The Unrestricted Simplex Protocol

Assuming:

• a (unidirectional) error free channel,
• that the sender's network layer has unlimited data to send (being "pushed down" from above), and
• that the receiver's network layer has an infinite buffer to receive the data (being "pushed up" from below).
• that the functions READ_xxx_LAYER() and WRITE_xxx_LAYER() block until their actions are complete - they execute synchronously.

In the sender:

FRAME frame;
int   len, link = 1;

while( true ) {
    READ_NETWORK_LAYER(frame.data, &len);
    frame.len = len;
    WRITE_PHYSICAL_LAYER(link, &frame, FRAME_SIZE(frame));
}

In the receiver:

FRAME frame;
int   len, link;

while( true ) {
    READ_PHYSICAL_LAYER(&link, &frame, &len);
    WRITE_NETWORK_LAYER(frame.data, frame.len);           
}

Note: when passing an array to a function in C, as we do for our frame's payloads, we do not need to place the '&' operator in front of the array's name.