Avaya Configuring OSI Services Bedienungsanleitung PDF herunterladen (Seite 41)

OSI Overview

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A router partitions a CLNP packet into two or more new packets (segments) if the

size of the packet is greater than the maximum size supported by the outbound

network. The values contained in the header fields of the segmented packets are

identical to those contained in the original packet (except for the segment length

and checksum fields). The router sends the partitioned packets out on the network.

When all of the packet segments finally arrive at the destination system, the

system reconstructs the original packet before sending it up to the next layer for

further processing.

To control data misdirection and congestion throughout the network, CLNP

includes a lifetime control function. The originating system can assign a specific

lifetime value (in units of 500 milliseconds) to the lifetime field of the packet

header before sending the system the packet out onto the network. Every system

that receives the packet decrements its lifetime. If the lifetime value reaches 0

before the packet reaches its destination system, the packet is dropped.

A system also discards a packet if its checksum is incorrect, if the destination

address is unknown, or if the network is too congested to process the packet.

CLNP includes an error reporting option that, when enabled, sends an error report

data packet back to the originating system whenever a data packet is lost or

discarded.

End System to Intermediate System Routing Exchange Protocol

The End System to Intermediate System Routing Exchange Protocol (ISO 9542)

defines the way end systems (computers, etc.) and intermediate systems (routers)

on the same subnetwork exchange configuration and routing information. (See

“Intermediate System to Intermediate System Intra-Domain Routing Exchange

Protocol” later in this chapter for information about communication between

routers.)

Configuration Reporting

The ISO 9542 configuration report function allows end systems and routers that

are attached to the same physical network (subnetwork) to dynamically discover

each other’s identity by periodically generating and exchanging Hello packets.

The Hello packet exchange process tells the router which NSAPs it can access.

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Avaya Configuring OSI Services Bedienungsanleitung Seite 41