[asterisk-dev] RTCP jitter computation

[Mark Kica]

Dear Sir or Mader ,



I would like to understand better to Asterisk's RTCP jitter
computation and improve its RTCP code (etc. rtcp transmission interval
computation)  , so  I found following code in res_rtp_asterisk.c



	gettimeofday(&now, NULL);
	timersub(&now, &rtp->rtcp->rxlsr, &dlsr);
	rtcpheader = (unsigned int *)bdata;
	rtcpheader[0] = htonl((2 << 30) | (1 << 24) | (RTCP_PT_RR << 16) |
((len/4)-1));
	rtcpheader[1] = htonl(rtp->ssrc);
	rtcpheader[2] = htonl(rtp->themssrc);
	rtcpheader[3] = htonl(((fraction & 0xff) << 24) | (lost & 0xffffff));
	rtcpheader[4] = htonl((rtp->cycles) | ((rtp->lastrxseqno & 0xffff)));
	rtcpheader[5] = htonl((unsigned int)(rtp->rxjitter * rate));
                              // <------------------------ Look here
----------------------
	rtcpheader[6] = htonl(rtp->rtcp->themrxlsr);
	rtcpheader[7] = htonl((((dlsr.tv_sec * 1000) + (dlsr.tv_usec / 1000))
* 65536) / 1000);



Why is the value of the jitter multiplied with rate ? Is it valid ?



There is also following code :



if ((!rtp->rxcore.tv_sec && !rtp->rxcore.tv_usec) || mark) {
		gettimeofday(&rtp->rxcore, NULL);
		rtp->drxcore = (double) rtp->rxcore.tv_sec + (double)
rtp->rxcore.tv_usec / 1000000;
		/* map timestamp to a real time */
		rtp->seedrxts = timestamp; /* Their RTP timestamp started with this */
		tmp = ast_samp2tv(timestamp, rate);
		rtp->rxcore = ast_tvsub(rtp->rxcore, tmp);
		/* Round to 0.1ms for nice, pretty timestamps */
		rtp->rxcore.tv_usec -= rtp->rxcore.tv_usec % 100;
	}

	gettimeofday(&now,NULL);
	/* rxcore is the mapping between the RTP timestamp and _our_ real
time from gettimeofday() */
	tmp = ast_samp2tv(timestamp, rate);
	*tv = ast_tvadd(rtp->rxcore, tmp);

	prog = (double)((timestamp-rtp->seedrxts)/(float)(rate));
	dtv = (double)rtp->drxcore + (double)(prog);
                            // Can somebody give me explanation what
is dtv ?
	current_time = (double)now.tv_sec + (double)now.tv_usec/1000000;
	transit = current_time - dtv;
	d = transit - rtp->rxtransit;
	rtp->rxtransit = transit;
	if (d<0) d=-d;
	rtp->rxjitter += (1./16.) * (d - rtp->rxjitter);



 Can somebody give me explanation what is dtv , prog a drxcore ?


yours sincerely

Mark Kica