[Users] real loading

[unplug]

I am using sipp to test the openser with the following result.  It
will create message 481 after a certain traffic.  In what conditioin
that openser will send 2 BYE to CISCO?  When I increase the concurrent
call to about 50, the system becomes unstable and almost hanged.  I am
concerning about the real loading of the openser.  Does anyone can
help me?

--------------my scenario----------
call: 3 / sec
concurrent call: 30
total call made: 1002
failed call: 21
failure rate: 21/1002
>From the log shown below, most of the error are created with message 481.
SIP/2.0 481 Call Leg/Transaction Does Not Exist

10.30.18.235                   10.30.18.234         10.30.0.211
UA-------------------BYE--> openser -- BYE--> CISCO
                                 	         -- BYE --> CISCO
                                                    <-- 481 -- CISCO
UA <------------------- 481--openser

U 10.30.18.235:5060 -> 10.30.18.234:5060
BYE sip:936418703 at 10.30.0.211:5060 SIP/2.0.
Via: SIP/2.0/UDP 10.30.18.235:5060;branch=z9hG4bK-104-10.
Route: <sip:10.30.18.234;ftag=104;lr=on;nat=yes>.
From: sipp <sip:889642688756 at 10.30.18.235:5060>;tag=104.
To: sut <sip:36418703 at 10.30.18.234:5060>;tag=145085F8-1DF8.
Call-ID: 104-13187 at 10.30.18.235.
CSeq: 2 BYE.
Contact: sip:889642688756 at 10.30.18.235:5060.
Max-Forwards: 70.
Subject: Performance Test.
Content-Length: 0.

U 10.30.18.234:5060 -> 10.30.0.211:5060
BYE sip:936418703 at 10.30.0.211:5060 SIP/2.0.
Record-Route: <sip:10.30.18.234;ftag=104;lr=on>.
Via: SIP/2.0/UDP 10.30.18.234;branch=z9hG4bKadce.370d22c5.0.
Via: SIP/2.0/UDP 10.30.18.235:5060;rport=5060;branch=z9hG4bK-104-10.
From: sipp <sip:889642688756 at 10.30.18.235:5060>;tag=104.
To: sut <sip:36418703 at 10.30.18.234:5060>;tag=145085F8-1DF8.
Call-ID: 104-13187 at 10.30.18.235.
CSeq: 2 BYE.
Contact: sip:889642688756 at 10.30.18.235:5060.
Max-Forwards: 69.
Subject: Performance Test.
Content-Length: 0.

U 10.30.18.234:5060 -> 10.30.0.211:5060
BYE sip:936418703 at 10.30.0.211:5060 SIP/2.0.
Record-Route: <sip:10.30.18.234;ftag=104;lr=on>.
Via: SIP/2.0/UDP 10.30.18.234;branch=z9hG4bKadce.370d22c5.0.
Via: SIP/2.0/UDP 10.30.18.235:5060;rport=5060;branch=z9hG4bK-104-10.
From: sipp <sip:889642688756 at 10.30.18.235:5060>;tag=104.
To: sut <sip:36418703 at 10.30.18.234:5060>;tag=145085F8-1DF8.
Call-ID: 104-13187 at 10.30.18.235.
CSeq: 2 BYE.
Contact: sip:889642688756 at 10.30.18.235:5060.
Max-Forwards: 69.
Subject: Performance Test.
Content-Length: 0.

U 10.30.0.211:53419 -> 10.30.18.234:5060
SIP/2.0 481 Call Leg/Transaction Does Not Exist.
Via: SIP/2.0/UDP
10.30.18.234;branch=z9hG4bKadce.370d22c5.0,SIP/2.0/UDP
10.30.18.235:5060;rport=5060;branch=z9hG4bK-104-10.
From: sipp <sip:889642688756 at 10.30.18.235:5060>;tag=104.
To: sut <sip:36418703 at 10.30.18.234:5060>;tag=145085F8-1DF8.
Call-ID: 104-13187 at 10.30.18.235.
CSeq: 2 BYE.
Content-Length: 0.

U 10.30.18.234:5060 -> 10.30.18.235:5060
SIP/2.0 481 Call Leg/Transaction Does Not Exist.
Via: SIP/2.0/UDP 10.30.18.235:5060;rport=5060;branch=z9hG4bK-104-10.
From: sipp <sip:889642688756 at 10.30.18.235:5060>;tag=104.
To: sut <sip:36418703 at 10.30.18.234:5060>;tag=145085F8-1DF8.
Call-ID: 104-13187 at 10.30.18.235.
CSeq: 2 BYE.
Content-Length: 0.