Networking on z/OS
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APPN databases

Networking on z/OS

APPN provides automatic network topology and directory support within APPN networks that simplifies network definition and permits dynamic selection of network routes. Some of the functions of APPN are topology and route selection services, and distributed directory searches.

APPN topology and route selection services

Topology and route selection services selects the best route to access a remote LU based on a set of user-specified criteria. Using the properties of the nodes and links in the network that are maintained in a network node topology database, a network node server calculates the best route from the local control point of the primary LU to the control point of the secondary LU according to the class of service selected by the LU initiating the session.

Topology and routing services is responsible for three functions:
  1. Maintaining the topology database
  2. Maintaining the LOGMODE-to-APPNCOS mapping table
  3. Calculating session routes based on (1) and (2).

Distributed directory searches

Distributed directory searches determine (through a network search) the current node location of any remote logical unit (LU) that is known locally only by name. This alleviates the need to define routing or location information for every remote LU with which a local LU can establish a session. The information collected during the directory search is stored in the network node's directory database.

You can compare the directory services database to a telephone book or address book, in which you look up a name (an LU's name) and determine its address.

An SNA session is the logical connection between two LUs. The LU that originates the session is named the originating LU (OLU), and its session partner is the destination LU (DLU). As its name implies, the originating LU initiates the session by sending a BIND to the destination LU.

The topology database is like a map, in which the APPN network nodes furnish the two addresses (the locations of the originating logical unit, OLU, and destination logical unit, DLU) and decide what the best route is between them based on your "driving requirements" (or "class of service"). That is, do you want to take the high speed route, the scenic route, or the most direct route? The BIND sent by the originating logical unit uses the best route that is available at the time the session is setup.

  • Topology database (TOPO DB)
    The topology database consists of a local topology database, unique to a node, and a network topology database, whose entries are replicated across all network nodes in the same topology subnetwork. The topology database stores and maintains the nodes and the links (transmission groups or TGs) in the networks and their characteristics.
  • Directory services database (DS DB)
    An APPN network node provides directory services to its locally resident LUs and to the LUs in its client end nodes. It also assists other network nodes in the network in their search for resources. When network nodes learn the location of resources by searching, they cache the location of these resources in the local directory database.
Parent topic: SNA Advanced Peer-to-Peer Networking (APPN)

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