[asterisk-dev] Q.921 strange behaviour

[Tzafrir Cohen]

On Thu, Jun 19, 2008 at 12:05:44PM +0000, Tony Mountifield wrote:
> While doing some testing of a PRI-sniffer I am working on, I discovered
> some strange operation at the Q.921 level that didn't seem right.
> 
> Just a bit of background first:
> 
> My test setup is a PC containing a TE405P, currently running
> zaptel-1.2.21, libpri-1.2.5 and asterisk-1.2.24.  I am running E1
> EuroISDN with spans 1/2 set to CPE and spans 3/4 set to NET. Using E1/T1
> crossover cables, span 1 is connected to span 3 and span 2 to span 4. I
> can make calls from one span to another with no problems.
> 
> The span 1 to 3 connection goes via a breakout box which in turn is
> connected to a Sangoma A102 (2-port E1) in high-impedance mode, enabling
> both directions of the link to be sniffed. The sniffer system with the
> Sangoma card is not running asterisk, but a custom app, which uses parts
> of libpri to understand the D-channel traffic received from the wanpipe
> sockets.
> 
> Now to the observed behaviour:
> 
> Today, the Asterisk system appears to be running normally. When idle,
> every 10 seconds the CPU port sends an RR, and in response receives
> an RR from the NET port.
> 
> However, yesterday, the Q.921 layer appeared to have got into a strange
> state where both sides were sending and receiving RR frames every few
> milliseconds.
> 
> This did not seem to stop the signalling of actual calls, which could
> still be made, but it certainly made the logger work hard!
> 
> Looking at one of the log files from the passive monitor, it appears
> that it got into this state after being required to do Q921
> retransmissions in response to a REJ packet. The asterisk box is rather
> under-powered (533MHz celeron), and when the D-channel decided to send
> a burst of RESTART messages, the receiving process seemed to miss the
> occasional frame, and so sent a REJ when it got the I-frame after the
> one it was expecting. Even though the wanted frame was correctly
> retransmitted, the RR-storm continued for ever.
> 
> All the above was observed by the passive monitor, not by using pri
> debug on the Asterisk.
> 
> Has this behaviour been observed before, and perhaps fixed in later
> versions of asterisk and/or libpri? I can try testing later versions,
> but was wondering if such an issue had been knowingly corrected.

Not an RR storm, but check http://bugs.digium.com/12861 for better 
handling of REJ and such. I believe it may help there.

-- 
               Tzafrir Cohen
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