ATTDBA

The deblocking algorithm (ATTDBA) determines the data blocking and deblocking algorithm to be used.

This algorithm, along with an indication of whether MFS is being used, determines the amount of data presented to or from a process for a single get or put operation.

When IMS is sending messages and MFS is not being used, each segment of the message on the message queue becomes a data entity within the RU (or chain of RUs). If a segment of the message spans message queue records (LRECLs), then each spanned portion of the segment becomes a data entity. If MFS is used, the segments are blocked and sent as a data entity according to the definition of the MFS record.

When IMS receives messages, each data entity of the input message is inserted in the message queue as a segment of the message. Because ISC does not support spanned queue segments on input, the largest message queue LRECL must be large enough to handle the largest data entity or MFS data record received. If this LRECL definition is exceeded, an error is detected and an appropriate error message is produced (DFS074). Also, the actual space available in the message queue is reduced by any variable prefix items on the message.

IMS implements four of the algorithms defined by SNA:
UNDEFINED
Same as RU, below, for IMS.
RU
Input RU is the entity presented to the attached process. IMS receives this DBA value but does not send it.
VARIABLE LENGTH, VARIABLE BLOCKED (VLVB)
Each data entity sent or received is preceded by a 2-byte length field that allows it independence from RU size or boundaries. Several data entities can be blocked into a single RU, or a data entity can span RUs. The length includes the 2 bytes for the length field itself.
CHAIN OF RUs
Each data entity is sent or received as a single SNA chain.

Each message received by IMS is deblocked based on the ATTDBA field in the ATTACH FM header. The input ATTDBA field value must indicate a supported algorithm, or the input is rejected.

The input ATTACH DBA parameter remains in effect until it is:
  • Changed by another ATTACH
  • Backed out to its value at the last sync point by the ERP
  • Reset at between-brackets state
  • Reset by the RAP FM header
  • Reset by a session failure

The input reset state for the ATTDBA parameter is UNDEFINED (equivalent to RU).

Because IMS does not remember the active input ATTDBA across IMS system failures, the ATTDBA must be provided using an ATTACH FM header to restore the receiving process after a bind or negotiable bind response indicating in-brackets (possible restart).

With the exception of output using MFS stream mode, output from IMS is automatically sent indicating the variable-length, variable blocked (VLVB) algorithm in the ATTACH or SCHEDULER FM header. This output is sent indicating chain assembly.