These functions are used to compute the ones-complement checksum required by IP and IPv6. The
in4_cksum() function is used to compute the transport-layer checksum required by
tcp(4) and
udp(4) over a range of bytes starting at
off and continuing on for
len bytes within the mbuf
m.
If the
nxt parameter is non-zero, it is assumed to be an IP protocol number. It is also assumed that the data within
m starts with an IP header, and the transport-layer header starts at
off; a pseudo-header is constructed as specified in RFC768 and RFC793, and the pseudo-header is prepended to the data covered by the checksum.
The
in6_cksum() function is similar; if
nxt is non-zero, it is assumed that
m starts with an IPv6 header, and that the transport-layer header starts after
off bytes.
The
in_cksum() function is equivalent to
in4_cksum(
m,
0,
0,
len).
These functions are always performance critical and should be reimplemented in assembler or optimized C for each platform; when available, use of repeated full-width add-with-carry followed by reduction of the sum to a 16 bit width usually leads to best results. See RFC's 1071, 1141, 1624, and 1936 for more information about efficient computation of the internet checksum.