Setting initial pacing values

The type of session-level pacing and pacing window sizes are determined by the BIND request when a session is established. When an extended BIND is used, adaptive session-level pacing is requested. If adaptive-session pacing is not supported or required by the session partner, the BIND response will be sent indicating fixed-session pacing. (XBSI is the extended BIND indicator in the session initiation request. XBSI=ON means that extended BIND is used to establish the session.) The actual values used are those in the BIND response.

The pacing values are carried in the BIND request flowing from the primary LU to the boundary function (if one exists for this session), and then to the secondary LU. In this way, all the participants in the session are informed of the pacing values to be used.

With this in mind, the possible pacing windows for a session are:

PS: Primary send
SR: Secondary receive
SS: Secondary send
PR: Primary receive

The direction of the data being sent determines which pacing window values are used.

When data is sent from the primary LU to the secondary LU, the PS and SR values are used.
- The primary send count controls the flow of requests from the primary logical unit to the boundary function,
- The secondary receive count controls the flow from the boundary function to the secondary logical unit.
When data is sent from the secondary LU to the primary LU, the SS and PR values are used.
- The secondary send count controls the flow of requests from the secondary logical unit to the boundary function.
- The primary receive count controls the flow from the boundary function to the primary logical unit.

The values for the four pacing windows are derived from:

The following operands on the MODEENT macro instruction for the logon mode entry:
- PSNDPAC for primary send
- SRCVPAC for secondary receive
- SSNDPAC for secondary send
The following operands on the LU or CDRSC definition statement:
- PACING
- VPACING
The following operands on the APPL definition statement:
- AUTH=NVPACE|VPACE
- VPACING

Note: An application program acting as a primary end of a session can obtain information about pacing counts using the INQUIRE macro instruction or the LOGON exit routine. For more information, see z/OS Communications Server: SNA Programming.

The sources used to determine pacing-window values are dependent on the configuration and the secondary LU (SLU). Figure 1 and Figure 2 describe the environment, and Table 1 identifies the sources used in determining the initial pacing window value when the primary LU (PLU) is an application program. For example, if the SLU is an application program, the pacing sources, regardless of whether an extended BIND is sent, are described in Table 1 under:

Letter I for primary to secondary flow
Letter D for secondary to primary flow

Notes:

For fixed-session pacing, the pacing window values derived are the actual values used.
For adaptive-session pacing, the pacing window values derived represent the minimum pacing window size to be used.

Table 1. Correspondence of methods to letters
Letters	Methods used by VTAM
A	If SRCVPAC is not 0, use it else use SLU VPACING (default is 7)
B	If SRCVPAC is not 0, use it else use SLU PACING (default is 1)
C	Use SRCVPAC
D	If SSNDPAC is 0, use it else use PLU VPACING (default 7)
E	Use SSNDPAC
F	If PLU AUTH=NVPACE, use 0 else if PSNDPAC is not 0, use it else use SLU VPACING (default is 2)
G	If PLU AUTH=NVPACE, use X'5F' else if PSNDPAC is not 0, use it else use SLU VPACING (default 1)
H	Use PSNDPAC
I	If PLU AUTH=NVPACE, use 0 else if SRCVPAC is not 0, use it else use SLU VPACING
J	Use PLU VPACING (default 7)

Figure 1. Pacing windows for SNA LUs. Coding is from the perspective of the owning SSCP acting as the PLU.

Figure 2. Pacing windows for non-SNA LUs. Coding is from the perspective of the owning SSCP acting as the PLU.