Network Layer

ICMP

Internet Control Message

ICMP is used for communicating with hosts and router.

There are two types of usage.

• error report
• query message for dianosing network problems

Error report

• discard packet
  • destination is not in routing table
  • time exceeded; TTL is equal 1
  • source quench
  • parameter issue
• redirection

Content

• 8bytes is transport layer header
• IP header following ICMP header is datagram received by router

prior IP packet

• IP header + data(8bytes + rest)

add ICMP header

• ICMP header + IP header + data(8bytes + rest)

and then, add IP header

• IP header + ICMP header + IP header + data(8bytes + rest)

Query message

• echo request and reply; Ex> ping command
• etc..

ICMP type and code table

There are two application, ping and traceroute using ICMP packet.