Simplified communication protocol model
We simplify the model to the four lowest software layers - user applications use the process layer and the remaining three are usually included in the operating system, such as Unix, which has an OSI stack to process messages through the layers.
Host-to-host layer: may establish virtual circuit for messages
Determines route to transmit packets
Transforms raw messages into one that appears free of transmission errors, transmits data cells and processes acknowledgement cells.