Computer Networks 0111-ICMP Gergely Windisch

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

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

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

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

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

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

ICMP types with numbers There is more where these came from: Source of the image: Petrényi József: TCP/IP alapok I. kötet

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

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

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

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

Destination unreachable - Codes

Destination unreachable example

ICMP Source Quench The don't shoot packet

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)

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

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

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

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

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

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

ARP

MAC to IP – request – reply

ARP Request

ARP Reply