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

[David Miller <davem@davemloft.net>]

From: Steffen Klassert <steffen.klassert@secunet.com>
Date: Wed, 13 Jun 2012 12:07:09 +0200

> On Wed, Jun 13, 2012 at 02:42:25AM -0700, David Miller wrote:
>> From: Steffen Klassert <steffen.klassert@secunet.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.

We don't update the PMTU.

But we behave as if we did.

The only effect the IP_PMTUDISC_* values have is in deciding whether
to set the DF flag in the outgoing packets.