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functional perspective, BER employs a least to most significant bit ordering scheme
(little-endian), using 8-bit octets. ASN.1 data types are encoded using three
variable length fields. The first field, defined as the tag field, indicates the ASN.1
data type. The second field, the length field, indicates how many octets make up the
actual ASN.1 data type. The third field is the value field, and it consists of x amount
of octets that make up the actual ASN.1 data.
ASN.1
ASN.1 is a platform and implementation-independent data description language
developed by the ISO for unambiguous data representation. ASN.1 presents data in
an abstract form using a highly structured syntax that only describes the ASN.1 data
types and the assignment of values to those data types. ASN.1 is not interested in
how the data will be processed, only with its correct presentation. ASN.1 is a very
complex language that offers a broad set of data type definitions. This makes it an
ideal tool for representing data on any system type. The SNMP SMI defines only a
small group of ASN.1 data types for use with SNMP. ASN.1 uses the following basic
structure to define data types and values:
data type value name | data type identifier ::= data value or { data type
identifier
(data value)}
Data type name labels can be specifically defined through the use of a unique data
identifier that appears in initial lowercase, uppercase form (for example,
sysContact). The data type identifier is either a named data type expressed in title
form (such as MacAddress) or as an ASN.1-defined data type, expressed as an
ASN.1 keyword in uppercase form (for example, INTEGER). Data values are
expressed as alphanumeric characters in all lowercase form (for example,
linkstatus 0).
NOTE
ASN.1 is a programming language, and a rather complicated one at that. Its
complete syntax, grammar, and implementation are far beyond the scope of this
book. Unless you are planning to develop your own MIBs, an intimate knowledge of
ASN.1 is not required. Table 11.3 lists some of the more commonly used ASN.1
keywords used with SNMP MIB's. The goal in this section is to provide you with a
basic understanding of how SNMP messages are constructed. Additional references
on SNMP are listed at the end of the chapter.