Recall that the network layer includes IP protocol + routing algorithm + ICMP protocol.

ICMP - Internet Control Message Protocol.

• Hand-in-hand with IP protocol - ICMPv4, ICMPv6 correspond to IPv4, IPv6
• Used for sending control messages on the internet (i.e. network-layer information)
  • e.g. error reporting (unreachable host, network, port, protocol)
  • e.g. echoing a request/reply - used by ping to check reachability
• Is in the network-layer “above” IP - ICMP messages are carried in IP datagrams
• ICMP messages include type, code, first 8 bytes of IP datagram causing an error

Traceroute & ICMP

Recall that traceroute is a program that checks what delays are over a particular path to a destination (hop-by-hop delay).

• Note - different implementations exist
  • Sent packets are UDP ⇒ when getting return message, some implementations encapsulate UDP packet in IP datagram
• Source sends sets of UDP segments to destination
  • 1st set has TTL = 1, 2nd set has TTL = 2, etc.
• When the datagram in the $n$th set arrives to the $n$th router:
  • Router discards datagram + sends source ICMP message (type 11, code 0)
  • ICMP message may contain name of router + IP address
• When ICMP return message arrives at source, record RTT
• Stopping criteria for packets:
  • UDP segments eventually arrive at destination host
  • Destination returns ICMP “port unreachable” message (type 3, code 3)
  • Source stops