计算机网络期末复习.doc

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IntroductionNetwork hardware组成：1.Broadcast links （一台发送，其他的都知道了）A single communication channel that is shared by all the machines on the networkSending a packet implies that all others receive it.2.Point-to-point links (点对点) 范围:(scale)LAN Local Area Networks 局域网MAN Metropolitan Area Networks 城域网WAN Wide Area Networks 广域网Wireless network 无线电网络Internetworks - Home networks Network software名词：Layer n on one machine carries on a conversation with layer n on another machine.The rules and conventions used in this conversation are collectively known as the layer n protocol.Layers: To reduce their design complexity,most networks are organized as a stack of layers of levels.Protocol: an agreement between the communicating parties on how communication is to proceed.Interface（接口）: between each pair of adjacent layers,it defines which primitive operations and services the lower layer makes available to the upper one.Network architecture(网络体系结构): A set of layers and protocols.Error control:Flow control:Connection-oriented service(连接服务) is modeled after the telephone system.First establishes a connection ,uses the connection,and then releases the connection.(连接、通信、释放)Connectionless service(无连接服务) is modeled after the postal system.Each message carries the full destination address,and each one is routed through the system independent of all the others.(独立地被路由)OSI Reference Model7 layersThe physical layer:Describes the transmission of raw bits in terms of mechanical, electrical, and timing interfaces, and the physical transmission issues.The data link layer:transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer=send bits in frames that add redundancy to detect something went wrongprovide the mechanisms so that fast senders dont overwhelm slow receivers (flow control)broadcast networks have an additional issue in the data link layer: how to control access to the shared channel. The network layer:Controls the operation of the subnet. Describes how routing (and congestion) is to be done and allow heterogeneous networks to be interconnected.The transport layer:Specify how to handle details of reliable transfer, this layer provides the actual network interface to applicationsThe transport layer is a true end-to-end layerThe transport layer also determines what type of service to provide to the session layer, and, ultimately, to the users of the network. The session layer:tells how applications can set up “long-lasting”communications, The presentation layer:Describes everything that is needed to exchange data in a platform-independent way. The application layer:contains a variety of protocols that are commonly needed by users.TCP/IP Reference Model4 layersThe Internet layer:The job of the internet layer is to deliver IP packets where they are supposed to go. Packet routing is clearly the major issue here, as is avoiding congestion. The transport layer:It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer.The application layer:T he TCP/IP model does not have session or presentation layers. they are of little use to most applications.The Host-to network layer:T he TCP/IP reference model does not really say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP packets to it. OSI和TCP/IP Reference model的优缺点:p45-49OSI的缺点：Bad timing ，Bad technology， Bad implementations ，Bad politicsTCP/IP的缺点：Service, interface, and protocol not distinguishedNot a general modelHost-to-network “layer” not really a layer, no mention of physical and data link layers Minor protocols deeply entrenched, hard to replaceTCP/IP的优点：it was there when neededfreely distributed with the UNIX operating system. Networks standardizationWhos Who in the Telecommunications World ?Whos Who in the International Standards World ?Whos Who in the Internet Standards ? 2. The Physical layer1. The physical layer is the basis of all networks.2. Nature imposes two fundamental limits on all channels, and these determine there bandwidth.-the Nyquist limit: deals with noiseless channels. Maximum data rate=2Hlog2V b/s,-the Shannon limit: deals with noise channels Maximum data rate= Hlog2(1+S/N)3. terms-band rate, bit rate-digital, analog-full duplex, half duplex, simplex: 双工、半双工、单工 p129-E1, T1 E1: 32 voice channels Europe and China 2.048MbpsT1: 24 voice channels North America and Japan 1.544Mpbs-bandwidth: The range of frequencies transmitted without being strongly attenuated is called the bandwidth. Data Transmission-Analog signals Transmission for digital data ：ASK, FSK, PSK, QPSK-Digital signals Transmission for digital data ：Manchester encoding，Differential Manchester encoding-Digital signals Transmission for analog data ：PCM, codecPCM (Pulse Code Modulation:脉冲编码调制): Analog Digital (AD) Transmission: (1) Sampling 采样 （2）Quantifying 量化 （3）Encoding 编码Three basic forms of modulation: （下面的三个词要会默写） p126-amplitude modulation ：幅度调制-frequency modulation ：频率调制-phase modulation ：像位调制4. Multiplexing: 复用-FDM 频分复用-TDM 时分复用-CDMA 码分复用-WDM 波分复用Switching-Circuit switching -Message switching-Packet switching ：VC, datagram3 Data Link LayerData Link Layer Design Issues-Services Provided to the Network Layer-Framing如何进行比特填充？-Error ControlCRC校验-Flow ControlSliding windowStop-and-Wait: Sending window=1; receiving window=1Go-Back-NSendingwindo1 ;receiving window=1Note that a window size of 1 means that the data link layer only accepts frames in orderSelective RepeatSending window1; receiving window1HDLC High Level Data Link Controla classical bit-oriented protocol whose variants have been in use for decades in many applications.PPP Point-to-point ProtocolA data link protocol used to connect home computers to the Internet.P2394 The Medium Access ControlSublayerData link layer:LLC+MAC (?)Channel Allocation:CSMA/CDCSMA(载波增听)-全称：Carrier Sense Multiple Access Protocolsmonitor the channel before transmission1-persistent:Listen whether the channel is free before transmitting.If busy, wait until it becomes free and then immediately start your transmission.Nonpersistent:Less greedy when the channel is busy, wait a random period of time before trying again. If you wait too long, the channel utilization drops.p-Persistent:Used with slotted systems.If you find the channel idle during the current slot, you transmit with probability p, If the station initially senses the channel busy, it waits until the next slot and applies the above algorithm.CSMA/CD (CSMA with Collision Detection) : For CSMA, stations listen to the cable while they are transmitting, and abort their transmissions as soon as they detect a collision rather than finish transmitting their frames.Collision Detection（边说边听）: The stations hardware must listen to the cable while it is transmitting. If what it reads back is different from what it is putting out, it knows that a collision is occurring.How long will it take them to realize that there has been a collision?In the worst case is 2 which is the network full cable round-trip propagation time. -Note:contention interval (?)Ethernet/IEEE802.3 -physical layer （用到什么协议？速率范围？传输媒介？） cabling (1)Ethernet systems use Manchester encoding with the high signal + 0.85 volts and the low signal -0.85 volts (2)Different Ethernets use different connection and cabling schemes四种常见的cabling:Thick coax,Thin coax,Twisted pair,Fiber opticsManchester encodingP275 -MAC sublayer Frame format -MAC Address: Generally 48-bit fields. The high-order bit of the destination address indicates ordinary or group addresses. ( 0 for ordinary addresses and 1 for group addresses).a Group addresses allow multiple stations to listen to a single address. When a frame is sent to a group address, all the stations in the group receive it.Sending to a group of stations is called multicast.The address consisting of all 1 bits is reserved for broadcast. A frame containing all 1s in the destination field is accepted by all stations on the network. bit 46 (adjacent to the high-order bit) indicates global or local address.Local addresses are assigned by each network administrator and have no significance outside the local network.Global addresses, in contrast, are assigned centrally by IEEE to ensure that no two stations anywhere in the world have the same global address.With 48 - 2 = 46 bits available, there are about 7 x 1013 global addresses.-Minimum and maximum frame length Data field: ranges from 0-1500.Frames should always be at least 64 bytesIf the data portion of a frame is less than 46 bytes, the Pad field is used to fill out the frame to the minimum size. All frames must take more than 2to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case.CSMA/CD ,BinaryExponentiaBackoff Time is divided into discrete slots whose length is equal to the worst-case round-trip propagation time on the other (2, 512 bit times, or 51.2 sec for 10Mbps)After the first collision, each station waits either 0 or 1 slot times before trying again.After i collisions, a random number between 0 and 2i -1 is chosen, and that number of slots is skipped.After ten collisions have been reached, the randomization interval is frozen at a maximum of 1023 slots.After 16 collisions, the controller throws in the towel and reports failure back to the computer.Switch/Bridge (网桥/交换机) 工作原理：This decision is made by looking up the destination address in a big table inside the bridge. As follows: If destination and source LANs are the same, discard the frame. If the destination and source LANs are different, forward the frame. If the destination LAN is unknown, use flooding.Spanning Tree Bridges?IEEE 802.11The use of virtual channel sensing using CSMA/CA.5 The Network Layer Virtual-Circuit and Datagram P344No advance setup is needed. Packets are injected into the subnet individually and routed independently of each other.The packets are called datagrams and the subnet is called a datagram subnet.图见346页Routing within a diagram subnet.A route from the source machine to the destination machine must be set up in advance, and stored in tables inside the routers. That route is used for all traffic flowing over the connection.This connection is called a VC (virtual circuit) and the subnet is called a virtual-circuit subnet.图见348页 Routing within a virtual-circuit subnet IP Address Classful Address P437 Subnet, Subnet Mask CIDR：allocate the IP addresses in variable-sized blocks, without regard to the classes. Network ID, Host ID, Broadcasting Address, availabe host address（C5-x_3.pdf的P49-50） NAT and Private Address P446(背出来)Private Address: 55 (or 10/8 ) (16,777,216 hosts) 55 (or 172.16/12 ) (1,048,576 hosts) 55(or 192.168/16 ) (65,536 hosts) ICMP, ARP(P450用文字描述工作原理), RARPARP ：Map IP addresses onto data link layer addresses Routing What is routing? Static Routing : ip route ”destination net id” ”subnet mask” ”next hop” Dynamic Routing6 The Transport LayerThe key position of the transport layerA key position Transport layer forms the major boundary between the provider and user of the data transmission service, also the major boundary between Resources Subnet and Communication Subnet.Socket Socket=an IP address with a port numberWell-known-port 几个常用的UDP 无连接的，头部非常短，8个字节 什么时候用到，哪些性能属于哪个UDP (User Datagram Protocol) is simple: no flow control, no error control, no retransmissions The main value of having UDP over just using raw IP is the addition of the source and destination ports to identify the end points within the source and destination machines.UDP is especially useful in client-server situations(RemoteProcedure Call) and multimedia(TheReal-Time Transport )The UDP headerTCP 面向连接，头部固定，24字节-Format-Connection EslabtishmentTCP Connection EstablishmentUses three-way handshake protocol. A SYN segment consumes 1 byte of sequence space so that it can be acknowledged unambiguously. (a)TCP connection establishment in the normal case. Connection EstablishmentBasic idea:To establish a connection, you send off a connection request to the other end. The other end then accepts the connection, and returns an acknowledgment.Big problem:Suppose the ackis delayed, so you (application) do another request, then you establishes two connections. No one knows you didnt do this on purpose.Main cause:The network has storage capabilities, and unpredictable delays. This may lead to delayed duplicates.-Connection release 对称释放，如何释放，描述出来，（汉字成图）TCP Connection ReleaseAlthough TCP connections are full duplex ,it is better to be thought of as independent releases of two simplex connections.Data may continue to flow indefinitely in one direction when the other direction is shut down.To avoid the two-army problem, timers are used. Asymmetric release: one party just closes down the connection. May result in loss of data:Abrupt disconnection with loss of data.symmetric release:each direction is released independently of the other one. A host can continue to receive data even after it has sent a DISCONNECT TPDU. -Window ManagementTCP Transmission Policy-TCP Window ManagementTCP uses dynamic buffer allocation and decouple the buffering from the acknowledgements.The receiver sends an acknowledgment and the max