netdev - Re: UDP path MTU discovery

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Message-ID: <87bpe825z2.fsf@basil.nowhere.org>
Date:	Sun, 28 Mar 2010 10:41:37 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	David Miller <davem@...emloft.net>
Cc:	gdt@....id.au, netdev@...r.kernel.org
Subject: Re: UDP path MTU discovery

David Miller <davem@...emloft.net> writes:

> From: Glen Turner <gdt@....id.au>
> Date: Fri, 26 Mar 2010 10:32:31 +1030
>
>> This differs from TCP, where it is the kernel -- and not
>> the application -- which organises retransmission. On
>> receiving a ICMP Fragmentation Needed the kernel can
>> immediately re-probe the path MTU wiht no waiting for
>> an exponential timer to expire.
>
> So the argument is, the kernel TCP does retransmission smart,
> userspace UDP apps do it stupidly, so let's turn off the feature
> instead of fixing userspace.
>
> Right?
>
> Sorry, fix this correctly in the user apps.  Putting the
> blame on UDP path MTU discovery is placing it in the
> wrong spot.

It means though that all IPv6 UDP applications essentially have
to implement path mtu discovery support (which is non trivial) 

Will be likely a long time until they're all fixed.

Seems like a big hole not considered by the IPv6 designers?

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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