User Agent Client (UAC)

Definition - What does User Agent Client (UAC) mean?

A User Agent Client (UAC) is a Session Initiation Protocol (SIP) or Voice over Internet Protocol (VoIP) application that serves as a peer-to-peer (P2P) communication gateway and generates distributed network service requests.

A UAC is a network server, email client, search engine or Web browser.

Techopedia explains User Agent Client (UAC)

Request for Comments (RFC) 1945 requires:

  • All UAC formats must include a product and optional comment string.
  • The user agent (UA) header field is used by HTTP, SIP and Simple Mail Transfer Protocol/Network News Transfer Protocol (SMTP/NNTP).


The UAC submits its ID string to the User Agent Server (UAS) with application type, OS or client/server revisions.

In a Web browser, the UA string format reads as follows:

Mozilla/[version] ([system and browser information]) [platform] ([platform details]) [extensions]

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