Used to communicate IP status and error messages between hosts and routers Uses IP to route its messages between hosts Must be implemented with IP remember, IP is just a packet delivery system transmits and routes datagrams from sources to destinations through a series of interconnected networks it has a checksum in the IP header to detect lost bits no error detection on the datagram payload though but has no native mechanism for source host notification This is where ICMP comes in its used to report IP errors to the source host ICMP data is carried as the payload of an IP datagram specifies additional message formats within this area . | Internet Command Message Protocol (ICMP) CS-431 Dick Steflik ICMP Internet Command Message Protocol (ICMP) RFC 792 Used to communicate IP status and error messages between host and routers ICMP Used to communicate IP status and error messages between hosts and routers Uses IP to route its messages between hosts Must be implemented with IP remember, IP is just a packet delivery system transmits and routes datagrams from sources to destinations through a series of interconnected networks it has a checksum in the IP header to detect lost bits no error detection on the datagram payload though but has no native mechanism for source host notification This is where ICMP comes in its used to report IP errors to the source host ICMP data is carried as the payload of an IP datagram specifies additional message formats within this area Basic ICMP Header Headers are 32 bits in length; all contain same three fields type - 8 bit message type code thirteen message type are defined code - 8 bit; . | Internet Command Message Protocol (ICMP) CS-431 Dick Steflik ICMP Internet Command Message Protocol (ICMP) RFC 792 Used to communicate IP status and error messages between host and routers ICMP Used to communicate IP status and error messages between hosts and routers Uses IP to route its messages between hosts Must be implemented with IP remember, IP is just a packet delivery system transmits and routes datagrams from sources to destinations through a series of interconnected networks it has a checksum in the IP header to detect lost bits no error detection on the datagram payload though but has no native mechanism for source host notification This is where ICMP comes in its used to report IP errors to the source host ICMP data is carried as the payload of an IP datagram specifies additional message formats within this area Basic ICMP Header Headers are 32 bits in length; all contain same three fields type - 8 bit message type code thirteen message type are defined code - 8 bit; indicating why message is being sent checksum - standard internet checksum 16 bit 1’s complement sum of the payload and header for purpose of calculation the checksum field is set to zero ICMP Message types 0 - Echo Reply 3 - Destination Unreachable 4 - Source Quench 5 - Redirect 8 - Echo 11 - Time Exceeded 12 - Parameter Problem 13 - Timestamp 14 - Timestamp Reply 15 - Information Request 16 - Information Reply 17 - Address Mask Request 18 - Address Mask Reply Destination Unreachable (3) ICMP header (4 bytes) + unused 32 bits (4 bytes) + IP header (24 bytes) + first 64 bits of data (8 bytes) = 40 bytes Codes: 0 - net unreachable ; 1 - host unreachable 2 - protocol unreachable ; 3 - port unreachable sent by destination host IP module 4 - fragmentation needed DF set ; 5 source route failed 6 - destination network unknown ; 7 destination host unknown 8 - source host isolated ; 9 - comm. with destn network prohibited 10 - comm. With dest host prohibited ; 11 - network unreachable for service
