tailieunhanh - Internetworking with TCP/IP- P23

Internetworking with TCP/IP- P23: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 188 Protocol Layering Chap. 11 Layering in a TCP IP Internet Environment Our statement of the layering principle is somewhat vague and the illustration in Figure skims over an important issue because it fails to distinguish between transfers from source to ultimate destination and transfers across multiple networks. Figure illustrates the distinction showing the path of a message sent from an application program on one host to an application on another through a router. As the figure shows message delivery uses two separate network frames one for the transmission from host A to router R and another from router R to host B. The network layering principle states that the frame delivered to R is identical to the frame sent by host A. By contrast the application and transport layers deal with end-to-end issues and are designed so the software at the source communicates with its peer at the ultimate destination. Thus the layering principle states that the packet received by the transport layer at the ultimate destination is identical to the packet sent by the transport layer at the original source. Host A HostB Figure The layering principle when a router is used. The frame delivered to router R is exactly the frame sent from host A but differs from the frame sent between R and B. Sec. The Protocol Layering Principle 189 It is easy to understand that in higher layers the layering principle applies across end-to-end transfers and that at the lowest layer it applies to a single machine transfer. It is not as easy to see how the layering principle applies to the Internet layer. On one hand we have said that hosts attached to an internet should view it as a large virtual network with the IP datagram taking the place of a network frame. In this view datagrams travel from original source to ultimate destination and the layering principle guarantees that the ultimate destination receives exactly the datagram that the original source sent. On the other