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Versions: 00 01 02 03 04                                                
Internet Engineering Task Force                             P. Sarolahti
INTERNET DRAFT                                     Nokia Research Center
File: draft-sarolahti-tsvwg-tcp-frto-04.txt                      M. Kojo
                                                  University of Helsinki
                                                              June, 2003
                                                 Expires: December, 2003

                   F-RTO: An Algorithm for Detecting
           Spurious Retransmission Timeouts with TCP and SCTP

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of [RFC2026].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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

   The list of Internet-Draft Shadow Directories can be accessed at


   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 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

   The F-RTO algorithm affects the TCP sender behavior only after a
   retransmission timeout. Otherwise the TCP behavior remains

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   unmodified.  This section describes a basic version of the F-RTO
   algorithm that does not require TCP options to work. 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
   could then 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.

   When the retransmission timer expires, the F-RTO algorithm takes the
   following steps at the TCP sender.

   1) When RTO expires, the TCP sender SHOULD retransmit the first
      unacknowledged segment and set SpuriousRecovery to FALSE.

      The highest sequence number transmitted so far is stored in
      variable "send_high". The TCP sender MAY postpone adjusting the
      congestion control parameters for the next two incoming ACKs,
      until it has got more input on whether the RTO was spurious or
      not. If the TCP sender adjusts the congestion control parameters
      at this point, it may store the earlier values of the parameters
      to be able to restore the values in case it detects that the RTO
      was spurious.

   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 transmitting unacknowledged data
         in slow start. The TCP sender does not enter step 3 of this
         algorithm, and the SpuriousRecovery variable remains as FALSE.

         If the acknowledgement covers the send_high point, 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.

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         If the acknowledgement is a duplicate ACK, it was triggered by
         a segment that was sent before the RTO retransmission. This can
         occur, for example, if the TCP sender is under fast recovery
         when the RTO occurs. In this situation reliably declaring a
         spurious RTO is difficult, hence the safest alternative is to
         follow the conventional RTO recovery.

      b) If the acknowledgement advances the window AND it is below the
         value of send_high, the TCP sender transmits up to two new
         (previously unsent) segments and enters step 3 of this

         Sending two new segments at this point is equally aggressive to
         the conventional RTO recovery algorithm, which would increase
         cwnd to 2 * MSS when the first valid ACK arrives after RTO. It
         is possible that the sender can transmit only one new segment
         at this time, because the receiver window limits it, or because
         the TCP sender does not have more data to send. This does not
         prevent the algorithm from working. In any case, the TCP sender
         SHOULD transmit at least one segment, either new data or from
         the retransmission queue. If the sender retransmits the next
         unacknowledged segment, it MUST NOT enter the step 3 of this
         algorithm, but continue retransmitting similarly to the
         conventional RTO recovery algorithm.

         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. Otherwise, a malicious
         receiver acknowledging partial segments could cause the sender
         to declare the RTO spurious in a case where data was lost. When
         receiving an acknowledgement for a partial segment, the TCP
         sender SHOULD revert to 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.

         The duplicate ACK indicates that at least one segment other
         than the segment which triggered RTO is lost in the last window
         of data. There is no sufficient evidence to assume that the RTO
         was spurious. Therefore, the sender proceeds with
         retransmissions similarly to the conventional RTO recovery
         algorithm, with the send_high variable stored when the

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         retransmission timer expired to avoid unnecessary fast

      b) If the acknowledgement advances the window and acknowledges
         data beyond the highest sequence number that was retransmitted
         on RTO, the TCP sender SHOULD declare the RTO spurious and set
         SpuriousRecovery to SPUR_TO.

         Because the TCP sender has retransmitted only one segment after
         the RTO, this acknowledgement indicates that an originally
         transmitted segment has arrived at the receiver. This is
         regarded as a strong indication of a spurious RTO. The TCP
         sender should assume that the unacknowledged segments are not
         lost and continue by sending new previously unsent segments.

         If this algorithm branch is taken, the TCP sender SHOULD set
         the value of send_high variable to SND.UNA in order to disable
         the NewReno "bugfix" [FH99]. The send_high variable was
         proposed for avoiding unnecessary multiple fast retransmits
         when RTO expires during fast recovery with NewReno TCP. As the
         sender has not retransmitted other segments but the one that
         triggered RTO, the problem addressed by the bugfix cannot
         occur. Therefore, if there are 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

   If the TCP sender does not have any new data to send in algorithm
   branch (2b), or the receiver window limits the transmission, it has
   to send something in order to prevent the transmission 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

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   - Retransmit data from the tail of the retransmission queue and
     continue with step 3 of the F-RTO algorithm. It is possible that
     the retransmission 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.

   After the RTO is declared spurious, the TCP sender cannot detect if
   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 avoids most of the unnecessary
   retransmissions after 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 spurious RTO. If packet reordering

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   or packet duplication occurs on the segment that triggered the
   spurious RTO, the F-RTO algorithm may not detect the spurious RTO.
   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,
   or when the TCP sender is under loss recovery. The difference to the
   basic F-RTO algorithm is that the sender may declare RTO spurious
   even when duplicate ACKs follow the RTO, if the SACK blocks
   acknowledge new data that was not transmitted after RTO. The
   algorithm presented in this section is also applicable to be used
   with the SCTP protocol.

   The SACK-enhanced F-RTO algorithm takes the following steps:

   1) When RTO expires, the TCP sender SHOULD retransmit first
      unacknowledged segment and set SpuriousRecovery to FALSE.

      A variable "SpuriousThreshold" is set to indicate the highest
      acknowledgement that is accepted for declaring the RTO spurious.
      If there was loss recovery ongoing at the time an RTO occurs,
      SpuriousThreshold is set to the value of RecoveryPoint that was
      valid at the time when RTO occurred. Otherwise, SpuriousThreshold
      is set to HighData, that indicates the highest sequence number
      transmitted. The SACK-based loss recovery algorithm describes
      RecoveryPoint and HighData in more detail [BAFW03].

   2) The first acknowledgement after RTO arrives at the sender.

      a) if the cumulative ACK acknowledges all segments up to
         SpuriousThreshold stored in algorithm step 1, 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 transmits up to two new (previously
         unsent) segments, within the limitations of the congestion
         window. If the TCP sender is not able to transmit previously
         unsent data due to receiver window limitation or because it

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         does not have 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 cannot be used for declaring a
         spurious RTO in step (3b).

   3) The second acknowledgement after RTO arrives at the sender.

      a) if the ACK acknowledges data above SpuriousThreshold, either in
         SACK blocks or as a cumulative ACK, the sender MUST set
         congestion window to no more than 3 * MSS and proceed with slow
         start, 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 SpuriousThreshold.

      b) if the ACK does not acknowledge data above SpuriousThreshold
         AND it acknowledges some previously unacknowledged data below
         SpuriousThreshold, 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. In addition,
         RecoveryPoint should be set to its earlier value, since no loss
         recovery was needed due to the RTO.

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 [LG02].

   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

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      RTO [LG02]. 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
      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.  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

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   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.


   We are grateful to Reiner Ludwig, Andrei Gurtov, Josh Blanton, Mark
   Allman, Sally Floyd, Yogesh Swami, Mika Liljeberg, and Ivan Arias
   Rodriguez 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.

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

   [LG02]    R. Ludwig and A. Gurtov. The Eifel Response Algorithm for
             TCP. Internet draft "draft-ietf-tsvwg-tcp-eifel-
             response-02.txt".  December 2002. 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

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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 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

   An unexpectedly long delay can trigger an RTO, should it occur on a
   single packet blocking the following packets, or appear as increased
   RTTs for several successive packets. The example below illustrates
   the sequence of packets and acknowledgements seen by the TCP sender
   that follows the F-RTO algorithm, when a sudden delay occurs
   triggering RTO but no packets are lost. For simplicity, delayed
   acknowledgements are not used in the example.

         ...                (cwnd = 6, ssthresh < 6, FlightSize = 5)
         1.  SEND(10)
         2.  ACK(6)
         3.  SEND(11)
         4.  <delay + RTO>  (set ssthresh <- 3)
         5.  SEND(6)
         6.  ACK(7)
         7.  SEND(12)
         8.  SEND(13)
         9.  ACK(8)         (set cwnd <- 3, FlightSize = 6)
         10. ACK(9)         (cwnd = 3,  FlightSize = 5)
         11. ACK(10)        (cwnd = 3,  FlightSize = 4)
         12. ACK(11)        (cwnd = 4,  FlightSize = 3)
         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 advances the cumulative ACK point, the TCP
   sender continues by sending two new data segments (steps 7, 8) and
   adjusts cwnd to 3 MSS. Because the second acknowledgement arriving
   after the RTO also advances the cumulative ACK point, the TCP sender
   exits the recovery and continues with the congestion avoidance. From
   this point on the retransmissions are invoked either by fast
   retransmit or when triggered by the retransmission timer. Because the
   TCP sender reduces cwnd when receiving the first ACK after RTO and
   sends the two new data segments at steps 7 and 8, it has to wait
   until the FlightSize is reduced to the level of congestion window

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   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 cumulatively. 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 = 5)
             <segment 6 lost>
         1.  SEND(10)
         2.  ACK(6)
         3.  SEND(11)
         4.  ACK(6)
         5.  ACK(6)
         6.  ACK(6)
         7.  SEND(6)        (set cwnd <- 6, set ssthresh <- 3)
             <segment 6 lost>
         8.  ACK(6)
         9.  <RTO>          (set ssthresh <- 2)
         10. SEND(6)
         11. ACK(9)
         12. SEND(12)
         13. SEND(13)
         14. ACK(9)         (set cwnd <- 3)
         15. SEND(9)
         16. SEND(10)
         17. SEND(11)
         18. 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
   advances the cumulative ACK point (step 11), the sender transmits two
   new segments. The second ACK in step 14 does not advance the
   cumulative ACK point, and the sender enters the slow start, sets cwnd
   to 3 * MSS, and retransmits the next three unacknowledged segments,
   as per the F-RTO algorithm description given in Section 2. After this
   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

   A performance study shows that F-RTO performs similarly to the
   regular recovery when consecutive packets are lost both up- and
   downstream as a result of link outage, triggering an RTO [SKR02].  If
   the RTO was not spurious but some data was actually lost, one of the
   next two ACKs after RTO does not advance the cumulative ACK point
   when RTO was caused by data loss, because the basic F-RTO retransmits
   only one segment after RTO. As a result, F-RTO sender continues by
   retransmitting unacknowledged segments similarly to the conventional
   RTO recovery.

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
   effects of packet reordering in F-RTO behavior to the cases where
   packet reordering occurs immediately after RTO. We consider the
   retransmission timeout due to packet reordering to be very rare case,
   since reordering often triggers fast retransmit due to duplicate ACKs
   caused by out-of-order segments. Should packet reordering occur after
   an RTO, duplicate ACKs arrive to the sender, taking the F-RTO
   algorithm to retransmit in slow start as a regular RTO recovery would
   do. Although this might not be the correct action, it is similar to
   the behavior of the regular TCP, making F-RTO a safe modification
   also in the presence of reordering.

Appendix B: On using the TCP timestamps with F-RTO

   The basic F-RTO algorithm suggests applying the conventional RTO
   recovery if the receiver window or application limits the
   transmission of new previously unsent data, and in such a case it is
   possible that the F-RTO algorithm cannot be used to detect a spurious
   RTO. The F-RTO sender can avoid the need of transmitting new
   previously unsent segments after RTO, if it has TCP timestamps
   [BBJ92] available. The Eifel detection algorithm [LK00] describes how
   the TCP timestamps can be used to avoid unnecessary retransmissions
   after a spurious RTO. However, if the RTO is declared spurious based
   on the timestamp echoed with the first acceptable ACK following the
   RTO, the TCP sender may falsely declare the RTO spurious and continue
   by transmitting new data when the RTO was caused by loss of
   acknowledgements. The Eifel algorithm may signal spurious RTO
   falsely, if the first data segment retransmitted after RTO was not
   lost, but the corresponding acknowledgement was, and the
   acknowledgement does not include DSACK option [FMMP00]. If sender and
   receiver implement DSACK, this problem can be avoided.

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   An alternative algorithm for detecting spurious RTOs by using TCP
   timestamps without DSACK is described below. When TCP timestamps are
   available, the F-RTO sender MAY apply the following algorithm.

   1) When RTO expires, retransmit first unacknowledged segment and
      store the timestamp of retransmitted segment in variable
      "RetransmitTS". Store the highest sequence number transmitted so
      far in variable "send_high".

   2) Wait until the first ACK that acknowledges previously
      unacknowledged data arrives at the sender. If duplicate ACKs
      arrive, they are processed normally while the sender stays in this
      step of the algorithm.

      a) if the timestamp echoed with the ACK is later or equal than
         what is stored in "RetransmitTS", the TCP sender SHOULD revert
         to the conventional RTO recovery and it MUST NOT enter step 3
         of this algorithm. The sender should adjust the congestion
         window according to the standard congestion control rules.

      b) if the timestamp echoed with the first ACK is earlier than what
         is stored in "RetransmitTS", the TCP sender SHOULD transmit the
         first unacknowledged segment and enter step 3 of this

   3) When the next acknowledgement arrives at the sender, it SHOULD
      apply one of the following branches of the algorithm.

      a) if the timestamp echoed with the ACK is later or equal than
         what is stored in "RetransmitTS", or if the acknowledgement is
         duplicate ACK, the TCP sender SHOULD revert to the conventional
         RTO recovery. The TCP sender MUST set the congestion window to
         no more than 2 * MSS.

      b) if the timestamp echoed with the ACK is earlier than what is
         stored in "RetransmitTS", the TCP sender SHOULD declare the RTO
         spurious. send_high SHOULD be set to the value of SND.UNA to
         cancel the NewReno bugfix, as described in Section 2.

   The drawback of this algorithm compared to the original Eifel
   detection is that the above-presented algorithm can make two
   unnecessary retransmissions instead of one. In addition, packet
   reordering, packet duplication, or packet loss for the next segment
   after the one that triggered RTO may prevent the detection of
   spurious RTO.  Therefore, it may be desirable to apply the basic F-
   RTO or the SACK-enhanced version of the F-RTO algorithm whenever the
   sender is able to transmit previously unsent data when the first ACK
   after RTO arrives. However, we believe the algorithm above

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   effectively avoids false spurious RTO signals.

Authors' Addresses

   Pasi Sarolahti
   Nokia Research Center
   P.O. Box 407

   Phone: +358 50 4876607
   EMail: pasi.sarolahti@nokia.com

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

   Phone: +358 9 1914 4179
   EMail: markku.kojo@cs.helsinki.fi

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