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