Internet Engineering Task Force                             P. Sarolahti
INTERNET DRAFT                                     Nokia Research Center
File: draft-ietf-tsvwg-tcp-frto-00.txt                           M. Kojo
                                                  University of Helsinki
                                                           October, 2003
                                                    Expires: April, 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
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   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

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   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

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   also be applied with the SCTP protocol.

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 amount 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 [GL02]. After a spurious RTO 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 RTO 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 RTOs. 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 RTOs caused by a delay in

   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

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   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 and 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.

   When an RTO occurs, the F-RTO sender retransmits the first
   unacknowledged segment normally, but tries to transmit new,
   previously unsent data after that. If the next two acknowledgements
   cover segments that were not retransmitted, the F-RTO sender can
   declare 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 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.

   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
   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

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   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

   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

      a) If the acknowledgement is a duplicate ACK OR it is
         acknowledging a sequence number equal to (or above) the value
         of send_high, 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

      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

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         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

   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 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

   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.
   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 must not
   enter step 3 of this algorithm, but revert 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 retransmit two
   segments after one round-trip has elapsed since the RTO. If sending
   new data is not possible in algorithm branch (2b), or the receiver

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   window limits the transmission, it has to send something in order to
   prevent the TCP transfer from stalling. In that case 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

   - 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
     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

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   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 have more accurate and 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
   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.

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   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

   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
         (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

   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.

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      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 start 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 suggested for adjusting the congestion
   control state after a spurious RTO. This document does not
   specifically recommend any of the alternatives below, but considers
   the response to spurious RTO as a subject of further research.

   1) Revert the congestion control parameters to the state before the
      RTO [LG03]. This appears to be a justified decision, because it is
      similar to the situation in which the RTO did not expire
      spuriously. However, two concerns exists with this approach:
      First, some detection mechanisms, such as F-RTO or the Eifel
      Detection algorithm, do not notice the loss of the spurious
      retransmission, thus introducing a small risk of violation of the
      congestion control principles. Second, a spurious RTO indicates
      that some part of the network was unable to deliver packets for a
      while, which can be considered as a potential indication of

   2) Reduce congestion window to half of its earlier value but revert
      slow start threshold to its earlier value [SL03].  This
      alternative takes measures to validate the congestion window after
      a period during which no data has been transmitted. This would be
      a justified action to take if the spurious RTO is assumed to be
      caused due to changes in the network conditions, such as a change

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      in the available bandwidth or a wireless handoff to another point
      of attachment in the network.

   3) Reduce ssthresh and congestion window when detecting a spurious
      RTO [SKR02]. For example, ssthresh and cwnd could be set to half
      of their earlier values, as done with the other congestion
      notification events. This alternative would be conservative enough
      considering the possibility of not detecting a packet loss of the
      RTO-triggered retransmission, but the TCP sender should avoid
      reducing the congestion window more than once in a round-trip
      time. Furthermore, if a spurious RTO occurs in the beginning of a
      TCP connection, this alternative causes the slow start to be
      canceled, which may sacrifice TCP performance.

5.  SCTP Considerations

   The SACK-enhanced F-RTO algorithm can be applied with the SCTP proto-
   col. 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
   RTO may remain undetected when applying the F-RTO algorithm and there
   was a delay spike that triggered the RTO. 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 parame-
   ters is too aggressive. Therefore, taking 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

   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

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   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.

6.  Security Considerations

   The main security threat regarding F-RTO is the possibility of
   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.


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   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.

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

   [BA02]    E. Blanton and M. Allman. On Making TCP more Robust to
             Packet Reordering. ACM 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

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             TCP in a GPRS Network. In Proc. of European Wireless, Flo-
             rence, Italy, February 2002

   [LG03]    R. Ludwig and A. Gurtov. The Eifel Response Algorithm for
             TCP. Internet draft "draft-ietf-tsvwg-tcp-eifel-
             response-03.txt".  March 2003. Work in progress.

   [LK00]    R. Ludwig and R.H. Katz. The Eifel Algorithm: Making TCP
             Robust Against Spurious Retransmissions. ACM Computer Com-
             munication Review, 30(1), January 2000.

   [LM03]    R. Ludwig and M. Meyer. The Eifel Detection Algorithm for
             TCP. RFC 3522, April 2003.

   [SKR02]   P. Sarolahti, M. Kojo, and K. Raatikainen. F-RTO: A New
             Recovery Algorithm for TCP Retransmission Timeouts. Univer-
             sity of Helsinki, Dept. of Computer Science. Series of Pub-
             lications C, No. C-2002-07. February 2002. Available at:

   [SL03]    Y. Swami and K. Le. DCLOR: De-correlated Loss Recovery
             using SACK option for spurious timeouts. Internet draft
             "draft-swami-tsvwg-tcp-dclor-01.txt". April 2003. Work in

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 [SKR02].

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,

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          ssthresh < 6,
          FlightSize = 5)
         1.  SEND 10 ---------------------------->
         2.          <---------------------------- ACK 6
         3.  SEND 11 ---------------------------->
         4.                       |
         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]

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   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
             [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

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   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

          (cwnd = 6,
          ssthresh < 6,
          FlightSize = 5)
             <segments 6-9 lost>
         1.  SEND 10 ---------------------------->
         2.          <---------------------------- ACK 6
         3.  SEND 11 ---------------------------->
         4.          <---------------------------- ACK 6
             [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

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   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.                       |
         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

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   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

   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

Authors' Addresses

   Pasi Sarolahti
   Nokia Research Center
   P.O. Box 407

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   Phone: +358 50 4876607

   Markku Kojo
   University of Helsinki
   Department of Computer Science
   P.O. Box 26

   Phone: +358 9 1914 4179

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