TCPIP是如何工作的

TCP/IP 是如何工作的 How TCP/IP Works Over the past few years, one of the most written-about network topics has been IP. But even with all this attention, few, if any, stories have traced the protocol/'s basic workings, that is, how routers and Layer 3 switches act upon IP information to move Ethernet packets across the network. As a point of referenced, bear in mind that IP is a member of the TCP/IP protocol suite. TCP functions at the Open Systems Interconnection(OSI)transport layer, or Layer 4.Its chief responsibility is to ensure reliable end-to-end connectivity. IP, located one layer down, at the OSI network layer, or Layer 3, communicates the addresses of each packet/'s sender and receiver to the routers along the way. Routers and Layer 3 switches can read IP and other Layer 3 protocols .This information, combined with routing tables and other network intelligence, is all it takes to get across the room or around the world via TCP/IP. The routing process begins with an IP address that is unique to the sending end station .End stations may be assigned permanent IP addresses or they may borrow them as needed from a Dynamic Host Configuration Protocol(DHCP)server or other service. Each packet carries a source address, which under current(IPv4)specifications is 32 bits long .In its header, each packet also carries the IP address of the final destination. If the sending end station determines that the destination address is not local, the packet goes to a first-hop router, typically one that is close and has been preassigned to the sender. The router inspects the packet/'s IP address and performs a route table lookup to see if the destination end station resides on the local(physically connected)network, typically called an IP subnet .An IP subnet usually is assigned to each of the router/'s network interfaces. If the destination IP address is local, the router searches an internal store of IP addresses and local-device Media Access Control(MAC)addresses. This store is known as the Address Resolution Protocol(ARP)cache. ARP is the universal tool for matching IP addresses to MAC addresses. If the destination/'s MAC address appears, the router installs that MAC address in the packet header(removing its own MAC address because that/'s no longer needed)and sends the packet to the destination end station. In the event that the destination MAC address does not appear in the ARP cache ―it might have timed out, for instance― the router must broadcast an ARP request to the subnet referenced by the packet/'s destination IP address. The end station with that IP address responds, sending back its MAC address .The router updates its cache, installs the new MAC address into the packet header and launches the packet. If the route table lookup shows that the packet is destined for a non-local subnet, the router forwards the packet to the next-hop router using the next-hop router/'s MAC address. Routing tables are continuously built and rebuilt by intelligent discovery protocols, such as Routing Information Protocol or Open Shortest Path First(OSPF). Each router/'s routing table shows the best route to the destination address; for addresses that may be several hops away, it shows the best next-hop router. 在过去几年中，写得最多的网络题目之一就是 IP。但是，即使引起了这但是，即使引起了这 么多的注意，仍很少有(如果有的话)文章介绍此协议的基本工作原理，即路 由器和第三层交换器是如何对 IP 信息起作用，以便在整个网络中传递以太网 包的。但是，即使引起了这 作为参考，请记住，IP 是 TCP/IP 整套协议中的一个协议。但是，即使引起了这 TCP 完成开放系统互连(OSI)模型中传输层，即第四层的功能。但是，即使引起了这它的主要 责任是确保端至端之间的可靠连接。但是，即使引起了这IP 位于下一层，在 OSI 的网络层，即第 三层，把每个包的发送者和接收者地址告诉一路上各个路由器。但是，即使引起了这路由器和第 三层交换器可以读出 IP 和其他的第三层协议。但是，即使引起了这这些信息与路由表以及其他网 络智能结合在一起，能通过 TCP/IP 在整个房间或围绕地球进行传递。但是，即使引起了这 路由过程始于对发送端站而言是唯一的一个 IP 地址，端站可以被分配永 久的 IP 地址，或者按需要从动态主机配置协议(DHCP)服务器或其他服务中 借用。但是，即使引起了这 每个包携带一个源地址，在目前(IPv4)规范下，包长度是 32 位。但是，即使引起了这在报头 中，每个包也携带最终目的地的 IP 地址。但是，即使引起了这 如果发送端站确定目的地址不在本地，该包就被送到第一跳路由器，一 般来说，该路由器是在附近的，并已被预先分配给发送者。但是，即使引起了这 该路由器检查包的 IP 地址，查找路由表，看看目的端点是否位于本地 (物理位置)网络，它通常叫做 IP 子网，IP 子网一般被分配到路由器的每个网 络接口上。但是，即使引起了这 如果目的 IP 地址为本地的，该路由器就搜寻存储着 IP 地址和本地设备 介质访问控制(MAC)地址的内部存储器，这个存储器叫作地址分辨协议(ARP) 高速缓存。但是，即使引起了这ARP 是把 IP 地址映射到 MAC 地址的通用工具。但是，即使引起了这如果目的地的 MAC 地址出现了，该路由器就把这个 MAC 地址装进包的报头中(取掉自己的 MAC 地址，因为不再需要它了)，把该包送到目的端站。但是，即使引起了这 万一目的地的 MAC 地址没有出现在 ARP 高速缓存中(例如，有可能超时 了)，路由器必须向与包的目的 IP 地址有关的子网广播一个 ARP 请求，拥有 此 IP 地址的端站就作出响应，把 MAC 地址发送回去，路由器因此更新高速 缓存，把新的 MAC 地址装进包的报头并发送该包。但是，即使引起了这 如果查找路由表显示包的目的地不在本地子网，路由器就利用下一跳路 由器的 MAC 地址把包转发给下一跳路由器。但是，即使引起了这路由表由智能发现协议，如路 由信息协议或者开放最短路径优先协议，不断地被建立和再建立。但是，即使引起了这每个路由 器的路由表显示了到达目的地址的最佳路径，对于要有几跳的地址，它显示 出最佳的下一跳路由器。但是，即使引起了这 电子墨水与纸 Electronic Ink and Paper Cambridge, Mass.-based E Ink Corp.is a leader in the development of electronic ink and “paper”that could replace newspapers and books as we know them today. The use of electronic ink and two-way wireless communication could lead to the creation of electronic books that will renew themselves with new selections when readers are finished with the current book——or newspapers that update themselves with the latest news while being read. Electronic ink, as devised by E Ink, is a clear, liquid plastic in which there are microcapsules that contain white chips in a blue dye.The microcapsules are suspended in a substance similar to vegetable cooking oil.The white chips are negatively charged so they react to electrical stimulus. This ink can be spread on any surface——from walls to computer screens——says Russell Wilcox, vice president and general manager at E Ink.However, the writing surface would look similar to a very thin laptop display screen with a clear surface on the front and circuitry on the back. A positive charge applied on the top surface of the ink will allow the white to show, making the surface as white as a sheet of paper.If the charge moves to the bottom, the dark particles will show, giving the appearance of blue ink.Electronic ink uses less power than a PalmPilot, and the message remains displayed even after the power is turned off. The ultimate goal is for the electronic pages to look and feel like paper.However, for the foreseeable future, these new books are likely to be bulkier than paperback books. Electronic ink will have interactive qualities, although E Ink isn't sure people will be able to write with it for a while——they will mainly receive messages.Xerox Corp.is also working on a technology that could replace paper as portable, renewable reading matter. The Xerox technology is called Gyricon.It's composed of a silicon rubber compound with the thickness and flexilility of poster board.The Gyricon sheets have thousands of plastic balls suspended in oil.Each ball is black on one side and white on the other and together they act as pixels to display images.Images can be updated much the same way as with a monitor.The beads are embedded in a large sheet, with each microcapsule suspended in oil to allow the beads to rotate in their orbits, says Robert Sprague, manager of the document hardware lab and electronic paper projects at Xerox's Palo Alto Research Center.The paper could be powered by a matrix of transistors, such as those in laptop computer screens.Gyricon uses reflective light, like real paper, so it would use less electricity. A Gyricon book will eventually be connected with a wireless device that will enable a reader to download content from the Internet. Xerox will also make the Gyricon interactive, so a user could write on it and reuse it. (美国) 麻省坎布里奇市的 E Ink 公司是开发电子墨水和“纸”的先导，这种 墨水和纸将替代今天我们所知道的报纸与书籍。但是，即使引起了这 利用电子墨水和双向无线通信有可能导致产生电子书籍 (当读者看完当 前的书本时，可以选择新书刷新) 或者产生能边读边用最新消息自我更新的 报纸。但是，即使引起了这 由 E Ink 公司发明的电子墨水是一种透明的液体塑料，里面有微型封装 块，封装块内含有在蓝色染料中的白色小片。但是，即使引起了这微型封装块悬浮在像食用油那 样 的物质中。但是，即使引起了这白色小片带负电荷，因而它们能对电刺激作出反应。但是，即使引起了这 据 E Ink 公司的副总裁兼总经理 Russell Wilcox 称，这种墨水可以涂在任 何表面，从墙到计算机屏幕。但是，即使引起了这然而，书写面看上去很像极薄的便携机显示屏， 在正面有一透明的表面，在背面有线路。但是，即使引起了这 加上墨水表面上方的正电荷将使白色显现出来，使表面像一张纸那样白。但是，即使引起了这 如果正电荷加到下方，黑粒子将显现出来，给出蓝墨水的痕迹。但是，即使引起了这 电子墨水耗电比 PalmPilot 掌上型电脑还要少，甚至在断电之后信息仍 能显示出来。但是，即使引起了这 这种电子页面的最终目标是看上去和感觉上都像纸。但是，即使引起了这但在可预见到的未 来，这些新型书很可能比真正纸的书体积更大。但是，即使引起了这 电子墨水具有交互的品性，虽然 E Ink 公司吃不准人们是否会用它写字 —它们主要用来接收信息。但是，即使引起了这 施乐公司也在对一种能替代纸的技术——便携式、可刷新的读物，进行 研究。但是，即使引起了这 施乐的这项技术叫 Gyricon。但是，即使引起了这它由硅胶化合物组成，具有像广告宣传牌 那样的厚度和柔软性。但是，即使引起了这Gyricon 片有成千上万个悬浮在油中的塑料球。但是，即使引起了这每个 球一面是黑的，另一面是白的，两者共同起到显示图形的像素的作用。但是，即使引起了这图形 能以监示器那样的 同样方法进行更新。但是，即使引起了这 施乐的 Palo Alto 研究中心文档硬件实验室和电子纸项目经理 Robert Sprague 称，小球被嵌在很大的片子中，每个微型封装块悬浮在油中，允许 小球在自己的轨道中旋转。但是，即使引起了这这种纸能由晶体管阵列供电，如便携机屏幕那样。但是，即使引起了这 Gyricon 像真正的 纸那样利用反射光，因而它用较少的电。但是，即使引起了这 Gyricon 书最终将与无线设备相连，能使读者从因特网上下载内容。但是，即使引起了这 施乐也将使 Gyricon 具有交互功能，因而用户能在它上面写字和反复使 用。但是，即使引起了这