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ROPE Language Reference - "$ip_check".

The "$ip_check" register holds the checksum value for the IP(v4) packet being inspected. It is a read-only integer value.

RFC_791 describes this value in these terms..

Header Checksum:  16 bits

  A checksum on the header only.  Since some header fields change
  (e.g., time to live), this is recomputed and verified at each point
  that the internet header is processed.

  The checksum algorithm is:

    The checksum field is the 16 bit one's complement of the one's
    complement sum of all 16 bit words in the header.  For purposes of
    computing the checksum, the value of the checksum field is zero.

  This is a simple to compute checksum and experimental evidence
  indicates it is adequate, but it is provisional and may be replaced
  by a CRC procedure, depending on further experience.

See Also

RFC_791, $ip_daddr, $ip_dont_frag, $ip_frag_off, $ip_id, $ip_ihl, $ip_protocol, $ip_saddr, $ip_tos, $ip_tot_len, $ip_ttl, $ip_version

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