IPv6 TCP transfers are hanging

[Kevin Oberman]

> Date: Thu, 13 Jan 2005 07:51:16 +0100
> From: Marian Durkovic <md at bts.sk>
> 
> Hi,
> 
> > 08:20:12.680313 aaa.es.net.ssh > bbb.es.net.54854: . 10145:11573(1428) 
> > ack 31040 win 58548 <nop,nop,timestamp 9068204 69095244> [flowlabel 0x6ae53] 
> > len 1460, hlim 64)
> 
> 1428 bytes is the proper payload length for TCPv6 with options
> 1448 bytes is the same for TCPv4
> 
> 1440 bytes is the payload length for TCPv6 without options
> 1460 byts is the same for TCPv4
> 
> If such packets are failing, your path MTU is probably not 1500 bytes but
> smaller.

Indeed, that is the case. One link is via a GRE tunnel and that is
restricting MTU.

What I don't understand is how this is happening. tcp.mssdflt is set to
1400 bytes and tcp.v6mssdflt is only 1024. If PMTU discovery was
working, this should not be a problem. If PMTU discovery is not used,
the MSS defaults should keep the packets small enough to work over the
tunnel.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634