ICMP

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ICMP
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Internet Control Message Protocol (ICMP)

• ICMP is used by IP to send management messages
• The ICMP header contains a Type and Code field
  that identifies the type of message being sent
• Type identifies the type of message and code
  identifies a subtype

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ICMP Messages
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Destination Unreachable Codes
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ICMP Messages
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Time Exceeded Message

• One of the codes for the Time Exceeded message is
  the Time To Live (TTL) code
• This code is used with the IP header TTL field
• Each router that processes a packet decrements
  the TTL field by 1
• When this value reaches 0 the packet is discarded
  
• The router which discards the packet sends a Time
  Exceeded ICMP packet to the sending host

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Trace Command
RouterA trace 10.1.3.1 Type escape sequence to
abort. Tracing the route 10.1.3.1 1 10.1.6.253 8
msec 4 msec 4 msec 2 10.1.3.1 12 msec 8 msec 4
msec RouterB debug ip icmp ICMP time exceeded
(time to live) sent to 10.1.6.251
(dest was 10.1.3.1) ICMP time exceeded (time to
live) sent to 10.1.6.251 (dest was
10.1.3.1) ICMP time exceeded (time to live) sent
to 10.1.6.251 (dest was 10.1.3.1)
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ICMP Redirect Message
Router A
Router B