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NAME

ip4_checksum
icmp_checksum
icmp6_checksum
igmp_checksum
tcp_checksum
udp_checksum
pim_checksum

fcs16    (used if PPP frame check sequence is 16 bits long)
fcs32    (used if PPP frame check sequence is 32 bits long)

SYNOPSIS

chk=ip4_checksum(pkt,offset,header_length)
chk=icmp_checksum(pkt,offset)
chk=icmp6_checksum(pkt,offset)
chk=igmp_checksum(pkt,offset)
chk=tcp_checksum(pkt,offset)
chk=udp_checksum(pkt,offset)
chk=pim_checksum(pkt,offset)

chk=fcs16(pkt)
chk=fcs32(pkt)

DESCRIPTION

checksum protocol dependant computation

funtion accepts two (or three in ip4 case) arguments:
    pkt is a packet reference containing the whole already dissected protocol packet
    offset is the start protocol position inside packet to be checked
    header_length is the ip version 4 header length (just for ip4_checksum routine)

funtion returns one boolean value:
    chk is TRUE if computation is correct otherwise FALSE

once you have dissected one protocol you have to check checksum value before proceding to dissect next one