The Internet Control Message Protocol (ICMP) is a network-layer Internet protocol that provides
message packets to report errors and other information regarding IP packet processing back to the
source. ICMP is documented in RFC 792.
ICMP Messages
ICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request
and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP
message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP
messages.
When an ICMP destination-unreachable message is sent by a router, it means that the router is unable
to send the package to its final destination. The router then discards the original packet. Two reasons
exist for why a destination might be unreachable. Most commonly, the source host has specified a
nonexistent address. Less frequently, the router does not have a route to the destination.
Destination-unreachable messages include four basic types: network unreachable, host unreachable,
protocol unreachable, and port unreachable. Network-unreachable messages usually mean that a
failure has occurred in the routing or addressing of a packet. Host-unreachable messages usually
indicates delivery failure, such as a wrong subnet mask. Protocol-unreachable messages generally
mean that the destination does not support the upper-layer protocol specified in the packet.
Port-unreachable messages imply that the TCP socket or port is not available.
An ICMP echo-request message, which is generated by the ping command, is sent by any host to test
node reachability across an internetwork. The ICMP echo-reply message indicates that the node can
be successfully reached.
An ICMP Redirect message is sent by the router to the source host to stimulate more efficient
routing. The router still forwards the original packet to the destination. ICMP redirects allow host
routing tables to remain small because it is necessary to know the address of only one router, even if
that router does not provide the best path. Even after receiving an ICMP Redirect message, some
message packets to report errors and other information regarding IP packet processing back to the
source. ICMP is documented in RFC 792.
ICMP Messages
ICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request
and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP
message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP
messages.
When an ICMP destination-unreachable message is sent by a router, it means that the router is unable
to send the package to its final destination. The router then discards the original packet. Two reasons
exist for why a destination might be unreachable. Most commonly, the source host has specified a
nonexistent address. Less frequently, the router does not have a route to the destination.
Destination-unreachable messages include four basic types: network unreachable, host unreachable,
protocol unreachable, and port unreachable. Network-unreachable messages usually mean that a
failure has occurred in the routing or addressing of a packet. Host-unreachable messages usually
indicates delivery failure, such as a wrong subnet mask. Protocol-unreachable messages generally
mean that the destination does not support the upper-layer protocol specified in the packet.
Port-unreachable messages imply that the TCP socket or port is not available.
An ICMP echo-request message, which is generated by the ping command, is sent by any host to test
node reachability across an internetwork. The ICMP echo-reply message indicates that the node can
be successfully reached.
An ICMP Redirect message is sent by the router to the source host to stimulate more efficient
routing. The router still forwards the original packet to the destination. ICMP redirects allow host
routing tables to remain small because it is necessary to know the address of only one router, even if
that router does not provide the best path. Even after receiving an ICMP Redirect message, some
devices might continue using the less-efficient route.
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