The Illustrated Network- P23:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER Internet Control Message Protocol 7 What You Will Learn In this chapter you will learn about ICMP messages their types and in many cases the codes used in each type. We ll look at which ICMP messages are routinely blocked at firewalls and which are essential for proper device operation. You will learn about the common ping utility for determining device accessibility reachability on an IP network. We ll discuss the mechanics of both ping and traceroute and use several ping examples to illustrate ICMP on the network. The only function of the IP layer is to provide addressing for and route the IP packet. That s all. Once an IP packet has been dealt with the IP layer just looks for the next packet. But IP is a connectionless best effort or unreliable method of packet delivery. The terms best effort and unreliable often make it sound like IP is casual about the delivery of packets which is why they are in quotes so that no one takes them too literally. IP s best effort is usually just fine given the low error rates on modern transports and it is mostly unreliable with regard to a lack of guarantees as has been pointed out. Besides there is nothing wrong with letting other layers such as the TCP segments or the Ethernet frames have the major responsibility for error detection and correction. This is not to say that IP should be oblivious to errors. The network layer in its ubiquitous and key position at the heart of the protocol stack should know about packet errors and is in a good position to let layers above know what s going on although IP lets the upper layers decide what to do about the condition . And there s plenty that can still go wrong and not just with regard to bit errors. A packet might wander the router cloud until the TTL field hits zero. A destination server might be down. A destination server might no longer exist. The do not fragment bit might forbid fragmentation when it is needed to send a packet stopping the routing process cold. In all of
