netdev - RE: UDP path MTU discovery

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Date:	Tue, 30 Mar 2010 08:58:32 -0700
From:	"Templin, Fred L" <Fred.L.Templin@...ing.com>
To:	Andi Kleen <andi@...stfloor.org>,
	"Edgar E. Iglesias" <edgar.iglesias@...il.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Rick Jones <rick.jones2@...com>, Glen Turner <gdt@....id.au>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: UDP path MTU discovery

> -----Original Message-----
> From: Andi Kleen [mailto:andi@...stfloor.org]
> Sent: Monday, March 29, 2010 11:20 PM
> To: Edgar E. Iglesias
> Cc: Andi Kleen; Templin, Fred L; Eric Dumazet; Rick Jones; Glen Turner; netdev@...r.kernel.org
> Subject: Re: UDP path MTU discovery
> 
> > If you don't want to hassle with all of that, the app can stick to
> > 1280 (or I guess for the extreme/lazy cases turn on fragmentation)..
> 
> See the early mails in this thread. This is about apps who can't
> limit themselves to 1280, but still don't want full blown PMTU.
> [They probably should, but it can be a lot of work]

Right. Some apps may need to send isolated packets that
are larger than the path MTU without invoking path MTU
discovery.

> The MTU would allow to force fragmentation on the sending host
> as a workaround similar to IPv4.

Right again. Unlike IPv4, however, IPv6 does not allow
in-the-network fragmentation. So when in doubt, apps
that need to send isolated packets that may violate the
path MTU should really perform host-based fragmentation
with a maximum fragment size of 1280. Isn't there a
socket option "IPV6_USE_MIN_MTU" that apps can use to
force fragmentation on large packets (RFC3542)?

Caveat - the app may have no way of knowing whether
the destination is capable of reassembling fragmented
packets larger than 1500...

Fred
fred.l.templin@...ing.com

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