Re: [PATCH 2/5] ipv4: Kill ip_rt_frag_needed().

From: David Miller <davem@davemloft.net>
Date: Wed, 13 Jun 2012 22:42:03 -0700 (PDT)

From: Steffen Klassert <steffen.klassert@secunet.com>
Date: Thu, 14 Jun 2012 07:35:29 +0200

quoted

With your patch applied, we stop setting the DF bit after we
received a 'need to frag' ICMP message, but we don't fragment. We
send the packets out unfragmented. Before we removed
ip_rt_frag_needed(), we did the fragmentation according to the pmtu
informations we got from the icmp message. Now the router with the
low mtu has to do the fragmentation.

Ok, then if we want to do the fragmentation locally then we have to
consider my initial patch which updates the PMTU in raw_err().

Did you test that?  I mean specifically, this patch:

http://marc.info/?l=linux-netdev&m=133945597319917&w=2

If it works for you, I will try to extend it to the other datagram
cases.

Actually, thinking some more, we could extend my inet->pmtudisc patch
to achieve a similar effect.

Essentially we'd have a socket local PMTU value for datagram sockets.

Would you be OK with that approach?

I like the inet->pmtudisc way of solving this problem, because it:

1) Requires no special code to "remember" the flow used for the last
   socket sendmsg() call.

2) In the events of a malicious attempt to poison the routing cache
   PMTU information, only one socket will be harmed, rather than
   the whole system.

I tried to look for inspiration in other systems, but all of them lack
source based routing and other things we support, so they just use
a purely destination address based cache for PMTU information.

Other systems also don't have to deal with SO_BINDTODEVICE which
influences the route.

So we absolutely have to make our PMTU operations with the full
context used to emit the packet.