#define MAX_PKT 4	/* 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