ICMP ( INTERNET CONTROL MESSAGE PROTOCOL) messages are divided into two broad categories : error-reporting messages and query messages.
The error reporting messages report the problems that a router or a host (destination) may encounter when it processes an IP packet. They would report back the problem back to the IP module at the sending system.
The query messages which occur impairs, help a host or a network manager get specific information from a router or another host. For example, this can be used by the hosts to discover the routers present in their network. The host would send a ICMP query asking for routers to respond. The outers present in the network will respond with an ICMP reply message. The host would get information about the router from this reply.
Under error-reporting type, the following messages are present :
- Destination unreachable
- Source quench
- Time exceeded
- Parameter problem
The query messages type has following messages :
- Echo request and reply
- Timestamp request and reply
- Address mask request and reply
- Router solicitation and advertisement