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Network Working Group P. Sarolahti
Request for Comments: 4138 Nokia Research Center
Category: Experimental M. Kojo
University of Helsinki
August 2005
Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP and the
Stream Control Transmission Protocol (SCTP)
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
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. The
F-RTO algorithm can also be applied to the Stream Control
Transmission Protocol (SCTP).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . 4
2. F-RTO Algorithm . . . . . . . . . . . . . . . . . . . . . 4
2.1. The Algorithm . . . . . . . . . . . . . . . . . . . 5
2.2. Discussion . . . . . . . . . . . . . . . . . . . . 6
3. SACK-Enhanced Version of the F-RTO Algorithm . . . . . . 8
4. Taking Actions after Detecting Spurious RTO . . . . . . . 10
5. SCTP Considerations . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References. . . . . . . . . . . . . . . . 12
8.2. Informative References. . . . . . . . . . . . . . . 13
Appendix A: Scenarios . . . . . . . . . . . . . . . . . . . . 15
Appendix B: SACK-Enhanced F-RTO and Fast Recovery . . . . . . 20
Appendix C: Discussion of Window-Limited Cases . . . . . . . 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) expires. 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
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].
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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, given a low-bandwidth link or some other change in
available bandwidth, arrival of competing traffic (possibly with
higher priority) can cause a sudden increase of 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. 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 an RTO expires, the F-RTO sender retransmits the first
unacknowledged segment as usual [APS99]. Deviating from 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
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SACK-enhanced version of the F-RTO algorithm, spurious timeouts may
be detected even if duplicate ACKs arrive after an RTO
retransmission.
The F-RTO algorithm can also be applied to the Stream Control
Transmission Protocol (SCTP) [Ste00], because SCTP has acknowledgment
and packet retransmission concepts similar to TCP. For convenience,
this document mostly refers to TCP, but the algorithms and other
discussion are valid for SCTP as well.
This document is organized as follows. Section 2 describes the basic
F-RTO algorithm. Section 3 outlines an optional enhancement to the
F-RTO algorithm that takes advantage of the TCP SACK option. Section
4 discusses the possible actions to be taken after detecting a
spurious RTO. Section 5 gives considerations on applying F-RTO with
SCTP, and Section 6 discusses the security considerations.
1.1. Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
2. 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 RTO
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 timeout, the F-RTO algorithm declares a timeout
spurious. The actions taken in response 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.
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2.1. The Algorithm
A TCP sender MAY implement the basic F-RTO algorithm. If it chooses
to apply the algorithm, the following steps MUST be taken after the
retransmission timer expires. 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.
1) When RTO expires, retransmit the first unacknowledged segment and
set SpuriousRecovery to FALSE. Also, store the highest sequence
number transmitted so far in variable "recover".
2) When the first acknowledgment after the RTO retransmission arrives
at the sender, the 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 it acknowledges a
sequence number equal to the value of "recover" OR it 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 it is below
the value of "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 C.
3) When the second acknowledgment after the RTO retransmission
arrives at the sender, the TCP sender either declares the timeout
spurious, or starts retransmitting the unacknowledged segments.
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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 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.
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
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revert to the conventional RTO recovery with slow start
retransmissions. Appendix C discusses some 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 RTO expires
during fast recovery with NewReno TCP. Because the 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 the 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
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F-RTO fails to detect the spurious timeout, it retransmits the
unacknowledged segments in slow start, and thus 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.
Given that the TCP Selective Acknowledgment Option [MMFR96] is
enabled for a TCP connection, a TCP sender MAY implement 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 SACK
loss recovery when retransmission timeout occurs. However, when
retransmission timeout occurs during existing loss recovery, it
should be possible to apply the principle of F-RTO within certain
limitations. This is a topic for further research. Appendix B
briefly discusses the related issues.
The steps of the SACK-enhanced version of the F-RTO algorithm are as
follows.
1) When the RTO expires, retransmit the first unacknowledged segment
and set SpuriousRecovery to FALSE. Set variable "recover" to
indicate the highest segment transmitted so far. Following the
recommendation in SACK specification [MMFR96], reset the SACK
scoreboard.
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 RTO expires
again, go to step 1 of the algorithm.
a) if a cumulative ACK acknowledges a sequence number equal to
"recover", 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.
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b) else, if a cumulative ACK acknowledges a sequence number
(smaller than "recover", but 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 C. In this
case, the TCP sender should follow the recommendations
concerning acknowledgments of retransmitted segments given in
Appendix B.
3) The next acknowledgment arrives at the sender. Either a duplicate
ACK or a new cumulative ACK (advancing the window) applies in this
step.
a) if the ACK acknowledges a sequence number above "recover",
either in SACK blocks or as a cumulative ACK, 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 "recover" in the SACK blocks. Leave
SpuriousRecovery set to FALSE.
b) if the ACK does not acknowledge sequence numbers above
"recover" 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, "recover" is set to SND.UNA, thus
allowing fast recovery on incoming duplicate acknowledgments.
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4. Taking Actions after Detecting Spurious RTO
Upon 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 after
detecting a spurious RTO. This document does not describe the
response to spurious timeouts, but a response algorithm is described
in RFC 4015 [LG04].
Additionally, different response variants to spurious retransmission
timeout have been discussed in various research papers [SKR03, GL03,
Sar03] and IETF documents [SL03]. The different response
alternatives vary in whether the spurious retransmission timeout
should be taken as a congestion signal, thus causing the congestion
window or slow start threshold to be reduced at the sender, or
whether the congestion control state should be fully reverted to the
state valid prior to the retransmission timeout.
5. SCTP Considerations
SCTP has similar retransmission algorithms and congestion control to
TCP. The SCTP T3-rtx timer for one destination address is maintained
in the same way as the TCP retransmission timer, and after a T3-rtx
expires, an SCTP sender retransmits unacknowledged data chunks in
slow start like TCP does. Therefore, SCTP is vulnerable to the
negative effects of the spurious retransmission timeouts similarly to
TCP. Due to similar RTO recovery algorithms, F-RTO algorithm logic
can be applied also to SCTP. Since SCTP uses selective
acknowledgments, the SACK-based variant of the algorithm is
recommended, although the basic version can also be applied to SCTP.
However, SCTP contains features that are not present with TCP that
need to be discussed when applying the F-RTO algorithm.
SCTP associations can be multi-homed. The current retransmission
policy states that retransmissions should go to alternative
addresses. If the retransmission was due to spurious timeout caused
by a delay spike, it is possible that the acknowledgment for the
retransmission arrives back at the sender before the acknowledgments
of the original transmissions arrive. If this happens, a possible
loss of the original transmission of the data chunk that was
retransmitted due to the spurious timeout may remain undetected when
applying the F-RTO algorithm. Because the timeout was caused by a
delay spike, and it was spurious in that respect, a suitable response
is to continue by sending new data. However, if the original
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transmission was lost, fully reverting the congestion control
parameters is too aggressive. Therefore, taking conservative actions
on congestion control is recommended, if the SCTP association is
multi-homed and retransmissions go to alternative addresses. The
information in duplicate TSNs can be then used for reverting
congestion control, if desired [BA04].
Note that the forward transmissions made in F-RTO algorithm step (2b)
should be destined to the primary address, since they are not
retransmissions.
When making a retransmission, an SCTP sender can bundle a number of
unacknowledged data chunks and include them in the same packet. This
needs to be considered when implementing F-RTO for SCTP. The basic
principle of F-RTO still holds: in order to declare the timeout
spurious, the sender must get an acknowledgment for a data chunk that
was not retransmitted after the retransmission timeout. In other
words, acknowledgments of data chunks that were bundled in RTO
retransmission must not be used for declaring the timeout spurious.
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 expires during
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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.
7. Acknowledgements
We are grateful 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 for the discussion and feedback
contributed to this text.
8. References
8.1. Normative References
[APS99] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[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.
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RFC 4138 Forward RTO-Recovery August 2005
[Ste00] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
L., and V. Paxson, "Stream Control Transmission Protocol",
RFC 2960, October 2000.
8.2. Informative References
[ABF01] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
TCP's Loss Recovery Using Limited Transmit", RFC 3042,
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.
[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] A. Gurtov and R. Ludwig. Evaluating the Eifel Algorithm
for TCP in a GPRS Network. In Proc. of European Wireless,
Florence, Italy, February 2002.
[GL03] A. Gurtov and R. Ludwig, Responding to Spurious Timeouts in
TCP. In Proceedings of IEEE INFOCOM 03, San Francisco, CA,
USA, March 2003.
[Jac88] V. Jacobson. Congestion Avoidance and Control. In
Proceedings of ACM SIGCOMM 88.
[LG04] Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm for
TCP", RFC 4015, February 2005.
[LK00] R. Ludwig and R.H. Katz. The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions. ACM SIGCOMM
Computer Communication Review, 30(1), January 2000.
[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.
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RFC 4138 Forward RTO-Recovery August 2005
[SKR03] P. Sarolahti, M. Kojo, 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. Congestion Control on Spurious TCP
Retransmission Timeouts. In Proceedings of IEEE Globecom
2003, San Francisco, CA, USA. December 2003.
[SL03] Y. Swami and K. Le, "DCLOR: De-correlated Loss Recovery
using SACK Option for Spurious Timeouts", work in progress,
September 2003.
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Appendix A: Scenarios
This section discusses different scenarios where RTOs occur and how
the basic F-RTO algorithm performs in those scenarios. The
interesting scenarios are: a sudden delay triggering retransmission
timeout, loss of a retransmitted packet during fast recovery, link
outage causing the loss of several packets, and packet reordering. A
performance evaluation with a more thorough analysis on a real
implementation of F-RTO is given in [SKR03].
A.1. Sudden Delay
The main motivation behind the F-RTO algorithm is to improve TCP
performance when a delay spike triggers a spurious retransmission
timeout. The example below illustrates the segments and
acknowledgments transmitted by the TCP end hosts when a spurious
timeout occurs, but no packets are lost. For simplicity, delayed
acknowledgments are not used in the example. The example below
applies the Eifel Response Algorithm [LG04] after detecting a
spurious timeout.
...
(cwnd = 6, ssthresh < 6, FlightSize = 6)
1. <---------------------------- ACK 5
2. SEND 10 ---------------------------->
(cwnd = 6, ssthresh < 6, FlightSize = 6)
3. <---------------------------- ACK 6
4. SEND 11 ---------------------------->
(cwnd = 6, ssthresh < 6, FlightSize = 6)
5. |
[delay]
|
[RTO]
[F-RTO step (1)]
6. SEND 6 ---------------------------->
(cwnd = 6, ssthresh = 3, FlightSize = 6)
<earlier xmitted SEG 6> --->
7. <---------------------------- ACK 7
[F-RTO step (2b)]
8. SEND 12 ---------------------------->
9. SEND 13 ---------------------------->
(cwnd = 7, ssthresh = 3, FlightSize = 7)
<earlier xmitted SEG 7> --->
10. <---------------------------- ACK 8
[F-RTO step (3b)]
[SpuriousRecovery <- SPUR_TO]
(cwnd = 7, ssthresh = 6, FlightSize = 6)
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11. SEND 14 ---------------------------->
(cwnd = 7, ssthresh = 6, FlightSize = 7)
12. <---------------------------- ACK 9
13. SEND 15 ---------------------------->
(cwnd = 7, ssthresh = 6, FlightSize = 7)
14. <---------------------------- ACK 10
15. SEND 16 ---------------------------->
(cwnd = 7, ssthresh = 6, FlightSize = 7)
...
When a sudden delay (long enough to trigger timeout) occurs at step
5, the TCP sender retransmits the first unacknowledged segment (step
6). The next ACK covers the RTO retransmission because the
originally transmitted segment 6 arrived at the receiver, and the TCP
sender continues by sending two new data segments (steps 8, 9). Note
that on F-RTO steps (1) and (2b), congestion window and FlightSize
are not yet reset because in the case of spurious timeout, the
segments sent before the timeout are still in the network. However,
the sender should still be equally aggressive toward conventional
TCP. Because the second acknowledgment arriving after the RTO
retransmission acknowledges data that was not retransmitted due to
timeout (step 10), the TCP sender declares the timeout to be spurious
and continues by sending new data on the next acknowledgments. Also,
the congestion control state is reversed, as required by the Eifel
Response Algorithm.
A.2. Loss of a Retransmission
If a retransmitted segment is lost, the only way to retransmit it is
to wait for the timeout to trigger the retransmission. Once the
segment is successfully received, the receiver usually acknowledges
several segments at once, because other segments in the same window
have been successfully delivered before the retransmission arrives at
the receiver. The example below shows a scenario where
retransmission (of segment 6) is lost, as well as a later segment
(segment 9) in the same window. The limited transmit [ABF01] or SACK
TCP [MMFR96] enhancements are not in use in this example.
...
(cwnd = 6, ssthresh < 6, FlightSize = 6)
<segment 6 lost>
<segment 9 lost>
1. <---------------------------- ACK 5
2. SEND 10 ---------------------------->
(cwnd = 6, ssthresh < 6, FlightSize = 6)
3. <---------------------------- ACK 6
4. SEND 11 ---------------------------->
(cwnd = 6, ssthresh < 6, FlightSize = 6)
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5. <---------------------------- ACK 6
6. <---------------------------- ACK 6
7. <---------------------------- ACK 6
8. SEND 6 --------------X
(cwnd = 6, ssthresh = 3, FlightSize = 6)
<segment 6 lost>
9. <---------------------------- ACK 6
10. SEND 12 ---------------------------->
(cwnd = 7, ssthresh = 3, FlightSize = 7)
11. <---------------------------- ACK 6
12. SEND 13 ---------------------------->
(cwnd = 8, ssthresh = 3, FlightSize = 8)
[RTO]
13. SEND 6 ---------------------------->
(cwnd = 8, ssthresh = 2, FlightSize = 8)
14. <---------------------------- ACK 9
[F-RTO step (2b)]
15. SEND 14 ---------------------------->
16. SEND 15 ---------------------------->
(cwnd = 7, ssthresh = 2, FlightSize = 7)
17. <---------------------------- ACK 9
[F-RTO step (3a)]
[SpuriousRecovery <- FALSE]
(cwnd = 3, ssthresh = 2, FlightSize = 7)
18. SEND 9 ---------------------------->
19. SEND 10 ---------------------------->
20. SEND 11 ---------------------------->
...
In the example above, segment 6 is lost and the sender retransmits it
after three duplicate ACKs in step 8. However, the retransmission is
also lost, and the sender has to wait for the RTO to expire before
retransmitting it again. Because the first ACK following the RTO
retransmission acknowledges the RTO retransmission (step 14), the
sender transmits two new segments. The second ACK in step 17 does
not acknowledge any previously unacknowledged data. Therefore, the
F-RTO sender enters the slow start and sets cwnd to 3 * MSS. The
congestion window can be set to three segments, because two round-
trips have elapsed after the retransmission timeout. Finally, the
receiver acknowledges all segments transmitted prior to entering
recovery and the sender can continue transmitting new data in
congestion avoidance.
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A.3. Link Outage
The example below illustrates the F-RTO behavior when 4 consecutive
packets are lost in the network causing the TCP sender to fall back
to RTO recovery. Limited transmit and SACK are not used in this
example.
...
(cwnd = 6, ssthresh < 6, FlightSize = 6)
<segments 6-9 lost>
1. <---------------------------- ACK 5
2. SEND 10 ---------------------------->
(cwnd = 6, ssthresh < 6, FlightSize = 6)
3. <---------------------------- ACK 6
4. SEND 11 ---------------------------->
(cwnd = 6, ssthresh < 6, FlightSize = 6)
5. <---------------------------- ACK 6
|
|
[RTO]
6. SEND 6 ---------------------------->
(cwnd = 6, ssthresh = 3, FlightSize = 6)
7. <---------------------------- ACK 7
[F-RTO step (2b)]
8. SEND 12 ---------------------------->
9. SEND 13 ---------------------------->
(cwnd = 7, ssthresh = 3, FlightSize = 7)
10. <---------------------------- ACK 7
[F-RTO step (3a)]
[SpuriousRecovery <- FALSE]
(cwnd = 3, ssthresh = 3, FlightSize = 7)
11. SEND 7 ---------------------------->
12. SEND 8 ---------------------------->
13. SEND 9 ---------------------------->
Again, F-RTO sender transmits two new segments (steps 8 and 9) after
the RTO retransmission is acknowledged. Because the next ACK does
not acknowledge any data that was not retransmitted after the
retransmission timeout (step 10), the F-RTO sender proceeds with
conventional recovery and slow start retransmissions.
A.4. Packet Reordering
Because F-RTO modifies the TCP sender behavior only after a
retransmission timeout and it is intended to avoid unnecessary
retransmissions only after spurious timeout, we limit the discussion
on the effects of packet reordering on F-RTO behavior to the cases
where it occurs immediately after the retransmission timeout. When
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the TCP receiver gets an out-of-order segment, it generates a
duplicate ACK. If the TCP sender implements the basic F-RTO
algorithm, this may prevent the sender from detecting a spurious
timeout.
However, if the TCP sender applies the SACK-enhanced F-RTO, it is
possible to detect a spurious timeout when packet reordering occurs.
Below, we illustrate the behavior of SACK-enhanced F-RTO when segment
8 arrives before segments 6 and 7, and segments starting from segment
6 are delayed in the network. In this example the TCP sender reduces
the congestion window and slow start threshold in response to
spurious timeout.
...
(cwnd = 6, ssthresh < 6, FlightSize = 6)
1. <---------------------------- ACK 5
2. SEND 10 ---------------------------->
(cwnd = 6, ssthresh < 6, FlightSize = 6)
3. <---------------------------- ACK 6
4. SEND 11 ---------------------------->
5. |
[delay]
|
[RTO]
6. SEND 6 ---------------------------->
(cwnd = 6, ssthresh = 3, FlightSize = 6)
<earlier xmitted SEG 8> --->
7. <---------------------------- ACK 6
[SACK 8]
[SACK F-RTO stays in step 2]
8. <earlier xmitted SEG 6> --->
9. <---------------------------- ACK 7
[SACK 8]
[SACK F-RTO step (2b)]
10. SEND 12 ---------------------------->
11. SEND 13 ---------------------------->
(cwnd = 7, ssthresh = 3, FlightSize = 7)
12. <earlier xmitted SEG 7> --->
13. <---------------------------- ACK 9
[SACK F-RTO step (3b)]
[SpuriousRecovery <- SPUR_TO]
(cwnd = 7, ssthresh = 6, FlightSize = 6)
14. SEND 14 ---------------------------->
(cwnd = 7, ssthresh = 6, FlightSize = 7)
15. <---------------------------- ACK 10
16. SEND 15 ---------------------------->
...
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After RTO expires and the sender retransmits segment 6 (step 6), the
receiver gets segment 8 and generates duplicate ACK with SACK for
segment 8. In response to the acknowledgment, the TCP sender does
not send anything but stays in F-RTO step 2. Because the next
acknowledgment advances the cumulative ACK point (step 9), the sender
can transmit two new segments according to SACK-enhanced F-RTO. The
next segment acknowledges new data between 7 and 11 that was not
acknowledged earlier (segment 7), so the F-RTO sender declares the
timeout spurious.
Appendix B: SACK-enhanced F-RTO and Fast Recovery
We believe that a slightly modified, SACK-enhanced F-RTO algorithm
can be used to detect spurious timeouts also when RTO expires while
an earlier loss recovery is underway. However, there are issues that
need to be considered if F-RTO is applied in this case.
In step 3, the original SACK-based F-RTO algorithm requires that an
ACK acknowledges previously unacknowledged non-retransmitted data
between SND.UNA and send_high. If RTO expires during earlier
(SACK-based) loss recovery, the F-RTO sender must use only
acknowledgments for non-retransmitted segments transmitted before the
SACK-based loss recovery started. This means that in order to
declare timeout spurious, the TCP sender must receive an
acknowledgment for non-retransmitted segment between SND.UNA and
RecoveryPoint in algorithm step 3. RecoveryPoint is defined in
conservative SACK-recovery algorithm [BAFW03], and it is set to
indicate the highest segment transmitted so far when SACK-based loss
recovery begins. In other words, if the TCP sender receives
acknowledgment for a segment that was transmitted more than one RTO
ago, it can declare the timeout spurious. Defining an efficient
algorithm for checking these conditions remains a future work item.
When spurious timeout is detected according to the rules given above,
it may be possible that the response algorithm needs to consider this
case separately, for example, in terms of which segments to
retransmit after RTO expires, and whether it is safe to revert the
congestion control parameters. This is considered a topic for future
research.
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Appendix C: 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 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].
- 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. Thus, this option
is not encouraged, except for hosts that are known to operate in an
environment that is prone to spurious timeouts. On the other hand,
with this method it is possible to limit unnecessary
retransmissions due to spurious timeout to one retransmission.
- Send a zero-sized segment below SND.UNA, similar to 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.
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Authors' Addresses
Pasi Sarolahti
Nokia Research Center
P.O. Box 407
FIN-00045 NOKIA GROUP
Finland
Phone: +358 50 4876607
EMail: pasi.sarolahti@nokia.com
http://www.cs.helsinki.fi/u/sarolaht/
Markku Kojo
University of Helsinki
Department of Computer Science
P.O. Box 68
FIN-00014 UNIVERSITY OF HELSINKI
Finland
Phone: +358 9 191 51305
EMail: kojo@cs.helsinki.fi
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