User Agent Server (UAS) is a Voice over Internet Protocol (VoIP) application that responds to User Agent Client (UAC) service requests based on input or other external stimuli in Session Initiation Protocol (SIP) systems.
Pursuant to RFC 1945, the User Agent (UA) header field is used by HTTP, SIP and Simple Mail Transfer Protocol/Network News Transfer Protocol (SMTP/NNTP).
The SIP UA uses the client-server protocol to facilitate peer-to-peer (PTP) calls. During the client-server conversation, the UA's identity is transmitted in HTTP or SIP via the header's UA field. This information is used to set optional communication session parameters, such as providing properly formatted content to desktops or smartphones.
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