Appendix C:
Glossary
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entire message has arrived and is in the right order. Network applications that want to save
processing time because they have very small data units to exchange may prefer UDP to
TCP.
UDP provides two services not provided by the IP layer. It provides port numbers to help
distinguish different user requests and, optionally, a checksum capability to verify that the
data arrived intact.
Common network applications that use UDP include the Domain Name System (DNS),
streaming media applications such as IPTV, Voice over IP (VoIP), and Trivial File Transfer
Protocol (TFTP).
UPnP
UPnP is an acronym for
U
niversal
P
lug and
P
lay. The goals of UPnP are to allow devices to
connect seamlessly and to simplify the implementation of networks in the home (data sharing,
communications, and entertainment) and in corporate environments for simplified installation
of computer components
User Priority
User Priority is a 3-bit field storing the priority level for the 802.1Q frame. It is also known
as PCP.
V
VLAN
Virtual LAN. A method to restrict communication between switch ports. VLANs can be used
for the following applications:
VLAN unaware switching:
This is the default configuration. All ports are VLAN unaware with
Port VLAN ID 1 and members of VLAN 1. This means that MAC addresses are learned in
VLAN 1, and the switch does not remove or insert VLAN tags.
VLAN aware switching:
This is based on the IEEE 802.1Q standard. All ports are VLAN
aware. Ports connected to VLAN aware switches are members of multiple VLANs and
transmit tagged frames. Other ports are members of one VLAN, set up with this Port VLAN ID,
and transmit untagged frames.
Provider switching:
This is also known as Q-in-Q switching. Ports connected to subscribers
are VLAN unaware, members of one VLAN, and set up with this unique Port VLAN ID. Ports
connected to the service provider are VLAN aware, members of multiple VLANs, and set up
to tag all frames. Untagged frames received on a subscriber port are forwarded to the
provider port with a single VLAN tag. Tagged frames received on a subscriber port are
