diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/checksum.c | 65 |
1 files changed, 57 insertions, 8 deletions
diff --git a/lib/checksum.c b/lib/checksum.c index 4dfa252..94cf71e 100644 --- a/lib/checksum.c +++ b/lib/checksum.c @@ -11,6 +11,62 @@ #include "nest/bird.h" #include "checksum.h" +static u16 /* One-complement addition */ +add16(u16 sum, u16 x) +{ + u16 z = sum + x; + return z + (z < sum); +} + +static u32 +add32(u32 sum, u32 x) +{ + u32 z = sum + x; + return z + (z < sum); +} + +static u16 +ipsum_calc_block(u16 *x, unsigned len, u16 sum) +{ + int rest; + u32 tmp, *xx; + + /* + * A few simple facts about the IP checksum (see RFC 1071 for detailed + * discussion): + * + * o It's associative and commutative. + * o It's byte order independent. + * o It's word size independent. + * + * This gives us a neat 32-bits-at-a-time algorithm which respects + * usual alignment requirements and is reasonably fast. + */ + + ASSERT(!(len % 2)); + if (!len) + return sum; + len >>= 1; + if ((unsigned long) x & 2) /* Align to 32-bit boundary */ + { + sum = add16(sum, *x++); + len--; + } + rest = len & 1; + len >>= 1; + tmp = 0; + xx = (u32 *) x; + while (len) + { + tmp = add32(tmp, *xx++); + len--; + } + sum = add16(sum, add16(tmp & 0xffff, tmp >> 16U)); + if (rest) + sum = add16(sum, *(u16 *) xx); + return sum; +} + static u16 ipsum_calc(void *frag, unsigned len, va_list args) { @@ -18,14 +74,7 @@ ipsum_calc(void *frag, unsigned len, va_list args) for(;;) { - u16 *x = frag; - ASSERT(!(len % 2)); - while (len) - { - u16 z = sum + *x++; - sum = z + (z < sum); - len -= 2; - } + sum = ipsum_calc_block(frag, len, sum); frag = va_arg(args, void *); if (!frag) break; |