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User agents have client and server components. The user agent client (UAC) sends SIP requests. The user agent server (UAS) receives requests and returns a SIP response. Unlike other network protocols that fix the roles of client and server, e.g., in HTTP, in which a web browser only acts as a client, and never as a server, SIP requires both ...
The client should try at the address in the Contact field. If an Expires field is present, the client may cache the result for that period of time. [1]: §21.3.3 305 Use Proxy The Contact field details a proxy that must be used to access the requested destination. [1]: §21.3.4 380 Alternative Service
JsSIP uses the SIP over WebSocket transport for sending and receiving SIP requests and responses, and thus, it requires a SIP proxy/server with WebSocket support. Currently the following SIP servers have been tested and are using JsSIP as the basis for their WebRTC Gateway functionality:
Linux Netfilter's SIP conntrack helper fully understands SIP and can classify (for QOS) and NAT all related traffic Netopia Netopia supports ALG PF , built-in OpenBSD firewall PF can handle the NAT through the "static-port" directive and the bandwidth control through the built-in queuing system of SIP connections
SIP is a signaling protocol for managing multimedia Voice over Internet Protocol (VoIP) telephone calls. A back-to-back user agent operates between both end points of a communications session and divides the communication channel into two call legs, and mediates all SIP signaling between the endpoints of the session, from establishment to ...
For example, most SBCs are implemented as back-to-back user agent. A B2BUA is a proxy-like server that splits a SIP transaction in two call legs: on the side facing the user agent client (UAC), it acts as server, on the side facing user agent server (UAS) it acts as a client.
MSRP can be used within a SIP session: to do instant messaging in a one-to-one or one-to-many mode; to do an attachment file transfer; to do some photo sharing (e.g., Image Share) based on prior exchange of capabilities between the user endpoints; MSRP session is set up through SIP's offer-answer [3] model.
oSIP is a free software library for VoIP applications implementing lower layers of Session Initiation Protocol (SIP). The library includes the minimal codebase required by any SIP application and offers enough flexibility to implement any SIP extension or behavior.