A.3.4 Service Advertisement Protocol (SAP)

IPX uses numeric addresses to keep track of the networks and devices available, although these numeric addresses are not often seen by end-users. Instead, they see the "names" of the devices and services on the network (such as the name of the server they are connecting to). NetWare uses a protocol called SAP that runs on NetWare servers and routers, and is used to map the device names to their IPX addresses. This concept is illustrated in Figure A.8 below:

Figure A.8 When a client needs to locate a device (or service) by name, it queries the nearest SAP server. These queries could be for a NetWare server, or for an application server using SPX (such as the Fax server in Figure A.6).

When a service becomes available on the network, the SAP servers make a note of the service's name and the IPX address that it is hosting the service. The various systems running SAP exchange their databases once every sixty seconds, guaranteeing that each system will see all of the services available across the entire network. On small or medium LANs, this does not consume a significant amount of bandwidth, although on a large nationwide network with hundreds of sites, this constant exchange of data can cripple a network.

Although SAP is really an application-level protocol, it uses IPX directly and does not use SPX or PEP. Indeed, SPX relies on SAP to locate servers by name instead of by their physical address.

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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