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6-6 CHAPTER 6: ROUTING WITH APPLETALK
The AppleTalk Address Resolution Protocol (AARP), which translates
hardware addresses to AppleTalk addresses, also exists at the datalink layer
because it is closely related to the Ethernet and token ring LAPs. This
protocol is usually included in the definition of each LAP, so it does not
appear in the reference model. Refer to the section “About AARP” for more
information about this protocol.
The Datagram
Delivery Protocol
(DDP)
The network layer accepts data from the layers above it and divides the data
into packets that can be sent over the network through the layers below it.
One protocol is present at the AppleTalk network layer: the Datagram
Delivery Protocol (DDP).
The DDP transfers data in packets called datagrams. Datagram delivery is
the basis for building other AppleTalk services, such as electronic mail. The
DDP allows AppleTalk to run as a process-to-process, best-effort delivery
system where the processes running in the nodes of interconnected
networks can exchange packets with each other.
End-to-End Services The transport layer and the session layer provide end-to-end services in the
AppleTalk network. These services ensure that routers transmit data
accurately between one another. Each layer includes four protocols that
work together to support these services. This section describes all these
protocols, and provides more detail for those that you can view using
Administration Console.
Transport Layer Protocols
The four transport layer protocols are these:
■ The Routing Table Maintenance Protocol (RTMP)
■ The AppleTalk Echo Protocol (AEP)
■ The Apple Talk Transaction Protocol (ATP)
■ The Name Binding Protocol (NBP)
The Routing Table Maintenance Protocol (RTMP). The RTMP maintains
information about AppleTalk addresses and connections between different
networks. The RTMP specifies that each router learns about new routes from
the other routers and deletes routes after a certain period if the local router
no longer broadcasts the route to the network.