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How a computer "knows" what to respond to particular ICMP messages? As is known there are various ICMP messages that differ by type and / or code. For example ICMP type 0 is "echo reply", type 13 /14 timestamp reply. So whenever a PC receives a ICMP message with valu "0" as a type or any other particular type - how does it know what to respond ? Where is that instruction written ?
Thank you so much
ICMP is part of the OS's network stack. It belongs to the network layer.
Each ICMP message type requires specific handling. An ICMP echo request triggers an appropriate echo reply, redirects or router advertisements feed the local routing table, destination unreachable messages are passed "up" the stack to the transport-layer protocol, and so on.
An echo reply (type 0) that is not expected is silently dropped.
Most ICMP messages can be divided into three categories, queries, replies and errors.
Queries and their corresponding replies have an identifier and sequence number, these can be used to match up replies to the corresponding query. A I understand it in practice ICMP query clients are usually implemented using "raw sockets" to receive all incoming ICMP packets and filter them in the application. This is inefficient but the amounts of traffic involved are too small for anyone to care.
ICMP errors contain a portion of the packet that caused the error (originally the internet header and the first 64 bits of payload, however later RFCs suggested including as much of the original packet as possible to support tunneling scenarios). This portion of the packet that caused the error can be used to pass the error up the stack to the appropriate transport protcol, which in turn can pass it to the appropriate socket.
