Defining an APPC Logon Mode

A logon mode contains a set of parameters and protocols that determines the communication characteristics of a session. Logon modes are entries in a logon mode table, a compiled version of which exists in SYS1.VTAMLIB.

To enable LU 6.2 on MVS, you need the VTAM® logon mode SNASVCMG that is in the SYS1.SAMPLIB logon mode table sample named ISTINCLM.

Further requirements depend on the version of VTAM your installation is using:

With VTAM 4.3 or earlier, you must define at least one logon mode entry other than SNASVCMG. To define additional logon modes, you may copy SNASVCMG, rename it and, if desired, alter the values.
With VTAM 4.4 or later, you may define additional logon modes, but are not required to do so. However, consider defining additional modes, to ensure that the system provides the session characteristics that are appropriate for your installation's APPC/MVS work. If you do not provide any logon modes other than SNASVCMG, APPC/MVS uses logon mode ATB#MODE, which means that session characteristics are determined by the value specified, or by the default, for the DLOGMOD parameter on the VTAM APPL statement.

For an explanation of the parameters in SNASVCMG, and a description of the DLOGMOD parameter of the APPL statement, see z/OS Communications Server: SNA Resource Definition Reference.

Figure 1 shows a logon mode that controls session level pacing. Controlling pacing is especially important for sessions between unlike systems that have differing processing capabilities.

Figure 1. Example logon mode (APPCPCLM)

*********************************************************************
*       LOGON MODE TABLE ENTRY FOR PC SESSIONS                      *
*********************************************************************
APPCPCLM MODEENT
             LOGMODE=APPCPCLM,                                      X
             RUSIZES=X'8787',                                       X
             SRCVPAC=X'00',                                         X
             SSNDPAC=X'01'

The maximum length of data that can be sent over a session is specified by the RUSIZES parameter. VTAM formats data into RUs and sends those RUs across the network. Pacing parameter SSNDPAC controls the number of RUs sent before the sending VTAM waits for a response from the receiver. When the receiver cannot accept the RUs fast enough, VTAM buffers become flooded with data. By specifying the appropriate pacing parameters, you can prevent the flooding of buffers.

Figure 2 shows a logon mode that enables dependent LU support.

Figure 2. Example logon mode (MVSAPPC)

*************************************************************************
*       LOGON MODE TABLE ENTRY FOR PC SESSIONS, DEPENDENT LU            *
*************************************************************************
MVSAPPC MODEENT LOGMODE=MVSAPPC,      APPC/MVS SESSION - DEPENDENT LU   X
                TYPE=0,                 NEGOTIATED BIND                 X
                FMPROF=X'13',                                           X
                TSPROF=X'07',                                           X
                PRIPROT=X'B0',          EX/DEF RESPONSE                 X
                PRIPROT=X'B0',          EX/DEF RESPONSE                 X
                SECPROT=X'B0',          EX/DEF RESPONSE                 X
                COMPROT=X'50A0',                                        X
                SSNDPAC=X'01',                                          X
                SRCVPAC=X'00',                                          X
                RUSIZES=X'8989',        4096 BYTE MAX RU SIZE BOTH DIR  X
                PSERVIC=X'060200000000000000000000'    LU TYPE 6.2

In Figure 2, important parameters to note are:

LOGMODE: Specifies the logon mode name to be used as a key for the session parameters in this table entry. This logon mode name corresponds to the logon mode an application programmer specifies in side information or in an APPC/MVS Allocate call.
RUSIZES: Specifies the maximum length of data in bytes that can be sent. The suggested value of X‘8989’ translates to a maximum length of 4096 bytes of data that can be sent at a time from each direction. There is no limit on how much total information can be sent.
SRCVPAC: Specifies the secondary receive pacing count. The suggested value is X‘00’. If zero, the value of the VPACING operand on the VTAM APPL statement controls both send and receive pacing for all sessions in all modes. A value of zero makes it easier to predict pacing results and makes it easier to maintain pacing definitions.
If non-zero, the VPACING value controls pacing in one direction, and the SRCVPAC value controls it in the other. LU 6.2 protocols make it difficult to predict which parameter will be in control at any given time.
SSNDPAC: Specifies the secondary send pacing count. Do not specify zero. If zero is used, outbound pacing for sessions is disabled, which can result in problems with IOBUF storage.

For more detailed information about logon mode parameters, see z/OS Communications Server: SNA Resource Definition Reference.