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.