Editing Session Initiation Protocol (section)

===User agent===
A user agent is a logical network endpoint that sends or receives SIP messages and manages SIP sessions. 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 peers to implement both roles. The roles of UAC and UAS only last for the duration of a SIP transaction.<ref name="RFC 3261"/>

A SIP phone is an [[IP phone]] that implements client and server functions of a SIP user agent and provides the traditional call functions of a telephone, such as dial, answer, reject, call hold, and call transfer.<ref name="Azzedine">{{cite book|last=Azzedine|title=Handbook of algorithms for wireless networking and mobile computing|publisher=CRC Press|year=2006|page=774|url=https://books.google.com/books?id=b8oisvv6fDAC&pg=PT774 | isbn=978-1-58488-465-1}}</ref><ref>{{cite book|last=Porter|first=Thomas |author2=Andy Zmolek |author3=Jan Kanclirz |author4=Antonio Rosela |title=Practical VoIP Security|publisher=Syngress |year=2006|pages=76–77|url=https://books.google.com/books?id=BYxdyekyRlwC&pg=PA76 | isbn=978-1-59749-060-3}}</ref> SIP phones may be implemented as a hardware device or as a [[softphone]]. As vendors increasingly implement SIP as a standard telephony platform, the distinction between hardware-based and software-based SIP phones is blurred and SIP elements are implemented in the basic firmware functions of many IP-capable communications devices such as [[smartphone]]s.

In SIP, as in HTTP, the [[user agent]] may identify itself using a message header field (''User-Agent''), containing a text description of the software, hardware, or the product name. The user agent field is sent in request messages, which means that the receiving SIP server can evaluate this information to perform device-specific configuration or feature activation. Operators of SIP network elements sometimes store this information in customer account portals,<ref>{{cite web |url=http://www.voipuser.org/forum_topic_14998.html |archive-date=2011-07-16 |archive-url=https://web.archive.org/web/20110716170218/http://www.voipuser.org/forum_topic_14998.html |title=User-Agents We Have Known |publisher=VoIP User}}</ref> where it can be useful in diagnosing SIP compatibility problems or in the display of service status.