IPv6 TCP transfers are hanging

[Kevin Oberman]

I think I have found a problem with TCP when run over IPv6.

I set my MSS for TCP to 1460 to allow a full 1500 byte MTU to be
utilized on my systems. (Yes, I see that this does break some things
like communicating via links where PMTUD is blocked and one or more
links restrict MTU to some size less than 1500 bytes.

What I am specifically seeing is a packet being sent out with a TCP
length of 1460. While this is fine for IPv4, it's too back for IPv6 and,
as you might expect, the far end never receives this packet.

There is a sysctl for net.inet.tcp.v6mssdflt which is set to 1024. This
should be fine, but it appears that it is not being honored and the V4
value is always used.

Am I mis-analyzing things or is TCP at least a bit broken when running
over V6? (Or am I at fault for setting the large MSS because ti is
honored with v6 even though there is a separate sysctl for IPv6?
-- 
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