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ICMP

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					Internet Control Message
         Protocol


               ICMP
               ICMP

•   ICMP has two major purposes:
    – To report erroneous conditions
    – To diagnose network problems
         ICMP Message Format
 1 byte      Type     Which type of message?

 1 byte     Code      Which sub-type of message?


2 bytes    Checksum   Checksum of entire ICMP message


     ?       Data     Extra data (specific to the type of message)
Important ICMP Messages

•   Echo Request/Reply
•   Destination Unreachable
•   Redirect
•   Time Exceeded
•   Parameter Problem
•   Router Advertisement/Discovery
    Echo Request/Reply

•   Echo Request (Type: 8, Code: 0):
    –   Requests that the receiving host send a copy
        (echo) of the request back to the sender
•   Echo Response (Type: 0, Code: 0):
    –   The response from the receiving host
•   Using these 2 commands is one way that
    the ping command can perform its function
Destination Unreachable
•       Network Unreachable (Type: 3, Code: 0):
    –       The network of the packet’s destination address field could not be reached
•       Network Unknown (Type: 3, Code: 6):
    –       The network of the packet’s destination address field could not be located
•       Host Unreachable (Type: 3, Code: 1):
    –       The host specified by the destination address could not be reached
•       Host Unknown (Type: 3, Code: 7):
    –       The host specified by the destination address could not be located
•       Protocol Unreachable (Type: 3, Code: 2):
    –       The specified protocol (e.g. TCP or UDP) could not be established with the
            specified host
•       Port Unreachable (Type: 3, Code: 3):
    –       The specified port number could not be reached
               Redirect

•   Redirect (Type: 5, Code: 0):
    –   This router knows of a better path to the
        specified destination
    –   The next router in the new path is provided
         Time Exceeded

•   Time Exceeded (Type: 11, Code: 0):
    –   The router has decremented the hop limit of a
        packet to zero and dropped the packet
        Parameter Problem

•   Parameter Problem (Type: 12, Code: 0):
    –    Part of all of the packet’s IP header was invalid
Router Advertisement/Discovery

  •   Router Advertisement (Type: 9, Code: 0):
      –   A router announces its existence using this type
          of message
  •   Router Discovery (Type: 10, Code: 0):
      –   A host explicitly searches for a router
                Ping

         ICMP Echo Request (Type: 8, Code: 0)




                                                 Destination
Source




         ICMP Echo Response (Type: 0, Code: 0)
                                      Traceroute
         Packet w/ Hop Limit = 1

         ICMP Time Limit Exceeded



                                               Packet w/ Hop Limit = 2

                                               ICMP Time Limit Exceeded




                                                                                                                                                     Destination
                                    Router 1




                                                                          Router 2




                                                                                                                Router 3
Source




                                                                                     Packet w/ Hop Limit = 3

                                                                                     ICMP Time Limit Exceeded



                                                                                                                           Packet w/ Hop Limit = 4


                                                                                                                            Response to Packet

				
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posted:8/20/2011
language:English
pages:12
Description: computer networking