I found out that this had to do with the packet size. The MTU size was 1500 and the NOTIFY request was close to that size. So I believe Kamailio uses TCP transport in this case.

From: sr-users [mailto:sr-users-bounces@lists.kamailio.org] On Behalf Of Haggerty, Paul (phaggert)
Sent: Thursday, April 20, 2017 1:58 PM
To: sr-users@lists.kamailio.org
Subject: [SR-Users] NOTIFY request from PCSCF is on TCP transport rather than UDP

Hello,

I’ve got an IMS client which uses UDP for transport. Our application server is also using UDP. We are using a Kamailio IMS Core.  The client REGISTERS with UDP and also does a SUBSCRIBE. Our application server responds with a NOTIFY to the client. Looking at the trace. The messages between the PCSCF and SCSCF are TCP. The REGISTER and SUBSCRIBE responses to the client from the PCSCF are UDP (which I would expect). However, when the PCSCF get the NOTIFY is tries to open a TCP connection to the Client. (I assume to send a NOTIFY).

Any ideas of what is happening here? I’m fairly new to the Kamailio IMS. I am more familiar with the Open IMS Core. (which is not having this issue).

Thanks,

Paul