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 ICMPv6 "type" Numbers Type codes (Error messages) (Informational messages) Destination Unreachable 2Packet Too Big 3Time Exceeded 4Parameter Problem 100 Private experimentation Unassigned Echo Request 129 Echo Reply 130 Multicast Listener Query 131 Multicast Listener Report …

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

Traceroute  Traceroute works by increasing the "time-to- live" value of each successive batch of packets sent. When a packet passes through a router, the router decrements the TTL value by one. When a packet with a TTL of one reaches a router, the packet is discarded and an ICMP time exceeded (type 11) packet is sent back to the sender. 16

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

18 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