UA (User Agent)

As defined for SIP in IETF RFC 3261, the UA (User Agent) represents an endpoint in the IP domain, a logical entity that can act as both a UAC (User Agent Client) that sends requests, and as UAS (User Agent Server) responding to requests.

UAC (User Agent Client)

Refer to IETF RFC 3261 for the following definition. “A user agent client is a logical entity that creates a new request, and then uses the client transaction state machinery to send it. The role of UAC lasts only for the duration of that transaction. In other words, if a piece of software initiates a request, it acts as a UAC for the duration of that transaction. If it receives a request later, it assumes the role of a user agent server for the processing of that transaction.&quot;

UAS (User Agent Server)

Refer to IETF RFC 3261 for the following definition. “A user agent server is a logical entity that generates a response to a SIP request. The response accepts, rejects, or redirects the request. This role lasts only for the duration of that transaction. In other words, if a piece of software responds to a request, it acts as a UAS for the duration of that transaction. If it generates a request later, it assumes the role of a user agent client for the processing of that transaction.&quot;

NENA-STA-010, NENA i3 Standard for Next Generation 9-1-1

RFC 3261, SIP: Session Initiation Protocol