Compression level negotiation
If the value specified on the COMPRES operand of the MODEENT macroinstruction allows negotiation, the compression levels for a session are negotiated through the BIND and BIND(RSP) RUs.
The data compression information is carried on a control vector X'66'.
Control vector X'66' is used only for compression-level negotiation.
The control vector is composed of these subvectors:
- X'80' is passed on a CDCINIT and a CINIT.
- X'81' is passed on a BIND to carry the requested compression levels.
- X'82' is passed on a BIND(RSP) to carry the actual compression levels.
The SLU first requests a compression level on the CDCINIT. The PLU responds with requested levels specified on the BIND. The SLU responds with actual levels on the BIND(RSP).
Note: There are limited negotiation capabilities for sessions using
a nonextended BIND. Bits 6 and 7 in byte 25 of the BIND are reserved
for data compression, and are used to negotiate whether compression
is acceptable or not. No level negotiation is possible. In the PLU-to-SLU
direction, level 2 is the highest possible compression level (If VTAM® owns the PLU, setting CMPVTAM
= 1 further limits the PLU-to-SLU level to 1). In the SLU-to-PLU direction,
level 1 is the highest possible compression level.