Discussion of Window-Limited Cases. . . . . . . . . . . . . . 15
B. Changes since RFC 4138. . . . . . . . . . . . . . . . . . . . 16
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Normative References . . . . . . . . . . . . . . . . . . . . . . 17
Informative References . . . . . . . . . . . . . . . . . . . . . 17
AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . . . 19
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
The Transmission Control Protocol (TCP) [Pos81] has two methods for
triggering retransmissions. First, the TCP sender relies on
incoming duplicate ACKs, which indicate that the receiver is missing
some of the data. After a required number of successive duplicate
ACKs have arrived at the sender, it retransmits the first
unacknowledged segment [APS99] and continues with a loss recovery
algorithm such as NewReno [FHG04] or SACK-based loss recovery
[BAFW03]. Second, the TCP sender maintains a retransmission timer
which triggers retransmission of segments, if they have not been
acknowledged before the retransmission timeout (RTO) occurs. When
the retransmission timeout occurs, the TCP sender enters the RTO
recovery where the congestion window is initialized to one segment
and unacknowledged segments are retransmitted using the slow-start
algorithm. The retransmission timer is adjusted dynamically, based
on the measured round-trip times [PA00].
It has been pointed out that the retransmission timer can expire
spuriously and cause unnecessary retransmissions when no segments
have been lost [LK00, GL02, LM03]. After a spurious retransmission
timeout, the late acknowledgments of the original segments arrive at
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the sender, usually triggering unnecessary retransmissions of a
whole window of segments during the RTO recovery. Furthermore,
after a spurious retransmission timeout, a conventional TCP sender
increases the congestion window on each late acknowledgment in slow
start. This injects a large number of data segments into the
network within one round-trip time, thus violating the packet
conservation principle [Jac88].
There are a number of potential reasons for spurious retransmission
timeouts. First, some mobile networking technologies involve sudden
delay spikes on transmission because of actions taken during a hand-
off. Second, a hand-off may take place from a low latency path to a
high latency path, suddenly increasing the round-trip time beyond
the current RTO value. Third, on a low-bandwidth link the arrival
of competing traffic (possibly with higher priority), or some other
change in available bandwidth, can cause a sudden increase of the
round-trip time. This may trigger a spurious retransmission
timeout. A persistently reliable link layer can also cause a sudden
delay when a data frame and several retransmissions of it are lost
for some reason. This document does not distinguish between the
different causes of such a delay spike. Rather, it discusses the
spurious retransmission timeouts caused by a delay spike in general.
This document describes the F-RTO detection algorithm for TCP. It is
based on the detection mechanism of the "Forward RTO-Recovery" (F-
RTO) algorithm [SKR03] that is used for detecting spurious
retransmission timeouts and thus avoids unnecessary retransmissions
following the retransmission timeout. When the timeout is not
spurious, the F-RTO algorithm reverts back to the conventional RTO
recovery algorithm, and therefore has similar behavior and
performance. In contrast to alternative algorithms proposed for
detecting unnecessary retransmissions (Eifel [LK00], [LM03] and
DSACK-based algorithms [BA04]), F-RTO does not require any TCP
options for its operation, and it can be implemented by modifying
only the TCP sender. The Eifel algorithm uses TCP timestamps
[BBJ92] for detecting a spurious timeout upon arrival of the first
acknowledgment after the retransmission. The DSACK-based algorithms
require that the TCP Selective Acknowledgment Option [MMFR96], with
the DSACK extension [FMMP00], is in use. With DSACK, the TCP
receiver can report if it has received a duplicate segment, enabling
the sender to detect afterwards whether it has retransmitted
segments unnecessarily. The F-RTO algorithm only attempts to detect
and avoid unnecessary retransmissions after an RTO. Eifel and DSACK
can also be used for detecting unnecessary retransmissions caused by
other events, such as packet reordering.
When the retransmission timer expires, the F-RTO sender retransmits
the first unacknowledged segment as usual [APS99]. Deviating from
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the normal operation after a timeout, it then tries to transmit new,
previously unsent data for the first acknowledgment that arrives
after the timeout, given that the acknowledgment advances the
window. If the second acknowledgment that arrives after the timeout
advances the window (i.e., acknowledges data that was not
retransmitted), the F-RTO sender declares the timeout spurious and
exits the RTO recovery. However, if either of these two
acknowledgments is a duplicate ACK, there will not be sufficient
evidence of a spurious timeout. Therefore, the F-RTO sender
retransmits the unacknowledged segments in slow start similarly to
the traditional algorithm. With a SACK-enhanced version of the F-RTO
algorithm, spurious timeouts may be detected even if duplicate ACKs
arrive after an RTO retransmission.
This document specifies the F-RTO algorithm for TCP only, replacing
the F-RTO functionality with TCP in RFC 4138 [SK05] and moving it
from Experimental to Standards Track status. The algorithm can also
be applied to the Stream Control Transmission Protocol (SCTP)
[Ste07] that has acknowledgment and packet retransmission concepts
similar to TCP. The considerations on applying F-RTO to SCTP are
discussed in RFC 4138, but the F-RTO support for SCTP remains with
Experimental status.
This document is organized as follows. Section 2 describes the basic
F-RTO algorithm, and the SACK-enhanced F-RTO algorithm is given in
Section 3. Section 4 discusses the possible actions to be taken
after detecting a spurious RTO and Section 5 discusses the security
considerations.
1.1. Conventions and 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 BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for protocols.
2. Basic F-RTO Algorithm
A timeout is considered spurious if it would have been avoided had
the sender waited longer for an acknowledgment to arrive [LM03]. F-
RTO affects the TCP sender behavior only after a retransmission
timeout. Otherwise, the TCP behavior remains the same. When the
retransmission timer expires, the F-RTO algorithm monitors incoming
acknowledgments and if the TCP sender gets an acknowledgment for a
segment that was not retransmitted due to the timeout, the F-RTO
algorithm declares a timeout spurious. The actions taken in response
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to a spurious timeout are not specified in this document, but we
discuss some alternatives in Section 4. This section introduces the
algorithm and then discusses the different steps of the algorithm in
more detail.
Following the practice used with the Eifel Detection algorithm
[LM03], we use the "SpuriousRecovery" variable to indicate whether
the retransmission is declared spurious by the sender. This variable
can be used as an input for a corresponding response algorithm. With
F-RTO, the value of SpuriousRecovery can be either SPUR_TO
(indicating a spurious retransmission timeout) or FALSE (indicating
that the timeout is not declared spurious and the TCP sender should
follow the conventional RTO recovery algorithm). In addition, we use
the "recover" variable specified in the NewReno algorithm [FHG04].
2.1. The Algorithm
A TCP sender implementing the basic F-RTO algorithm MUST take the
following steps after the retransmission timer expires. If the
retransmission timer expires again during the execution of the F-RTO
algorithm, the TCP sender MUST re-start the algorithm processing
from step 1. If the sender implements some loss recovery algorithm
other than Reno or NewReno [FHG04], the F-RTO algorithm SHOULD NOT
be entered when earlier fast recovery is underway.
The F-RTO algorithm takes different actions based on whether an
incoming acknowledgement advances the cumulative acknowledgement
point for a received in-order segment, or whether it is a duplicate
acknowledgement to indicate an out-of-order segment. Duplicate
acknowledgement is defined in [APB08]. The F-RTO algorithm does not
specify actions for receiving a segment that neither acknowledges
new data nor is a duplicate acknowledgement. The TCP sender SHOULD
ignore such segments and wait for a segment that either acknowledges
new data or is a duplicate acknowledgment.
1) When the retransmission timer expires, retransmit the first
unacknowledged segment and set SpuriousRecovery to FALSE. If the
TCP sender is already in RTO recovery AND "recover" is larger
than or equal to SND.UNA (the oldest unacknowledged sequence
number [Pos81]), do not enter step 2 of this algorithm. Instead,
store the highest sequence number transmitted so far in variable
"recover" and continue with slow start retransmissions following
the conventional RTO recovery algorithm.
2) When the first acknowledgment after the RTO retransmission
arrives at the TCP sender, store the highest sequence number
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transmitted so far in variable "recover". The TCP sender chooses
one of the following actions, depending on whether the ACK
advances the window or whether it is a duplicate ACK.
a) If the acknowledgment is a duplicate ACK OR the
Acknowledgement field covers "recover" but not more than
"recover" OR the acknowledgment does not acknowledge all of
the data that was retransmitted in step 1, revert to the
conventional RTO recovery and continue by retransmitting
unacknowledged data in slow start. Do not enter step 3 of
this algorithm. The SpuriousRecovery variable remains as
FALSE.
b) Else, if the acknowledgment advances the window AND the
Acknowledgement field does not cover "recover", transmit up to
two new (previously unsent) segments and enter step 3 of this
algorithm. If the TCP sender does not have enough unsent data,
it can send only one segment. In addition, the TCP sender MAY
override the Nagle algorithm [Nag84] and immediately send a
segment if needed. Note that sending two segments in this step
is allowed by TCP congestion control requirements [APS99]: An
F-RTO TCP sender simply chooses different segments to
transmit.
If the TCP sender does not have any new data to send, or the
advertised window prohibits new transmissions, the recommended
action is to skip step 3 of this algorithm and continue with
slow start retransmissions, following the conventional RTO
recovery algorithm. However, alternative ways of handling the
window-limited cases that could result in better performance
are discussed in Appendix A.
3) When the second acknowledgment after the RTO retransmission
arrives at the TCP sender, the TCP sender either declares the
timeout spurious, or starts retransmitting the unacknowledged
segments.
a) If the acknowledgment is a duplicate ACK, set the congestion
window to no more than 3 * MSS, and continue with the slow
start algorithm retransmitting unacknowledged segments. The
congestion window can be set to 3 * MSS, because two round-
trip times have elapsed since the RTO, and a conventional TCP
sender would have increased cwnd to 3 during the same time.
Leave SpuriousRecovery set to FALSE.
b) If the acknowledgment advances the window (i.e., if it
acknowledges data that was not retransmitted after the
timeout), declare the timeout spurious, set SpuriousRecovery
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to SPUR_TO, and set the value of the "recover" variable to
SND.UNA (the oldest unacknowledged sequence number [Pos81]).
2.2. Discussion
The F-RTO sender takes cautious actions when it receives duplicate
acknowledgments after a retransmission timeout. Because duplicate
ACKs may indicate that segments have been lost, reliably detecting a
spurious timeout is difficult due to the lack of additional
information. Therefore, it is prudent to follow the conventional
TCP recovery in those cases.
The condition in step 1 prevents the execution of the F-RTO
algorithm in case a previous RTO recovery is underway when the
retransmission timer expires, except in case the retransmission
timer expires multiple times for the same segment. If the
retransmission timer expires during an earlier RTO-based loss
recovery, acknowledgements for retransmitted segments may falsely
lead the TCP sender to declare the timeout spurious.
If the first acknowledgment after the RTO retransmission covers the
"recover" point at algorithm step (2a), there is not enough evidence
that a non-retransmitted segment has arrived at the receiver after
the timeout. This is a common case when a fast retransmission is
lost and has been retransmitted again after an RTO, while the rest
of the unacknowledged segments were successfully delivered to the
TCP receiver before the retransmission timeout. Therefore, the
timeout cannot be declared spurious in this case.
If the first acknowledgment after the RTO retransmission does not
acknowledge all of the data that was retransmitted in step 1, the
TCP sender reverts to the conventional RTO recovery. Otherwise, a
malicious receiver acknowledging partial segments could cause the
sender to declare the timeout spurious in a case where data was
lost.
The TCP sender is allowed to send two new segments in algorithm
branch (2b) because the conventional TCP sender would transmit two
segments when the first new ACK arrives after the RTO
retransmission. If sending new data is not possible in algorithm
branch (2b), or if the receiver window limits the transmission, the
TCP sender has to send something in order to prevent the TCP
transfer from stalling. If no segments were sent, the pipe between
sender and receiver might run out of segments, and no further
acknowledgments would arrive. Therefore, in the window-limited
case, the recommendation is to revert to the conventional RTO
recovery with slow start retransmissions. Appendix A discusses some
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alternative solutions for window-limited situations.
If the retransmission timeout is declared spurious, the TCP sender
sets the value of the "recover" variable to SND.UNA in order to
allow fast retransmit [FHG04]. The "recover" variable was proposed
for avoiding unnecessary, multiple fast retransmits when the
retransmission timer expires during fast recovery with NewReno TCP.
Because the F-RTO sender retransmits only the segment that triggered
the timeout, the problem of unnecessary multiple fast retransmits
[FHG04] cannot occur. Therefore, if three duplicate ACKs arrive at
the sender after the timeout, they probably indicate a packet loss,
and thus fast retransmit should be used to allow efficient recovery.
If there are not enough duplicate ACKs arriving at the sender after
a packet loss, the retransmission timer expires again and the sender
enters step 1 of this algorithm.
When the timeout is declared spurious, the TCP sender cannot detect
whether the unnecessary RTO retransmission was lost. In principle,
the loss of the RTO retransmission should be taken as a congestion
signal. Thus, there is a small possibility that the F-RTO sender
will violate the congestion control rules, if it chooses to fully
revert congestion control parameters after detecting a spurious
timeout. The Eifel detection algorithm has a similar property,
while the DSACK option can be used to detect whether the
retransmitted segment was successfully delivered to the receiver.
The F-RTO algorithm has a side-effect on the TCP round-trip time
measurement. Because the TCP sender can avoid most of the
unnecessary retransmissions after detecting a spurious timeout, the
sender is able to take round-trip time samples on the delayed
segments. If the regular RTO recovery was used without TCP
timestamps, this would not be possible due to the retransmission
ambiguity. As a result, the RTO is likely to have more accurate and
larger values with F-RTO than with the regular TCP after a spurious
timeout that was triggered due to delayed segments. We believe this
is an advantage in networks that are prone to delay spikes.
There are some situations where the F-RTO algorithm may not avoid
unnecessary retransmissions after a spurious timeout. If packet
reordering or packet duplication occurs on the segment that
triggered the spurious timeout, the F-RTO algorithm may not detect
the spurious timeout due to incoming duplicate ACKs. Additionally,
if a spurious timeout occurs during fast recovery, the F-RTO
algorithm often cannot detect the spurious timeout because the
segments that were transmitted before the fast recovery trigger
duplicate ACKs. However, we consider these cases rare, and note
that in cases where F-RTO fails to detect the spurious timeout, it
retransmits the unacknowledged segments in slow start, and thus
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performs similarly to the regular RTO recovery.
3. SACK-Enhanced Version of the F-RTO Algorithm
This section describes an alternative version of the F-RTO algorithm
that uses the TCP Selective Acknowledgment Option [MMFR96]. By
using the SACK option, the TCP sender detects spurious timeouts in
most of the cases when packet reordering or packet duplication is
present. If the SACK blocks acknowledge new data that was not
transmitted after the RTO retransmission, the sender may declare the
timeout spurious, even when duplicate ACKs follow the RTO.
3.1. The Algorithm
Given that the TCP Selective Acknowledgment Option [MMFR96] is
enabled for a TCP connection, a TCP sender MAY apply the SACK-
enhanced F-RTO algorithm. If the sender applies the SACK-enhanced
F-RTO algorithm, it MUST follow the steps below. This algorithm
SHOULD NOT be applied if the TCP sender is already in loss recovery
when a retransmission timeout occurs.
The steps of the SACK-enhanced version of the F-RTO algorithm are as
follows. If the retransmission timer expires again during the
execution of the SACK-enhanced F-RTO algorithm, the TCP sender MUST
re-start the algorithm processing from step 1.
1) When the retransmission timer expires, retransmit the first
unacknowledged segment and set SpuriousRecovery to FALSE.
Following the recommendation in the SACK specification [MMFR96],
reset the SACK scoreboard. If "RecoveryPoint" is larger than or
equal to SND.UNA, do not enter step 2 of this algorithm. Instead,
set variable "RecoveryPoint" to indicate the highest sequence
number transmitted so far and continue with slow start
retransmissions following the conventional RTO recovery
algorithm.
2) Wait until the acknowledgment of the data retransmitted due to
the timeout arrives at the sender. If duplicate ACKs arrive
before the cumulative acknowledgment for retransmitted data,
adjust the scoreboard according to the incoming SACK information.
Stay in step 2 and wait for the next new acknowledgment. If the
retransmission timeout expires again, go to step 1 of the
algorithm. When a new acknowledgment arrives, set variable
"RecoveryPoint" to indicate the highest sequence number
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transmitted so far.
a) If the Cumulative Acknowledgement field covers "RecoveryPoint"
but not more than "RecoveryPoint", revert to the conventional
RTO recovery and set the congestion window to no more than 2 *
MSS, like a regular TCP would do. Do not enter step 3 of this
algorithm.
b) Else, if the Cumulative Acknowledgement field does not cover
"RecoveryPoint" but is larger than SND.UNA, transmit up to two
new (previously unsent) segments and proceed to step 3. If
the TCP sender is not able to transmit any previously unsent
data -- either due to receiver window limitation or because it
does not have any new data to send -- the recommended action
is to refrain from entering step 3 of this algorithm. Rather,
continue with slow start retransmissions following the
conventional RTO recovery algorithm.
It is also possible to apply some of the alternatives for
handling window-limited cases discussed in Appendix A.
3) The next acknowledgment arrives at the sender. Either a
duplicate ACK or a new cumulative ACK (advancing the window)
applies in this step. Other types of ACKs are ignored without any
action.
a) If the Cumulative Acknowledgement field or a SACK block covers
more than "RecoveryPoint", set the congestion window to no
more than 3 * MSS and proceed with the conventional RTO
recovery, retransmitting unacknowledged segments. Take this
branch also when the acknowledgment is a duplicate ACK and it
does not acknowledge any new, previously unacknowledged data
below "RecoveryPoint" in the SACK blocks. Leave
SpuriousRecovery set to FALSE.
b) If the Cumulative Acknowledgement field or a SACK block in the
ACK does not cover more than "RecoveryPoint" AND it
acknowledges data that was not acknowledged earlier (either
with cumulative acknowledgment or using SACK blocks), declare
the timeout spurious and set SpuriousRecovery to SPUR_TO. The
retransmission timeout can be declared spurious, because the
segment acknowledged with this ACK was transmitted before the
timeout.
If there are unacknowledged holes between the received SACK blocks,
those segments are retransmitted similarly to the conventional SACK
recovery algorithm [BAFW03]. If the algorithm exits with
SpuriousRecovery set to SPUR_TO, "RecoveryPoint" is set to SND.UNA,
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thus allowing fast recovery on incoming duplicate acknowledgments.
3.2. Discussion
The SACK enhanced algorithm works on the same principle as the basic
algorithm, but by utilizing the additional information from the SACK
option. When a genuine retransmission timeout occurs during a steady
state of a connection, it can be assumed that there are no segments
left in the pipe. Otherwise, the acknowledgments triggered by these
segments would have triggered the SACK loss recovery or transmission
of new segments. Therefore, if the F-RTO sender receives
acknowledgements for segments transmitted before the retransmission
timeout in response to the two new segments sent at the algorithm
step 2, the normal operation of TCP has been just delayed, and the
retransmission timeout is considered spurious. Note that this
reasoning works only when the TCP sender is not in loss recovery at
the time the retransmission timeout occurs. The condition in step 1
checking that "RecoveryPoint" is larger than or equal to SND.UNA
prevents the execution of the F-RTO algorithm in case a previous
loss recovery, either RTO recovery or SACK loss recovery, is
underway when the retransmission timer expires. It, however, allows
the execution of the F-RTO algorithm, if the retransmission timer
expires multiple times for the same segment.
4. Taking Actions after Detecting Spurious RTO
Upon a retransmission timeout, a conventional TCP sender assumes
that outstanding segments are lost and starts retransmitting the
unacknowledged segments. When the retransmission timeout is
detected to be spurious, the TCP sender should not continue
retransmitting based on the timeout. For example, if the sender was
in congestion avoidance phase transmitting new, previously unsent
segments, it should continue transmitting previously unsent segments
in congestion avoidance.
There are currently two alternatives specified for a spurious
timeout response algorithm, the Eifel Response Algorithm [LG04], and
an algorithm for adapting the retransmission timeout after a
spurious RTO [BBA06]. If no specific response algorithm is
implemented, the TCP SHOULD respond to spurious timeout
conservatively, applying the TCP congestion control specification
[APS99]. Different response algorithms for spurious retransmission
timeouts have been analyzed in some research papers [GL03, Sar03]
and IETF documents [SL03].
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5. Evaluation of RFC 4138
F-RTO was first specified in an Experimental RFC 4138 that has been
implemented in a number of operating systems since it was published.
Gained experience has been documented in a separate document
[KYHS07], and can be summarized as follows.
If the TCP sender employs F-RTO, it is able to detect spurious RTOs
and avoid the unnecessary retransmission of the whole window of
data. Because F-RTO avoids the unnecessary retransmissions after a
spurious RTO, it is able to adhere to the packet conservation
principle, unlike a regular TCP that enters the slow-start recovery
unnecessarily an inappropriately restarts the ACK clock while there
are segments outstanding in the network. When a spurious RTO has
been detected, a sender can select an appropriate congestion control
response instead of setting the congestion window to one segment.
Because F-RTO avoids unnecessary retransmissions, it is able to take
the RTT of the delayed segments into account when calculating the
RTO estimate, which may help in avoiding further spurious
retransmission timeouts.
Experimental results with the basic F-RTO have been reported in an
emulated network using a Linux implementation [SKR03]. Also
different congestion control responses along with the SACK-enhanced
version of F-RTO were tested in a similar environment [Sar03]. There
are publications analyzing F-RTO performance over commercial W-CDMA
networks, and in an emulated HSDPA network [Yam05, Hok05]. Also
Microsoft reported positive experiences with their implementation of
F-RTO in the IETF-68 meeting.
It is known that some spurious RTOs may remain undetected by F-RTO
if duplicate acknowledgements arrive at the sender immediately after
the spurious RTO, for example due to packet reordering or packet
loss. There are rare corner cases where F-RTO could "hide" a packet
loss and therefore lead to inappropriate behavior with non-
conservative congestion control response: first, if a massive packet
reordering occurred so that the acknowledgement of RTO
retransmission arrived at the sender before the acknowledgments of
original transmissions, the sender might not detect the loss of the
segment that triggered the RTO. Second, a malicious receiver could
lead F-RTO to make a wrong conclusion after an RTO by acknowledging
segments it has not received. Such receiver would, however, risk
breaking the consistency of the TCP state between the sender and
receiver, causing the connection to become unusable, which cannot be
of any benefit to the receiver. Therefore we believe it is not
likely that receivers would start employing such tricks in a
significant scale. Finally, loss of the unnecessary RTO
retransmission cannot be detected without using some explicit
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acknowledgement scheme such as DSACK. This is common to the other
mechanisms for detecting spurious RTO, as well as to regular TCP
that does not use DSACK. We note that if the congestion control
response to spurious RTO is conservative enough, the above corner
cases do not cause problems due to increased congestion.
6. Security Considerations
The main security threat regarding F-RTO is the possibility that a
receiver could mislead the sender into setting too large a
congestion window after an RTO. There are two possible ways a
malicious receiver could trigger a wrong output from the F-RTO
algorithm. First, the receiver can acknowledge data that it has not
received. Second, it can delay acknowledgment of a segment it has
received earlier, and acknowledge the segment after the TCP sender
has been deluded to enter algorithm step 3.
If the receiver acknowledges a segment it has not really received,
the sender can be led to declare spurious timeout in the F-RTO
algorithm, step 3. However, because the sender will have an
incorrect state, it cannot retransmit the segment that has never
reached the receiver. Therefore, this attack is unlikely to be
useful for the receiver to maliciously gain a larger congestion
window.
A common case for a retransmission timeout is that a fast
retransmission of a segment is lost. If all other segments have
been received, the RTO retransmission causes the whole window to be
acknowledged at once. This case is recognized in F-RTO algorithm
branch (2a). However, if the receiver only acknowledges one segment
after receiving the RTO retransmission, and then the rest of the
segments, it could cause the timeout to be declared spurious when it
is not. Therefore, it is suggested that, when an RTO occurs during
the fast recovery phase, the sender would not fully revert the
congestion window even if the timeout was declared spurious.
Instead, the sender would reduce the congestion window to 1.
If there is more than one segment missing at the time of a
retransmission timeout, the receiver does not benefit from
misleading the sender to declare a spurious timeout because the
sender would have to go through another recovery period to
retransmit the missing segments, usually after an RTO has elapsed.
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7. IANA Considerations
This document has no actions for IANA.
8. Acknowledgements
The authors would like to thank Alfred Hoenes, Ilpo Jarvinen and
Murari Sridharan for the comments on this document.
We are also thankful to Reiner Ludwig, Andrei Gurtov, Josh Blanton,
Mark Allman, Sally Floyd, Yogesh Swami, Mika Liljeberg, Ivan Arias
Rodriguez, Sourabh Ladha, Martin Duke, Motoharu Miyake, Ted Faber,
Samu Kontinen, and Kostas Pentikousis who gave valuable feedback
during the preparation of RFC 4138, the precursor of this document.
Appendix
A. Discussion of Window-Limited Cases
When the advertised window limits the transmission of two new
previously unsent segments, or there are no new data to send, it is
recommended in F-RTO algorithm step (2b) that the TCP sender
continue with the conventional RTO recovery algorithm. The
disadvantage is that the sender may continue unnecessary
retransmissions due to possible spurious timeout. This section
briefly discusses the options that can potentially improve
performance when transmitting previously unsent data is not
possible.
- The TCP sender could reserve an unused space of a size of one or
two segments in the advertised window to ensure the use of
algorithms such as F-RTO or Limited Transmit [ABF01] in receiver
window-limited situations. On the other hand, while doing this,
the TCP sender should ensure that the window of outstanding
segments is large enough for proper utilization of the available
pipe.
- Use additional information if available, e.g., TCP timestamps with
the Eifel Detection algorithm, for detecting a spurious timeout.
However, Eifel detection may yield different results from F-RTO
when ACK losses and an RTO occur within the same round-trip time
[SKR03].
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- Retransmit data from the tail of the retransmission queue and
continue with step 3 of the F-RTO algorithm. It is possible that
the retransmission will be made unnecessarily. Furthermore, the
operation of the SACK-based F-RTO algorithm would need to consider
this case separately, to not use the retransmitted segment to
indicate spurious timeout. Given these considerations, this option
is not recommended.
- Send a zero-sized segment below SND.UNA, similar to a TCP Keep-
Alive probe, and continue with step 3 of the F-RTO algorithm.
Because the receiver replies with a duplicate ACK, the sender is
able to detect whether the timeout was spurious from the incoming
acknowledgment. This method does not send data unnecessarily, but
it delays the recovery by one round-trip time in cases where the
timeout was not spurious. Therefore, this method is not
encouraged.
- In receiver-limited cases, send one octet of new data, regardless
of the advertised window limit, and continue with step 3 of the F-
RTO algorithm. It is possible that the receiver will have free
buffer space to receive the data by the time the segment has
propagated through the network, in which case no harm is done. If
the receiver is not capable of receiving the segment, it rejects
the segment and sends a duplicate ACK.
B. Changes since RFC 4138
Changes from RFC 4138 are summarized below, apart from minor editing
and language improvements.
* Modified the basic F-RTO algorithm and the SACK-enhanced F-RTO
algorithm to prevent the TCP sender from applying the F-RTO
algorithm if the retransmission timer expires when an earlier RTO
recovery is underway, except when the retransmission timer expires
multiple times for the same segment.
* Clarified behavior on multiple timeouts.
* Added a paragraph on acknowledgements that do not acknowledge new
data but are not duplicate acknowledgements.
* Clarified the SACK-algorithm a bit, and added one paragraph of
description of the basic idea of the algorithm.
* Removed SCTP considerations.
Sarolahti/Kojo/Yamamoto/Hata Section B. [Page 16]
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* Removed earlier Appendix sections, except Appendix C from RFC
4138, which is now Appendix A.
* Clarified text about the possible response algorithms.
* Added section that summarizes the evaluation of RFC 4138.
References
Normative References
[APS99] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[APB08] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", Internet-Draft "draft-ietf-tcpm-
rfc2581bis-04.txt",
April 2008.
[BAFW03] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, April 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[FHG04] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782,
April 2004.
[MMFR96] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgement Options", RFC 2018,
October 1996.
[PA00] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[Pos81] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
Informative References
[ABF01] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
TCP's Loss Recovery Using Limited Transmit", RFC 3042,
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January 2001.
[BA04] Blanton, E. and M. Allman, "Using TCP Duplicate Selective
Acknowledgement (DSACKs) and Stream Control Transmission
Protocol (SCTP) Duplicate Transmission Sequence Numbers
(TSNs) to Detect Spurious Retransmissions", RFC 3708,
February 2004.
[BBA06] Blanton, J., Blanton, E., and M. Allman, "Using Spurious
Retransmissions to Adapt the Retransmission Timeout",
Internet-Draft "draft-allman-rto-backoff-04.txt", December
2006. Work in progress.
[BBJ92] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[FMMP00] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, July 2000.
[GL02] Gurtov A. and R. Ludwig, "Evaluating the Eifel Algorithm
for TCP in a GPRS Network", In Proc. European Wireless,
Florence, Italy, February 2002.
[GL03] Gurtov A. and R. Ludwig, "Responding to Spurious Timeouts
in TCP", In Proc. IEEE INFOCOM 03, San Francisco, CA, USA,
March 2003.
[Jac88] Jacobson, V., "Congestion Avoidance and Control", In
Proc. ACM SIGCOMM 88.
[Hok05] Hokamura, A., et al., "Performance Evaluation of F-RTO and
Eifel Response Algorithms over W-CDMA packet network", In
Proc. Wireless Personal Multimedia Communications
(WPMC'05),
Sept. 2005.
[KYHS07] Kojo, M., Yamamoto, K., Hata, M., and P. Sarolahti,
"Evaluation of RFC 4138", Internet-draft
"draft-kojo-tcpm-frto-eval-00.txt", June 2007. Work
in progress.
[LG04] Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
for TCP", RFC 4015, February 2005.
[LK00] Ludwig R. and R.H. Katz, "The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions", ACM SIGCOMM
Computer Communication Review, 30(1), January 2000.
Sarolahti/Kojo/Yamamoto/Hata [Page 18]
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[LM03] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
for TCP", RFC 3522, April 2003.
[Nag84] Nagle, J., "Congestion Control in IP/TCP Internetworks",
RFC 896, January 1984.
[SK05] Sarolahti, P. and M. Kojo, "Forward RTO-Recovery (F-RTO):
An Algorithm for Detecting Spurious Retransmission
Timeouts with TCP and the Stream Control Transmission
Protocol (SCTP)", RFC 4138, August 2005.
[SKR03] Sarolahti, P., Kojo, M., and K. Raatikainen, "F-RTO: An
Enhanced Recovery Algorithm for TCP Retransmission
Timeouts", ACM SIGCOMM Computer Communication Review,
33(2), April 2003.
[Sar03] P. Sarolahti, P., "Congestion Control on Spurious TCP
Retransmission Timeouts", In Proc. of IEEE Globecom
2003, San Francisco, CA, USA. December 2003.
[SL03] Swami Y. and K. Le, "DCLOR: De-correlated Loss Recovery
using SACK Option for Spurious Timeouts", Expired
Internet-Draft, September 2003.
[Ste07] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[Yam05] Yamamoto, K., et al., "Effects of F-RTO and Eifel Response
Algorithms for W-CDMA and HSDPA networks", In Proc.
Wireless
Personal Multimedia Communications (WPMC'05), September
2005.
AUTHORS' ADDRESSES
Pasi Sarolahti
Nokia Research Center
P.O. Box 407
FI-00045 NOKIA GROUP
Finland
Phone: +358 50 4876607
Email: pasi.sarolahti@nokia.com
Markku Kojo
University of Helsinki
Sarolahti/Kojo/Yamamoto/Hata [Page 19]
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P.O. Box 68
FI-00014 UNIVERSITY OF HELSINKI
Finland
Email: kojo@cs.helsinki.fi
Kazunori Yamamoto
NTT Docomo, Inc.
3-5 Hikarinooka, Yokosuka, Kanagawa, 239-8536, Japan
Phone: +81-46-840-3812
Email: yamamotokaz@nttdocomo.co.jp
Max Hata
NTT Docomo, Inc.
3-5 Hikarinooka, Yokosuka, Kanagawa, 239-8536, Japan
Phone: +81-46-840-3812
Email: hatama@s1.nttdocomo.co.jp
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