One document matched: draft-ietf-tsvwg-tcp-frto-01.txt
Differences from draft-ietf-tsvwg-tcp-frto-00.txt
Internet Engineering Task Force P. Sarolahti
INTERNET DRAFT Nokia Research Center
File: draft-ietf-tsvwg-tcp-frto-01.txt M. Kojo
University of Helsinki
February, 2004
Expires: August, 2004
F-RTO: An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP and SCTP
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
Spurious retransmission timeouts (RTOs) cause suboptimal TCP
performance, because they often result in unnecessary retransmission
of the last window of data. This document describes the "Forward RTO
Recovery" (F-RTO) algorithm for detecting spurious TCP RTOs. 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 an RTO, the F-RTO algorithm at a TCP sender monitors the
incoming acknowledgements to determine whether the timeout was
spurious and to decide whether to send new segments or retransmit
unacknowledged segments. The algorithm effectively helps to avoid
additional unnecessary retransmissions and thereby improves TCP
performance in case of a spurious timeout. The F-RTO algorithm can
Expires: August 2004 [Page 1]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
also be applied with the SCTP protocol.
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].
1. Introduction
The TCP protocol [Pos81] has two methods for triggering
retransmissions. Primarily, 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]. Secondarily, the TCP sender maintains a
retransmission timer which triggers retransmission of segments, if
they have not been acknowledged within the retransmission timer
expiration period. When the retransmission timer expires, the TCP
sender enters the RTO recovery where 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 trigger unnecessary retransmissions when no segments
have been lost [LK00, GL02, LM03]. After a spurious retransmission
timeout the late acknowledgements of original segments arrive at the
sender, usually triggering unnecessary retransmissions of 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 acknowledgement in slow start,
injecting a large number of data segments to the network within one
round-trip time.
There are a number of potential reasons for spurious retransmission
timeouts. First, some mobile networking technologies involve sudden
delay peaks on transmission because of actions taken during a hand-
off. Second, arrival of competing traffic, possibly with higher
priority, on a low-bandwidth link or some other change in available
bandwidth involves a sudden increase of round-trip time which may
trigger a spurious retransmission timeout. A persistently reliable
link layer can also cause a sudden delay when several data frames are
lost for some reason. This document does not distinguish the
different causes of such a delay, but discusses the spurious
Expires: August 2004 [Page 2]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
retransmission timeouts caused by a delay in general.
This document describes an alternative RTO recovery algorithm called
"Forward RTO-Recovery" (F-RTO) to be used for detecting spurious RTOs
and thus avoiding unnecessary retransmissions following the RTO. When
the RTO is not spurious, the F-RTO algorithm reverts back to the
conventional RTO recovery algorithm and should have similar behavior
and performance. F-RTO does not require any TCP options in its
operation, and it can be implemented by modifying only the TCP
sender. This is different from alternative algorithms (Eifel [LK00],
[LM03] and DSACK-based algorithms [BA02]) that have been suggested
for detecting unnecessary retransmissions. The Eifel algorithm uses
TCP timestamps [BBJ92] for detecting a spurious timeout upon arrival
of the first acknowledgement after the retransmission. The DSACK-
based algorithms require that the TCP Selective Acknowledgment Option
[MMFR96] with DSACK extension [FMMP00] is in use. With DSACK, the TCP
receiver can report if it has received a duplicate segment, making it
possible for the sender to detect afterwards whether it has
retransmitted segments unnecessarily. In addition, the F-RTO
algorithm only attempts to detect and avoid unnecessary
retransmissions after an RTO. Eifel and DSACK can also be used in
detecting unnecessary retransmissions in other events, for example
due to packet reordering.
When an RTO occurs, the F-RTO sender retransmits the first
unacknowledged segment as usual. Deviating from the normal operation
after a timeout, it then tries to transmit new, previously unsent
data, for the first acknowledgement that arrives after the timeout
given that the acknowledgement advances the window. If the second
acknowledgement that arrives after the timeout also advances the
window, i.e., acknowledges data that was not retransmitted, the F-RTO
sender declares the RTO spurious and exit the RTO recovery. However,
if either of the next two acknowledgements is a duplicate ACK, there
was no sufficient evidence of spurious RTO; 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 RTOs may be detected even if
duplicate ACKs arrive after an RTO.
The F-RTO algorithm can also be applied with the SCTP protocol
[Ste00], because SCTP has similar acknowledgement and packet
retransmission concepts as TCP. When a SCTP retransmission timeout
occurs, the SCTP sender is required to retransmit the outstanding
data similarly to TCP, thus being prone to unnecessary
retransmissions and congestion control adjustments, if delay spikes
occur in the network. The SACK-enhanced version of F-RTO should be
directly applicable to SCTP, which has selective acknowledgements as
a built-in feature. For simplicity, this document mostly refers to
Expires: August 2004 [Page 3]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
TCP, but the algorithms and other discussion should be applicable
also to SCTP.
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 leverage on the TCP SACK option. Section 4
discusses the possible actions to be taken after detecting a spurious
RTO, and Section 5 discusses the security considerations.
2. F-RTO Algorithm
An RTO is spurious if there are segments outstanding in the network
that would have prevented the RTO, had their acknowledgements arrived
earlier at the sender. F-RTO affects the TCP sender behavior only
after a retransmission timeout, otherwise the TCP behavior remains
unmodified. When RTO expires the F-RTO algorithm monitors incoming
acknowledgements and declares an RTO spurious, if the TCP sender gets
an acknowledgement for a segment that was not retransmitted due to
RTO. The actions taken in response to spurious RTO are not specified
in this document, but we discuss the different alternatives for
congestion control in Section 4.
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 related response algorithm. With F-RTO,
the outcome of SpuriousRecovery can either be SPUR_TO, indicating a
spurious retransmission timeout; or FALSE, when the RTO is not
declared spurious, and the TCP sender should follow the conventional
RTO recovery algorithm.
A TCP sender MAY implement the basic F-RTO algorithm, and if it
chooses to apply the algorithm, the following steps MUST be taken
after the retransmission timer expires.
1) When RTO expires, the TCP sender SHOULD retransmit the first
unacknowledged segment and set SpuriousRecovery to FALSE. Store
the highest sequence number transmitted so far in variable
"send_high".
2) When the first acknowledgement after the RTO arrives at the
sender, the sender chooses the following actions depending on
whether the ACK advances the window or whether it is a duplicate
ACK.
a) If the acknowledgement is a duplicate ACK OR it is
acknowledging a sequence number equal to (or above) the value
Expires: August 2004 [Page 4]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
of send_high OR it does not acknowledge all of the data that
was retransmitted in step 1, the TCP sender MUST revert to the
conventional RTO recovery and continue by retransmitting
unacknowledged data in slow start. The TCP sender MUST NOT
enter step 3 of this algorithm, and the SpuriousRecovery
variable remains as FALSE.
b) If the acknowledgement advances the window AND it is below the
value of send_high, the TCP sender SHOULD 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 SHOULD send only one segment. In addition, the TCP sender
MAY override the Nagle algorithm and send immediately an
undersized segment if needed. If the TCP sender does not have
any new data to send, the TCP sender SHOULD transmit a segment
from the retransmission queue. If TCP sender retransmits the
first unacknowledged segment, it MUST NOT enter step 3 of this
algorithm but continue with the conventional RTO recovery
algorithm. In this case acknowledgement of the next segment
would not unambiguously indicate that the original transmission
arrived at the receiver.
3) When the second acknowledgement after the RTO arrives at the
sender, either declare the RTO spurious, or start retransmitting
the unacknowledged segments.
a) If the acknowledgement is a duplicate ACK, the TCP sender MUST
set congestion window to no more than 3 * MSS, and continue
with the slow start algorithm retransmitting unacknowledged
segments. The sender leaves SpuriousRecovery set to FALSE.
b) If the acknowledgement advances the window, i.e. it
acknowledges data that was not retransmitted after the RTO, the
TCP sender SHOULD declare the RTO spurious, set
SpuriousRecovery to SPUR_TO and set the value of send_high
variable to SND.UNA.
The F-RTO sender takes cautious actions when it receives duplicate
acknowledgements after an RTO. Since duplicate ACKs may indicate that
segments have been lost, reliably detecting a spurious RTO is
difficult in the lack of additional information. Therefore the safest
alternative is to follow the conventional TCP recovery in those
cases.
If the first acknowledgement after RTO covers the send_high point at
algorithm step (2a), there is not enough evidence that a non-
retransmitted segment has arrived at the receiver after the RTO.
Expires: August 2004 [Page 5]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
This is a common case when a fast retransmission is lost and it has
been retransmitted again after an RTO, while the rest of the
unacknowledged segments have successfully been delivered to the TCP
receiver before the RTO. Therefore the RTO cannot be declared
spurious in this case.
If the first acknowledgement after RTO 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
RTO 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. If sending new
data is not possible in algorithm branch (2b), or 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 may run out of segments,
and no further acknowledgements arrive. If transmitting previously
unsent data is not possible, the following options are available for
the sender.
- Continue with the conventional RTO recovery algorithm and do not
try to detect the spurious RTO. The disadvantage is that the sender
may do unnecessary retransmissions due to possible spurious RTO. On
the other hand, we believe that the benefits of detecting spurious
RTO in an application limited or receiver limited situations are
not very remarkable.
- Use additional information if available, e.g. TCP timestamps with
the Eifel Detection algorithm, for detecting a spurious RTO.
However, Eifel detection may yield different results from F-RTO
when ACK losses and a 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 is unnecessarily made, hence this option is not
encouraged, except for hosts that are known to operate in an
environment that is highly likely to have spurious RTOs. On the
other hand, with this method it is possible to avoid several
unnecessary retransmissions due to spurious RTO by doing only one
retransmission that may be unnecessary.
- Send a zero-sized segment below SND.UNA similar to TCP Keep-Alive
probe and continue with step 3 of the F-RTO algorithm. Since the
receiver replies with a duplicate ACK, the sender is able to detect
Expires: August 2004 [Page 6]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
from the incoming acknowledgement whether the RTO was spurious.
While this method does not send data unnecessarily, it delays the
recovery by one round-trip time in cases where the RTO was not
spurious, and therefore 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 has 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.
If the RTO is declared spurious, the TCP sender sets the value of
send_high variable to SND.UNA in order to disable the NewReno
"bugfix" [FH99]. The send_high variable was proposed for avoiding
unnecessary multiple fast retransmits when RTO expires during fast
recovery with NewReno TCP. As the sender has not retransmitted other
segments but the one that triggered RTO, the problem addressed by the
bugfix cannot occur. Therefore, if there are three duplicate ACKs
arriving at the sender after the RTO, they are likely to indicate a
packet loss, hence 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 another
time and the sender enters step 1 of this algorithm.
When the RTO 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, and thus there is a small possibility that the F-RTO sender
violates the congestion control rules, if it chooses to fully revert
congestion control parameters after detecting a spurious RTO. 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 RTO, 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 retransmission ambiguity. As a result, the RTO
estimator is likely to be more accurate and have larger values with
F-RTO than with the regular TCP after a spurious RTO that was
triggered due to delayed segments. We believe this is an advantage in
the networks that are prone to delay spikes.
It is possible that the F-RTO algorithm does not always avoid
Expires: August 2004 [Page 7]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
unnecessary retransmissions after a spurious RTO. If packet
reordering or packet duplication occurs on the segment that triggered
the spurious RTO, the F-RTO algorithm may not detect the spurious RTO
due to incoming duplicate ACKs. Additionally, if a spurious RTO
occurs during fast recovery, the F-RTO algorithm often cannot detect
the spurious RTO. However, we consider these cases relatively rare,
and note that in cases where F-RTO fails to detect the spurious RTO,
it performs similarly to the regular RTO recovery.
3. A SACK-enhanced version of the F-RTO algorithm
This section describes an alternative version of the F-RTO algorithm,
that makes use of TCP Selective Acknowledgement Option [MMFR96]. By
using the SACK option the TCP sender can detect spurious RTOs in most
of the cases when packet reordering or packet duplication is present.
The difference to the basic F-RTO algorithm is that the sender may
declare RTO spurious even when duplicate ACKs follow the RTO, if the
SACK blocks acknowledge new data that was not transmitted after RTO.
The algorithm principle presented in this section is also applicable
to be used with the SCTP protocol.
Given that the TCP Selective Acknowledgement Option [MMFR96] is
enabled for a TCP connection, 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 loss recovery when
RTO occurs. However, it should be possible to apply the principle of
F-RTO within certain limitations also when RTO occurs during existing
loss recovery. While this is a topic of further research, Appendix B
briefly discusses the related issues.
1) When RTO expires, the TCP sender SHOULD retransmit first
unacknowledged segment and set SpuriousRecovery to FALSE. Variable
"send_high" is set to indicate the highest segment transmitted so
far.
2) Wait until the acknowledgement for the segment retransmitted due
to RTO arrives at the sender. If duplicate ACKs arrive, store the
incoming SACK information but stay in step 2. If RTO expires,
restart the algorithm.
a) if the cumulative ACK acknowledges all segments up to
send_high, the TCP sender SHOULD revert to the conventional RTO
recovery and it MUST set congestion window to no more than 2 *
MSS. The sender does not enter step 3 of this algorithm.
b) otherwise, the TCP sender SHOULD transmit up to two new
Expires: August 2004 [Page 8]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
(previously unsent) segments, within the limitations of the
congestion window. If the TCP sender is not able to transmit
any previously unsent data due to receiver window limitation or
because it does not have any new data to send, it MAY follow
one of the options presented in Section 2. However, if the TCP
sender chooses to retransmit a data segment here, SACK of that
segment MUST NOT be used for declaring a spurious RTO in step
(3b).
3) When the next acknowledgement arrives at the sender.
a) if the ACK acknowledges data above send_high, either in SACK
blocks or as a cumulative ACK, the sender MUST set congestion
window to no more than 3 * MSS and proceed with conventional
recovery, retransmitting unacknowledged segments. The sender
SHOULD take this branch also when the acknowledgement is a
duplicate ACK and it does not contain any new SACK blocks for
previously unacknowledged data below send_high.
b) if the ACK does not acknowledge data above send_high AND it
acknowledges some previously unacknowledged data below
send_high, the TCP sender SHOULD declare the RTO spurious and
set SpuriousRecovery to SPUR_TO.
If there are unacknowledged holes between the received SACK blocks,
those segments SHOULD be retransmitted similarly to the conventional
SACK recovery algorithm [BAFW03]. If the algorithm exits with
SpuriousRecovery set to SPUR_TO, send_high SHOULD be set to SND.UNA,
thus allowing fast recovery on incoming duplicate acknowledgements.
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 RTO is detected to be spurious, the
TCP sender should not continue retransmitting based on the RTO. For
example, if the sender was in congestion avoidance phase transmitting
new previously unsent segments, it should continue transmitting
previously unsent segments after detecting spurious RTO. In addition,
it is suggested that the RTO estimation is reinitialized and the RTO
timer is adjusted to a more conservative value in order to avoid
subsequent spurious RTOs [LG03].
Different approaches have been discussed for adjusting the congestion
control state after a spurious RTO in various research papers [SKR03,
GL03, Sar03] and Internet-Drafts [SL03, LG03]. The different response
suggestions vary in whether the spurious retransmission timeout
Expires: August 2004 [Page 9]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
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.
This document does not give recommendation on selecting the response
alternative, but considers the response to spurious RTO as a subject
of further research.
5. SCTP Considerations
The basic F-RTO or the SACK-enhanced F-RTO algorithm can be applied
with the SCTP protocol. However, SCTP contains features that are not
present with TCP that need to be discussed when applying the F-RTO
algorithm.
SCTP association can be multi-homed. The current retransmission pol-
icy states that retransmissions should go to alternative addresses.
If the retransmission was due to spurious RTO caused by a delay
spike, it is possible that the acknowledgement for the retransmission
arrives back at the sender before the acknowledgements of the origi-
nal transmissions arrive. If this happens, a possible loss of the
original transmission of the data chunk that was retransmitted due to
the spurious RTO may remain undetected when applying the F-RTO algo-
rithm. Because the RTO was caused by the delay, and it was spurious
in that respect, a suitable response is to continue by sending new
data. However, if the original transmission was lost, fully reverting
the congestion control parameters is too aggressive. Therefore, tak-
ing conservative actions on congestion control is recommended, if the
SCTP association is multi-homed and retransmissions go to alternative
address. The information in duplicate TSNs can be then used for
reverting congestion control, if desired [BA02].
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, a 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 RTO spurious,
the sender must get an acknowledgement for a data chunk that was not
retransmitted after the RTO. In other words, acknowledgements of data
chunks that were bundled in RTO retransmission must not be used for
declaring the RTO spurious.
Expires: August 2004 [Page 10]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
6. Security Considerations
The main security threat regarding F-RTO is the possibility of a
receiver misleading the sender to set 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
acknowledgement 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 lead to declare RTO spurious in F-RTO algorithm
step 3. However, since this causes the sender to have 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 of an RTO is that a fast retransmission of a segment is
lost. If all other segments have been received, the RTO retransmis-
sion 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 RTO to be
declared spurious when it is not. Therefore, it is suggested that
when an RTO expires during fast recovery phase, the sender would not
fully revert the congestion window even if the RTO was declared spu-
rious, but reduce the congestion window to 1. However, the sender can
take actions to avoid unnecessary retransmissions normally. If a TCP
sender implements a burst avoidance algorithm that limits the sending
rate to be no higher than in slow start, this precaution is not
needed, and the sender may apply F-RTO normally.
If there are more than one segments missing at the time when an RTO
occurs, the receiver does not benefit from misleading the sender to
declare a spurious RTO, because the sender would then have to go
through another recovery period to retransmit the missing segments,
usually after an RTO.
Acknowledgements
We are grateful to Reiner Ludwig, Andrei Gurtov, Josh Blanton, Mark
Allman, Sally Floyd, Yogesh Swami, Mika Liljeberg, Ivan Arias
Rodriguez, Sourabh Ladha, and Martin Duke for the discussion and
feedback contributed to this text.
Expires: August 2004 [Page 11]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
Normative References
[APS99] M. Allman, V. Paxson, and W. Stevens. TCP Congestion Con-
trol. RFC 2581, April 1999.
[BAFW03] E. Blanton, M. Allman, K. Fall, and L. Wang. A Conservative
Selective Acknowledgment (SACK)-based Loss Recovery Algo-
rithm for TCP. RFC 3517. April 2003.
[MMFR96] M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow. TCP Selec-
tive Acknowledgement Options. RFC 2018, October 1996.
[PA00] V. Paxson and M. Allman. Computing TCP's Retransmission
Timer. RFC 2988, November 2000.
[Pos81] J. Postel. Transmission Control Protocol. RFC 793, Septem-
ber 1981.
[Ste00] R. Stewart, et. al. Stream Control Transmission Protocol.
RFC 2960, October 2000.
Informative References
[ABF01] M. Allman, H. Balakrishnan, and S. Floyd. Enhancing TCP's
Loss Recovery Using Limited Transmit. RFC 3042, January
2001.
[BA02] E. Blanton and M. Allman. On Making TCP more Robust to
Packet Reordering. ACM SIGCOMM Computer Communication
Review, 32(1), January 2002.
[BBJ92] D. Borman, R. Braden, and V. Jacobson. TCP Extensions for
High Performance. RFC 1323, May 1992.
[FH99] S. Floyd and T. Henderson. The NewReno Modification to
TCP's Fast Recovery Algorithm. RFC 2582, April 1999.
[FMMP00] S. Floyd, J. Mahdavi, M. Mathis, and M. Podolsky. An Exten-
sion to the Selective Acknowledgement (SACK) Option to 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, Flo-
rence, Italy, February 2002
[GL03] A. Gurtov and R. Ludwig, Responding to Spurious Timeouts in
TCP, In Proceedings of IEEE INFOCOM 03, March 2003.
Expires: August 2004 [Page 12]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
[LG03] R. Ludwig and A. Gurtov. The Eifel Response Algorithm for
TCP. Internet draft "draft-ietf-tsvwg-tcp-eifel-
response-04.txt". October 2003. Work in progress.
[LK00] R. Ludwig and R.H. Katz. The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions. ACM SIGCOMM Com-
puter Communication Review, 30(1), January 2000.
[LM03] R. Ludwig and M. Meyer. The Eifel Detection Algorithm for
TCP. RFC 3522, April 2003.
[SKR03] P. Sarolahti, M. Kojo, and K. Raatikainen. F-RTO: An
Enhanced Recovery Algorithm for TCP Retransmission Time-
outs. ACM SIGCOMM Computer Communication Review, 33(2),
April 2003.
[Sar03] P. Sarolahti. Congestion Control on Spurious TCP Retrans-
mission Timeouts. In Proceedings of IEEE Globecom 2003.
December 2003.
[SL03] Y. Swami and K. Le. DCLOR: De-correlated Loss Recovery
using SACK option for spurious timeouts. Internet draft
"draft-swami-tsvwg-tcp-dclor-02.txt". September 2003. Work
in progress.
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 RTO, 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 of F-RTO algorithm is to improve TCP performance
when a delay spike triggers a spurious retransmission timeout. The
example below illustrates the segments and acknowledgements
transmitted by the TCP end hosts when a spurious RTO occurs, but no
packets are lost. For simplicity, delayed acknowledgements are not
used in the example. The example below reduces the congestion window
and slow start threshold by half after detecting a spurious RTO.
...
(cwnd = 6,
Expires: August 2004 [Page 13]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
ssthresh < 6,
FlightSize = 5)
1. SEND 10 ---------------------------->
2. <---------------------------- ACK 6
3. SEND 11 ---------------------------->
4. |
[delay]
|
[RTO]
5. SEND 6 ---------------------------->
<earlier xmitted SEG 6> --->
6. <---------------------------- ACK 7
[F-RTO step (2b)]
7. SEND 12 ---------------------------->
8. SEND 13 ---------------------------->
<earlier xmitted SEG 7> --->
9. <---------------------------- ACK 8
[F-RTO step (3b)]
[SpuriousRecovery <- SPUR_TO]
[cwnd <- 3, ssthresh <- 3]
10. <---------------------------- ACK 9
11. <---------------------------- ACK 10
12. <---------------------------- ACK 11
13. SEND 14 ---------------------------->
...
When a sudden delay long enough to trigger RTO occurs at step 4, the
TCP sender retransmits the first unacknowledged segment (step 5).
Because the next ACK covers the RTO retransmission because originally
transmitted segment 6 arrives at the receiver, the TCP sender
continues by sending two new data segments (steps 7, 8). Because the
second acknowledgement arriving after the RTO acknowledges data that
was not retransmitted due to RTO (step 9), the TCP sender declares
the RTO as spurious and continues by sending new data. Because the
TCP sender reduces cwnd when it detects the spurious RTO, it has to
wait for some outstanding segments to leave the network before it can
continue transmitting again at step 13.
A.2. Loss of a retransmission
If a retransmitted segment is lost, the only way to retransmit it
again is to wait for the RTO 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]
Expires: August 2004 [Page 14]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
enhancements are not in use in this example.
...
(cwnd = 6,
ssthresh < 6,
FlightSize = 5)
<segment 6 lost>
<segment 9 lost>
1. SEND 10 ---------------------------->
2. <---------------------------- ACK 6
3. SEND 11 ---------------------------->
4. <---------------------------- ACK 6
5. <---------------------------- ACK 6
6. <---------------------------- ACK 6
7. SEND 6 --------------X
<segment 6 lost>
[ssthresh <- 3, cwnd <- ssthresh + 3 = 6]
8. <---------------------------- ACK 6
|
|
[RTO]
[ssthresh <- 2]
9. SEND 6 ---------------------------->
10. <---------------------------- ACK 9
[F-RTO step (2b)]
11. SEND 12 ---------------------------->
12. SEND 13 ---------------------------->
13. <---------------------------- ACK 9
[F-RTO step (3a)]
[SpuriousRecovery <- FALSE]
[cwnd <- 3]
14. SEND 9 ---------------------------->
15. SEND 10 ---------------------------->
16. SEND 11 ---------------------------->
17. <---------------------------- ACK 11
...
In the example above, segment 6 is lost and the sender retransmits it
after three duplicate ACKs in step 7. 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
acknowledges the RTO retransmission (step 10), the sender transmits
two new segments. The second ACK in step 13 does not acknowledge any
previously unacknowledged data. Therefore the F-RTO sender enters the
slow start and sets cwnd to 3 * MSS. Congestion window can be set to
three segments, because two round-trips have elapsed after the RTO.
After this the receiver acknowledges all segments transmitted prior
to entering recovery and the sender can continue transmitting new
Expires: August 2004 [Page 15]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
data in congestion avoidance.
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 = 5)
<segments 6-9 lost>
1. SEND 10 ---------------------------->
2. <---------------------------- ACK 6
3. SEND 11 ---------------------------->
4. <---------------------------- ACK 6
|
|
[RTO]
[ssthresh <- 3]
5. SEND 6 ---------------------------->
6. <---------------------------- ACK 7
[F-RTO step (2b)]
7. SEND 12 ---------------------------->
8. SEND 13 ---------------------------->
9. <---------------------------- ACK 7
[F-RTO step (3a)]
[SpuriousRecovery <- FALSE]
[cwnd <- 3]
10. SEND 7 ---------------------------->
11. SEND 8 ---------------------------->
12. SEND 9 ---------------------------->
13. <---------------------------- ACK 14
Again, F-RTO sender transmits two new segments (steps 7 and 8) after
the RTO retransmission is acknowledged. Because the next ACK does not
acknowledge any data that was not retransmitted after the RTO (step
9), the F-RTO sender proceeds with conventional recovery and slow
start retransmissions.
A.4. Packet reordering
Since F-RTO modifies the TCP sender behavior only after a
retransmission timeout and it is intended to avoid unnecessary
retransmits only after spurious RTO, we limit the discussion on the
Expires: August 2004 [Page 16]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
effects of packet reordering in F-RTO behavior to the cases where
packet reordering occurs immediately after the RTO. When 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 RTO.
However, if the TCP sender applies the SACK-enhanced F-RTO, it is
possible to detect a spurious RTO also when packet reordering occurs.
We illustrate the behavior of SACK-enhanced F-RTO below 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 RTO.
...
(cwnd = 6,
ssthresh < 6,
FlightSize = 5)
1. SEND 10 ---------------------------->
2. <---------------------------- ACK 6
3. SEND 11 ---------------------------->
4. |
[delay]
|
[RTO]
5. SEND 6 ---------------------------->
<earlier xmitted SEG 8> --->
6. <---------------------------- ACK 6
[SACK 8]
[SACK F-RTO stays in step 2]
7. <earlier xmitted SEG 6> --->
8. <---------------------------- ACK 7
[SACK 8]
[SACK F-RTO step (2b)]
9. SEND 12 ---------------------------->
10. SEND 13 ---------------------------->
11. <earlier xmitted SEG 7> --->
12. <---------------------------- ACK 9
[SACK F-RTO step (3b)]
[SpuriousRecovery <- SPUR_TO]
[ssthresh <- 3, cwnd <- 3]
13. <---------------------------- ACK 10
14. <---------------------------- ACK 11
15. SEND 14 ---------------------------->
...
After RTO expires and the sender retransmits segment 6 (step 5), the
receiver gets segment 8 and generates duplicate ACK with SACK for
Expires: August 2004 [Page 17]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
segment 8. In response to the acknowledgement the TCP sender does not
send anything but stays in F-RTO step 2. Because the next
acknowledgement advances the cumulative ACK point (step 8), 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 RTO spurious.
Appendix B: Applying SACK-enhanced F-RTO when RTO occurs during loss
recovery
We believe that slightly modified SACK-enhanced F-RTO algorithm can
be used to detect spurious RTOs also when RTO occurs while an earlier
loss recovery is underway. However, there are issues that need to be
considered if F-RTO is applied in this case.
The original SACK-based F-RTO requires in algorithm step 3 that an
ACK acknowledges previously unacknowledged non-retransmitted data
between SND.UNA and send_high. If RTO takes place during earlier
(SACK-based) loss recovery, the F-RTO sender must only use
acknowledgements for non-retransmitted segments transmitted before
the SACK-based loss recovery started. This means that in order to
declare RTO spurious the TCP sender must receive an acknowledgement
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 acknowledgement for segment
that was transmitted more than one RTO ago, it can declare the RTO
spurious. Defining an efficient algorithm for checking these
conditions remains as a future work item.
When spurious RTO 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 what segments to retransmit
after RTO, and whether it is safe to revert the congestion control
parameters in this case. This is considered as a topic of future
research.
Authors' Addresses
Pasi Sarolahti
Nokia Research Center
P.O. Box 407
FIN-00045 NOKIA GROUP
Finland
Expires: August 2004 [Page 18]
draft-ietf-tsvwg-tcp-frto-01.txt February 2004
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 26
FIN-00014 UNIVERSITY OF HELSINKI
Finland
Phone: +358 9 1914 4179
EMail: markku.kojo@cs.helsinki.fi
Expires: August 2004 [Page 19]
| PAFTECH AB 2003-2026 | 2026-04-22 08:26:48 |