Q252648: XGEN: A Brief Introduction to ASN.1 and BER
Article: Q252648
Product(s): Windows for Workgroups and Windows NT Networking Issues
Version(s): ; winnt:5.5
Operating System(s):
Keyword(s): exc55
Last Modified: 29-OCT-2000
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The information in this article applies to:
- Microsoft Windows 2000 Professional
- Microsoft Windows 2000 Server
- Microsoft Windows 2000 Advanced Server
- Microsoft Exchange Server, version 5.5
- Microsoft Exchange 2000 Server
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SUMMARY
=======
This article describes Abstract Syntax Notation One (ASN.1) and Basic Encoding
Rules (BER), and clarifies the differences between the two.
Many information technology workers, including seasoned professionals, are often
confused about what ASN.1 and BER are, the differences between the two, or even
why the distinction is important. Their confusion is understandable when even
those people who know ASN.1 and BER well frequently use the terms incorrectly.
Because ASN.1 is the language of standards, it is very common to find standards
recommendations written in ASN.1. Support professionals, consultants, and
information technology specialists alike benefit from knowing ASN.1 because it
allows them to dig deep into the standards and to decode and understand the
content of the data that they troubleshoot. The most common standards written in
ASN.1 are those produced by the International Telegraph and Telephone
Consultative Committee (CCITT) and the International Telecommunications Union
(ITU). However, members of the Internet Engineering Task Force (IETF) are
increasingly using ASN.1 to succinctly state their recommendations. ASN.1 says
in one page what otherwise takes five or six pages to write out in some other
format.
The following is a list of commonly used standards that are written in ASN.1:
- X.400 (Electronic Messaging)
- X.500 (Directory Services)
- X.200 (Network communications)
- Request for Comments (RFCs) 2251-2256 (Lightweight Directory Access Protocol,
or LDAP)
- Too many other RFCs to mention
Any computer or data communications specialists who wishes to really set
themselves apart need to have at least a basic understanding of ASN.1 and BER.
Because both Microsoft Windows 2000 and Microsoft Exchange 2000 Server implement
the above-mentioned standards in one form or another, fluency in reading and
understanding ASN.1 and BER makes troubleshooting those two products that much
easier.
MORE INFORMATION
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ASN.1
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Abstract Syntax Notation One (more commonly known as ASN.1) is a language for
defining standards without regard to the implementation. It is the language of
standards writers. When John Smith at CalTech wants to write a recommendation
for standardizing the procedures that one component follows for talking to
another component, he writes the recommendation in ASN.1 notation, and submits
the recommendation to a standards body such as the ITU. ASN.1 facilitates
communication between professionals and committee members by offering a common
language for describing a standard. ASN.1 is defined in ITU-T Recommendations
X.209 and X.690.
For example, ASN.1 defines:
- What a "type" is.
- What a "module" is and how it should look.
- What an INTEGER is.
- What a BOOLEAN is.
- What a "structured type" is.
- What certain keywords mean (for example, BEGIN, END, IMPORT, EXPORT,
EXTERNAL, and so on).
- How to "tag" a type so that it can be properly encoded.
ASN.1 has no regard to any specific standard, encoding method, programming
language, or hardware platform. It is simply a language for defining standards.
Or in other words, standards are written in ASN.1.
BER
---
BER (pronounced "burr") is the common name for the Basic Encoding Rules of ASN.1.
BER is defined in ITU-T Recommendations X.209 and X.690. BER is one set of rules
for encoding ASN.1 data to a stream of octets that can be transmitted over a
communications link. Other methods of encoding ASN.1 data include Distinguished
Encoding Rules (DER), Canonical Encoding Rules (CER), and Packing Encoding Rules
(PER). Each encoding method has its application, but BER tends to be the
encoding method most commonly used and most commonly talked about.
BER defines:
- Methods for encoding ASN.1 values.
- Rules for deciding when to use a given method.
- The format of specific octets in the data.
Contrasting ASN.1 and BER
-------------------------
ASN.1 is like a programming language (such as C), whereas BER is like a compiler
for that language. Compilers are platform-specific, whereas many high-level
programming languages are not. C defines the rules and language for writing a
program. A program is not C; it is written in C. The program is not useful until
it is compiled for a specific platform (such as Intel x86). So it is with ASN.1
and BER. ASN.1 is the language for writing a standard. A standard is not ASN.1;
it is written in ASN.1. Data which is generated from a program that complies
with the standard may loosely be termed "ASN.1 data." ASN.1 data is not useful
(that is, it cannot be transmitted across a LAN) until the ASN.1 data is encoded
into a stream of octets which can be easily decoded at the destination. To
illustrate, the following is an example of these concepts derived from ITU-T
Recommendation X.209, Appendix I. This is an informal description of a personnel
data record:
Name: John P Smith
Date of Birth: 17 July 1959
(other data)
The ASN.1 description of a personnel record (the standard) might be:
PersonnelRecord ::= [APPLICATION 0] IMPLICIT SET {
Name,
title [0] VisibleString,
dateOfBirth [1] Date,
(other types defined) }
Name ::= [APPLICATION 1] IMPLICIT SEQUENCE {
givenName VisibleString,
initial VisibleString,
familyName VisibleString }
This ASN.1 description describes what a PersonnelRecord looks like. But more
importantly, it describes how application data should be formatted so that it
becomes ASN.1 data before encoding takes place.
Next, the application maps the personnel data into the personnel record structure
(ASN.1 data format), and then applies the Basic Encoding Rules (BER) to the
ASN.1 data. This is what it might look like (with the exception that the names
would be converted to ASCII):
Personnel
Record Length Contents
60 8185
Name Length Contents
61 10
VisibleString Length Contents
1A 04 "John"
VisibleString Length Contents
1A 01 "P"
VisibleString Length Contents
1A 05 "Smith"
DateofBirth Length Contents
A0 0A
Date Length Contents
43 08 "19590717"
When all is said and done, what actually gets transmitted (or more specifically,
what becomes the data portion for the packet at the next layer down) is:
60 81 85 61 10 1A 04 ....
.... 0A 43 08 19 59 07 17
REFERENCES
==========
For more information on ASN.1, consult the recommendation itself (ITU-T
Recommendation X.208 or X.690).
For more information on BER, consult the recommendation itself (ITU-T
Recommendation X.209 or X.691).
Additional query words:
======================================================================
Keywords : exc55
Technology : kbwin2000AdvServ kbwin2000AdvServSearch kbwin2000Serv kbwin2000ServSearch kbwin2000Search kbwin2000ProSearch kbwin2000Pro kbExchangeSearch kbExchange550 kbZNotKeyword2 kbExchange2000Search kbWinAdvServSearch kbExchange2000Serv
Version : :; winnt:5.5
Issue type : kbinfo
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