Understanding the network.pdf - Back to Home

interaction. Each element can be used to modify network behavior
and performance. ZIP is covered in the Layer 5 session protocols
section.
AppleTalk Transaction Protocol
AppleTalk Transaction Protocol (ATP) provides acknowledged
transmission service between sockets. Each network transaction
consists of two actions: request and response. In most cases, the
transmission is a client end-station interacting with a file server or
printer. Each ATP request and response must be acknowledged with a
transaction request and transaction response to report the outcome
of the action. This approach is used by ATP to provide data
acknowledgment, packet sequencing, data segmentation, and
reassembly, which is needed to handle data loss due to network
transmission errors. There are three types of ATP transactions:
request, response, and release. The release transaction is used to end
an ATP session.
ATP uses two types of transaction services to handle error correction:
• At-Least-Once (ALO)
• Exactly-Once (EO)
ALO transaction services are used by applications that return the
same outcome if the transaction is executed more than once. For
example, if a host performs a name lookup, the response is the same
regardless of which transaction is successful. EO transaction services
are used if duplicate requests would affect the success of the
transaction. With EO, a transactions list is maintained, so duplicate
transactions are performed only once if a data loss condition exists.
All ATP transmissions are timed, and the duration varies depending on
the type of ATP transaction.
AppleTalk Layer 5 (Session), Layer 6 (Presentation),
and Layer 7 (Application) Protocols
Six protocols make up AppleTalk's upper layer protocol suite: four
session layer protocols (Layer 5) and two presentation layer protocols
(Layer 6). The session layer protocols are used for session negotiation
and communication between the lower layer network protocols and
end-node application data, which is provided by the presentation and