.\"	$NetBSD: in4_cksum.9,v 1.6 2008/04/30 13:10:58 martin Exp $
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.\" Copyright (c) 2001 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Bill Sommerfeld.
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.Dd May 22, 2001
.Dt IN_CKSUM 9
.Os
.Sh NAME
.Nm in_cksum ,
.Nm in4_cksum ,
.Nm in6_cksum
.Nd compute Internet checksum
.Sh SYNOPSIS
.Ft uint16_t
.Fn in_cksum "struct mbuf *m" "int len"
.Ft uint16_t
.Fn in4_cksum "struct mbuf *m" "uint8_t nxt" "int off" "int len"
.Ft uint16_t
.Fn in6_cksum "struct mbuf *m" "uint8_t nxt" "int off" "int len"
.Sh DESCRIPTION
These functions are used to compute the ones-complement checksum
required by IP and IPv6.
The
.Fn in4_cksum
function is used to compute the transport-layer checksum required by
.Xr tcp 4
and
.Xr udp 4
over a range of bytes starting at
.Fa off
and continuing on for
.Fa len
bytes within the mbuf
.Fa m .
.Pp
If the
.Fa nxt
parameter is non-zero, it is assumed to be an IP protocol number.
It is also assumed that the data within
.Fa m
starts with an IP header, and the transport-layer header starts at
.Fa 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.
.Pp
The
.Fn in6_cksum
function is similar; if
.Fa nxt
is non-zero, it is assumed that
.Fa m
starts with an IPv6 header, and that the transport-layer header starts
after
.Fa off
bytes.
.Pp
The
.Fn in_cksum
function is equivalent to
.Fn in4_cksum "m" "0" "0" "len" .
.Pp
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.
.Sh RETURN VALUES
All three functions return the computed checksum value.
.Sh SEE ALSO
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr protocols 5 ,
.Xr mbuf 9
.Sh STANDARDS
These functions implement the Internet transport-layer checksum as specified
in RFC768, RFC793, and RFC2460.
.Sh BUGS
The
.Fn 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.