Alias selection function (function code X'07')

This function is processed along the session setup path during session initiation. The alias selection function can be called during:

INIT OTHER CD processing to translate the PLU and the SLU names, SLU logon mode name, PLU- and SLU-owning SSCP names, and SLU class-of-service (CoS) name.
CDINIT request routing processing to translate the DLU real names, OLU alias name, DLU logon mode and CoS names, and associated LU alias names, and to determine the DLU-owning SSCP name.
CDINIT response time to translate the CoS names, associated LU alias names, and USERVAR names, and when requested by the setting of start option TRANSLAT=USERVAR. TRANSLAT=USERVAR allows the exit to be invoked even when translation is not required, so that USERVAR information can be passed to the exit.
DSRLST routing time to translate the DLU real names and the USERVAR names, and to determine the DLU-owning SSCP names.

If the exit returns a translated name, subsequent hosts do not request translation for that name. The translated names that the exit returns vary, depending on the network configuration and the prior session establishment.

When the first logon to a DLU is attempted, VTAM® invokes the alias selection function to translate the DLU name. After the session is established, logon attempts for subsequent sessions continue to cause VTAM to invoke the alias function, but VTAM does not pass the DLU name to the exit for translation. This is because the DLU real network ID and real name are already known, either through prior translation or a predefined real CDRSC.

Examples of session flows for the alias selection function are shown in Session flows for alias selection function.

Table 1 shows the alias selection function parameter list pointed to by register 1. The parameters are described in Parameter descriptions.

Table 1. Alias selection function parameter list
Dec (Hex) Offset	Size (Bytes)	Description
0 (0)	4	Address of environment vectors
4 (4)	4	Address of function code and related session information
8 (8)	4	Address of user data field
12 (C)	4	Address of PLU resource identifier control vector
16 (10)	4	Address of SLU resource identifier control vector
20 (14)	4	Address of session ID
24 (18)	4	Reserved
28 (1C)	4	Address of OLU gateway information vector
32 (20)	4	Address of DLU gateway information vector
36 (24)	4	Address of OLU adjacent SSCP vector
40 (28)	4	Address of DLU adjacent SSCP vector
44 (2C)	4	Address of alias selection input parameter list
48 (30)	4	Address of alias selection output parameter list
52 (34)	4	Reserved
56 (38)	4	Address of USERVAR resource identifier control vector (can be 0)
68 (44)	4	Address of session authorization data vector
72 (48)	4	Address of VTAM exit services parameter list
76 (4C)	4	Address of session management data area