- What's the difference between a stateless and a stateful proxy server?
- Stateless proxies forget about the SIP request once it has been forwarded. Stateful proxies remember the request after it has been forwarded, so they can associate the response with some internal state. In other words, stateful proxies maintain transaction state. Stateful implies transaction state, not call state.
Stateless proxies scale very well, and can be very fast. They are good for network cores. Stateful proxies can do more (they can fork, for example, see the next question) and can provide services stateless ones can't (call forward busy, for example). They don't scale as much as stateless ones. An admininstrator gets to decide which to use. These are also logical entities; a physical proxy is likely to act as a stateless proxy for some calls, stateful for others, and as a redirect server for even others.
Neither stateful nor stateless proxies need to maintain call state, although they can, but will need to make sure that they are part of subsequent transactions via the Record-Route header.
Proxies must be stateful if one of the following conditions hold:
- uses TCP,
- uses multicast,
- forks.
- Why can a forking SIP proxy not be stateless?
- A forking SIP proxy cannot be stateless because it needs to perform a filtering operation, returning (in general) one response out of the many it receives. For example, a forking proxy with three branches, that receives a 200-class, 400-class, and 500-class response on each branch respectively, should return only the 200-class response upstream. If the proxy were stateless, it would end up returning all three of the responses upstream (since it won't remember that it had received prior responses when it gets another one). The result of this is (1) response implosion at the client, and (2) inconsistent responses at the client. (In this example, depending on the order the responses would be received, the client would think that the call failed, just to get a success indication some time later.) Thus, a forking proxy must be stateful.
Also note that a proxy that uses TCP must be stateful as well, whether it forks or not. This has to do with reliability issues.
Why do you want state in a proxy? Certain services (like forking) simply require it. A sequential search proxy requires state; sequential search is the heart of services like follow-me and personal mobility. It's at the discretion of the implementor whether to use a stateful or stateless proxy. You can even be "super stateful", and use the Record-Route header to allow a proxy to be on the signaling path of all subsequent exchanges. This allows a stateful proxy to maintain call state in addition to transaction state.
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