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Date: Wed, 13 Jun 2012 22:59:41 -0700 (PDT)
From: David Miller <davem@...emloft.net>
To: steffen.klassert@...unet.com
Cc: netdev@...r.kernel.org
Subject: Re: [PATCH 2/5] ipv4: Kill ip_rt_frag_needed().
From: David Miller <davem@...emloft.net>
Date: Wed, 13 Jun 2012 22:42:03 -0700 (PDT)
> From: Steffen Klassert <steffen.klassert@...unet.com>
> Date: Thu, 14 Jun 2012 07:35:29 +0200
>
>> 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.
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