Request unit (RU)
The Request Unit is the information that is being sent between the two network addressable units (NAU) on the session.
Depending on the session the RU can take different forms.
- The first bytes of the RU indicate a request or response code. Depending on the code, the rest of the RU takes a format specific for that code. For example, a CDINIT (Cross Domain INITiate) has a code of X'818641' while a SESSST (session started) RU has X'810686' as the first 3 bytes of the RU.
LU to LU sessions:
- The first byte might contain a code like the code in the SSCP sessions. The BIND has a code of X'31' and has a standard format.
The first bytes of the RU might be a Function Management Header (FMH). The distributed data facility (DDF) uses FMH5 to start conversations and for CNOS. It also uses FMH7 for errors. Usually, the FMH is followed by other data that is being sent.
The request unit (RU) can be data only, in a format (or it might be unformatted) that is recognized only by the applications on the session.