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IETF RFC 2923
TCP Problems with Path MTU Discovery
Last modified on Thursday, September 14th, 2000
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Network Working Group K. Lahey
Request for Comments: 2923 dotRocket, Inc.
Category: Informational September 2000
TCP Problems with Path MTU Discovery
 Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2000). All Rights Reserved.
Abstract
This memo catalogs several known Transmission Control Protocol (TCP)
implementation problems dealing with Path Maximum Transmission Unit
Discovery (PMTUD), including the long-standing black hole problem,
stretch acknowlegements (ACKs) due to confusion between Maximum
Segment Size (MSS) and segment size, and MSS advertisement based on
PMTU.
1. Introduction
This memo catalogs several known TCP implementation problems dealing
with Path MTU Discovery [RFC 1191], 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 [RFC 1981] 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.
Lahey Informational PAGE 1 
RFC 2923 TCP Problems with Path MTU Discovery September 2000
Classification
One or more problem categories for which the problem is
classified: "congestion control", "performance", "reliability",
"non-interoperation -- connectivity failure".
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.
Lahey Informational PAGE 2
RFC 2923 TCP Problems with Path MTU Discovery September 2000
2. Known implementation problems
2.1.
Name of Problem
Black Hole Detection
Classification
Non-interoperation -- connectivity failure
Description
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.
As was pointed out in [RFC 1435], 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 [RFC 2401] and IP-in-IP [RFC 2003] tunnels
shouldn't cause these sorts of problems, if the implementations
follow the advice in the appropriate documents.
PMTUD, as documented in [RFC 1191], 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 hole.
Significance
When PMTUD fails due to the lack of ICMP messages, TCP will also
completely fail under some conditions.
Implications
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
Lahey Informational PAGE 3
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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
RFC 1191 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)
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>
Lahey Informational PAGE 4
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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
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 [RFC 1122].
References
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.
Lahey Informational PAGE 5
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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.
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.
2.2.
Name of Problem
Stretch ACK due to PMTUD
Classification
Congestion Control / Performance
Description
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.
Significance
Stretch ACKs have a variety of unfortunate effects, more fully
outlined in [RFC 2525]. 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.
Implications
The complete implications of stretch ACKs are outlined in
[RFC 2525].
Lahey Informational PAGE 6
RFC 2923 TCP Problems with Path MTU Discovery September 2000
Relevant RFCs
RFC 1122 outlines the requirements for frequency of ACK
generation. [RFC 2581] expands on this and clarifies that delayed
ACK is a SHOULD, not a MUST.
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)
Lahey Informational PAGE 7
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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.
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)
Lahey Informational PAGE 8
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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.)
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 connection.
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 [RFC 813].
2.3.
Name of Problem
Determining MSS from PMTU
Classification
Performance
Description
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.
Significance
The advertised MSS is an indication to the remote system about the
largest TCP segment that can be received [RFC 879]. If this value is
too small, the remote system will be forced to use a smaller segment
size when sending, purely because the local system found a particular
PMTU earlier.
Lahey Informational PAGE 9
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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.
Implications
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. [RFC 879] 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: 129.99.238.5 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)
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
Lahey Informational PAGE 10
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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
packets.
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: 129.99.238.5 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
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)
Lahey Informational PAGE 11
RFC 2923 TCP Problems with Path MTU Discovery September 2000
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 out.
How to fix
The MSS should be determined based on the MTUs of the interfaces on
the system, as outlined in [RFC 1122] and [RFC 1191].
3. 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.
4. 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 [RFC 2525]. Special thanks to
Amy Bock, who helped perform the PMTUD tests which discovered these
bugs.
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RFC 2923 TCP Problems with Path MTU Discovery September 2000
5. References
[RFC 2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC 1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC 813] Clark, D., "Window and Acknowledgement Strategy in TCP",
RFC 813, July 1982.
[Jacobson89] V. Jacobson, C. Leres, and S. McCanne, tcpdump, June
1989, ftp.ee.lbl.gov
[RFC 1435] Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", RFC 1435, March 1993.
[RFC 1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC
1191, November 1990.
[RFC 1981] McCann, J., Deering, S. and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.
[Paxson96] V. Paxson, "End-to-End Routing Behavior in the
Internet", IEEE/ACM Transactions on Networking (5),
pp.~601-615, Oct. 1997.
[RFC 2525] Paxon, V., Allman, M., Dawson, S., Fenner, W., Griner,
J., Heavens, I., Lahey, K., Semke, I. and B. Volz,
"Known TCP Implementation Problems", RFC 2525, March
1999.
[RFC 879] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC 879, November 1983.
[RFC 2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001,
January 1997.
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RFC 2923 TCP Problems with Path MTU Discovery September 2000
6. Author's Address
Kevin Lahey
dotRocket, Inc.
1901 S. Bascom Ave., Suite 300
Campbell, CA 95008
USA
Phone: +1 408-371-8977 x115
email: kml@dotrocket.com
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RFC 2923 TCP Problems with Path MTU Discovery September 2000
7. Full Copyright Statement
Copyright © The Internet Society (2000). 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
English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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TCP Problems with Path MTU Discovery
RFC TOTAL SIZE: 30976 bytes
PUBLICATION DATE: Thursday, September 14th, 2000
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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