Network Working Group                                          R. Ludwig
Request for Comments: 3522                                      M. Meyer
Category: Experimental                                 Ericsson Research
                                                              April 2003

The Eifel Detection Algorithm for TCP

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

Abstract

   The Eifel detection algorithm allows a TCP sender to detect a
   posteriori whether it has entered loss recovery unnecessarily.  It
   requires that the TCP Timestamps option defined in RFC 1323 be
   enabled for a connection.  The Eifel detection algorithm makes use of
   the fact that the TCP Timestamps option eliminates the retransmission
   ambiguity in TCP.  Based on the timestamp of the first acceptable ACK
   that arrives during loss recovery, it decides whether loss recovery
   was entered unnecessarily.  The Eifel detection algorithm provides a
   basis for future TCP enhancements.  This includes response algorithms
   to back out of loss recovery by restoring a TCP sender's congestion
   control state.

Terminology

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

   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, with
   repacketization, 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
   algorithm, this makes no difference as it also operates 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 a 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 a TCP sender to trigger a fast
   retransmit.  Currently, DupThresh is specified as a fixed value of
   three [RFC2581].  Future TCPs might implement an adaptive DupThresh.

1. Introduction

   The retransmission ambiguity problem [Zh86], [KP87] is a TCP sender's
   inability to distinguish whether the first acceptable ACK that
   arrives after a retransmit was sent in response to the original
   transmit or the retransmit.  This problem occurs after a timeout-
   based retransmit and after a fast retransmit.  The Eifel detection
   algorithm uses the TCP Timestamps option defined in [RFC1323] to
   eliminate the retransmission ambiguity.  It thereby allows a TCP
   sender to detect a posteriori whether it has entered loss recovery
   unnecessarily.

   This added capability of a TCP sender is useful in environments where
   TCP's loss recovery and congestion control algorithms may often get
   falsely triggered.  This can be caused by packet reordering, packet
   duplication, or a sudden delay increase in the data or the ACK path
   that results in a spurious timeout.  For example, such sudden delay
   increases can often occur in wide-area wireless access networks due
   to handovers, resource preemption due to higher priority traffic
   (e.g., voice), or because the mobile transmitter traverses through a
   radio coverage hole (e.g., see [Gu01]).  In such wireless networks,
   the often unnecessary go-back-N retransmits that typically occur
   after a spurious timeout create a serious problem.  They decrease
   end-to-end throughput, are useless load upon the network, and waste
   transmission (battery) power.  Note that across such networks the use
   of timestamps is recommended anyway [RFC3481].

   Based on the Eifel detection algorithm, a TCP sender may then choose
   to implement dedicated response algorithms.  One goal of such a
   response algorithm would be to alleviate the consequences of a
   falsely triggered loss recovery.  This may include restoring the TCP
   sender's congestion control state, and avoiding the mentioned
   unnecessary go-back-N retransmits.  Another goal would be to adapt
   protocol parameters such as the duplicate acknowledgement threshold
   [RFC2581], and the RTT estimators [RFC2988].  This is to reduce the
   risk of falsely triggering TCP's loss recovery again as the
   connection progresses.  However, such response algorithms are outside

   the scope of this document.  Note: The original proposal, the "Eifel
   algorithm" [LK00], comprises both a detection and a response
   algorithm.  This document only defines the detection part.  The
   response part is defined in [LG03].

   A key feature of the Eifel detection algorithm is that it already
   detects, upon the first acceptable ACK that arrives during loss
   recovery, whether a fast retransmit or a timeout was spurious.  This
   is crucial to be able to avoid the mentioned go-back-N retransmits.
   Another feature is that the Eifel detection algorithm is fairly
   robust against the loss of ACKs.

   Also the DSACK option [RFC2883] can be used to detect a posteriori
   whether a TCP sender has entered loss recovery unnecessarily [BA02].
   However, the first ACK carrying a DSACK option usually arrives at a
   TCP sender only after loss recovery has already terminated.  Thus,
   the DSACK option cannot be used to eliminate the retransmission
   ambiguity.  Consequently, it cannot be used to avoid the mentioned
   unnecessary go-back-N retransmits.  Moreover, a DSACK-based detection
   algorithm is less robust against ACK losses.  A recent proposal based
   on neither the TCP timestamps nor the DSACK option does not have the
   limitation of DSACK-based schemes, but only addresses the case of
   spurious timeouts [SK03].

2. Events that Falsely Trigger TCP Loss Recovery

   The following events may falsely trigger a TCP sender's loss recovery
   and congestion control algorithms.  This causes a so-called spurious
   retransmit, and an unnecessary reduction of the TCP sender's
   congestion window and slow start threshold [RFC2581].

      -  Spurious timeout

      -  Packet reordering

      -  Packet duplication

   A spurious timeout is a timeout that would not have occurred had the
   sender "waited longer".  This may be caused by increased delay that
   suddenly occurs in the data and/or the ACK path.  That in turn might
   cause an acceptable ACK to arrive too late, i.e., only after a TCP
   sender's retransmission timer has expired.  For the purpose of
   specifying the algorithm in Section 3, we define this case as SPUR_TO
   (equal 1).

      Note: There is another case where a timeout would not have
      occurred had the sender "waited longer": the retransmission timer
      expires, and afterwards the TCP sender receives the duplicate ACK

      that would have triggered a fast retransmit of the oldest
      outstanding segment.  We call this a 'fast timeout', since in
      competition with the fast retransmit algorithm the timeout was
      faster.  However, a fast timeout is not spurious since apparently
      a segment was in fact lost, i.e., loss recovery was initiated
      rightfully.  In this document, we do not consider fast timeouts.

   Packet reordering in the network may occur because IP [RFC791] does
   not guarantee in-order delivery of packets.  Additionally, a TCP
   receiver generates a duplicate ACK for each segment that arrives
   out-of-order.  This results in a spurious fast retransmit if three or
   more data segments arrive out-of-order at a TCP receiver, and at
   least three of the resulting duplicate ACKs arrive at the TCP sender.
   This assumes that the duplicate acknowledgement threshold is set to
   three as defined in [RFC2581].

   Packet duplication may occur because a receiving IP does not (cannot)
   remove packets that have been duplicated in the network.  A TCP
   receiver in turn also generates a duplicate ACK for each duplicate
   segment.  As with packet reordering, this results in a spurious fast
   retransmit if duplication of data segments or ACKs results in three
   or more duplicate ACKs to arrive at a TCP sender.  Again, this
   assumes that the duplicate acknowledgement threshold is set to three.

   The negative impact on TCP performance caused by packet reordering
   and packet duplication is commonly the same: a single spurious
   retransmit (the fast retransmit), and the unnecessary halving of a
   TCP sender's congestion window as a result of the subsequent fast
   recovery phase [RFC2581].

   The negative impact on TCP performance caused by a spurious timeout
   is more severe.  First, the timeout event itself causes a single
   spurious retransmit, and unnecessarily forces a TCP sender into slow
   start [RFC2581].  Then, as the connection progresses, a chain
   reaction gets triggered that further decreases TCP's performance.
   Since the timeout was spurious, at least some ACKs for original
   transmits typically arrive at the TCP sender before the ACK for the
   retransmit arrives.  (This is unless severe packet reordering
   coincided with the spurious timeout in such a way that the ACK for
   the retransmit is the first acceptable ACK to arrive at the TCP
   sender.)  Those ACKs for original transmits then trigger an implicit
   go-back-N loss recovery at the TCP sender [LK00].  Assuming that none
   of the outstanding segments and none of the corresponding ACKs were
   lost, all outstanding segments get retransmitted unnecessarily.  In
   fact, during this phase, a TCP sender violates the packet
   conservation principle [Jac88].  This is because the unnecessary go-
   back-N retransmits are sent during slow start.  Thus, for each packet
   that leaves the network and that belongs to the first half of the

   original flight, two useless retransmits are sent into the network.
   In addition, some TCPs suffer from a spurious fast retransmit.  This
   is because the unnecessary go-back-N retransmits arrive as duplicates
   at the TCP receiver, which in turn triggers a series of duplicate
   ACKs.  Note that this last spurious fast retransmit could be avoided
   with the careful variant of 'bugfix' [RFC2582].

   More detailed explanations, including TCP trace plots that visualize
   the effects of spurious timeouts and packet reordering, can be found
   in the original proposal [LK00].

3. The Eifel Detection Algorithm

3.1 The Idea

   The goal of the Eifel detection algorithm is to allow a TCP sender to
   detect a posteriori whether it has entered loss recovery
   unnecessarily.  Furthermore, the TCP sender should be able to make
   this decision upon the first acceptable ACK that arrives after the
   timeout-based retransmit or the fast retransmit has been sent.  This
   in turn requires extra information in ACKs by which the TCP sender
   can unambiguously distinguish whether that first acceptable ACK was
   sent in response to the original transmit or the retransmit.  Such
   extra information is provided by the TCP Timestamps option [RFC1323].
   Generally speaking, timestamps are monotonously increasing "serial
   numbers" added into every segment that are then echoed within the
   corresponding ACKs.  This is exploited by the Eifel detection
   algorithm in the following way.

   Given that timestamps are enabled for a connection, a TCP sender
   always stores the timestamp of the retransmit sent in the beginning
   of loss recovery, i.e., the timestamp of the timeout-based retransmit
   or the fast retransmit.  If the timestamp of the first acceptable
   ACK, that arrives after the retransmit was sent, is smaller then the
   stored timestamp of that retransmit, then that ACK must have been
   sent in response to an original transmit.  Hence, the TCP sender must
   have entered loss recovery unnecessarily.

   The fact that the Eifel detection algorithm decides upon the first
   acceptable ACK is crucial to allow future response algorithms to
   avoid the unnecessary go-back-N retransmits that typically occur
   after a spurious timeout.  Also, if loss recovery was entered
   unnecessarily, a window worth of ACKs are outstanding that all carry
   a timestamp that is smaller than the stored timestamp of the
   retransmit.  The arrival of any one of those ACKs is sufficient for
   the Eifel detection algorithm to work.  Hence, the solution is fairly

   robust against ACK losses.  Even the ACK sent in response to the
   retransmit, i.e., the one that carries the stored timestamp, may get
   lost without compromising the algorithm.

3.2 The Algorithm

   Given that the TCP Timestamps option [RFC1323] is enabled for a
   connection, a TCP sender MAY use the Eifel detection algorithm as
   defined in this subsection.

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

      (1)     Set a "SpuriousRecovery" variable to FALSE (equal 0).

      (2)     Set a "RetransmitTS" variable to the value of the
              Timestamp Value field of the Timestamps option included in
              the retransmit sent when loss recovery is initiated.  A
              TCP sender must ensure that RetransmitTS does not get
              overwritten as loss recovery progresses, e.g., in case of
              a second timeout and subsequent second retransmit of the
              same octet.

      (3)     Wait for the arrival of an acceptable ACK.  When an
              acceptable ACK has arrived, proceed to step (4).

      (4)     If the value of the Timestamp Echo Reply field of the
              acceptable ACK's Timestamps option is smaller than the
              value of RetransmitTS, then proceed to step (5),

              else proceed to step (DONE).

      (5)     If the acceptable ACK carries a DSACK option [RFC2883],
              then proceed to step (DONE),

              else if during the lifetime of the TCP connection the TCP
              sender has previously received an ACK with a DSACK option,
              or the acceptable ACK does not acknowledge all outstanding
              data, then proceed to step (6),

              else proceed to step (DONE).

      (6)     If the loss recovery has been initiated with a timeout-
              based retransmit, then set
                  SpuriousRecovery <- SPUR_TO (equal 1),

              else set
                  SpuriousRecovery <- dupacks+1

      (RESP)  Do nothing (Placeholder for a response algorithm).

      (DONE)  No further processing.

   The comparison "smaller than" in step (4) is conservative.  In
   theory, if the timestamp clock is slow or the network is fast,
   RetransmitTS could at most be equal to the timestamp echoed by an ACK
   sent in response to an original transmit.  In that case, it is
   assumed that the loss recovery was not falsely triggered.

   Note that the condition "if during the lifetime of the TCP connection
   the TCP sender has previously received an ACK with a DSACK option" in
   step (5) would be true in case the TCP receiver would signal in the
   SYN that it is DSACK-enabled.  But unfortunately, this is not
   required by [RFC2883].

3.3 A Corner Case: "Timeout due to loss of all ACKs" (step 5)

   Even though the oldest outstanding segment arrived at a TCP receiver,
   the TCP sender is forced into a timeout if all ACKs are lost.
   Although the resulting retransmit is unnecessary, such a timeout is
   unavoidable.  It should therefore not be considered spurious.
   Moreover, the subsequent reduction of the congestion window is an
   appropriate response to the potentially heavy congestion in the ACK
   path.  The original proposal [LK00] does not handle this case well.
   It effectively disables this implicit form of congestion control for
   the ACK path, which otherwise does not exist in TCP.  This problem is
   fixed by step (5) of the Eifel detection algorithm as explained in
   the remainder of this section.

   If all ACKs are lost while the oldest outstanding segment arrived at
   the TCP receiver, the retransmit arrives as a duplicate.  In response
   to duplicates, RFC 1323 mandates that the timestamp of the last
   segment that arrived in-sequence should be echoed.  That timestamp is
   carried by the first acceptable ACK that arrives at the TCP sender
   after loss recovery was entered, and is commonly smaller than the
   timestamp carried by the retransmit.  Consequently, the Eifel
   detection algorithm misinterprets such a timeout as being spurious,
   unless the TCP receiver is DSACK-enabled [RFC2883].  In that case,
   the acceptable ACK carries a DSACK option, and the Eifel algorithm is
   terminated through the first part of step (5).

      Note: Not all TCP implementations strictly follow RFC 1323.  In
      response to a duplicate data segment, some TCP receivers echo the
      timestamp of the duplicate.  With such TCP receivers, the corner
      case discussed in this section does not apply.  The timestamp
      carried by the retransmit would be echoed in the first acceptable
      ACK, and the Eifel detection algorithm would be terminated through
      step (4).  Thus, even though all ACKs were lost and independent of
      whether the DSACK option was enabled for a connection, the Eifel
      detection algorithm would have no effect.

   With TCP receivers that are not DSACK-enabled, disabling the
   mentioned implicit congestion control for the ACK path is not a
   problem as long as data segments are lost, in addition to the entire
   flight of ACKs.  The Eifel detection algorithm misinterprets such a
   timeout as being spurious, and the Eifel response algorithm would
   reverse the congestion control state.  Still, the TCP sender would
   respond to congestion (in the data path) as soon as it finds out
   about the first loss in the outstanding flight.  I.e., the TCP sender
   would still halve its congestion window for that flight of packets.
   If no data segment is lost while the entire flight of ACKs is lost,
   the first acceptable ACK that arrives at the TCP sender after loss
   recovery was entered acknowledges all outstanding data.  In that
   case, the Eifel algorithm is terminated through the second part of
   step (5).

   Note that there is little concern about violating the packet
   conservation principle when entering slow start after an unavoidable
   timeout caused by the loss of an entire flight of ACKs, i.e., when
   the Eifel detection algorithm was terminated through step (5).  This
   is because in that case, the acceptable ACK corresponds to the
   retransmit, which is a strong indication that the pipe has drained
   entirely, i.e., that no more original transmits are in the network.
   This is different with spurious timeouts as discussed in Section 2.

3.4 Protecting Against Misbehaving TCP Receivers (the Safe Variant)

   A TCP receiver can easily make a genuine retransmit appear to the TCP
   sender as a spurious retransmit by forging echoed timestamps.  This
   may pose a security concern.

   Fortunately, there is a way to modify the Eifel detection algorithm
   in a way that makes it robust against lying TCP receivers.  The idea
   is to use timestamps as a segment's "secret" that a TCP receiver only
   gets to know if it receives the segment.  Conversely, a TCP receiver
   will not know the timestamp of a segment that was lost.  Hence, to
   "prove" that it received the original transmit of a segment that a
   TCP sender retransmitted, the TCP receiver would need to return the
   timestamp of that original transmit.  The Eifel detection algorithm

   could then be modified to only decide that loss recovery has been
   unnecessarily entered if the first acceptable ACK echoes the
   timestamp of the original transmit.

   Hence, implementers may choose to implement the algorithm with the
   following modifications.

   Step (2) is replaced with step (2'):

      (2')    Set a "RetransmitTS" variable to the value of the
              Timestamp Value field of the Timestamps option that was
              included in the original transmit corresponding to the
              retransmit.  Note: This step requires that the TCP sender
              stores the timestamps of all outstanding original
              transmits.

   Step (4) is replaced with step (4'):

      (4')    If the value of the Timestamp Echo Reply field of the
              acceptable ACK's Timestamps option is equal to the value
              of the variable RetransmitTS, then proceed to step (5),

              else proceed to step (DONE).

   These modifications come at a cost: the modified algorithm is fairly
   sensitive against ACK losses since it relies on the arrival of the
   acceptable ACK that corresponds to the original transmit.

      Note: The first acceptable ACK that arrives after loss recovery
      has been unnecessarily entered should echo the timestamp of the
      original transmit.  This assumes that the ACK corresponding to the
      original transmit was not lost, that that ACK was not reordered in
      the network, and that the TCP receiver does not forge timestamps
      but complies with RFC 1323.  In case of a spurious fast
      retransmit, this is implied by the rules for generating ACKs for
      data segments that fill in all or part of a gap in the sequence
      space (see section 4.2 of [RFC2581]) and by the rules for echoing
      timestamps in that case (see rule (C) in section 3.4 of
      [RFC1323]).  In case of a spurious timeout, it is likely that the
      delay that has caused the spurious timeout has also caused the TCP
      receiver's delayed ACK timer [RFC1122] to expire before the
      original transmit arrives.  Also, in this case the rules for
      generating ACKs and the rules for echoing timestamps (see rule (A)
      in section 3.4 of [RFC1323]) ensure that the original transmit's
      timestamp is echoed.

   A remaining problem is that a TCP receiver might guess a lost
   segment's timestamp from observing the timestamps of recently
   received segments.  For example, if segment N was lost while segment
   N-1 and N+1 have arrived, a TCP receiver could guess the timestamp
   that lies in the middle of the timestamps of segments N-1 and N+1,
   and echo it in the ACK sent in response to the retransmit of segment
   N.  Especially if the TCP sender implements timestamps with a coarse
   granularity, a misbehaving TCP receiver is likely to be successful
   with such an approach.  In fact, with the 500 ms granularity
   suggested in [WS95], it even becomes quite likely that the timestamps
   of segments N-1, N, N+1 are identical.

   One way to reduce this risk is to implement fine grained timestamps.
   Note that the granularity of the timestamps is independent of the
   granularity of the retransmission timer.  For example, some TCP
   implementations run a timestamp clock that ticks every millisecond.
   This should make it more difficult for a TCP receiver to guess the
   timestamp of a lost segment.  Alternatively, it might be possible to
   combine the timestamps with a nonce, as is done for the Explicit
   Congestion Notification (ECN) [RFC3168].  One would need to take
   care, though, that the timestamps of consecutive segments remain
   monotonously increasing and do not interfere with the RTT timing
   defined in [RFC1323].

4. 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 http://www.ietf.org/ipr.

   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
   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.

5. Security Considerations

   There do not seem to be any security considerations associated with
   the Eifel detection algorithm.  This is because the Eifel detection
   algorithm does not alter the existing protocol state at a TCP sender.
   Note that the Eifel detection algorithm only requires changes to the
   implementation of a TCP sender.

   Moreover, a variant of the Eifel detection algorithm has been
   proposed in Section 3.4 that makes it robust against lying TCP
   receivers.  This may become relevant when the Eifel detection
   algorithm is combined with a response algorithm such as the Eifel
   response algorithm [LG03].

Acknowledgments

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

Normative References

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

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

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

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

Informative References

   [BA02]    Blanton, E. and M. Allman, "Using TCP DSACKs and SCTP
             Duplicate TSNs to Detect Spurious Retransmissions", Work in
             Progress.

   [RFC1122] Braden, R., "Requirements for Internet Hosts -
             Communication Layers", STD 3, RFC 1122, October 1989.

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

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

   [RFC3481] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F.
             Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
             Wireless Networks", RFC 3481, February 2003.

   [Jac88]   Jacobson, V., "Congestion Avoidance and Control", In
             Proceedings of ACM SIGCOMM 88.

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

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

   [LG03]    Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm for
             TCP", Work in Progress.

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

   [RFC791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
             1981.

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

   [SK03]    Sarolahti, P. and M. Kojo, "F-RTO: A TCP RTO Recovery
             Algorithm for Avoiding Unnecessary Retransmissions", Work
             in Progress.

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

   [Zh86]    Zhang, L., "Why TCP Timers Don't Work Well", In Proceedings
             of ACM SIGCOMM 86.

Authors' Addresses

   Reiner Ludwig
   Ericsson Research
   Ericsson Allee 1
   52134 Herzogenrath, Germany

   EMail: Reiner.Ludwig@eed.ericsson.se

   Michael Meyer
   Ericsson Research
   Ericsson Allee 1
   52134 Herzogenrath, Germany

   EMail: Michael.Meyer@eed.ericsson.se

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