U
UA	User agent. Combination of UAS and UAC that initiates and receives calls. See UAS and UAC.
UAC	User-agent client. Client application that initiates a SIP request.
UAL	User Adaptation Layer.
UAS	User-agent server (or user agent). Server application that contacts the user when a SIP request is received, then returns a response on behalf of the user. The response accepts, rejects, or redirects the request.
UC	Unified communications. Convergence of multiple media types on top of a common IP infrastructure.
UDP	User Datagram Protocol. Connectionless transport-layer protocol for exchanging datagrams without acknowledgments or guaranteed delivery. Part of the TCP/IP protocol stack.
UDPTL	User Datagram Protocol transport layer. Transport layer that is used on top of UDP. Makes packet delivery more reliable by providing data redundancy.
ULP	Upper Layer Protocol. Logical higher-layer application that uses SCTP services.
upspeed	Method used to dynamically change the codec type and speed to meet network conditions.
URI	Uniform (or Universal) Resource Identifier. Takes a form similar to an e-mail address, indicates the user's SIP identity, and is used for redirection of SIP messages.
URI	Uniform Resource Identifier. Type of formatted identifier that encapsulates the name of an Internet object, and labels it with an identification of the name space, thus producing a member of the universal set of names in registered name spaces and of addresses referring to registered protocols or name spaces.
URL	Universal (or Uniform) Resource Locator. Standard address of any resource on the Internet that is part of the World Wide Web.
URQ message	Unregistration Request RAS message.