gtps2m0x | ACF/SNA Data Communications Reference |
TPFs support of the NPSI request unit (RU) chaining allows messages of up to 32K in length to be transmitted and received through an X.25 network. Input arriving in 4K blocks is deblocked into a chain of 1055-byte blocks to insulate the application from the usage of 4K blocks. Messages chained in 4K blocks can be queued and dequeued. The NPSI RU chaining function allows chaining of 1055-byte blocks and 4K blocks to virtual circuits. This RU chaining function is optional on a multi-channel link basis and is specified in the NPSI generation by using the MBITCHN=YES/NO keyword. You can use the CHAIN parameter in OSTG to specify the NPSI resources using the RU chaining function. (For additional information on the CHAIN parameter, see the TPF ACF/SNA Network Generation.)
There is a restriction of chained 381-byte or single 1055-byte or 4K blocks for NPSI resources that OSTG has defined as not supporting RU chaining.
Support of the NPSI RU chaining function also affects TPF's management of maximum RU sizes. Maximum RU sizes, or maximum input buffer sizes, are negotiated at session initiation time. The maximum RU size dictates the size of PIU segments in an RU chained message.
Bind processing uses the SESINIT and CDCINIT values to fill RV1MAXIP in the RVT. These values are used by bind processing to specify TPF's maximum input buffer size. This value is the least of:
To fill in RV1MAXDT, the remote resources maximum input buffer size, bind processing uses the SESINIT and CDCINIT values. This value is less than:
OMT creates PIU segments equal to the remote resources maximum RU size (RV1MAXDT) whenever possible. Because of core block size restrictions, values greater than or less than 4027 bytes may not be honored. OMT constructs segments of 4027 bytes when a value greater than this is found. This allows applications that do not have the capability to chain 1055-byte or 4K blocks to take advantage of NPSI's ability to handle 4K PIUs. For instance, if an application issues ROUTC with an output message in chained 381-byte blocks, OMT packs the output into a 4K block and issues SOUTC of the PIU segment. This significantly reduces the number of PIUs flowing in the SNA network.
You can tune the SNA network performance by updating the VTAM LOGMODE table. TPF uses its bind image table as absolute maximum values to prevent you from specifying a value that is too high for TPF.