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


Type8 bitsIndicate type of ICMP message
Code8 bitsIndicates subtype of message
Checksum16 bitsAllows receiving node to determine if packet became corrupted during transmission
Rest of header32 bitsVaries depending on type/subtype
DataVariableUsually contains IP header and first 8 bytes of data portion of IP packet that triggered ICMP message

Networking Computer Science