Comment 0 for bug 154271

etup: amd64, xen0 = ubuntu 7.10 ("gutsy"), xenU = ubuntu 7.10 ("gutsy"),
network config with xenU routing via xen0.

scp xenU:file . very slow, ~50KB/s
scp xen0:file . several MB/s

on xen0:
scp xenU:file . many MB/s

had a look at tcpdump, I see lines like this:
10:55:30.319389 IP (tos 0x8, ttl 64, id 59286, offset 0, flags [DF], proto TCP
(6), length 2948) xenU.22 > desktop: . 47785:50681(2896) ack 0 win 404
<nop,nop,timestamp 20681576 687514747>
10:55:30.319399 IP (tos 0xc8, ttl 64, id 52994, offset 0, flags [none], proto
ICMP (1), length 576) xen0 > xenU: ICMP desktop unreachable - need to frag
(mtu 1500), length 556 IP (tos 0x8, ttl 64, id 59286, offset 0, flags [DF],
proto TCP (6), length 2948) xenU.22 > desktop.41056: . 47785:50681(2896) ack
0 win 404 <nop,nop,timestamp 20681576 687514747>[|icmp]

so the xenU is sending packets with size 2948? why?
ifconfig on xenU:

eth0 Link encap:Ethernet HWaddr 00:16:3E:6D:03:2D
          inet addr:xenU Bcast:... Mask:255.255.255.255
          inet6 addr: fe80::216:3eff:fe6d:32d/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
          RX packets:34918213 errors:0 dropped:0 overruns:0 frame:0
          TX packets:34262839 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:12337072144 (11.4 GB) TX bytes:48486812526 (45.1 GB)

so the MTU is 1500 which is fine. xenU sending too big packets.