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Intro to the Internet Control Message Protocol (ICMP)

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• Managing the Internet
  • Errors:

    Sometimes, errors occur while transmitting IP-packets

    Sample errors:

    The destination network ID is not found in the routing table Such router does not have a default entry in the routing table !!!!      The TTL field has counted down to 0 and the IP packet is discarded The most common casue of this phenonemon is a routing loop in the routing tables) Etc, etc

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  • Reporting errors:

    IP network errors are reported using the Internet Control Message Protocol (ICMP)

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• Transmitting ICMP messages
  • How to send an ICMP message through the Internet:

    The ICMP information (packet) is transmitted inside the data portion of an IP packet: The protocol field is set to 1 (one) to indicate that the data is an ICMP message

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• The ICMP data packet format
  • Structure of an ICMP data packet:

    +------------+------------+----------------------------+ | type | code | checksum | +------------+------------+----------------------------+ | ICMP data (depending of the type of the ICMP msg) | +------------------------------------------------------+

    Meaning of the type code:

    Type code	ICMP packet type
    0	Echo reply (for echo request)
    3	Destination unreachable
    4	Source quench
    5	Redirect
    8	Echo request
    9	Router advertisement
    10	Router solicitation
    11	Time exceeded (TTL=0)
    etc etc

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• Network jargon: "out-of-band" communication
  • Normal communication between 2 nodes on the Internet:

    Normally, nodes on the Internet transmits data messages (such as email or webpages) to each outer

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  • Out-of-band communication:

    Out-of-band communication = abnormal communication The term out-of-band refers to communications which occur outside of a previously established communication method or channel. Out-of-band communications are messages that are not part of the normal messages that are transmitted between the nodes Out-of-band messages are not generated by users Out-of-band messages are the result of some unexpected event (like errors)

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  • Example of out-of-band communication:

    The Internet Control Message Protocol (ICMP) messages