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Versions: 00 01 02 03 04 rfc2923                           Informational
Network Working Group                                           K. Lahey
Internet Draft                                                 June 2000
                                                Expires:   November 2000

                  TCP Problems with Path MTU Discovery

1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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''.

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   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.

Lahey                                                           [Page 1]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

2. Introduction

   This memo catalogs several known TCP implementation problems dealing
   with Path MTU Discovery [RFC1191], including the long-standing black
   hole problem, stretch ACKs due to confusion between MSS and segment
   size, and MSS advertisement based on PMTU.  The goal in doing so is
   to improve conditions in the existing Internet by enhancing the
   quality of current TCP/IP implementations.

   While Path MTU Discovery (PMTUD) can be used with any upper-layer
   protocol, it is most commonly used by TCP;  this document does not
   attempt to treat problems encountered by other upper-layer protocols.
   Path MTU Discovery for IPv6 [RFC1981] treats only IPv6-dependent
   issues, but not the TCP issues brought up in this document.

   Each problem is defined as follows:

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

     One or more problem categories for which the problem is classified:
     "congestion control", "performance", "reliability",
     "non-interoperation -- connectivity failure".

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

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

     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.

Lahey                                                           [Page 2]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

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

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

     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.

3. Known implementation problems


Name of Problem
     Black Hole Detection

     Non-interoperation -- connectivity failure

     A host performs Path MTU Discovery by sending out as large a packet
     as possible, with the Don't Fragment (DF) bit set in the IP header.
     If the packet is too large for a router to forward on to a
     particular link, the router must send an ICMP Destination
     Unreachable -- Fragmentation Needed message to the source address.
     The host then adjusts the packet size based on the ICMP message.

Lahey                                                           [Page 3]

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     As was pointed out in [RFC1435], routers don't always do this
     correctly -- many routers fail to send the ICMP messages, for a
     variety of reasons ranging from kernel bugs to configuration
     problems.  Firewalls are often misconfigured to suppress all ICMP
     messages.  IPsec [RFC2401] and IP-in-IP [RFC2003] tunnels shouldn't
     cause these sorts of problems, if the implementations follow the
     advice in the appropriate documents.

     PMTUD, as documented in [RFC1191], fails when the appropriate ICMP
     messages are not received by the originating host.  The upper-layer
     protocol continues to try to send large packets and, without the
     ICMP messages, never discovers that it needs to reduce the size of
     those packets.  Its packets are disappearing into a PMTUD black

     When PMTUD fails due to the lack of ICMP messages, TCP will also
     completely fail under some conditions.

     This failure is especially difficult to debug, as pings and some
     interactive TCP connections to the destination host work.  Bulk
     transfers fail with the first large packet and the connection
     eventually times out.

     These situations can almost always be blamed on a misconfiguration
     within the network, which should be corrected.  However it seems
     inappropriate for some TCP implementations to suffer
     interoperability failures over paths which do not affect other TCP
     implementations (i.e. those without PMTUD).  This creates a market
     disincentive for deploying TCP implementation with PMTUD enabled.

Relevant RFCs
     RFC1191 describes Path MTU Discovery.  RFC 1435 provides an early
     description of these sorts of problems.

Trace file demonstrating the problem
     Made using tcpdump [Jacobson89] recording at an intermediate host.

     20:12:11.951321 A > B: S 1748427200:1748427200(0)
          win 49152 <mss 1460>
     20:12:11.951829 B > A: S 1001927984:1001927984(0)

Lahey                                                           [Page 4]

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          ack 1748427201 win 16384 <mss 65240>
     20:12:11.955230 A > B: . ack 1 win 49152 (DF)
     20:12:11.959099 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:12:13.139074 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:12:16.188685 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:12:22.290483 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:12:34.491856 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:12:58.896405 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:13:47.703184 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:14:52.780640 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:15:57.856037 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:17:02.932431 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:18:08.009337 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:19:13.090521 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:20:18.168066 A > B: . 1:1461(1460) ack 1 win 49152 (DF)
     20:21:23.242761 A > B: R 1461:1461(0) ack 1 win 49152 (DF)

     The short SYN packet has no trouble traversing the network, due to
     its small size.  Similarly, ICMP echo packets used to diagnose
     connectivity problems will succeed.

     Large data packets fail to traverse the network.  Eventually the
     connection times out.  This can be especially confusing when the
     application starts out with a very small write, which succeeds,
     following up with many large writes, which then fail.

Trace file demonstrating correct behavior

     Made using tcpdump recording at an intermediate host.

     16:48:42.659115 A > B: S 271394446:271394446(0)
          win 8192 <mss 1460> (DF)
     16:48:42.672279 B > A: S 2837734676:2837734676(0)
          ack 271394447 win 16384 <mss 65240>
     16:48:42.676890 A > B: . ack 1 win 8760 (DF)
     16:48:42.870574 A > B: . 1:1461(1460) ack 1 win 8760 (DF)
     16:48:42.871799 A > B: . 1461:2921(1460) ack 1 win 8760 (DF)
     16:48:45.786814 A > B: . 1:1461(1460) ack 1 win 8760 (DF)
     16:48:51.794676 A > B: . 1:1461(1460) ack 1 win 8760 (DF)
     16:49:03.808912 A > B: . 1:537(536) ack 1 win 8760
     16:49:04.016476 B > A: . ack 537 win 16384
     16:49:04.021245 A > B: . 537:1073(536) ack 1 win 8760
     16:49:04.021697 A > B: . 1073:1609(536) ack 1 win 8760
     16:49:04.120694 B > A: . ack 1609 win 16384
     16:49:04.126142 A > B: . 1609:2145(536) ack 1 win 8760

     In this case, the sender sees four packets fail to traverse the

Lahey                                                           [Page 5]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

     network (using a two-packet initial send window) and turns off
     PMTUD.  All subsequent packets have the DF flag turned off, and the
     size set to the default value of 536 [RFC1122].

     This problem has been discussed extensively on the tcp-impl mailing
     list;  the name "black hole" has been in use for many years.

How to detect

     This shows up as a TCP connection which hangs (fails to make
     progress) until closed by timeout (this often manifests itself as a
     connection that connects and starts to transfer, then eventually
     terminates after 15 minutes with zero bytes transfered).  This is
     particularly annoying with an application like ftp, which will work
     perfectly while it uses small packets for control information, and
     then fail on bulk transfers.

     A series of ICMP echo packets will show that the two end hosts are
     still capable of passing packets,  a series of MTU-sized ICMP echo
     packets will show some fragmentation, and a series of MTU-sized
     ICMP echo packets with DF set will fail.  This can be confusing for
     network engineers trying to diagnose the problem.

     There are several traceroute implementations that do PMTUD, and can
     demonstrate the problem.

How to fix
     TCP should notice that the connection is timing out.  After several
     timeouts, TCP should attempt to send smaller packets, perhaps
     turning off the DF flag for each packet.  If this succeeds, it
     should continue to turn off PMTUD for the connection for some
     reasonable period of time, after which it should probe again to try
     to determine if the path has changed.

     Note that, under IPv6, there is no DF bit -- it is implicitly on at
     all times.  Fragmentation is not allowed in routers, only at the
     originating host.  Fortunately, the minimum supported MTU for IPv6
     is 1280 octets, which is significantly larger than the 68 octet
     minimum in IPv4.  This should make it more reasonable for IPv6 TCP
     implementations to fall back to 1280 octet packets, when IPv4
     implementations will probably have to turn off DF to respond to
     black hole detection.

     Ideally, the ICMP black holes should be fixed when they are found.

Lahey                                                           [Page 6]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

     If hosts start to implement black hole detection, it may be that
     these problems will go unnoticed and unfixed.  This is especially
     unfortunate, since detection can take several seconds each time,
     and these delays could result in a significant, hidden degradation
     of performance.  Hosts that implement black hole detection should
     probably log detected black holes, so that they can be fixed.


Name of Problem
     Stretch ACK due to PMTUD

     Congestion Control / Performance

     When a naively implemented TCP stack communicates with a PMTUD
     equipped stack, it will try to generate an ACK for every second
     full-sized segment.  If it determines the full-sized segment based
     on the advertised MSS, this can degrade badly in the face of PMTUD.

     The PMTU can wind up being a small fraction of the advertised MSS;
     in this case, an ACK would be generated only very infrequently.


     Stretch ACKs have a variety of unfortunate effects, more fully
     outlined in [RFC2525].  Most of these have to do with encouraging a
     more bursty connection, due to the infrequent arrival of ACKs.
     They can also impede congestion window growth.


     The complete implications of stretch ACKs are outlined in

Relevant RFCs
     RFC 1122 outlines the requirements for frequency of ACK generation.
     [RFC2581] expands on this and clarifies that delayed ACK is a
     SHOULD, not a MUST.

Lahey                                                           [Page 7]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

Trace file demonstrating it

     Made using tcpdump recording at an intermediate host.  The
     timestamp options from all but the first two packets have been
     removed for clarity.

     18:16:52.976657 A > B: S 3183102292:3183102292(0) win 16384
          <mss 4312,nop,wscale 0,nop,nop,timestamp 12128 0> (DF)
     18:16:52.979580 B > A: S 2022212745:2022212745(0) ack 3183102293 win 49152
          <mss 4312,nop,wscale 1,nop,nop,timestamp 1592957 12128> (DF)
     18:16:52.979738 A > B: . ack 1 win 17248  (DF)
     18:16:52.982473 A > B: . 1:4301(4300) ack 1 win 17248  (DF)
     18:16:52.982557 C > A: icmp: B unreachable -
          need to frag (mtu 1500)! (DF)
     18:16:52.985839 B > A: . ack 1 win 32768  (DF)
     18:16:54.129928 A > B: . 1:1449(1448) ack 1 win 17248  (DF)
     18:16:58.507078 A > B: . 1463941:1465389(1448) ack 1 win 17248  (DF)
     18:16:58.507200 A > B: . 1465389:1466837(1448) ack 1 win 17248  (DF)
     18:16:58.507326 A > B: . 1466837:1468285(1448) ack 1 win 17248  (DF)
     18:16:58.507439 A > B: . 1468285:1469733(1448) ack 1 win 17248  (DF)
     18:16:58.524763 B > A: . ack 1452357 win 32768  (DF)
     18:16:58.524986 B > A: . ack 1461045 win 32768  (DF)
     18:16:58.525138 A > B: . 1469733:1471181(1448) ack 1 win 17248  (DF)
     18:16:58.525268 A > B: . 1471181:1472629(1448) ack 1 win 17248  (DF)
     18:16:58.525393 A > B: . 1472629:1474077(1448) ack 1 win 17248  (DF)
     18:16:58.525516 A > B: . 1474077:1475525(1448) ack 1 win 17248  (DF)
     18:16:58.525642 A > B: . 1475525:1476973(1448) ack 1 win 17248  (DF)
     18:16:58.525766 A > B: . 1476973:1478421(1448) ack 1 win 17248  (DF)
     18:16:58.526063 A > B: . 1478421:1479869(1448) ack 1 win 17248  (DF)
     18:16:58.526187 A > B: . 1479869:1481317(1448) ack 1 win 17248  (DF)
     18:16:58.526310 A > B: . 1481317:1482765(1448) ack 1 win 17248  (DF)
     18:16:58.526432 A > B: . 1482765:1484213(1448) ack 1 win 17248  (DF)
     18:16:58.526561 A > B: . 1484213:1485661(1448) ack 1 win 17248  (DF)
     18:16:58.526671 A > B: . 1485661:1487109(1448) ack 1 win 17248  (DF)
     18:16:58.537944 B > A: . ack 1478421 win 32768  (DF)
     18:16:58.538328 A > B: . 1487109:1488557(1448) ack 1 win 17248  (DF)

     Note that the interval between ACKs is significantly larger than
     two times the segment size;  it works out to be almost exactly two
     times the advertised MSS.  This transfer was long enough that it
     could be verified that the stretch ACK was not the result of lost
     ACK packets.

Lahey                                                           [Page 8]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

Trace file demonstrating correct behavior

     Made using tcpdump recording at an intermediate host.  The
     timestamp options from all but the first two packets have been
     removed for clarity.

     18:13:32.287965 A > B: S 2972697496:2972697496(0)
          win 16384 <mss 4312,nop,wscale 0,nop,nop,timestamp 11326 0> (DF)
     18:13:32.290785 B > A: S 245639054:245639054(0)
          ack 2972697497 win 34496 <mss 4312> (DF)
     18:13:32.290941 A > B: . ack 1 win 17248 (DF)
     18:13:32.293774 A > B: . 1:4313(4312) ack 1 win 17248 (DF)
     18:13:32.293856 C > A: icmp: B unreachable -
          need to frag (mtu 1500)! (DF)
     18:13:33.637338 A > B: . 1:1461(1460) ack 1 win 17248 (DF)
     18:13:35.561691 A > B: . 1514021:1515481(1460) ack 1 win 17248 (DF)
     18:13:35.561814 A > B: . 1515481:1516941(1460) ack 1 win 17248 (DF)
     18:13:35.561938 A > B: . 1516941:1518401(1460) ack 1 win 17248 (DF)
     18:13:35.562059 A > B: . 1518401:1519861(1460) ack 1 win 17248 (DF)
     18:13:35.562174 A > B: . 1519861:1521321(1460) ack 1 win 17248 (DF)
     18:13:35.564008 B > A: . ack 1481901 win 64680 (DF)
     18:13:35.564383 A > B: . 1521321:1522781(1460) ack 1 win 17248 (DF)
     18:13:35.564499 A > B: . 1522781:1524241(1460) ack 1 win 17248 (DF)
     18:13:35.615576 B > A: . ack 1484821 win 64680 (DF)
     18:13:35.615646 B > A: . ack 1487741 win 64680 (DF)
     18:13:35.615716 B > A: . ack 1490661 win 64680 (DF)
     18:13:35.615784 B > A: . ack 1493581 win 64680 (DF)
     18:13:35.615856 B > A: . ack 1496501 win 64680 (DF)
     18:13:35.615952 A > B: . 1524241:1525701(1460) ack 1 win 17248 (DF)
     18:13:35.615966 B > A: . ack 1499421 win 64680 (DF)
     18:13:35.616088 A > B: . 1525701:1527161(1460) ack 1 win 17248 (DF)
     18:13:35.616105 B > A: . ack 1502341 win 64680 (DF)
     18:13:35.616211 A > B: . 1527161:1528621(1460) ack 1 win 17248 (DF)
     18:13:35.616228 B > A: . ack 1505261 win 64680 (DF)
     18:13:35.616327 A > B: . 1528621:1530081(1460) ack 1 win 17248 (DF)
     18:13:35.616349 B > A: . ack 1508181 win 64680 (DF)
     18:13:35.616448 A > B: . 1530081:1531541(1460) ack 1 win 17248 (DF)
     18:13:35.616565 A > B: . 1531541:1533001(1460) ack 1 win 17248 (DF)
     18:13:35.616891 A > B: . 1533001:1534461(1460) ack 1 win 17248 (DF)

     In this trace, an ACK is generated for every two segments that
     arrive.  (The segment size is slightly larger in this trace, even
     though the source hosts are the same, because of the lack of
     timestamp options in this trace.)

Lahey                                                           [Page 9]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

How to detect
     This condition can be observed in a packet trace when the
     advertised MSS is significantly larger than the actual PMTU of a

How to fix
     Several solutions for this problem have been proposed:

     A simple solution is to ACK every other packet, regardless of size.
     This has the drawback of generating large numbers of ACKs in the
     face of lots of very small packets;  this shows up with
     applications like the X Window System.

     A slightly more complex solution would monitor the size of incoming
     segments and try to determine what segment size the sender is
     using.  This requires slightly more state in the receiver, but has
     the advantage of making receiver silly window syndrome avoidance
     computations more accurate [RFC813].


Name of Problem
     Determining MSS from PMTU


     The MSS advertised at the start of a connection should be based on
     the MTU of the interfaces on the system.  (For efficiency and other
     reasons this may not be the largest MSS possible.)  Some systems
     use PMTUD determined values to determine the MSS to advertise.

     This results in an advertised MSS that is smaller than the largest
     MTU the system can receive.

     The advertised MSS is an indication to the remote system about the
     largest TCP segment that can be received [RFC879].  If this value
     is too small, the remote system will be forced to use a smaller

Lahey                                                          [Page 10]

Internet Draft    TCP Problems with Path MTU Discovery         June 2000

     segment size when sending, purely because the local system found a
     particular PMTU earlier.

     Given the asymmetric nature of many routes on the Internet
     [Paxson97], it seems entirely possible that the return PMTU is
     different from the sending PMTU.  Limiting the segment size in this
     way can reduce performance and frustrate the PMTUD algorithm.

     Even if the route was symmetric, setting this artificially lowered
     limit on segment size will make it impossible to probe later to
     determine if the PMTU has changed.

     The whole point of PMTUD is to send as large a segment as possible.
     If long-running connections cannot successfully probe for larger
     PMTU, then potential performance gains will be impossible to
     realize.  This destroys the whole point of PMTUD.

Relevant RFCs
     RFC 1191.  [RFC879] provides a complete discussion of MSS
     calculations and appropriate values.  Note that this practice does
     not violate any of the specifications in these RFCs.

Trace file demonstrating it
     This trace was made using tcpdump running on an intermediate host.
     Host A initiates two separate consecutive connections, A1 and A2,
     to host B.  Router C is the location of the MTU bottleneck.  As
     usual, TCP options are removed from all non-SYN packets.

     22:33:32.305912 A1 > B: S 1523306220:1523306220(0)
          win 8760 <mss 1460> (DF)
     22:33:32.306518 B > A1: S 729966260:729966260(0)
          ack 1523306221 win 16384 <mss 65240>
     22:33:32.310307 A1 > B: . ack 1 win 8760 (DF)
     22:33:32.323496 A1 > B: P 1:1461(1460) ack 1 win 8760 (DF)
     22:33:32.323569 C > A1: icmp: unreachable -
          need to frag (mtu 1024) (DF) (ttl 255, id 20666)
     22:33:32.783694 A1 > B: . 1:985(984) ack 1 win 8856 (DF)
     22:33:32.840817 B > A1: . ack 985 win 16384
     22:33:32.845651 A1 > B: . 1461:2445(984) ack 1 win 8856 (DF)
     22:33:32.846094 B > A1: . ack 985 win 16384
     22:33:33.724392 A1 > B: . 985:1969(984) ack 1 win 8856 (DF)
     22:33:33.724893 B > A1: . ack 2445 win 14924
     22:33:33.728591 A1 > B: . 2445:2921(476) ack 1 win 8856 (DF)
     22:33:33.729161 A1 > B: . ack 1 win 8856 (DF)

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     22:33:33.840758 B > A1: . ack 2921 win 16384


     22:33:34.238659 A1 > B: F 7301:8193(892) ack 1 win 8856 (DF)
     22:33:34.239036 B > A1: . ack 8194 win 15492
     22:33:34.239303 B > A1: F 1:1(0) ack 8194 win 16384
     22:33:34.242971 A1 > B: . ack 2 win 8856 (DF)
     22:33:34.454218 A2 > B: S 1523591299:1523591299(0)
          win 8856 <mss 984> (DF)
     22:33:34.454617 B > A2: S 732408874:732408874(0)
          ack 1523591300 win 16384 <mss 65240>
     22:33:34.457516 A2 > B: . ack 1 win 8856 (DF)
     22:33:34.470683 A2 > B: P 1:985(984) ack 1 win 8856 (DF)
     22:33:34.471144 B > A2: . ack 985 win 16384
     22:33:34.476554 A2 > B: . 985:1969(984) ack 1 win 8856 (DF)
     22:33:34.477580 A2 > B: P 1969:2953(984) ack 1 win 8856 (DF)


     Notice that the SYN packet for session A2 specifies an MSS of 984.

Trace file demonstrating correct behavior

     As before, this trace was made using tcpdump running on an
     intermediate host.  Host A initiates two separate consecutive
     connections, A1 and A2, to host B.  Router C is the location of the
     MTU bottleneck.  As usual, TCP options are removed from all non-SYN

     22:36:58.828602 A1 > B: S 3402991286:3402991286(0) win 32768
          <mss 4312,wscale 0,nop,timestamp 1123370309 0,
           echo 1123370309> (DF)
     22:36:58.844040 B > A1: S 946999880:946999880(0)
          ack 3402991287 win 16384
          <mss 65240,nop,wscale 0,nop,nop,timestamp 429552 1123370309>
     22:36:58.848058 A1 > B: . ack 1 win 32768  (DF)
     22:36:58.851514 A1 > B: P 1:1025(1024) ack 1 win 32768  (DF)
     22:36:58.851584 C > A1: icmp: unreachable -
          need to frag (mtu 1024) (DF)
     22:36:58.855885 A1 > B: . 1:969(968) ack 1 win 32768  (DF)
     22:36:58.856378 A1 > B: . 969:985(16) ack 1 win 32768  (DF)
     22:36:59.036309 B > A1: . ack 985 win 16384
     22:36:59.039255 A1 > B: FP 985:1025(40) ack 1 win 32768  (DF)
     22:36:59.039623 B > A1: . ack 1026 win 16344
     22:36:59.039828 B > A1: F 1:1(0) ack 1026 win 16384

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     22:36:59.043037 A1 > B: . ack 2 win 32768  (DF)
     22:37:01.436032 A2 > B: S 3404812097:3404812097(0) win 32768
          <mss 4312,wscale 0,nop,timestamp 1123372916 0,
           echo 1123372916> (DF)
     22:37:01.436424 B > A2: S 949814769:949814769(0)
          ack 3404812098 win 16384
          <mss 65240,nop,wscale 0,nop,nop,timestamp 429562 1123372916>
     22:37:01.440147 A2 > B: . ack 1 win 32768  (DF)
     22:37:01.442736 A2 > B: . 1:969(968) ack 1 win 32768  (DF)
     22:37:01.442894 A2 > B: P 969:985(16) ack 1 win 32768  (DF)
     22:37:01.443283 B > A2: . ack 985 win 16384
     22:37:01.446068 A2 > B: P 985:1025(40) ack 1 win 32768  (DF)
     22:37:01.446519 B > A2: . ack 1025 win 16384
     22:37:01.448465 A2 > B: F 1025:1025(0) ack 1 win 32768  (DF)
     22:37:01.448837 B > A2: . ack 1026 win 16384
     22:37:01.449007 B > A2: F 1:1(0) ack 1026 win 16384
     22:37:01.452201 A2 > B: . ack 2 win 32768  (DF)

     Note that the same MSS was used for both session A1 and session A2.

How to detect

     This can be detected using a packet trace of two separate
     connections;  the first should invoke PMTUD; the second should
     start soon enough after the first that the PMTU value does not time

How to fix
     The MSS should be determined based on the MTUs of the interfaces on
     the system, as outlined in [RFC1122] and [RFC1191].

4. Security Considerations

   The one security concern raised by this memo is that ICMP black holes
   are often caused by over-zealous security administrators who block
   all ICMP messages.  It is vitally important that those who design and
   deploy security systems understand the impact of strict filtering on
   upper-layer protocols.  The safest web site in the world is worthless
   if most TCP implementations cannot transfer data from it.  It would
   be far nicer to have all of the black holes fixed rather than fixing
   all of the TCP implementations.

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5. Acknowledgements

   Thanks to Mark Allman, Vern Paxson, and Jamshid Mahdavi for generous
   help reviewing the document, and to Matt Mathis for early suggestions
   of various mechanisms that can cause PMTUD black holes, as well as
   review.  The structure for describing TCP problems, and the early
   description of that structure is from [RFC2525].  Special thanks to
   Amy Bock, who helped perform the PMTUD tests which discovered these

6. References

     M. Allman, V. Paxson, and W. Stevens, "TCP Congestion Control",
     April 1999.

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

     D. Clark, "Window and Acknowledgement Strategy in TCP," July 1982.

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

     S. Knowles, "IESG Advice from Experience with Path MTU Discovery,"
     March 1993.

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

     J. McCann, S. Deering & J. Mogul, "Path MTU Discovery for IP
     version 6", August 1996.

     V. Paxson, "End-to-End Routing Behavior in the Internet", IEEE/ACM
     Transactions on Networking (5), pp.~601-615, Oct. 1997.

     V. Paxon, Editor, M. Allman, S. Dawson, W. Fenner, J. Griner, I.

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     Heavens, K. Lahey, J. Semke, and B. Volz, "Known TCP Implementation
     Problems", March 1999.

     J. Postel, "The TCP Maximum Segment Size and Related Topics,"
     November, 1983.

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

6.1. Author's Address

   Kevin Lahey <kml@logictier.com
   LogicTier, Inc.
   Suite 100
   2 Waters Park Drive
   San Mateo, CA 94403
   Phone: +1 650/678-7033

7. Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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       This draft was created in June 2000.
       It expires in November 2000.

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