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. 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.
RETURN VALUES
All three functions return the computed checksum value.
SEE ALSO
inet(4),
inet6(4),
tcp(4),
udp(4),
protocols(5),
mbuf(9)
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.