lib/csum.c

   1 /*
   2  * Copyright (c) 2008, 2009 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
  20 /* Returns the IP checksum of the 'n' bytes in 'data'.
  21  *
  22  * The return value has the same endianness as the data.  That is, if 'data'
  23  * consists of a packet in network byte order, then the return value is a value
  24  * in network byte order, and if 'data' consists of a data structure in host
  25  * byte order, then the return value is in host byte order. */
  26 uint16_t
  27 csum(const void *data, size_t n)
  28 {
  29     return csum_finish(csum_continue(0, data, n));
  30 }
  31
  32 /* Adds the 16 bits in 'new' to the partial IP checksum 'partial' and returns
  33  * the updated checksum.  (To start a new checksum, pass 0 for 'partial'.  To
  34  * obtain the finished checksum, pass the return value to csum_finish().) */
  35 uint32_t
  36 csum_add16(uint32_t partial, uint16_t new)
  37 {
  38     return partial + new;
  39 }
  40
  41 /* Adds the 32 bits in 'new' to the partial IP checksum 'partial' and returns
  42  * the updated checksum.  (To start a new checksum, pass 0 for 'partial'.  To
  43  * obtain the finished checksum, pass the return value to csum_finish().) */
  44 uint32_t
  45 csum_add32(uint32_t partial, uint32_t new)
  46 {
  47     return partial + (new >> 16) + (new & 0xffff);
  48 }
  49
  50
  51 /* Adds the 'n' bytes in 'data' to the partial IP checksum 'partial' and
  52  * returns the updated checksum.  (To start a new checksum, pass 0 for
  53  * 'partial'.  To obtain the finished checksum, pass the return value to
  54  * csum_finish().) */
  55 uint32_t
  56 csum_continue(uint32_t partial, const void *data_, size_t n)
  57 {
  58     const uint16_t *data = data_;
  59
  60     for (; n > 1; n -= 2) {
  61         partial = csum_add16(partial, *data++);
  62     }
  63     if (n) {
  64         partial += *(uint8_t *) data;
  65     }
  66     return partial;
  67 }
  68
  69 /* Returns the IP checksum corresponding to 'partial', which is a value updated
  70  * by some combination of csum_add16(), csum_add32(), and csum_continue().
  71  *
  72  * The return value has the same endianness as the checksummed data.  That is,
  73  * if the data consist of a packet in network byte order, then the return value
  74  * is a value in network byte order, and if the data are a data structure in
  75  * host byte order, then the return value is in host byte order. */
  76 uint16_t
  77 csum_finish(uint32_t partial)
  78 {
  79     return ~((partial & 0xffff) + (partial >> 16));
  80 }
  81
  82 /* Returns the new checksum for a packet in which the checksum field previously
  83  * contained 'old_csum' and in which a field that contained 'old_u16' was
  84  * changed to contain 'new_u16'. */
  85 uint16_t
  86 recalc_csum16(uint16_t old_csum, uint16_t old_u16, uint16_t new_u16)
  87 {
  88     /* Ones-complement arithmetic is endian-independent, so this code does not
  89      * use htons() or ntohs().
  90      *
  91      * See RFC 1624 for formula and explanation. */
  92     uint16_t hc_complement = ~old_csum;
  93     uint16_t m_complement = ~old_u16;
  94     uint16_t m_prime = new_u16;
  95     uint32_t sum = hc_complement + m_complement + m_prime;
  96     uint16_t hc_prime_complement = sum + (sum >> 16);
  97     return ~hc_prime_complement;
  98 }
  99
 100 /* Returns the new checksum for a packet in which the checksum field previously
 101  * contained 'old_csum' and in which a field that contained 'old_u32' was
 102  * changed to contain 'new_u32'. */
 103 uint16_t
 104 recalc_csum32(uint16_t old_csum, uint32_t old_u32, uint32_t new_u32)
 105 {
 106     return recalc_csum16(recalc_csum16(old_csum, old_u32, new_u32),
 107                          old_u32 >> 16, new_u32 >> 16);
 108 }