SIP Proxy—Expected Behavior

It is important to understand the basic processing of a SIP proxy according to RFC3261. Without this understanding it will be very difficult to configure a Proxy server.

Each proxy will take routing decisions, modifying the request before sending it to the next element. The responses will be routed over the same set of proxies traversed by the original request in the reverse order.

A SIP proxy can operate in stateless or stateful mode. When a SIP proxy works as a simple SIP packet forwarder, it forwards the packets to a single element determined by the request. A proxy working in stateless mode discards any information about the message after the message has been forwarded. This characteristic limits the failure treatment and billing.

When OpenSER knows that the message 200 OK corresponds to a specific INVITE we say that it is working in stateful mode. This means simply that you can now manage the response in an onreply_route() block. With stateless processing each message is handled without a context. Stateless processing is used in applications like load balancing; it uses the command forward() in the script.

When you need more sophisticated resources like billing, call forward, and voicemail, you will need to use stateful processing. Each transaction will be maintained in memory and failures, responses, and retransmissions will be associated with this specific transaction. Stateful transactions are handled by the TM (transaction) module and usually use the t_relay() command.

An often misunderstood concept is that the processing is stateful by transaction and not by dialog. Thus, it is the stateful processing of an INVITE request until the 200 OK response (transaction) and not from the INVITE to the BYE request (dialog).