An APPN class of service (COS) defines the required or requested characteristics of a route for a session. A COS consists of a set of ranges of acceptable values for the characteristics of links and nodes to be used for a session specifying that particular COS. APPN Classes of Service are defined in a VTAMLST definition list.

Unlike the COS for the subarea network, where the COS is actually a list of VRs that are acceptable for a particular COS, APPN COS specifies the types of routes that are acceptable for a Class of Service.

Class of Service database

The COS database exists on all VTAM® network nodes (NNs) and end nodes (ENs). It is the same on both ENs and NNs, and it is built when VTAM is initialized.

The COS database contains the following items:

• COS definitions from VTAMLST definition files

  IBM® provides three sets of COS definitions: COSAPPN, ISTACST2. and ISTACST3. Each set contains the same seven default APPN COS definitions. However, differences exist in the way the seven Classes of Service are defined in each set. See What are the IBM-supplied default classes of service? for information about the differences between the three sets and how to determine which one is appropriate for your configuration. It is recommended that you use the defaults wherever possible and that the COS definitions be the same throughout the network. The default APPN COS definitions in z/OS® Communications Server (COSAPPN) are the same default Classes of Service provided by other APPN products.

  One or all of the IBM-supplied sets of COS definitions can be included in VTAMLST. User-defined Classes of Service can also be included. For information about how to define your own COS, see the z/OS Communications Server: SNA Resource Definition Reference.

  You can have only one set of COS definitions in VTAMLST active at any time. The IBM-supplied COS definitions in APPNCOS are automatically activated during z/OS Communication Server initialization. COS definitions can also be activated with a VARY command or by specifying the name of the VTAMLST member in a CONFIG list. Entries can be added or modified by activating or reactivating the VTAMLST definitions.

• A list of logon mode names and the corresponding APPN COS

  A set of default mappings for the seven IBM-supplied classes of service is included in the default logon mode table. It is recommended that users also code one of the seven standard APPN COS names in any user defined mode tables. For information about defining user-specified modes and corresponding Classes of Service, see z/OS Communications Server: SNA Resource Definition Reference.

• A list of calculated routes

  As an NN calculates routes for particular classes of service, the routes are stored in VTAM memory. Storing routes allows them to be reused, rather than being recalculated each time a route is requested.

  Routes are periodically recalculated to allow for distribution of sessions over equally weighted paths. Start options control this process; see z/OS Communications Server: SNA Resource Definition Reference.