#define MAX_PKT 1024	/* determines packet size in bytes */typedef enum {false, true} boolean;	/* boolean type */typedef unsigned int seq_nr;	/* sequence or ack numbers */typedef struct {unsigned char data[MAX_PKT];} packet;	/*    packet definition */typedef enum {data, ack, nak} frame_kind;	/* frame_kind definition */typedef struct {	/* frames are transported in this layer */  frame_kind kind;	/* what kind of a frame is it? */  seq_nr seq;   	/* sequence number */  seq_nr ack;   	/* acknowledgement number */  packet info;  	/* the network layer packet */} frame;/* Wait for an event to happen; return its type in event. */void wait_for_event(event_type *event);/* Fetch a packet from the network layer for transmission on the channel. */void from_network_layer(packet *p);/* Deliver information from an inbound frame to the network layer. */void to_network_layer(packet *p);/* Go get an inbound frame from the physical layer and copy it to r. */void from_physical_layer(frame *r);/* Pass the frame to the physical layer for transmission. */void to_physical_layer(frame *s);/* Start the clock running and enable the timeout event. */void start_timer(seq_nr k);/* Stop the clock and disable the timeout event. */void stop_timer(seq_nr k);/* Start an auxiliary timer and enable the ack_timeout event. */void start_ack_timer(void);/* Stop the auxiliary timer and disable the ack_timeout event. */void stop_ack_timer(void);/* Allow the network layer to cause a network_layer_ready event. */void enable_network_layer(void);/* Forbid the network layer from causing a network_layer_ready event. */void disable_network_layer(void);/* Macro inc is expanded in-line: Increment k circularly. */#define inc(k) if (k < MAX_SEQ) k = k + 1; else k = 0