Internet Control Message Protocol - Technical Details - ICMP Segment Structure - Header


The ICMP header starts after the IPv4 header. All ICMP packets will have an 8-byte header and variable-sized data section. The first 4 bytes of the header will be consistent. The first byte is for the ICMP type. The second byte is for the ICMP code. The third and fourth bytes are a checksum of the entire ICMP message. The contents of the remaining 4 bytes of the header will vary based on the ICMP type and code.

ICMP error messages contain a data section that includes the entire IP header plus the first 8 bytes of data from the IP datagram that caused the error message. The ICMP datagram is then encapsulated in a new IP datagram.

Bits 0–7 8–15 16–23 24–31
0 Type Code Checksum
32 Rest of Header
  • Type – ICMP type as specified below.
  • Code – Subtype to the given type.
  • Checksum – Error checking data. Calculated from the ICMP header+data, with value 0 for this field. The checksum algorithm is specified in RFC 1071.
  • Rest of Header – Four byte field. Will vary based on the ICMP type and code.

Read more about this topic:  Internet Control Message Protocol, Technical Details, ICMP Segment Structure

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