ASN.1全面指南：协议规范与编码解析

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"ASN1_Complete" 本书《ASN.1 Complete》由John Larmouth教授撰写，专注于ASN.1（抽象语法标记一号）的全面介绍。ASN.1是一种用于定义数据结构的标准，常用于通信协议的规范，特别是电信和网络领域。这本书可能是为那些与作者的孩子在Withington Girls' School和The Manchester Grammar School一起学习的人准备的，希望对他们有所帮助。书中内容分为多个部分，首先介绍了全球通信基础设施，然后阐述了ASN.1的本质。在发展过程中，ASN.1扮演着关键角色，它允许不同系统间的数据交换具有明确的结构和语义。ASN.1的文本结构也进行了详述，帮助读者理解其组织方式。在“ASN.1概述”这一章节中，作者探讨了协议的定义，指出协议是规定不同系统间通信规则的一组指令。协议的规格化涉及一些基本概念，如分层和协议“洞”（即不同层次之间的接口）。早期的分层模型被讨论，同时指出了过分依赖分层架构的缺点，强调应保持设计简单。协议的可扩展性是另一个关键点，使得协议能够适应未来的需求。 asn.1中的抽象和传输语法是核心概念，它们分离了协议数据的逻辑表示（抽象语法）和实际传输时的编码表示（传输语法）。书中有对比了命令行和基于语句的方法，并引入了接口定义语言（IDL）的概念，IDL用于定义不同系统间的接口，这在分布式系统和网络通信中至关重要。更深入地，书中讨论了抽象值和类型，以及如何编码这些抽象值。这部分帮助读者理解如何将抽象的协议数据转化为实际的二进制编码，以便在网络中传输。最后，书中对这些概念进行了评估讨论，指出在选择表示和编码方法时有许多不同的路径和考虑因素。《ASN.1 Complete》是一本深入浅出的教程，旨在帮助读者理解和应用ASN.1，这对于理解网络通信协议和开发相关软件至关重要。无论是初学者还是经验丰富的专业人员，都能从中获益。

16 © OSS,31 May 1999

• Rapid and precise specification of computer exchanges by a standardization body.

• Easy and bug-free implementation of the resulting standard by those producing products to

support the application.

In a number of industrial sectors, but particularly in the telecommunications sector, in security-

related exchanges, and in multimedia exchanges, ASN.1 is the dominant means of specifying

application protocols. (The only other major contender is the character-based approach often used

by IETF, but which is less suitable for complex structures, and which usually produces a much

less compact set of encodings). A description of some of the applications where ASN.1 has been

used as the specification language is given in Chapter of Section IV.

2 What exactly is ASN.1?

The term "TCP/IP" can be used to describe two protocol specifications (Transmission Control

Protocol - TCP, and Internet Protocol - IP), or more broadly to describe the complete set of

protocols and supporting software that are based around TCP/IP. Similarly, the term "ASN.1" can

be used narrowly to describe a notation or language called "Abstract Syntax Notation One", or can

be used more broadly to describe the notation, the associated encoding rules, and the software tools

that assist in its use.

The things that make ASN.1 important, and unique, are:

• It is an internationally-standardised, vendor-independent, platform-independent and

language-independent notation for specifying data-structures at a high level of abstraction.

(The notation is described in Sections I and II).

• It is supported by rules which determine the precise bit-patterns (again platform-

independent and language-independent) to represent values of these data-structures when

they have to be transferred over a computer network, using encodings that are not

unnecessarily verbose. (The encoding rules are described in Section III).

• It is supported by tools available for most platforms and several programming languages

that map the ASN.1 notation into data-structure definitions in a computer programming

language of choice, and which support the automatic conversion between values of those

data-structures in memory and the defined bit-patterns for transfer over a communications

line. (The tools are described in Chapter 6 of Section I).

There are a number of other subtle features of ASN.1 that are important and are discussed later in

this text. Some of these are:

• It addresses the problem of, and provides support for, interworking between deployed

"version 1" systems and "version 2" systems that are designed and deployed many years

apart. (This is called "extensibility").

• It provides mechanisms to enable partial or generic specification by one standards group,

with other standards groups developing (perhaps in very different ways) specific

specifications.

18 © OSS,31 May 1999

• It recognises the potential for interworking problems between large systems capable of

handling long strings, large integer values, large iterative structures, and small systems

that may have a lesser capability.

• It provides a range of data-structures which is generally much richer than that of normal

programming languages, such as the size of integers, naming structures, and character

string types. This enables precision in the specification of the range of values that need to

be transferred, and hence production of more optimal encodings.

3 The development process with ASN.1

The flow diagram illustrates the development process from inception to deployment of initial

systems.

(But it must be remembered that this process is frequently an iterative one, with both early

revisions by the standardization group to "get it right" and with more substantial revisions some

years later when a "version 2" standard is produced.)

Some key points to note from the diagram:

• The decision to employ ASN.1 as the notation for defining a standard is a key one. It

requires a good understanding of the ASN.1 notation by the standardization group, but

provides a rich set of facilities for a clear specification. Alternative means of protocol

specification are discussed in Chapter 1 of Section I.

• There is no need for the standardization group (or implementors) to be concerned with the

detailed bit-patterns to be used to communicate the desired semantics: details of encoding

are "hidden" in the ASN.1 encoding rule specifications and in the run-time support

provided by the ASN.1 tools.

• The implementation task is a simple one: the only code that needs to be written (and

debugged and tested) is the code to perform the semantic actions required of the

application. There is no need to write and debug complex parsing or encoding code.

4 Structure of the text.

Section I covers the most commonly encountered features of the ASN.1 notation. It also briefly

introduces all other aspects of the notation, with full coverage in Section II. It is intended that

those who are not primarily responsible for writing specifications using ASN.1 or for coding

implementations, but who need a basic understanding to assist in or to manage development (of

standards or implementations), will obtain all that they need from Section I. Those with primary

responsibility for writing or coding will need Section II also.

Section III describes the principles behind the ASN.1 encoding rules, and much of the detail.

However, this text is really only for the curious! There is no need for standards' writers or coders

to know about these encodings (provided that a tool is used for the implementation).

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