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Feng Lin Sichuan Univ. Layered Reference Model Data Transfer in Layered Model Network Devices TCP & UDP More on IP Link Layer Tech. Networks are complex!! Dealing with complex systems: ◦ explicit structure allows identification, relationship of complex system’s pieces layered reference model for discussion ◦ modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system Internet protocol stack ISO/OSI reference model Application layer: supporting network applications ◦ POP, SMTP, HTTP Transport layer: process-process data transfer ◦ TCP, UDP Network layer: routing of datagrams from source to destination ◦ IP, routing protocols Link layer: data transfer between neighboring network elements ◦ PPP, Ethernet Physical layer: bits “on the wire” A layer N+1 entity sees the lower layers only as a service provider The unit of data send between peer entities is called a Protocol Data Unit (PDU,协议数据单元 ) A N Layer Entity PDU (at layer N) N Layer Entity B Scenario: Layer-N at A sends a layer-N PDU to layer-N at B 8 As data is moving down the protocol stack, each protocol is adding layer-specific control information User data HTTP HTTP Header User data TCP Header HTTP Header User data TCP IP TCP segment IP Header Ethernet TCP Header HTTP Header User data IP datagram Ethernet Header IP Header TCP Header HTTP Header User data Ethernet frame 9 Ethernet Trailer HTTP HTTP protocol HTTP TCP TCP protocol TCP IP Ethernet IP IP protocol Ethernet argon.tcpiplab.edu 128.143.137.144 Ethernet IP IP protocol Ethernet Ethernet Ethernet router71.tcpip- router137.tcpiplab.edu lab.edu 128.143.137.1 128.143.71.1 00:e0:f9:23:a8:20 neon.tcpip-lab.edu 128.143.71.21 10 HTTP TCP IP Frame is an IP datagram Ethernet Send HTTP Request to neon HTTP Establish a connection to Open TCPatconnection 128.143.71.21 port 80 to TCP 128.143.71.21 port 80 IP datagram is a TCP segment for port 80 Send IP data-gram Send a datagram (which contains a Send IP datagram to IP to IP to128.143.71.21 128.143.71.21 connection request) 128.143.71.21 Frame is an IP datagram Send the Send the datagram to 128.143.137.1 Ethernet Ethernet Ethernet datagram to 128.143.7.21 argon.tcpipneon.tcpip-lab.edu router71.tcpip- router137.tcpipSend Ethernet frame Send Ethernet frame lab.edu 128.143.71.21 lab.edu lab.edu to to 128.143.137.144 128.143.137.1 128.143.71.1 00:20:af:03:98:28 00:e0:f9:23:a8:20 00:e0:f9:23:a8:20 11 Network devices are components used to connect computers or other electronic devices together Besides the various modems for connecting into Internet, the following devices are most commonly used. ◦ ◦ ◦ ◦ NIC( Network Interface Card) Hub Switch Router NIC is a hardware card installed in a computer so it can communicate on a network Layer 1 & Layer 2 A hub is a device that connects several nodes of a LAN A repeater amplifies and regenerates signals so they can retain the strength to reach their destinations Hub is basically a multiport repeater. Layer 1 device Switches are a special type of hub that offers an additional layer of intelligence to basic, physical-layer repeater hubs. Layer 2 device A router is a device that is connected to at least two networks and makes decisions about the best route for data In general, a router stores a routing table which is generated by routing algorithm. When pkts arrive, the router will forward pkts by routing table. Working on Layer 3 Port number—the id of network process ◦ Example port numbers: HTTP server: 80 Mail server: 25 Two type of transport services provided by Internet ◦ Connectionless transport service—UDP (User Datagram Protocol) Unreliable data transfer Simple, high transfer speed ◦ Connection-Oriented transport service—TCP(Transmission Control Protocol) Reliable data transfer Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol Underlying transport protocol SMTP Telnet HTTP FTP HTTP (eg Youtube), RTP SIP, RTP, proprietary (e.g., Skype) TCP TCP TCP TCP TCP or UDP typically UDP Application Layer 218 Internet Protocol (IP协议) :One of the most important protocols Uses connectionless(无连接) delivery – it does not guarantee delivery Each packet that travels through the Internet is treated as an independent unit of data without any relation to any other unit of data. 20 IP Address (IP地址): An identifier for a computer or device on a TCP/IP network. The format of an IP address version 4 is a 32-bit numeric address written as four numbers separated by periods (点分十进制表示法). Each number can be zero to 255. For example: 10000001 00001011 00000011 00011111 129 11 3 IP=129.11.3.31 31 IP address consists of two parts ◦ The network portion is used to route packets between networks ◦ The host portion identifies the particular device on the host network There are five classes of IP address The class of address is identified by the first bits of the address Three classes are used for networks. The last two are reserved for special purposes (Class D is for multicast, and Class E is for experimentation and future use) Byte 1 Byte 2 Byte 3 Byte 4 Class A 0 Network Host Class B 10 Network Host Class C 110 Network Host Class A – First eight bits identify network, the first bit is fixed at 0, giving 27 (128) networks, and 224 (16,777,316) hosts Class B – First 16 bits identify network, the first two bits are fixed at 10, giving 214 (16,384) networks, and 216 (65,536) hosts Class C – First 21 bits identify network, the first three bits are fixed at 110, giving 221 (2,097,152) networks and 28 (256) hosts 23 For any organization the Network portion of the address remains fixed, the network administrator can assign the host address Assignment can be manual or through Dynamic Host Configuration Dynamic Host Configuration Protocol (动态主机配置 协议) ◦ Internet protocol for automating the configuration of computers that use TCP/IP. Larger classes of networks (A and B) are usually divided into smaller networks ◦ This is achieved by dividing up the host portion of the address ◦ This process is known as subnetting A ‘subnet mask’ uses some of the bits from the host address to create subnets ◦ The number of bits used determines the number of subnets which are generated ◦ The remaining bits in the host portion identify the number of host IP addresses in each subnet <network portion><subnet portion><host portion> ◦ For Example: IP Address: 218.194.22.96 Subnet mask: 255.255.255.224 25 26 ipconfig Configure the network parameters on localhost Ping (Packet Internet Groper) – sends a signal to a specific address and waits for a reply ◦ If it gets a response, Ping Reports computer is online Displays the elapsed time (latency) for the round-trip message Traceroute – records a packet’s path from your computer to its destination ◦ Hop跳数 is an intermediate connection in a string of connections linking two network devices. ◦ Traceroute utilities to trace the hop Using Ping or Traceroute, you can discover how long data is in transit On average, data within China usually arrives at its destination 110-120 ms (milliseconds) after it is sent Overseas transmission require a little more time Collision: When two devices on a sharing channel send packets at the same instant, it’s called a collision. Network access method ◦ determined by type of network interface card and cables ◦ Ethernet ◦ Token Ring Ethernet 以太网: IEEE 802.3 ◦ The most popular LAN tech. ◦ Uses Bus or Star Topology ◦ Supports data transfer rate of 10/100/1000 Mbps ◦ Use CSMA/CD to avoid collision 31 Token ring 令牌环网: IEEE 802.5 ◦ The second most popular network access method ◦ transmits data at 4 or 16 megabits per second over twisted-pair wire. ◦ uses ring topology ◦ uses token (special message) to prevent collisions 32 Based on Ring Topology Makes use of Tokens ◦ A token is a special series of bits that travels around a tokenring network. ◦ The token acts like a ticket, enabling its owner to send a message across the network 33 Layered Reference Model Network Devices How data travels in the layered model Difference between UDP and TCP IP and IP address Tools to check your network Ethernet, Token Ring what did you know? ◦ Fundamental concepts of Communication, Computer Networks, and Internet What don’t you know? ◦ How to work with computer networks ◦ How the computer networks work in detail You can reveal questions above in the course, named computer networks.