A VTAM® application program and an LU exchange requests and responses to requests. A request normally contains data. In addition to data, or instead of data, a request can contain control information (described in VTAM publications as control requests or indicators). A response normally contains information about whether a particular request arrived and was processed successfully or unsuccessfully. The response can also contain control information. As explained later in this chapter, a response does not have to be returned for every request; it is possible for an application program and an LU to communicate without either side ever sending a response. (In this book, a request corresponds to an SNA request unit [RU] and associated request header [RH]. A response corresponds to an SNA response unit [RU] and associated response header [RH].)

In this chapter, the legend in Figure 1 is used in figures that depict request and response flows:

Figure 2 illustrates an exchange of requests and responses between an application program and an LU.