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ICMP Protocol - Internet Control Message Protocol
The Internet Control Message Protocol communicates errors messages between systems that require attention. This error is not usually at the user level, the error is handled by the application layer. In the OSI Reference model, ICMPs are generally considered part of the IP layer. To put to plain and simple, ICMP messages are sent "behind the scenes" to your applications to tell them if there is a problem with the connection or transfer of data. These messages tell your application to change what they are doing, change how they are sending and basically give them an error code on how they can successfully fix their problems. ICMPs' are sent in a standard IP datagram, this datagram contains an IP header of twenty (20) bytes and then the ICMP message. There are fifteen values for the type field of the datagram which identifies the type of ICMP message. The "code" portion of the datagram specifies the condition of the "type" even further. This sounds a bit confusing at first, but you have two major categories, the primary "problem," and then a secondary more in depth look at that problem. This will all make more sense when you look at information below. Type 0, code 0 - echo reply Type 3 (destination unreachable) Type 4, code 0 - source quench Type 5 (redirects) Type 8, code 0 - echo request Type 9, code 0 - router advertisement Type 10, code 0 - router solicitation Type 11 (time exceeded) Type 12 (parameter problem) Type 14, code 0 - timestamp reply Type 15, code 0 - information request Type 16, code 0 - information reply Type 17, code 0 - address mask request Type 18, code 0 - address mask reply As you can see from the above information, the primary "type" groups are the main problem, with the "code" further specifying the problem. Much of this stuff doesn't make sense to the human end-user anyway, but it means a lot to the software developers who use these requests to fix problems in their coding. ICMP's are a nice behind the scenes helper that are good to have their, but be thankful you don't really have to deal with them. For even more detailed information on the Internet Control Message Protocol (ICMP) the protocol is defined in RFC 792. Ken Dennis
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