A user agent is a client application implementing a network protocol used in communications within a client-server distributed computing system. The term most notably refers to applications that access the World Wide Web, but other systems, such as the Session Initiation Protocol (SIP), use the term user agent to refer to both end points of a communications session.[1]
Web user agents range from web browsers to search engine crawlers (spiders), as well as mobile phones, screen readers and braille browsers used by people with disabilities. When a user agent operates, it typically identifies itself, its application type, operating system, software vendor, or software revision, by submitting a characteristic identification string to its operating peer. In the HTTP and SIP protocols, this is transmitted in a header field User-Agent. Bots, such as web crawlers, often also include a URL and/or e-mail address so that the webmaster can contact the operator of the bot.
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