* Problems with tg3 driver after lowering the MTU
@ 2010-01-21 15:21 Szilveszter Ordog
2010-01-21 15:27 ` Ben Hutchings
0 siblings, 1 reply; 6+ messages in thread
From: Szilveszter Ordog @ 2010-01-21 15:21 UTC (permalink / raw)
To: netdev
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?
slipszi
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Problems with tg3 driver after lowering the MTU
2010-01-21 15:21 Problems with tg3 driver after lowering the MTU Szilveszter Ordog
@ 2010-01-21 15:27 ` Ben Hutchings
2010-01-21 15:42 ` Szilveszter Ordog
0 siblings, 1 reply; 6+ messages in thread
From: Ben Hutchings @ 2010-01-21 15:27 UTC (permalink / raw)
To: Szilveszter Ordog; +Cc: netdev
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.
Ben.
--
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Problems with tg3 driver after lowering the MTU
2010-01-21 15:27 ` Ben Hutchings
@ 2010-01-21 15:42 ` Szilveszter Ordog
2010-01-21 17:47 ` Templin, Fred L
0 siblings, 1 reply; 6+ messages in thread
From: Szilveszter Ordog @ 2010-01-21 15:42 UTC (permalink / raw)
To: netdev
On Thu, Jan 21, 2010 at 16:27, Ben Hutchings <bhutchings@solarflare.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.
slipszi
^ permalink raw reply [flat|nested] 6+ messages in thread
* RE: Problems with tg3 driver after lowering the MTU
2010-01-21 15:42 ` Szilveszter Ordog
@ 2010-01-21 17:47 ` Templin, Fred L
2010-01-21 18:30 ` Ben Hutchings
0 siblings, 1 reply; 6+ messages in thread
From: Templin, Fred L @ 2010-01-21 17:47 UTC (permalink / raw)
To: Szilveszter Ordog, netdev@vger.kernel.org
> -----Original Message-----
> From: netdev-owner@vger.kernel.org [mailto:netdev-owner@vger.kernel.org] On Behalf Of Szilveszter
> Ordog
> Sent: Thursday, January 21, 2010 7:42 AM
> To: netdev@vger.kernel.org
> Subject: Re: Problems with tg3 driver after lowering the MTU
>
> On Thu, Jan 21, 2010 at 16:27, Ben Hutchings <bhutchings@solarflare.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@boeing.com
> slipszi
> --
> To unsubscribe from this list: send the line "unsubscribe netdev" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
^ permalink raw reply [flat|nested] 6+ messages in thread
* RE: Problems with tg3 driver after lowering the MTU
2010-01-21 17:47 ` Templin, Fred L
@ 2010-01-21 18:30 ` Ben Hutchings
2010-01-21 23:02 ` Templin, Fred L
0 siblings, 1 reply; 6+ messages in thread
From: Ben Hutchings @ 2010-01-21 18:30 UTC (permalink / raw)
To: Templin, Fred L; +Cc: Szilveszter Ordog, netdev@vger.kernel.org
On Thu, 2010-01-21 at 09:47 -0800, Templin, Fred L wrote:
> > -----Original Message-----
> > From: netdev-owner@vger.kernel.org [mailto:netdev-owner@vger.kernel.org] On Behalf Of Szilveszter
> > Ordog
> > Sent: Thursday, January 21, 2010 7:42 AM
> > To: netdev@vger.kernel.org
> > Subject: Re: Problems with tg3 driver after lowering the MTU
> >
> > On Thu, Jan 21, 2010 at 16:27, Ben Hutchings <bhutchings@solarflare.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.
In theory, MRU and MTU could be quite separate. However, there is no
standard interface to set an interface's MRU, so where the hardware
supports a variable MRU drivers normally set it based on the MTU.
Ben.
--
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.
^ permalink raw reply [flat|nested] 6+ messages in thread
* RE: Problems with tg3 driver after lowering the MTU
2010-01-21 18:30 ` Ben Hutchings
@ 2010-01-21 23:02 ` Templin, Fred L
0 siblings, 0 replies; 6+ messages in thread
From: Templin, Fred L @ 2010-01-21 23:02 UTC (permalink / raw)
To: Ben Hutchings; +Cc: Szilveszter Ordog, netdev@vger.kernel.org
Ben,
> -----Original Message-----
> From: Ben Hutchings [mailto:bhutchings@solarflare.com]
> Sent: Thursday, January 21, 2010 10:30 AM
> To: Templin, Fred L
> Cc: Szilveszter Ordog; netdev@vger.kernel.org
> Subject: RE: Problems with tg3 driver after lowering the MTU
>
> On Thu, 2010-01-21 at 09:47 -0800, Templin, Fred L wrote:
> > > -----Original Message-----
> > > From: netdev-owner@vger.kernel.org [mailto:netdev-owner@vger.kernel.org] On Behalf Of Szilveszter
> > > Ordog
> > > Sent: Thursday, January 21, 2010 7:42 AM
> > > To: netdev@vger.kernel.org
> > > Subject: Re: Problems with tg3 driver after lowering the MTU
> > >
> > > On Thu, Jan 21, 2010 at 16:27, Ben Hutchings <bhutchings@solarflare.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.
>
> In theory, MRU and MTU could be quite separate. However, there is no
> standard interface to set an interface's MRU, so where the hardware
> supports a variable MRU drivers normally set it based on the MTU.
In RFC1122, EMTU_S (i.e., the MTU) and EMTU_R (i.e., the
MRU) are maintained separately and are not specified as
dependent on one another directly. Intuitively however,
the relationship MTU <= MRU seems appropriate, and consistent
with a number of specs that require the reassembly size to
be no smaller than the interface MTU. The term "MRU" itself
came from RFC1661 (PPP) where it is the MRU and not MTU that
is negotiated.
So I see evidence to support the assertion that MTU <= MRU,
but I don't see much evidence to support the assertion that
MTU == MRU. Besides, if a packet arrives that is larger
than MTU but no larger than MRU, why throw away good data?
Fred
fred.l.templin@boeing.com
> Ben.
>
> --
> Ben Hutchings, Senior Software Engineer, Solarflare Communications
> Not speaking for my employer; that's the marketing department's job.
> They asked us to note that Solarflare product names are trademarked.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2010-01-21 15:21 Problems with tg3 driver after lowering the MTU Szilveszter Ordog
2010-01-21 15:27 ` Ben Hutchings
2010-01-21 15:42 ` Szilveszter Ordog
2010-01-21 17:47 ` Templin, Fred L
2010-01-21 18:30 ` Ben Hutchings
2010-01-21 23:02 ` Templin, Fred L
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