1、ICMP校验算法代码
unsigned short cal_chksum(unsigned short *addr,int len) { int nleft = len; int sum = 0; unsigned short *w = addr; unsigned short answer =0; /*把ICMP报头二进制数据以2字节为单位累加起来*/ while(nleft > 1) { sum += *w++; nleft -= 2; } /*若ICMP报头为奇数个字节,会剩下最后一字节。把最后一个字节视为一个2字节数据的高字节,这个2字节数据的低字节为0,继续累加*/ if(nleft == 1) { *(unsigned char *)(&answer)=*(unsigned char *)w; sum+=answer; } /*校验和是以16位为单位进行求和计算的,sum是32位的,sum&0xffff是低16位,sum>>16是右移16位,取到的是高16位,相加就是高16位和低16位的和。*/ sum = (sum >> 16) + (sum & 0xffff); /*这一步是有可能上面加的时候有进位到高16位的,再把高16位加进来。*/ sum += (sum >> 16); /*上一步肯定不会再有进位了,即使上面sum高16位非0也不要紧,此处sum只能把低字节数赋值给answer,因为answer为16位*/ answer =~ sum; return answer; }