Transcript Chapter 23
Chapter 23 Internetworking Part 3 (Control Messages, Error Handling, ICMP) 1 IP Semantics IP is best-effort Datagrams can be Lost Delayed Duplicated Delivered out of order Corrupted 2 Error Detection IP does not Introduce errors Ignore all errors Errors detected Corrupted bits Illegal addresses Routing loops Fragment loss 3 Problems and Solutions Corrupted header bits Header checksum Illegal destination address Routing tables Routing loop Time-To-Live (TTL) field Fragment loss Timeout 4 Internet Control Message Protocol (ICMP) Separate protocol for Errors Information Required part of IP Sends error messages to original source 5 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 Problem 100 Private experimentation 102-126 Unassigned ----------------------------------------------128 Echo Request 129 Echo Reply 130 Multicast Listener Query 131 Multicast Listener Report … 6 Example ICMP Messages Source Quench Sent by router Triggered by datagram overrun Requests sending host(s) to slow data 7 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) 8 Example ICMP Messages (continued) Destination unreachable Specifies whether Destination network unreachable Destination host unreachable Protocol port on destination unreachable 9 Example ICMP Messages (continued) Redirect Sent by router Goes to host on local network Host used incorrect initial router Requests host to change routes 10 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 11 ICMP Message Transport Error messages go back to original source (may cross internet) Messages carried in IP 12 Illustration of ICMP Message Encapsulation Two levels of encapsulation IP type field specifies ICMP 13 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 No problem error messages about ICMP error messages 14 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 15 Traceroute Traceroute works by increasing the "time-tolive" 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 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 18