[理学]【计算机网络课程教学课件】ch5-2.ppt

Computer Networks - 5 - 2 COMPUTER NETWORKSChapter 05 The Network Layer Part 2 Shoubao Yang Tel: 3601540 syang@ 1/~syang September 2006 5.5 Internetworking Problem: Great! We have all these networks, with all different protocol stacks, and now we just to let them talk to each other. Non-Solution: Enforce all networks to run the same protocol stack. That’s asking for a lot of trouble, and effectively saying that we are not allowed to make any progress. Solution: Construct all kinds of gateways that connect to different kinds of networks Connecting Networks A collection of interconnected networks. 5.5.1 How Networks Differ Don’t worry: it’s impossible to resolve all differences. The solution is to just take a simple approach (like the Internet). We now only consider the starred (*) issues. The relay systems for internetworking repeaters at the physical layer for boosting signals. bridges to make the interconnection at the data link layer. multiprotocol routers for forwarding, and possibly splitting up packets (bridges can’t do the latter). transport gateways for coupling byte streams in different networks. application gateways, e.g., for handling electronic mail between OSI and TCP/IP networks. 5.5.2 How Networks Can Be Connected (a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers. 5.5.3 Concatenated Virtual Circuits Basic idea: Assume the constituent networks support virtual circuits. In that case, the internet can use virtual circuit technology through concatenation. Note: If one of the constituent networks does not support virtual circuits, or, for example, provides only unreliable data transport, simple concatenation will be hard. 5.5.4 Connectionless Internetworking Basic idea: Have the network layer offer only datagram services: unreliable, unordered packet flow. Often, connection-oriented services are not even supported (Internet). Main problem: Addressing – different networks use completely different addresses, so