[asterisk-dev] IAX internet draft (draft-guy-iax-00)

[Derek Smithies]

Hi,

> >   Consequently, there are times when the timestamp (on the sending  
> > side)
> >    has to be "tweaked" high by 3, to differentiate between two  
> > different
> >    frames.  A lagrq frame does not increase the oseqno at the sending
> >    side.
> 
> I don't see why you say that. I think the +3 stuff is a work around  
> for a
> specific implementation problem, not a protocol requirement.
> The combination of oseqno with ack'ness makes a packet unique.
> 
I am sorry - I do remember reading something about this +3 business was a 
jiter buffer saviour. From the code:

/* On a dataframe, use last value + 3 (to accomodate jitter buffer 
               shrinking) if appropriate unless it's a genuine frame */
if (genuine) {
	/* genuine (IAX LAGRQ etc) must keep their clock-based stamps */
	if (ms <= p->lastsent)
		ms = p->lastsent + 3;
} else if (abs(ms - p->lastsent) <= MAX_TIMESTAMP_SKEW) {
  /* non-genuine frames (!?) (DTMF, CONTROL) should be pulled into 
                              the predicted stream stamps */
	ms = p->lastsent + 3;
}


=====================

> >  A timestamp MUST also be assigned for the
> >    call, beginning at 0 and incrementing each millisecond
> > is unclear.
> > Incrementing by what value??
> >
> > In fact::
> >  The timestamp has  units of milliseconds.
> >  Thus, the maximum timestamp in a 16bit field is 65.535 seconds.
> 
> No, timestamp is either a 32 bit field (fullframe) or a 15 bit field
> (miniframes).
> 
> If you wrap a 32 bit counter in a single call, you have other  
> problems.......
> 
No, you misunderstand me completetly. completely.

The maximum timestamp value you can squeeze into a 16 bit field, such as 
in a mini frame, 65.535 ms. From iax2 packet dumps, immediately prior 
to this value wrapping around when this value wraps, one full frame 
of voice is sent.

I was not talking about the 32 bit value wrapping around.
I was talking about the 16 bit value wrappng around.


Derek.

-- 
Derek Smithies Ph.D.                 Any fool can write code that 
IndraNet Technologies Ltd.                a computer can understand.        
Email: derek at indranet.co.nz         Good programmers write code 
ph +64 3 365 6485                          that humans can understand.
Web: http://www.indranet-technologies.com/            Martin Fowler