ICMP (Internet Control Message Protocol)

ICMP packet

  • Network layer core protocol
  • Reports on success/failure of data delivery
  • Can indicate:
    • When part of a network is congested
    • When data fails to reach its destination
    • When data has been discarded because TTL has expired
  • Announces transmission failures to sender
    • Does not correct detected errors
  • Provides critical information for troubleshooting network problems
  • ICMPv6 on IPv6 networks perform the functions of ICMP and ARP on IPv4 networks


Field Length Function
Type 8 bits Indicate type of ICMP message
Code 8 bits Indicates subtype of message
Checksum 16 bits Allows receiving node to determine if packet became corrupted during transmission
Rest of header 32 bits Varies depending on type/subtype
Data Variable Usually contains IP header and first 8 bytes of data portion of IP packet that triggered ICMP message

Networking Computer Science