How to compute checksum for TCP packets?

24Jun09

Given code can help you to compute checksum.

/* pseudo header for checksum calculation */
typedef struct pseudoh {
    guint32 src_addr;
    guint32 dest_addr;
    guint8 zero;
    guint8 protocol;
    guint16 length;
} PSEUDO_HDR, * PPSEUDO_HDR;

/* Compute Checksum for TCP packets */
GUINT16 ComputeChecksum(PGUINT16 pseudo_hdr, PGUINT16 ptcp_hdr, PGUINT16 pdata, GUINT32 dataSize) {

guint32 checksum = 0;
guint32 nleft = 0;

nleft = SIZE_PSEUDO_HDR;
while(nleft > 1) {
    /* This is the inner loop */
    checksum = checksum + *(pseudo_hdr++);
    nleft = nleft – sizeof(guint16);
}

nleft = SIZE_TCP_HEADER;
while(nleft > 1) {
    /* This is the inner loop */
    checksum = checksum + *(ptcp_hdr++);
    nleft = nleft – sizeof(guint16);
}

nleft = dataSize;
while(nleft > 1) {
    /* This is the inner loop */
    checksum = checksum + *(pdata++);
    nleft = nleft – sizeof(guint16);
}
if(nleft)
    checksum = checksum + *((pguint8)pdata);
/* Fold 32-bit sum to 16 bits */
checksum = (0xFFFF&(checksum >> 16)) + (checksum & 0xffff);
checksum = (checksum & 0xffff) + (0xFFFF&(checksum >> 16));
return (guint16)(~checksum);
}