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.
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。ARP是把IP地址映射到MAC地址的通用工具。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。
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。
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。
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。微型封装块悬浮在像食用油那样 的物质中。白色小片带负电荷，因而它们能对电刺激作出反应。
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。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。
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。这种纸能由晶体管阵列供电，如便携机屏幕那样。 Gyricon像真正的 纸那样利用反射光，因而它用较少的电。