Most of the issue arises with DIAMETER Connection Establishment, here we are giving some view on how does DIAMETER Connection take place. As we know; Diameter is an application layer protocol, therefore virtually we could distinguish into two connections.
1) Transport Connection
2) DIAMETER connection
1) Transport Connection:
When ever a DIAMETER Application comes up (Client/Server) first of all it brings its transport connection which can be TCP/SCTP on Port 3868 (By Default)or TLS/DTLS on PORT 5868 (By Default)( if security is applied). Before moving further we must check that Transport Connection is UP. For this we could check the message s that are exchanged during TCP/SCTP or TLS/DTLS connection establishment.
2) DIAMETER Connection
Once the Transport Connection is properly set up then Application initiates DIAMETER connection, For this First message that is triggered; is CER (Capabilities-Exchange-Request) with all supported application Ids. DIAMETER Connection said to be established when an Application receives CEA (Capabilities-Exchange-Answer) with Result-Code set to DIAMETER_SUCCESS.
According to RFC-6733 When secure transport is established then all messages shall be exchanged on secured transport including CER/CEA.
Your Comments /Suggestions and Questions are always welcome, shall clarify with best of our knowledge. So feel free to put Questions.