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The Illustrated Network- P38

The Illustrated Network- P38: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 13 Routing and Peering 339 different ISPs rather than keeping requests and replies all on one ISP s network. This common practice has real consequences for QoS enforcement. These drawbacks of the telephony settlements model resulted in a movement to more simplistic arrangements among ISP peers which usually means ISPs of roughly equal size. These are often called peering arrangements or just peering. There is no strict definition of what a peer is or is not but it often describes two ISPs that are directly connected and have instituted some routing policies between them. In addition there is nearly endless variation in settlement arrangements. These are just some of the broad categories. The key is that any traffic that a small network can offload onto a peer costs less than traffic that stays on internal transit links. Economically there is often also a sender-keeps-all arrangement in place and no money changes hands. An ISP that is not a peer is just another customer of the ISP and customers pay for services rendered. An interesting and common situation arises when three peers share a transit peer member. This situation is shown in Figure 13.4. There are typically no financial arrangements for peer ISPs providing transit services to the third peer so peer ISPs will not provide transit to a third peer ISP unless of course the third peer ISP is willing to pay and become a customer of one of the other ISPs . FIGURE 13.4 ISPs do not provide free transit services and generally are either peers or customers of other ISPs. Unless arrangements are made ISP B will routinely block transit traffic between ISP A and ISP C. 340 PART III Routing and Routing Protocols All three of these ISPs are peers in the sense that they are roughly equal in terms of network resources. They could all be small or regional or national ISPs. ISP A peers with ISP B and ISP B peers with ISP C but ISP A has no peering arrangement or direct link with ISP C. So packet deliveries from hosts .