Differences between Strict Routing and Loose Routing

Loose and strict are different methods of routing SIP messages. Loose routing is new in SIP version 2. When you use loose routing, the R-URI is never changed and backwards compatibility is maintained with the older method (strict routing RFC2543).

The problem with strict routing is in the process of specifying the entire proxy set in the initial request before starting the SIP dialog. The processing throws away the information contained in the received R-URI. The behavior of UAs with outbound-proxy was problematic. The whole system would fail if there was a failure in one of the elements.

The solution, is that loose routing is the correct method. It keeps the target separated from the route. It allows each destination to route the packet and has a mechanism to keep backward compatibility with strict routing. The support of loose routing is indicated by the parameter ;lr.

When the SIP server receives a message, it can decide if it wants to stay in the middle or not (record-route). If the SIP server does not want to stay in the middle, it can pass the information to the user agents' UAs to connect each other. After this process the messages follow between the user agents.

If OpenSER wants to stay in the middle of the conversation (that is, for billing purposes) it should insert a ROUTE header field using the function record_route().