Re: [Bluez-devel] Re: [Bluez-users] How do I obtain a free PSM automatically?

[Max Krasnyansky <maxk@qualcomm.com>]

At 06:14 PM 2/8/2003 +0200, Alexandros Karypidis wrote:
>BTW, it dawned on me that:
>
>1) connect() succeeds even if bind() wasn't previously called for the client 
>socket.
>2) accept() also produces a new socket.
>
>I assumed that these operations evidently select a free PSM for the sockets 
>they use/produce. I could not find any such thing in the code however (but I 
>don't really know the BlueZ code).
>
>Then, I did the simple tests of:
>
>A. Calling getsockname() after a connect() with no previous bind()
>B. Calling getsockname() after a accept()
>
>Case A reported PSM zero (0) !!! This is clearly a bug!
No it's _not_. 
L2CAP PSM != TCP/UDP port. Although it may look like it's the same thing. It's not. 

>Case B reported PSM 5000. This is the PSM where the server socket on which 
>accept() was called was bound. So:
>
>        clsock = accept (srvsock, ...);
>
>Produces a clsock whose PSM is equal to that of srvsock. I don't know if this 
>is correct or not.
Yes it is correct.

L2CAP PSM identifies a protocol or a service. CID identifies a logical channel.
Since both client and server sockets use the _same_ protocol they must have the 
same PSM.

TCP/UDP
        [client port]:[client IP]  ---> [server port]:[server IP]

L2CAP
        [service PSM]:[client CID] ---> [service PSM]:[server CID]

See the difference ?
Don't ask me why L2CAP designers didn't stick to common port concept.
Bluetooth spec is full of "innovations" ;-).

So the current code is correct.

Now if people really want dynamic PSMs. We could introduce something like
bind(psm == 0xffff).

Max



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