tailieunhanh - Internetworking with TCP/IP- P17

Internetworking with TCP/IP- P17: 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 | 9 Internet Protocol Error And Control Messages ICMP Introduction The previous chapter shows how the Internet Protocol software provides an unreliable connectionless datagram delivery service by arranging for each router to forward datagrams. A datagram travels from router to router until it reaches one that can deliver the datagram directly to its final destination. If a router cannot route or deliver a datagram or if the router detects an unusual condition that affects its ability to forward the datagram . network congestion the router needs to inform the original source to take action to avoid or correct the problem. This chapter discusses a mechanism that internet routers and hosts use to communicate such control or error information. We will see that routers use the mechanism to report problems and hosts use it to test whether destinations are reachable. The Internet Control Message Protocol In the connectionless system we have described so far each router operates autonomously routing or delivering datagrams that arrive without coordinating with the original sender. The system works well if all machines operate correctly and agree on routes. Unfortunately no large communication system works correctly all the time. Besides failures of communication lines and processors IP fails to deliver datagrams when the destination machine is temporarily or permanently disconnected from the network when the time-to-live counter expires or when intermediate routers become so 129 130 Internet Protocol Error And Control Messages ICMP Chap. 9 congested that they cannot process the incoming traffic. The important difference between having a single network implemented with dedicated hardware and an internet implemented with software is that in the former the designer can add special hardware to inform attached hosts when problems arise. In an internet which has no such hardware mechanism a sender cannot tell whether a delivery failure resulted from a local malfunction