From: Rick Jones <rick.jones2@hp.com>
To: Lloyd Standish <lloyd@crnatural.net>
Cc: netfilter@vger.kernel.org
Subject: Re: MTU question
Date: Fri, 20 Apr 2012 10:30:19 -0700 [thread overview]
Message-ID: <4F919D2B.70206@hp.com> (raw)
In-Reply-To: <op.wc2nftsax1lyi3@debiandesk2.net>
On 04/20/2012 09:15 AM, Lloyd Standish wrote:
> Hi,
>
> I have a question about MTU. Suppose my PC's Ethernet interface is
> configured at 1500 MTU, and it connects to a router interface configured
> to use 1492 MTU.
>
> Suppose my PC needs to send packets to the router. Given these MTU
> settings, which of these 2 is correct?
>
> (a) my PC will only send 1492 byte packets maximum (because the
> destination interface is only 1492), or
> (b) the PC will send packets up to 1500 bytes in size, but the router
> will split the packets before sending them on
>
> I have been assuming (a) is correct.
I suspect the answer is some variation on:
c) Since the two systems are in the same broadcast domain, with no
intervening router to perform PathMTU discovery, the PC will send IP
datagrams up to 1500 bytes, which likely as not will be dropped by the
"router" as being too large before it even gets to IP on the router.
I would expect that only if your "router" has a 1500 byte MTU on the
interface facing the PC, and a 1492 byte MTU on the interface facing the
"internet" would you see the PathMTU discovery activity which would
cause the PC to adjust the effective MTU it was using to match 1492.
Only if the IPv4 DF (Don't Fragment) bit is clear should the "router"
split the packets.
rick jones
> Thanks in advance.
>
> --
> Lloyd
> --
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next prev parent reply other threads:[~2012-04-20 17:30 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-20 16:15 MTU question Lloyd Standish
2012-04-20 17:30 ` Rick Jones [this message]
2012-04-22 14:32 ` Jan Engelhardt
2012-04-20 17:41 ` Steven Kath
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