Re: Re: 6to4/SIT and IP DF

Lähettäjä: Pekka Savola [off-list ref]
Päiväys: 2003/10/15 ke AM 09:28:28 GMT+03:00
Vastaanottaja: David Stevens [off-list ref]
Kopio: r.venning@telstra.com,  [off-list ref],  [off-list ref], 
        [off-list ref]
Aihe: Re: 6to4/SIT and IP DF

On Tue, 14 Oct 2003, David Stevens wrote:

quoted

I was trying out 6to4 and noticed that the v4 encapsulating header has DF
set, which RFC3056 says should not be set.

Because ICMPv4 won't, in general, include enough packet to determine the
original v6 sender, end-to-end PMTU won't work. The possible use I could
see is if the tunnel MTU is modified based on the PTMU (I didn't check),
but  that's probably not a good idea for any tunnels  that have "any" as
the remote v4 address. Doing that would force all MTU's to the lowest of
any v4 destination's path.

So, I think it's appropriate to always clear IP DF in the IPv4 header
generated by SIT, but I thought I'd see if anyone else has a comment on
that before I submit the trivial patch. :-)

Any thoughts?

Seems like a good idea.  The only thing I'm worried about is when someone
is attached to a network of at least 1500 MTU (at IPv6 level), and uses
6to4 -- then basically every IPv6 packet over 1480 bytes will be
fragmented in the network, even though it could potentially be chopped to
smaller pieces already in the end-nodes.

Just wondering how our 6to4 implementation handles this case at the 
moment..

-- 

I think we send icmp TOOBIG by default for packets longer than (pmtu - iphdr).

--Mika