[asterisk-dev] DUNDi transaction ID generation

[Russell Bryant]

I'm just poking around pbx_dundi, and I can't seem to figure out this function 
that generates a new transaction ID.  A transaction ID in DUNDi is kind of like 
a call number in IAX2.  It is the identifier use to uniquely identify a session 
between two peers.

The transaction ID is a 16-bit number.  The DUNDi draft that Mark put together 
when he wrote pbx_dundi says:

    A transaction is uniquely identified at
    each end by an arbitrary 16-bit transaction identifier from 1 to
    65535 which SHOULD be selected as a random number for increased
    security.  Each message contains both a source and destination
    transaction number so that a received message can be uniquely
    identified by the transaction number alone.  The transaction
    identifier "0" is reserved for the destination transaction for the
    opening message of a dialog, since the destination transaction is not
    known at that time.

So, it seems that a valid transaction ID would be from 1 to 65535.  However, the 
function that generates the transaction ID uses a magic number, 32766, which I 
can't determine the reasoning for.  32766 is 0x7FFE, which is basically 16 bits 
with the high and low bit set to 0.

Any ideas?


References:

DUNDi protocol draft: http://www.dundi.com/dundi.txt

Relevant code:

static int get_trans_id(void)
{
         struct dundi_transaction *t;
         int stid = (ast_random() % 32766) + 1;
         int tid = stid;

         do {
                 AST_LIST_TRAVERSE(&alltrans, t, all) {
                         if (t->strans == tid)
                                 break;
                 }
                 if (!t)
                         return tid;
                 tid = (tid % 32766) + 1;
         } while (tid != stid);

         return 0;
}

-- 
Russell Bryant
Software Engineer
Digium, Inc.
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