The Internet Control Message Protocol

1
The Internet Control Message Protocol

• Chapter 23

2
ICMP

• In TCP/IP there is no intelligence in the network
• Thus, if something fails, there is no easy way to
  report that failure
• A special message mechanism allows routers to
  report nasty events

3
Reporting or Correction?

• Errors are reported, but they are not corrected
• Errors about error messages are not reported for
  obvious reasons
• Errors are reported to IP software, not to a
  particular protrocol
• Errors are reported to the source

4
Examples

• A Header Checksum Error
• The datagram must be discarded
• Why?
• Less severe errors involve ICMP
• IP type 1
• IP uses ICMP and ICMP uses IP

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A List of ICMP Messages
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Examples - I

• Source Quench
• A router has run out of buffer space and must
  discard datagrams
• When it discards a datagram, it sends a source
  quench message to the host that created the
  datagram. Why that host?
• The host must reduce its transmission rate

7
Examples - II

• Time exceeded.
• TTL field gets to zero
• Datagram discarded
• ICMP message is sent
• Destination Unreachable
• When a router discovers that a datagram cannot be
  delivered, it sends a message
• Network unreachable
• Host unreachable

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Examples - III

• Redirect causes a host to change its route
• Eg, the extra hop problem
• Parameter problem
• Echo request/reply
• Address Mask request/reply
• A host boots and needs to know the network mask

9
ICMP Message Transport

• ICMP uses IP
• Encapsulation
• Error messages and replies are sent to the
  sending host
• No special priority
• No error messages sent when ICMP messages are
  dropped

10
Ping and Traceroute

• Ping tests reachability!
• Traceroute sends datagrams with TTL varying from
  1 to n
• At each host that discards one of these
  datagrams, an ICMP time exceeded message is sent
• Most useful in a stable internet

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Traceroute - The Last Address

• Two ways to do traceroute
• Send ICMP echo request messages
• Send a datagram to a non-existent service (port)
• Windows use the first approach, and always uses
  the information the user typed on the command
  line
• Unix uses the actual interface on which the
  datagram arrived
• Two answers are possible for a multi-homed host

12
Path MTU Discovery

• IP software sends a sequence of probes with the
  Dont Fragment bit set
• If it arrives at the destination, then that
  datagrams size is acceptable
• If not, the software reduces the size of the
  datagram and retries

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Echo Request and Reply Format

• Type (8 or 0)
• Code
• Checksum
• Identifier
• Sequence Number
• Optional Data

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Unreachable Destinations

• Type is 3
• First 64 bits of the datagram
• Code
• 0 network unreachable
• 1 host unreachable
• 2 protocol unreachable
• 3 port unreachable

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Codes (continued)

• 4 fragmentation needed and DF bit set
• 5 source route failed
• 6 destination network unknown
• 7 Destination host unknown

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Congestion and Flow Control

• Because IP is connectionless, there is no way to
  reserve resources on routers in advance
• As a result routers can be overrun with traffic

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Congestion

• A computer may be able to generate traffic faster
  than a network can transfer it
• If many computers simultaneously need to send
  datagrams through a single router, the router may
  experience congeston

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Whats a Router to do?

• Small bursts can be solved by buffering
• If traffic continues, a source quench message is
  sent to the original source to reduce its rate of
  transmission

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More Sophisticated Routers

• Monitor incoming traffic and quench sources with
  the highest rates
• Send requests before the queue actually overflows

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Congestion (continued)

• There is no ICMP message to reverse the effect of
  a source quench
• A source lowers its rate until it stops receiving
  quench messages and then gradually increases the
  rate
• Rely on higher level protocols!

21
Source Quench Format

• Type 4
• Code 0
• Checksum
• Unused
• Internet Header first 64 bits

22
Route Change Requests

• Routers are assumed to know correct routes
  http//www.calvin.edu/lave/figure-8.7.pdf
• Hosts begin with minimal routing information and
  learn new routes from routers
• ICMP redirect allows a host to boot knowing the
  address of only one router

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Code Field in a redirecct

• 0 redirect for the net (obsolete)
• 1 redirect for the host
• 2 redirect for the type of service and net
• 3 redirect for the type of service and host

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Circular or Excessively Long Routes

• Datagrams are prevented from circling forever by
  TTL
• ICMP time exceeded message
• Code 0 Time-to-live count
• Fragment reassembly time exceeded

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Reporting Other Problems

• ICMP parameter problem message
• Only severe problems
• Arguments to an option are incorrect
• Pointer field identifies the octet that caused
  the problem

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Clock Synchronization and Transit Time Estimation

• ICMP timestamp request message
• Time in milliseconds since midnight Universal
  Time
• Round trip delay can be difficult to estimate
• Varies greatly
• averaged