Information Object Class (ASN.1)

ASN.1 Information Object Class is a concept widely used in ASN.1 specifications to address issues related to protocol specification similar to issues addressed by CORBA/IDL specifications.

Information Object Classes are used for example to specify ROSE (Remote Operations Service Element) protocol in ASN.1.

Abbreviations

Abbreviations used throughout this article:

ASN.1 - Abstract Syntax Notation OneIOC - Information Object ClassIOS - Information Object SetIO - Information ObjectSQL - Structured Query LanguagePER - Packed Encoding RulesBER - Basic Encoding RulesIDL - Interface Definition LanguageCORBA - Common Object Request Broker ArchitectureIIOP - Internet Inter-ORB Protocol

Introduction

The simplest way of thinking of ASN.1 Information Object Classes is to regard them as a way to represent IDL specification in ASN.1 using concepts derived from the relational databases theory and SQL syntax in particular.

The concepts used in ASN.1 are more flexible than the ones used in IDL, because, continuing the analogy, they allow to "customize grammar" of the "IDL specification". ASN.1 encoding rules are used as a transfer syntax for remote invocations that resemble CORBA/IIOP.

In the light of this comparison, we can draw an approximate analogy between concepts used in Information Object Classes and SQL and IDL concepts as shown in Table 1.

Analogy by example

Table 2 illustrates by example correspondence of ASN.1 concepts to similar constructs found in SQL and IDL.

Parameterization

If you carefully examine the ASN.1 example presented in Table 2 and compare it to IDL concepts, you will see one important limitation on the ASN.1 side.

Our example ASN.1 data types which we agreed to compare to a high-level CORBA/IDL transfer syntax specification are limited to definition of such transfer syntax only for a single instance of what we compared to an IDL interface (Information Object Set in ASN.1 terms).

In other words, such transfer syntax is not generic and it is not reusable.

With the current set of known tools you can't define such a transfer syntax in a generic way in, say, ASN.1 specification A and then reuse it in ASN.1 specifications B and C that define concrete application-specific "IDL interfaces" on which A does not depend.

The reason for the current limitation is that we currently hard-code our Information Object Set (MyWarehouseOps in case of OPERATION, or MyErrorSet in case of ERROR) into our ASN.1 data types (high-level transfer syntax specification).

Now we need to make one last step to have a complete and fully-functioning system. We need to introduce a concept of type parameterization using Information Object Set as a type formal parameter.

Here is our Request type rewritten with the concept of parameterization in mind:

Request {OPERATION : OpSet} ::= SEQUENCE{ invokeId INTEGER,">
 opcode OPERATION.&operationCode ({OpSet}),
 req-pars OPERATION.&InvocationParsType ({OpSet} {@opcode})}