System information

Troubleshooting AppleTalk 9-183
ATP
ATP
ATP is suitable for transaction-based applications such as those found in banks or retail stores. ATP
transactions consist of requests (from clients) and replies (from servers). Each request/reply pair has
a particular transaction ID. Transactions occur between two socket clients. ATP uses exactly once
(XO) and at-least-once (ALO) transactions. XO transactions are used in situations where performing
the transaction more than once would be unacceptable. Banking transactions are examples of
transactions that, if performed more than once, result in invalid data.
ATP is capable of most important transport-layer functions, including data acknowledgment and
retransmission, packet sequencing, and fragmentation and reassembly. ATP limits message
segmentation to eight packets, and ATP packets cannot contain more than 578 data bytes.
Upper-Layer Protocols
AppleTalk supports several upper-layer protocols:
• AppleTalk Data Stream Protocol (ADSP) establishes and maintains full-duplex data streams
between two sockets in an AppleTalk internetwork. ADSP is a reliable protocol in that it
guarantees that data bytes are delivered in the same order as sent and that they are not duplicated.
ADSP numbers each data byte to keep track of the individual elements of the data stream. ADSP
also specifies a flow-control mechanism. The destination can essentially slow source
transmissions by reducing the size of its advertised receive window. ADSP also provides an
out-of-band control message mechanism. Attention packets are used as the vehicle for moving
out-of-band control messages between two AppleTalk entities. These packets use a separate
sequence number stream to differentiate them from normal ADSP data packets.
• The AppleTalk Session Protocol (ASP) establishes and maintains sessions (logical
conversations) between an AppleTalk client and a server.
• AppleTalk’s Printer Access Protocol (PAP) is a connection-oriented protocol that establishes and
maintains connections between clients and servers. (Use of the term printer in this protocol’s title
is purely historical.)
• The AppleTalk Filing Protocol (AFP) helps clients share server files across a network.
Troubleshooting AppleTalk
This section presents protocol-related troubleshooting information for AppleTalk connectivity and
performance problems. In addition to general AppleTalk problems, this chapter also covers
AppleTalk Enhanced IGRP, AppleTalk Remote Access (ARA), AURP, and FDDITalk problems.
The section “AppleTalk Configuration and Troubleshooting Tips” discusses preventive measures
and tips to help you configure and troubleshoot your AppleTalk internetwork. The remaining
sections describe specific AppleTalk symptoms, the problems that are likely to cause each symptom,
and the solutions to those problems.
The following sections cover the most common network issues in AppleTalk environments:
• AppleTalk: Users Cannot Access Zones or Services
• AppleTalk: Zones Missing from Chooser
• AppleTalk: No Devices in Chooser
• AppleTalk: Network Services Intermittently Unavailable
• AppleTalk: Old Zone Names Appear in Chooser (Phantom Zones)