netdev - Re: [PATCH 2/5] ipv4: Kill ip_rt_frag

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Date:	Wed, 13 Jun 2012 12:07:09 +0200
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 2/5] ipv4: Kill ip_rt_frag_needed().

On Wed, Jun 13, 2012 at 02:42:25AM -0700, David Miller wrote:
> From: Steffen Klassert <steffen.klassert@...unet.com>
> Date: Wed, 13 Jun 2012 10:01:52 +0200
> 
> > I think an application that sets IP_PMTUDISC_WANT explicitly will
> > rely on the fact that the kernel does pmtu discovery. Changing
> > the socket setting to IP_PMTUDISC_DONT the first time we get into
> > trouble makes IP_PMTUDISC_WANT pointless for udp and raw sockets.
> 
> How so?
> 
> We are mimicking exactly what would happen if we had just created
> a new routing cache entry when the application openned the socket.
> 
> There is no behavioral difference whatsoever.
> 
> We absolutely do perform PMTU discovery, the first large packet
> will trigger it.  And then, as if we had lowered the PMTU in
> the routing cache entry, we will stop setting DF in the packets.

Maybe I missunderstood what you meant. I thought that you don't want
to update the pmtu cache informations at all on udp and raw.
If we update the pmtu cache informations with first large packet,
I agree absolutely.

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