原始套接字之PING（6）

发送ICMP数据的时候要进行，ICMP报头填充：

/*设置ICMP报头*/
static void icmp_pack(struct icmp *icmph, int seq, struct timeval *tv, int length )
{
	unsigned char i = 0;
	/*设置报头*/
	icmph->icmp_type = ICMP_ECHO;	/*ICMP回显请求*/
	icmph->icmp_code = 0;			/*code值为0*/
	icmph->icmp_cksum = 0;	  /*先将cksum值填写0，便于之后的cksum计算*/
	icmph->icmp_seq = seq;			/*本报的序列号*/
	icmph->icmp_id = pid &0xffff;	/*填写PID*/
	for(i = 0; i< length; i++)
		icmph->icmp_data[i] = i;
									/*计算校验和*/
	icmph->icmp_cksum = icmp_cksum((unsigned char*)icmph, length);
}

其中会进行头部字段和数据的校验（数据在内的整个ICMP包的校验）：

/*设置ICMP报头*/
static void icmp_pack(struct icmp *icmph, int seq, struct timeval *tv, int length )
{
	unsigned char i = 0;
	/*设置报头*/
	icmph->icmp_type = ICMP_ECHO;	/*ICMP回显请求*/
	icmph->icmp_code = 0;			/*code值为0*/
	icmph->icmp_cksum = 0;	  /*先将cksum值填写0，便于之后的cksum计算*/
	icmph->icmp_seq = seq;			/*本报的序列号*/
	icmph->icmp_id = pid &0xffff;	/*填写PID*/
	for(i = 0; i< length; i++)
		icmph->icmp_data[i] = i;
									/*计算校验和*/
	icmph->icmp_cksum = icmp_cksum((unsigned char*)icmph, length);
}