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ICMP Usage in Scanning 42 Copyright Ó Ofir Arkin, 2000 http://www.sys-security.com Type Name Code 15 Information Request                  0    No Code 16 Information Reply                      0    No Code 17 Address Mask Request             0    No Code 18 Address Mask Reply                  0    No Code 19 Reserved (for Security)              0    No Code 20-29 reserved (for Robustness Experiment) 30 Traceroute                                  31 Datagram Conversion Error       32 Mobile Host Redirect                  33 IPv6 Where-Are-You                  34 IPv6 I-Am-Here                       35 Mobile Registration Request      36 Mobile Registration Reply          39 SKIP 40 Photuris 0    Reserved 1    unknown security parameters index 2    valid security parameters, but authentication          failed 3    valid security parameters, but decryption failed Table 2: ICMP Types & Codes Checksum  –  contains  the  16bit  one’s  complement  of  the  one’s  complement  sum  of  the ICMP message starting with the ICMP Type field. For computing this checksum, the checksum field is assumed to be zero. Data – Will contain a part of the original IP message for which this ICMP message was generated. The length of the DATA field equals the IP datagram length less the IP header length. Every ICMP error message includes the Internet (IP) Header and at least the first 8 data octets (bytes) of the datagram that triggered the error; more than 8 octets (bytes) may be sent; this header and data must be unchanged from the received datagram.