U

UDP

User Datagram Protocol: A connectionless Transport layer protocol in the TCP/IP protocol stack that simply allows datagrams to be exchanged without acknowledgments or delivery guarantees, requiring other protocols to handle error processing and retransmission. UDP is defined in RFC 768.

unnumbered frames

HDLC frames used for control management purposes, such as link startup and shutdown or mode specification.

update interval

Typically used with routing protocols, update interval specifies the time between route updates.

uptime

The length or percentage of time during which a computer system is functioning and available for use.