Application tag in ASN.1 Declaration

asn.1, mib, snmp

Solution

Note: This question is tagged SNMP, but this is not the proper declaration for SNMP's IpAddress type. This is actually a Counter32 type:

IpAddress ::= [APPLICATION 0] IMPLICIT OCTET STRING (SIZE (4))

Counter32 ::= [APPLICATION 1] IMPLICIT INTEGER (0..4294967295)

RFC 2578

However;

An ASN.1 type has both a notation and an on-the-wire encoding. New types can be defined based on built-in types. In your question, a new type "IpAddress" is being defined as a finite-ranged (0..4294967295) derivation of the built-in "INTEGER" type, which has no built-in range restriction.

An ASN.1 encoding consists of three parts: a one-byte tag indicating the data type, a length for the content, and the content (value) itself. Without the part in [brackets], the encoding of the new type would be identical to the base type--in this case INTEGER, which has a tag value of 0x02 (hex). [APPLICATION 1] indicates that the value is to be encoded with a different tag so that you can distinguish its encoding from an INTEGER.

This part in [brackets] consists of an Tag Class and a Number. The Class can be UNIVERSAL (only used for built-in ASN.1 types), APPLICATION (which means application specific, i.e. [APPLICATION 1] to SNMP may mean something entirely different to [APPLICATION 1] in another protocol), PRIVATE for enterprise-specific, or may be omitted (in which case it is context-specific).

The Tag Class and Number are essentially bit flags that are or'ed together to form the tag value in the encoding. APPLICATION = 0x40, so [APPLICATION 1] means the value defined as in your question is encoded with a tag byte of 0x41.

However, as I mentioned above, this would actually be taken by SNMP to be a Counter32 value, not an IpAddress. 0x40 would be an IpAddress, and the contents would be encoded as an OCTET STRING rather than an INTEGER.

Problem

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