Re: [Opalvoip-devel] Bug in rtp.cxx

[Robert J. <ro...@vo...>]

As "variance" is a signed value the "unsigned wraparound" problem should 
not occur. However, I can see in the log what you are saying.

The only thing I can think of is you are using a 64 bit compiler and 
"variance" is a 64 bit signed integer. Then, if the compiler does not do 
the sign extension for the unsigned expression (diff - lastTransitTime) 
then I can see how it could occur.

I have changed the code to guarantee a positive number.

*Robert Jongbloed*
/OPAL/OpenH323/PTLib Architect and Co-founder./
Commercial support at http://www.voxlucida.com.au

On 16/02/2013 6:09 AM, Southanon Ratsamy wrote:
> There seems to be a bug in rtp.cxx that causes the jitterLevel to jump 
> up in some cases. The bug is seen when the difference in time goes 
> from large to small to large again. This seems to be present in the 
> opal 3.10.9 release.
>
> This is the line of the bug in rtp.cxx, function is 
> Internal_OnReceiveData.
>
>         // As per RFC3550 Appendix 8
>         diff *= GetJitterTimeUnits(); // Convert to timestamp units
>         long variance = diff - lastTransitTime;
>
> Both "diff" and "lastTransitTime" are unsigned. If lastTransitTime is 
> bigger than diff, this produces a very large unsigned number. Not the 
> small negative number we are expecting.
>
> Here is sample output demonstrating what is happening:
>
>   0:11.293        RTP Jitter:0xe3494910 rtp.cxx(1346)  RTP     Session 
> 1, ssrc=3735373445, jitterLevel = 21
>   0:11.318        RTP Jitter:0xe3494910          rtp.cxx(1305) RTP     
> Session 1, ssrc=3735373445, tick = 170:25:21.136, 
> lastReceivedPacketTime = 170:25:21.110, diff = 26
>   0:11.318        RTP Jitter:0xe3494910          rtp.cxx(1324) RTP     
> Session 1, ssrc=3735373445, diff = 208, lastTransitTime = 152, 
> variance = 56, variance2 = 56
>
>   0:11.318        RTP Jitter:0xe3494910          rtp.cxx(1346) RTP     
> Session 1, ssrc=3735373445, jitterLevel = 76
>   0:11.333        RTP Jitter:0xe3494910          rtp.cxx(1305) RTP     
> Session 1, ssrc=3735373445, tick = 170:25:21.151, 
> lastReceivedPacketTime = 170:25:21.136, diff = 15
>   0:11.333        RTP Jitter:0xe3494910          rtp.cxx(1324) RTP     
> Session 1, ssrc=3735373445, diff = 120, lastTransitTime = 208, 
> variance = 4294967208, variance2 = -88
>
>   0:11.333        RTP Jitter:0xe3494910          rtp.cxx(1346) RTP     
> Session 1, ssrc=3735373445, jitterLevel = 4294967279
>   0:11.353        RTP Jitter:0xe3494910          rtp.cxx(1305) RTP     
> Session 1, ssrc=3735373445, tick = 170:25:21.170, 
> lastReceivedPacketTime = 170:25:21.151, diff = 19
>   0:11.353        RTP Jitter:0xe3494910          rtp.cxx(1324) RTP     
> Session 1, ssrc=3735373445, diff = 152, lastTransitTime = 120, 
> variance = 32, variance2 = 32
>
> Notice the jump in the variance value when diff is smaller than 
> lastTransitTime. variance2 is the same calculation with 
> lastTransitTime type casted to a long. Since this variance is part of 
> the jitterLevel calculation, this subsequently causes the jitterLevel 
> to jump in value dramatically.
>
> Best Regards,
> Southanon
>
>
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