External specification of execution modes

Messages in ISC VTAM® use the SNA-defined function management headers. ISC messages that are routed by VTAM can be processed either synchronously or asynchronously, as determined primarily by the type of FM header and secondarily by the VTAM bracket protocols that are sent with the message.

Messages that use ISC TCP/IP for communication between IMS and IBM® CICS® Transaction Server for z/OS® use the IPIC message format, which is an internal protocol that is defined by CICS. ISC TCP/IP messages can be processed only asynchronously.

The following table provides a summary of supported VTAM protocols. Using FM headers and VTAM bracket protocols to establish the mode of the message (synchronous or asynchronous) can be considered to be external to the message.

Table 1. Processing mode requested by FM headers
FM header with VTAM bracket protocol Synchronous Asynchronous
ATTACH with CD X  
ATTACH with EB   X
ATTACH without either EB or CD X  
SCHEDULER with CD X X
SCHEDULER with EB   X
SCHEDULER without either EB or CD X X
Messages that are received by IMS with the ATTACH FM header are processed:
  • Synchronously, if the session state is left in-brackets after the message is received.
  • Asynchronously, if either of the following occurs:
    • The session state is left between-brackets after the message is received.
    • The message is sent with EB on the first- or only-in-chain.

During synchronous processing, unsolicited or asynchronous output is not sent.

All messages that are received by IMS with the SCHEDULER FM header are processed asynchronously by the receiver with respect to the session.