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TCPM                                                    R. Scheffenegger
Internet-Draft                                                    NetApp
Intended status: Experimental                             March 12, 2021
Expires: September 13, 2021

           Simple Lost Retransmission Detection with SACK TCP


   Lost Retransmissions are a major source of latency for TCP transfers.
   This note specifies how selective acknowledgment (SACK) information
   can be used to timely recover from lost retransmissions.  In
   addition, it codifies the congestion control reaction on lost

Note to Readers

   Discussion of this draft takes place on the TCPM working group
   mailing list [1], which is archived at

   Working Group information can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on September 13, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Design Considerations . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Recovery Initiation . . . . . . . . . . . . . . . . . . .   4
     5.2.  Detection of lost retransmissions . . . . . . . . . . . .   4
     5.3.  Reordering  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.4.  Ordering of retransmitted segments  . . . . . . . . . . .   6
   6.  Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Lost Retransmission Detection . . . . . . . . . . . . . .   7
     6.2.  LRD Algorithm Detail  . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Lost Retransmission Detection Example  . . . . . . .  12
     A.1.  Lost Retransmission, Mid-Stream . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Selective Acknowledgement (SACK) is widely used to identify exactly
   which TCP segment was lost and only send these missing segments
   during a recovery episode.  This helps improve the effectiveness of
   loss recovery and aligns with the principle of packet conservation.
   In addition, SACK information can also be used to infer about lost
   retransmissions.  When this information is not used, TCP senders
   revert to the retransmission timeout (RTO) scheme to recover from
   lost retransmissions.

   Current SACK implementations, with one widely deployed exception, do
   not perform lost retransmission detection.  Lost retransmission

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   detection (LRD) in the one implementation that performs it was
   described as an emergent feature due to the way the sender is
   handling SACK.  Therefore, LRD is handled in that stack within the
   current regime of loss recovery, but without any additional
   congestion control reaction.

   This note specifies the use of SACK to detect and recover from lost
   retransmissions.  Using this scheme, a RTO is only required to
   recover from excessive loss of segments, or ACKs.  The intention of
   this note is to enhance SACK loss recovery so that most RTO events
   can be mitigated.  Only during episodes of pathological network
   impediments, RTO are still necessary to achieve forward progress.

   The mechanism described adheres strictly to the principle of packet
   conservation.  It also requires the use of the forward
   acknowledgement (FACK) mechanism, described in more detail in [MM96a]
   and [TLP].

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Overview

   TCP Selective Acknowledgement [RFC2018] was designed to provide
   detailed information to the sender about the segements already
   received.  Based on this information, a sender can reduce the number
   of unnecessary retransmissions to close to zero and also recover from
   a loss of multiple segments within a single round trip time (RTT),
   and without reverting to a retransmission timeout (RTO).

   To that end, [RFC6675] describes the necessary data structures a
   sender has to maintain to keep track of incoming SACK information.
   However, no explicit attempt was made to specify how to use the
   information gained during the recovery episode to detect lost

   In addition, [RFC2018] specifically stipulated up to which point a
   SACK enabled sender may promote segments to become eligible for
   retransmission under the SACK scheme.  This heuristic works very well
   during bulk transfers, where the sender always has additional data to
   send.  Close to the end of a stream, when there is no more data in
   the socket to send, current SACK implementations fail to promote
   still outstanding and never acknowledged segments to become eligible

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   for retransmission.  When this happens, the performance of a TCP SACK
   implementation adhereing to [RFC3517] degrades and is lower than the
   performance of TCP NewReno [RFC3782], which can recovery this
   particular event without an RTO.

   The introduction of a rescue retransmission, as described in
   [RFC6675], addresses this particular issue.

   This document is concerned with the behavior of a TCP SACK sender,
   when after retransmission of all ourstanding segments, and the
   transmission of new data, the recovery state persists (SND.UNA does
   not advance to SND.MAX at the time of loss recovery initiation, also
   known as Recovery Point).

4.  Definitions

   This document uses the terms SND.UNA, SND.NXT, SND.MAX as defined in

   SND.FACK (forward acknowledgment) is used to describe the highest
   sequence number that has been SACKed by the receiver and subsequently
   seen by the sender.  The full definition can be found in [MM96a] and
   [MM96b].  The FACK mechanism is further described in [TLP].

5.  Design Considerations

   The algorithm described in this document has to adhere to the
   principle of packet conservation.  Detection and recovery from lost
   retransmissions is plagued with the same set of problems that can
   become worrysome during regular loss detection and loss recovery.
   Especially heavy reordering and recovery at the end-of-stream can
   make it hard to achive good efficiency during loss recovery.

5.1.  Recovery Initiation

   The algorithm outlined does not speak about the engagement of the
   loss recovery state by the sender TCP.  It is assumed, that the
   methods outlined in Congestion Control [RFC5681], Early Retransmit
   [RFC5827] and [SRE], now incorporated into [RFC6675] are used to
   engage in loss recovery.  This leaves only the case where all
   segments between SND.UNA and SND.MAX are lost to be recovered from by
   means of retransmission timeout.

5.2.  Detection of lost retransmissions

   The intuition behind the scheme is that if a retransmission succeeds,
   then the cumulative ack should increase one round trip time after the
   retransmission was sent.  Otherwise, the retransmission must have

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   been lost.  The key is to have a unambiguous signal which indicates
   that at least one RTT has passed after a retransmission was sent out.

   As long as the sending TCP has still unsent data available, an
   unabigious signal can be deducted by using the FACK mechanism.  After
   the first round of sending retransmissions, the sender MAY send
   previously unsent data.  Once SND.FACK advances and encompasses this
   newly sent data, the sender can deduct with high probability, that
   any still outstanding packets have been dropped by the network.  The
   sender MAY start retransmitting all still outstanding packets.  If
   the sender chooses to do so, it MUST take an appropriate congestion
   control action.  This action is prudent, as the loss of retransmitted
   packets can be a signal of persistent congestion in the network, that
   lasts even after the initial congestion control reaction at least one
   RTT before.

   Note that the one popular stack performing LRD already does not react
   by reducing the congestion window before starting the next cycle of
   retransmissions.  It is therefore more aggressive that the mechanism
   described herein.  Nevertheless, no network instabilities have been
   reported since that stack started using LRD more than two decades

5.3.  Reordering

   Without making use of additional information not contained in the
   SACK entries, only reordered ACKs can be discriminated.

   If a single data segment is delayed, and later resent, it is not
   possible by using only information available within SACK entries to
   distinguish if the original or retransmitted segment was SACKed.
   Thus lost retransmission detection can fall victim to reordered data
   segments, if it were to use retransmitted segments as signal to
   detemine lost retransmissions.

   The use of an SACK acknowledging data that was not sent at the
   initiation of the recovery episode prevents this issue.

   On the return path, reordered ACKs may be recognized, by comparing
   the SACK entries contained in the ACK.  The original ACK from the in-
   sequence, original transmission does not contain any SACK entries
   beyond SND.FACK, while the ACK for a retransmitted segment would
   likely contain SACK blocks of segments higher than the newly SACKed

   Also, if an ACK does not contain any newly SACKed segments than
   already known in the senders scoreboard, ACK reordering is likely to
   have occured.  For example, the SACK entry may contain only a part of

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   an entry already in the scoreboard.  However, such a simple heuristic
   is not enough to discriminate properly the ACK for a retransmitted
   data segment from the ACK of the original data segment.

5.4.  Ordering of retransmitted segments

   There are a number of choices when it comes to deciding which packet
   to transmit at what time and also in what order.  With TCP SACK, the
   decision of what to send has been decoupled from the decision when
   (and how much) to send.

   In the context of lost retransmission detection, there are at least
   four broad approaches, each of which has a different figure of merit:

   o  Stricty enqueue all known lost segments first in the range
      [SND.UNA ... SND.FACK].  Only when the last enqueued segment has
      been retransmitted at least once, segments which are found to be
      still missing may be enqueued for a 2nd cycle, again from the
      newer [SND.UNA ... SND.FACK].  This is the most conservative
      approach, and would ensure the least amount of spurious
      (unnecessary) retransmissions.

   o  A second approach would be for the sender to re-enqueue an already
      retransmitted segment as soon as it receives positive proof that
      at least 3 segments have been received, which were sent after the
      segment in question.  This is the approach choosen by one popular
      stack.  However, it assumes a continous data stream so that at any
      later time, there will still be enough data segments around that
      the criteria can be matched for a lost retransmission.  The delay
      on the receiver side, before some new data can be delivered up the
      stack to the application can be reduced somewhat over option 1.
      This approach still maintains nearly optimal efficiency and very
      few spurious retransmissions.

   o  Third, a slightly more relaxed criteria for detection of lost
      retransmissions can be applied.  As soon as any data segment is
      positively acknowledged (SACKed), that was sent at least dupthresh
      segments later, a retransmitted segment can be considered lost.
      Note that dupthresh is not necessarily constant in this approach,
      as the same guidelines as defined in Early Retransmit [RFC5827]
      may be applied once the retransmitted segment closes in on

   o  Last, a sender could assume strictly in-sequence delivery of
      retransmitted segments.  During loss recovery, the transmission
      rate of the sent segments is slower than just prior to the
      detection of the loss, in particular when PRR [RFC6937] is in use.
      This may reduce load-induced reordering to some extent.  This

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      approach would allow the most timely delivery of data only blocked
      by a few lost segments on the receiver side, but would also have
      the least efficiency in terms of packet conversation.

   Furthermore, the senders congestion window might not allow for many
   re-retransmissions before a stall.  Therefore, additional steps would
   be necessary on the sender side, to ensure continous, paced
   transmission even after the ACK clock has stopped.  This limits the
   usefulness of this approach, and addressing congestion control and
   timing related issues are outside the scope of this note.  However,
   this is effectively implemented when using RACK [RFC8985].

6.  Algorithm

   Section 5 in [RFC2018] seems to have been interpreted as an exlusive
   list of which segments may become elegible for retransmission, but
   can also be interpreted as an inclusive list:

   After the SACKed bit is turned on (as the result of processing a
   received SACK option), the data sender will skip that segment
   during any later retransmission.  Any segment that has the SACKed
   bit turned off and is less than the highest SACKed segment is
   available for retransmission.

6.1.  Lost Retransmission Detection

   In order to track if a retransmitted segment might have been lost,
   the sender requires additional state while in the recovery state.

   Once TCP has established that genuine loss exists in the network, it
   enters loss recovery.  At this point, the current value of SND.MAX is
   stored ("Recover" in NewReno [RFC6582]).  Thus it is enough to check
   if SND.FACK advances beyond "Recover".  Once that becomes true, some
   previously unsent data was acknowledged by the receiver.  By that
   time, any outstanding retransmissions should have been received as
   well.  Thus the sender MAY retransmit the outstanding data from the
   SACK scoreboard again, after taking appropriate congestion control
   action (i.e. reducing the congestion window).

   The retransmission SHOULD proceed in order of ascending sequence
   numbers across the unfilled holes of the SACK scoreboard, to maximize
   the chance that a delayed segment closes still outstanding holes.

   Note that implementations tracking sequence-number ranges in their
   scoreboard only need to track a single sequence number per recovery
   episode.  Multiple cycles of SACK loss recover, without leaving loss
   recovery in between, are possible by tracking the relevant "Recovery"
   in the scoreboard data structure.

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   Implicitly, this rule will also make sure, that all the segments
   which had become elegible for retransmission will have been sent at
   least one time, before any additional round of retransmissions is
   initiated.  If the entire flight of data except a small number of
   segments at the end were lost, it takes at least one RTT for the
   information about successfully received segments to reach the sender.
   By that time, the first round of retransmissions is already completed
   (and additional data segments with sequence numbers higher than
   SND.MAX at the start of the recovery episode start may have been
   already been sent.)

   In order to guarantee a timely delivery at end-of-stream, a TCP
   sender implementing LRD SHOULD also make use of the "Rescue
   Retransmission" as defined in [RFC6675].

6.2.  LRD Algorithm Detail

   1.:  On entering Loss Recovery, store SND.MAX to Recover

   2.:  After retransmission of the last segment of a hole in the
      scoreboard, store Recover to Hole.Rxmit

   3.:  Once SND.FACK advances beyond Recover, while there are holes in
      the scoreboard:

   3.1.:  store SND.MAX to Recover

   3.2.:  perform adequate congestion control reaction (i.e. reduce the
      congestion window)

   3.3.:  retransmit each hole in the scoreboard, where Hole.Rxmit <
      Recover, when appropriate to do so.

7.  Security Considerations

   The algorithm presented in this paper shares security considerations
   with [RFC2018] and [RFC6675].

8.  IANA Considerations

   This document does not require any IANA actions.

9.  Acknowledgements

   The author would like to thank Matt Mathis for the insightful
   discussions about SACK and it's intended behavior and the spirit
   driving the design of SACK.

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   Dragana Damjanovic was very helpful in reviewing an earlier version
   of this text and point out numerous clarifications.

   Furthermore, valuable feedback was received from John Heffner, Jeff
   Prem and Anumita Biswas.

10.  References

10.1.  Normative References

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC5827]  Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
              P. Hurtig, "Early Retransmit for TCP and Stream Control
              Transmission Protocol (SCTP)", RFC 5827,
              DOI 10.17487/RFC5827, May 2010,

   [RFC6582]  Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
              NewReno Modification to TCP's Fast Recovery Algorithm",
              RFC 6582, DOI 10.17487/RFC6582, April 2012,

   [RFC6675]  Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
              and Y. Nishida, "A Conservative Loss Recovery Algorithm
              Based on Selective Acknowledgment (SACK) for TCP",
              RFC 6675, DOI 10.17487/RFC6675, August 2012,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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10.2.  Informative References

   [DAC]      Beomjoon Kim, . and . Jaiyong Lee, "Retransmission Loss
              Recovery by Duplicate Acknowledgement Counting", IEEE
              Communications Letters, vol. 8, no. 1, pp. 69-71 , January

   [LRD]      Beomjoon Kim, ., Dongmin Kim, ., and . Jaiyong Lee, "Lost
              Retransmission Detection for TCP SACK", IEEE
              Communications Letters, vol. 8, no. 9, pp. 600-602 ,
              September 2004.

   [LRD2]     Beomjoon Kim, ., Yong-Hoon Choi, ., Jaiyong Lee, ., Min-
              Seok Oh, ., and . Jin-Sung Choi, "["Lost Retransmission
              Detection for TCP Part 2", "TCP using SACK option"]",
              Proceedings of IFIP-TC6 Networking 2004, LNCS 3042,
              Springer-Verlag, vol. 3042, pp. 88-99 , May 2004.

   [LRSF]     Hurtig, P, ., Garcia, J, ., and A. Brunstrom, "Loss
              Recovery in Short TCP/SCTP Flows", Karlstad University
              Studies 2006:71 , December 2006.

   [MM96a]    Mathis, M, . and J. Mahdavi, "["Forward Acknowledgment",
              "Refining TCP Congestion Control"]", Proceedings of
              SIGCOMM 1996 , August 1996.

   [MM96b]    Mathis, M, . and J. Mahdavi, "TCP Rate-Halving with
              Bounding Parameters", September 2004,

   [RFC3517]  Blanton, E., Allman, M., Fall, K., and L. Wang, "A
              Conservative Selective Acknowledgment (SACK)-based Loss
              Recovery Algorithm for TCP", RFC 3517,
              DOI 10.17487/RFC3517, April 2003,

   [RFC3782]  Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
              Modification to TCP's Fast Recovery Algorithm", RFC 3782,
              DOI 10.17487/RFC3782, April 2004,

   [RFC6937]  Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional
              Rate Reduction for TCP", RFC 6937, DOI 10.17487/RFC6937,
              May 2013, <https://www.rfc-editor.org/info/rfc6937>.

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   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,

   [RFC8985]  Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
              RACK-TLP Loss Detection Algorithm for TCP", RFC 8985,
              DOI 10.17487/RFC8985, February 2021,

              Jarvinen, I, . and M. Kojo, "Using TCP Selective
              Acknowledgement (SACK) Information to Determine Duplicate
              Acknowledgements for Loss Recovery Initiation", March
              2010, <http://tools.ietf.org/html/draft-ietf-tcpm-sack-

              Kodama, Y, ., Takano, R, ., Okazaki, F, ., and T. Kudoh,
              "Improvement of Communication Performance of Linux TCP/IP
              by Fixing a Problem in Detection of Loss of
              Retransmission", March 2008,

   [SRE]      Jarvinen, I. and M. Kojo, "Using TCP Selective
              Acknowledgement (SACK) Information to Determine Duplicate
              Acknowledgements for Loss Recovery Initiation", draft-
              ietf-tcpm-sack-recovery-entry-01 (work in progress), March

   [TCPLat]   Cardwell, N, ., Savage, S, ., and T. Anderson, "Modeling
              TCP Latency", Proceedings IEEE INFOCOM , March 2000.

   [TLP]      Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
              "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
              Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
              in progress), February 2013.

10.3.  URIs

   [1] mailto:tcpm@ietf.org

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Appendix A.  Lost Retransmission Detection Example

   The following lengthy graph shows the intended behavior under
   pathological packet loss, where every third segment is lost.  Note
   that SACK LRD will not be able to recover, if the loss ratio during
   recovery is higher than about 50%, due to the congestion window

   For clarity, each segment is denoted only via a single number.  Note
   that the ACKs are also given with the segement they ack, not the next
   sequence number.

A.1.  Lost Retransmission, Mid-Stream

       ACK          Transmitted  Received    ACK Sent
       Received     Segment      Segment     (Including SACK Blocks)

                    5000-5499    5000-5499   (delayed ACK)
                    5500-5999    5500-5999
                    6000-6499    (dropped)
                    6500-6999    (dropped)
                    7000-7499    (dropped)
                    7500-7999    (dropped)
                    8000-8499    (dropped)
                    8500-8999    (dropped)
                    9000-9499    9000-9499
                                             6000; 9000-9500
                    9500-9999    9500-9999
                                             6000; 9000-10000
                    10000-10499  10000-10499
                                             6000; 9000-10500
                    10500-10999  10000-10999
                                             6000; 9000-11000

       6000; 9000-9500
       (lim. tr.)   11000-11499  11000-11499
                                             6000; 9000-11500
       6000; 9000-10000
       (lim. tr.)   11500-11999  11500-11999 (end-of-stream)
                                             6000; 9000-12000

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       6000; 9000-10500
       (fast retr.) 6000-6499    (dropped)   [^1]

       6000; 9000-11000

       6000; 9000-11500
                    6500-6999    6500-6999   [^2]
                                             6000; 6500-7000,9000-12000
       6000; 9000-12000

       6000; 6500-7000,9000-12000
                    7000-7499    7000-7499   [^2]
                                             6000; 6500-7500,9000-12000
       6000; 6500-7500,9000-12000
                    7500-7999    7500-7999   \[mark 8999*\]
                                             6000; 6500-8000,9000-12000
       6000; 6500-8000,9000-12000            (trigger 6000-6499)
                    6000-6499    6000-6499   \[mark 8999*\]
                                             8000; 9000-12000
       8000; 9000-12000
                    8000-8499    8000-8499   \[mark 8999*\]
                                             8500; 9000-12000
       8500; 9000-12000
                    8500-8999    8500-8999   \[mark 12499**\]
       12000                                 (exit loss recovery)

                                 Figure 1

Author's Address

   Richard Scheffenegger
   Am Europlatz 2
   Vienna  1120

   Email: rs.ietf@gmx.at

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