NAME
in_cksum,
in4_cksum, in6_cksum
— compute Internet
checksum
SYNOPSIS
uint16_t
in_cksum(struct
mbuf *m, int
len);
uint16_t
in4_cksum(struct
mbuf *m, uint8_t
nxt, int off,
int len);
uint16_t
in6_cksum(struct
mbuf *m, uint8_t
nxt, int off,
int len);
DESCRIPTION
These functions are used to compute the ones-complement checksum required by IP and IPv6. Thein4_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.
RETURN VALUES
All three functions return the computed checksum value.
SEE ALSO
STANDARDS
These functions implement the Internet transport-layer checksum as specified in RFC768, RFC793, and RFC2460.
BUGS
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.