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

Categories: Networking