Hi,
I think there is a problem with Path MTU discovery and Linux 2.2.5.
It could be incorrect ICMP being sent or a bug in TCP.
Please contact me if you want more info. Raw tcpdump data available.
Greetings,
Rob van Nieuwkerk
The three machines involved:
----------------------------
delphi: dual CPU Alpha, Linux-2.2.5-ac5 SMP
wormhole: Cyrix 486 DX2, Linux-2.2.5-ac5
cyborg: HPUX 9.*
network setup:
--------------
delphi
|
| Ethernet MTU=1500
|
blackhole
|
| Ethernet MTU=1500
|
wormhole
|
| PPP MTU=576
|
blackdance
|
| Ethernet MTU=1500
|
cyborg
Problem description:
--------------------
Delphi starts a newsfeed connection to cyborg.
Wormhole sends a ICMP "Dest unreachable - need to frag" when delphi
tries to send something bigger than the PPP MTU of 576.
Delphi seems to ignore this and does not lower its Path MTU for the
connection.
tcpdump taken on delphi:
------------------------
22:30:54.000000 delphi.1771 > cyborg.nntp: S 1178249675:1178249675(0) win 32120 <mss
1460,sackOK,timestamp 85429269 0,nop,wscale 0> (DF) [tos 0x2]
22:30:54.000000 cyborg.nntp > delphi.1771: S 1292608000:1292608000(0) ack 1178249676
win 8192 <mss 1460> [tos 0x2]
22:30:54.000000 delphi.1771 > cyborg.nntp: . ack 1 win 32120 (DF) [tos 0x2]
22:30:54.000000 cyborg.nntp > delphi.1771: P 1:71(70) ack 1 win 24576 [tos 0x2]
22:30:54.000000 delphi.1771 > cyborg.nntp: . ack 71 win 32120 (DF) [tos 0x2]
22:30:54.000000 delphi.1771 > cyborg.nntp: P 1:14(13) ack 71 win 32120 (DF) [tos 0x2]
22:30:55.000000 cyborg.nntp > delphi.1771: P 71:104(33) ack 14 win 24576 [tos 0x2]
22:30:55.000000 delphi.1771 > cyborg.nntp: P 14:49(35) ack 104 win 32120 (DF) [tos 0x2]
22:30:55.000000 cyborg.nntp > delphi.1771: P 104:109(5) ack 49 win 24576 [tos 0x2]
22:30:55.000000 delphi.1771 > cyborg.nntp: P 49:1363(1314) ack 109 win 32120 (DF) [tos
0x2]
22:30:55.000000 wormhole > delphi: icmp: cyborg unreachable - need to frag (mtu 576)
[tos 0xc2]
22:30:55.000000 cyborg.nntp > delphi.1771: P 104:109(5) ack 49 win 24576 [tos 0x2]
22:30:55.000000 delphi.1771 > cyborg.nntp: . ack 109 win 32120 (DF) [tos 0x2]
22:30:55.000000 delphi.1771 > cyborg.nntp: P 49:1363(1314) ack 109 win 32120 (DF) [tos
0x2]
22:30:55.000000 wormhole > delphi: icmp: cyborg unreachable - need to frag (mtu 576)
[tos 0xc2]
22:30:57.000000 delphi.1771 > cyborg.nntp: P 49:1363(1314) ack 109 win 32120 (DF) [tos
0x2]
22:30:57.000000 wormhole > delphi: icmp: cyborg unreachable - need to frag (mtu 576)
[tos 0xc2]
22:31:00.000000 delphi.1771 > cyborg.nntp: P 49:1363(1314) ack 109 win 32120 (DF) [tos
0x2]
22:31:00.000000 wormhole > delphi: icmp: cyborg unreachable - need to frag (mtu 576)
[tos 0xc2]
22:31:06.000000 delphi.1771 > cyborg.nntp: P 49:1363(1314) ack 109 win 32120 (DF) [tos
0x2]
22:31:06.000000 wormhole > delphi: icmp: cyborg unreachable - need to frag (mtu 576)
[tos 0xc2]
22:31:11.000000 wormhole > delphi: icmp: cyborg unreachable - need to frag (mtu 576)
[tos 0xc2]
22:31:17.000000 delphi.1771 > cyborg.nntp: P 49:1363(1314) ack 109 win 32120 (DF) [tos
0x2]
22:31:17.000000 wormhole > delphi: icmp: cyborg unreachable - need to frag (mtu 576)
[tos 0xc2]
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