Lecture 21: Network Management #1

Subranges and "Derived Types" in ASN.1

In "pure" ASN.1, there are no constraints on the range of values that a variable can take. This is in contrast to "real world" languages such as Java and C where data items are usually associated with "reserved storage" -- eg, defining an integer typically reserves 4 bytes (32 bits) of memory in the program's data area.

Most ASN.1-based applications (eg SNMP) use derived types, which are specified using the sub-range (or sub-type) capabilities of the language^[3]. These types usually match up with the storage which the application will reserve for them. Some examples include:

 
Month ::= INTEGER (1..12) 
Day ::= INTEGER (1..31) 
Daily-temperatures ::= SEQUENCE SIZE (31) OF INTEGER 
Name ::= PrintableString (SIZE (1..20 ))

Digression on ASN.1 Syntax Conventions

In writing ASN.1 specifications, the case of the characters used conventionally indicates to a human reader the part of the ASN.1 grammar in use. Thus:

The keywords of the ASN.1 language, and macros, are written entirely in uppercase, eg OCTET STRING. Amazingly, each of these keywords can be separated by an optional space character.
A type in ASN.1 is indicated by starting its name with an uppercase letter, eg: Gauge.
for a value (an instance of a type), the word starts with a lowercase letter, eg internet. Note however, that what ASN.1 calls a value we would (in programming) call a variable^[4].

Note that in these notes, an attempt will also be made to present all ASN.1 (and other) syntactic elements in the fixed-width courier typeface. Occasional lapses from this convention may have to be forgiven by the reader... Examples:

MyType ::= TYPE
nameOfValue MyType ::= VALUE

Note also that, where appropriate, we adopt the OSI convention of referring to "octets", instead of "bytes", even though the terms have almost universally identical meaning.

^[3] These are usually given APPLICATION-specific tags, as noted earlier, although some are sufficiently important to be defined in the standard.
^[4] Reminding us of the quote from Though The Looking Glass by Lewis Carroll: "When I use a word," Humpty Dumpty said in rather a scornful tone, "it means just what I choose it to mean -- neither more nor less."...

Lecture 21: Network Management #1

The tutorial for this lecture is Tutorial #21.
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