1. Computer Networks 0111-ICMP Gergely Windisch windisch.gergely@nik.uni-obuda.hu

2. Control protocols Protocols that don't actually take part in transmitting the message, but rather help with the data transfer ICMP, ARP

3. ICMP What is ICMP? Why do we need it? What do we use it for? Where does it fit?

4. ICMP Transfer protocols (next week) are fun, we can choose from having one that is reliable but chatty or one that is efficient but unreliable With IP we don't have that option, all we have is the unreliable. Thus ICMP

5. ICMP Errors in the connection can be stochastic (random) or deterministic (random, but with a high probability of being repeated). ICMP helps with the latter – when something breaks, don't force the communication, give up

6. ICMP layout Source of the image: Petrényi József: TCP/IP alapok I. kötet

7. ICMP types Source of the image: Petrényi József: TCP/IP alapok I. kötet

8. ICMP types with numbers There is more where these came from: http://www.iana.org/assignments/icmp-parameters Source of the image: Petrényi József: TCP/IP alapok I. kötet

9. ICMP messages: Ping Used to test the internet echo request, echo reply Type: 0 or 8 Code: 0 Checksum - CRC ID&Seq: helps connect the request and the reply Optional data: anything goes (usually letters) traceroute Source of the image: Petrényi József: TCP/IP alapok I. kötet

10. ICMP echo request Source of the image: Petrényi József: TCP/IP alapok I. kötet

11. ICMP echo reply Source of the image: Petrényi József: TCP/IP alapok I. kötet

12. ICMP destination unreachable Type: 3 Code: lots and lots Checksum: CRC IP header and first 8 bytes…

13. Destination unreachable - Codes

14. Destination unreachable example

15. ICMP Source Quench The don't shoot packet

16. ICMP Redirect Routers can tell the user a different way that is better (the next hop is still on the same network) Type: 5 Code: 0-3 (redirect host or complete network)

17. ICMP router discovery Get the router (gateway) automatically – Router Advertisement – Router Solicitation

18. ICMP router discovery Get the router (gateway) automatically Router Advertisement Router sends packet periodically – Type: 9 – Code: 0

19. ICMP router discovery Get the router (gateway) automatically Router Solicitation When enabled, host sends periodically – Type: 10 – Code: 0

20. ICMP time exceeded Fragmentation TTL Type: 11 Code: 0: TTL exceeded 1: Fragment reassembly time exceeded

21. ICMP parameter problem There is something wrong with the IP packet header, but I don't really know what and I can't find a matching ICMP – Type: 12 – Code: 0-2 0: Error at pointer byte 1: Missing field 2: Length error

22. ARP We know the IP address But we need to talk in the local environment How do we find the MAC?

23. ARP

25. MAC to IP – request – reply

26. ARP Request

27. ARP Reply