Using function management headers

SNA architecture defines a particular type of header field that accompanies some messages, called a function management header (FMH).

The header conveys information about the message and how it should be handled. For some logical units, use of an FMH is mandatory, for others it is optional, and in some cases FMHs cannot be used at all. In particular, FMHs do not apply to LU type 2 and LU type 3 terminals, which are the most common 3270 devices.