IPV4转发实验

复习用

/** THIS FILE IS FOR IP FORWARD TEST*/#include "sysInclude.h"#include <vector>using std::vector;// system supportextern void fwd_LocalRcv(char *pBuffer, int length);extern void fwd_SendtoLower(char *pBuffer, int length, unsigned int nexthop);extern void fwd_DiscardPkt(char *pBuffer, int type);extern unsigned int getIpv4Address( );// implemented by students//一个路由节点 struct route_table{int dest;int nexthop;};//路由表 vector<route_table> mytable;void stud_Route_Init(){return;}// 添加函数就是从proute中取出dest和nexthop，构成一个路由节点，然后加入路由表 void stud_route_add(stud_route_msg *proute){int dest;route_table t;dest = ntohl(proute->dest) & ((1<<31)>>(ntohl(proute->masklen)-1));t.dest = dest;t.nexthop = ntohl(proute->nexthop);mytable.push_back(t);return;}int stud_fwd_deal(char *pBuffer, int length){int version = pBuffer[0] >> 4; int headlength = pBuffer[0] & 0xf; int timetolive = (int)pBuffer[8]; int headerChecksum = ntohs(*(short unsigned int*)(pBuffer+10)); int destinationAddress = ntohl(*(unsigned int*)(pBuffer+16)); if (destinationAddress == getIpv4Address())//destination地址和本机地址相同 {//调用fwd_LocalRcv把ip包交给上层协议函数 fwd_LocalRcv(pBuffer,length);return 0;}if (timetolive <= 0)//TTL小于等于0时丢弃该包 {fwd_DiscardPkt(pBuffer,STUD_FORWARD_TEST_TTLERROR);return 1;}vector<route_table>::iterator ii;for (ii = mytable.begin(); ii!=mytable.end(); ii++)//在路由表中查找下一跳地址 {if (ii->dest == destinationAddress)//如果能找到 {char *buffer = new char[length];memcpy(buffer,pBuffer,length);buffer[8]--;int sum = 0;//重新计算校验和 unsigned short int localCheckSum = 0;for(int i = 0;i < 2 * headlength;i ++){if (i == 5)continue;sum = sum + (buffer[i*2]<<8) + (buffer[i*2+1]); sum %= 65535;}localCheckSum = htons(0xffff - (unsigned short int)sum);memcpy(buffer+10, &localCheckSum, sizeof(short unsigned int));fwd_SendtoLower(buffer, length, ii->nexthop);//发送至下一跳 return 0;}}fwd_DiscardPkt(pBuffer,STUD_FORWARD_TEST_NOROUTE);//路由表中不存在下一跳，丢弃该包 return 1;}