lib/csum.c

   1 /*
   2  * Copyright (c) 2008, 2009, 2010 Nicira Networks.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 #include <config.h>
  18 #include "csum.h"
  19 #include "unaligned.h"
  20
  21 /* Returns the IP checksum of the 'n' bytes in 'data'.
  22  *
  23  * The return value has the same endianness as the data.  That is, if 'data'
  24  * consists of a packet in network byte order, then the return value is a value
  25  * in network byte order, and if 'data' consists of a data structure in host
  26  * byte order, then the return value is in host byte order. */
  27 uint16_t
  28 csum(const void *data, size_t n)
  29 {
  30     return csum_finish(csum_continue(0, data, n));
  31 }
  32
  33 /* Adds the 16 bits in 'new' to the partial IP checksum 'partial' and returns
  34  * the updated checksum.  (To start a new checksum, pass 0 for 'partial'.  To
  35  * obtain the finished checksum, pass the return value to csum_finish().) */
  36 uint32_t
  37 csum_add16(uint32_t partial, uint16_t new)
  38 {
  39     return partial + new;
  40 }
  41
  42 /* Adds the 32 bits in 'new' to the partial IP checksum 'partial' and returns
  43  * the updated checksum.  (To start a new checksum, pass 0 for 'partial'.  To
  44  * obtain the finished checksum, pass the return value to csum_finish().) */
  45 uint32_t
  46 csum_add32(uint32_t partial, uint32_t new)
  47 {
  48     return partial + (new >> 16) + (new & 0xffff);
  49 }
  50
  51
  52 /* Adds the 'n' bytes in 'data' to the partial IP checksum 'partial' and
  53  * returns the updated checksum.  (To start a new checksum, pass 0 for
  54  * 'partial'.  To obtain the finished checksum, pass the return value to
  55  * csum_finish().) */
  56 uint32_t
  57 csum_continue(uint32_t partial, const void *data_, size_t n)
  58 {
  59     const uint16_t *data = data_;
  60
  61     for (; n > 1; n -= 2, data++) {
  62         partial = csum_add16(partial, get_unaligned_u16(data));
  63     }
  64     if (n) {
  65         partial += *(uint8_t *) data;
  66     }
  67     return partial;
  68 }
  69
  70 /* Returns the IP checksum corresponding to 'partial', which is a value updated
  71  * by some combination of csum_add16(), csum_add32(), and csum_continue().
  72  *
  73  * The return value has the same endianness as the checksummed data.  That is,
  74  * if the data consist of a packet in network byte order, then the return value
  75  * is a value in network byte order, and if the data are a data structure in
  76  * host byte order, then the return value is in host byte order. */
  77 uint16_t
  78 csum_finish(uint32_t partial)
  79 {
  80     while (partial >> 16) {
  81         partial = (partial & 0xffff) + (partial >> 16);
  82     }
  83     return ~partial;
  84 }
  85
  86 /* Returns the new checksum for a packet in which the checksum field previously
  87  * contained 'old_csum' and in which a field that contained 'old_u16' was
  88  * changed to contain 'new_u16'. */
  89 uint16_t
  90 recalc_csum16(uint16_t old_csum, uint16_t old_u16, uint16_t new_u16)
  91 {
  92     /* Ones-complement arithmetic is endian-independent, so this code does not
  93      * use htons() or ntohs().
  94      *
  95      * See RFC 1624 for formula and explanation. */
  96     uint16_t hc_complement = ~old_csum;
  97     uint16_t m_complement = ~old_u16;
  98     uint16_t m_prime = new_u16;
  99     uint32_t sum = hc_complement + m_complement + m_prime;
 100     return csum_finish(sum);
 101 }
 102
 103 /* Returns the new checksum for a packet in which the checksum field previously
 104  * contained 'old_csum' and in which a field that contained 'old_u32' was
 105  * changed to contain 'new_u32'. */
 106 uint16_t
 107 recalc_csum32(uint16_t old_csum, uint32_t old_u32, uint32_t new_u32)
 108 {
 109     return recalc_csum16(recalc_csum16(old_csum, old_u32, new_u32),
 110                          old_u32 >> 16, new_u32 >> 16);
 111 }