[lvs-users] lvs-tun MTU and fragmentation

[Sameer Garg]

Hi All,

   In my quest for setting up LVS-Tun we encountered another problem.
MTU and fragmentation is a known problem. First some back ground
information.

1. We can't use LVS-DR because our machines are all over the place.
(Thank the ISP)
2. The setup comprises of 2 machines. 1 director and 1 Real Server.
3. We are using RHEL 4.


The setup works when the packet size is small. When the packet size
increase (because of cookies) we experience timeouts.

I have gone through the MTU fragmentation section of the Howto but
haven't been able to solve the problem. We changed the MTU value on
the director, RS and  tunl0 individually and combined but that didn't
help either.

Am pasting the tcpdump output on the director.

17:09:49.348719 122.162.81.169.50120 > 198.65.11.8.http: S [tcp sum
ok] 2024274182:2024274182(0) win 65535 <mss 1452,nop,wscale
1,nop,nop,timestamp 51880267 0,sackOK,eol> (DF) (ttl 51, id 43796, len
64)
17:09:49.645469 122.162.81.169.50120 > 198.65.11.8.http: . [tcp sum
ok] ack 2909078 win 33120 <nop,nop,timestamp 51880565 881526975> (DF)
(ttl 51, id 43809, len 52)
17:09:49.670865 122.162.81.169.50120 > 198.65.11.8.http: .
0:1440(1440) ack 1 win 33120 <nop,nop,timestamp 51880565 881526975>
(DF) (ttl 51, id 43810, len 1492)
17:09:49.670892 198.65.11.8 > 122.162.81.169: icmp: 198.65.11.8
unreachable - need to frag (mtu 1480) [tos 0xc0]  (ttl 64, id 64006,
len 576)
17:09:49.670867 122.162.81.169.50120 > 198.65.11.8.http: P [tcp sum
ok] 1440:1446(6) ack 1 win 33120 <nop,nop,timestamp 51880565
881526975> (DF) (ttl 51, id 43811, len 58)
17:09:50.770837 122.162.81.169.50120 > 198.65.11.8198.65.11.8.http: .
0:1440(1440) ack 1 win 33120 <nop,nop,timestamp 51881662 881527298>
(DF) (ttl 51, id 43856, len 1492)
17:09:50.770859 198.65.11.8 > 122.162.81.169: icmp: 198.65.11.8
unreachable - need to frag (mtu 1480) [tos 0xc0]  (ttl 64, id 64007,
len 576)
17:09:52.758639 122.162.81.169.50120 > 198.65.11.8.http: .
0:1440(1440) ack 1 win 33120 <nop,nop,timestamp 51883656 881527298>
(DF) (ttl 51, id 43938, len 1492)
17:09:52.758660 198.65.11.8 > 122.162.81.169: icmp: 198.65.11.8
unreachable - need to frag (mtu 1480) [tos 0xc0]  (ttl 64, id 64008,
len 576)
17:09:56.548754 122.162.81.169.50120 > 198.65.11.8.http: .
0:1440(1440) ack 1 win 33120 <nop,nop,timestamp 51887444 881527298>
(DF) (ttl 51, id 44078, len 1492)
17:09:56.548781 198.65.11.8 > 122.162.81.169: icmp: 198.65.11.8
unreachable - need to frag (mtu 1480) [tos 0xc0]  (ttl 64, id 64009,
len 576)

If needed I can post the configuration file of ldirectord.

Thanks.

Sameer