Chapter 11. SIP Proxy: Marshal Server
The Proxy server is a creature of the SIP RFC, 3261, and, as its name implies, is intended to act on behalf of other network endpoints rather than on its own. In VOCAL, we implemented two flavors of the SIP Proxy server as distinct entities and called them the Marshal server and the Feature server. There are also SIP Proxy servers built into the MGCP-SIP and H.323-SIP translators. See Chapters 15 and 16 for more information about the translators.
The Marshal server is the first point of contact for all traffic entering the VOCAL system: it authenticates users, provides data that is useful for call detail record collection, and forwards SIP request and response messages to other network entities. The Feature server provides core network features, such as call forwarding, and is the subject of Chapter 13.
“Why do we need Marshal servers?” This question was raised repeatedly during our initial white-boarding of VOCAL. Logically, we could have used the Redirect server (RS) to provide authentication and call detail record (CDR) data, but combining those elements into the RS would have negatively impacted our ability to scale the system. While you can add additional RSs to the system to provide redundancy, you can’t easily add more hardware to the RS to scale the system because of its requirement to keep all of the registration information together in the same place. To promote scalability , we decided to decentralize the authentication ...