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Tags in ASN.1
Note that in the previous slides, each element of a structured data
item was identified using a TAG.
There are four classes of tags:
- UNIVERSAL
- a universal tag is defined in ISO/IEC 8824 and gives a
machine-readable identification of all of the basic types of an
ASN.1 specification. The tags attached to the basic data types in
slides 2 & 3 of this lecture are an example.
- APPLICATION
- these tags are only required to be unique within a particular
application. An example is a tag representing a particular packet
type (PDU) sent by an application.
- CONTEXT-SPECIFIC
- these tags are only required to be unique within an already
tagged type. The tags attached to values in the
CHOICE and ENUMERATED
examples in slides 2 & 3 are an example.
- PRIVATE
- these are defined for the use of a particular organisation and
have no meaning outside of that organisation.
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