In an APPN network, LU-LU session initiation requires that the control point serving the requesting LU obtain partner LU location and route information to establish the session. For example, when the control point in the end node of the requesting LU receives the session initiation request, it forwards a Locate request to its network node server to locate the partner LU. If the resource is known to the network node server, the network node server sends a verification search directly to the resource. If the resource is not known to the network node server (that is, if the resource information has not been previously cached), the network node server conducts a network-wide search for the resource.

After information about a resource has been cached, the resource verification reduction function can be used to eliminate the verification search for the partner LU. Either the network node server of the requesting LU or the network node server of the partner LU may use the cached information to build a reply on behalf of the destination LU when resource verification reduction is requested. As long as the cache information is accurate and complete, session setup time is reduced. The maximum amount of time cached information will be used for resource verification reduction is controlled by the VFYREDTI start option. If the information is older than the value of the VFYREDTI start option value, resource verification will be performed upon receiving the next request for the resource. For more information about the VFYREDTI start option, see the z/OS Communications Server: SNA Resource Definition Reference.

Cached information that is not accurate may cause the session setup to fail. When this occurs, VTAM® will redrive the session initiation request, forcing verification of the resource.

If the requesting application has a LOGON exit, it is driven for a second time when the second initiation request is successful. However, the CINIT information for the second request contains different session information than that of the initial LOGON exit invocation. Thus, an application program with a LOGON exit must be able to have its LOGON exit driven multiple times as a result of one logical request (for example, an ALLOCATE). Application programs that support having their LOGON exits driven multiple times must indicate this support, by coding the application capabilities vector with the multiple logon exit support indicator on, to take advantage of resource verification reduction. For more information about how to code the application capabilities vector, see z/OS Communications Server: SNA Programmer's LU 6.2 Guide.

The benefit of resource verification reduction is limited for session requests traversing subnetwork boundaries (border nodes and interchange nodes). Resource verification reduction can eliminate a verification search across only one of the traversed subnetworks.