GeoDesy GD-P1000E1000 - Manual (Page 48)

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A MIB can be depicted as an abstract tree with an unnamed root. Individual data
items make up the leaves of the tree. Object identifiers (IDs) uniquely identify or
name MIB objects in the tree. Object IDs are like telephone numbers -- they are
organized hierarchically with specific digits assigned by different organizations.
The object ID structure of an SNMP MIB defines three main branches: Consultative
Committee for International Telegraph and Telephone (CCITT), International
Organization for Standardization (ISO), and joint ISO/CCITT. Much of the current
MIB activity occurs in the portion of the ISO branch defined by object identifier 1.3.6.1
and dedicated to the Internet community.
The current Internet-standard MIB, MIB -II, is defined in RFC 1213 and contains 171
objects. These objects are grouped by protocol (including TCP, IP, User Datagram
Protocol [UDP], SNMP, and others) and other categories, including "system" and
"interfaces."
SMI Definitions
The SMI specifies that all managed objects should have a name, a syntax, and an
encoding. The name is the object ID, which was discussed in the preceding section.
The syntax defines the object's data type (for example, "integer" or "string"). A subset
of ASN.1 definitions are used for the SMI syntax. The encoding describes how the
information associated with the managed object is formatted as a series of data items
for transmission on the network. Another ISO specification, called the Basic Encoding
Rules (BERs), details SMI encodings.
SMI data types are divided into three categories: simple types, application-wide
types, and simply constructed types.
Simple types include four primitive ASN.1 types:
Integers -- Unique values that are positive or negative whole numbers, including
zero.
Octet strings -- Unique values that are an ordered sequence of zero or more octets.
Object IDs -- Unique values from the set of all object identifiers allocated according to
the rules specified in ASN.1.
Bit strings -- New in SNMPv2, these comprise zero or more named bits that specify a
value.
Application-wide data types refer to special data types defined by the SMI:
Network addresses -- Represent an address from a particular protocol family.
Counters -- Non-negative integers that increment by positive one until they reach a
maximum value, when they are reset to zero. The total number of bytes received on
an interface is an example of a counter. In SNMPv1, counter size was not specified.
In SNMPv2, 32-bit and 64-bit counters are defined.