Message switching

Message switching is part of the implicit APPC interface and allows IMS terminals and LU 6.2 application programs to exchange messages. Messages routed to an LU 6.2 application program initiate LU 6.2 application programs.

When using DFSAPPC, the remote device can choose to route a message using either the LTERM or LU 6.2 TPN option. Messages sent with the LTERM option are directed to IMS-managed local or remote LTERMs.1 Messages sent without the LTERM option are sent to the specified LU 6.2 application program.

The message format for DFSAPPC is shown in the following figure.

Figure 1. DFSAPPC message format
Read syntax diagramSkip visual syntax diagramDFSAPPC1 (2 3LTERM= valueTPN= value LU= valueMODE= valueTYPE=BMSIDE= valueSYNC=NC4 )User-data
Notes:
  • 1 A mandatory blank is required between DFSAPPC and the options.
  • 2 Use blanks anywhere within the DFSAPPC options except within keywords or values. Use commas as delimiters between keyword-parameter sets along with or in place of blanks. However, because the TP name character set allows commas, at least one blank must be used to terminate the TPN value.
  • 3 You can specify either the LTERM= or the TPN= option, but not both. Only use the other keyword options when you specify the TPN= option.
  • 4 Use the IMS default values for the DFSAPPC options to establish an LU 6.2 conversation with a partner program when the values are not provided by another source. If the DFSAPPC service is coded without specifying any options, use IMS default LU 6.2 conversation characteristics.
The DFSAPPC option keywords are defined as follows in order of occurrence (except for the keywords following TPN=, which are listed alphabetically):
LTERM=
The 1- to 8-character LTERM option is the name of an IMS LTERM. Messages sent with the LTERM option are directed to an IMS-managed local or remote LTERM. If the LTERM is associated with the LU 6.2 descriptor, it is treated as if an LU 6.2 application program has been explicitly selected. LTERM names can contain uppercase alphabetic, numeric, and national characters ('@', '$', '#'). When LTERM is specified, other keywords cannot be specified.
TPN=
The 1- to 64-character TPN option is the partner's transaction program name used with the logical unit name to establish an LU 6.2 conversation with a partner program. Because the TP name character set allows commas, at least one blank must be used to terminate the TPN value.

TP names can contain any character from the 00640 character set except a blank. The 00640 character set, documented in the CPI Communications Specification includes uppercase and lowercase letters A through Z, numerals 0-9, and 20 special characters.

When the TPN and SIDE options are specified, the TPN name overrides the TP name contained in the side information entry.

Although DFSAPPC allows the use of the 00640 character set, IMS commands do not use this character set. IMS commands can only operate on TPNs that use uppercase alphabetic, numeric, and national characters ('@', '$', '#'). IMS commands cannot operate on extended TPNs.

LU=
The 1- to 17-character LU option is the partner's logical unit name used with the transaction program name (TPN) to establish an LU 6.2 conversation with a partner program.

LU names can contain any character from the APPC/MVS™ Type A character set. LU names can contain uppercase alphabetic, numeric, and national characters ('@', '$', '#'), and must begin with an alphabetic or national character. You can also use a 17-byte network-qualified LU name in the LU field.

When the LU and SIDE options are specified, the LU name overrides the LU name contained in the side information entry.

MODE=
The 1- to 8-character MODE option is the partner's mode name used with the logical unit name and transaction program name to establish an LU 6.2 conversation with a partner program.

MODE names can contain any character from the APPC/MVS Type A character set. MODE names can contain uppercase alphabetic, numeric, and national characters ('@', '$', '#'), and must begin with an alphabetic or national character.

When the MODE= and SIDE options are specified, the MODE name overrides the mode name contained in the side information entry.

SIDE=
The 1- to 8-character SIDE option is the side information entry name used to establish an LU 6.2 conversation with a partner program.

SIDE names can contain any character from the 01134 character set. The 01134 character set, documented in the CPI Communications Specification includes uppercase alphabetic characters A through Z and numeric characters 0-9.

When the SIDE option is specified, the LU, TPN, and MODE options can also be specified to override the values in the side information entry.

SYNC=
The SYNC option allows the application to override the LU 6.2 conversation sync level default provided by IMS.
SYNC=N
Sync_Level is NONE.
SYNC=C
Sync_Level is CONFIRM.
TYPE=
The TYPE option allows the application to override the LU 6.2 conversation type default provided by IMS.
TYPE=B
Conversation type is BASIC.
TYPE=M
Conversation type is MAPPED.

If an error is found while processing the options list, error message DFS1957 DFSAPPC ERROR is sent to the terminal.

Related reading: For more information on Type A character sets, see z/OS® MVS Programming: Writing Transaction Programs for APPC/MVS.

1 If the LTERM is associated with an LU 6.2 destination, the message is sent as if an LU 6.2 application program had been explicitly selected.