netdev - Re: UDP path MTU discovery

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Message-ID: <20100329201431.GH20695@one.firstfloor.org>
Date:	Mon, 29 Mar 2010 22:14:31 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Rick Jones <rick.jones2@...com>
Cc:	Andi Kleen <andi@...stfloor.org>, Glen Turner <gdt@....id.au>,
	netdev@...r.kernel.org
Subject: Re: UDP path MTU discovery

On Mon, Mar 29, 2010 at 10:01:42AM -0700, Rick Jones wrote:
> >In theory one could probably add some hack in the the kernel UDP code
> >to hold one packet and retransmit it immediately with fragments when
> >the ICMP comes in. However that would be quite far in behaviour from
> >traditional UDP and be considered very ugly. It could also mess up
> >congestion avoidance schemes done by the application. 
> >
> >Still might be preferable over rewriting zillions of applications?
> 
> But which of the last N datagrams sent by the application should be 
> retained for retransmission?  It could be scores if not hundreds of 
> datagrams depending on the behaviour of the application and the latency to 
> the narrow part of the network.

Yes, if there's a large window you lose. I guess it would make protocols
like DHCP work at least ("transactional UDP" as the original poster called it)

I don't know if it would fix enough applications to be worth 
implementing. The only way to find out would be to try I guess.
I don't have any better ideas.

> That the IPv6 specification was heavily "influenced" by "the router guys" 
> seems increasingly clear...

Yes it sounds like the IETF didn't completely think that through.

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