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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
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
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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
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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
equipment Two standards:
OSI Reference model
Never lived up to（符合） early promises
TCP/IP protocol suite
Most widely used
Also: IBM Systems Network Architecture
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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
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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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
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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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
Protocol specification must be precise Functional description of what is providrenced by SAPs
寻址
由服务访问点引用
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Service Primitives （原语） and Parameters
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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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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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
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
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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
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
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
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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
Destination SAP Sequence number Error detection code