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What is SLP and Why Is It Needed?

The Service Location Protocol (SLP) is a protocol for service discovery. It allows clients to locate servers and other services on the network. Reliable service discovery used to be accomplished using IPX SAP (Service Advertising Protocol) broadcast messages. Since IPX is no longer routed through NCSU's network, SLP must be used in order for clients to reliably locate services.

Clients that have not been configured to use SLP may indeed be able to locate services without IPX routing. This may be the result of a hosts file on the client that has been populated with the IP addresses of the services the client needs. This may also be the result of multicast messages the client is sending out to request services. Neither of these methods are as robust as SLP.

If SLP is not configured, intermittent service location problems may occur which are difficult to diagnose. Symptoms of service location problems may include error messages on the client such as "Tree Not Found", "Tree Unknown", or "Server Not Found".

For more information concerning the requirement of SLP in an IP-only network, see:
Is SLP required in a pure IP NetWare 5 Network? - TID 10024578

For more information concerning how a client may be able to locate services without SLP see:
Understanding the NetWare5 Client SLP Discovery/Login Process. - TID 10014303

SLP basically consists of 4 different components. They are:

• User Agents (UA)- these are the consumers of SLP. UAs make requests for particular services as needed. A UA is built into Novell's client software.
• Service Agents (SA)- these are associated with providers of particular services. Each service that is compatible with SLP has an SA associated with it. If DAs do not exist on the network or if clients are not configured to use DAs, the SAs will answer UA requests directly. If a DA exists and is properly configured, SAs will register their services with the DA and the DA will reply to client service requests. SAs are built into many programs and devices. Netware 5.1 and later includes built-in SAs (there are multiple different SAs built into Netware). Printers and other devices also include SAs.
• Directory Agents (DA)- these are the gatherers of services. SAs register their services with DAs. The DA will keep track of all of the services. Depending on how the SAs and DAs are configured, the DA will organize the services into scopes. When a client requests a particular service, the DA will answer the request. This frees the SAs from this task. ITD provides DA services to NCSU. Colleges and departments should not have to deploy their own DAs.
• Scopes- these are sets of services. The SAs can dictate which scopes they want their services registered in. DAs are configured to only service certain scopes. The DA services that ITD provides service the scope called "NCSU". Any services registered in this scope should be available to clients configured to use this scope.
  

For more advanced information about SLP, see the Links and Resources page.

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