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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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