Service Advertising Protocol (SAP)

The Service Advertising Protocol (SAP) allows service-providing nodes -- such as file servers, print servers, gateway servers, and application servers -- to advertise their services and addresses. SAP makes the process of adding and removing services on an internetwork dynamic. As servers are booted up, they advertise their services using SAP; when they are brought down, they use SAP to indicate that their services will no longer be available.

SAP broadcasts

Through SAP, clients on the network can determine what services are available on the network and obtain the internetwork address of the nodes (servers) where they can access those services. This is an important function, because a workstation cannot initiate a session with a service provider without first having that server's address.

A gateway server, for instance, will broadcast a SAP packet every 60 seconds (the period defined for all servers advertising with SAP) onto the network segment to which it is connected. The SAP agent in each router on that segment copies the information contained in the SAP packet into an internal table called the server information table. Because the SAP agent in each router keeps up-to-date information on available servers, a client wanting to locate the gateway server can access a nearby router for the correct IPX address.

SAP packet structure

Like RIP, SAP uses IPX and a medium-access protocol for its transport. Figure 4-6 illustrates the structure of a SAP packet.

Figure 4-6 SAP Packet Structure

A SAP packet contains the following fields:

Operation

Specifies the operation that the packet is performing.

Service type

Specifies the type of service offered by the server.

Server name

Specifies the name of the broadcasting server.

Network address

Specifies the network number of the broadcasting server.

Node address

Specifies the node number of the broadcasting server.

Socket address

Specifies the socket number of the service provider.

Hops to server

Specifies the number of hops to the broadcasting server.

The ``Operation'' field can specify the following operations:

• a workstation request for the name and address of the nearest server of a certain type

• a general request, by a router, for the names and addresses of either all the servers or all the servers of a certain type on the internetwork

• a response to either a "Get Nearest Server" request or a general request

• periodic broadcasts by servers and routers

• changed server information broadcasts

There can be one or more sets of fields following the ``Operation'' field. If the packet contains information about more than one server, it will contain more than one set of fields (n sets of fields). Each SAP packet can contain information about up to seven servers.

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