(usagi-users 02427) PMTU discovery broken?

[Sebastian Zimmermann <S.Zimmermann@xxxxxxxxxxxxx>]

Hi,

I have a strange problem here. I try to download a large file using
IPv6. Between both hosts is a firewall using the latest usagi stable
kernel. The end hosts use a vanilla 2.4.20 kernel.
The first packets are transmitted just fine, but then some router
between the hosts begins to generate ICMPv6 Type 2 packets (announcing
MTU 1476) and sends them to the sender of the file. In the firewall logs
I can see the ICMPv6 packet passing the firewall - they are *not*
blocked.
If I understand correctly, the sender should be reducing the packet size
accordingly. However, the MTU doesn't change and all subsequent packets
again cause ICMPv6 Type 2 packets. The download stalls (but sometimes
packets still go through!?).

When I do a separate ping6 from the receiver to the sender, it goes
through until I start the download and begin to see the ICMPv6 Type 2
packets. When I abort the download, ping6 works again.

When I set IPv6 MTU on the firewall to a value smaller than 1480, IPv6
doesn't work at all any more.

Something seems to be really broken with PMTU discovery. However,
everything worked fine before I put the firewall in between the two
hosts. The firewall only blocks some IPv4 traffic and UDP, but neither
ICMPv6 nor TCP/v6.

Can somebody explain to me what is going wrong here?

Thanks

Sebastian

P.S.: Please Cc: to my address as I am not subscribed to this list.
Thanks!