Does a diameter client always send CER to diameter server or vice-versa is possible ?

Question

How the scenario will be handled in case both client as well as server sends CER towards each other ?

Answer 1

Not necessarily, it is not hard and fast that only client can send the CER. Node acting as a server can also send the CER.

Some time it is difficult to judge which is server and which is client; because both are intermediate nodes. So server can also trigger the CER message.

When both sides issue CER about same time then only one session is created i.e. both will result in the single session.

Answer 2

Once the transport connection is established, both client as well as server can trigger first diameter message i.e. Capability Exchange Request (CER) message towards each other. Even though, If both the nodes receive CER only one connection can be established and continued. Receiver lexicographically compares received origin-Host Id with its own Origin-Host Id. If it finds that its own Origin-Host id is higher than the received one, it drops the CER message and other end will send CEA. Fro more details refer 6733.