[Bluez-devel] Zero length L2CAP packets

[Bluez-devel] Zero length L2CAP packets

[Till Harbaum]

Hi,

while working on the avetana jsr82 implementation we discovered, that the bt 
spec as well as jsr82 allow to transmit zero length l2cap packets. IMHO the 
linux socket interface does not support zero length packets and a read 
returning zero indicates an end of file (in this case meaning that the 
connection has been closed).

What happens if the other side actually sends a zero length packet? Is this 
dropped inside the kernel? Or will read return zero which i'll misinterpret 
as the eof marker? How can i distinguish between eof and a zero length 
packet?

Regards,
  Till

-- 
Dr.-Ing. Till Harbaum                       Tel.:  +49 721 4998963
BeeCon GmbH                                 Fax:   +49 721 4998962
Haid-und-Neu Strasse 7, 76131 Karlsruhe     Mobil: +49 179 9087904
harbaum@beecon.de                           http://www.beecon.de


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Re: [Bluez-devel] Zero length L2CAP packets

[Marcel Holtmann]

Hi Till,

> while working on the avetana jsr82 implementation we discovered, that the bt 
> spec as well as jsr82 allow to transmit zero length l2cap packets. IMHO the 
> linux socket interface does not support zero length packets and a read 
> returning zero indicates an end of file (in this case meaning that the 
> connection has been closed).
> 
> What happens if the other side actually sends a zero length packet? Is this 
> dropped inside the kernel? Or will read return zero which i'll misinterpret 
> as the eof marker? How can i distinguish between eof and a zero length 
> packet?

actually I see no problem with zero length L2CAP packets. I modified the
l2test program to actually be able to send and receive them, but there
is no problem at all. This is a socket and not a file and so you might
use poll() or select() to detect the hangup of the connection and not
some weird zero length magic.

The kernel btw is dropping no packets. I see all of these zero packets
flying from one end to the other.

Regards

Marcel




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Re: [Bluez-devel] Zero length L2CAP packets

[Marcel Holtmann]

Hi Moritz,

> I just tried writing a 0-length packet on my open L2CAP Connection and 
> I cannot see anything happening with hcidump. The remote end is a Mac. 
> But I guess that I should see anything regardless of the remote end. 
> 1-byte packets and more get transferred correctly but
> 
> write (fd, "foo", 0);
> 
> on my open L2CAP Descriptor fd does not give any reaction in hcidump.
> 
> my version of bluez is libbluetooth1 2.11-1

update the library and utils and also the kernel. Maybe you need to use
send() for sending 0 byte payload, because this is what is used in the
l2test program.

Regards

Marcel




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