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Glossary

Ping

Packet Internet Groper: A Unix-based Internet diagnostic tool, con-

sisting of a message sent to test the accessibility of a particular device on the
IP network. The acronym (from which the "full name" was formed) reflects
the underlying metaphor of submarine sonar. Just as the sonar operator
sends out a signal and waits to hear it echo ("ping") back from a submerged
object, the network user can ping another node on the network and wait to
see if it responds.

pinhole congestion

A problem associated with distance-vector routing

protocols if more than one connection to a remote network is known, but
they are different bandwidths.

pleisochronous

Nearly synchronous, except that clocking comes from an

outside source instead of being embedded within the signal as in synchro-
nous transmissions.

PLP

Packet Level Protocol: Occasionally called X.25 level 3 or X.25 Pro-

tocol, a network-layer protocol that is part of the X.25 stack.

PNNI

Private Network-Network Interface: An ATM Forum specification

for offering topology data used for the calculation of paths through the net-
work, among switches and groups of switches. It is based on well-known
link-state routing procedures and allows for automatic configuration in net-
works whose addressing scheme is determined by the topology.

point-to-multipoint connection

In ATM, a communication path going

only one way, connecting a single system at the starting point, called the
"root node," to systems at multiple points of destination, called "leaves."
See also: point-to-point connection.

point-to-point connection

In ATM, a channel of communication that

can be directed either one way or two ways between two ATM end systems.
See also: point-to-multipoint connection.

poison reverse updates

These update messages are transmitted by a

router back to the originator (thus ignoring the split-horizon rule) after
route poisoning has occurred. Typically used with DV routing protocols
in order to overcome large routing loops and offer explicit information
when a subnet or network is not accessible (instead of merely suggesting
that the network is unreachable by not including it in updates). See also:
route poisoning.

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