Forward RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious Retransmission Timeouts with TCP and the Stream Control Transmission Protocol (SCTP)
RFC 4138

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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   [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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   [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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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

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Acknowledgement

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