Information Technology Reference
In-Depth Information
The Transport Layer
AppleTalk's transport layer is implemented by several protocols: Routing Table Maintenance Protocol
(RTMP), AppleTalk Update Routing Protocol (AURP), AppleTalk Echo Protocol (AEP), AppleTalk
Transaction Protocol (ATP), and Name Binding Protocol (NBP).
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.
Figure9-4
A Sample AppleTalk Routing Table
Network 4
Router 3
Network 2
Network 3
2
3
Backbone
Router 1
Router 2
1
Network 1
Router 1 Routing Table
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
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.
