[Bluez-users] L2CAP MTU and packet fragmentation?

[Bluez-users] L2CAP MTU and packet fragmentation?

[Stephen Strowes]

Hi,

Forgive me if this has been answered time and again, but I can't seem to
access the archives just now. Google's not helping me much either.

I have a bit of BlueZ code, which I have to make use of L2CAP sockets to
communicate between devices. I've seen that I can alter the MTU for
packets, but I'm surprised that packets don't get fragmented as per
plain UDP when a packet larger than the MTU is passed to send().

Have I missed a configuration option somewhere?

I could live with the MTU being 64k with no fragmentation, but I seem to
get quite a lot of corrupt packets over around 2kB.


Cheers,
Stephen.



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Re: [Bluez-users] L2CAP MTU and packet fragmentation?

[Marcel Holtmann]

Hi Stephen,

> Forgive me if this has been answered time and again, but I can't seem to
> access the archives just now. Google's not helping me much either.
> 
> I have a bit of BlueZ code, which I have to make use of L2CAP sockets to
> communicate between devices. I've seen that I can alter the MTU for
> packets, but I'm surprised that packets don't get fragmented as per
> plain UDP when a packet larger than the MTU is passed to send().
> 
> Have I missed a configuration option somewhere?

the L2CAP socket is a SOCK_SEQPACKET and you have to honor that MTU. It
is not a stream or a connection-less socket.

> I could live with the MTU being 64k with no fragmentation, but I seem to
> get quite a lot of corrupt packets over around 2kB.

I have no idea what you are talking about. The L2CAP layer is reliable
and the underlaying ACL link, too.

Regards

Marcel



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Re: [Bluez-users] L2CAP MTU and packet fragmentation?

[Stephen Strowes]

Hi,

On Mon, 2006-06-26 at 14:17 +0200, Marcel Holtmann wrote:
> > I could live with the MTU being 64k with no fragmentation, but I seem to
> > get quite a lot of corrupt packets over around 2kB.
> 
> I have no idea what you are talking about. The L2CAP layer is reliable
> and the underlaying ACL link, too.

Sorry, correction, I get a lot of lost packets. I assume either because
they've become corrupted, or perhaps because they never arrive.

I get between 20-40% packet loss with packet sizes around 3000 bytes,
and more loss as packet sizes increase. l2ping says 56% loss at 3500
bytes.

This loss is what prompted my question. This behaviour, actually, is why
I'm using L2CAP -- because it was likened to UDP in what I'd read
previously. Ironically, perhaps, it's just a little *too* lossy  :-)

I'll either bolt on some packet segmentation to my app, or use RFCOMM
for now.


Cheers,
S.



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Re: [Bluez-users] L2CAP MTU and packet fragmentation?

[Marcel Holtmann]

Hi Stephen,

> > > I could live with the MTU being 64k with no fragmentation, but I seem to
> > > get quite a lot of corrupt packets over around 2kB.
> > 
> > I have no idea what you are talking about. The L2CAP layer is reliable
> > and the underlaying ACL link, too.
> 
> Sorry, correction, I get a lot of lost packets. I assume either because
> they've become corrupted, or perhaps because they never arrive.
> 
> I get between 20-40% packet loss with packet sizes around 3000 bytes,
> and more loss as packet sizes increase. l2ping says 56% loss at 3500
> bytes.
> 
> This loss is what prompted my question. This behaviour, actually, is why
> I'm using L2CAP -- because it was likened to UDP in what I'd read
> previously. Ironically, perhaps, it's just a little *too* lossy  :-)

with l2ping you use the L2CAP signal channel and it happens that some
remote implementation don't handle this very well. Hence you might see a
loss percentage. This actually means unanswered echo requests.

If you actually use L2CAP data channel with SOCK_SEQPACKET then you
won't see a loss at all. It is a reliable channel.

> I'll either bolt on some packet segmentation to my app, or use RFCOMM
> for now.

Or help Martin Roehricht to extend L2CAP with retransmission and flow
control. Once this has been implemented it will be possible to have a
SOCK_STREAM on L2CAP like you have with RFCOMM.

Regards

Marcel



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Re: [Bluez-users] L2CAP MTU and packet fragmentation?

[Stephen D. Strowes]

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Hi,

> If you actually use L2CAP data channel with SOCK_SEQPACKET then you
> won't see a loss at all. It is a reliable channel.

Yeah, this is what I'm doing, but I'm seeing loss.

I'm following the code from http://people.csail.mit.edu/albert/bluez-intro/x556.html, but not fiddling with anything related to unreliability in section 4.3.2.

Trying the same between two different machines gets me the reliable data transfer.

Am I dealing with bad hardware here? Hardware with bad drivers (I'm dealing with Bluetooth built into a HP iPaq running Familiar Linux, kernel version 2.6.12)? Or an out of date bluez-utils (bluez-utils-nodbus package is version 2.14-r4)?


Cheers,
S.

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Re: [Bluez-users] L2CAP MTU and packet fragmentation?

[Marcel Holtmann]

Hi Stephen,

> > If you actually use L2CAP data channel with SOCK_SEQPACKET then you
> > won't see a loss at all. It is a reliable channel.
> 
> Yeah, this is what I'm doing, but I'm seeing loss.
> 
> I'm following the code from http://people.csail.mit.edu/albert/bluez-intro/x556.html, but not fiddling with anything related to unreliability in section 4.3.2.
> 
> Trying the same between two different machines gets me the reliable data transfer.
> 
> Am I dealing with bad hardware here? Hardware with bad drivers (I'm dealing with Bluetooth built into a HP iPaq running Familiar Linux, kernel version 2.6.12)? Or an out of date bluez-utils (bluez-utils-nodbus package is version 2.14-r4)?

you can't see loss on the L2CAP data channel. This is impossible.

Regards

Marcel



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Re: [Bluez-users] L2CAP MTU and packet fragmentation?

[Stephen D. Strowes]

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Hi,
> > Am I dealing with bad hardware here? Hardware with bad drivers (I'm dealing with Bluetooth 
> built into a HP iPaq running Familiar Linux, kernel version 2.6.12)? Or an out of date 
> bluez-utils (bluez-utils-nodbus package is version 2.14-r4)?
> 
> you can't see loss on the L2CAP data channel. This is impossible.

I'll put the packet loss down to bad hardware support then, since it's not a bluez problem.


Cheers,
S.

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