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Glossary
GL-21
Cisco ASA 5500 Series Configuration Guide using ASDM
tunnel
A method of transporting data in one protocol by encapsulating it in another protocol. Tunneling is
used for reasons of incompatibility, implementation simplification, or security. For example, a tunnel
lets a remote VPN client have encrypted access to a private network.
Turbo ACL
Increases ACL lookup speeds by compiling them into a set of lookup tables. Packet headers are used
to access the tables in a small, fixed number of lookups, independent of the existing number of ACL
entries.
U
UDP
User Datagram Protocol. A connectionless transport layer protocol in the IP protocol stack. UDP is a
simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, which
requires other protocols to handle error processing and retransmission. UDP is defined in RFC 768.
UMTS
Universal Mobile Telecommunication System. An extension of GPRS networks that moves toward an
all-IP network by delivering broadband information, including commerce and entertainment services,
to mobile users via fixed, wireless, and satellite networks.
Unicast RPF
Unicast Reverse Path Forwarding. Unicast RPF guards against spoofing by ensuring that packets have
a source IP address that matches the correct source interface according to the routing table.
URL
Uniform Resource Locator. A standardized addressing scheme for accessing hypertext documents and
other services using a browser. For example, http://www.cisco.com.
user EXEC mode
The lowest privilege level at the ASA CLI. The user EXEC mode prompt appears as follows when you
first access the ASA:
hostname>
See also command-specific configuration mode, global configuration mode, and privileged EXEC
mode.
UTC
Coordinated Universal Time. The time zone at zero degrees longitude, previously called Greenwich
Mean Time (GMT) and Zulu time. UTC replaced GMT in 1967 as the world time standard. UTC is
based on an atomic time scale rather than an astronomical time scale.
UTRAN
Universal Terrestrial Radio Access Network. Networking protocol used for implementing wireless
networks in UMTS. GTP allows multi-protocol packets to be tunneled through a UMTS/GPRS
backbone between a GGSN, an SGSN and the UTRAN.
UUIE
User-User Information Element. An element of an H.225 packet that identifies the users implicated in
the message.
V
VLAN
Virtual LAN. A group of devices on one or more LANs that are configured (using management
software) so that they can communicate as if they were attached to the same physical network cable,
when they are located on a number of different LAN segments. Because VLANs are based on logical
instead of physical connections, they are extremely flexible.