Public bug reported:
three domUs:
gateway, loginserver, webserver
mtu of webserver is 1500
[EMAIL PROTECTED]:~ $ ifconfig eth0
eth0 Protokoll:Ethernet Hardware Adresse 00:16:3E:39:F9:F4
inet Adresse:137.226.18.3 Bcast:137.226.18.255 Maske:255.255.255.0
inet6 Adresse: fe80::216:3eff:fe39:f9f4/64
Gültigkeitsbereich:Verbindung
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:3206004 errors:0 dropped:0 overruns:0 frame:0
TX packets:4090292 errors:0 dropped:0 overruns:0 carrier:0
Kollisionen:0 Sendewarteschlangenlänge:1000
RX bytes:317519642 (302.8 MiB) TX bytes:6339718363 (5.9 GiB)
Nevertheless during a TCP transfer:
16:16:24.303263 IP webserver.umic-mesh.net.www >
loginserver.umic-mesh.net.54364: . 10472538:10485570(13032) ack 114 win 91
<nop,nop,timestamp 63828762 63825762>
an IP packet of size 13032! I assume this is meant to improve TCP throughput
but this is totally broken! One can't just override the actual
MTU setting and begin to send with whatever sized packets seems fortunate.
As if this weren't bad enough, it is even ignoring ICMP fragmentatiation needed
messages... leading to a maximal TCP throughput of around 8 Kilobyte per
second, where the gateway is another domU.
After one ICMP the packet is sent with correct MTU, however the next packet
will be again sent with illegal MTU:
During a transfer from webserver through gateway to an outside machine:
16:24:33.788608 IP 137.226.12.150.33012 > 137.226.18.3.80: S
2873656164:2873656164(0) win 5840 <mss 1460,sackOK,timestamp 5405857
0,nop,wscale 6>
16:24:33.800995 IP 137.226.18.3.80 > 137.226.12.150.33012: S
3221591711:3221591711(0) ack 2873656165 win 5792 <mss 1460,sackOK,timestamp
63951128 5405857,nop,wscale 6>
16:24:33.788877 IP 137.226.12.150.33012 > 137.226.18.3.80: . ack 1 win 92
<nop,nop,timestamp 5405857 63951128>
16:24:33.788986 IP 137.226.12.150.33012 > 137.226.18.3.80: P 1:114(113) ack 1
win 92 <nop,nop,timestamp 5405857 63951128>
16:24:33.788994 IP 137.226.18.3.80 > 137.226.12.150.33012: . ack 114 win 91
<nop,nop,timestamp 63951128 5405857>
16:24:33.789171 IP 137.226.18.3.80 > 137.226.12.150.33012: . 1:2897(2896) ack
114 win 91 <nop,nop,timestamp 63951128 5405857>
16:24:33.789232 IP 137.226.18.1 > 137.226.18.3: ICMP 137.226.12.150 unreachable
- need to frag (mtu 1500), length 556
16:24:33.991430 IP 137.226.18.3.80 > 137.226.12.150.33012: . 1:1449(1448) ack
114 win 91 <nop,nop,timestamp 63951179 5405857>
16:24:33.992082 IP 137.226.12.150.33012 > 137.226.18.3.80: . ack 1449 win 137
<nop,nop,timestamp 5405908 63951179>
16:24:33.992091 IP 137.226.18.3.80 > 137.226.12.150.33012: . 2897:5793(2896)
ack 114 win 91 <nop,nop,timestamp 63951179 5405908>
16:24:33.992205 IP 137.226.18.1 > 137.226.18.3: ICMP 137.226.12.150 unreachable
- need to frag (mtu 1500), length 556
16:24:34.195401 IP 137.226.18.3.80 > 137.226.12.150.33012: . 1449:2897(1448)
ack 114 win 91 <nop,nop,timestamp 63951230 5405908>
16:24:34.195982 IP 137.226.12.150.33012 > 137.226.18.3.80: . ack 2897 win 182
<nop,nop,timestamp 5405959 63951230>
16:24:34.195989 IP 137.226.18.3.80 > 137.226.12.150.33012: . 5793:8689(2896)
ack 114 win 91 <nop,nop,timestamp 63951230 5405959>
16:24:34.196086 IP 137.226.18.1 > 137.226.18.3: ICMP 137.226.12.150 unreachable
- need to frag (mtu 1500), length 556
...
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
--
xen kernel does not honor mtu setting (network stack totally broken)
https://bugs.launchpad.net/bugs/238573
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