softmac mtu

softmac mtu

[Matthieu CASTET]

Hi,

why softmac (and maybe device using linux 80211 stack) can't increase
their mtu above 1500 ?

IRRC 802.11 allow to send bigger frame. Moreover some driver like airo
allow to use mtu biger than 2000.

thanks,


Matthieu

Re: softmac mtu

[Larry Finger]

Matthieu CASTET wrote:
> Hi,
> 
> why softmac (and maybe device using linux 80211 stack) can't increase
> their mtu above 1500 ?
> 
> IRRC 802.11 allow to send bigger frame. Moreover some driver like airo
> allow to use mtu biger than 2000.

The maximum value for MTU is set in include/linux/if_ether.h for all ethernet-type communications, 
not in softmac or ieee80211. I doubt that one could easily change the number. It may be that the 
802.11 standard allows bigger frames, but it looks to me as if Linux does not.

Larry

Re: softmac mtu

[David Miller]

From: Larry Finger <Larry.Finger@lwfinger.net>
Date: Sat, 23 Sep 2006 16:40:15 -0500

> The maximum value for MTU is set in include/linux/if_ether.h for all
> ethernet-type communications, not in softmac or ieee80211. I doubt
> that one could easily change the number. It may be that the 802.11
> standard allows bigger frames, but it looks to me as if Linux does
> not.


Not correct.  Linux is perfectly fine with setting 9000 byte MTU on
ethernet devices that support it, and in fact just about every
gigabit ethernet driver supports it.

That macro you see in if_ether.h is just the value of the base MTU
limit, so larger MTU settings are easily allowable on a per-device
basis.

Re: softmac mtu

[Larry Finger]

David Miller wrote:
> From: Larry Finger <Larry.Finger@lwfinger.net>
> Date: Sat, 23 Sep 2006 16:40:15 -0500
> 
>> The maximum value for MTU is set in include/linux/if_ether.h for all
>> ethernet-type communications, not in softmac or ieee80211. I doubt
>> that one could easily change the number. It may be that the 802.11
>> standard allows bigger frames, but it looks to me as if Linux does
>> not.
> 
> 
> Not correct.  Linux is perfectly fine with setting 9000 byte MTU on
> ethernet devices that support it, and in fact just about every
> gigabit ethernet driver supports it.
> 
> That macro you see in if_ether.h is just the value of the base MTU
> limit, so larger MTU settings are easily allowable on a per-device
> basis.

Where/how does the device allow it? When I tried 'ifconfig eth0 mtu 2000' on my VIA Technologies, 
Inc. VT6102 [Rhine-II] wired controller, I got a 'SIOCSIFMTU: Invalid argument' message, which is 
the same message I get on my BCM4306 wireless card.

Larry

Re: softmac mtu

[Arnaldo Carvalho de Melo]

On 9/23/06, Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com> wrote:
> On 9/23/06, Larry Finger <Larry.Finger@lwfinger.net> wrote:
> > David Miller wrote:
> > > From: Larry Finger <Larry.Finger@lwfinger.net>
> > > Date: Sat, 23 Sep 2006 16:40:15 -0500
> > >
> > >> The maximum value for MTU is set in include/linux/if_ether.h for all
> > >> ethernet-type communications, not in softmac or ieee80211. I doubt
> > >> that one could easily change the number. It may be that the 802.11
> > >> standard allows bigger frames, but it looks to me as if Linux does
> > >> not.
> > >
> > >
> > > Not correct.  Linux is perfectly fine with setting 9000 byte MTU on
> > > ethernet devices that support it, and in fact just about every
> > > gigabit ethernet driver supports it.
> > >
> > > That macro you see in if_ether.h is just the value of the base MTU
> > > limit, so larger MTU settings are easily allowable on a per-device
> > > basis.
> >
> > Where/how does the device allow it? When I tried 'ifconfig eth0 mtu 2000' on my VIA Technologies,
> > Inc. VT6102 [Rhine-II] wired controller, I got a 'SIOCSIFMTU: Invalid argument' message, which is
> > the same message I get on my BCM4306 wireless card.
>
> David didn't said 1500 all the way to 9000, he said that some drivers
> support 9000, some don't, lemme check for ya which one does...


drivers/net/8139cp.c: max is 4096
drivers/net/acenic.c: 9000

just do a:

vi $(find drivers/net | xargs grep -l change_mtu)

and check the rest :-)

- Arnaldo

Re: softmac mtu

[David Miller]

From: Larry Finger <Larry.Finger@lwfinger.net>
Date: Sat, 23 Sep 2006 16:59:48 -0500

> Where/how does the device allow it? When I tried 'ifconfig eth0 mtu
> 2000' on my VIA Technologies, Inc. VT6102 [Rhine-II] wired
> controller, I got a 'SIOCSIFMTU: Invalid argument' message, which is
> the same message I get on my BCM4306 wireless card.

It allows it in the device specific ->change_mtu() method.
Tigon3, for example, overrides this with it's own function
called tg3_change_mtu() which checks if the particular model
of the chip supports jumbo MTU and if so allows such a setting.

The VIA driver simply doesn't override that function, and uses
the default ethernet one because either that ethernet chip doesn't
support the larger MTU or the author simply hasn't gotten around
to implementing the override.

Re: softmac mtu

[Matthieu CASTET]

So why a special mtu handler is not implemented in 802.11 stack :
- 802.11 stack doesn't support bigger packet
- 802.11 stack doesn't support very bigger packet (with
802.11 fragmentation)
- 802.11 writers were lazy

Matthieu

Re: softmac mtu

[John W. Linville]

On Mon, Sep 25, 2006 at 07:17:31PM +0000, Matthieu CASTET wrote:
> So why a special mtu handler is not implemented in 802.11 stack :
> - 802.11 stack doesn't support bigger packet
> - 802.11 stack doesn't support very bigger packet (with
> 802.11 fragmentation)
> - 802.11 writers were lazy

I suspect the latter...patches welcome! :-)

-- 
John W. Linville
linville@tuxdriver.com

Re: softmac mtu

[Jiri Benc]

On Mon, 25 Sep 2006 16:01:32 -0400, John W. Linville wrote:
> On Mon, Sep 25, 2006 at 07:17:31PM +0000, Matthieu CASTET wrote:
> > So why a special mtu handler is not implemented in 802.11 stack :
> > - 802.11 stack doesn't support bigger packet
> > - 802.11 stack doesn't support very bigger packet (with
> > 802.11 fragmentation)
> > - 802.11 writers were lazy
> 
> I suspect the latter...patches welcome! :-)

d80211 supports larger MTU.

 Jiri

-- 
Jiri Benc
SUSE Labs

Re: softmac mtu

[castet.matthieu]

Hi,

Selon Jiri Benc <jbenc@suse.cz>:

> On Mon, 25 Sep 2006 16:01:32 -0400, John W. Linville wrote:
> > On Mon, Sep 25, 2006 at 07:17:31PM +0000, Matthieu CASTET wrote:
> > > So why a special mtu handler is not implemented in 802.11 stack :
> > > - 802.11 stack doesn't support bigger packet
> > > - 802.11 stack doesn't support very bigger packet (with
> > > 802.11 fragmentation)
> > > - 802.11 writers were lazy
> >
> > I suspect the latter...patches welcome! :-)
>
> d80211 supports larger MTU.
>
I have a patch for 802.11 linux stack.
It works for mtu like 2000 but if the packet is fragmented (with mtu = 3000 for
example) my usb dongle (zd1211) crash (doesn't work anymore until I replug it).
I need to investigate a bit.

Also I wonder what should be the max mtu.

Matthieu

Re: softmac mtu

[Johannes Berg]

On Wed, 2006-09-27 at 15:57 +0200, castet.matthieu@free.fr wrote:

> Also I wonder what should be the max mtu.

2304, I think, as that's
synonym sMaxMsduLng Integer = 2304; /* max octets in an MSDU */

But maybe I'm interpreting the spec wrongly?

johannes

Re: softmac mtu

[matthieu castet]

[-- Attachment #1: Type: text/plain, Size: 647 bytes --]

Johannes Berg wrote:
> On Wed, 2006-09-27 at 15:57 +0200, castet.matthieu@free.fr wrote:
> 
> 
>>Also I wonder what should be the max mtu.
> 
> 
> 2304, I think, as that's
> synonym sMaxMsduLng Integer = 2304; /* max octets in an MSDU */
> 

Yes but if it is bigger the frame get framented at the 802.11 layer : in 
theory we could put mtu (IP max packet size) a big as we want [1].
Also if I understand it correctly if encryption is used there is less 
space for IP data ?

If people want to play with it, I attach the current version of my 
(trivial) patch.


Matthieu

[1]
if mtu > 2304, (struct ieee80211_txb*)->nr_frags is bigger than one ;)

[-- Attachment #2: ieee80211mtu --]
[-- Type: text/plain, Size: 759 bytes --]

Index: linux/net/ieee80211/ieee80211_module.c
===================================================================
--- linux.orig/net/ieee80211/ieee80211_module.c	2006-09-25 22:37:20.000000000 +0200
+++ linux/net/ieee80211/ieee80211_module.c	2006-09-25 22:39:07.000000000 +0200
@@ -118,6 +118,14 @@
 			      &ieee->network_free_list);
 }
 
+static int ieee80211_change_mtu(struct net_device *dev, int new_mtu)
+{
+	if ((new_mtu < 68) || (new_mtu > 2400))
+		return -EINVAL;
+	dev->mtu = new_mtu;
+	return 0;
+}
+
 struct net_device *alloc_ieee80211(int sizeof_priv)
 {
 	struct ieee80211_device *ieee;
@@ -133,6 +141,7 @@
 	}
 	ieee = netdev_priv(dev);
 	dev->hard_start_xmit = ieee80211_xmit;
+	dev->change_mtu = ieee80211_change_mtu;
 
 	ieee->dev = dev;
 

Re: softmac mtu

[Johannes Berg]

On Wed, 2006-09-27 at 19:17 +0200, matthieu castet wrote:

> > 2304, I think, as that's
> > synonym sMaxMsduLng Integer = 2304; /* max octets in an MSDU */
> > 
> 
> Yes but if it is bigger the frame get framented at the 802.11 layer : in 
> theory we could put mtu (IP max packet size) a big as we want [1].

No, you're confusing MPDU and MSDU. Or I am :) Please try to read the
spec as well.

johannes

Re: softmac mtu

[matthieu castet]

Johannes Berg wrote:
> On Wed, 2006-09-27 at 19:17 +0200, matthieu castet wrote:
> 
> 
>>>2304, I think, as that's
>>>synonym sMaxMsduLng Integer = 2304; /* max octets in an MSDU */
>>>
>>
>>Yes but if it is bigger the frame get framented at the 802.11 layer : in 
>>theory we could put mtu (IP max packet size) a big as we want [1].
> 
> 
> No, you're confusing MPDU and MSDU. Or I am :) Please try to read the
> spec as well.
> 
Right,

Thanks for the clarification.


Matthieu

Re: softmac mtu

[Michael Buesch]

On Wednesday 27 September 2006 15:55, Johannes Berg wrote:
> On Wed, 2006-09-27 at 15:57 +0200, castet.matthieu@free.fr wrote:
> 
> > Also I wonder what should be the max mtu.
> 
> 2304, I think, as that's
> synonym sMaxMsduLng Integer = 2304; /* max octets in an MSDU */
> 
> But maybe I'm interpreting the spec wrongly?

Yes, I think so. ;)

I am pretty sure this is the maximum _fragment_ size.
But each packet (which max size is defined by the MTU) can
be fragmented. I don't know if there's a limit on the max
number of frags.

-- 
Greetings Michael.

Re: softmac mtu

[Jouni Malinen]

On Thu, Sep 28, 2006 at 05:35:17PM +0200, Michael Buesch wrote:
> On Wednesday 27 September 2006 15:55, Johannes Berg wrote:
> > On Wed, 2006-09-27 at 15:57 +0200, castet.matthieu@free.fr wrote:
> > > Also I wonder what should be the max mtu.
> > 
> > 2304, I think, as that's
> > synonym sMaxMsduLng Integer = 2304; /* max octets in an MSDU */
> > 
> > But maybe I'm interpreting the spec wrongly?
> 
> Yes, I think so. ;)
> 
> I am pretty sure this is the maximum _fragment_ size.
> But each packet (which max size is defined by the MTU) can
> be fragmented. I don't know if there's a limit on the max
> number of frags.

No it's not. How about reading the standard.. ;-) Take a look at how
MSDU and MPDU are defined.. MSDU can be fragmented into smaller MPDUs,
but the limit here is indeed for MSDU, not MPDU. In other words,
fragmentation in IEEE 802.11 is different from fragmentation in IP and
it is also used for different purpose.

-- 
Jouni Malinen                                            PGP id EFC895FA

Re: softmac mtu

[Johannes Berg]

On Thu, 2006-09-28 at 17:35 +0200, Michael Buesch wrote:

> I am pretty sure this is the maximum _fragment_ size.

But then why does it talk of MPDU and MSDU?

from 802.11:

synonym sMaxMsduLng Integer = 2304; /* max octets in an MSDU */

synonym sMaxMpduLng Integer =         /* max octets in an MPDU */
  (sMaxMsduLng + sMacHdrLng + sWdsAddLng + sWepAddLng + sCrcLng);

synonym sMinFragLng Integer = 256; /* min value for aMpduMaxLength */
synonym sMaxFragNum Integer =        /* maximum fragment number */
  (sMaxMsduLng / (sMinFragLng - sMacHdrLng - sCrcLng));

The last calculation implies that each frame can only be up to
sMaxMsduLng octets long even when fragmented.

also cf. 9.1.4:

   9.1.4 Fragmentation/defragmentation overview

   The process of partitioning a MAC service data unit (MSDU) or a MAC
   management protocol data unit (MMPDU) into smaller MAC level frames,
   MAC protocol data units (MPDUs), is called fragmentation.
   Fragmentation creates MPDUs smaller than the original MSDU or MMPDU
   length to increase reliability, by increasing the probability of
   successful transmission of the MSDU or MMPDU in cases where channel
   characteristics limit reception reliability for longer frames.

IOW, I read it as an MSDU is the data that is supposed to be transmitted
over the link and that can be at most 2304 bytes long, and then it may
be fragmented or not and is then copied into (an) MPDU(s).

> But each packet (which max size is defined by the MTU) can
> be fragmented. I don't know if there's a limit on the max
> number of frags.

There's only a 4 bit fragment counter ;) However, see above.

johannes

Re: softmac mtu

[Jouni Malinen]

On Thu, Sep 28, 2006 at 05:47:12PM +0200, Johannes Berg wrote:
> On Thu, 2006-09-28 at 17:35 +0200, Michael Buesch wrote:
> 
> > I am pretty sure this is the maximum _fragment_ size.
> 
> But then why does it talk of MPDU and MSDU?

Maybe because your understanding is closer to what the standard says..
;-)

> IOW, I read it as an MSDU is the data that is supposed to be transmitted
> over the link and that can be at most 2304 bytes long, and then it may
> be fragmented or not and is then copied into (an) MPDU(s).

Yes.

> > But each packet (which max size is defined by the MTU) can
> > be fragmented. I don't know if there's a limit on the max
> > number of frags.
> 
> There's only a 4 bit fragment counter ;) However, see above.

And 4 bits is quite enough for sending the maximum number of fragments
which is currently about 10 (256 octet min frag len and 2304 max MSDU
len with some extra needed to cover the extra header in each fragment).

-- 
Jouni Malinen                                            PGP id EFC895FA

Re: softmac mtu

[Daniel Drake]

castet.matthieu@free.fr wrote:
> I have a patch for 802.11 linux stack.
> It works for mtu like 2000 but if the packet is fragmented (with mtu = 3000 for
> example) my usb dongle (zd1211) crash (doesn't work anymore until I replug it).
> I need to investigate a bit.

zd1211rw requires explicit support for large frames due to the USB 
protocol in use. This is supported by the vendor driver but not zd1211rw 
just yet.

Daniel

softmac mtu

[Matthieu CASTET]

Hi,

why softmac (and maybe device using linux 80211 stack) can't increase
their mtu above 1500 ?

IRRC 802.11 allow to send bigger frame. Moreover some driver like airo
allow to use mtu biger than 2000.

thanks,


Matthieu