Take the 2-minute tour ×
Server Fault is a question and answer site for professional system and network administrators. It's 100% free, no registration required.

I have a server with a (sixxs) IPv6 tunnel and a local network behind it. The tunnel has MTU of 1470, and a prefix with this MTU is advertised by radvd, and picked up by the local client:

root@host:~# ip -6 route
2001:xxxx:xxxx::/64 dev eth1  proto kernel  metric 256  expires 298sec mtu 1470
fe80::/64 dev eth1  proto kernel  metric 256  mtu 1470
default via fe80::dad3:85ff:feaf:7e77 dev eth1  proto kernel  metric 1024  expires 28sec mtu 1470 hoplimit 64

The interface of the client has MTU of 1500, as usual. Now, when I transfer a file to a remote IPv6 host, the following happens (wireshark packet dump on the server, LAN interface, of relevant part):

15.034320 host -> remote SSHv2 Encrypted request packet len=2796
15.034408 server -> host ICMPv6 Too big
15.241163 host -> remote SSHv2 [TCP Retransmission] Encrypted request packet len=1398
15.252193 remote -> host TCP ssh > 58188 [ACK] Seq=2658 Ack=121902 Win=64128 Len=0 TSV=2205083594 TSER=4294965684
15.252480 host -> remote SSHv2 [TCP Retransmission] Encrypted request packet len=2796
15.252558 server -> host ICMPv6 Too big
15.461151 host -> remote SSHv2 [TCP Retransmission] Encrypted request packet len=1398

So, the host sends a packet of size 2796 (should not even be possible, the link MTU is 1500), and the server correctly replies with ICMPv6 Too big. The packet is then re-transmitted with the correct size and acknowledged. But then, the next packet is again too big, and the process repeats indefinitely, while the file is transferred at a snail's pace... What is happening here? The route cache shows that the MTU of the route is picked up correctly (IPv6 addresses replaced with names):

root@host:~# ip -6 route show cached
remote via fe80::dad3:85ff:feaf:7e77 dev eth1  metric 0 
    cache  mtu 1470 hoplimit 64
server via server dev eth1  metric 0 
    cache  mtu 1470
share|improve this question
add comment

1 Answer 1

up vote 1 down vote accepted

Okay, at home more weird issues were happening in the network. I took the Microsoft way and rebooted the server. Problem seems gone.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.