ICMP: Internet Control Message Protocol

A protocol used by hosts and routers to identify and troubleshoot network problems.

• ICMP reports errors such as unreachable hosts, networks, ports, and protocols.

• ICMP echo request and reply messages can be used to check network connectivity.

1. The source sends a sequence of UDP segments to the destination.
   • The first segment has a time to live (TTL) of 1.
   • The second segment has a TTL of 2.
   • And so on.
   • An uncommon port number is used.
2. When the nth set of datagrams reaches the nth router:
   • The router discards the datagram and sends an ICMP message (type 11, code 0).
   • The ICMP message includes the router's name and IP address.
3. When the ICMP message arrives, the source records the round-trip time (RTT).
4. Termination conditions
   • The source terminates when the UDP segment reaches the destination.
   • The source terminates upon receiving an ICMP port-unreachable message (type 3, code 3) from the destination.
   • The source stops.