/*該協(xié)議是搭載ack的回退n步協(xié)議*/
#include?
#include?“protocol.h“

#define?max_seq?7
#define?flag?126?????
#define?ESC?100
#define?wait_time?2700					//發(fā)送計(jì)時(shí)器等待的時(shí)間
#define?ack_wait_time?280

static?int?phl_ready?=?0;
unsigned?char?buf[max_seq+1][270];
unsigned?char?ack[8];????//發(fā)送空的ack幀

unsigned?char?in_buf[600]?last_buf[520];//接收時(shí)的緩沖區(qū);去掉冗余之后的緩沖區(qū)為防備因誤碼兩幀合并為一幀而定義了很大一個(gè)數(shù)組
int?nbuffered=0;??????????????//發(fā)送的幀數(shù)
int?buf_size[max_seq+1];??????//記下以發(fā)送各幀的幀長(zhǎng)
int?next_frame_to_send=0;?
int?frame_in_phl=0;???????????//用于成幀
int?frame_expected=0;
int?ack_expected=0;

int?between（int?aint?bint?c）
{
	if（?（（a<=b）&&（b		return?1;
	else?return?0;
}
//判斷幀尾防止出現(xiàn)誤判esc?esc?flag為數(shù)據(jù)的情況
int?end_flag（int?in_len）
{
	int?count=0;
	int?i;
	if（in_len<=0）
		return?1;
	for（i=in_len-1;in_buf[i]==ESC&&i>=0;i--）//記錄flag前的esc數(shù)目
		count++;
	return?count%2;????????????????????//若flag前的esc為偶數(shù)，則為幀尾
}//成幀函數(shù)--數(shù)據(jù)幀
void?send_frame（char?*my_bufint?len）
{				
	int?n;							
	buf[frame_in_phl][0]=（frame_expected+max_seq）%（max_seq+1）;	//ack
	buf[frame_in_phl][1]=frame_in_phl;					//發(fā)送幀的幀號(hào)
	for（n=0;n		buf[frame_in_phl][n+2]=my_buf[n];				//將處理過(guò)的新幀賦值到緩沖區(qū)中
	len=len+2;
	*（unsigned?int?*）（buf[frame_in_phl]+len）?=?crc32（buf[frame_in_phl]len）;		//在原始幀的基礎(chǔ)上加檢驗(yàn)和
	buf_size[frame_in_phl]=len+4;						//記錄當(dāng)前幀的長(zhǎng)度包括3個(gè)幀頭，4個(gè)檢驗(yàn)和
	nbuffered=nbuffered+1;				//緩沖區(qū)占用數(shù)加一
	frame_in_phl=（frame_in_phl+1）%（max_seq+1）;
}
//成幀函數(shù)--ack幀
void?send_ack（）									//ack幀的處理
{
	ack[0]=（frame_expected+max_seq）%（max_seq+1）;
	ack[1]=max_seq+10;					//ack幀的序號(hào)位，使ack[1]==frame_expected恒不成立
	*（unsigned?int?*）（ack+2）?=?crc32（ack2）;		//在原始幀的基礎(chǔ)上加檢驗(yàn)和
}
//主函數(shù)
int?main（int?argc?char?**argv）
{
????int?event?arg?n??m??i??j??len?=?0?in_len?=?0;	
	unsigned?char?my_buf[260];
	int?phl_wait=0;????//在物理層中還沒(méi)有被發(fā)送的幀

????protocol_init（argc?argv）;?
????enable_network_layer（）;

????for?（;;）?{
????????event?=?wait_for_event（&arg）;

????????switch?（event）?{
????????case?NETWORK_layer_READY:
????????????len?=?get_packet（my_buf）;?
			phl_wait++;
			send_frame（my_buflen）;
			stop_ack_timer（）;

????????????if?（phl_ready）?{
????????????????for（j=0;j				{	
					//幀結(jié)構(gòu)為?ack?seq?內(nèi)容
					//每幀尾部加發(fā)送一個(gè)flag
					
					for?（i?=?0;?i?ame_to_send];?i++）{
						if?（（buf[next_frame_to_send][i]==flag）?||?（buf[next_frame_to_send][i]==ESC）?）
							send_byte（ESC）;
						send_byte（buf[next_frame_to_send][i]）;
					}
					send_byte（flag）;//幀尾

					start_timer（next_frame_to_sendwait_time）;	//發(fā)送完啟動(dòng)相應(yīng)的定時(shí)器
					next_frame_to_send=（next_frame_to_send+1）%（max_seq+1）;
				}
????????????????phl_ready??=?0;				//記錄物理層當(dāng)前狀態(tài)為
				phl_wait?=?phl_wait-j;
????????????}
????????????break;

????????case?PHYSICAL_layer_READY:
????????????if?（phl_wait）?{					//若有未發(fā)送的幀便發(fā)送
				for（j=0;j				{
				
					for?（i?=?0;?i?ame_to_send];?i++）{
						if?（?（buf[next_fram