We are using SIP Communicator(Version-sip-communicator-src-1.0-alpha3-nightly.build.2164) with a locally installed SIP server in our Intranet and it works perfectly fine for Third-party Call Control scenario orchestrated by a Application Server.
However, when we perform the same setup in a isolated Lab network with no DNS server and using Statically configured IPs on SIP server and Client machines, we are facing a strange behavior with Sip Communicator. It generates a 100 Trying message immediately on incoming INVITE, but notifies user of incoming call (as well as generates 180 Ringing) only after a long delay of 6-10 seconds. Also once user accepts the call (for which 200 OK is generated instantaneously), the ringing continues for many-many seconds because the Ack sent back by SIP server is not processed right away.
We are not able to understand the reason for such a behavior. Noticed that Sip Server is sending Notify of Sip-Message-Summary to Sip Communicator after initial REGISTER which seems to be forcing Sip Communicator to send out multiple DNS SRV queries for _sip._udp.<ip-address>. Can this be hogging the threads of Sip Communicator and reason for the delay? Also, as we are using just IP Address in isolated Lab network; why is DNS lookup done at all?