1 Chapter 23 Internetworking Part 3 (Control Messages, Error Handling, ICMP)

2 IP Semantics  IP is best-effort  Datagrams can be  Lost  Delayed  Duplicated  Delivered out of order  Corrupted

3 Error Detection  IP does not  Introduce errors  Ignore all errors  Errors detected  Corrupted bits  Illegal addresses  Routing loops  Fragment loss

4 Problems and Solutions  Corrupted header bits  Header checksum  Illegal destination address  Routing tables  Routing loop  Time-To-Live (TTL) field  Fragment loss  Timeout

5 Internet Control Message Protocol (ICMP)  Separate protocol for  Errors  Information  Required part of IP  Sends error messages to original source

6 Example ICMP Message Types

7 Example ICMP Messages  Source Quench  Sent by router  Triggered by datagram overrun  Requests sending host(s) to slow data

8 Example ICMP Messages (continued)  Time exceeded  Sent by router  TTL on datagram reached zero  Not a request for retransmission  Sent by host  Reassembly timeout (some fragments lost)

9 Example ICMP Messages (continued)  Destination unreachable  Specifies whether  Destination network unreachable  Destination host unreachable  Protocol port on destination unreachable

10 Example ICMP Messages (continued)  Redirect  Sent by router  Goes to host on local network  Host used incorrect initial router  Requests host to change routes

11 Example ICMP Messages (continued)  Echo request and reply  Not an error  Tests whether destination reachable  Request sent by ping program  Reply sent by ICMP on destination computer

12 ICMP Message Transport  Error messages go back to original source (may cross internet)  Messages carried in IP

13 Illustration of ICMP Message Encapsulation  Two levels of encapsulation  IP type field specifies ICMP

14 Avoiding an Infinite Loop  What happens if:  Datagram D causes an ICMP error message, I 1  Error message I 1 causes another error, which generates ICMP message I 2  Message I 2 generates another error, I 3  Error messages cascade  To avoid the problem  No error messages about ICMP error messages

15 Path MTU Discovery  IP datagram header contains a bit to specify no fragmentation allowed  ICMP sends an error message when fragmentation required but not permitted  Technique  Probe to find largest MTU that does not generate an error message  Note: MTU not generated if routes change

16 Summary  IP uses best-effort delivery semantics  IP includes mechanisms to detect errors  Header checksum  Time-to-live field

17 Summary (continued)  Internet Control Message Protocol  Has both error and informational messages  Closely integrated with IP  ICMP messages  Encapsulated in IP  Sent back to original source  Used by diagnostic programs like ping