MTU over 1500 on WLAN ?

Hi!

WLAN packets are supposedly about 2300 bytes long, but
usually
the user data size is about 1500 bytes.
Is the difference 2300-1500 some kind of protocol overhead ?
Could MTU larger than 1500 be used ?

Thanks for any reply,
David Balažic

PS: Sending this for the third time, hopefully it makes it
to the list.

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On Wed, Jun 28, 2006 at 09:52:22AM +0200, David Balazic
wrote:
> WLAN packets are supposedly about 2300 bytes long, but
usually
> the user data size is about 1500 bytes.
> Is the difference 2300-1500 some kind of protocol
overhead ?
> Could MTU larger than 1500 be used ?

The only reason to clamp the MTU at 1500 bytes is that
people typically 
have 802.11 networks bridged with wired networks, which have
an MTU of 
1500.  If you're doing pure wireless-wireless (eg in an
adhoc situation) 
you can crank the MTU up higher, if so desired.  

Also, if you're routing rather than bridging, you can get
away with a
larger MTU, if only because the routers generally do the
right thing,
fragmenting the frames if necessary.

 - Solomon
-- 
Solomon Peachy                        solomonlinux-wlan.com
AbsoluteValue Systems                 http://www.linux-wlan.com
721-D North Drive                     +1 (321) 259-0737 
(office)
Melbourne, FL 32934                   +1 (321) 259-0286 
(fax)
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Quoting Solomon Peachy <solomonlinux-wlan.com>:

> On Wed, Jun 28, 2006 at 09:52:22AM +0200, David Balazic
wrote:
> > WLAN packets are supposedly about 2300 bytes long,
but usually
> > the user data size is about 1500 bytes.
> > Is the difference 2300-1500 some kind of protocol
overhead ?
> > Could MTU larger than 1500 be used ?
>
> The only reason to clamp the MTU at 1500 bytes is that
people typically
> have 802.11 networks bridged with wired networks, which
have an MTU of
> 1500.  If you're doing pure wireless-wireless (eg in
an adhoc situation)
> you can crank the MTU up higher, if so desired.

No I can't. That's why I'm asking.

ifconfig ra0 mtu 2000

returns an error (for all value over 1500).
Don't have the exact error message here, sorry.

This is with a ralink 2500 chipset (PCI card).
OS is Ubuntu 6.06.

Is this becasue of the driver ?
Or ifconfig limitation ?

Regards,
David

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On Thu, Jun 29, 2006 at 03:50:36PM +0200, David Balazic
wrote:
> No I can't. That's why I'm asking.
> 
> ifconfig ra0 mtu 2000

Yeah, but that's not linux-wlan-ng/prism2, which doesn't
have this
limitation.

Other chipsets may or may not impose MTU limitations, but
according to
802.11 the max frame length is 2336 bytes, including the
full 802.11
header and FCS, leaving 2312 bytes for the payload.  (well,
technically
2310 bytes if you're using QoS/802.11e/WME, and encryption
overhead may
eat up to 16 more bytes -- So call it 2294 bytes max
payload, worst
case..)

 - Solomon
-- 
Solomon Peachy                        solomonlinux-wlan.com
AbsoluteValue Systems                 http://www.linux-wlan.com
721-D North Drive                     +1 (321) 259-0737 
(office)
Melbourne, FL 32934                   +1 (321) 259-0286 
(fax)
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