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1、Computer Networks Computer Networks and Internetsand Internets计算机网络与因特网计算机网络与因特网课件课件厦门大学软件林坤辉 PART II Packet TransmissionChapter 9 Hardware Addressing and Frame Type Identification硬件编址与帧类型标识9.1 IntroductionlPhysically,any signal sent across a shared network reaches all attached stations. lAt a given
2、 station,the network interface hardware detects the electrical signal and extracts a copy of the frame. lCommunication does not usually involve all stations. Instead,most communication occurs because an application program on one computer sends data directly to an application program on another. 9.2
3、 Specifying A Recipient指定接收方lMost LAN technologies use an addressing scheme to provide direct communication. lEach station on the LAN is assigned a unique numeric value called a physical address(物理地址),hardware address(硬 件地址),or media access address(MAC). lA frame sent across a LAN contains the addre
4、ss of the sending computer,called a source address,and the address of the intended recipient,called the destination address.9.3 How LAN Hardware Uses Addresses To Filter Packets 局域网硬件如何使用地址过滤包lThe LAN hardware in a computer as completely separate from the computers CPU and memory.lThe LAN interface
5、hardware handles all the details of sending and receiving frame on the shared medium without using the computers CPU.l lThe LAN interface hardware uses The LAN interface hardware uses physical addressing to prevent the physical addressing to prevent the computer from receiving all packets computer f
6、rom receiving all packets that travel across the LAN.that travel across the LAN.l lOnce it has captured a complete Once it has captured a complete frame, the interface hardware frame, the interface hardware compares the destination address in compares the destination address in the frame to the stat
7、ions physical the frame to the stations physical address.address.l网卡的任务：l检测帧是否存在l有效性检验l判断是否接收 CPU的任务l判断是否传给上层处理l分层处理的目的：减少CPU的负荷l数据包过滤是通信接收方的动作l通过检测数据的标识，对符合接收条件的：l复制帧l中断CPU，将复制后的帧交给CPUl继续等待下一个帧 对不符合接收条件的：放弃接收，继续等待下一个帧9.4 Format of A Physical AddresslPhysical Address 必须是所在局域网内的唯一标识 局域网内要对物理地址提供检索的功能
8、 lThree broad categories:l lStatic addressStatic address（静态地址）静态地址）l l出厂前由生产厂家一次性配置完成出厂前由生产厂家一次性配置完成l l全球唯一硬件标识，不会出现地址冲突全球唯一硬件标识，不会出现地址冲突l l一次生成，永久使用一次生成，永久使用l l地址较长，查询慢地址较长，查询慢l lConfigurable address Configurable address （可配置地址）可配置地址）l l系统出厂后，用户动态分配，保持局域网内系统出厂后，用户动态分配，保持局域网内 唯一唯一l l地址短，查询快地址短，查询快l
9、l地址可以永久有效，也可以随需求改变地址可以永久有效，也可以随需求改变l lDynamic address Dynamic address （动态地址）动态地址）l l每当系统启动的时候动态分配，局域网内唯每当系统启动的时候动态分配，局域网内唯 一一l l地址短，查询快地址短，查询快l l有可能出现地址冲突有可能出现地址冲突l l地址动态改变，不利于地址映射表的维护地址动态改变，不利于地址映射表的维护9.5 Broadcasting 广播lBroadcasting was originally applied to radio and television transmission. lThe
10、 network designers define a special,reserved address known as a broadcast address(广播地址). lThe hardware interface on a computer is built to recognize the special broadcast address as well as the stations physical address.l l当一台计算机需要向多台计算机广播信息时，若当一台计算机需要向多台计算机广播信息时，若 使用单播技术，必须向每一台计算机分别发送数使用单播技术，必须向每一台
11、计算机分别发送数 据，将造成网络负载的成倍增长。据，将造成网络负载的成倍增长。l l采用广播实现方式，发送方按照一个广播地址向采用广播实现方式，发送方按照一个广播地址向 共享介质发送数据帧，网上所有设备的网卡分别共享介质发送数据帧，网上所有设备的网卡分别 进行数据帧的拷贝，并交给进行数据帧的拷贝，并交给CPUCPU处理。需要接收处理。需要接收 数据帧的设备由数据帧的设备由CPUCPU完成向上层的传递，不需要完成向上层的传递，不需要 数据帧的设备由数据帧的设备由CPUCPU丢弃该数据帧丢弃该数据帧 通过广播，实现了一对多的数据通信，比单点传通过广播，实现了一对多的数据通信，比单点传 输方式减少了
12、负载，但所有计算机输方式减少了负载，但所有计算机CPUCPU都要决都要决 定接收还是丢弃数据帧，这样增加了无关设备定接收还是丢弃数据帧，这样增加了无关设备 的的CPUCPU负载。负载。9.6 Multicasting 组播l lBroadcasting is extremely inefficient.Broadcasting is extremely inefficient.l lWhen a frame arrives,the network When a frame arrives,the network interface hardware places the contents in
13、terface hardware places the contents in memory,interrupts(in memory,interrupts(中断中断) ) the CPU,and the CPU,and allows system software to determine allows system software to determine whether the frame should be ignored.whether the frame should be ignored.l lDiscarding frame involves using the Discar
14、ding frame involves using the CPU to make a decision.CPU to make a decision.l lA restricted form of broadcasting A restricted form of broadcasting known as multicasting.known as multicasting.l lA network interface does not A network interface does not automatically forward multicast frame automatica
15、lly forward multicast frame to the CPU.to the CPU.l lThe interface hardware must be The interface hardware must be programmed with specification of programmed with specification of which multicast frame to accept and which multicast frame to accept and which to reject.which to reject.l lThe interfac
16、e hardware makes the The interface hardware makes the decision and only accepts those frames decision and only accepts those frames that match the specification.that match the specification.9.7 Multicast Addressing 组播编址 lWhen the computer boots,the interface is programmed to recognize only the computers address and broadcast address. lIf an application on the computer wishes to
