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IN_CKSUM(9) NetBSD Kernel Developer's Manual IN_CKSUM(9)
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in_cksum, in4_cksum, in6_cksum -- compute Internet checksum
in_cksum(struct mbuf *m, int len);
in4_cksum(struct mbuf *m, uint8_t nxt, int off, int len);
in6_cksum(struct mbuf *m, uint8_t nxt, int off, int len);
These functions are used to compute the ones-complement checksum required
by IP and IPv6. The in4_cksum() function is used to compute the trans-
port-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 num-
ber. 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 con-
structed 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 reimple-
mented 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
All three functions return the computed checksum value.
inet(4), inet6(4), tcp(4), udp(4), protocols(5), mbuf(9)
These functions implement the Internet transport-layer checksum as speci-
fied in RFC768, RFC793, and RFC2460.
The in6_cksum() function currently requires special handling of link-
local addresses in the pseudo-header due to the use of embedded scope-
id's within link-local addresses.
NetBSD 9.0 May 22, 2001 NetBSD 9.0