数据与计算机通信 2

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Protocol Architectures and Networks
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Addressing Requirements
Two levels of addressing required Each computer needs unique
network address Each application on a (multitasking) computer needs a unique address within the computer
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Operation of a Protocol Architecture
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Standardized Protocol Architectures
Required for devices to communicate Vendors have more marketable products Customers can insist on standards based
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OSI - The Model
A layer model Each layer performs a subset of the
required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not 22 require changes in other layers
Task broken into subtasks Implemented separately in layers in stack Functions needed in both systems 3 Peer layers communicate
Key Elements of a Protocol
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Primitive Types
REQUEST
请求
INDICATION
A primitive issued by a service user to invoke some service and to pass the parameters needed to specify fully the requested service
Exchange of data between the
computer and the network Sending computer provides address of destination May invoke levels of service（调用 某些级别的服务） Dependent on type of network used (LAN, packet switched etc.)
数据通信与网络
Data Communications & Networking 2. 协议体系结构
Protocol Architecture

为什么需要协议体系结构 简化的三层协议体系 OSI模型的体系结构 TCP/IP协议体系结构
1
Agenda
为什么需要协议体系结构 简化的协议体系结构 OSI模型的体系结构 TCP/IP协议体系结构
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Simplified File Transfer Architecture
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A Three Layer Model
Application Layer （应用层） Transport Layer
（传输层） Network Access Layer (网络接入层)
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Network Access Layer
OSI Layers
Application Presentation Session Transport Network Data Link Physical
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OSI模型中协议分层的理解
请举一日常生活中的例子来形象地描述该模型中
各层的peer-to-peer protocol（对等协议）和 上下层之间的interface（接口）。 北大学生张明给理工大学生李强写信的例子
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Transport Layer
Reliable data exchange Independent of network being
used Independent of application
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Application Layer
Support for different user
applications e.g. e-mail, file transfer
(SNA)
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思考题
协议是什么？为什么需要协议？ 协议构成的基本要素是什么？ 协议体系结构为什么采用分层的方法？优缺点？ 三层简单协议体系结构中，各层协议完成的功
能？ 网络系统中的两级地址是？ 靠什么来传递对等层协议相关信息？ 协议体系结构的工作？
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Agenda
为什么需要协议体系结构 简化的协议体系结构 OSI模型的体系结构 TCP/IP协议体系结构
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Protocol Data Units (PDU)
At each layer, protocols are used to
communicate Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added
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2.1 Need For Protocol Architecture
E.g. File transfer
Source must activate comms. Path or inform network of destination Source must check destination is prepared to receive File transfer application on source must check destination file management system will accept and store file for his user May need file format translation
Protocol specification must be precise Functional description of what is provided 提供何物的功能描 述
Addressing
Referenced by SAPs
寻址
由服务访问点引用
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Service Primitives （原语） and Parameters
张明写信 封入信封 投入信箱 收信分检装邮包 邮包送转运站 装列车箱发运
中文 姓名地址 目的地址 目的站
李强读信 拆开信封 从信箱中取信件 开邮包选路投递 从转运站取回邮包 列车箱卸包送转运站
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张明给李强写信的通信协议体系结构图例
张明（发送方）
写信 第4层 封信 第4层对等协议 拆信 4/3接口
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作为标准的框架结构的OSI体系结构
Layer Specific Standards特定层的标准
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Elements of Standardization
Protocol specification 协议规范
Operates between the same layer on two systems
李强（接收方）
读信
第3层对等协议
第3层 投信 取信 3/2接口
第2层
收信 分检 装包
第2层对等协议 转运邮包
投递 选路 拆包
取回邮包 2/1接口
装车 第1层
发车
第1层对等协议
到站
卸车 26
铁轨（传输媒体）
The OSI Environment
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OSI as Framework for Standardization
Services between adjacent layers
expressed in terms of primitives and parameters Primitives specify function to be performed Parameters pass data and control info
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2.3 OSI
Open Systems Interconnection Developed by the International
Organization for Standardization (ISO) Seven layers A theoretical system delivered too late! TCP/IP is the de facto （actually） standard
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2.2 Protocol Architecture
Task of communication broken up
into modules For example file transfer could use three modules
File transfer application Communication service module Network access module
Destination SAP Sequence number Error detection code
15 This gives a transport protocol data unit
Network PDU
Adds network header
network address for destination computer Facilities requests
The service access point or SAP
上层访问下层所提供的服务 是上下层之间的接口
The port on TCP/IP stacks
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Protocols in Simplified Architecture
请与前面简化的文件传送体系结构进行对比
•前者是此协议体系结构的一个例子 •网络接入模块即网络接入层 •通信服务模块对应于传输层 •文件传送应用属应用层
在两个系统的同层之间工作
May involve different operating system可涉及不同的操作系统

协议规范必须准确 Format of data units 数据单元的格式 Semantics of all fields 所有字段的语义 allowable sequence of PDUs 协议数据单元认可的序号 Service definition 服务定义
1 2 3
Syntax Data formats Signal levels
Semantics Control information Error handling
Timing Speed matching Sequencing
语法
语义
同步
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Agenda
为什么需要协议体系结构 简化的协议体系结构 OSI模型的体系结构 TCP/IP协议体系结构
指示
A primitive issued by a service provider either to: indicate that a procedure has been invoked by the peer service user on the connection and to provide the associated parameters, or notify the service user of a provider-initiated action
equipment Two standards:
OSI Reference model
Never lived up to（符合） early promises
TCP/IP protocol suite
Most widely used
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Also: IBM Systems Network Architecture