How AppleTalk Routers Function in Internet Communications

SUMMARY

LocalTalk networks are different from PC networks and require you to consider special concepts and issues when setting them up. Most LocalTalk networks are internets--separate physical networks connected by routers that enable network communications by maintaining a map of the physical network on the internet and forwarding data to the correct network destinations.

Seed routers provide other specialized internet functions. For more information on them see 105155.

ROUTING TABLE MAINTENANCE PROTOCOL (RTMP)

AppleTalk uses routing table maintenance protocol (RTMP) to maintain information about internetworking addresses and connections. Routers provide interfaces for RTMP and numerous other protocols and processes.

The routing table contains all possible destination network numbers (or the network range) and five entries necessary for forwarding datagrams:

• the number of the data link port that connects to the local network's DDP
• the destination network number
• the next router's node ID
• the number of hops needed to reach the destination network
• a cross-reference into the zone information table

The routers use RTMP to exchange this routing information, keeping their tables current and minimizing internetwork delays.

ZONE INFORMATION PROTOCOL (ZIP)

The zone information protocol (ZIP) maintains a zone information table that the name binding protocol (NBP) uses to match networks with their zones. ZIP also helps routers maintain their tables.

EXAMPLE

If you send a job across a router to a printer. A sniff of the request will show you the process stages:

• When the printing process begins, you will see RTMP and ZIP packets. These update the routers.
• Then, an NBP request to find the printer, something resembling:

         NBP Request ID=XX (=:LaserWriter@zonename)
  						

• After the logical connection is initiated, the printer access protocol (PAP) starts a connection.
• When this connection is confirmed, data transmission starts.

The process for a file request is very similar, although it uses the file access protocol (FAP) instead of the printer access protocol (PAP). If you are looking for network or broadcast problems, use the ZIP and RTMP packets to verify that the information (number of hops and network ranges) is correct.