Internet Control Message Protocol (ICMP) messages:
• ICMP messages are used to communicate the network layer information among the hosts and routers.
Different types of ICMP messages:
Source quench message:
• It is used for performing congestion control.
• This message is sent to the host from the congested router to force the host to reduce its transmission rate.
Echo reply message:
• When an echo request of type 8 code 0 is sent to the destination, the destination host sends back the reply of Type 0 Code 0 ICMP.
TTL (Time to live) expired message:
• It is a warning message sent to the source.
• When the router notices that the TTL has expired, it discards the datagram and sends the warning message of Type 11 Code 0 to the source.
• Warning message includes the name of the router and its IP address.
Port unreachable message:
• Host sends port unreachable message of Type 3 Code 3 to the source, when the datagram contains an unlikely port number.
• When the message reaches source, source stops to send additional probe packets.
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