| Changing from ASN.1:1988 to ASN.1:2002 |
No Change in the bits on the wire
Please note that there is no change in the "bits on the
wire" when changing from using ASN.1:1988 as defined in CCITT Rec. X.208 and BER:1988
as defined in CCITT Rec. X.209 to using
ASN.1:2002 as defined ITU-T Rec. X.680-X.683|ISO/IEC 8824-1,2,3,4 and BER:2002 as
defined in ITU-T Rec. X.690|ISO/IEC8825-1. In fact, it is impossible for a
communicating peer to determine whether ASN.1:1988 or
ASN.1:2002 is being used by another communicating peer
since the bits on the wire do not change.
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Differences between ASN.1:1988 and ASN.1:2002
The main difference is that the multitude of defect
reports that were issued against CCITT Rec. X.208
have been corrected in ITU-T Rec. X.680 - X.683|ISO/IEC 8824-1,2,3,4.
With X.208, there were many ambiguities in
the notation that made it possible to write ASN.1
modules which, when implemented, resulted in
non-interoperability even though both peers were fully
conformant to X.208 and X.209. The most visible
manifestations of these bug fixes are the replacement of
the macro notation and ANY/ANY DEFINED BY, a change in
the CHOICE value notation, and the mandatory presence of
identifiers in the definitions of SET, SEQUENCE and
CHOICE types.
The ability to smoothly extend types in future versions
of a protocol has been added by introducing the extension marker ("..."
notation) for indicating where extensions are permitted
to occur. Several new character string types were also
introduced for better support of international languages
or alphabets, including one new type CHARACTER STRING
that allows the alphabet to be determined at runtime.
Compared to the 1997 edition, the 2002 edition of ASN.1 provides the following new features:
- an XML notation that can be used to define abstract values in an ASN.1 module;
- decimal notation (e.g., 1.5e3) for REAL values;
- a new RELATIVE-OID type for relative object identifiers;
- a new subtype constraint (CONTAINING/ENCODED BY) to constrain the content of an octet string or a bit string;
- a subtype constraint by regular expressions (PATTERN) for character string types;
- a semantic model which clearly defines the rules for type and values compatibilities;
- multi-line comments à la C or Java (i.e., between /* and */);
- the new X.692 Recommendation which provides an Encoding Control Notation (ECN) for the definition of proprietary encoding rules.
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Advantages of changing from ASN.1:1988 to ASN.1:2002
Advantages of changing from ASN.1:1988 to ASN.1:2002
include better readability, more precision, greater
flexibility and much greater ease of implementation. For
example, when macros are used in ASN.1:1988 modules, you
cannot determine the purpose of a macro without its
author describing it to you using textual comments. With
ASN.1:2002, the notation is much more precise, making
the author’s description unnecessary. Indeed, the ASN.1:2002
replacement for ANY DEFINED BY (now called an open
type) and macros such as OPERATION is so much better
that applications can be written more quickly, simply
and in significantly fewer lines of code than when
ASN.1:1988 is used.
With the ASN.1:2002, it is possible for encoders and
decoders to fully decode a message, including open types
nested in open types nested in open types (i.e., ANY
nested in ANY nested in ANY...), via a single call to
the encoder or decoder, and without any custom
modifications to the ASN.1 to accomplish this. Contrast
this to the 1988 approach which either requires the user to
repeatedly call the encoder or decoder for each level of
ANY that is to be encoded or decoded, or which requires
you to make custom modifications to the ASN.1 and
utilize special ASN.1 tools that understand such
customizations.
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How to change ASN.1:1988 to ASN.1:2002
In a nutshell, the main changes you will need to make are:
- Ensure that the components of SET, SEQUENCE and
CHOICE all have identifiers.
- Include a colon after the identifier in CHOICE
values.
- Change ANY and ANY DEFINED BY to use the more
descriptive open type notation.
- Change the macro notation to the information
object class notation (e.g., if macro OPERATION is used) or
parameterized types (e.g., if macro SIGNED is used).
Note: Annex A (Use of ASN.1:1988/1990 notation) and
Annex E (Superseded features) of the 1997
edition of X.680 were providing details on changing from ASN.1:1988 to ASN.1:1997/2002. They have not
been kept in the 2002 edition, for X.208 (ASN.1:1988) has been withdrawn at the same time when the 2002
edition was published. Those annexes are provided as a separate webpage.
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Mixing ASN.1:1988 and ASN.1:2002
Please note that you cannot mix 1988 and post-1988 ASN.1
syntax in the same ASN.1 module. You can import types
and values from ASN.1:1988 modules into 1997 and vice
versa as long they follow a few guidelines as explained
in A.2 (of X.680:1997). For example, an ASN.1:2002 module
cannot import macros, and a 1988 ASN.1 module cannot
import types or values containing new types such as
UTF8String.
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Free ASN.1:2002 syntax checkers
There are two free ASN.1 syntax checkers available for
your use in checking your ASN.1:2002 specifications.
Please see http://www.oss.com/products/checksyntax.html
and http://asn1.elibel.tm.fr/tools/asnp/.
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Downloadable books on ASN.1
There are also two books on ASN.1 that you can download
Please see http://www.oss.com/asn1/booksintro.html
(English) and http://asn1.elibel.tm.fr/book/(French and English)
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