Copyright (c) 2001 The NetBSD Foundation, Inc.
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.Dd May 22, 2001 .Dt in4_cksum 9 .Os .Sh NAME .Nm in4_cksum .Nd Compute checksum for IPv4 packets. .Sh SYNOPSIS .Ft uint16_t .Fn in4_cksum "struct mbuf *m" "uint8_t nxt" "int off" "int len" .Sh DESCRIPTION The .Fn in4_cksum function is used to compute the ones-complement checksum required by the .Xr inet 4 protocol family over a range of bytes starting at .Fa off and continuing on for .Fa len bytes within the mbuf .Fa m .
p 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; a pseudo-header is constructed as specified in RFC768 and RFC793, and the pseudo-header is prepended to the data covered by the checksum.
p This function is almost always performance critical and should be reimplemented in assembler or optimized C for each platform; use of repeated full-width add-with-carry followed by reduction of the sum to a 16 bit width usually leads to best results. .Sh RETURN VALUES The .Nm function returns the computed checksum value. .Sh SEE ALSO .Xr inet 4 , .Xr mbuf 9 , .Xr protocols 5 , .Xr tcp 4 , .Xr udp 4 . .Sh STANDARDS The .Nm function implements the checksum specified in RFC768 and RFC793
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