SIP servlets differ from typical HTTP servlets used in web applications in the following ways:
HTTP servlets have a particular context (called the context-root) in which they run, while SIP servlets have no context.
HTTP servlets typically return HTML pages to the requesting client, while SIP servlets typically connect SIP-enabled clients to enable telecommunications between the client and server.
SIP is a peer-to-peer protocol, unlike HTTP, and SIP servlets can originate SIP requests, unlike HTTP servlets which only send responses to the originating client.
SIP servlets often act as proxies to other SIP endpoints, while HTTP servlets are typically the final endpoint for incoming HTTP requests.
SIP servlets can generate multiple responses for a particular request.
SIP servlets can communicate asynchronously, and are not obligated to respond to incoming requests.
SIP servlets often work in concert with other SIP servlets to respond to particular SIP requests, unlike HTTP servlets which typically are solely responsible for responding to HTTP requests.