Скачать презентацию How the Internet works Lab How the
How_the_Internet_works.pptx
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Скачать презентацию How the Internet works Lab How the
Скачать презентацию How the Internet works Lab How the
How the Internet works Lab
How the Internet works • Internet is like a global phone system for computer • www. nytimes. com -- placing a "call" in effect - and sends a request for the main web page. • The request is small, about 1 KB (1 kilobyte). • The www. nytimes. com machine sends back a large response which is the web page -- maybe 200 KB -- and ends the call.
ETHERNET • Ethernet is a LAN (Local Area Network) networking technology, used to connect small groups of, say, 20 computers. • Ethernet cable lengths are typically limited to 100 meters • A typical LAN application is networking the computers in one room or in one floor of a building. • The most common form of ethernet wiring is 100 base-T which uses wires with a "RJ-45" connector
Packets • Image - 50 KB - divided into packets. • Each packet is about 1500 bytes (a typical ethernet size) the 50 KB image could be divided into about 32 packets
Packets • A packet of 1500 bytes of information we want to send. • 1 byte = 8 bits so that's 12000 bits to send, where each bit is a 0 or 1. • The sending computer could read through the 12000 bits in order for each 1 bit, put 3 volts between the wires, and for each 0 bit, put 0 volts between the wires.
Why Packets? • Packet Granularity / Sharing • ethernet can operate like a big input hopper. . all the packets to send are dumped in there, and the ethernet can work through them, sending one packet at a time
Why Packets? • Packet Checksum Re-Send • Each packet includes extra checksum bytes, so the receiver of the packet can detect if some of the bits in the packet got corrupted in transmission • Scheme: - go through all the bytes, and add them all up. The checksum is the last 3 digits of the sum of all the bytes; - send that checksum along with the rest of the packet data. - The receiver can do the same computation -- adding up all the bytes -- to check that they get the same checksum
Landmark Ethernet Design
Landmark Ethernet Design • Shared -- there's just the one wire and everybody uses it • Distributed and cooperative -- no central control, depends on each computer following the cooperative protocol in good faith • Insecure -- not to hard to listen and pick up packets not intended for you • Performance degrades as more computers use the shared medium.
IP Address • 4 bytes, written between dots, like "171. 64. 2. 3". • 23. 75. 345. 200 What’s wrong?
Router • 171. 64. 166 wants to send a packet to a computer at 173. 255. 219. 70 • IP Packet From: 171. 64. 166 To: 173. 255. 219. 70 data
Router • Each computer on the LAN has a unique address. This is know as its MAC address (Media Access Control). The MAC address is burned in at the factory • Подключение по локальной сети - Ethernet адаптер: . . . Физический адрес. . : 00 -0 C-F 1 -AE-B 1 -34
Router
ACK Re-Send • As part of TCP/IP routing, the ultimate recipient sends back to the sender ACK (acknowledgement) information, letting the sender know which packets have been successfully received (the packets have serial numbers 0, 1, 2. . ).
Wi-Fi Networking Wi-Fi is an enormously successful LAN standard Wi-Fi refers to the 802. 11 set of standards 100 feet Wi-Fi uses the "unlicensed" 2. 4 Ghz and 5 Ghz radio bands, which anyone in the US is free to use • 802. 11 g - 54 mbps (millions of bits per second) • 802. 11 n - 150 Mbps and 300 Mbps • •
URL (Uniform Resource Locator)) • http: //www. cmu. edu/class/ssd 2 - http at the start is the networking scheme to use - www. stanford. edu is the domain name of the computer on the internet that hosts the web page -the web server - /class/ssd 2 path indicates essentially which directory and file we want specifically from that web server
Web client • In networking terminology, the browser is the "client" which makes requests and displays what it gets back.
Web Server • Web server -- a machine which hosts a set of web pages, and waits for requests to come in for those pages. • The phrase "web server" can refer to the physical machine, or it can mean the program that responds to requests. • The url http: //example. com/a. html means to get the file a. html from the web-root directory.
HTTP Request / Response
Скачать презентацию How the Internet works Lab How the
How_the_Internet_works.pptx