ICMP can also be used to diagnose network related problem. This is accomplished through the query message, a group of four different pair of message.
A query message is encapsulated in an IP packet, which in tum is encapsulated in a data link layer frame.
Four pairs of Query message are:
Echo Request and Reply:
- This is used to determine whether two system(host or routers) can communicate with each other.
- It is proof that intermediate routers are receiving, processing and forwarding the IP datagram.
- It is used to test upto network layers of all the device from source to destination.
- Ping command can be used to generate a series of each request and reply.
- Type is 8 and 0 for request and reply respectively.
Timestamp request and reply:
- Two machines (hosts or routers) can use the timestamp request and timestamp reply messages to determine the round-trip time needed for an IP datagram to travel between them.
- It can also be used to synchronize the clocks in two machines
- Type is 13 and 14 for request and reply respectively.
Address mask request and reply:
- A host may send a mask request to the router to get its mask if it knows the address of the router or may send broadcast address to fetch its unknown mask.
- Type is 17 and 18 for request and reply respectively.
Router Solicitation and Advertisement:
- It is a router discovery protocol to locate routers in the network using ICMP queries.
- Router solicitation is a request message(broadcast or multicast) send by host for getting the address of the router.
- It is used when the host became active for the first time on the network.
- Host can use the response to update their routing table as higher priority router become available in the network.
- Router Advertisement: This is the response message for router solicitation message. The router or routers that receive the solicitation message broadcast their routing information using the router-advertisement message.
- A router can also periodically send router-advertisement messages even if no host has solicited.
- Note that when a router sends out an advertisement, it announces not only its own presence but also the presence of all routers on the network of which it is aware
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