NETWORK MANAGEMENT PROTOCOL



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Introduction

Although some RS-232 connectors still cling desperately
to audio hardware products around the world,
for years Ethernet and the Internet Protocol (IP) have
been replacing older serial communication formats as
the connection of choice for monitoring and controlling
audio as well as video, networking, and industrial
equipment. Since the childhood of computer networking,
network designers envisioned a world where a
person’s audio system, video system, HVAC system,
and toaster all connect to the same network. To that
end, computer scientists developed a protocol capable
of managing any network device.


SNMPSNMP:
Overview

SNMP is the protocol that allows an SNMP manager
(the controller) to control an SNMP agent (the controlee)
by exchanging SNMP messages. An SNMP message
is a packet sent over UDP/IP to port 161. UDP/IP is
the User Datagram Protocol over IP. To learn how to
get your hardware to send your SNMP message using
UDP/IP to port 161, pick your favorite programming
language and hit the help files. To learn how to construct
an SNMP message, read on.


Basic Encoding Rules

Follow the Basic Encoding Rules when laying out the
bytes of an SNMP message. The most fundamental
rule states that each field is encoded in three parts:
Type, Length, and Data. Type specifies the data type of
the field using a single byte identifier. For a brief table
of some data types and their identifiers, see Table 1.
Length specifies the length in bytes of the following
Data section, and Data is the actual value communicated
(the number, string, OID, etc).



SNMP Message Format

The SNMP message format specifies which fields to include in the message and in what order. Ultimately, the
message is made of several layers of nested fields. At the outer-most layer, the SNMP message is a single field,
of the Sequence type. The entire message is a Sequence of three smaller fields: the SNMP Version (Integer), the
SNMP Community String (Octet String), and the SNMP PDU (GetRequest, or SetRequest).



Actual Bytes
To all the readers joining us at this point, because they
are in a hurry, or because they enjoy reading the last
chapter of their mystery novels first, welcome. Now
is the time to create the SNMP message by applying
the Basic Encoding Rules to the fields and laying out
the bytes in the correct order. As an example, Figure 3
shows an SNMP GetRequest packet for the microphoneMute
parameter on a Rane NM 1 (OID: 1.3.6.1.4.1.26
80.1.2.7.3.2.0). It is important to remember that changing
the number of bytes of any field in the SNMP message
requires changing the Length byte of all the outer
layers that enclose the edited field.