tcp/ip校驗和計算

#include <cassert>
#include <cstdlib>

#include "network.h"

unsigned short ChecksumTcpUdp(unsigned int saddr, unsigned int daddr,
        unsigned short len, unsigned char proto, const unsigned char *data)
{
    assert(NULL != data);

    const sniff_tcp *tcpHeader = (const sniff_tcp *)data;

    unsigned char protoStr[2] = { 0x00, proto };
    long sum = (saddr & 0xffff) + (saddr >> 16) + (daddr & 0xffff)
        + (daddr >> 16) + len + *(unsigned short *)protoStr;
 
    int count = ntohs(len);
 
    while (count > 1) {
        sum += *(unsigned short *)data;
        data += 2;
        count -= 2;
    }
    if (count > 0) {
        sum += *(unsigned char *)data;
    }
 
    /* Remove the existing checksum from result.
     */
    sum -= tcpHeader->th_sum;
 
    while (sum >> 16) {
        sum = (sum & 0xffff) + (sum >> 16);
    }
 
    return ~sum;
}
 
unsigned short ChecksumIp(const unsigned char *addr, int count)
{
    assert(NULL != addr);

    const sniff_ip *ipHeader = (const sniff_ip *)addr;

    long sum = 0;
    while (count > 1) {
        sum += *(unsigned short *)addr;
        addr += 2;
        count -= 2;
    }
    if (count > 0) {
        sum += *(unsigned char *)addr;
    }

    /* Remove the existing checksum from result.
     */
    sum -= ipHeader->ip_sum;

    while (sum >> 16) {
        sum = (sum & 0xffff) + (sum >> 16);
    }

    return ~sum;
}