I have a question regarding kamailio transaction processing. Any help is greatly appreciated.

We are load testing kamailio using SIPP tool. Kamailio is working in outbound proxy mode and routing SIP messages in-between SIPP servers. SIPP generates REGISTER requests and SIP calls.

Registration load is 100 registrations per second. A steady stream of registration requests are generated throughout the test duration.

Call load is 100 calls per second with average call duration of 3.5mins with a max load of 10,000 concurrent calls. SIPP starts with a call rate of 100 call/sec and makes 10,000 concurrent calls. Calls terminate at 3.5mins and SIPP will make new calls to maintain 10,000 concurrent call load.

We monitored kamailio transactions using “kamctl moni” for current transaction in use. If kamailio has more than 1000 standing transaction in memory, we notice REGISTRATION retransmissions. Kamailio takes more than 300ms to process some registration requests. However requests related to call transactions are not affected. No INVITE or 200OK retransmissions were noticed in this process. By decreased call load to 10 call/second and registration retransmissions reduced considerably. Again by increased call load to 100 call/sec and we started noticing REGISTER retransmissions.

Here is our question. If number of transactions in kamailio memory contributes to processing delay should not it affect both REGISTRATION and CALL transactions equally? Why only REGISTRATION are getting affected while calls are fine without any retransmissions.

Is kamailio prioritizing processing call transactions over REGISTRATION transactions? Can someone throw some light on this please?