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RFC 2525

Informational
Pages: 61
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Known TCP Implementation Problems

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Network Working Group                                          V. Paxson
Request for Comments: 2525                                        Editor
Category: Informational                                     ACIRI / ICSI
                                                               M. Allman
                            NASA Glenn Research Center/Sterling Software
                                                               S. Dawson
                                          Real-Time Computing Laboratory
                                                               W. Fenner
                                                              Xerox PARC
                                                               J. Griner
                                              NASA Glenn Research Center
                                                              I. Heavens
                                                    Spider Software Ltd.
                                                                K. Lahey
                                           NASA Ames Research Center/MRJ
                                                                J. Semke
                                        Pittsburgh Supercomputing Center
                                                                 B. Volz
                                            Process Software Corporation
                                                              March 1999


                   Known TCP Implementation Problems

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Table of Contents

   1.  INTRODUCTION....................................................2
   2.  KNOWN IMPLEMENTATION PROBLEMS...................................3
     2.1  No initial slow start........................................3
     2.2  No slow start after retransmission timeout...................6
     2.3  Uninitialized CWND...........................................9
     2.4  Inconsistent retransmission.................................11
     2.5  Failure to retain above-sequence data.......................13
     2.6  Extra additive constant in congestion avoidance.............17
     2.7  Initial RTO too low.........................................23
     2.8  Failure of window deflation after loss recovery.............26
     2.9  Excessively short keepalive connection timeout..............28
     2.10 Failure to back off retransmission timeout..................31

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     2.11 Insufficient interval between keepalives....................34
     2.12 Window probe deadlock.......................................36
     2.13 Stretch ACK violation.......................................40
     2.14 Retransmission sends multiple packets.......................43
     2.15 Failure to send FIN notification promptly...................45
     2.16 Failure to send a RST after Half Duplex Close...............47
     2.17 Failure to RST on close with data pending...................50
     2.18 Options missing from TCP MSS calculation....................54
   3.  SECURITY CONSIDERATIONS........................................56
   4.  ACKNOWLEDGEMENTS...............................................56
   5.  REFERENCES.....................................................57
   6.  AUTHORS' ADDRESSES.............................................58
   7.  FULL COPYRIGHT STATEMENT.......................................60

1. Introduction

   This memo catalogs a number of known TCP implementation problems.
   The goal in doing so is to improve conditions in the existing
   Internet by enhancing the quality of current TCP/IP implementations.
   It is hoped that both performance and correctness issues can be
   resolved by making implementors aware of the problems and their
   solutions.  In the long term, it is hoped that this will provide a
   reduction in unnecessary traffic on the network, the rate of
   connection failures due to protocol errors, and load on network
   servers due to time spent processing both unsuccessful connections
   and retransmitted data.  This will help to ensure the stability of
   the global Internet.

   Each problem is defined as follows:

   Name of Problem
      The name associated with the problem.  In this memo, the name is
      given as a subsection heading.

   Classification
      One or more problem categories for which the problem is
      classified:  "congestion control", "performance", "reliability",
      "resource management".

   Description
      A definition of the problem, succinct but including necessary
      background material.

   Significance
      A brief summary of the sorts of environments for which the problem
      is significant.

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   Implications
      Why the problem is viewed as a problem.

   Relevant RFCs
      The RFCs defining the TCP specification with which the problem
      conflicts.  These RFCs often qualify behavior using terms such as
      MUST, SHOULD, MAY, and others written capitalized.  See RFC 2119
      for the exact interpretation of these terms.

   Trace file demonstrating the problem
      One or more ASCII trace files demonstrating the problem, if
      applicable.

   Trace file demonstrating correct behavior
      One or more examples of how correct behavior appears in a trace,
      if applicable.

   References
      References that further discuss the problem.

   How to detect
      How to test an implementation to see if it exhibits the problem.
      This discussion may include difficulties and subtleties associated
      with causing the problem to manifest itself, and with interpreting
      traces to detect the presence of the problem (if applicable).

   How to fix
      For known causes of the problem, how to correct the
      implementation.

2. Known implementation problems

2.1.

   Name of Problem
      No initial slow start

   Classification
      Congestion control

   Description
      When a TCP begins transmitting data, it is required by RFC 1122,
      4.2.2.15, to engage in a "slow start" by initializing its
      congestion window, cwnd, to one packet (one segment of the maximum
      size).  (Note that an experimental change to TCP, documented in
      [RFC2414], allows an initial value somewhat larger than one
      packet.)  It subsequently increases cwnd by one packet for each
      ACK it receives for new data.  The minimum of cwnd and the

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      receiver's advertised window bounds the highest sequence number
      the TCP can transmit.  A TCP that fails to initialize and
      increment cwnd in this fashion exhibits "No initial slow start".

   Significance
      In congested environments, detrimental to the performance of other
      connections, and possibly to the connection itself.

   Implications
      A TCP failing to slow start when beginning a connection results in
      traffic bursts that can stress the network, leading to excessive
      queueing delays and packet loss.

      Implementations exhibiting this problem might do so because they
      suffer from the general problem of not including the required
      congestion window.  These implementations will also suffer from
      "No slow start after retransmission timeout".

      There are different shades of "No initial slow start".  From the
      perspective of stressing the network, the worst is a connection
      that simply always sends based on the receiver's advertised
      window, with no notion of a separate congestion window.  Another
      form is described in "Uninitialized CWND" below.

   Relevant RFCs
      RFC 1122 requires use of slow start.  RFC 2001 gives the specifics
      of slow start.

   Trace file demonstrating it
      Made using tcpdump [Jacobson89] recording at the connection
      responder.  No losses reported by the packet filter.

   10:40:42.244503 B > A: S 1168512000:1168512000(0) win 32768
                           <mss 1460,nop,wscale 0> (DF) [tos 0x8]
   10:40:42.259908 A > B: S 3688169472:3688169472(0)
                           ack 1168512001 win 32768 <mss 1460>
   10:40:42.389992 B > A: . ack 1 win 33580 (DF) [tos 0x8]
   10:40:42.664975 A > B: P 1:513(512) ack 1 win 32768
   10:40:42.700185 A > B: . 513:1973(1460) ack 1 win 32768
   10:40:42.718017 A > B: . 1973:3433(1460) ack 1 win 32768
   10:40:42.762945 A > B: . 3433:4893(1460) ack 1 win 32768
   10:40:42.811273 A > B: . 4893:6353(1460) ack 1 win 32768
   10:40:42.829149 A > B: . 6353:7813(1460) ack 1 win 32768
   10:40:42.853687 B > A: . ack 1973 win 33580 (DF) [tos 0x8]
   10:40:42.864031 B > A: . ack 3433 win 33580 (DF) [tos 0x8]

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      After the third packet, the connection is established.  A, the
      connection responder, begins transmitting to B, the connection
      initiator.  Host A quickly sends 6 packets comprising 7812 bytes,
      even though the SYN exchange agreed upon an MSS of 1460 bytes
      (implying an initial congestion window of 1 segment corresponds to
      1460 bytes), and so A should have sent at most 1460 bytes.

      The ACKs sent by B to A in the last two lines indicate that this
      trace is not a measurement error (slow start really occurring but
      the corresponding ACKs having been dropped by the packet filter).

      A second trace confirmed that the problem is repeatable.

   Trace file demonstrating correct behavior
      Made using tcpdump recording at the connection originator.  No
      losses reported by the packet filter.

   12:35:31.914050 C > D: S 1448571845:1448571845(0)
                            win 4380 <mss 1460>
   12:35:32.068819 D > C: S 1755712000:1755712000(0)
                            ack 1448571846 win 4096
   12:35:32.069341 C > D: . ack 1 win 4608
   12:35:32.075213 C > D: P 1:513(512) ack 1 win 4608
   12:35:32.286073 D > C: . ack 513 win 4096
   12:35:32.287032 C > D: . 513:1025(512) ack 1 win 4608
   12:35:32.287506 C > D: . 1025:1537(512) ack 1 win 4608
   12:35:32.432712 D > C: . ack 1537 win 4096
   12:35:32.433690 C > D: . 1537:2049(512) ack 1 win 4608
   12:35:32.434481 C > D: . 2049:2561(512) ack 1 win 4608
   12:35:32.435032 C > D: . 2561:3073(512) ack 1 win 4608
   12:35:32.594526 D > C: . ack 3073 win 4096
   12:35:32.595465 C > D: . 3073:3585(512) ack 1 win 4608
   12:35:32.595947 C > D: . 3585:4097(512) ack 1 win 4608
   12:35:32.596414 C > D: . 4097:4609(512) ack 1 win 4608
   12:35:32.596888 C > D: . 4609:5121(512) ack 1 win 4608
   12:35:32.733453 D > C: . ack 4097 win 4096

   References
      This problem is documented in [Paxson97].

   How to detect
      For implementations always manifesting this problem, it shows up
      immediately in a packet trace or a sequence plot, as illustrated
      above.

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   How to fix
      If the root problem is that the implementation lacks a notion of a
      congestion window, then unfortunately this requires significant
      work to fix.  However, doing so is important, as such
      implementations also exhibit "No slow start after retransmission
      timeout".

2.2.

   Name of Problem
      No slow start after retransmission timeout

   Classification
      Congestion control

   Description
      When a TCP experiences a retransmission timeout, it is required by
      RFC 1122, 4.2.2.15, to engage in "slow start" by initializing its
      congestion window, cwnd, to one packet (one segment of the maximum
      size).  It subsequently increases cwnd by one packet for each ACK
      it receives for new data until it reaches the "congestion
      avoidance" threshold, ssthresh, at which point the congestion
      avoidance algorithm for updating the window takes over.  A TCP
      that fails to enter slow start upon a timeout exhibits "No slow
      start after retransmission timeout".

   Significance
      In congested environments, severely detrimental to the performance
      of other connections, and also the connection itself.

   Implications
      Entering slow start upon timeout forms one of the cornerstones of
      Internet congestion stability, as outlined in [Jacobson88].  If
      TCPs fail to do so, the network becomes at risk of suffering
      "congestion collapse" [RFC896].

   Relevant RFCs
      RFC 1122 requires use of slow start after loss.  RFC 2001 gives
      the specifics of how to implement slow start.  RFC 896 describes
      congestion collapse.

      The retransmission timeout discussed here should not be confused
      with the separate "fast recovery" retransmission mechanism
      discussed in RFC 2001.

   Trace file demonstrating it
      Made using tcpdump recording at the sending TCP (A).  No losses
      reported by the packet filter.

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   10:40:59.090612 B > A: . ack 357125 win 33580 (DF) [tos 0x8]
   10:40:59.222025 A > B: . 357125:358585(1460) ack 1 win 32768
   10:40:59.868871 A > B: . 357125:358585(1460) ack 1 win 32768
   10:41:00.016641 B > A: . ack 364425 win 33580 (DF) [tos 0x8]
   10:41:00.036709 A > B: . 364425:365885(1460) ack 1 win 32768
   10:41:00.045231 A > B: . 365885:367345(1460) ack 1 win 32768
   10:41:00.053785 A > B: . 367345:368805(1460) ack 1 win 32768
   10:41:00.062426 A > B: . 368805:370265(1460) ack 1 win 32768
   10:41:00.071074 A > B: . 370265:371725(1460) ack 1 win 32768
   10:41:00.079794 A > B: . 371725:373185(1460) ack 1 win 32768
   10:41:00.089304 A > B: . 373185:374645(1460) ack 1 win 32768
   10:41:00.097738 A > B: . 374645:376105(1460) ack 1 win 32768
   10:41:00.106409 A > B: . 376105:377565(1460) ack 1 win 32768
   10:41:00.115024 A > B: . 377565:379025(1460) ack 1 win 32768
   10:41:00.123576 A > B: . 379025:380485(1460) ack 1 win 32768
   10:41:00.132016 A > B: . 380485:381945(1460) ack 1 win 32768
   10:41:00.141635 A > B: . 381945:383405(1460) ack 1 win 32768
   10:41:00.150094 A > B: . 383405:384865(1460) ack 1 win 32768
   10:41:00.158552 A > B: . 384865:386325(1460) ack 1 win 32768
   10:41:00.167053 A > B: . 386325:387785(1460) ack 1 win 32768
   10:41:00.175518 A > B: . 387785:389245(1460) ack 1 win 32768
   10:41:00.210835 A > B: . 389245:390705(1460) ack 1 win 32768
   10:41:00.226108 A > B: . 390705:392165(1460) ack 1 win 32768
   10:41:00.241524 B > A: . ack 389245 win 8760 (DF) [tos 0x8]

      The first packet indicates the ack point is 357125.  130 msec
      after receiving the ACK, A transmits the packet after the ACK
      point, 357125:358585.  640 msec after this transmission, it
      retransmits 357125:358585, in an apparent retransmission timeout.
      At this point, A's cwnd should be one MSS, or 1460 bytes, as A
      enters slow start.  The trace is consistent with this possibility.

      B replies with an ACK of 364425, indicating that A has filled a
      sequence hole.  At this point, A's cwnd should be 1460*2 = 2920
      bytes, since in slow start receiving an ACK advances cwnd by MSS.
      However, A then launches 19 consecutive packets, which is
      inconsistent with slow start.

      A second trace confirmed that the problem is repeatable.

   Trace file demonstrating correct behavior
      Made using tcpdump recording at the sending TCP (C).  No losses
      reported by the packet filter.

   12:35:48.442538 C > D: P 465409:465921(512) ack 1 win 4608
   12:35:48.544483 D > C: . ack 461825 win 4096
   12:35:48.703496 D > C: . ack 461825 win 4096
   12:35:49.044613 C > D: . 461825:462337(512) ack 1 win 4608

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   12:35:49.192282 D > C: . ack 465921 win 2048
   12:35:49.192538 D > C: . ack 465921 win 4096
   12:35:49.193392 C > D: P 465921:466433(512) ack 1 win 4608
   12:35:49.194726 C > D: P 466433:466945(512) ack 1 win 4608
   12:35:49.350665 D > C: . ack 466945 win 4096
   12:35:49.351694 C > D: . 466945:467457(512) ack 1 win 4608
   12:35:49.352168 C > D: . 467457:467969(512) ack 1 win 4608
   12:35:49.352643 C > D: . 467969:468481(512) ack 1 win 4608
   12:35:49.506000 D > C: . ack 467969 win 3584

      After C transmits the first packet shown to D, it takes no action
      in response to D's ACKs for 461825, because the first packet
      already reached the advertised window limit of 4096 bytes above
      461825.  600 msec after transmitting the first packet, C
      retransmits 461825:462337, presumably due to a timeout.  Its
      congestion window is now MSS (512 bytes).

      D acks 465921, indicating that C's retransmission filled a
      sequence hole.  This ACK advances C's cwnd from 512 to 1024.  Very
      shortly after, D acks 465921 again in order to update the offered
      window from 2048 to 4096.  This ACK does not advance cwnd since it
      is not for new data.  Very shortly after, C responds to the newly
      enlarged window by transmitting two packets.  D acks both,
      advancing cwnd from 1024 to 1536.  C in turn transmits three
      packets.

   References
      This problem is documented in [Paxson97].

   How to detect
      Packet loss is common enough in the Internet that generally it is
      not difficult to find an Internet path that will force
      retransmission due to packet loss.

      If the effective window prior to loss is large enough, however,
      then the TCP may retransmit using the "fast recovery" mechanism
      described in RFC 2001.  In a packet trace, the signature of fast
      recovery is that the packet retransmission occurs in response to
      the receipt of three duplicate ACKs, and subsequent duplicate ACKs
      may lead to the transmission of new data, above both the ack point
      and the highest sequence transmitted so far.  An absence of three
      duplicate ACKs prior to retransmission suffices to distinguish
      between timeout and fast recovery retransmissions.  In the face of
      only observing fast recovery retransmissions, generally it is not
      difficult to repeat the data transfer until observing a timeout
      retransmission.

Top      ToC       Page 9 
      Once armed with a trace exhibiting a timeout retransmission,
      determining whether the TCP follows slow start is done by
      computing the correct progression of cwnd and comparing it to the
      amount of data transmitted by the TCP subsequent to the timeout
      retransmission.

   How to fix
      If the root problem is that the implementation lacks a notion of a
      congestion window, then unfortunately this requires significant
      work to fix.  However, doing so is critical, for reasons outlined
      above.

2.3.

   Name of Problem
      Uninitialized CWND

   Classification
      Congestion control

   Description
      As described above for "No initial slow start", when a TCP
      connection begins cwnd is initialized to one segment (or perhaps a
      few segments, if experimenting with [RFC2414]).  One particular
      form of "No initial slow start", worth separate mention as the bug
      is fairly widely deployed, is "Uninitialized CWND".  That is,
      while the TCP implements the proper slow start mechanism, it fails
      to initialize cwnd properly, so slow start in fact fails to occur.

      One way the bug can occur is if, during the connection
      establishment handshake, the SYN ACK packet arrives without an MSS
      option.  The faulty implementation uses receipt of the MSS option
      to initialize cwnd to one segment; if the option fails to arrive,
      then cwnd is instead initialized to a very large value.

   Significance
      In congested environments, detrimental to the performance of other
      connections, and likely to the connection itself.  The burst can
      be so large (see below) that it has deleterious effects even in
      uncongested environments.

   Implications
      A TCP exhibiting this behavior is stressing the network with a
      large burst of packets, which can cause loss in the network.

   Relevant RFCs
      RFC 1122 requires use of slow start.  RFC 2001 gives the specifics
      of slow start.

Top      ToC       Page 10 
   Trace file demonstrating it
      This trace was made using tcpdump running on host A.  Host A is
      the sender and host B is the receiver.  The advertised window and
      timestamp options have been omitted for clarity, except for the
      first segment sent by host A.  Note that A sends an MSS option in
      its initial SYN but B does not include one in its reply.

   16:56:02.226937 A > B: S 237585307:237585307(0) win 8192
         <mss 536,nop,wscale 0,nop,nop,timestamp[|tcp]>
   16:56:02.557135 B > A: S 1617216000:1617216000(0)
         ack 237585308 win 16384
   16:56:02.557788 A > B: . ack 1 win 8192
   16:56:02.566014 A > B: . 1:537(536) ack 1
   16:56:02.566557 A > B: . 537:1073(536) ack 1
   16:56:02.567120 A > B: . 1073:1609(536) ack 1
   16:56:02.567662 A > B: P 1609:2049(440) ack 1
   16:56:02.568349 A > B: . 2049:2585(536) ack 1
   16:56:02.568909 A > B: . 2585:3121(536) ack 1

      [54 additional burst segments deleted for brevity]

   16:56:02.936638 A > B: . 32065:32601(536) ack 1
   16:56:03.018685 B > A: . ack 1

      After the three-way handshake, host A bursts 61 segments into the
      network, before duplicate ACKs on the first segment cause a
      retransmission to occur.  Since host A did not wait for the ACK on
      the first segment before sending additional segments, it is
      exhibiting "Uninitialized CWND"

   Trace file demonstrating correct behavior

      See the example for "No initial slow start".

   References
      This problem is documented in [Paxson97].

   How to detect
      This problem can be detected by examining a packet trace recorded
      at either the sender or the receiver.  However, the bug can be
      difficult to induce because it requires finding a remote TCP peer
      that does not send an MSS option in its SYN ACK.

   How to fix
      This problem can be fixed by ensuring that cwnd is initialized
      upon receipt of a SYN ACK, even if the SYN ACK does not contain an
      MSS option.

Top      ToC       Page 11 
2.4.

   Name of Problem
      Inconsistent retransmission

   Classification
      Reliability

   Description
      If, for a given sequence number, a sending TCP retransmits
      different data than previously sent for that sequence number, then
      a strong possibility arises that the receiving TCP will
      reconstruct a different byte stream than that sent by the sending
      application, depending on which instance of the sequence number it
      accepts.

      Such a sending TCP exhibits "Inconsistent retransmission".

   Significance
      Critical for all environments.

   Implications
      Reliable delivery of data is a fundamental property of TCP.

   Relevant RFCs
      RFC 793, section 1.5, discusses the central role of reliability in
      TCP operation.

   Trace file demonstrating it
      Made using tcpdump recording at the receiving TCP (B).  No losses
      reported by the packet filter.

   12:35:53.145503 A > B: FP 90048435:90048461(26)
                             ack 393464682 win 4096
                                        4500 0042 9644 0000
                    3006 e4c2 86b1 0401 83f3 010a b2a4 0015
                    055e 07b3 1773 cb6a 5019 1000 68a9 0000
   data starts here>504f 5254 2031 3334 2c31 3737*2c34 2c31
                    2c31 3738 2c31 3635 0d0a
   12:35:53.146479 B > A: R 393464682:393464682(0) win 8192
   12:35:53.851714 A > B: FP 90048429:90048463(34)
                          ack 393464682 win 4096
                                        4500 004a 965b 0000
                    3006 e4a3 86b1 0401 83f3 010a b2a4 0015
                    055e 07ad 1773 cb6a 5019 1000 8bd3 0000
   data starts here>5041 5356 0d0a 504f 5254 2031 3334 2c31
                    3737*2c31 3035 2c31 3431 2c34 2c31 3539
                    0d0a

Top      ToC       Page 12 
      The sequence numbers shown in this trace are absolute and not
      adjusted to reflect the ISN.  The 4-digit hex values show a dump
      of the packet's IP and TCP headers, as well as payload.  A first
      sends to B data for 90048435:90048461.  The corresponding data
      begins with hex words 504f, 5254, etc.

      B responds with a RST.  Since the recording location was local to
      B, it is unknown whether A received the RST.

      A then sends 90048429:90048463, which includes six sequence
      positions below the earlier transmission, all 26 positions of the
      earlier transmission, and two additional sequence positions.

      The retransmission disagrees starting just after sequence
      90048447, annotated above with a leading '*'.  These two bytes
      were originally transmitted as hex 2c34 but retransmitted as hex
      2c31.  Subsequent positions disagree as well.

      This behavior has been observed in other traces involving
      different hosts.  It is unknown how to repeat it.

      In this instance, no corruption would occur, since B has already
      indicated it will not accept further packets from A.

      A second example illustrates a slightly different instance of the
      problem.  The tracing again was made with tcpdump at the receiving
      TCP (D).

   22:23:58.645829 C > D: P 185:212(27) ack 565 win 4096
                                        4500 0043 90a3 0000
                    3306 0734 cbf1 9eef 83f3 010a 0525 0015
                    a3a2 faba 578c 70a4 5018 1000 9a53 0000
   data starts here>504f 5254 2032 3033 2c32 3431 2c31 3538
                    2c32 3339 2c35 2c34 330d 0a
   22:23:58.646805 D > C: . ack 184 win 8192
                                        4500 0028 beeb 0000
                    3e06 ce06 83f3 010a cbf1 9eef 0015 0525
                    578c 70a4 a3a2 fab9 5010 2000 342f 0000
   22:31:36.532244 C > D: FP 186:213(27) ack 565 win 4096
                                        4500 0043 9435 0000
                    3306 03a2 cbf1 9eef 83f3 010a 0525 0015
                    a3a2 fabb 578c 70a4 5019 1000 9a51 0000
   data starts here>504f 5254 2032 3033 2c32 3431 2c31 3538
                    2c32 3339 2c35 2c34 330d 0a

Top      ToC       Page 13 
      In this trace, sequence numbers are relative.  C sends 185:212,
      but D only sends an ACK for 184 (so sequence number 184 is
      missing).  C then sends 186:213.  The packet payload is identical
      to the previous payload, but the base sequence number is one
      higher, resulting in an inconsistent retransmission.

      Neither trace exhibits checksum errors.

   Trace file demonstrating correct behavior
      (Omitted, as presumably correct behavior is obvious.)

   References
      None known.

   How to detect
      This problem unfortunately can be very difficult to detect, since
      available experience indicates it is quite rare that it is
      manifested.  No "trigger" has been identified that can be used to
      reproduce the problem.

   How to fix
      In the absence of a known "trigger", we cannot always assess how
      to fix the problem.

      In one implementation (not the one illustrated above), the problem
      manifested itself when (1) the sender received a zero window and
      stalled; (2) eventually an ACK arrived that offered a window
      larger than that in effect at the time of the stall; (3) the
      sender transmitted out of the buffer of data it held at the time
      of the stall, but (4) failed to limit this transfer to the buffer
      length, instead using the newly advertised (and larger) offered
      window.  Consequently, in addition to the valid buffer contents,
      it sent whatever garbage values followed the end of the buffer.
      If it then retransmitted the corresponding sequence numbers, at
      that point it sent the correct data, resulting in an inconsistent
      retransmission.  Note that this instance of the problem reflects a
      more general problem, that of initially transmitting incorrect
      data.

2.5.

   Name of Problem
      Failure to retain above-sequence data

   Classification
      Congestion control, performance

Top      ToC       Page 14 
   Description
      When a TCP receives an "above sequence" segment, meaning one with
      a sequence number exceeding RCV.NXT but below RCV.NXT+RCV.WND, it
      SHOULD queue the segment for later delivery (RFC 1122, 4.2.2.20).
      (See RFC 793 for the definition of RCV.NXT and RCV.WND.)  A TCP
      that fails to do so is said to exhibit "Failure to retain above-
      sequence data".

      It may sometimes be appropriate for a TCP to discard above-
      sequence data to reclaim memory.  If they do so only rarely, then
      we would not consider them to exhibit this problem.  Instead, the
      particular concern is with TCPs that always discard above-sequence
      data.

   Significance
      In environments prone to packet loss, detrimental to the
      performance of both other connections and the connection itself.

   Implications
      In times of congestion, a failure to retain above-sequence data
      will lead to numerous otherwise-unnecessary retransmissions,
      aggravating the congestion and potentially reducing performance by
      a large factor.

   Relevant RFCs
      RFC 1122 revises RFC 793 by upgrading the latter's MAY to a SHOULD
      on this issue.

   Trace file demonstrating it
      Made using tcpdump recording at the receiving TCP.  No losses
      reported by the packet filter.

      B is the TCP sender, A the receiver.  A exhibits failure to retain
      above sequence-data:

   10:38:10.164860 B > A: . 221078:221614(536) ack 1 win 33232 [tos 0x8]
   10:38:10.170809 B > A: . 221614:222150(536) ack 1 win 33232 [tos 0x8]
   10:38:10.177183 B > A: . 222150:222686(536) ack 1 win 33232 [tos 0x8]
   10:38:10.225039 A > B: . ack 222686 win 25800

      Here B has sent up to (relative) sequence 222686 in-sequence, and
      A accordingly acknowledges.

   10:38:10.268131 B > A: . 223222:223758(536) ack 1 win 33232 [tos 0x8]
   10:38:10.337995 B > A: . 223758:224294(536) ack 1 win 33232 [tos 0x8]
   10:38:10.344065 B > A: . 224294:224830(536) ack 1 win 33232 [tos 0x8]
   10:38:10.350169 B > A: . 224830:225366(536) ack 1 win 33232 [tos 0x8]
   10:38:10.356362 B > A: . 225366:225902(536) ack 1 win 33232 [tos 0x8]

Top      ToC       Page 15 
   10:38:10.362445 B > A: . 225902:226438(536) ack 1 win 33232 [tos 0x8]
   10:38:10.368579 B > A: . 226438:226974(536) ack 1 win 33232 [tos 0x8]
   10:38:10.374732 B > A: . 226974:227510(536) ack 1 win 33232 [tos 0x8]
   10:38:10.380825 B > A: . 227510:228046(536) ack 1 win 33232 [tos 0x8]
   10:38:10.387027 B > A: . 228046:228582(536) ack 1 win 33232 [tos 0x8]
   10:38:10.393053 B > A: . 228582:229118(536) ack 1 win 33232 [tos 0x8]
   10:38:10.399193 B > A: . 229118:229654(536) ack 1 win 33232 [tos 0x8]
   10:38:10.405356 B > A: . 229654:230190(536) ack 1 win 33232 [tos 0x8]

      A now receives 13 additional packets from B.  These are above-
      sequence because 222686:223222 was dropped.  The packets do
      however fit within the offered window of 25800.  A does not
      generate any duplicate ACKs for them.

      The trace contributor (V. Paxson) verified that these 13 packets
      had valid IP and TCP checksums.

   10:38:11.917728 B > A: . 222686:223222(536) ack 1 win 33232 [tos 0x8]
   10:38:11.930925 A > B: . ack 223222 win 32232

      B times out for 222686:223222 and retransmits it.  Upon receiving
      it, A only acknowledges 223222.  Had it retained the valid above-
      sequence packets, it would instead have ack'd 230190.

   10:38:12.048438 B > A: . 223222:223758(536) ack 1 win 33232 [tos 0x8]
   10:38:12.054397 B > A: . 223758:224294(536) ack 1 win 33232 [tos 0x8]
   10:38:12.068029 A > B: . ack 224294 win 31696

      B retransmits two more packets, and A only acknowledges them.
      This pattern continues as B retransmits the entire set of
      previously-received packets.

      A second trace confirmed that the problem is repeatable.

   Trace file demonstrating correct behavior
      Made using tcpdump recording at the receiving TCP (C).  No losses
      reported by the packet filter.

   09:11:25.790417 D > C: . 33793:34305(512) ack 1 win 61440
   09:11:25.791393 D > C: . 34305:34817(512) ack 1 win 61440
   09:11:25.792369 D > C: . 34817:35329(512) ack 1 win 61440
   09:11:25.792369 D > C: . 35329:35841(512) ack 1 win 61440
   09:11:25.793345 D > C: . 36353:36865(512) ack 1 win 61440
   09:11:25.794321 C > D: . ack 35841 win 59904

      A sequence hole occurs because 35841:36353 has been dropped.

Top      ToC       Page 16 
   09:11:25.794321 D > C: . 36865:37377(512) ack 1 win 61440
   09:11:25.794321 C > D: . ack 35841 win 59904
   09:11:25.795297 D > C: . 37377:37889(512) ack 1 win 61440
   09:11:25.795297 C > D: . ack 35841 win 59904
   09:11:25.796273 C > D: . ack 35841 win 61440
   09:11:25.798225 D > C: . 37889:38401(512) ack 1 win 61440
   09:11:25.799201 C > D: . ack 35841 win 61440
   09:11:25.807009 D > C: . 38401:38913(512) ack 1 win 61440
   09:11:25.807009 C > D: . ack 35841 win 61440
   (many additional lines omitted)
   09:11:25.884113 D > C: . 52737:53249(512) ack 1 win 61440
   09:11:25.884113 C > D: . ack 35841 win 61440

      Each additional, above-sequence packet C receives from D elicits a
      duplicate ACK for 35841.

      09:11:25.887041 D > C: . 35841:36353(512) ack 1 win 61440
      09:11:25.887041 C > D: . ack 53249 win 44032

      D retransmits 35841:36353 and C acknowledges receipt of data all
      the way up to 53249.

   References
      This problem is documented in [Paxson97].

   How to detect
      Packet loss is common enough in the Internet that generally it is
      not difficult to find an Internet path that will result in some
      above-sequence packets arriving.  A TCP that exhibits "Failure to
      retain ..." may not generate duplicate ACKs for these packets.
      However, some TCPs that do retain above-sequence data also do not
      generate duplicate ACKs, so failure to do so does not definitively
      identify the problem.  Instead, the key observation is whether
      upon retransmission of the dropped packet, data that was
      previously above-sequence is acknowledged.

      Two considerations in detecting this problem using a packet trace
      are that it is easiest to do so with a trace made at the TCP
      receiver, in order to unambiguously determine which packets
      arrived successfully, and that such packets may still be correctly
      discarded if they arrive with checksum errors.  The latter can be
      tested by capturing the entire packet contents and performing the
      IP and TCP checksum algorithms to verify their integrity; or by
      confirming that the packets arrive with the same checksum and
      contents as that with which they were sent, with a presumption
      that the sending TCP correctly calculates checksums for the
      packets it transmits.

Top      ToC       Page 17 
      It is considerably easier to verify that an implementation does
      NOT exhibit this problem.  This can be done by recording a trace
      at the data sender, and observing that sometimes after a
      retransmission the receiver acknowledges a higher sequence number
      than just that which was retransmitted.

   How to fix
      If the root problem is that the implementation lacks buffer, then
      then unfortunately this requires significant work to fix.
      However, doing so is important, for reasons outlined above.



(page 17 continued on part 2)

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