Several types of SIP Servers

Several types of SIP Servers

  • User agent server runs on a SIP terminal (could be a SIP phone, a PDA, laptop, …) - it consists of two parts:
    • User Agent Client (UAC): initiates requests
    • User Agent Server (UAS): responds to requests
  • SIP proxy - interprets (if necessary, rewrites specific parts of a SIP request message) before forwarding it to a server closer to the destination:
    • SIP stateful proxy server - remembers its queries and answer; can also forward several queries in parallel (can be Transaction Stateful or Call Stateful).
    • SIP stateless proxy server
    • Proxies ignore SDP and do not handle any media (content)
    • Outgoing proxy: used by a user agent to route an outgoing request
    • Incoming proxy: proxy server which supports a domain (receives incoming requests)
  • SIP redirect server - directs the client to contact an alternate URI
  • Registrar server - receives SIP REGISTER requests updates LS
  • Location server (LS) - knows the current binding and queried by Proxies to do their routing
    • SIP can also use DNS SRV (Service) Records used to locate (inbound) proxy.
    • note in RFC 2543: a location server is a generic term for a database

Transcript

[slide138] So, we see SIP's really simple. Now we make a little more complicated, because we have the user agent for the caller, and we have a user agent for the callee. So, these two caller and callee UAs. But we also can now have proxies between them. So, the result is, for instance, KTH, which is a proxy, so when I want to make my call, instead of my having to go and do all of the work, I send my INVITE to the proxy, and the proxy figures out how should it forward the INVITE. It might know, hey, we actually have a separate network path that we use if you're making a call to Chalmers in Gothenburg, and so we'll route the traffic over that particular path. Or we have a special agreement for calls going to Bolivia, and we use this particular operator to carry those, so we send the INVITE by that operator. And not surprisingly, we can have outgoing proxies and incoming proxies. We can have redirect servers, the registrar we talked about, and the location server.