Internet Protocol: Error and Control Messages (ICMP)

1. Internet Protocol: Error and Control Messages (ICMP) Chapter 8

2. We have • Unreliable • Connectionless datagram delivery… • Need a way to inform original source if: • Router cannot forward or deliver a datagram • Router detects unusual condition affecting the ability to forward a datagram • Original source will need to avoid or correct the problem

3. Internet Control Message Protocol • Routers operate autonomously • If everything works correctly, all ok. But: • Communication lines fail • Processors fail • Destination machines get disconnected from the network • TTL counters expire • Intermediate routers get congested

4. In an internet, no special hardware mechanisms to inform hosts of problems • Don’t know if failure is local or remote problem • Debugging is difficult • IP has nothing itself to help out • Added special-purpose message mechanism • Internet Control Message Protocol (ICMP) • Considered a required part of IP

5. ICMP messages travel in data portion of IP datagrams • ICMP: • Allows sending of error or control messages to other routers or hosts • Provides communication between IP software on one machine and IP software on another • ICMP module handles ICMP messages • Any machine can send an ICMP message

6. Error Reporting vs Correction • ICMP is technically a reporting mechanism • ICMP only reports error condition to source • Source must relate error to a specific application program or take other action • Most errors stem from original source • Some do not • ICMP only informs original source • Source may not be able to determine problem

7. Have to restrict communication to original source: • Datagrams don’t usually record complete route • Can’t know path taken to a given router • So cannot inform intermediate routers • Inform original source • Hope host administrators and network administrators will cooperate to solve problem

8. ICMP Message Delivery • ICMP uses two levels of encapsulation • ICMP message in datagram data portion • Datagram in frame data portion • Datagrams carrying ICMP messages: • Are forwarded the same way as others • May themselves get lost or discarded • May cause further congestion

9. Figure 8.1

10. Exception for error messages • ICMP messages not generated if error results from datagram carrying ICMP error message • ICMP not a higher-level protocol • It is a required part of IP • IP used to carry because messages may have to travel across several networks • Cannot be delivered by physical transport alone

11. ICMP Message Format • Each ICMP message has own format • But, all begin with three fields: • 8-bit integer message TYPE • 8-bit CODE field • 16-bit CHECKSUM • Also includes first 64 data bits of datagram causing the problem

12. Echo Reply and Request (0 & 8) • Command on many systems is ping • Sends echo request to a machine • Receiver sends echo reply to sender • Tests if destination is reachable & responding • Verifies source, outbound path, destination, and return path • Sophisticated versions send a series of requests and provide statistics about datagram loss

13. C:\>ping google.com Pinging google.com [64.233.187.99] with 32 bytes of data: Reply from 64.233.187.99: bytes=32 time=31ms TTL=239 Reply from 64.233.187.99: bytes=32 time=32ms TTL=239 Reply from 64.233.187.99: bytes=32 time=31ms TTL=239 Reply from 64.233.187.99: bytes=32 time=30ms TTL=239 Ping statistics for 64.233.187.99: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 30ms, Maximum = 32ms, Average = 31ms C:\>

14. Echo request and reply message format:

15. Unreachable Destination (3) • Sent when router cannot forward or deliver a datagram

16. Discarding datagrams not taken lightly • Messages sent when router determines destination unreachable • Network unreachable: usually forwarding failure • Host unreachable: usually delivery failure • 13 types possible; specified in CODE field • However, routers cannot know all delivery failures • Destination on Ethernet • Network HW doesn’t provide acknowledgements

17. Source Quench (4) • Congestion: routers overrun with traffic • Can happen for two reasons: • High-speed computer generate traffic too fast for network to handle • Many computers sending thru same router • Datagrams buffered if arrive too fast • Helps with small burst • If continues, memory gets exhausted

18. Source quench used to report congestion • Requests source to reduce rate of transmission • Usually one message per discarded datagram • May be more sophisticated • Quench source with highest transmission rate • Avoid congestion by sending before overflow

19. Redirect (Route Change)(5) • Routers are assumed to know correct routes • Hosts begin with minimal forwarding info • Usually know the address of single router • Learn new routes from routers • One way is the ICMP redirect message • Router detects host using a non-optimal route • Redirect requests the host change its route • Host routing table stays small but optimal

20. Limitation: • Only have interactions between directly connected host and router • Later routers cannot send ICMP redirect • Don’t know non-directly connected router’s address • Redirect not used to solve general problem of propagating routing information Code 0: redirect for net Code 2: redirect for TOS and net Code 1: redirect for host Code 3: redirect for TOS and host

21. Figure 8.7

22. Time Exceeded (11) • Can get routing cycles • R1 sends datagrams for D to R2 • R2 sends datagrams for D to R1 • TTL (hop count) timer used to stop circle • ICMP time exceededmessage: • Sent when datagram discarded due to TTL=0 • Or, when timed out waiting for datagram fragments

23. Code 0: TTL count exceeded Code 1: Fragment reassembly time exceeded

24. Parameter Problem (12) • Problem exists with a datagram • Severe enough to cause it to be discarded • Uses pointer to ID octet that caused the problem Code 1: required option is missing (pointer field not used)

25. Timestamp Request & Reply (13 or 14) • Can have clock synchronization problem • Widely different clocks can confuse operations • Several protocols exist to synchronize clocks • One of simplest: • Machine sends ICMP timestamp request message • Receiver returns a timestamp reply

26. From originate time: - can compute total time From receive & transmit times: - can compute network transit time Then estimate differences in clocks

27. Information Request and Reply (15 or 16)*OBSOLETE* • Intended to allow hosts to discover internet address at startup • Made obsolete by DHCP

28. Address Mask Request & Reply (17 or 18)*OBSOLETE* • Chap 9 – motivation for subnets • Understand for now: • Some bits in hostid identify a physical network • Host needs to know: • Which bits correspond to the physical network • Which bits correspond to the host identifiers • Subnet mask: • 32-bit quantity allowing address interpretation • Intended to allow host to obtain the address mask used on the local network • Made obsolete by DHCP

29. Router Solicitation & Advertisement*OBSOLETE* • Intended to allow a host to discover routers available on the local network • Made obsolete by DHCP • Two differences from DHCP: • Host got info directly from router • Prevented hosts from retaining routes after a router crashes (soft state technique) • Routers advertise their information periodically • Host discards route when timer for it expires

30. Summary • Routers may need to communicate with network software on a particular host • Report abnormal condition • Send control information • Internet Control Message Protocol is used • Travels in data area of IP datagram • Three fixed-length fields at the beginning • Message type determines rest of format