Computer Network Chapter IPV4 Datagram Format

Computer Network Chapter IPV4 Datagram Format

struct iphdr {
    
    
    uint8_t version : 4;		//4位版本字段表示Internet标头的格式,ipv4值是4
    uint8_t ihl : 4;			//4位因特网报头长度,ip报头中32位字的数量
    uint8_t tos;				//服务类型字段,该字段传达IP数据报的服务质量,源于第一个IP规范,在以后的规范中,它被划分为较小的字段,为了简单起见,我们按照原始规范中的定义该字段
    uint16_t len;				//总长度字段,len传达整个ip数据报的长度
    uint16_t id;				//数据报索引,用于重新组装分段的IP数据报,字段的值是一个计数器,由发送方递增,接收方对接受的片段进行排序
    uint16_t flags : 3;			//标志字段,定义数据报的各种控制标志,比如指定是否允许对数据报进行分段,是最后一个分段还是有更多的分段传入
    uint16_t frag_offset : 13;	//片段偏移字段,指示片段在数据报中的位置,比如第一个数据报将该索引设置为0
    uint8_t ttl;				//生存时间,通常由原始发送器设置为64,每个接收器将该计数器递减一,当它达到零时,数据报将被丢弃,并且可能会回复ICMP消息以指示错误
    uint8_t proto;				//上层协议类型,比如UDP是16,TCP是6
    uint16_t csum;				//标头校验用于验证ip标头的完整性
    uint32_t saddr;				//源地址,32位的ipv4地址
    uint32_t daddr;				//目的地址,32的ipv4地址
} __attribute__((packed));

The csum checksum field is used to check the integrity of IP datagrams.
The definition of the original IP specification: The checksum field is the 16-bit one's complement of the one's complement sum of all 16-bit words in the header. To calculate the checksum, The checksum field has a value of zero

actual code

uint16_t checksum(void *addr, int count)
{
    
    
    /* Compute Internet Checksum for "count" bytes
     *         beginning at location "addr".
     * Taken from https://tools.ietf.org/html/rfc1071
     */

    register uint32_t sum = 0;
    uint16_t * ptr = addr;

    while( count > 1 )  {
    
    
        // 每两位一组,计算和
        sum += * ptr++;
        count -= 2;
    }

    // 如果有剩余字段,累加
    if( count > 0 )
        sum += * (uint8_t *) ptr;

    /*  Fold 32-bit sum to 16 bits */
    // 将其转换为补码, 进位比特添加到前16比特
    while (sum>>16)
        sum = (sum & 0xffff) + (sum >> 16);


    return ~sum;
}

Take the IP header 45 00 00 54 41 e0 40 00 40 01 00 00 0a 00 04 0a 00 00 05 as an example:

Adding these fields together gives the two's complement sum 01 1b 3e

Then, to convert it to two's complement, a carry bit is added to the first 16 bits: 1b 3e+01=1b 3f

Finally, taking the 1's complement of the sum, we get the checksum value e4c0

The IP header becomes 45 00 00 54 41 e0 40 00 40 01 (e4 c0) 0a 00 04 0a 00 00 05

The checksum can be verified by applying the algorithm again, if the result is 0, the data is probably correct

Simple is better than complex, that’s all