From Zenitel Wiki
DIP is the control protocol used between STENTOFON IP stations and AlphaCom E exchanges. It is used for all data-communication with the IP stations, such as call control, LED settings and display text to the station, and key presses and inputs from the station.
DIP is short for "Display-data over IP", and refers to the ASLT display data protocol. This protocol was based on 9600 baud one-way transmission while DIP is transmitted both ways over IP (TCP and UDP).
See also ACDP message: DIP FWD