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Internet Networking Spring 2002

Internet Networking Spring 2002. Tutorial 4 ICMP (Internet Control Message Protocol). ICMP - Introduction. Defined in RFCs 792/1122 Allow routers to send error or control messages to other router or hosts Considered as a required part of IP layer Although implemented above IP layer

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Internet Networking Spring 2002

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  1. Internet Networking Spring 2002 Tutorial 4 ICMP (Internet Control Message Protocol)

  2. ICMP - Introduction • Defined in RFCs 792/1122 • Allow routers to send error or control messages to other router or hosts • Considered as a required part of IP layer • Although implemented above IP layer • Not making IP service more reliable, but provide feedback about network problems • Destination of ICMP packets is ICMP software module on another machine

  3. Typical Network Error and Control Cases • Reporting errors: • Unreachable destination (network, host, port) • Traffic overrun on router (congestion) • Detecting Circular or excessively long routes • Information exchanging: • Testing destination reachability • Clock synchronization • Discovering the local router

  4. Errors During Sending ICMP Message • ICMP messages are routed as usual packets • There is no additional reliability or priority • Thus, error messages themselves may be lost or discarded • New ICMP error message is not generated for: • ICMP error messages (however, may be generated for ICMP queries) • Fragment other than the first • Broadcast or Multicast messages

  5. ICMP Packet Encapsulation ICMP Header ICMP Data IP Header Datagram Data Area Frame Header Frame Data Area

  6. ICMP Header Structure 0 7 8 15 16 31 • TYPE field identifies type of service • There are 15 different values for the TYPE field • CODE - further specifies some types of services • CHECKSUM - for discovering errors. • Same algorithm as in IP checksum • Covers the entire ICMP message Type Code Checksum

  7. ICMP Data • Content of DATA depends on TYPE and CODE fields • In case of an error, DATA field contains IP header and 8 first bytes of a datagram that caused the problem • In hope to help for the Receiver to determine what caused the problem

  8. ICMP Message Types

  9. ICMP Message Types (cont.)

  10. Source Quench Message • Congestion will occur if a router gets datagrams in faster rate than it can process. • In such cases router must discards some of the arriving datagrams • It sends then ICMP “source quench” message to report congestion to the original source • A source quench message is a request for the source to reduce its current rate of datagram transmission

  11. Source Quench Message • There is no ICMP message to reverse the effect of a source quench Source Quench Message Format 0 7 8 15 16 31 Type (4) Code (0) Checksum Unused (Must be zero) IP header + first 8 bytes of datagram

  12. Clock Synchronization • ICMP timestamp request allows a system to query another for the current time ICMP Timestamp Request and Reply Message Format 0 7 8 15 16 31 Type (13 or 14) Code (0) Checksum Identifier Sequence number 32-bit originate timestamp 32-bit receive timestamp 32-bit transmit timestamp

  13. Clock Synchronization • The requestor fills in the originatetimestamp and sends the request • Timestamp values are in milliseconds past midnight • The replying system fills in receivetimestamp when it receives the request and transmit timestamp when it sends the reply Example: sun % icmptime gemini orig = 83601883, recv = 83598140, xmit = 83598140 rtt = 247 ms, difference = -3743 ms

  14. MTU Discovering • When a router get a datagram that requires fragmentation, but the IP header fragment flag is turned on, than the router drop the packet and sends ICMP unreachable error - Fragmentation Required to the sender • Newer router also returns next hop MTUvalue that caused the packet dropping, insides of ICMP message 0 7 8 15 16 31 Type (3) Code (4) Checksum Unused (must be 0) MTU of next-hop network IP header + first 8 bytes of datagram

  15. MTU Discovering • Algorithm: • We send packets with don’t fragment bit set • The size of the first packet we send will be equal the MTU of the outgoing interface • Whenever we receive an ICMP “can’t fragment” error we will reduce the size of the packet: • If the router sending ICMP error, returns MTU that caused the drop than we will use this value • Otherwise we we will try the next smallest MTU (RFC defines only a limited number of MTUs)

  16. Ping Program • A program for checking if host is alive • Exists in most Operation Systems • Sends ICMP message of type Echo Request • Receiver answers with ICMP messages of type Echo Reply • Enables also to see the Round Trip Time from a sender to a destination

  17. Ping (Example)

  18. Traceroute Program • Lets see the route that IP datagrams follow from one host to another • There is no guaranty that two that two consecutive IP datagrams from the same source to the same destination follow the same route, but most of the time they do • Sends a sequence of datagrams with TTL set to 1,2,etc. • These datagrams are UDP packets sent to some unused port.

  19. Traceroute Program (cont.) • When intermediate router receives a packet with TTL=1 it throws the packet and sends back ICMP “time exceeded” message • In such way we can discover all routers in the was between source and destination • The process finishes, when a destination host gets the packet and sends back ICMP “port unreachable” message • Many sites now put firewalls that don’t give traceroute/ping packets get through

  20. Traceroute (Example)

  21. Traceroute (Example) traceroute from ack.berkeley.edu to www.technion.ac.il 1 vlan206.inr-203-eva.Berkeley.EDU (128.32.206.1) 0.573 ms 0.595 ms 0.507 ms 2 vlan210.inr-202-doecev.Berkeley.EDU (128.32.255.9) 0.816 ms 0.546 ms 0.553 ms 3 gigE3-0.inr-000-eva.Berkeley.EDU (128.32.0.201) 0.357 ms 0.253 ms 0.242 ms 4 pos3-0.c2-berk-gsr.Berkeley.EDU (128.32.0.90) 0.345 ms 0.345 ms 0.294 ms 5 SUNV--BERK.POS.calren2.net (198.32.249.14) 1.565 ms 1.670 ms 1.515 ms 6 Abilene--QSV.POS.calren2.net (198.32.249.162) 1.853 ms 1.716 ms 1.725 ms 7 losa-snva.abilene.ucaid.edu (198.32.8.18) 9.297 ms 9.087 ms 9.143 ms 8 hstn-losa.abilene.ucaid.edu (198.32.8.22) 40.695 ms 40.786 ms 40.651 ms 9 atla-hstn.abilene.ucaid.edu (198.32.8.34) 59.921 ms 59.719 ms 59.941 ms 10 ipls-atla.abilene.ucaid.edu (198.32.8.41) 69.950 ms 79.609 ms 69.786 ms 11 ILAN-Abeline.ilan.net.il (192.114.98.2) 74.505 ms 74.324 ms 74.205 ms 12 chi-gp3-fe-i2.ilan.net.il (192.114.101.33) 75.376 ms 74.741 ms 74.375 ms 13 tau-gp2-s0.ilan.net.il (192.114.99.66) 265.863 ms 264.125 ms 264.264 ms 14 tau-gp1-fe-i2.ilan.net.il (192.114.99.34) 264.943 ms 265.664 ms 265.047 ms 15 technion-gp1-mag.ilan.net.il (128.139.203.13) 271.842 ms 269.802 ms 286.051 ms

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