ASN.1详解：ITU-T X.208标准解析

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"X.208(ASN.1).pdf" 这篇文档是关于国际电信联盟(ITU)的ITU-T X.208推荐标准，它详细阐述了抽象语法表示法一（ASN.1）。ASN.1是一种在开放系统互连（OSI）模型中用于数据表示的标准，尤其适用于网络协议和数据交换的底层实现。对于那些希望深入理解网络通信底层机制的人来说，这是一个重要的学习资源。 ASN.1的主要目标是提供一种标准化的方式来定义数据结构，使得不同系统间的数据交换变得更为简单和一致。它定义了一种抽象语法，允许开发者独立于任何特定的编程语言或机器架构来描述数据。这使得ASN.1广泛应用于各种网络协议，如TCP/IP、 Diameter、SS7等，以及在证书、消息传递和其他网络通信中。 X.208标准中包含了ASN.1的基本元素和规则，如基本数据类型（如整型、字符串、布尔值）、复合数据类型（如序列和选择）、以及编码规则。这些编码规则，如BER（Basic Encoding Rules）、DER（Distinguished Encoding Rules）和PER（Packed Encoding Rules），定义了如何将ASN.1定义的抽象数据转化为二进制形式进行传输。文档提取自所谓的"蓝皮书"的第八卷第四分册，并且尽管排版可能与原版有所不同，但内容保持不变。此版本仍然受ITU的版权保护，未经书面许可，不得以任何形式复制或利用。 ASN.1的使用不仅限于电信领域，也广泛应用于其他行业，如医疗信息交换、航空航天通信以及物联网设备间的通信。理解并掌握ASN.1对于网络工程师、系统架构师以及从事相关开发工作的人来说至关重要，因为它提供了在不同系统间进行高效、可靠数据交换的通用语言。通过学习这个文档，读者可以了解ASN.1的基本概念、语法结构，以及如何使用ASN.1来定义复杂的数据结构和协议。此外，读者还将熟悉ASN.1的编码机制，这对于解析和创建符合ASN.1规范的数据包至关重要。X.208标准是深入理解和应用ASN.1不可或缺的参考资料。

3.39 user (of ASN.1)

The individual or organization that defines the abstract syntax of a particular piece of information using

ASN.1.

3.40 subtype (of a parent type)

A type whose values are specified as a subset of the values of some other type (the parent type).

3.41 parent type (of a subtype)

Type used to define a subtype.

Note –

The parent type may itself be a subtype of some other type.

3.42 subtype specification

A notation which can be used in association with the notation for a type, to define a subtype of that type.

3.43 subtype value set

A notation forming part of a subtype specification, specifying a set of values of the parent type which are to be

included in the subtype.

3.44 This Recommendation uses the following terms defined in Recommendation X.216:

a) presentation data value; and

b) (an) abstract syntax; and

c) abstract syntax name; and

d) transfer syntax name.

3.45 This Recommendation also uses the following terms defined in ISO 6523;

a) issuing organization; and

b) organization code; and

c) International Code Designator.

3.46 This Recommendation uses the following term defined in Recommendation X.226:

a) presentation context identifier.

4 Abbreviations

ASN.1 Abstract Syntax Notation One.

UTC Coordinated Universal Time.

ICD International Code Designator.

DCC Data Country Code

DNIC Data Network Identification Code.

5 Notation used in this Recommendation

The ASN.1 notation consists of a sequence of characters from the ASN.1 character set specified in § 7.

Each use of the ASN.1 notation contains characters from the ASN.1 character set grouped into items. Clause 8

specifies all the sequences of characters forming ASN.1 items, and names each item.

The ASN.1 notation is specified in § 9 (and following clauses) by specifying the collection of sequences of

items which form valid instances of the ASN.1 notation, and by specifying the semantics of each sequence.

Fascicle VIII.4 - Rec. X.208 11

In order to specify these collections, this Recommendation uses a formal notation defined in the following sub-

clauses.

5.1 Productions

A new (more complex) collection of ASN.1 sequences is defined by means of a production. This uses the

names of collections of sequences defined in this Recommendation and forms a new collection of sequences by

specifying either

a) that the new collection of sequences is to consist of any of the original collections; or

b) that the new collection is to consist of any sequence which can be generated by taking exactly one

sequence from each collection, and juxtaposing them in a specified order.

Each production consists of the following parts, on one or several lines, in order:

a) a name for the new collection of sequences;

b) the characters

::=

c) one or more alternative collections of sequences, defined as in § 5.2, separated by the character

A sequence is present in the new collection if it is present in one or more of the alternative collections. The

new collection is referenced in this Recommendation by the name in a) above.

Note –

If the same sequence appears in more than one alternative, any semantic ambiguity in the resulting

notation is resolved by other parts of the complete ASN.1 sequence.

5.2 The alternative collections

Each of the alternative collections of sequences in “

ne or more alternative collections of”

is specified by a list

of names. Each name is either the name of an item, or is the name of a collection of sequences defined by a production in

this Recommendation.

The collection of sequences defined by the alternative consists of all sequences obtained by taking any one of

the sequences (or the item) associated with the first name, in combination with (and followed by) any one of the

sequences (or item) associated with the second name, in combination with (and followed by) any one of the sequences

(or item) associated with the third name, and so on up to and including the last name (or item) in the alternative.

5.3 Example of a production

BitStringValue ::=

bstring |

hstring |

{IdentifierList}

is a production which associates with the name BitStringValue the following sequences:

a) any bstring (an item); and

b) any hstring (an item); and

c) any sequence associated with IdentifierList, preceded by a { and followed by a }

Note –

{ and } are the names of items containing the single characters { and } (see § 8).

In this example, IdentifierList would be defined by a further production, either before or after production

defining BitStringValue.

5.4 Layout

Each production used in this Recommendation is preceded and followed by an empty line. Empty lines do not

appear within productions. The production may be on a single line, or may be spread over several lines. Layout is not

significant.

12 Fascicle VIII.4 - Rec. X.208

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