tcp checksum

[junlon2006]

Tcp checksum实现原理(可根据各arch下SMID指令集优化）

unsigned short checksum1(const char *buf, unsigned size)
{
    unsigned sum = 0;
    int i;

    /* Accumulate checksum */
    for (i = 0; i < size - 1; i += 2)
    {
        unsigned short word16 = *(unsigned short *) &buf[i];
        sum += word16;
    }

    /* Handle odd-sized case */
    if (size & 1)
    {
        unsigned short word16 = (unsigned char) buf[i];
        sum += word16;
    }

    /* Fold to get the ones-complement result */
    while (sum >> 16) sum = (sum & 0xFFFF)+(sum >> 16);

    /* Invert to get the negative in ones-complement arithmetic */
    return ~sum;
}

[junlon2006]

Lwip checksum实现

#define SWAP_BYTES_IN_WORD(w) (((w) & 0xff) << 8) | (((w) & 0xff00) >> 8)
#define FOLD_U32T(u)          ((u32_t)(((u) >> 16) + ((u) & 0x0000ffffUL)))
u16_t
lwip_standard_chksum(const void *dataptr, int len)
{
  const u8_t *pb = (const u8_t *)dataptr;
  const u16_t *ps;
  u16_t t = 0;
  u32_t sum = 0;
  int odd = ((unsigned long)pb & 1);

  /* Get aligned to u16_t */
  if (odd && len > 0) {
    ((u8_t *)&t)[1] = *pb++;
    len--;
  }

  /* Add the bulk of the data */
  ps = (const u16_t *)(const void *)pb;
  while (len > 1) {
    sum += *ps++;
    len -= 2;
  }

  /* Consume left-over byte, if any */
  if (len > 0) {
    ((u8_t *)&t)[0] = *(const u8_t *)ps;
  }

  /* Add end bytes */
  sum += t;

  /* Fold 32-bit sum to 16 bits
     calling this twice is probably faster than if statements... */
  sum = FOLD_U32T(sum);
  sum = FOLD_U32T(sum);

  /* Swap if alignment was odd */
  if (odd) {
    sum = SWAP_BYTES_IN_WORD(sum);
  }

  return (u16_t)sum;
}