IN_CKSUM(9) | Kernel Developer's Manual | IN_CKSUM(9)

# 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.

May 22, 2001 | NetBSD 8.2