Selecting RU Size for SNA Sessions

The request/response unit (RU) size for IBM® Connect:Direct® is specified in the VTAM RUSIZES parameter in the VTAM log mode table. The following example shows a mode table entry with a secondary and primary logical unit RU size of 1,024 bytes.

CDLOGM   MODEENT LOGMODE=CDLOGM,                                    X

When selecting an RU size for IBM Connect:Direct, it is important to know how RU size relates to the NCP MAXDATA parameter.

The NCP MAXDATA value is specified in the NCP physical unit (PU) macro definition. It specifies, in bytes, the maximum amount of data that the NCP can receive from the PU at one time. This amount includes the transmission header and the request/response header, totaling 29 bytes for IBM Connect:Direct.

For example, if the IBM Connect:Direct VTAM log mode table entry specifies 4,096 bytes, the NCP MAXDATA value must be at least 4,125 bytes for IBM Connect:Direct to function correctly. In other words, the NCP MAXDATA value must be at least 29 bytes larger than the RU size specified in the IBM Connect:Direct VTAM log mode table.

  • The minimum RU size value for IBM Connect:Direct API-to-DTF transmissions is 512 bytes. The maximum RU size value is 64 KB bytes. For best results, use a value of 7 KB.
Note: For locally attached non-SNA 3270 connections, specify RUSIZES =X‘87C7’.