TCP/IP First-Step [Electronic resources] نسخه متنی

What does the acronym ICMP stand for?

Answer: ICMP is the name of a component internal to the Internet Protocol (IP). Its full name is the Internet Control Message Protocol.

Which layer of the OSI reference model does ICMP occupy?

Answer: ICMP occupies Layer 3, the network layer, of the OSI reference model.

What is the purpose of ICMP?

Answer: ICMP was specifically designed to enable communications between the various machines that comprise a network. Computers that speak IP, as well as the devices in a network, can all communicate via ICMP and share news about problems within the network or within a single IP packet.

Is ICMP useful to end users?

Answer: Yes it is, just not directly. For one thing, ICMP enables a network's computers and other devices to operate more efficiently by sharing information about error conditions within the network. Users benefit indirectly from this by having a network that is more operationally effective.

Users can also benefit from some of ICMP's capabilities by using utilities that employ ICMP mechanisms. Two such utilities bundled into TCP/IP are ping and traceroute. They appear similar, but work in different ways and employ different ICMP message types. Consequently, they gather different data and perform different functions.

What are some of the tools you can use that rely upon ICMP?

Answer: Although ICMP gets used extensively in an IP network, it was designed as a behind-the-scenes communications protocol that only is used in system-to-system communications. End-user functionality just wasn't designed into ICMP. Having said that, two wonderful tools enable you to benefit from ICMP. These tools are ping and traceroute.

What does PING do and why is it useful?

Answer: ping isn't part of ICMP. It's a utility, bundled with TCP/IP, that uses ICMP messages. Specifically, ping enables you to check whether any given machine in the network is reachable. If it replies to your ping, chances are fairly good that it is alive and well and ready to serve you. If it doesn't respond to your ping, all you really know is you can't get there from here! The machine might be down or a network problem might prevent you from reaching that device.

Which ICMP message types does PING utilize?

Answer: ping actually uses two ICMP message types: Echo and Echo Reply. When you run the ping utility, you are causing an ICMP Echo message to be sent. The machine you are trying to reach is obligated by the ICMP protocol to respond by sending you an ICMP Echo Reply message.

What does TRACEROUTE do and why is it useful?

Answer: traceroute is another TCP/IP utility that takes advantage of ICMP's capabilities. This program doesn't generate an ICMP message, but it causes ICMP messages to be returned.

If you know what you are looking at, you can map out all the connections that make up your network and get a good perspective on how busy or idle each connection is at any given time.

Which ICMP message type does TRACEROUTE utilize?

Answer: traceroute works quite a bit differently from ping. Whereas PING employs two ICMP message types, traceroute uses just oneand indirectly at that! When you run traceroute you are not generating an ICMP message. You are generating series of IP packets with a short Time-To-Live (TTL) interval. As these packets expire during their journey through the network, network devices send an ICMP Time Exceeded message back to you. traceroute interprets the contents of these Time Exceeded messages and displays them on your computer screen.

Can using a system-communications mechanism such as ICMP make IP a reliable protocol?

Answer: No! IP was not designed to be reliable. ICMP makes IP more effective by enabling machines that speak IP to share information about either system- or packet-level problems in the network. That's not how you ensure reliable delivery of packets.

Besides, ICMP is the lowest priority of any IP-based protocol, so it is not uncommon to see network devices discarding ICMP messages due to the load they are experiencing. If ICMP is always the first casualty of a busy network, there's no practical way it could augment IP to ensure reliable IP packet delivery.