计算机网络 科大精品课程ch5-1.pptVIP

Computer Networks - 05 -1 COMPUTER NETWORKSChapter 05 The Network Layer Part 1 Shoubao Yang Tel: 3601540 syang@ 1/~syang September 2006 Chapter 5THE NETWORK LAYER Provide facilities for getting data from a source to a destination May require making many hops at intermediate routers along the way 5.1 Network Layer Design Issues 5.1.1 Store-and-Forward Packet Switching A host with a packet to send transmits it to the nearest router, either on its own LAN or over a point-to-point link to the carrier. The packet is stored there until it has fully arrived so the checksum can be verified. Then it is forwarded to the next router along the path until it reaches the destination host, where it is delivered. The environment of the network layer protocols 5.1.2 Services provided to the Transport Layer Service goals: Services should be independent of technology Transport layer is shielded from number, type, and topology of the subnet Uniform numbering plan across LANs and WANs Issues about connection-oriented and connectionless services, the bottom line is about where to put the complexity. In connection-oriented service, it is in the network layer (subnet) In connectionless service, it is in the transport layer(hosts). The subnet is inherently unreliable The Internet community argues that the routers’ job is moving packets around and nothing else. The subnet is inherently unreliable, no matter how it is designed. Therefore, the hosts should accept the fact that the network is unreliable and do error control, i.e., error detection and correction and flow control themselves. Network service should be connectionless, with primitives SEND PACKET and RECEIVE PACKET and little else. No packet ordering and flow control should be done, the hosts are going to do that anyway; each packet must carry the full destination address, for it is carried independently of its predecessors. Subnet should provide a reliable, connection-oriented service? The telephone companies argue that