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Glossary

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PIM SM

Protocol Independent Multicast sparse mode: PIM SM utilizes the

unicast route table and relies on the shared root distribution architecture for
multicast data forwarding.

PIM sparse-dense mode

An interface configuration that allows the inter-

face to choose the method of PIM operation.

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.

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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