System information

The Transport Layer
Book Title
9-180
RTMP
The protocol that establishes and maintains AppleTalk routing tables is RTMP. RTMP routing tables
contain an entry for each network that a datagram can reach. Each entry includes the router port that
leads to the destination network, the node ID of the next router to receive the packet, the distance in
hops to the destination network, and the current state of the entry (good, suspect, or bad). Periodic
exchange of routing tables allows the routers in an internetwork to ensure that they supply current
and consistent information. Figure 9-4 shows a sample RTMP table and the corresponding network
architecture.
Figure 9-4 A Sample AppleTalk Routing Table
AppleTalk’s NBP associates AppleTalk names (expressed as network-visible entities, or NVEs) with
addresses. An NVE is an AppleTalk network-addressable service, such as a socket. NVEs are
associated with one or more entity names and attribute lists. Entity names are character strings such
as printer@net1, whereas attribute lists specify NVE characteristics.
Named NVEs are associated with network addresses through the process of name binding. Name
binding can be done when the user node is first started up, or dynamically, immediately before first
use. NBP orchestrates the name binding process, which includes name registration, name
confirmation, name deletion, and name lookup.
Zones allow name lookup in a group of logically related nodes. To look up names within a zone, an
NBP lookup request is sent to a local router, which sends a broadcast request to all networks that
have nodes belonging to the target zone. The Zone Information Protocol (ZIP) coordinates this
effort.
ZIP maintains network number–to–zone name mappings in zone information tables (ZITs). ZITs are
stored in routers, which are the primary users of ZIP, but end nodes use ZIP during the startup
process to choose their zone and to acquire internetwork zone information. ZIP uses RTMP routing
tables to keep up with network topology changes. When ZIP finds a routing table entry that is not in
the ZIT, it creates a new ZIT entry. Figure 9-5 shows a sample ZIT.
Backbone
Router 1 Routing Table
Network 3
Network 2
Network 1
2
3
1
Network Distance Port Next router Entry state
1 0 1 0 Good
2 0 2 0 Good
3 1 3 Router2 Good
4 2 3 Router2 Good
Network 4
Router 2Router 1
Router 3