ICMP

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ICMP
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Errors

• Many different kinds of errors
• Checksums, networks/hosts not reachable, etc.
• Internet Control Message Protocol - at layer 3
  with IP
• IP uses ICMP to send messages
• ICMP uses IP for transport
• Error messages not allowed to create new error
  messages

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(No Transcript)
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Examples

• Source quench
• Sent by routers when overwhelmed no more
  buffers
• Sender must slow down
• Time exceeded
• TTL decremented to zero
• Loop in route or traceroute

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Examples Continued

• Destination unreachable
• Network, host, protocol
• Redirect
• Used for route changes
• Now not used, typically an attack
• Parameter problem bad parameter in message
• Two non-error types
• Echo Request/Reply
• Address mask request

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Encapsulation
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Uses of ICMP Messaging

• Ping uses echo requests
• Traceroute uses TTL field
• Datagram sent with TTL 1, error comes back from
  first router (with routers IP address)
• Datagram sent with TTL 2, error comes back from
  second router
• Etc.
• Path MTU determination

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Traceroute Usage Continued

• Traceroute does not discover absolute route as
  routes change on a per-datagram basis
• Its data most meaningful in cases where NW routes
  are stable
• Getting last address message to ultimate
  destination
• Microsoft echo usage to ultimate destination
  returns same address as sent
• Unix datagram to non-existent application UDP
  to app level get back destination unreachable
  with address of interface over which error
  message is sent
• Difference if ultimate destination is multi-homed

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Path MTU Trying to Preempt Fragmentation

• Communication most efficient if sender creates
  datagrams of smallest MTU along path
• Routers dont have to fragment
• Destination doesnt have to reassemble
• Learn path MTU iteratively trial and error
  probing repeated sending
• Send datagram of native LAN size with Dont
  Fragment bit set

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Path MTU Continued

• If receive error, reduce size
• Repeat until no errors

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Exercises

• 23.2, 23.4