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Internet Protocol: Error and Control Messages Chapter 9 Internet Control Message Protocol • IP fails to deliver datagrams when: – the destination machine is temporarily or permanently disconnected from the network – the TTL reaches zero – intermediate routers are so congested that they can’t process incoming data • The sender does not know if failure to deliver is the result of a local or a remote problem • IP does not provide for the sender to test for connectivity or reachability Internet Control Message Protocol • ICMP is a special-purpose message mechanism which: – allows routers to report errors and provide information about unexpected circumstances – is required by IP and included in all IP implementations • ICMP messages travel across the internet in the data portion of IP datagrams • The messages are bound for IP, but may be passed to a higher-level protocol or application Error Reporting vs. Error Correcting • When a router encounters an error, it can report it to the original source • The source can then respond and take corrective action • Why are intermediate routers not notified of the error? ICMP Message Delivery • ICMP messages require two levels of encapsulation as shown in Figure 9.1 ICMP hdr IP hdr Frame hdr ICMP data Datagram data area Frame data area ICMP Message Delivery • ICMP messages are routed like other datagrams and may themselves be lost or discarded – In an already congested network, the error message may cause additional congestion – If a datagram carrying an ICMP message causes an error, another ICMP message is not created. Why? • Even though encapsulated, ICMP is not considered a higher level protocol than IP – The messages may need to travel over several physical networks to get back to the original source…and? ICMP Message Format • All ICMP messages begin with the same 3 fields: – Type field - 8 bits – Code field - 8 bits providing more information about the type – Checksum field - 16 bits • ICMP messages also contain the header and the first 64 data bits of the datagram causing the problem – Good information in those first 64 bits about protocols and application programs ICMP Message Types Type Field Message Type 0 3 4 5 8 9 10 11 Echo Reply Destination Unreachable Source Quench Redirect (change a route) Echo Request Router Advertisement Router Solicitation Time Exceeded for a Datagram ICMP Message Types Type Field Message Type 12 13 14 15 16 17 18 Parameter Problem on a Datagram Timestamp Request Timestamp Reply Information Request (not used) Information Reply (not used) Address Mask Request Address Mask Reply Summary • Normal communication across an internet involves sending messages from an application on one host to an application on another host • ICMP allows for communication from (usually) a router to a host • Types of messages: – – – – source quench for slowing down rate of transmission redirection for requesting a change in a routing table echo request/reply for checking destination reachability solicitation and advertisement for defaults in routing For Next Time • Exam • Read Chapter 10 for the next class period