1. Internet Control Message Protocol ICMP

2. ICMP has two major purposes: –To report erroneous conditions –To diagnose network problems ICMP has two major purposes: –To report erroneous conditions –To diagnose network problems

3. ICMP Message Format Type Code Checksum Data 1 byte 2 bytes ? Which type of message? Which sub-type of message? Checksum of entire ICMP message Extra data (specific to the type of message)

4. Important ICMP Messages Echo Request/Reply Destination Unreachable Redirect Time Exceeded Parameter Problem Router Advertisement/Discovery Echo Request/Reply Destination Unreachable Redirect Time Exceeded Parameter Problem Router Advertisement/Discovery

5. Echo Request/Reply Echo Request (Type: 8, Code: 0): –Requests that the receiving host send a copy (echo) of the request back to the sender Echo Response (Type: 0, Code: 0): –The response from the receiving host Using these 2 commands is one way that the ping command can perform its function Echo Request (Type: 8, Code: 0): –Requests that the receiving host send a copy (echo) of the request back to the sender Echo Response (Type: 0, Code: 0): –The response from the receiving host Using these 2 commands is one way that the ping command can perform its function

6. Destination Unreachable Network Unreachable (Type: 3, Code: 0): –The network of the packet’s destination address field could not be reached Network Unknown (Type: 3, Code: 6): –The network of the packet’s destination address field could not be located Host Unreachable (Type: 3, Code: 1): –The host specified by the destination address could not be reached Host Unknown (Type: 3, Code: 7): –The host specified by the destination address could not be located Protocol Unreachable (Type: 3, Code: 2): –The specified protocol (e.g. TCP or UDP) could not be established with the specified host Port Unreachable (Type: 3, Code: 3): –The specified port number could not be reached Network Unreachable (Type: 3, Code: 0): –The network of the packet’s destination address field could not be reached Network Unknown (Type: 3, Code: 6): –The network of the packet’s destination address field could not be located Host Unreachable (Type: 3, Code: 1): –The host specified by the destination address could not be reached Host Unknown (Type: 3, Code: 7): –The host specified by the destination address could not be located Protocol Unreachable (Type: 3, Code: 2): –The specified protocol (e.g. TCP or UDP) could not be established with the specified host Port Unreachable (Type: 3, Code: 3): –The specified port number could not be reached

7. Redirect Redirect (Type: 5, Code: 0): –This router knows of a better path to the specified destination –The next router in the new path is provided Redirect (Type: 5, Code: 0): –This router knows of a better path to the specified destination –The next router in the new path is provided

8. Time Exceeded Time Exceeded (Type: 11, Code: 0): –The router has decremented the hop limit of a packet to zero and dropped the packet Time Exceeded (Type: 11, Code: 0): –The router has decremented the hop limit of a packet to zero and dropped the packet

9. Parameter Problem Parameter Problem (Type: 12, Code: 0): –Part of all of the packet’s IP header was invalid Parameter Problem (Type: 12, Code: 0): –Part of all of the packet’s IP header was invalid

10. Router Advertisement/Discovery Router Advertisement (Type: 9, Code: 0): –A router announces its existence using this type of message Router Discovery (Type: 10, Code: 0): –A host explicitly searches for a router Router Advertisement (Type: 9, Code: 0): –A router announces its existence using this type of message Router Discovery (Type: 10, Code: 0): –A host explicitly searches for a router

11. Ping ICMP Echo Request (Type: 8, Code: 0) ICMP Echo Response (Type: 0, Code: 0) DestinationSource

12. Traceroute Destination Router 1Router 2Router 3 Packet w/ Hop Limit = 1 ICMP Time Limit Exceeded Packet w/ Hop Limit = 2 ICMP Time Limit Exceeded Packet w/ Hop Limit = 3 Response to Packet Packet w/ Hop Limit = 4 Source