The IPX/SPX protocol suite isn’t nearly as orderly as others when it comes to mapping upper layer protocols to the OSI model. Many of the protocols found above the network layer span multiple OSI layers between Transport and Application. The mapping of the suite to the OSI model at the beginning of this section helps to illustrate this. While the actual mappings may not be so clear-cut, the purpose of these protocols is what is most important. We’ll take a look at a number of these here including SAP, NCP, NetWare Shell, and NetBIOS.
Service Advertising Protocol (SAP)
In older version of NetWare (those prior to version 4.0), servers made other systems aware of the services that they offered through the use of Service Advertising Protocol (SAP) broadcasts. For example, a NetWare server might be functioning as a both a file and print server. In this case, the server would broadcast SAP advertisements onto the network, making clients aware that these services are offered at its network address. SAP advertisements are also used by clients to find their nearest server at startup, in the form of a Get Nearest Server (GNS) request. SAP identifiers are hexadecimal numbers that represent the services. Common examples of SAP identifiers are listed below.
File Server 0x4
Print Server 0x7
Servers send out SAP broadcasts every 60 seconds by default. Since these advertisements are broadcasts, they are limited in scope to the local broadcast domain. For this reason, clients in other broadcast domains will not hear these advertisements. To account for this, Cisco routers can be configured with access lists that can selectively propagate or ignore SAP advertisements from different systems. Similarly, when a local NetWare router is not available, a Cisco router can be configured to respond to GNS requests. Access lists and their configuration will be looked at in detail in Chapter 9.
Newer versions of NetWare that use eDirectory or NetWare Directory Services (NDS) rely on SAP broadcasts to a lesser degree, since the location of servers can be found via a query to an NDS/eDirectory server.
NetWare Core Protocol (NCP)
The NetWare Core Protocol (NCP) is a set of procedures followed in IPX-based communication. When a client using the NetWare Shell attempts to access file, print, or directory resources on a NetWare server, NCP processes the request.
In a Windows environment, the line between client and server roles is blurred. NetWare clients, on the other hand, are those running another operating system such as Windows 2000. The NetWare Shell is additional redirector software installed on a client to allow it to make requests to NCP on a NetWare server. For example, when a client wishes to access a network drive, the request is passed to the NetWare shell, which creates an NCP request. It then passes the NCP data to IPX to be forwarded over the network. In this way, a client application is unaware that the request is actually for a remote resource.
NetWare also provides an emulated environment in which NetBIOS applications can be run. NetBIOS is a broadcast-based Session layer protocol developed by Microsoft and IBM that uses names instead of numerical addresses to represent clients. NetBIOS traffic can be encapsulated in IPX packets to allow NetBIOS-based communication across an internetwork.