Chapter 23

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 0-127 (Error messages) 128-255 (Informational messages) ----------------------------------------------- 1 Destination Unreachable 2 Packet Too Big 3 Time Exceeded 4 Parameter Problem100 Private experimentation 102-126 Unassigned ----------------------------------------------- 128 Echo Request 129 Echo Reply130 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, I1 • Error message I1 causes another error, which generates ICMP message I2 • Message I2 generates another error, I3 • 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. 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.

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