A.3.3 Packet Exchange Protocol (PEP) and the NetWare Core Protocol (NCP)

Another higher-level transport protocol similar to SPX is PEP. PEP is used exclusively for the delivery of the NCP packets used for file reads and writes on a NetWare server.

NCPs provide a "dictionary" of I/O-related network commands, and are the heart of the NetWare server's distributed filesystem. Standard NCP commands exist for opening a file on the server, or for writing to a file, etcetera. Whenever a client wishes to open a file on the server, the NetWare client intercepts the system command and issues the equivalent NCP command to the NetWare server. The server receives the command, opens the file, and passes the information back to the client (this procedure is illustrated in Figure A.7 below). By using a standardized set of commands, the NetWare clients are not dependent upon the operating system's calls, and the network can be optimized around a consistent set of services.

Figure A.7 NCPs provide the client-independent file- and print-access commands commonly used in NetWare networks.

NCPs are sent using the PEP protocol, which provides extremely high levels of reliability. Each PEP packet is verified independently. When a packet is sent using PEP, an internal timer is started, and no other NCP packets can be transmitted until a response is received. If the timer expires, PEP rebuilds and retransmits the packet, and restarts the NCP timer. This handshaking and waiting consumes a lot of network bandwidth, but absolutely guarantees that the packets get delivered. On small LANs, this goes unnoticed, but in large or exceedingly complex networks, performance can suffer tremendously from these long waiting periods.

For more information on IPX, refer to section A.3.1 Internetwork Packet Exchange (IPX). For more information on SPX, refer to section A.3.2 Sequenced Packet Exchange (SPX). For more information on SAP, refer to section A.3.4 Service Advertisement Protocol (SAP). For more information on IPX routing, refer to section A.3.5 Routing Information Protocol (RIP) and NetWare Link State Protocol (NLSP).

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