Channel exit programs

Channel exit programs are programs that are called at defined places in the processing sequence of an MCA. Users and vendors can write their own channel exit programs. Some are supplied by IBM®.

There are several types of channel exit program, but only four have a role in providing link level security:
  • Security exit
  • Message exit
  • Send exit
  • Receive exit
These four types of channel exit program are illustrated in Figure 1 and are described in the following topics.
Figure 1. Security, message, send, and receive exits on a message channel
This diagram shows security, message, send, and receive exits on a message channel. The exits are called by the MCA at each end of the channel. The security exits are called before any messages start to flow and communicate with each other by sending security messages. A message exit is called by the sending MCA after it has got a message from the transmission queue and, at the receiving end, a message exit is called before the MCA puts a message on its destination queue. The sending MCA might need to split a message to send it over the communications connection. It then issues a communications send for each portion of the message. As a result, a send exit is called for each portion of the message and, at the receiving end, a receive exit is called for each portion.