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Versions: 00 01 02 03 04 05 RFC 2525

Network Working Group                                  V. Paxson, Editor
Internet Draft                                                 M. Allman
                                                               S. Dawson
                                                               W. Fenner
                                                               J. Griner
                                                              I. Heavens
                                                                K. Lahey
                                                                J. Semke
                                                                 B. Volz
Expiration Date: May 1999                                  November 1998


                   Known TCP Implementation Problems
                    <draft-ietf-tcpimpl-prob-05.txt>


1. Status of this Memo

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

   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''.

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

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














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Table of Contents

   1.  STATUS OF THIS MEMO.............................................1
   2.  INTRODUCTION....................................................2
   3.  KNOWN IMPLEMENTATION PROBLEMS...................................4
     3.1  No initial slow start........................................4
     3.2  No slow start after retransmission timeout...................6
     3.3  Uninitialized CWND...........................................9
     3.4  Inconsistent retransmission.................................11
     3.5  Failure to retain above-sequence data.......................14
     3.6  Extra additive constant in congestion avoidance.............18
     3.7  Initial RTO too low.........................................24
     3.8  Failure of window deflation after loss recovery.............27
     3.9  Excessively short keepalive connection timeout..............30
     3.10 Failure to back off retransmission timeout..................32
     3.11 Insufficient interval between keepalives....................35
     3.12 Window probe deadlock.......................................38
     3.13 Stretch ACK violation.......................................42
     3.14 Retransmission sends multiple packets.......................45
     3.15 Failure to send FIN notification promptly...................48
     3.16 Failure to send a RST after Half Duplex Close...............50
     3.17 Failure to RST on close with data pending...................53
     3.18 Options missing from TCP MSS calculation....................57
   4.  SECURITY CONSIDERATIONS........................................59
   5.  ACKNOWLEDGEMENTS...............................................59
   6.  REFERENCES.....................................................60
   7.  AUTHORS' ADDRESSES.............................................62


2. 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



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        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.

   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.






Paxson, Editor                                                  [Page 3]


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3. Known implementation problems


3.1.

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 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



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     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]

     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



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     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.

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".


3.2.

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



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     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.

     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]




Paxson, Editor                                                  [Page 7]


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     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
     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.



Paxson, Editor                                                  [Page 8]


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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.

     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.


3.3.

Name of Problem
     Uninitialized CWND


Classification
     Congestion control


Description
     As described above for "No initial slow start", when a TCP



Paxson, Editor                                                  [Page 9]


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     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.

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]




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     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.


3.4.

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.



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     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

     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.



Paxson, Editor                                                 [Page 12]


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     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

     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.




Paxson, Editor                                                 [Page 13]


ID                 Known TCP Implementation Problems       November 1998


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.


3.5.

Name of Problem
     Failure to retain above-sequence data


Classification
     Congestion control, performance


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



Paxson, Editor                                                 [Page 14]


ID                 Known TCP Implementation Problems       November 1998


     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]
     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



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ID                 Known TCP Implementation Problems       November 1998


     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.

     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



Paxson, Editor                                                 [Page 16]


ID                 Known TCP Implementation Problems       November 1998


     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.

     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.










Paxson, Editor                                                 [Page 17]


ID                 Known TCP Implementation Problems       November 1998


3.6.

Name of Problem
     Extra additive constant in congestion avoidance


Classification
     Congestion control / performance


Description
     RFC 1122 section 4.2.2.15 states that TCP MUST implement Jacobson's
     "congestion avoidance" algorithm [Jacobson88], which calls for
     increasing the congestion window, cwnd, by:

         MSS * MSS / cwnd

     for each ACK received for new data [RFC2001].  This has the effect
     of increasing cwnd by approximately one segment in each round trip
     time.

     Some TCP implementations add an additional fraction of a segment
     (typically MSS/8) to cwnd for each ACK received for new data
     [Stevens94, Wright95]:

         (MSS * MSS / cwnd) + MSS/8

     These implementations exhibit "Extra additive constant in
     congestion avoidance".


Significance
     May be detrimental to performance even in completely uncongested
     environments (see Implications).

     In congested environments, may also be detrimental to the
     performance of other connections.


Implications
     The extra additive term allows a TCP to more aggressively open its
     congestion window (quadratic rather than linear increase).  For
     congested networks, this can increase the loss rate experienced by
     all connections sharing a bottleneck with the aggressive TCP.

     However, even for completely uncongested networks, the extra
     additive term can lead to diminished performance, as follows.  In
     congestion avoidance, a TCP sender probes the network path to



Paxson, Editor                                                 [Page 18]


ID                 Known TCP Implementation Problems       November 1998


     determine its available capacity, which often equates to the number
     of buffers available at a bottleneck link.  With linear congestion
     avoidance, the TCP only probes for sufficient capacity (buffer) to
     hold one extra packet per RTT.

     Thus, when it exceeds the available capacity, generally only one
     packet will be lost (since on the previous RTT it already found
     that the path could sustain a window with one less packet in
     flight).  If the congestion window is sufficiently large, then the
     TCP will recover from this single loss using fast retransmission
     and avoid an expensive (in terms of performance) retransmission
     timeout.

     However, when the additional additive term is used, then cwnd can
     increase by more than one packet per RTT, in which case the TCP
     probes more aggressively.  If in the previous RTT it had reached
     the available capacity of the path, then the excess due to the
     extra increase will again be lost, but now this will result in
     multiple losses from the flight instead of a single loss.  TCPs
     that do not utilize SACK [RFC2018] generally will not recover from
     multiple losses without incurring a retransmission timeout
     [Fall96,Hoe96], significantly diminishing performance.


Relevant RFCs
     RFC 1122 requires use of the "congestion avoidance" algorithm.  RFC
     2001 outlines the fast retransmit/fast recovery algorithms.  RFC
     2018 discusses the SACK option.


Trace file demonstrating it

     Recorded using tcpdump running on the same FDDI LAN as host A.
     Host A is the sender and host B is the receiver.  The connection
     establishment specified an MSS of 4,312 bytes and a window scale
     factor of 4.  We omit the establishment and the first 2.5 MB of
     data transfer, as the problem is best demonstrated when the window
     has grown to a large value.  At the beginning of the trace excerpt,
     the congestion window is 31 packets.  The connection is never
     receiver-window limited, so we omit window advertisements from the
     trace for clarity.

     11:42:07.697951 B > A: . ack 2383006
     11:42:07.699388 A > B: . 2508054:2512366(4312)
     11:42:07.699962 A > B: . 2512366:2516678(4312)
     11:42:07.700012 B > A: . ack 2391630
     11:42:07.701081 A > B: . 2516678:2520990(4312)
     11:42:07.701656 A > B: . 2520990:2525302(4312)



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ID                 Known TCP Implementation Problems       November 1998


     11:42:07.701739 B > A: . ack 2400254
     11:42:07.702685 A > B: . 2525302:2529614(4312)
     11:42:07.703257 A > B: . 2529614:2533926(4312)
     11:42:07.703295 B > A: . ack 2408878
     11:42:07.704414 A > B: . 2533926:2538238(4312)
     11:42:07.704989 A > B: . 2538238:2542550(4312)
     11:42:07.705040 B > A: . ack 2417502
     11:42:07.705935 A > B: . 2542550:2546862(4312)
     11:42:07.706506 A > B: . 2546862:2551174(4312)
     11:42:07.706544 B > A: . ack 2426126
     11:42:07.707480 A > B: . 2551174:2555486(4312)
     11:42:07.708051 A > B: . 2555486:2559798(4312)
     11:42:07.708088 B > A: . ack 2434750
     11:42:07.709030 A > B: . 2559798:2564110(4312)
     11:42:07.709604 A > B: . 2564110:2568422(4312)
     11:42:07.710175 A > B: . 2568422:2572734(4312) *

     11:42:07.710215 B > A: . ack 2443374
     11:42:07.710799 A > B: . 2572734:2577046(4312)
     11:42:07.711368 A > B: . 2577046:2581358(4312)
     11:42:07.711405 B > A: . ack 2451998
     11:42:07.712323 A > B: . 2581358:2585670(4312)
     11:42:07.712898 A > B: . 2585670:2589982(4312)
     11:42:07.712938 B > A: . ack 2460622
     11:42:07.713926 A > B: . 2589982:2594294(4312)
     11:42:07.714501 A > B: . 2594294:2598606(4312)
     11:42:07.714547 B > A: . ack 2469246
     11:42:07.715747 A > B: . 2598606:2602918(4312)
     11:42:07.716287 A > B: . 2602918:2607230(4312)
     11:42:07.716328 B > A: . ack 2477870
     11:42:07.717146 A > B: . 2607230:2611542(4312)
     11:42:07.717717 A > B: . 2611542:2615854(4312)
     11:42:07.717762 B > A: . ack 2486494
     11:42:07.718754 A > B: . 2615854:2620166(4312)
     11:42:07.719331 A > B: . 2620166:2624478(4312)
     11:42:07.719906 A > B: . 2624478:2628790(4312) **

     11:42:07.719958 B > A: . ack 2495118
     11:42:07.720500 A > B: . 2628790:2633102(4312)
     11:42:07.721080 A > B: . 2633102:2637414(4312)
     11:42:07.721739 B > A: . ack 2503742
     11:42:07.722348 A > B: . 2637414:2641726(4312)
     11:42:07.722918 A > B: . 2641726:2646038(4312)
     11:42:07.769248 B > A: . ack 2512366

     The receiver's acknowledgment policy is one ACK per two packets
     received.  Thus, for each ACK arriving at host A, two new packets
     are sent, except when cwnd increases due to congestion avoidance,



Paxson, Editor                                                 [Page 20]


ID                 Known TCP Implementation Problems       November 1998


     in which case three new packets are sent.

     With an ack-every-two-packets policy, cwnd should only increase one
     MSS per 2 RTT.  However, at the point marked "*" the window
     increases after 7 ACKs have arrived, and then again at "**" after 6
     more ACKs.

     While we do not have space to show the effect, this trace suffered
     from repeated timeout retransmissions due to multiple packet losses
     during a single RTT.


Trace file demonstrating correct behavior

     Made using the same host and tracing setup as above, except now A's
     TCP has been modified to remove the MSS/8 additive constant.
     Tcpdump reported 77 packet drops; the excerpt below is fully self-
     consistent so it is unlikely that any of these occurred during the
     excerpt.

     We again begin when cwnd is 31 packets (this occurs significantly
     later in the trace, because the congestion avoidance is now less
     aggressive with opening the window).

     14:22:21.236757 B > A: . ack 5194679
     14:22:21.238192 A > B: . 5319727:5324039(4312)
     14:22:21.238770 A > B: . 5324039:5328351(4312)
     14:22:21.238821 B > A: . ack 5203303
     14:22:21.240158 A > B: . 5328351:5332663(4312)
     14:22:21.240738 A > B: . 5332663:5336975(4312)
     14:22:21.270422 B > A: . ack 5211927
     14:22:21.271883 A > B: . 5336975:5341287(4312)
     14:22:21.272458 A > B: . 5341287:5345599(4312)
     14:22:21.279099 B > A: . ack 5220551
     14:22:21.280539 A > B: . 5345599:5349911(4312)
     14:22:21.281118 A > B: . 5349911:5354223(4312)
     14:22:21.281183 B > A: . ack 5229175
     14:22:21.282348 A > B: . 5354223:5358535(4312)
     14:22:21.283029 A > B: . 5358535:5362847(4312)
     14:22:21.283089 B > A: . ack 5237799
     14:22:21.284213 A > B: . 5362847:5367159(4312)
     14:22:21.284779 A > B: . 5367159:5371471(4312)
     14:22:21.285976 B > A: . ack 5246423
     14:22:21.287465 A > B: . 5371471:5375783(4312)
     14:22:21.288036 A > B: . 5375783:5380095(4312)
     14:22:21.288073 B > A: . ack 5255047
     14:22:21.289155 A > B: . 5380095:5384407(4312)
     14:22:21.289725 A > B: . 5384407:5388719(4312)



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ID                 Known TCP Implementation Problems       November 1998


     14:22:21.289762 B > A: . ack 5263671
     14:22:21.291090 A > B: . 5388719:5393031(4312)
     14:22:21.291662 A > B: . 5393031:5397343(4312)
     14:22:21.291701 B > A: . ack 5272295
     14:22:21.292870 A > B: . 5397343:5401655(4312)
     14:22:21.293441 A > B: . 5401655:5405967(4312)
     14:22:21.293481 B > A: . ack 5280919
     14:22:21.294476 A > B: . 5405967:5410279(4312)
     14:22:21.295053 A > B: . 5410279:5414591(4312)
     14:22:21.295106 B > A: . ack 5289543
     14:22:21.296306 A > B: . 5414591:5418903(4312)
     14:22:21.296878 A > B: . 5418903:5423215(4312)
     14:22:21.296917 B > A: . ack 5298167
     14:22:21.297716 A > B: . 5423215:5427527(4312)
     14:22:21.298285 A > B: . 5427527:5431839(4312)
     14:22:21.298324 B > A: . ack 5306791
     14:22:21.299413 A > B: . 5431839:5436151(4312)
     14:22:21.299986 A > B: . 5436151:5440463(4312)
     14:22:21.303696 B > A: . ack 5315415
     14:22:21.305177 A > B: . 5440463:5444775(4312)
     14:22:21.305755 A > B: . 5444775:5449087(4312)
     14:22:21.308032 B > A: . ack 5324039
     14:22:21.309525 A > B: . 5449087:5453399(4312)
     14:22:21.310101 A > B: . 5453399:5457711(4312)
     14:22:21.310144 B > A: . ack 5332663           ***

     14:22:21.311615 A > B: . 5457711:5462023(4312)
     14:22:21.312198 A > B: . 5462023:5466335(4312)
     14:22:21.341876 B > A: . ack 5341287
     14:22:21.343451 A > B: . 5466335:5470647(4312)
     14:22:21.343985 A > B: . 5470647:5474959(4312)
     14:22:21.350304 B > A: . ack 5349911
     14:22:21.351852 A > B: . 5474959:5479271(4312)
     14:22:21.352430 A > B: . 5479271:5483583(4312)
     14:22:21.352484 B > A: . ack 5358535
     14:22:21.353574 A > B: . 5483583:5487895(4312)
     14:22:21.354149 A > B: . 5487895:5492207(4312)
     14:22:21.354205 B > A: . ack 5367159
     14:22:21.355467 A > B: . 5492207:5496519(4312)
     14:22:21.356039 A > B: . 5496519:5500831(4312)
     14:22:21.357361 B > A: . ack 5375783
     14:22:21.358855 A > B: . 5500831:5505143(4312)
     14:22:21.359424 A > B: . 5505143:5509455(4312)
     14:22:21.359465 B > A: . ack 5384407
     14:22:21.360605 A > B: . 5509455:5513767(4312)
     14:22:21.361181 A > B: . 5513767:5518079(4312)
     14:22:21.361225 B > A: . ack 5393031
     14:22:21.362485 A > B: . 5518079:5522391(4312)



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ID                 Known TCP Implementation Problems       November 1998


     14:22:21.363057 A > B: . 5522391:5526703(4312)
     14:22:21.363096 B > A: . ack 5401655
     14:22:21.364236 A > B: . 5526703:5531015(4312)
     14:22:21.364810 A > B: . 5531015:5535327(4312)
     14:22:21.364867 B > A: . ack 5410279
     14:22:21.365819 A > B: . 5535327:5539639(4312)
     14:22:21.366386 A > B: . 5539639:5543951(4312)
     14:22:21.366427 B > A: . ack 5418903
     14:22:21.367586 A > B: . 5543951:5548263(4312)
     14:22:21.368158 A > B: . 5548263:5552575(4312)
     14:22:21.368199 B > A: . ack 5427527
     14:22:21.369189 A > B: . 5552575:5556887(4312)
     14:22:21.369758 A > B: . 5556887:5561199(4312)
     14:22:21.369803 B > A: . ack 5436151
     14:22:21.370814 A > B: . 5561199:5565511(4312)
     14:22:21.371398 A > B: . 5565511:5569823(4312)
     14:22:21.375159 B > A: . ack 5444775
     14:22:21.376658 A > B: . 5569823:5574135(4312)
     14:22:21.377235 A > B: . 5574135:5578447(4312)
     14:22:21.379303 B > A: . ack 5453399
     14:22:21.380802 A > B: . 5578447:5582759(4312)
     14:22:21.381377 A > B: . 5582759:5587071(4312)
     14:22:21.381947 A > B: . 5587071:5591383(4312) ****

     "***" marks the end of the first round trip.  Note that cwnd did
     not increase (as evidenced by each ACK eliciting two new data
     packets).  Only at "****", which comes near the end of the second
     round trip, does cwnd increase by one packet.

     This trace did not suffer any timeout retransmissions.  It
     transferred the same amount of data as the first trace in about
     half as much time.  This difference is repeatable between hosts A
     and B.


References
     [Stevens94] and [Wright95] discuss this problem.  The problem of
     Reno TCP failing to recover from multiple losses except via a
     retransmission timeout is discussed in [Fall96,Hoe96].


How to detect
     If source code is available, that is generally the easiest way to
     detect this problem.  Search for each modification to the cwnd
     variable; (at least) one of these will be for congestion avoidance,
     and inspection of the related code should immediately identify the
     problem if present.




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ID                 Known TCP Implementation Problems       November 1998


     The problem can also be detected by closely examining packet traces
     taken near the sender.  During congestion avoidance, cwnd will
     increase by an additional segment upon the receipt of (typically)
     eight acknowledgements without a loss.  This increase is in
     addition to the one segment increase per round trip time (or two
     round trip times if the receiver is using delayed ACKs).

     Furthermore, graphs of the sequence number vs. time, taken from
     packet traces, are normally linear during congestion avoidance.
     When viewing packet traces of transfers from senders exhibiting
     this problem, the graphs appear quadratic instead of linear.

     Finally, the traces will show that, with sufficiently large
     windows, nearly every loss event results in a timeout.


How to fix
     This problem may be corrected by removing the "+ MSS/8" term from
     the congestion avoidance code that increases cwnd each time an ACK
     of new data is received.


3.7.

Name of Problem
     Initial RTO too low


Classification
     Performance


Description
     When a TCP first begins transmitting data, it lacks the RTT
     measurements necessary to have computed an adaptive retransmission
     timeout (RTO).  RFC 1122, 4.2.3.1, states that a TCP SHOULD
     initialize RTO to 3 seconds.  A TCP that uses a lower value
     exhibits "Initial RTO too low".


Significance
     In environments with large RTTs (where "large" means any value
     larger than the initial RTO), TCPs will experience very poor
     performance.


Implications
     Whenever RTO < RTT, very poor performance can result as packets are



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ID                 Known TCP Implementation Problems       November 1998


     unnecessarily retransmitted (because RTO will expire before an ACK
     for the packet can arrive) and the connection enters slow start and
     congestion avoidance.  Generally, the algorithms for computing RTO
     avoid this problem by adding a positive term to the estimated RTT.
     However, when a connection first begins it must use some estimate
     for RTO, and if it picks a value less than RTT, the above problems
     will arise.

     Furthermore, when the initial RTO < RTT, it can take a long time
     for the TCP to correct the problem by adapting the RTT estimate,
     because the use of Karn's algorithm (mandated by RFC 1122, 4.2.3.1)
     will discard many of the candidate RTT measurements made after the
     first timeout, since they will be measurements of retransmitted
     segments.


Relevant RFCs
     RFC 1122 states that TCPs SHOULD initialize RTO to 3 seconds and
     MUST implement Karn's algorithm.


Trace file demonstrating it
     The following trace file was taken using tcpdump at host A, the
     data sender.  The advertised window and SYN options have been
     omitted for clarity.

     07:52:39.870301 A > B: S 2786333696:2786333696(0)
     07:52:40.548170 B > A: S 130240000:130240000(0) ack 2786333697
     07:52:40.561287 A > B: P 1:513(512) ack 1
     07:52:40.753466 A > B: . 1:513(512) ack 1
     07:52:41.133687 A > B: . 1:513(512) ack 1
     07:52:41.458529 B > A: . ack 513
     07:52:41.458686 A > B: . 513:1025(512) ack 1
     07:52:41.458797 A > B: P 1025:1537(512) ack 1
     07:52:41.541633 B > A: . ack 513
     07:52:41.703732 A > B: . 513:1025(512) ack 1
     07:52:42.044875 B > A: . ack 513
     07:52:42.173728 A > B: . 513:1025(512) ack 1
     07:52:42.330861 B > A: . ack 1537
     07:52:42.331129 A > B: . 1537:2049(512) ack 1
     07:52:42.331262 A > B: P 2049:2561(512) ack 1
     07:52:42.623673 A > B: . 1537:2049(512) ack 1
     07:52:42.683203 B > A: . ack 1537
     07:52:43.044029 B > A: . ack 1537
     07:52:43.193812 A > B: . 1537:2049(512) ack 1

     Note from the SYN/SYN-ACK exchange, the RTT is over 600 msec.
     However, from the elapsed time between the third and fourth lines



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     (the first packet being sent and then retransmitted), it is
     apparent the RTO was initialized to under 200 msec.  The next line
     shows that this value has doubled to 400 msec (correct exponential
     backoff of RTO), but that still does not suffice to avoid an
     unnecessary retransmission.

     Finally, an ACK from B arrives for the first segment.  Later two
     more duplicate ACKs for 513 arrive, indicating that both the
     original and the two retransmissions arrived at B.  (Indeed, a
     concurrent trace at B showed that no packets were lost during the
     entire connection).  This ACK opens the congestion window to two
     packets, which are sent back-to-back, but at 07:52:41.703732 RTO
     again expires after a little over 200 msec, leading to an
     unnecessary retransmission, and the pattern repeats.  By the end of
     the trace excerpt above, 1536 bytes have been successfully
     transmitted from A to B, over an interval of more than 2 seconds,
     reflecting terrible performance.


Trace file demonstrating correct behavior
     The following trace file was taken using tcpdump at host C, the
     data sender.  The advertised window and SYN options have been
     omitted for clarity.

     17:30:32.090299 C > D: S 2031744000:2031744000(0)
     17:30:32.900325 D > C: S 262737964:262737964(0) ack 2031744001
     17:30:32.900326 C > D: . ack 1
     17:30:32.910326 C > D: . 1:513(512) ack 1
     17:30:34.150355 D > C: . ack 513
     17:30:34.150356 C > D: . 513:1025(512) ack 1
     17:30:34.150357 C > D: . 1025:1537(512) ack 1
     17:30:35.170384 D > C: . ack 1025
     17:30:35.170385 C > D: . 1537:2049(512) ack 1
     17:30:35.170386 C > D: . 2049:2561(512) ack 1
     17:30:35.320385 D > C: . ack 1537
     17:30:35.320386 C > D: . 2561:3073(512) ack 1
     17:30:35.320387 C > D: . 3073:3585(512) ack 1
     17:30:35.730384 D > C: . ack 2049

     The initial SYN/SYN-ACK exchange shows that RTT is more than 800
     msec, and for some subsequent packets it rises above 1 second, but
     C's retransmit timer does not ever expire.


References
     This problem is documented in [Paxson97].





Paxson, Editor                                                 [Page 26]


ID                 Known TCP Implementation Problems       November 1998


How to detect
     This problem is readily detected by inspecting a packet trace of
     the startup of a TCP connection made over a long-delay path.  It
     can be diagnosed from either a sender-side or receiver-side trace.
     Long-delay paths can often be found by locating remote sites on
     other continents.


How to fix
     As this problem arises from a faulty initialization, one hopes
     fixing it requires a one-line change to the TCP source code.


3.8.

Name of Problem
     Failure of window deflation after loss recovery


Classification
     Congestion control / performance


Description
     The fast recovery algorithm allows TCP senders to continue to
     transmit new segments during loss recovery.  First, fast
     retransmission is initiated after a TCP sender receives three
     duplicate ACKs.  At this point, a retransmission is sent and cwnd
     is halved.  The fast recovery algorithm then allows additional
     segments to be sent when sufficient additional duplicate ACKs
     arrive.  Some implementations of fast recovery compute when to send
     additional segments by artificially incrementing cwnd, first by
     three segments to account for the three duplicate ACKs that
     triggered fast retransmission, and subsequently by 1 MSS for each
     new duplicate ACK that arrives.  When cwnd allows, the sender
     transmits new data segments.

     When an ACK arrives that covers new data, cwnd is to be reduced by
     the amount by which it was artificially increased.  However, some
     TCP implementations fail to "deflate" the window, causing an
     inappropriate amount of data to be sent into the network after
     recovery.  One cause of this problem is the "header prediction"
     code, which is used to handle incoming segments that require little
     work.  In some implementations of TCP, the header prediction code
     does not check to make sure cwnd has not been artificially
     inflated, and therefore does not reduce the artificially increased
     cwnd when appropriate.




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ID                 Known TCP Implementation Problems       November 1998


Significance
     TCP senders that exhibit this problem will transmit a burst of data
     immediately after recovery, which can degrade performance, as well
     as network stability.  Effectively, the sender does not reduce the
     size of cwnd as much as it should (to half its value when loss was
     detected), if at all.  This can harm the performance of the TCP
     connection itself, as well as competing TCP flows.


Implications
     A TCP sender exhibiting this problem does not reduce cwnd
     appropriately in times of congestion, and therefore may contribute
     to congestive collapse.


Relevant RFCs
     RFC 2001 outlines the fast retransmit/fast recovery algorithms.
     [Brakmo95] outlines this implementation problem and offers a fix.


Trace file demonstrating it
     The following trace file was taken using tcpdump at host A, the
     data sender.  The advertised window (which never changed) has been
     omitted for clarity, except for the first packet sent by each host.

     08:22:56.825635 A.7505 > B.7505: . 29697:30209(512) ack 1 win 4608
     08:22:57.038794 B.7505 > A.7505: . ack 27649 win 4096
     08:22:57.039279 A.7505 > B.7505: . 30209:30721(512) ack 1
     08:22:57.321876 B.7505 > A.7505: . ack 28161
     08:22:57.322356 A.7505 > B.7505: . 30721:31233(512) ack 1
     08:22:57.347128 B.7505 > A.7505: . ack 28673
     08:22:57.347572 A.7505 > B.7505: . 31233:31745(512) ack 1
     08:22:57.347782 A.7505 > B.7505: . 31745:32257(512) ack 1
     08:22:57.936393 B.7505 > A.7505: . ack 29185
     08:22:57.936864 A.7505 > B.7505: . 32257:32769(512) ack 1
     08:22:57.950802 B.7505 > A.7505: . ack 29697 win 4096
     08:22:57.951246 A.7505 > B.7505: . 32769:33281(512) ack 1
     08:22:58.169422 B.7505 > A.7505: . ack 29697
     08:22:58.638222 B.7505 > A.7505: . ack 29697
     08:22:58.643312 B.7505 > A.7505: . ack 29697
     08:22:58.643669 A.7505 > B.7505: . 29697:30209(512) ack 1
     08:22:58.936436 B.7505 > A.7505: . ack 29697
     08:22:59.002614 B.7505 > A.7505: . ack 29697
     08:22:59.003026 A.7505 > B.7505: . 33281:33793(512) ack 1
     08:22:59.682902 B.7505 > A.7505: . ack 33281
     08:22:59.683391 A.7505 > B.7505: P 33793:34305(512) ack 1
     08:22:59.683748 A.7505 > B.7505: P 34305:34817(512) ack 1 ***
     08:22:59.684043 A.7505 > B.7505: P 34817:35329(512) ack 1



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ID                 Known TCP Implementation Problems       November 1998


     08:22:59.684266 A.7505 > B.7505: P 35329:35841(512) ack 1
     08:22:59.684567 A.7505 > B.7505: P 35841:36353(512) ack 1
     08:22:59.684810 A.7505 > B.7505: P 36353:36865(512) ack 1
     08:22:59.685094 A.7505 > B.7505: P 36865:37377(512) ack 1

     The first 12 lines of the trace show incoming ACKs clocking out a
     window of data segments.  At this point in the transfer, cwnd is 7
     segments.  The next 4 lines of the trace show 3 duplicate ACKs
     arriving from the receiver, followed by a retransmission from the
     sender.  At this point, cwnd is halved (to 3 segments) and
     artificially incremented by the three duplicate ACKs that have
     arrived, making cwnd 6 segments.  The next two lines show 2 more
     duplicate ACKs arriving, each of which increases cwnd by 1 segment.
     So, after these two duplicate ACKs arrive the cwnd is 8 segments
     and the sender has permission to send 1 new segment (since there
     are 7 segments outstanding).  The next line in the trace shows this
     new segment being transmitted.  The next packet shown in the trace
     is an ACK from host B that covers the first 7 outstanding segments
     (all but the new segment sent during recovery).  This should cause
     cwnd to be reduced to 3 segments and 2 segments to be transmitted
     (since there is already 1 outstanding segment in the network).
     However, as shown by the last 7 lines of the trace, cwnd is not
     reduced, causing a line-rate burst of 7 new segments.


Trace file demonstrating correct behavior
     The trace would appear identical to the one above, only it would
     stop after the line marked "***", because at this point host A
     would correctly reduce cwnd after recovery, allowing only 2
     segments to be transmitted, rather than producing a burst of 7
     segments.


References
     This problem is documented and the performance implications
     analyzed in [Brakmo95].


How to detect
     Failure of window deflation after loss recovery can be found by
     examining sender-side packet traces recorded during periods of
     moderate loss (so cwnd can grow large enough to allow for fast
     recovery when loss occurs).


How to fix
     When this bug is caused by incorrect header prediction, the fix is
     to add a predicate to the header prediction test that checks to see



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ID                 Known TCP Implementation Problems       November 1998


     whether cwnd is inflated; if so, the header prediction test fails
     and the usual ACK processing occurs, which (in this case) takes
     care to deflate the window.  See [Brakmo95] for details.


3.9.

Name of Problem
     Excessively short keepalive connection timeout


Classification
     Reliability


Description
     Keep-alive is a mechanism for checking whether an idle connection
     is still alive.  According to RFC 1122, keepalive should only be
     invoked in server applications that might otherwise hang
     indefinitely and consume resources unnecessarily if a client
     crashes or aborts a connection during a network failure.

     RFC 1122 also specifies that if a keep-alive mechanism is
     implemented it MUST NOT interpret failure to respond to any
     specific probe as a dead connection.  The RFC does not specify a
     particular mechanism for timing out a connection when no response
     is received for keepalive probes.  However, if the mechanism does
     not allow ample time for recovery from network congestion or delay,
     connections may be timed out unnecessarily.


Significance
     In congested networks, can lead to unwarranted termination of
     connections.


Implications
     It is possible for the network connection between two peer machines
     to become congested or to exhibit packet loss at the time that a
     keep-alive probe is sent on a connection.  If the keep-alive
     mechanism does not allow sufficient time before dropping
     connections in the face of unacknowledged probes, connections may
     be dropped even when both peers of a connection are still alive.


Relevant RFCs
     RFC 1122 specifies that the keep-alive mechanism may be provided.
     It does not specify a mechanism for determining dead connections



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ID                 Known TCP Implementation Problems       November 1998


     when keepalive probes are not acknowledged.


Trace file demonstrating it
     Made using the Orchestra tool at the peer of the machine using
     keep-alive.  After connection establishment, incoming keep-alives
     were dropped by Orchestra to simulate a dead connection.

     22:11:12.040000 A > B: 22666019:0 win 8192 datasz 4 SYN
     22:11:12.060000 B > A: 2496001:22666020 win 4096 datasz 4 SYN ACK
     22:11:12.130000 A > B: 22666020:2496002 win 8760 datasz 0 ACK
     (more than two hours elapse)
     00:23:00.680000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
     00:23:01.770000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
     00:23:02.870000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
     00:23.03.970000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
     00:23.05.070000 A > B: 22666019:2496002 win 8760 datasz 1 ACK

     The initial three packets are the SYN exchange for connection
     setup.  About two hours later, the keepalive timer fires because
     the connection has been idle.  Keepalive probes are transmitted a
     total of 5 times, with a 1 second spacing between probes, after
     which the connection is dropped.  This is problematic because a 5
     second network outage at the time of the first probe results in the
     connection being killed.


Trace file demonstrating correct behavior
     Made using the Orchestra tool at the peer of the machine using
     keep-alive.  After connection establishment, incoming keep-alives
     were dropped by Orchestra to simulate a dead connection.

     16:01:52.130000 A > B: 1804412929:0 win 4096 datasz 4 SYN
     16:01:52.360000 B > A: 16512001:1804412930 win 4096 datasz 4 SYN ACK
     16:01:52.410000 A > B: 1804412930:16512002 win 4096 datasz 0 ACK
     (two hours elapse)
     18:01:57.170000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:03:12.220000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:04:27.270000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:05:42.320000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:06:57.370000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:08:12.420000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:09:27.480000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:10:43.290000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:11:57.580000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
     18:13:12.630000 A > B: 1804412929:16512002 win 4096 datasz 0 RST ACK

     In this trace, when the keep-alive timer expires, 9 keepalive



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ID                 Known TCP Implementation Problems       November 1998


     probes are sent at 75 second intervals.  75 seconds after the last
     probe is sent, a final RST segment is sent indicating that the
     connection has been closed.  This implementation waits about 11
     minutes before timing out the connection, while the first
     implementation shown allows only 5 seconds.


References
     This problem is documented in [Dawson97].


How to detect
     For implementations manifesting this problem, it shows up on a
     packet trace after the keepalive timer fires if the peer machine
     receiving the keepalive does not respond.  Usually the keepalive
     timer will fire at least two hours after keepalive is turned on,
     but it may be sooner if the timer value has been configured lower,
     or if the keepalive mechanism violates the specification (see
     Insufficient interval between keepalives problem).  In this
     example, suppressing the response of the peer to keepalive probes
     was accomplished using the Orchestra toolkit, which can be
     configured to drop packets.  It could also have been done by
     creating a connection, turning on keepalive, and disconnecting the
     network connection at the receiver machine.


How to fix
     This problem can be fixed by using a different method for timing
     out keepalives that allows a longer period of time to elapse before
     dropping the connection.  For example, the algorithm for timing out
     on dropped data could be used.  Another possibility is an algorithm
     such as the one shown in the trace above, which sends 9 probes at
     75 second intervals and then waits an additional 75 seconds for a
     response before closing the connection.


3.10.

Name of Problem
     Failure to back off retransmission timeout


Classification
     Congestion control / reliability


Description
     The retransmission timeout is used to determine when a packet has



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ID                 Known TCP Implementation Problems       November 1998


     been dropped in the network.  When this timeout has expired without
     the arrival of an ACK, the segment is retransmitted. Each time a
     segment is retransmitted, the timeout is adjusted according to an
     exponential backoff algorithm, doubling each time.  If a TCP fails
     to receive an ACK after numerous attempts at retransmitting the
     same segment, it terminates the connection.  A TCP that fails to
     double its retransmission timeout upon repeated timeouts is said to
     exhibit "Failure to back off retransmission timeout".


Significance
     Backing off the retransmission timer is a cornerstone of network
     stability in the presence of congestion.  Consequently, this bug
     can have severe adverse affects in congested networks.  It also
     affects TCP reliability in congested networks, as discussed in the
     next section.


Implications
     It is possible for the network connection between two TCP peers to
     become congested or to exhibit packet loss at the time that a
     retransmission is sent on a connection.  If the retransmission
     mechanism does not allow sufficient time before dropping
     connections in the face of unacknowledged segments, connections may
     be dropped even when, by waiting longer, the connection could have
     continued.


Relevant RFCs
     RFC 1122 specifies mandatory exponential backoff of the
     retransmission timeout, and the termination of connections after
     some period of time (at least 100 seconds).


Trace file demonstrating it
     Made using tcpdump on an intermediate host:

     16:51:12.671727 A > B: S 510878852:510878852(0) win 16384
     16:51:12.672479 B > A: S 2392143687:2392143687(0) ack 510878853 win 16384
     16:51:12.672581 A > B: . ack 1 win 16384
     16:51:15.244171 A > B: P 1:3(2) ack 1 win 16384
     16:51:15.244933 B > A: . ack 3 win 17518  (DF)

     <receiving host disconnected>

     16:51:19.381176 A > B: P 3:5(2) ack 1 win 16384
     16:51:20.162016 A > B: P 3:5(2) ack 1 win 16384
     16:51:21.161936 A > B: P 3:5(2) ack 1 win 16384



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     16:51:22.161914 A > B: P 3:5(2) ack 1 win 16384
     16:51:23.161914 A > B: P 3:5(2) ack 1 win 16384
     16:51:24.161879 A > B: P 3:5(2) ack 1 win 16384
     16:51:25.161857 A > B: P 3:5(2) ack 1 win 16384
     16:51:26.161836 A > B: P 3:5(2) ack 1 win 16384
     16:51:27.161814 A > B: P 3:5(2) ack 1 win 16384
     16:51:28.161791 A > B: P 3:5(2) ack 1 win 16384
     16:51:29.161769 A > B: P 3:5(2) ack 1 win 16384
     16:51:30.161750 A > B: P 3:5(2) ack 1 win 16384
     16:51:31.161727 A > B: P 3:5(2) ack 1 win 16384

     16:51:32.161701 A > B: R 5:5(0) ack 1 win 16384

     The initial three packets are the SYN exchange for connection
     setup, then a single data packet, to verify that data can be
     transferred.  Then the connection to the destination host was
     disconnected, and more data sent.  Retransmissions occur every
     second for 12 seconds, and then the connection is terminated with a
     RST.  This is problematic because a 12 second pause in connectivity
     could result in the termination of a connection.


Trace file demonstrating correct behavior
     Again, a tcpdump taken from a third host:

     16:59:05.398301 A > B: S 2503324757:2503324757(0) win 16384
     16:59:05.399673 B > A: S 2492674648:2492674648(0) ack 2503324758 win 16384
     16:59:05.399866 A > B: . ack 1 win 17520
     16:59:06.538107 A > B: P 1:3(2) ack 1 win 17520
     16:59:06.540977 B > A: . ack 3 win 17518  (DF)

     <receiving host disconnected>

     16:59:13.121542 A > B: P 3:5(2) ack 1 win 17520
     16:59:14.010928 A > B: P 3:5(2) ack 1 win 17520
     16:59:16.010979 A > B: P 3:5(2) ack 1 win 17520
     16:59:20.011229 A > B: P 3:5(2) ack 1 win 17520
     16:59:28.011896 A > B: P 3:5(2) ack 1 win 17520
     16:59:44.013200 A > B: P 3:5(2) ack 1 win 17520
     17:00:16.015766 A > B: P 3:5(2) ack 1 win 17520
     17:01:20.021308 A > B: P 3:5(2) ack 1 win 17520
     17:02:24.027752 A > B: P 3:5(2) ack 1 win 17520
     17:03:28.034569 A > B: P 3:5(2) ack 1 win 17520
     17:04:32.041567 A > B: P 3:5(2) ack 1 win 17520
     17:05:36.048264 A > B: P 3:5(2) ack 1 win 17520
     17:06:40.054900 A > B: P 3:5(2) ack 1 win 17520

     17:07:44.061306 A > B: R 5:5(0) ack 1 win 17520



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     In this trace, when the retransmission timer expires, 12
     retransmissions are sent at exponentially-increasing intervals,
     until the interval value reaches 64 seconds, at which time the
     interval stops growing.  64 seconds after the last retransmission,
     a final RST segment is sent indicating that the connection has been
     closed.  This implementation waits about 9 minutes before timing
     out the connection, while the first implementation shown allows
     only 12 seconds.


References
     None known.


How to detect
     A simple transfer can be easily interrupted by disconnecting the
     receiving host from the network.  tcpdump or another appropriate
     tool should show the retransmissions being sent.  Several trials in
     a low-rtt environment may be required to demonstrate the bug.


How to fix
     For one of the implementations studied, this problem seemed to be
     the result of an error introduced with the addition of the Brakmo-
     Peterson RTO algorithm [Brakmo95], which can return a value of zero
     where the older Jacobson algorithm always returns a positive value.
     Brakmo and Peterson specified an additional step of min(rtt + 2,
     RTO) to avoid problems with this.  Unfortunately, in the
     implementation this step was omitted when calculating the
     exponential backoff for the RTO.  This results in an RTO of 0
     seconds being multiplied by the backoff, yielding again zero, and
     then being subjected to a later MAX operation that increases it to
     1 second, regardless of the backoff factor.

     A similar TCP persist failure has the same cause.


3.11.

Name of Problem
     Insufficient interval between keepalives


Classification
     Reliability


Description



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ID                 Known TCP Implementation Problems       November 1998


     Keep-alive is a mechanism for checking whether an idle connection
     is still alive.  According to RFC 1122, keep-alive may be included
     in an implementation.  If it is included, the interval between
     keep-alive packets MUST be configurable, and MUST default to no
     less than two hours.


Significance
     In congested networks, can lead to unwarranted termination of
     connections.


Implications
     According to RFC 1122, keep-alive is not required of
     implementations because it could: (1) cause perfectly good
     connections to break during transient Internet failures; (2)
     consume unnecessary bandwidth ("if no one is using the connection,
     who cares if it is still good?"); and (3) cost money for an
     Internet path that charges for packets.  Regarding this last point,
     we note that in addition the presence of dial-on-demand links in
     the route can greatly magnify the cost penalty of excess
     keepalives, potentially forcing a full-time connection on a link
     that would otherwise only be connected a few minutes a day.

     If keepalive is provided the RFC states that the required inter-
     keepalive distance MUST default to no less than two hours.  If it
     does not, the probability of connections breaking increases, the
     bandwidth used due to keepalives increases, and cost increases over
     paths which charge per packet.


Relevant RFCs
     RFC 1122 specifies that the keep-alive mechanism may be provided.
     It also specifies the two hour minimum for the default interval
     between keepalive probes.


Trace file demonstrating it
     Made using the Orchestra tool at the peer of the machine using
     keep-alive.  Machine A was configured to use default settings for
     the keepalive timer.

     11:36:32.910000 A > B: 3288354305:0      win 28672 datasz 4 SYN
     11:36:32.930000 B > A: 896001:3288354306 win 4096  datasz 4 SYN ACK
     11:36:32.950000 A > B: 3288354306:896002 win 28672 datasz 0 ACK

     11:50:01.190000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
     11:50:01.210000 B > A: 896002:3288354306 win 4096  datasz 0 ACK



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     12:03:29.410000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
     12:03:29.430000 B > A: 896002:3288354306 win 4096  datasz 0 ACK

     12:16:57.630000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
     12:16:57.650000 B > A: 896002:3288354306 win 4096  datasz 0 ACK

     12:30:25.850000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
     12:30:25.870000 B > A: 896002:3288354306 win 4096  datasz 0 ACK

     12:43:54.070000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
     12:43:54.090000 B > A: 896002:3288354306 win 4096  datasz 0 ACK

     The initial three packets are the SYN exchange for connection
     setup.  About 13 minutes later, the keepalive timer fires because
     the connection is idle.  The keepalive is acknowledged, and the
     timer fires again in about 13 more minutes.  This behavior
     continues indefinitely until the connection is closed, and is a
     violation of the specification.


Trace file demonstrating correct behavior
     Made using the Orchestra tool at the peer of the machine using
     keep-alive.  Machine A was configured to use default settings for
     the keepalive timer.

     17:37:20.500000 A > B: 34155521:0       win 4096 datasz 4 SYN
     17:37:20.520000 B > A: 6272001:34155522 win 4096 datasz 4 SYN ACK
     17:37:20.540000 A > B: 34155522:6272002 win 4096 datasz 0 ACK

     19:37:25.430000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
     19:37:25.450000 B > A: 6272002:34155522 win 4096 datasz 0 ACK

     21:37:30.560000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
     21:37:30.570000 B > A: 6272002:34155522 win 4096 datasz 0 ACK

     23:37:35.580000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
     23:37:35.600000 B > A: 6272002:34155522 win 4096 datasz 0 ACK

     01:37:40.620000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
     01:37:40.640000 B > A: 6272002:34155522 win 4096 datasz 0 ACK

     03:37:45.590000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
     03:37:45.610000 B > A: 6272002:34155522 win 4096 datasz 0 ACK

     The initial three packets are the SYN exchange for connection
     setup.  Just over two hours later, the keepalive timer fires
     because the connection is idle.  The keepalive is acknowledged, and
     the timer fires again just over two hours later.  This behavior



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     continues indefinitely until the connection is closed.


References
     This problem is documented in [Dawson97].


How to detect
     For implementations manifesting this problem, it shows up on a
     packet trace.  If the connection is left idle, the keepalive probes
     will arrive closer together than the two hour minimum.


3.12.

Name of Problem
     Window probe deadlock


Classification
     Reliability


Description
     When an application reads a single byte from a full window, the
     window should not be updated, in order to avoid Silly Window
     Syndrome (SWS; see [RFC813]).  If the remote peer uses a single
     byte of data to probe the window, that byte can be accepted into
     the buffer.  In some implementations, at this point a negative
     argument to a signed comparison causes all further new data to be
     considered outside the window; consequently, it is discarded (after
     sending an ACK to resynchronize).  These discards include the ACKs
     for the data packets sent by the local TCP, so the TCP will
     consider the data unacknowledged.

     Consequently, the application may be unable to complete sending new
     data to the remote peer, because it has exhausted the transmit
     buffer available to its local TCP, and buffer space is never being
     freed because incoming ACKs that would do so are being discarded.
     If the application does not read any more data, which may happen
     due to its failure to complete such sends, then deadlock results.


Significance
     It's relatively rare for applications to use TCP in a manner that
     can exercise this problem.  Most applications only transmit bulk
     data if they know the other end is prepared to receive the data.
     However, if a client fails to consume data, putting the server in



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     persist mode, and then consumes a small amount of data, it can
     mistakenly compute a negative window.  At this point the client
     will discard all further packets from the server, including ACKs of
     the client's own data, since they are not inside the (impossibly-
     sized) window.  If subsequently the client consumes enough data to
     then send a window update to the server, the situation will be
     rectified.  That is, this situation can only happen if the client
     consumes 1 < N < MSS bytes, so as not to cause a window update, and
     then starts its own transmission towards the server of more than a
     window's worth of data.


Implications
     TCP connections will hang and eventually time out.


Relevant RFCs
     RFC 793 describes zero window probing.  RFC 813 describes Silly
     Window Syndrome.


Trace file demonstrating it
     Trace made from a version of tcpdump modified to print out the
     sequence number attached to an ACK even if it's dataless.  An
     unmodified tcpdump would not print seq:seq(0); however, for this
     bug, the sequence number in the ACK is important for unambiguously
     determining how the TCP is behaving.

     [ Normal connection startup and data transmission from B to A.
       Options, including MSS of 16344 in both directions, omitted
       for clarity. ]
     16:07:32.327616 A > B: S 65360807:65360807(0) win 8192
     16:07:32.327304 B > A: S 65488807:65488807(0) ack 65360808 win 57344
     16:07:32.327425 A > B: . 1:1(0) ack 1 win 57344
     16:07:32.345732 B > A: P 1:2049(2048) ack 1 win 57344
     16:07:32.347013 B > A: P 2049:16385(14336) ack 1 win 57344
     16:07:32.347550 B > A: P 16385:30721(14336) ack 1 win 57344
     16:07:32.348683 B > A: P 30721:45057(14336) ack 1 win 57344
     16:07:32.467286 A > B: . 1:1(0) ack 45057 win 12288
     16:07:32.467854 B > A: P 45057:57345(12288) ack 1 win 57344

     [ B fills up A's offered window ]
     16:07:32.667276 A > B: . 1:1(0) ack 57345 win 0

     [ B probes A's window with a single byte ]
     16:07:37.467438 B > A: . 57345:57346(1) ack 1 win 57344

     [ A resynchronizes without accepting the byte ]



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     16:07:37.467678 A > B: . 1:1(0) ack 57345 win 0

     [ B probes A's window again ]
     16:07:45.467438 B > A: . 57345:57346(1) ack 1 win 57344

     [ A resynchronizes and accepts the byte (per the ack field) ]
     16:07:45.667250 A > B: . 1:1(0) ack 57346 win 0

     [ The application on A has started generating data.  The first
       packet A sends is small due to a memory allocation bug. ]
     16:07:51.358459 A > B: P 1:2049(2048) ack 57346 win 0

     [ B acks A's first packet ]
     16:07:51.467239 B > A: . 57346:57346(0) ack 2049 win 57344

     [ This looks as though A accepted B's ACK and is sending
       another packet in response to it.  In fact, A is trying
       to resynchronize with B, and happens to have data to send
       and can send it because the first small packet didn't use
       up cwnd. ]
     16:07:51.467698 A > B: . 2049:14337(12288) ack 57346 win 0

     [ B acks all of the data that A has sent ]
     16:07:51.667283 B > A: . 57346:57346(0) ack 14337 win 57344

     [ A tries to resynchronize.  Notice that by the packets
       seen on the network, A and B *are* in fact synchronized;
       A only thinks that they aren't. ]
     16:07:51.667477 A > B: . 14337:14337(0) ack 57346 win 0

     [ A's retransmit timer fires, and B acks all of the data.
       A once again tries to resynchronize. ]
     16:07:52.467682 A > B: . 1:14337(14336) ack 57346 win 0
     16:07:52.468166 B > A: . 57346:57346(0) ack 14337 win 57344
     16:07:52.468248 A > B: . 14337:14337(0) ack 57346 win 0

     [ A's retransmit timer fires again, and B acks all of the data.
       A once again tries to resynchronize. ]
     16:07:55.467684 A > B: . 1:14337(14336) ack 57346 win 0
     16:07:55.468172 B > A: . 57346:57346(0) ack 14337 win 57344
     16:07:55.468254 A > B: . 14337:14337(0) ack 57346 win 0


Trace file demonstrating correct behavior
     Made between the same two hosts after applying the bug fix
     mentioned below (and using the same modified tcpdump).

     [ Connection starts up with data transmission from B to A.



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       Note that due to a separate bug (the fact that A and B
       are communicating over a loopback driver), B erroneously
       skips slow start. ]
     17:38:09.510854 A > B: S 3110066585:3110066585(0) win 16384
     17:38:09.510926 B > A: S 3110174850:3110174850(0) ack 3110066586 win 57344
     17:38:09.510953 A > B: . 1:1(0) ack 1 win 57344
     17:38:09.512956 B > A: P 1:2049(2048) ack 1 win 57344
     17:38:09.513222 B > A: P 2049:16385(14336) ack 1 win 57344
     17:38:09.513428 B > A: P 16385:30721(14336) ack 1 win 57344
     17:38:09.513638 B > A: P 30721:45057(14336) ack 1 win 57344
     17:38:09.519531 A > B: . 1:1(0) ack 45057 win 12288
     17:38:09.519638 B > A: P 45057:57345(12288) ack 1 win 57344

     [ B fills up A's offered window ]
     17:38:09.719526 A > B: . 1:1(0) ack 57345 win 0

     [ B probes A's window with a single byte.  A resynchronizes
       without accepting the byte ]
     17:38:14.499661 B > A: . 57345:57346(1) ack 1 win 57344
     17:38:14.499724 A > B: . 1:1(0) ack 57345 win 0

     [ B probes A's window again.  A resynchronizes and accepts
       the byte, as indicated by the ack field ]
     17:38:19.499764 B > A: . 57345:57346(1) ack 1 win 57344
     17:38:19.519731 A > B: . 1:1(0) ack 57346 win 0

     [ B probes A's window with a single byte.  A resynchronizes
       without accepting the byte ]
     17:38:24.499865 B > A: . 57346:57347(1) ack 1 win 57344
     17:38:24.499934 A > B: . 1:1(0) ack 57346 win 0

     [ The application on A has started generating data.
       B acks A's data and A accepts the ACKs and the
       data transfer continues ]
     17:38:28.530265 A > B: P 1:2049(2048) ack 57346 win 0
     17:38:28.719914 B > A: . 57346:57346(0) ack 2049 win 57344

     17:38:28.720023 A > B: . 2049:16385(14336) ack 57346 win 0
     17:38:28.720089 A > B: . 16385:30721(14336) ack 57346 win 0
     17:38:28.720370 B > A: . 57346:57346(0) ack 30721 win 57344

     17:38:28.720462 A > B: . 30721:45057(14336) ack 57346 win 0
     17:38:28.720526 A > B: P 45057:59393(14336) ack 57346 win 0
     17:38:28.720824 A > B: P 59393:73729(14336) ack 57346 win 0
     17:38:28.721124 B > A: . 57346:57346(0) ack 73729 win 47104

     17:38:28.721198 A > B: P 73729:88065(14336) ack 57346 win 0
     17:38:28.721379 A > B: P 88065:102401(14336) ack 57346 win 0



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     17:38:28.721557 A > B: P 102401:116737(14336) ack 57346 win 0
     17:38:28.721863 B > A: . 57346:57346(0) ack 116737 win 36864


References
     None known.


How to detect
     Initiate a connection from a client to a server.  Have the server
     continuously send data until its buffers have been full for long
     enough to exhaust the window.  Next, have the client read 1 byte
     and then delay for long enough that the server TCP sends a window
     probe.  Now have the client start sending data.  At this point, if
     it ignores the server's ACKs, then the client's TCP suffers from
     the problem.


How to fix
     In one implementation known to exhibit the problem (derived from
     4.3-Reno), the problem was introduced when the macro MAX() was
     replaced by the function call max() for computing the amount of
     space in the receive window:

         tp->rcv_wnd = max(win, (int)(tp->rcv_adv - tp->rcv_nxt));

     When data has been received into a window beyond what has been
     advertised to the other side, rcv_nxt > rcv_adv, making this
     negative.  It's clear from the (int) cast that this is intended,
     but the unsigned max() function sign-extends so the negative number
     is "larger".  The fix is to change max() to imax():

         tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));

     4.3-Tahoe and before did not have this bug, since it used the macro
     MAX() for this calculation.


3.13.

Name of Problem
     Stretch ACK violation


Classification
     Congestion Control/Performance





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Description
     To improve efficiency (both computer and network) a data receiver
     may refrain from sending an ACK for each incoming segment,
     according to [RFC1122].  However, an ACK should not be delayed an
     inordinate amount of time.  Specifically, ACKs SHOULD be sent for
     every second full-sized segment that arrives.  If a second full-
     sized segment does not arrive within a given timeout (of no more
     than 0.5 seconds), an ACK should be transmitted, according to
     [RFC1122].  A TCP receiver which does not generate an ACK for every
     second full-sized segment exhibits a "Stretch ACK Violation".


Significance
     TCP receivers exhibiting this behavior will cause TCP senders to
     generate burstier traffic, which can degrade performance in
     congested environments.  In addition, generating fewer ACKs
     increases the amount of time needed by the slow start algorithm to
     open the congestion window to an appropriate point, which
     diminishes performance in environments with large bandwidth-delay
     products.  Finally, generating fewer ACKs may cause needless
     retransmission timeouts in lossy environments, as it increases the
     possibility that an entire window of ACKs is lost, forcing a
     retransmission timeout.


Implications
     When not in loss recovery, every ACK received by a TCP sender
     triggers the transmission of new data segments.  The burst size is
     determined by the number of previously unacknowledged segments each
     ACK covers.  Therefore, a TCP receiver ack'ing more than 2 segments
     at a time causes the sending TCP to generate a larger burst of
     traffic upon receipt of the ACK.  This large burst of traffic can
     overwhelm an intervening gateway, leading to higher drop rates for
     both the connection and other connections passing through the
     congested gateway.

     In addition, the TCP slow start algorithm increases the congestion
     window by 1 segment for each ACK received.  Therefore, increasing
     the ACK interval (thus decreasing the rate at which ACKs are
     transmitted) increases the amount of time it takes slow start to
     increase the congestion window to an appropriate operating point,
     and the connection consequently suffers from reduced performance.
     This is especially true for connections using large windows.


Relevant RFCs
     RFC 1122 outlines delayed ACKs as a recommended mechanism.




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Trace file demonstrating it
     Trace file taken using tcpdump at host B, the data receiver (and
     ACK originator).  The advertised window (which never changed) and
     timestamp options have been omitted for clarity, except for the
     first packet sent by A:

     12:09:24.820187 A.1174 > B.3999: . 2049:3497(1448) ack 1
         win 33580 <nop,nop,timestamp 2249877 2249914> [tos 0x8]
     12:09:24.824147 A.1174 > B.3999: . 3497:4945(1448) ack 1
     12:09:24.832034 A.1174 > B.3999: . 4945:6393(1448) ack 1
     12:09:24.832222 B.3999 > A.1174: . ack 6393
     12:09:24.934837 A.1174 > B.3999: . 6393:7841(1448) ack 1
     12:09:24.942721 A.1174 > B.3999: . 7841:9289(1448) ack 1
     12:09:24.950605 A.1174 > B.3999: . 9289:10737(1448) ack 1
     12:09:24.950797 B.3999 > A.1174: . ack 10737
     12:09:24.958488 A.1174 > B.3999: . 10737:12185(1448) ack 1
     12:09:25.052330 A.1174 > B.3999: . 12185:13633(1448) ack 1
     12:09:25.060216 A.1174 > B.3999: . 13633:15081(1448) ack 1
     12:09:25.060405 B.3999 > A.1174: . ack 15081

     This portion of the trace clearly shows that the receiver (host B)
     sends an ACK for every third full sized packet received.  Further
     investigation of this implementation found that the cause of the
     increased ACK interval was the TCP options being used.  The
     implementation sent an ACK after it was holding 2*MSS worth of
     unacknowledged data.  In the above case, the MSS is 1460 bytes so
     the receiver transmits an ACK after it is holding at least 2920
     bytes of unacknowledged data.  However, the length of the TCP
     options being used [RFC1323] took 12 bytes away from the data
     portion of each packet.  This produced packets containing 1448
     bytes of data.  But the additional bytes used by the options in the
     header were not taken into account when determining when to trigger
     an ACK.  Therefore, it took 3 data segments before the data
     receiver was holding enough unacknowledged data (>= 2*MSS, or 2920
     bytes in the above example) to transmit an ACK.


Trace file demonstrating correct behavior

     Trace file taken using tcpdump at host B, the data receiver (and
     ACK originator), again with window and timestamp information
     omitted except for the first packet:

     12:06:53.627320 A.1172 > B.3999: . 1449:2897(1448) ack 1
         win 33580 <nop,nop,timestamp 2249575 2249612> [tos 0x8]
     12:06:53.634773 A.1172 > B.3999: . 2897:4345(1448) ack 1
     12:06:53.634961 B.3999 > A.1172: . ack 4345
     12:06:53.737326 A.1172 > B.3999: . 4345:5793(1448) ack 1



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     12:06:53.744401 A.1172 > B.3999: . 5793:7241(1448) ack 1
     12:06:53.744592 B.3999 > A.1172: . ack 7241
     12:06:53.752287 A.1172 > B.3999: . 7241:8689(1448) ack 1
     12:06:53.847332 A.1172 > B.3999: . 8689:10137(1448) ack 1
     12:06:53.847525 B.3999 > A.1172: . ack 10137

     This trace shows the TCP receiver (host B) ack'ing every second
     full-sized packet, according to [RFC1122].  This is the same
     implementation shown above, with slight modifications that allow
     the receiver to take the length of the options into account when
     deciding when to transmit an ACK.


References
     This problem is documented in [Allman97] and [Paxson97].


How to detect
     Stretch ACK violations show up immediately in receiver-side packet
     traces of bulk transfers, as shown above.  However, packet traces
     made on the sender side of the TCP connection may lead to
     ambiguities when diagnosing this problem due to the possibility of
     lost ACKs.


3.14.

Name of Problem
     Retransmission sends multiple packets


Classification
     Congestion control


Description
     When a TCP retransmits a segment due to a timeout expiration or
     beginning a fast retransmission sequence, it should only transmit a
     single segment.  A TCP that transmits more than one segment
     exhibits "Retransmission Sends Multiple Packets".

     Instances of this problem have been known to occur due to
     miscomputations involving the use of TCP options.  TCP options
     increase the TCP header beyond its usual size of 20 bytes.  The
     total size of header must be taken into account when retransmitting
     a packet.  If a TCP sender does not account for the length of the
     TCP options when determining how much data to retransmit, it will
     send too much data to fit into a single packet.  In this case, the



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     correct retransmission will be followed by a short segment
     (tinygram) containing data that may not need to be retransmitted.

     A specific case is a TCP using the RFC 1323 timestamp option, which
     adds 12 bytes to the standard 20-byte TCP header.  On
     retransmission of a packet, the 12 byte option is incorrectly
     interpreted as part of the data portion of the segment.  A standard
     TCP header and a new 12-byte option is added to the data, which
     yields a transmission of 12 bytes more data than contained in the
     original segment.  This overflow causes a smaller packet, with 12
     data bytes, to be transmitted.


Significance
     This problem is somewhat serious for congested environments because
     the TCP implementation injects more packets into the network than
     is appropriate.  However, since a tinygram is only sent in response
     to a fast retransmit or a timeout, it does not effect the sustained
     sending rate.


Implications
     A TCP exhibiting this behavior is stressing the network with more
     traffic than appropriate, and stressing routers by increasing the
     number of packets they must process.  The redundant tinygram will
     also elicit a duplicate ACK from the receiver, resulting in yet
     another unnecessary transmission.


Relevant RFCs
     RFC 1122 requires use of slow start after loss; RFC 2001 explicates
     slow start; RFC 1323 describes the timestamp option that has been
     observed to lead to some implementations exhibiting this problem.


Trace file demonstrating it
     Made using tcpdump recording at a machine on the same subnet as
     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.  In addition,
     portions of the trace file not pertaining to the packet in question
     have been removed (missing packets are denoted by ``[...]'' in the
     trace).

     11:55:22.701668 A > B: . 7361:7821(460) ack 1
         win 49324 <nop,nop,timestamp 3485348 3485113>
     11:55:22.702109 A > B: . 7821:8281(460) ack 1




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     [...]

     11:55:23.112405 B > A: . ack 7821
     11:55:23.113069 A > B: . 12421:12881(460) ack 1
     11:55:23.113511 A > B: . 12881:13341(460) ack 1
     11:55:23.333077 B > A: . ack 7821
     11:55:23.336860 B > A: . ack 7821
     11:55:23.340638 B > A: . ack 7821
     11:55:23.341290 A > B: . 7821:8281(460) ack 1
     11:55:23.341317 A > B: . 8281:8293(12) ack 1
     11:55:23.498242 B > A: . ack 7821
     11:55:23.506850 B > A: . ack 7821
     11:55:23.510630 B > A: . ack 7821

     [...]

     11:55:23.746649 B > A: . ack 10581

     The second line of the above trace shows the original transmission
     of a segment which is later dropped.  After 3 duplicate ACKs, line
     9 of the trace shows the dropped packet (7821:8281), with a 460-
     byte payload, being retransmitted.  Immediately following this
     retransmission, a packet with a 12-byte payload is unnecessarily
     sent.


Trace file demonstrating correct behavior

     The trace file would be identical to the one above, with a single
     line:

     11:55:23.341317 A > B: . 8281:8293(12) ack 1

     omitted.


References
     [Brakmo95]


How to detect
     This problem can be detected by examining a packet trace of the TCP
     connections of a machine using TCP options, during which a packet
     is retransmitted.







Paxson, Editor                                                 [Page 47]


ID                 Known TCP Implementation Problems       November 1998


3.15.

Name of Problem
     Failure to send FIN notification promptly


Classification
     Performance


Description
     When an application closes a connection, the corresponding TCP
     should send the FIN notification promptly to its peer (unless
     prevented by the congestion window).  If a TCP implementation
     delays in sending the FIN notification, for example due to waiting
     until unacknowledged data has been acknowledged, then it is said to
     exhibit "Failure to send FIN notification promptly".

     Also, while not strictly required, FIN segments should include the
     PSH flag to ensure expedited delivery of any pending data at the
     receiver.


Significance
     The greatest impact occurs for short-lived connections, since for
     these the additional time required to close the connection
     introduces the greatest relative delay.

     The additional time can be significant in the common case of the
     sender waiting for an ACK that is delayed by the receiver.


Implications
     Can diminish total throughput as seen at the application layer,
     because connection termination takes longer to complete.


Relevant RFCs
     RFC 793 indicates that a receiver should treat an incoming FIN flag
     as implying the push function.


Trace file demonstrating it
     Made using tcpdump (no losses reported by the packet filter).

     10:04:38.68 A > B: S 1031850376:1031850376(0) win 4096
                     <mss 1460,wscale 0,eol> (DF)
     10:04:38.71 B > A: S 596916473:596916473(0) ack 1031850377



Paxson, Editor                                                 [Page 48]


ID                 Known TCP Implementation Problems       November 1998


                     win 8760 <mss 1460> (DF)
     10:04:38.73 A > B: . ack 1 win 4096 (DF)
     10:04:41.98 A > B: P 1:4(3) ack 1 win 4096 (DF)
     10:04:42.15 B > A: . ack 4 win 8757 (DF)
     10:04:42.23 A > B: P 4:7(3) ack 1 win 4096 (DF)
     10:04:42.25 B > A: P 1:11(10) ack 7 win 8754 (DF)
     10:04:42.32 A > B: . ack 11 win 4096 (DF)
     10:04:42.33 B > A: P 11:51(40) ack 7 win 8754 (DF)
     10:04:42.51 A > B: . ack 51 win 4096 (DF)
     10:04:42.53 B > A: F 51:51(0) ack 7 win 8754 (DF)
     10:04:42.56 A > B: FP 7:7(0) ack 52 win 4096 (DF)
     10:04:42.58 B > A: . ack 8 win 8754 (DF)

     Machine B in the trace above does not send out a FIN notification
     promptly if there is any data outstanding.  It instead waits for
     all unacknowledged data to be acknowledged before sending the FIN
     segment.  The connection was closed at 10:04.42.33 after requesting
     40 bytes to be sent.  However, the FIN notification isn't sent
     until 10:04.42.51, after the (delayed) acknowledgement of the 40
     bytes of data.


Trace file demonstrating correct behavior
     Made using tcpdump (no losses reported by the packet filter).

     10:27:53.85 C > D: S 419744533:419744533(0) win 4096
                     <mss 1460,wscale 0,eol> (DF)
     10:27:53.92 D > C: S 10082297:10082297(0) ack 419744534
                     win 8760 <mss 1460> (DF)
     10:27:53.95 C > D: . ack 1 win 4096 (DF)
     10:27:54.42 C > D: P 1:4(3) ack 1 win 4096 (DF)
     10:27:54.62 D > C: . ack 4 win 8757 (DF)
     10:27:54.76 C > D: P 4:7(3) ack 1 win 4096 (DF)
     10:27:54.89 D > C: P 1:11(10) ack 7 win 8754 (DF)
     10:27:54.90 D > C: FP 11:51(40) ack7 win 8754 (DF)
     10:27:54.92 C > D: . ack 52 win 4096 (DF)
     10:27:55.01 C > D: FP 7:7(0) ack 52 win 4096 (DF)
     10:27:55.09 D > C: . ack 8 win 8754 (DF)

     Here, Machine D sends a FIN with 40 bytes of data even before the
     original 10 octets have been acknowledged. This is correct behavior
     as it provides for the highest performance.


References
     This problem is documented in [Dawson97].





Paxson, Editor                                                 [Page 49]


ID                 Known TCP Implementation Problems       November 1998


How to detect
     For implementations manifesting this problem, it shows up on a
     packet trace.


3.16.

Name of Problem
     Failure to send a RST after Half Duplex Close


Classification
     Resource management


Description
     RFC 1122 4.2.2.13 states that a TCP SHOULD send a RST if data is
     received after "half duplex close", i.e. if it cannot be delivered
     to the application.  A TCP that fails to do so is said to exhibit
     "Failure to send a RST after Half Duplex Close".


Significance
     Potentially serious for TCP endpoints that manage large numbers of
     connections, due to exhaustion of memory and/or process slots
     available for managing connection state.


Implications
     Failure to send the RST can lead to permanently hung TCP
     connections.  This problem has been demonstrated when HTTP clients
     abort connections, common when users move on to a new page before
     the current page has finished downloading.  The HTTP client closes
     by transmitting a FIN while the server is transmitting images,
     text, etc.  The server TCP receives the FIN,  but its application
     does not close the connection until all data has been queued for
     transmission.  Since the server will not transmit a FIN until all
     the preceding data has been transmitted, deadlock results if the
     client TCP does not consume the pending data or tear down the
     connection: the window decreases to zero, since the client cannot
     pass the data to the application, and the server sends probe
     segments.  The client acknowledges the probe segments with a zero
     window. As mandated in RFC1122 4.2.2.17, the probe segments are
     transmitted forever.  Server connection state remains in
     CLOSE_WAIT, and eventually server processes are exhausted.

     Note that there are two bugs.  First, probe segments should be
     ignored if the window can never subsequently increase.  Second, a



Paxson, Editor                                                 [Page 50]


ID                 Known TCP Implementation Problems       November 1998


     RST should be sent when data is received after half duplex close.
     Fixing the first bug, but not the second, results in the probe
     segments eventually timing out the connection, but the server
     remains in CLOSE_WAIT for a significant and unnecessary period.


Relevant RFCs
     RFC 1122 sections 4.2.2.13 and 4.2.2.17.


Trace file demonstrating it
     Made using an unknown network analyzer.  No drop information
     available.

     client.1391 > server.8080: S 0:1(0) ack: 0 win: 2000 <mss: 5b4>
     server.8080 > client.1391: SA 8c01:8c02(0) ack: 1 win: 8000 <mss:100>
     client.1391 > server.8080: PA
     client.1391 > server.8080: PA 1:1c2(1c1) ack: 8c02 win: 2000
     server.8080 > client.1391: [DF] PA 8c02:8cde(dc) ack: 1c2 win: 8000
     server.8080 > client.1391: [DF] A 8cde:9292(5b4) ack: 1c2 win: 8000
     server.8080 > client.1391: [DF] A 9292:9846(5b4) ack: 1c2 win: 8000
     server.8080 > client.1391: [DF] A 9846:9dfa(5b4) ack: 1c2 win: 8000
     client.1391 > server.8080: PA
     server.8080 > client.1391: [DF] A 9dfa:a3ae(5b4) ack: 1c2 win: 8000
     server.8080 > client.1391: [DF] A a3ae:a962(5b4) ack: 1c2 win: 8000
     server.8080 > client.1391: [DF] A a962:af16(5b4) ack: 1c2 win: 8000
     server.8080 > client.1391: [DF] A af16:b4ca(5b4) ack: 1c2 win: 8000
     client.1391 > server.8080: PA
     server.8080 > client.1391: [DF] A b4ca:ba7e(5b4) ack: 1c2 win: 8000
     server.8080 > client.1391: [DF] A b4ca:ba7e(5b4) ack: 1c2 win: 8000
     client.1391 > server.8080: PA
     server.8080 > client.1391: [DF] A ba7e:bdfa(37c) ack: 1c2 win: 8000
     client.1391 > server.8080: PA
     server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c2 win: 8000
     client.1391 > server.8080: PA

     [ HTTP client aborts and enters FIN_WAIT_1 ]

     client.1391 > server.8080: FPA

     [ server ACKs the FIN and enters CLOSE_WAIT ]

     server.8080 > client.1391: [DF] A

     [ client enters FIN_WAIT_2 ]

     server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000




Paxson, Editor                                                 [Page 51]


ID                 Known TCP Implementation Problems       November 1998


     [ server continues to try to send its data ]

     client.1391 > server.8080: PA < window = 0 >
     server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
     client.1391 > server.8080: PA < window = 0 >
     server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
     client.1391 > server.8080: PA < window = 0 >
     server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
     client.1391 > server.8080: PA < window = 0 >
     server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
     client.1391 > server.8080: PA < window = 0 >

     [ ... repeat ad exhaustium ... ]



Trace file demonstrating correct behavior
     Made using an unknown network analyzer.  No drop information
     available.

     client > server D=80 S=59500 Syn Seq=337 Len=0 Win=8760
     server > client D=59500 S=80 Syn Ack=338 Seq=80153 Len=0 Win=8760
     client > server D=80 S=59500 Ack=80154 Seq=338 Len=0 Win=8760

     [ ... normal data omitted ... ]

     client > server D=80 S=59500 Ack=14559 Seq=596 Len=0 Win=8760
     server > client D=59500 S=80 Ack=596 Seq=114559 Len=1460 Win=8760

     [ client closes connection ]

     client > server D=80 S=59500 Fin Seq=596 Len=0 Win=8760
     server > client D=59500 S=80 Ack=597 Seq=116019 Len=1460 Win=8760

     [ client sends RST (RFC1122 4.2.2.13) ]

     client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
     server > client D=59500 S=80 Ack=597 Seq=117479 Len=1460 Win=8760
     client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
     server > client D=59500 S=80 Ack=597 Seq=118939 Len=1460 Win=8760
     client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
     server > client D=59500 S=80 Ack=597 Seq=120399 Len=892 Win=8760
     client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
     server > client D=59500 S=80 Ack=597 Seq=121291 Len=1460 Win=8760
     client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0

     "client" sends a number of RSTs, one in response to each incoming
     packet from "server".  One might wonder why "server" keeps sending



Paxson, Editor                                                 [Page 52]


ID                 Known TCP Implementation Problems       November 1998


     data packets after it has received a RST from "client"; the
     explanation is that "server" had already transmitted all five of
     the data packets before receiving the first RST from "client", so
     it is too late to avoid transmitting them.


How to detect
     The problem can be detected by inspecting packet traces of a large,
     interrupted bulk transfer.


3.17.

Name of Problem
     Failure to RST on close with data pending


Classification
     Resource management


Description
     When an application closes a connection in such a way that it can
     no longer read any received data, the TCP SHOULD, per section
     4.2.2.13 of RFC 1122, send a RST if there is any unread received
     data, or if any new data is received. A TCP that fails to do so
     exhibits "Failure to RST on close with data pending".

     Note that, for some TCPs, this situation can be caused by an
     application "crashing" while a peer is sending data.

     We have observed a number of TCPs that exhibit this problem.  The
     problem is less serious if any subsequent data sent to the now-
     closed connection endpoint elicits a RST (see illustration below).


Significance
     This problem is most significant for endpoints that engage in large
     numbers of connections, as their ability to do so will be curtailed
     as they leak away resources.


Implications
     Failure to reset the connection can lead to permanently hung
     connections, in which the remote endpoint takes no further action
     to tear down the connection because it is waiting on the local TCP
     to first take some action.  This is particularly the case if the
     local TCP also allows the advertised window to go to zero, and



Paxson, Editor                                                 [Page 53]


ID                 Known TCP Implementation Problems       November 1998


     fails to tear down the connection when the remote TCP engages in
     "persist" probes (see example below).


Relevant RFCs
     RFC 1122 section 4.2.2.13.  Also, 4.2.2.17 for the zero-window
     probing discussion below.


Trace file demonstrating it
     Made using tcpdump.  No drop information available.

     13:11:46.04 A > B: S 458659166:458659166(0) win 4096
                         <mss 1460,wscale 0,eol> (DF)
     13:11:46.04 B > A: S 792320000:792320000(0) ack 458659167
                         win 4096
     13:11:46.04 A > B: . ack 1 win 4096 (DF)
     13:11.55.80 A > B: . 1:513(512) ack 1 win 4096 (DF)
     13:11.55.80 A > B: . 513:1025(512) ack 1 win 4096 (DF)
     13:11:55.83 B > A: . ack 1025 win 3072
     13:11.55.84 A > B: . 1025:1537(512) ack 1 win 4096 (DF)
     13:11.55.84 A > B: . 1537:2049(512) ack 1 win 4096 (DF)
     13:11.55.85 A > B: . 2049:2561(512) ack 1 win 4096 (DF)
     13:11:56.03 B > A: . ack 2561 win 1536
     13:11.56.05 A > B: . 2561:3073(512) ack 1 win 4096 (DF)
     13:11.56.06 A > B: . 3073:3585(512) ack 1 win 4096 (DF)
     13:11.56.06 A > B: . 3585:4097(512) ack 1 win 4096 (DF)
     13:11:56.23 B > A: . ack 4097 win 0
     13:11:58.16 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
     13:11:58.16 B > A: . ack 4097 win 0
     13:12:00.16 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
     13:12:00.16 B > A: . ack 4097 win 0
     13:12:02.16 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
     13:12:02.16 B > A: . ack 4097 win 0
     13:12:05.37 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
     13:12:05.37 B > A: . ack 4097 win 0
     13:12:06.36 B > A: F 1:1(0) ack 4097 win 0
     13:12:06.37 A > B: . ack 2 win 4096 (DF)
     13:12:11.78 A > B: . 4096:4097(1) ack 2 win 4096 (DF)
     13:12:11.78 B > A: . ack 4097 win 0
     13:12:24.59 A > B: . 4096:4097(1) ack 2 win 4096 (DF)
     13:12:24.60 B > A: . ack 4097 win 0
     13:12:50.22 A > B: . 4096:4097(1) ack 2 win 4096 (DF)
     13:12:50.22 B > A: . ack 4097 win 0

     Machine B in the trace above does not drop received data when the
     socket is "closed" by the application (in this case, the
     application process was terminated). This occurred at approximately



Paxson, Editor                                                 [Page 54]


ID                 Known TCP Implementation Problems       November 1998


     13:12:06.36 and resulted in the FIN being sent in response to the
     close. However, because there is no longer an application to
     deliver the data to, the TCP should have instead sent a RST.

     Note: Machine A's zero-window probing is also broken.  It is
     resending old data, rather than new data. Section 3.7 in RFC 793
     and Section 4.2.2.17 in RFC 1122 discuss zero-window probing.


Trace file demonstrating better behavior
     Made using tcpdump.  No drop information available.

     Better, but still not fully correct, behavior, per the discussion
     below.  We show this behavior because it has been observed for a
     number of different TCP implementations.

     13:48:29.24 C > D: S 73445554:73445554(0) win 4096
                         <mss 1460,wscale 0,eol> (DF)
     13:48:29.24 D > C: S 36050296:36050296(0) ack 73445555
                         win 4096 <mss 1460,wscale 0,eol> (DF)
     13:48:29.25 C > D: . ack 1 win 4096 (DF)
     13:48:30.78 C > D: . 1:1461(1460) ack 1 win 4096 (DF)
     13:48:30.79 C > D: . 1461:2921(1460) ack 1 win 4096 (DF)
     13:48:30.80 D > C: . ack 2921 win 1176 (DF)
     13:48:32.75 C > D: . 2921:4097(1176) ack 1 win 4096 (DF)
     13:48:32.82 D > C: . ack 4097 win 0 (DF)
     13:48:34.76 C > D: . 4096:4097(1) ack 1 win 4096 (DF)
     13:48:34.84 D > C: . ack 4097 win 0 (DF)
     13:48:36.34 D > C: FP 1:1(0) ack 4097 win 4096 (DF)
     13:48:36.34 C > D: . 4097:5557(1460) ack 2 win 4096 (DF)
     13:48:36.34 D > C: R 36050298:36050298(0) win 24576
     13:48:36.34 C > D: . 5557:7017(1460) ack 2 win 4096 (DF)
     13:48:36.34 D > C: R 36050298:36050298(0) win 24576

     In this trace, the application process is terminated on Machine D
     at approximately 13:48:36.34.  Its TCP sends the FIN with the
     window opened again (since it discarded the previously received
     data).  Machine C promptly sends more data, causing Machine D to
     reset the connection since it cannot deliver the data to the
     application. Ideally, Machine D SHOULD send a RST instead of
     dropping the data and re-opening the receive window.

     Note: Machine C's zero-window probing is broken, the same as in the
     example above.



Trace file demonstrating correct behavior



Paxson, Editor                                                 [Page 55]


ID                 Known TCP Implementation Problems       November 1998


     Made using tcpdump.  No losses reported by the packet filter.

     14:12:02.19 E > F: S 1143360000:1143360000(0) win 4096
     14:12:02.19 F > E: S 1002988443:1002988443(0) ack 1143360001
                         win 4096 <mss 1460> (DF)
     14:12:02.19 E > F: . ack 1 win 4096
     14:12:10.43 E > F: . 1:513(512) ack 1 win 4096
     14:12:10.61 F > E: . ack 513 win 3584 (DF)
     14:12:10.61 E > F: . 513:1025(512) ack 1 win 4096
     14:12:10.61 E > F: . 1025:1537(512) ack 1 win 4096
     14:12:10.81 F > E: . ack 1537 win 2560 (DF)
     14:12:10.81 E > F: . 1537:2049(512) ack 1 win 4096
     14:12:10.81 E > F: . 2049:2561(512) ack 1 win 4096
     14:12:10.81 E > F: . 2561:3073(512) ack 1 win 4096
     14:12:11.01 F > E: . ack 3073 win 1024 (DF)
     14:12:11.01 E > F: . 3073:3585(512) ack 1 win 4096
     14:12:11.01 E > F: . 3585:4097(512) ack 1 win 4096
     14:12:11.21 F > E: . ack 4097 win 0 (DF)
     14:12:15.88 E > F: . 4097:4098(1) ack 1 win 4096
     14:12:16.06 F > E: . ack 4097 win 0 (DF)
     14:12:20.88 E > F: . 4097:4098(1) ack 1 win 4096
     14:12:20.91 F > E: . ack 4097 win 0 (DF)
     14:12:21.94 F > E: R 1002988444:1002988444(0) win 4096

     When the application terminates at 14:12:21.94, F immediately sends
     a RST.

     Note: Machine E's zero-window probing is (finally) correct.


How to detect
     The problem can often be detected by inspecting packet traces of a
     transfer in which the receiving application terminates abnormally.
     When doing so, there can be an ambiguity (if only looking at the
     trace) as to whether the receiving TCP did indeed have unread data
     that it could now no longer deliver.  To provoke this to happen, it
     may help to suspend the receiving application so that it fails to
     consume any data, eventually exhausting the advertised window.  At
     this point, since the advertised window is zero, we know that the
     receiving TCP has undelivered data buffered up.  Terminating the
     application process then should suffice to test the correctness of
     the TCP's behavior.









Paxson, Editor                                                 [Page 56]


ID                 Known TCP Implementation Problems       November 1998


3.18.

Name of Problem
     Options missing from TCP MSS calculation


Classification
     Reliability / performance


Description
     When a TCP determines how much data to send per packet, it
     calculates a segment size based on the MTU of the path.  It must
     then subtract from that MTU the size of the IP and TCP headers in
     the packet.  If IP options and TCP options are not taken into
     account correctly in this calculation, the resulting segment size
     may be too large.  TCPs that do so are said to exhibit "Options
     missing from TCP MSS calculation".


Significance
     In some implementations, this causes the transmission of strangely
     fragmented packets.  In some implementations with Path MTU (PMTU)
     discovery [RFC1191], this problem can actually result in a total
     failure to transmit any data at all, regardless of the environment
     (see below).

     Arguably, especially since the wide deployment of firewalls, IP
     options appear only rarely in normal operations.


Implications
     In implementations using PMTU discovery, this problem can result in
     packets that are too large for the output interface, and that have
     the DF (don't fragment) bit set in the IP header.  Thus, the IP
     layer on the local machine is not allowed to fragment the packet to
     send it out the interface.  It instead informs the TCP layer of the
     correct MTU size of the interface; the TCP layer again miscomputes
     the MSS by failing to take into account the size of IP options; and
     the problem repeats, with no data flowing.


Relevant RFCs
     RFC 1122 describes the calculation of the effective send MSS.  RFC
     1191 describes Path MTU discovery.


Trace file demonstrating it



Paxson, Editor                                                 [Page 57]


ID                 Known TCP Implementation Problems       November 1998


     Trace file taking using tcpdump on host C.  The first trace
     demonstrates the fragmentation that occurs without path MTU
     discovery:

     13:55:25.488728 A.65528 > C.discard:
             P 567833:569273(1440) ack 1 win 17520
             <nop,nop,timestamp 3839 1026342>
             (frag 20828:1472@0+)
             (ttl 62, optlen=8 LSRR{B#} NOP)

     13:55:25.488943 A > C:
             (frag 20828:8@1472)
             (ttl 62, optlen=8 LSRR{B#} NOP)

     13:55:25.489052 C.discard > A.65528:
             . ack 566385 win 60816
             <nop,nop,timestamp 1026345 3839> (DF)
             (ttl 60, id 41266)

     Host A repeatedly sends 1440-octet data segments, but these hare
     fragmented into two packets, one with 1432 octets of data, and
     another with 8 octets of data.

     The second trace demonstrates the failure to send any data
     segments, sometimes seen with hosts doing path MTU discovery:

     13:55:44.332219 A.65527 > C.discard:
             S 1018235390:1018235390(0) win 16384
             <mss 1460,nop,wscale 0,nop,nop,timestamp 3876 0> (DF)
             (ttl 62, id 20912, optlen=8 LSRR{B#} NOP)

     13:55:44.333015 C.discard > A.65527:
             S 1271629000:1271629000(0) ack 1018235391 win 60816
             <mss 1460,nop,wscale 0,nop,nop,timestamp 1026383 3876> (DF)
             (ttl 60, id 41427)

     13:55:44.333206 C.discard > A.65527:
             S 1271629000:1271629000(0) ack 1018235391 win 60816
             <mss 1460,nop,wscale 0,nop,nop,timestamp 1026383 3876> (DF)
             (ttl 60, id 41427)

     This is all of the activity seen on this connection.  Eventually
     host C will time out attempting to establish the connection.


How to detect
     The "netcat" utility [Hobbit96] is useful for generating source
     routed packets:



Paxson, Editor                                                 [Page 58]


ID                 Known TCP Implementation Problems       November 1998


     1% nc C discard
     (interactive typing)
     ^C
     2% nc C discard < /dev/zero
     ^C
     3% nc -g B C discard
     (interactive typing)
     ^C
     4% nc -g B C discard < /dev/zero
     ^C

     Lines 1 through 3 should generate appropriate packets, which can be
     verified using tcpdump.  If the problem is present, line 4 should
     generate one of the two kinds of packet traces shown.


How to fix
     The implementation should ensure that the effective send MSS
     calculation includes a term for the IP and TCP options, as mandated
     by RFC 1122.


4. Security Considerations

   This memo does not discuss any specific security-related TCP
   implementation problems, as the working group decided to pursue
   documenting those in a separate document.  Some of the implementation
   problems discussed here, however, can be used for denial-of-service
   attacks.  Those classified as congestion control present
   opportunities to subvert TCPs used for legitimate data transfer into
   excessively loading network elements.  Those classified as
   "performance", "reliability" and "resource management" may be
   exploitable for launching surreptitious denial-of-service attacks
   against the user of the TCP.  Both of these types of attacks can be
   extremely difficult to detect because in most respects they look
   identical to legitimate network traffic.


5. Acknowledgements

   Thanks to numerous correspondents on the tcp-impl mailing list for
   their input:  Steve Alexander, Larry Backman, Jerry Chu, Alan Cox,
   Kevin Fall, Richard Fox, Jim Gettys, Rick Jones, Allison Mankin, Neal
   McBurnett, Perry Metzger, der Mouse, Thomas Narten, Andras Olah,
   Steve Parker, Francesco Potorti`, Luigi Rizzo, Allyn Romanow, Al
   Smith, Jerry Toporek, Joe Touch, and Curtis Villamizar.

   Thanks also to Josh Cohen for the traces documenting the "Failure to



Paxson, Editor                                                 [Page 59]


ID                 Known TCP Implementation Problems       November 1998


   send a RST after Half Duplex Close" problem; and to John Polstra, who
   analyzed the "Window probe deadlock" problem.


6. References



[Allman97]
     M. Allman, "Fixing Two BSD TCP Bugs," Technical Report CR-204151,
     NASA Lewis Research Center, Oct. 1997.
     http://gigahertz.lerc.nasa.gov/~mallman/papers/bug.ps

[RFC2414]
     M. Allman, S. Floyd and C. Partridge, "Increasing TCP's Initial
     Window," Sep. 1998.

[RFC1122]
     R. Braden, Editor, "Requirements for Internet Hosts --
     Communication Layers," Oct. 1989.

[RFC2119]
     S. Bradner, "Key words for use in RFCs to Indicate Requirement
     Levels," Mar. 1997.

[Brakmo95]
     L. Brakmo and L. Peterson, "Performance Problems in BSD4.4 TCP,"
     ACM Computer Communication Review, 25(5):69-86, 1995.

[RFC813]
     D. Clark, "Window and Acknowledgement Strategy in TCP," Jul. 1982.

[Dawson97]
     S. Dawson, F. Jahanian, and T. Mitton, "Experiments on Six
     Commercial TCP Implementations Using a Software Fault Injection
     Tool," to appear in Software Practice & Experience, 1997.  A
     technical report version of this paper can be obtained at
     ftp://rtcl.eecs.umich.edu/outgoing/sdawson/CSE-TR-298-96.ps.gz.

[Fall96]
     K. Fall and S. Floyd, "Simulation-based Comparisons of Tahoe, Reno,
     and SACK TCP," ACM Computer Communication Review, 26(3):5-21, 1996.

[Hobbit96]
     Hobbit, Avian Research, netcat, available via anonymous ftp to
     ftp.avian.org, 1996.

[Hoe96]



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ID                 Known TCP Implementation Problems       November 1998


     J. Hoe, "Improving the Start-up Behavior of a Congestion Control
     Scheme for TCP," Proc. SIGCOMM '96.

[Jacobson88]
     V. Jacobson, "Congestion Avoidance and Control," Proc. SIGCOMM '88.
     ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z

[Jacobson89]
     V. Jacobson, C. Leres, and S. McCanne, tcpdump, available via
     anonymous ftp to ftp.ee.lbl.gov, Jun. 1989.

[RFC2018]
     M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, "TCP Selective
     Acknowledgement Options," Oct. 1996.

[RFC1191]
     J. Mogul and S. Deering, "Path MTU discovery," Nov. 1990.

[RFC896]
     J. Nagle, "Congestion Control in IP/TCP Internetworks," Jan. 1984.

[Paxson97]
     V. Paxson, "Automated Packet Trace Analysis of TCP
     Implementations," Proc. SIGCOMM '97, available from
     ftp://ftp.ee.lbl.gov/papers/vp-tcpanaly-sigcomm97.ps.Z.

[RFC793]
     J. Postel, Editor, "Transmission Control Protocol," Sep. 1981.

[RFC2001]
     W. Stevens, "TCP Slow Start, Congestion Avoidance, Fast Retransmit,
     and Fast Recovery Algorithms," Jan. 1997.

[Stevens94]
     W. Stevens, "TCP/IP Illustrated, Volume 1", Addison-Wesley
     Publishing Company, Reading, Massachusetts, 1994.

[Wright95]
     G. Wright and W. Stevens, "TCP/IP Illustrated, Volume 2", Addison-
     Wesley Publishing Company, Reading Massachusetts, 1995.











Paxson, Editor                                                 [Page 61]


ID                 Known TCP Implementation Problems       November 1998


7. Authors' Addresses

   Vern Paxson <vern@ee.lbl.gov>
   Network Research Group
   Lawrence Berkeley National Laboratory
   Berkeley, CA 94720
   USA
   Phone: +1 510/486-7504

   Mark Allman <mallman@lerc.nasa.gov>
   NASA Lewis Research Center/Sterling Software
   21000 Brookpark Road
   MS 54-2
   Cleveland, OH 44135
   USA
   Phone: +1 216/433-6586

   Scott Dawson <sdawson@eecs.umich.edu>
   Real-Time Computing Laboratory
   EECS Building
   University of Michigan
   Ann Arbor, MI  48109-2122
   USA
   Phone: +1 313/763-5363

   William C. Fenner <fenner@parc.xerox.com>
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   USA
   Phone: +1 650/812-4816

   Jim Griner <jgriner@lerc.nasa.gov>
   NASA Lewis Research Center
   21000 Brookpark Road
   MS 54-2
   Cleveland, OH 44135
   USA
   Phone: +1 216/433-5787

   Ian Heavens <ian@spider.com>
   Spider Software Ltd.
   8 John's Place, Leith
   Edinburgh EH6 7EL
   UK
   Phone: +44 131/475-7015

   Kevin Lahey <kml@nas.nasa.gov>



Paxson, Editor                                                 [Page 62]


ID                 Known TCP Implementation Problems       November 1998


   NASA Ames Research Center/MRJ
   MS 258-6
   Moffett Field, CA 94035
   USA
   Phone: +1 650/604-4334

   Jeff Semke <semke@psc.edu>
   Pittsburgh Supercomputing Center
   4400 Fifth Ave
   Pittsburgh, PA 15213
   USA
   Phone: +1 412/268-4960

   Bernie Volz <volz@process.com>
   Process Software Corporation
   959 Concord Street
   Framingham, MA 01701
   USA
   Phone: +1 508/879-6994
































Paxson, Editor                                                 [Page 63]


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