INTERNET CONTROL MESSAGE PROTCOL

ICMP n allows router to send error or control messages to another router or host n provides communication between IP software on one machine & IP software on another

ICMP Message Transport

ICMP Error Messages n Source Quench n Time Exceeded n Destination Unreachable n Redirect n Fragmentation Required

Source Quench n Sent by router whenever it has received so many datagrams that it has run out of buffer space n Process: –Incoming datagrams discarded –Source Quench sent to host –Host receives message & reduces rate of transmission

Time Exceeded n If router reduces TTL field to zero, datagram discarded & message sent to host n If reassemble timer expires before all fragments for a datagram arrive

Destination Unreachable n If a datagram cannot be delivered to its final source. n Distinction between: –Entire net down (router failure) –Host temporarily off-line

Redirect n If router determines a host has incorrectly sent a datagram that should be sent to a different router, the router uses a redirect to cause the host to change its route

Fragmentation Required n If router receives datagram that exceeds MTU of the net over which it will be sent, router discards datagram & sends a fragmentation request to sender