Gateway path selection

For cross-network sessions, the gateway path selection function of the session management exit routine can be used to modify a list of alternate gateway paths to distribute LU-LU sessions among two or more gateway NCPs. You can shorten the list by deleting entries or reorder it by putting the preferred paths at the top of the list.

If you do not provide this function in your session management exit routine, VTAM® uses the list of gateway paths determined by the GWPATH definition statements. However, the gateway NCP used for the SSCP-SSCP session is always the first one used for LU-LU sessions. For information about using the session management exit routine, see z/OS Communications Server: SNA Customization.

A gateway path is selected by one of the following methods:

SSCP-SSCP sessions

Each successive gateway path between two VTAMs is tried until either the SSCP-SSCP session is established or all of the possible session paths have been tried unsuccessfully.

The class of service name that is normally used is the COS name for SSCP sessions, ISTVTCOS. This is the default name that is used for establishing a cross-network SSCP-SSCP session and, if used, is the COS name that is passed to the NetView® alias name translation facility or the VTAM session management exit routine. You can specify a different Class of Service name by coding the value on the ADJNETCS operand on the GWPATH definition statement. VTAM uses that COS name to locate the virtual route list in the proper COS table as specified on the COSTAB operand of the BUILD or NETWORK definition statements in NCP. If the ADJNETCS operand is specified, VTAM does not invoke the NetView alias name translation facility or the VTAM session management exit routine. For more information about network routing in a multiple-network environment, see Cross-network routing.

LU-LU sessions

Each gateway NCP (defined by a GWPATH definition statement) between the two VTAMs is tried until one of the following occurs:

The requesting logical unit receives a positive response to the initiate request for the session, meaning that the requested session partner has been located.
All the possible gateway NCPs (defined by the GWPATH definition statements for the CDRM in another network) have been tried unsuccessfully.
A failure not related to the GWNCP is detected.

If multiple gateway paths are used, VTAM selects a particular gateway NCP for each LU-LU session. The gateway NCP used for the cross-network SSCP-SSCP session is always the one VTAM attempts first for LU-LU session setup. If this is unsuccessful, the gateway VTAM selects another gateway NCP for the session. If you coded a session management exit routine to shorten or reorder the list of gateway NCPs, the gateway VTAM uses that altered list to establish the LU-LU session path.

During session establishment of a cross-network LU-LU session, the gateway VTAM that is controlling gateway path selection gets the list of alternate gateway NCPs defined by the GWPATH definition statements. If the BIND RU that flows between the requesting logical unit and the session partner fails because a virtual route required by the session cannot be activated, no further gateway path selection is attempted. VTAM messages are issued to inform the operator that session establishment failed, and the requesting logical unit is informed.