SPX is a reliable transport protocol that uses IPX for delivery. Since IPX is a true "network" protocol that offers no reliability by itself, applications that use the network must have some form of reliable transport, which is a service provided by SPX. Examples of these applications might include a client-server database, or a server-based virus-protection product, or a fax gateway product similar to the one shown in Figure A.6 below. Rather than have the client application read and write data to a file on the server's volume, the client application speaks to a server-based application using tokenized commands sent over SPX.
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SPX achieves this connectivity through "virtual circuits", which provides an application-to- application connection across IPX. SPX does not perform error-checking and acknowledgment services for every packet sent or received, but instead acknowledges all the packets it has received within a certain "window." If an error is discovered in any packet sent or received within that window, then all of the packets sent or received during that time are flagged as bad and are retransmitted.
Since most of the networks that use IPX and SPX are local LANs that have high levels of reliability, this window technique allows large amounts of data to flow quickly with little risk of corruption. Therefore, the wholesale marking of bad packets within a window does not occur very often, and is a good way to get extra performance from the network.
For more information on IPX, refer to section A.3.1 Internetwork Packet Exchange (IPX). For more information on PEP and NCP, refer to section A.3.3 Packet Exchange Protocol (PEP) and the NetWare Core Protocol (NCP). 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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