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 Education Made Easy

Internet Control Message Protocol

        - It is used for error reporting and diagnostic

        - It sits on top of IP.


        - There are several types of ICMP messages.  They are used in different cases

        - ICMP Source Quench

        - It is sent to a host by a router when a packet has been dropped due to congestion


        - Assume that R2 is receiving more packets than the packets it can forward.  This is called congestion.  At some point the packet queue will overflow and packets will be dropped

        - In this case, R2 will send an ICMP Source Quench message to the source of the packet dropped (H1)

        - A Network administrator may have a program that collects these ICMP source quench packets

        - ICMP packets also may be lost and there is no higher level protocol to retransmit them

        - Also there cannot be ICMP messages sent about other ICMP messages.  This prevents infinite loops

        - No errors about other errors.

ICMP Time Exceeded

        - It is sent by a router to the source of a packet when the TTL reaches 0

        - Also it is sent to the source of a packet when the reassembly timer expires before completing reassembly



ICMP Message Types

        Source Quench

                - Sent by a router when a packet is dropped

        Time Exceeded

                - sent by router when TTL reaches zero

                - sent by a host when reassembly timer expires before reassembling packet

        Destination Unreachable

                - sent by a router when a packet cannot be delivered

                - Also it can be sent by a host if the destination port of a UDP/TCP packet does not exist

        Echo/Reply

                - It is also called a "ping"

                - Not an error message

                - It is used to test connectivity

                - The sender sends an ICMP echo request message and the receiver answers with an echo reply message

        ICMP Redirect

                - It is sent by a router to a host in the local network if the host used the router in the wrong way

Example 1


1.  A wants to send an IP packet to B but it sends it to R instead

2.  R delivers packet to B

3.  Since A could have sent the packet directly to B, R sends an ICMP redirect message to A

Example 2


1.  A wants to send a packet B.  A sends packet to R1

2.  R1 forwards packet to R2

3.  R1 sends redirect message to A telling to forward packets directed to B to R2 instead


ICMP Class Notes