ICMP: Internet Control Message Protocol

• used by hosts, routers, gateways to communication network-level information

  • error reporting: unreachable host, network, port, protocol
  • echo request/reply (used by ping)

• network-layer “above” IP:

  • ICMP msgs carried in IP datagrams

• ICMP message: type, code plus first 8 bytes of IP datagram causing error

Type Code description

0 0 echo reply (ping)

3 0 dest. network unreachable

3 1 dest host unreachable

3 2 dest protocol unreachable

3 3 dest port unreachable

3 6 dest network unknown

3 7 dest host unknown

4 0 source quench (congestion

control - not used)

8 0 echo request (ping)

9 0 route advertisement

10 0 router discovery

11 0 TTL expired

12 0 bad IP header

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