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Versions: 00 01 02 03 04 05 RFC 2525

Network Working Group                                  V. Paxson, Editor
Internet Draft
Expiration Date: September 1997                               March 1997


                   Known TCP Implementation Problems
                    <draft-ietf-tcpimpl-prob-00.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 learn the current status of any Internet Draft, please  check  the
   ``1id-abstracts.txt'' listing contained in the Internet Drafts shadow
   directories  on  ftp.is.co.za   (Africa),   nic.nordu.net   (Europe),
   munnari.oz.au  (Pacific  Rim),  ds.internic.net  (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.


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:




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

   Category
        One or more problem categories for which the problem is  classi-
        fied.   Categories  used  so far: "congestion control", "perfor-
        mance", "reliability".  Others anticipated: "security", "intero-
        perability", "configuration".

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

   Significance
        A quanitification as to how serious the problem  is  considered.
        Categories are "Non-critical", "Serious", and "Critical".

   Implications
        Why the problem is viewed as a problem.

   Relevant RFCs
        Brief discussion of the RFCs with respect to which  the  problem
        is viewed as an implementation error.

   Trace file demonstrating the problem
        One or more ASCII trace  files  demonstrating  the  problem,  if
        applicable.   These  may  in the future be replaced with URLs to
        on-line traces.

   Trace file demonstrating correct behavior
        One or more examples of how correct behavior appears in a trace,
        if applicable.  These may in the future be replaced with URLs to
        on-line traces.

   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 associ-
        ated with causing the  problem  to  manifest  itself,  and  with
        interpreting  traces  to  detect the presence of the problem (if
        applicable).  In the future, this may include URLs for  diagnos-
        tic tools.

   How to fix
        For known causes of the problem, how to correct the  implementa-
        tion.



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   Implementation specifics
        If it is viewed as beneficial to document particular implementa-
        tions exhibiting the problem, and if the corresponding implemen-
        tors approve, then this section gives  the  specifics  of  those
        implementations,  along with a contact address for the implemen-
        tors.


3. Known implementation problems


3.1. No initial slow start

Category
     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 conges-
     tion 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.  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
     Serious.

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.  Some other forms
     are described in "Uninitialized CWND" and "Initial CWND of 2  pack-
     ets".


Relevant RFCs' 5



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

Trace file demonstrating it
     Made using tcpdump/BPF recording at the connection  responder.   No
     losses reported.

     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 con-
     nection responder, begins transmitting to B, the connection initia-
     tor.  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/BPF recording at the connection originator.   No
     losses reported.

     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



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     12:35:32.433690 C > D: . 1537:2049(512) ack 1 win 4608
     12:35:32.434481 C > D: . 2049:2561(512) ack 1 win 4608
     12:35:32.435032 C > D: . 2561:3073(512) ack 1 win 4608
     12:35:32.594526 D > C: . ack 3073 win 4096
     12:35:32.595465 C > D: . 3073:3585(512) ack 1 win 4608
     12:35:32.595947 C > D: . 3585:4097(512) ack 1 win 4608
     12:35:32.596414 C > D: . 4097:4609(512) ack 1 win 4608
     12:35:32.596888 C > D: . 4609:5121(512) ack 1 win 4608
     12:35:32.733453 D > C: . ack 4097 win 4096


References
     This problem is documented in [Paxson97].

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

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,  as  such  implementa-
     tions  exhibit  "No slow start after retransmission timeout", which
     has a significance of "Critical".


3.2. No slow start after retransmission timeout

Category
     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
     Critical.

Implications
     Entering slow start upon timeout forms one of the  cornerstones  of



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     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 dis-
     cussed in RFC 2001.


Trace file demonstrating it
     Made using tcpdump/BPF recording at the sending TCP (A).  No losses
     reported.

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

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



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     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 incon-
     sistent with slow start.

     A second trace confirmed that the problem is repeatable.


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

     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.


References
     This problem is documented in [Paxson97].




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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 retransmis-
     sion 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, deter-
     mining  whether the TCP follows slow start is done by computing the
     correct progression of cwnd and comparing it to the amount of  data
     transmited by the TCP subsequent to the timeout rtransmission.


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. Inconsistent retransmission

Category
     Reliability

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

Significance
     Critical.

Implications



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     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/BPF recording at  the  receiving  TCP  (B).   No
     losses reported.

     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  posi-
     tions  below the earlier transmission, all 26 positions of the ear-
     lier transmission, and two additional sequence positions.

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

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

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



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     A second example illustrates a slightly different instance  of  the
     problem.   The  tracing  again  was  made  with  tcpdump/BPF 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  previ-
     ous  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 mani-
     fested.  No "trigger" has been  identified  that  can  be  used  to
     reproduce the problem.

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

     In one implementation (not the one illustrated above), the  problem
     manifested  itself  when  (1) the sender received a zero window and



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ID                 Known TCP Implementation Problems          March 1997


     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.


3.4. Failure to retain above-sequence data

Category
     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).
     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
     Serious.

Implications
     In times of congestion, a failure  to  retain  above-sequence  data
     will lead to numerous otherwise-unnecessary retransmissions, aggra-
     vating 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/BPF recording at the receiving TCP.   No  losses
     reported.




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ID                 Known TCP Implementation Problems          March 1997


     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 222676 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

     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.




Paxson, Editor                                                 [Page 12]


ID                 Known TCP Implementation Problems          March 1997


     A second trace confirmed that the problem is repeatable.


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

     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
     09:11:25.884113 D > C: . 52737:53249(512) ack 1 win 61440
     09:11:25.884113 C > D: . ack 35841 win 61440

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

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

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


References
     This problem is documented in [Paxson97].


How to detect
     Packet loss is common enough in the Internet that generally  it  is
     not  difficult  to  find  an Internet path that will result in some
     above-sequence packets arriving.  A TCP that exhibits  "Failure  to
     retain  ..."  may  not  generate  duplicate acks for these packets.
     However, some TCPs that do retain above-sequence data also  do  not



Paxson, Editor                                                 [Page 13]


ID                 Known TCP Implementation Problems          March 1997


     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 dis-
     carded 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 con-
     tents 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.


4. Security Considerations

   This version of this  memo  does  not  discuss  any  security-related
   implementation problems.  Futures versions most likely will, so secu-
   rity considerations will require revisiting.


5. Acknowledgements

   Thanks to numerous correspondents on the tcp-impl  mailing  list  for
   their  input: Steve Alexander, Mark Allman, Larry Backman, Jerry Chu,
   Alan Cox, Kevin Fall, Richard Fox, Jim Gettys,  Rick  Jones,  Allison
   Mankin,  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.






Paxson, Editor                                                 [Page 14]


ID                 Known TCP Implementation Problems          March 1997


6. References


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

[Paxson97]
     V. Paxson, "Automated Packet  Trace  Analysis  of  TCP  Implementa-
     tions," available in draft form from vern@ee.lbl.gov, Feb. 1997.

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

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

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


7. Author's Address

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




















Paxson, Editor                                                 [Page 15]


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