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[Linphone-users] ortp: RTP Data Header Validity Checks

From: Rudolf Svanda
Subject: [Linphone-users] ortp: RTP Data Header Validity Checks
Date: Thu, 14 May 2009 08:19:21 +0200
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i have a question to validity checks after sender change its SSRC. Ortp 
discarts SSRC_CHANGED_THRESHOLD RTP Pakets till 
declaring source as valid. In RFC 3550 is a code snippet with MIN_SEQUENTIAL 
set to 2. SSRC_CHANGED_THRESHOLD is set to 50 
Pakets. What is the reason for such a high value (in Time it's 1s)? 

And there ist possibly a bug in validity check. Sender sends following RTP 
SSRC:a;Seq: b;Time: c
SSRC:a;Seq: b+1;Time: c+160
SSRC:a;Seq: b+2;Time: c+320
SSRC:a;Seq: b+k;Time: c+(k*160)
now sender change it SSRC, Seq and Time
SSRC:x;Seq: y;Time: z
SSRC:x;Seq: y+1;Time: z+160
SSRC:x;Seq: y+2;Time: z+320

if z ist lower than c+(k*160), all RTP Pakets will be discarded till sender 
sends RTP Paket with Timestamp higher than c+(k*160). I think 
after SSRC is decrared as valid, Timestamp should be set to current RTP Paket 
Timestamp. Something like this (rtpparce.c in version 

*** 148,153 ****
--- 148,154 ----

+                               session->rtp.rcv_last_ts = rtp->timestamp;
                                /*discard the packet*/


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