12.7.1 Facility Specification and Negotiation

X.25 user facilities are specified on a per-call basis. The X25_SET_FACILITY ioctl is used to set facilities one at a time. The X25_GET_FACILITY ioctl is used to read facilities one at a time. These ioctl commands support all facilities (1980 and 1984 X.25).

Facilities are set in two places: before issuing a connect call, in order to request desired facilities in the Call Request packet; and before issuing a listen call, in order to negotiate the facilities proposed in an Incoming Call packet.

Facilities are usually read in two places: after a call to connect has succeeded, and after a call to accept has succeeded. This is done to determine the values of the facilities in effect for the resulting connection. Facilities can be read at any time, in general, to determine values which were previously set.