netdev - RE: Problems with tg3 driver after lowering the MTU

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Message-ID: <E1829B60731D1740BB7A0626B4FAF0A6467F0DA0D5@XCH-NW-01V.nw.nos.boeing.com>
Date:	Thu, 21 Jan 2010 09:47:03 -0800
From:	"Templin, Fred L" <Fred.L.Templin@...ing.com>
To:	Szilveszter Ordog <slipszi@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: Problems with tg3 driver after lowering the MTU

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On Behalf Of Szilveszter
> Ordog
> Sent: Thursday, January 21, 2010 7:42 AM
> To: netdev@...r.kernel.org
> Subject: Re: Problems with tg3 driver after lowering the MTU
> 
> On Thu, Jan 21, 2010 at 16:27, Ben Hutchings <bhutchings@...arflare.com> wrote:
> > On Thu, 2010-01-21 at 16:21 +0100, Szilveszter Ordog wrote:
> >> After the MTU is lowered (e.g. to 1420) on a tg3-driven interface,
> >> received packets larger than that (e.g. 1500 bytes) are silently
> >> discarded by the driver. Therefore the system doesn't send ICMP
> >> fragmentation-needed packets and the other side doesn't detect this
> >> condition.
> >>
> >> Is this a known bug?
> >
> > I don't believe this a bug.  Within a local network, MTU should be set
> > the same for all interfaces.  Routers that connect networks with
> > different MTUs will generate the fragmentation-needed message as
> > appropriate.
> 
> Other drivers do not behave like that. Most of them always allow
> packets smaller than 1500 bytes. That is why I think that this is a
> bug.

The question seems to be what happens when the
Maximum Transmission Unit (MTU) is smaller than
the Maximum Receive Unit (MRU)?

For TCP, MSS is calculated based on MTU (not MRU)
so there should not be any size mismatches with TCP.
For large packets of other IP protocols, it should
be OK for the end system to receive a packet larger
than it is capable of sending. So, it seems to me
that packets that are larger than MTU but no larger
than MRU should be accepted.

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

> slipszi
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