Network Working Group                                      Reiner Ludwig
INTERNET-DRAFT                                         Ericsson Research
Expires: September 2003                                    Andrei Gurtov
                                                      Sonera Corporation
                                                             March, 2003

                  The Eifel Response Algorithm for TCP

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
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   The Eifel response algorithm requires a detection algorithm to detect
   a posteriori whether the TCP sender has entered loss recovery
   unnecessarily. In response to a spurious timeout it adapts the
   retransmission timer to avoid further spurious timeouts, and can
   avoid - depending on the detection algorithm - the often unnecessary
   go-back-N retransmits that would otherwise be sent. Likewise, it
   adapts the duplicate acknowledgement threshold in response to a
   spurious fast retransmit. In both cases, the Eifel response algorithm
   restores the congestion control state in such a way that packet
   bursts are avoided.

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   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

   We refer to the first-time transmission of an octet as the 'original
   transmit'. A subsequent transmission of the same octet is referred to
   as a 'retransmit'. In most cases this terminology can likewise be
   applied to data segments as opposed to octets. However, when
   repacketization occurs, a segment can contain both first-time
   transmissions and retransmissions of octets. In that case this
   terminology is only consistent when applied to octets. For the Eifel
   detection and response algorithms this makes no difference as they
   also operate correctly when repacketization occurs.

   We use the term 'acceptable ACK' as defined in [RFC793]. That is an
   ACK that acknowledges previously unacknowledged data. We use the term
   'duplicate ACK', and the variable 'dupacks' as defined in [WS95]. The
   variable 'dupacks' is a counter of duplicate ACKs that have already
   been received by the TCP sender before the fast retransmit is sent.
   We use the variable 'DupThresh' to refer to the so-called duplicate
   acknowledgement threshold, i.e., the number of duplicate ACKs that
   need to arrive at the TCP sender to trigger a fast retransmit.
   Currently, DupThresh is specified as a fixed value of three

   Furthermore, we use the TCP sender state variables 'SND.UNA' and
   'SND.NXT' as defined in [RFC793]. SND.UNA holds the segment sequence
   number of the oldest outstanding segment. SND.NXT holds the segment
   sequence number of the next segment the TCP sender will
   (re-)transmit. In addition, we define as 'SND.MAX' the segment
   sequence number of the next original transmit to be sent. The
   definition of SND.MAX is equivalent to the definition of snd_max in

   We use the TCP sender state variables 'cwnd' (congestion window), and
   'ssthresh' (slow start threshold), and the terms 'SMSS',
   'FlightSize', and 'Initial Window (IW)' as defined in [RFC2581].
   FlightSize is the amount of outstanding data in the network, or
   alternatively, the difference between SND.MAX and SND.UNA at a given
   point in time. The IW is the size of the sender's congestion window
   after the three-way handshake is completed. We use the TCP sender
   state variables 'SRTT' and 'RTTVAR', and the term 'RTO' as defined in
   [RFC2988]. In addition, we assume that the TCP sender maintains in
   the variable 'RTT-SAMPLE' the value of the latest round-trip time
   (RTT) measurement.

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1. Introduction

   The Eifel response algorithm relies on a detection algorithm such as
   the Eifel detection algorithm defined in [RFC***B]. That document
   discusses the relevant background and motivation that also applies to
   this document. Hence, the reader is expected to be familiar with
   [RFC***B]. Note that alternative response algorithms have been
   proposed [BDA03] that could also rely on the Eifel detection
   algorithm, and vice versa alternative detection algorithms have been
   proposed [BA02b], [SK03] that could work together with the Eifel
   response algorithm.

   The Eifel response algorithm requires a detection algorithm to detect
   a posteriori whether the TCP sender has entered loss recovery
   unnecessarily. In response to a spurious timeout it adapts the
   retransmission timer to avoid further spurious timeouts, and can
   avoid - depending on the detection algorithm - the often unnecessary
   go-back-N retransmits that would otherwise be sent. Likewise, it
   adapts the duplicate acknowledgement threshold in response to a
   spurious fast retransmit. In both cases, the Eifel response algorithm
   restores the congestion control state in such a way that packet
   bursts are avoided.

2. Interworking with Detection Algorithms

   If the Eifel response algorithm is implemented at the TCP sender, it
   MUST be implemented together with a detection algorithm that is
   specified in an RFC.

   Designers of detection algorithms who want to offer the possibility
   that their detection algorithms can work together with the Eifel
   response algorithm MUST reuse the variable SpuriousRecovery with the
   semantics and defined values as specified in [RFC***B]. In addition,
   we define LATE_SPUR_TO (equal -1) as another possible value of the
   variable SpuriousRecovery. Detection algorithms must set the value of
   SpuriousRecovery to LATE_SPUR_TO if the detection is based upon
   receiving the ACK for the retransmit. For example, this applies to
   detection algorithms that are based on the DSACK option.

3. The Eifel Response Algorithm

   The complete algorithm is specified in section 2.1. In sections 2.2
   to 2.4, we motivate the different steps of the algorithm.

3.1. The Algorithm

   Given that a TCP sender has enabled a detection algorithm that
   complies with the requirements set in Section 2, a TCP sender MAY use
   the Eifel response algorithm as defined in this subsection.

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   If the Eifel response algorithm is used, the following steps MUST be
   taken by the TCP sender, but only upon initiation of loss recovery,
   i.e., when either the timeout-based retransmit or the fast retransmit
   is sent. Note: The algorithm MUST NOT be reinitiated after loss
   recovery has already started. In particular, it may not be
   reinitiated upon subsequent timeouts for the same segment, and not
   upon retransmitting segments other than the oldest outstanding

      (0)     Before the variables cwnd and ssthresh get updated when
              loss recovery is initiated, set a "pipe_prev" variable as
                  pipe_prev <- max (FlightSize, ssthresh)

      (DTCT)  This is a placeholder for a detection algorithm that must
              be executed at this point. In case [RFC***B] is used as
              the detection algorithm, steps (1) - (6) of that algorithm
              go here.

      (RESP)  If SpuriousRecovery equals FALSE, then proceed to step

              else if SpuriousRecovery equals SPUR_TO, then proceed to
              step (STO.1),

              else if SpuriousRecovery equals LATE_SPUR_TO, then proceed
              to step (STO.2),

              else (spurious fast retransmit) proceed to step (SFR).

      (STO.1) Resume transmission off the top:

                  SND.NXT <- SND.MAX

      (STO.2) Adapt the Conservativeness of the Retransmission Timer:

              If the retransmission timer is implemented according to
              [RFC2988], then change the calculation of SRTT to
                  SRTT <- SRTT + 1/FlightSize * (RTT-SAMPLE - SRTT)
              and set
                  SRTT <- RTT-SAMPLE
                  RTTVAR <- RTT-SAMPLE/2,
              recalculate the RTO, and restart the retransmission timer,

                  Note: Even after changing the calculation of SRTT, the
                  retransmission timer is considered as being
                  implemented according to [RFC2988].

              else adapt the conservativeness of the retransmission

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              Proceed to step (ReCC).

      (SFR)   Adapt the duplicate acknowledgement threshold:

                  DupThresh <- max (DupThresh, SpuriousRecovery)

              Proceed to step (ReCC).

      (ReCC)  Revert the congestion control state:

              If the acceptable ACK has the ECN-Echo flag [RFC3168] set
              OR the TCP sender has already taken more than three
              timeouts for the oldest outstanding segment, then proceed
              to step (DONE),

              else set
                  cwnd <- min (pipe_prev, (FlightSize + IW))
                  ssthresh <- pipe_prev

              Proceed to step (DONE).

      (DONE)  No further processing.

3.2 Responding to Spurious Timeouts

3.2.1 Suppressing the Unnecessary go-back-N Retransmits (step STO.1)

   Without the use of the TCP timestamps option, the TCP sender suffers
   from the retransmission ambiguity problem [Zh86], [KP87]. This means
   that when the first acceptable ACK arrives after a spurious timeout,
   the TCP sender must believe that that ACK was sent in response to the
   retransmit when in fact it was sent in response to the original
   transmit. Furthermore, the TCP sender must also believe that all
   other segments outstanding at that point were lost.

      Note: Except for certain cases where original ACKs were lost, that
      first acceptable ACK cannot carry any DSACK option [RFC2883].

   Consequently, once the TCP sender's state has been updated after the
   first acceptable ACK has arrived, SND.NXT equals SND.UNA. This is
   what causes the often unnecessary go-back-N retransmits. Now every
   arriving acceptable ACK that was sent in response to an original
   transmit will advance SND.NXT. But as long as SND.NXT is smaller than
   the value that SND.MAX had when the timeout occurred, those ACKs will
   clock out retransmits; whether those segments were lost or not.

   In fact, during this phase the TCP sender breaks 'packet
   conservation' [Jac88]. This is because the go-back-N retransmits are

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   sent during slow start. I.e., for each original transmit leaving the
   network, two retransmits are sent into the network as long as SND.NXT
   does not equal SND.MAX (see [LK00] for more detail).

   The use of the TCP timestamps option reliably eliminates the
   retransmission ambiguity problem. Thus, once the Eifel detection
   algorithm detected that a timeout was spurious, it is therefore safe
   to let the TCP sender resume the transmission with new data. Thus,
   the Eifel response algorithm changes the TCP sender's state by
   setting SND.NXT to SND.MAX in that case.

3.2.2 Adapting the Retransmission Timer (step STO.2)

   There is currently only one retransmission timer standardized for TCP
   [RFC2988]. We therefore only address that timer explicitly. Future
   standards that might define alternatives to [RFC2988] should propose
   similar measures to adapt the conservativeness of the retransmission

   Since the timeout was spurious, the TCP sender's RTT estimators are
   likely to be off. However, since timestamps are being used, a new and
   valid RTT measurement (RTT-SAMPLE) can be derived from the acceptable
   ACK. It is therefore suggested to reinitialize the RTT estimators
   from RTT-SAMPLE. Note that this RTT-SAMPLE will be relatively large
   since it will include the delay spike that caused the spurious
   timeout in the first place. To have the new RTO become effective, the
   retransmission timer needs to be restarted. This is consistent with
   [RFC2988] which recommends restarting the retransmission timer with
   the arrival of an acceptable ACK.

   When the path's RTT varies largely, it is recommended to take RTT
   samples more frequently than only once per RTT. This allows the TCP
   sender to track changes in the RTT more closely. In particular, a TCP
   sender can react more quickly to sudden increases of the RTT by
   sooner updating the RTO to a more conservative value. The TCP
   Timestamps option [RFC1323] provides this capability, allowing the
   TCP sender to sample the RTT from every segment that is acknowledged.
   Using timestamps across such paths leads to a more conservative TCP
   retransmission timer and reduces the risk of triggering spurious
   timeouts [IMLGK02].

   On the other hand, it is known that executing the RTO calculation
   defined in [RFC2988] more often than once per RTT leads to an RTO
   that decays too quickly, i.e., that converges to the RTT too quickly.
   This is because of the fixed gains (1/8 and 1/4) of RFC2988's RTT
   estimators. When timing every segment these gains are increasingly
   too large with an increasing FlightSize. This leads to the effect
   that the RTT estimators "lose" their memory too soon. This is a known
   conflict between [RFC2988] and [RFC1323]. Especially, a large RTO
   resulting from an RTT spike will decay within one or two RTTs (e.g.,
   see [LS00]). Hence, simply reinitializing RFC2988's RTT estimators

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   from RTT-SAMPLE is probably not enough to make the retransmission
   timer sufficiently conservative for at least the next couple of RTTs.
   A solution for the case when every segment is timed according to
   [RFC1323] is to make the gains adaptive to the FlightSize [LS00]. We
   suggest to adopt this solution for at least the SRTT.

3.3 Responding to Spurious Fast Retransmits (step SFR)

   The assumption behind the fast retransmit algorithm [RFC2581] is that
   a segment was lost if as many duplicate ACKs have arrived at the TCP
   sender as indicated by DupThresh. Currently, DupThresh is specified
   as a fixed value of three [RFC2581]. That value is assumed to be
   sufficiently conservative so that packet reordering and/or packet
   duplication does not falsely trigger the fast retransmit algorithm.
   Clearly, this assumption does not hold for a particular TCP
   connection once the TCP sender detects that the last fast retransmit
   was spurious. It is therefore suggested to dynamically adapt
   DupThresh to the reordering characteristics observed over the course
   of a particular connection.

   At the beginning of a connection DupThresh is initialized with three.
   Then for each spurious fast retransmit that is detected, DupThresh is
   set to the maximum of the previous DupThresh, and the lowest value
   that would have avoided that last spurious fast retransmit. Note that
   the Eifel detection algorithm records the latter value in
   SpuriousRecovery. This strategy ensures that the TCP sender is able
   to cope with the longest reordering length seen on a particular
   connection so far. However, the strategy may lead to fast timeouts
   [RFC***B], i.e., an event where the retransmission timer expires
   before the TCP sender receives the duplicate ACK that would trigger a
   fast retransmit of the oldest outstanding segment.

   Also, we believe that this strategy should be implemented together
   with an advanced version of the Limited Transmit algorithm [RFC3042].
   That is for each duplicate ACK that arrives until DupThresh is
   reached, the TCP sender should sent a new data segment if allowed by
   the TCP receiver's advertised window, and if new data is available.
   Although, the current Limited Transmit algorithm only allows this for
   the first two duplicate ACKs, we believe that such an advanced
   limited transmit strategy is safe. It is already implemented in
   widely deployed TCPs [SK02].

   Other alternatives for responding to spurious fast retransmits are
   discussed in [BA02a].

3.4 Reverting Congestion Control State (step ReCC)

   When a TCP sender enters loss recovery, it also assumes that is has
   received a congestion indication. In response to that it reduces
   cwnd, and ssthresh. However, once the TCP sender detects that the
   loss recovery has been falsely triggered, this reduction was

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   unnecessary. In fact, no congestion signal has been received. We
   therefore believe that it is safe to revert to the previous
   congestion control state.

   We suggest to restore cwnd to the minimum of the previous FlightSize,
   and the current FlightSize plus IW. The latter avoids large packet
   bursts that may occur with less careful variants for restoring
   congestion control state. For example, the original proposal [LK00]
   typically causes large bursts after packet reordering. The current
   proposal limits a potential packet burst to IW, which is considered
   an acceptable burst size. It is the amount of data that a TCP sender
   may send into a yet "unprobed" network at the beginning of a

   In addition, we suggest to restore ssthresh to pipe_prev, i.e., the
   maximum of the previous value of ssthresh and the value that
   FlightSize had when loss recovery was unnecessarily entered. As a
   result, the TCP sender either immediately resumes probing the network
   for more bandwidth in congestion avoidance, or it first slow starts
   until it has reached its previous share of the available bandwidth.

   Clearly, when the acceptable ACK signals congestion through the
   ECN-Echo flag [RFC3168], the TCP sender MUST refrain from reverting
   congestion control state. The same is true if the TCP sender has
   already taken more than three timeouts for the oldest outstanding
   segment. Allowing three timeouts while still reverting congestion
   control state goes beyond [RFC2581]. That standard recommends setting
   cwnd to no more than the restart window (one SMSS) if the TCP sender
   has not sent data in an interval exceeding the current RTO. That is
   done to restart the ACK clock which is believed to be lost. The case
   in step (ReCC) of the Eifel response algorithm is different. Since,
   an acceptable ACK corresponding to an original transmit has finally
   returned, the TCP has reason to believe that the ACK clock was merely
   interrupted but has now resumed "ticking" again.

4. Non-Conservative Advanced Loss Recovery after Spurious Timeouts

   A TCP sender MAY implement an optimistic form of advanced loss
   recovery after a spurious timeout has been detected as motivated in
   this section. Such a scheme MUST be terminated after the highest
   sequence number outstanding when the spurious timeout was detected
   has been acknowledged.

   We have studied environments where spurious timeouts and multiple
   losses from the same flight of packets often coincide [GL02]. Note
   that we refer to the case were the oldest outstanding segment does
   arrive at the TCP receiver but one or more packets from the remaining
   outstanding flight are lost. We found that in such a case TCP-Reno's
   performance can even suffer if the Eifel response algorithm is
   operated without an advanced loss recovery scheme such as NewReno
   [RFC2582], or SACK-based schemes [2018], [RFC***A]. The reason is

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   TCP-Reno's aggressiveness after a spurious timeout. Even though it
   breaks 'packet conservation' (see Section 2.2.1) when blindly
   retransmitting all outstanding segments, it usually recovers the
   back-to-back packet losses within a single round-trip time. On the
   contrary, the more conservative TCP-Reno/Eifel was forced into
   another (backed-off) timeout in that case.

   However, in a more recent study [GL03], we found that the mentioned
   advanced loss recovery schemes are often too conservative to compete
   against TCP-Reno's blind go-back-N in terms of quickly recovering
   multiple losses after a spurious timeout. The problem with the
   NewReno scheme is that it does not exploit knowledge (e.g., provided
   through SACK options) about which segments were lost. The problem
   with the conservative SACK-based scheme [RFC***A] is that it waits
   for three SACKs before it retransmits a lost segment. This may often
   lead to a second - and in this case genuine - (potentially backed-
   off) timeout. In those cases TCP-Reno's loss recovery is often
   quicker due the blind go-back-N. This could be viewed as a
   disincentive to the deployment of the Eifel response algorithm.

      [Making TCP (even) more conservative by fixing a misbehavior in
      the name of 'packet conservation' would probably at most result in
      credits in the academic world.]

   We therefore suggest that a TCP sender MAY implement an optimistic
   (non-conservative) form of advanced loss recovery after a spurious
   timeout has been detected, if the following guidelines are met:

      - Packet Conservation: The TCP sender may not have more segments
        (counting both original transmits and retransmits) in flight
        than indicated by the congestion window.

      - A retransmit may only be sent when a potential loss has been
        indicated. For example, a single duplicate ACK is such an
        indication; potentially with the corresponding SACK info in case
        the SACK option is enabled for the connection.

   We have developed and evaluated such a scheme (a variant of NewReno
   that exploits SACK info) in [GL03] that shows good results.

5. IPR Considerations

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document. For more information consult the online list of claimed
   rights at

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights

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   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights. Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11. Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

6. Security Considerations

   There is a risk that a detection algorithm is fooled by spoofed ACKs
   that make genuine retransmits appear to the TCP sender as spurious
   retransmits. When such a detection algorithm is run together with the
   Eifel response algorithm, this could effectively disable congestion
   control at the TCP sender. Should this become a concern, the Eifel
   response algorithm SHOULD only be run together with detection
   algorithms that are known to be safe against such "ACK spoofing

   For example, the safe variant of the Eifel detection algorithm
   [RFC***B], is a reliable method to protect against this risk.


   Many thanks to Keith Sklower, Randy Katz, Michael Meyer, Stephan
   Baucke, Sally Floyd, Vern Paxson, Mark Allman, Ethan Blanton, Pasi
   Sarolahti, and Alexey Kuznetsov for very useful discussions that
   contributed to this work.

Normative References

   [RFC2581] M. Allman, V. Paxson, W. Stevens, TCP Congestion Control,
             RFC 2581, April 1999.

   [RFC3042] M. Allman, H. Balakrishnan, S. Floyd, Enhancing TCP's Loss
             Recovery Using Limited Transmit, RFC 3042, January 2001.

   [RFC2119] S. Bradner, Key words for use in RFCs to Indicate
             Requirement Levels, RFC 2119, March 1997.

   [RFC2582] S. Floyd, T. Henderson, The NewReno Modification to TCP's
             Fast Recovery Algorithm, RFC 2582, April 1999.

   [RFC2883] S. Floyd, J. Mahdavi, M. Mathis, M. Podolsky, A. Romanow,
             An Extension to the Selective Acknowledgement (SACK) Option
             for TCP, RFC 2883, July 2000.

   [RFC1323] V. Jacobson, R. Braden, D. Borman, TCP Extensions for High
             Performance, RFC 1323, May 1992.

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   [RFC***B] R. Ludwig, M. Meyer, The Eifel Detection Algorithm for TCP,
             RFC***B, March 2003.

   [RFC2018] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, TCP Selective
             Acknowledgement Options, RFC 2018, October 1996.

   [RFC2988] V. Paxson, M. Allman, Computing TCP's Retransmission Timer,
             RFC 2988, November 2000.

   [RFC793]  J. Postel, Transmission Control Protocol, RFC793, September

   [RFC3168] K. Ramakrishnan, S. Floyd, D. Black, The Addition of
             Explicit Congestion Notification (ECN) to IP, RFC 3168,
             September 2001

Informative References

   [BA02a]   E. Blanton, M. Allman, On Making TCP More Robust to Packet
             Reordering, ACM Computer Communication Review, Vol. 32,
             No. 1, January 2002.

   [BA02b]   E. Blanton, M. Allman, Using TCP DSACKs and SCTP Duplicate
             TSNs to Detect Spurious Retransmissions, draft-blanton-
             dsack-use-02.txt (work in progress), October 2002.

   [BDA03]   E. Blanton, R. Dimond, M. Allman. Practices for TCP Senders
             in the Face of Segment Reordering, draft-blanton-tcp-
             reordering-00.txt (work in progress), February 2003..

   [RFC***A] E. Blanton, M. Allman, K. Fall, L. Wang, A Conservative
             SACK-based Loss  Recovery Algorithm for TCP, RFC***A,
             March 2003.

   [Gu01]    A. Gurtov, Effect of Delays on TCP Performance, In
             Proceedings of IFIP Personal Wireless Conference,
             August 2001.

   [GL02]    A. Gurtov, R. Ludwig, Evaluating the Eifel Algorithm for
             TCP in a GPRS Network, In Proceedings of the European
             Wireless Conference, February 2002.

   [GL03]    A. Gurtov, R. Ludwig, Responding to Spurious Timeouts in
             TCP, In Proceedings of IEEE INFOCOM 03, .

   [RFC3481] H. Inamura, G. Montenegro, R. Ludwig, A. Gurtov,
             F. Khafizov, TCP over Second (2.5G) and Third (3G)
             Generation Wireless Networks, RFC3481, February 2003.

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INTERNET-DRAFT            TCP - Eifel Response               March, 2003

   [KP87]    P. Karn, C. Partridge, Improving Round-Trip Time Estimates
             in Reliable Transport Protocols, In Proceedings of ACM
             SIGCOMM 87.

   [LK00]    R. Ludwig, R. H. Katz, The Eifel Algorithm: Making TCP
             Robust Against Spurious Retransmissions, ACM Computer
             Communication Review, Vol. 30, No. 1, January 2000.

   [LS00]    R. Ludwig, K. Sklower, The Eifel Retransmission Timer, ACM
             Computer Communication Review, Vol. 30, No. 3, July 2000.

   [SK02]    P. Sarolahti, A. Kuznetsov, Congestion Control in Linux
             TCP, In Proceedings of USENIX, June 2002.

   [SK03]    P. Sarolahti, M. Kojo, F-RTO: A TCP RTO Recovery Algorithm
             for Avoiding Unnecessary Retransmissions, draft-sarolahti-
             tsvwg-tcp-frto-03.txt (work in progress), January 2003.

   [WS95]    G. R. Wright, W. R. Stevens, TCP/IP Illustrated, Volume 2
             (The Implementation), Addison Wesley, January 1995.

   [Zh86]    L. Zhang, Why TCP Timers Don't Work Well, In Proceedings of
             ACM SIGCOMM 88.

Author's Address

     Reiner Ludwig
     Ericsson Research (EED)
     Ericsson Allee 1
     52134 Herzogenrath, Germany

     Andrei Gurtov
     Cellular Systems Development
     P.O. Box 970, FIN-00051 Sonera
     Helsinki, Finland
     Phone: +358(0)20401
     Fax:   +358(0)204064365

This Internet-Draft expires in September 2003.

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