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ICMP: reporting a better route to the destination

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• Example of a better route
  • Consider the following scenario:

    Suppose a host contains only the default route: But there are multiple router (R1 and R2) on the IP network Specifically: Another (internal) IP network is connected to the second router R2 Suppose he host H1 sends a packet to the host H2 on the other IP network (network Nx): According the routing table in H1: The IP packet will be transmitted to (default) router R1 The router R1 forwards the packet (H1,H2) (H2 is on network Nx) to router R2:

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  • Better route:

    The host H1 can transmit the IP packet to Router R2 directly:

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• Detecting a better route
  • How to detect a better route:

    When some router R1 forwards an IP packet to another router on the same IP network Example: (Clearly, the transmission should use the other router !!!)

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• Reporting a better route
  • Reporting a better route:

    When a router discovers a better route: The router transmits an ICMP re-direct control message to the source (sender) of the IP packet

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  • Structure of the ICMP redirect control message:

    Comment:

    The ICMP redirect control message contains: The IP address of the "better" router for the IP packet !!! Another piece of information that is sent back is: The (network ID) of the IP destination (This information is in the IP header portion of the ICMP packet)

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• Processing an ICMP redirect control message

  • Processing an ICMP redirect control message:

    The ICMP redirect message will insert the following routing entry into the routing table of the source: (NetID of the destination IP dest,    IP address of the better router)

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  • Example:

    The destination (H1) receives an ICMP redirect control message: The host H1 will insert the routing table entry (netID, R2):

  • Result:

    Now host H1 now has a better route to the destination H2

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• Security Problems with ICMP-redirect
  • Fact:

    The ICMP redirect messages pose major security risks...

  • But this topic (security) is beyond the scope of this course...

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  • If you want to explore some spoofing technqiues, read this excellent article: click here

    (I have a local copy: click here )