IPv6 MTU discrovery -- how should it work?

[Lev Serebryakov]

Hello, Freebsd-net.

 My  home network is connected to IPv6 world with Hurricane Electric's
tunnel (IPv6 over IPv6, "gif").
 It has MTU 1280.

 Everything  works till packets are small -- for example, interactive
shell (ssh) session over IPv6 works great. But when I need to transfer
buil  of  bytes  from  server  to  local  network  (scp,  or even fast
scrolling of man page in interactive session) traffic hangs. It starts
to work after couple (5-10) minutes, but many applications has timeouts
less, that that, and it is very annoying in any case.

  When I connect to external server twice, and run tcpdump in one
session for another one, and then try to use second (sniffed) session
for bulk transfer, I can clearly see, that server tries to send IPv6
packets with size 1420 to my local  network  many  times  and  doesn't
get any answer, so it is MTU problem for sure, but how it could be fixed?

 Also, Youtube doesn't work over IPv6 with same symptoms, so it is not
only  my servers' problem, it looks like my local network (and tunnel)
problem.

 How should it work? Maybe, I'm filtering out something mandatory on firewall?

-- 
// Black Lion AKA Lev Serebryakov <lev at FreeBSD.org>