tailieunhanh - Internetworking with TCP/IP- P16

Internetworking with TCP/IP- P16: 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 | 118 Internet Protocol Routing IP Datagrams Chap. 8 Because the internet addresses of all machines on a single network include a common network prefix and extracting that prefix requires only a few machine instructions testing whether a machine can be reached directly is extremely efficient. From an internet perspective it is easiest to think of direct delivery as the final step in any datagram transmission even if the datagram traverses many networks and intermediate routers. The final router along the path between the datagram source and its destination will connect directly to the same physical network as the destination. Thus the final router will deliver the datagram using direct delivery. We can think of direct delivery between the source and destination as a special case of general purpose routing - in a direct route the datagram does not happen to pass through any intervening routers. Indirect Delivery Indirect delivery is more difficult than direct delivery because the sender must identify a router to which the datagram can be sent. The router must then forward the datagram on toward its destination network. To visualize how indirect routing works imagine a large internet with many networks interconnected by routers but with only two hosts at the far ends. When one host wants to send to the other it encapsulates the datagram and sends it to the nearest router. We know that the host can reach a router because all physical networks are interconnected so there must be a router attached to each network. Thus the originating host can reach a router using a single physical network. Once the frame reaches the router software extracts the encapsulated datagram and the IP software selects the next router along the path towards the destination. The datagram is again placed in a frame and sent over the next physical network to a second router and so on until it can be delivered directly. These ideas can be summarized Routers in a TCP IP internet form a .