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<header>
<copyright>
<year>1997</year><year>2018</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
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Unless required by applicable law or agreed to in writing, software
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<title>Introduction</title>
<prepared>Martin Björklund</prepared>
<docno></docno>
<date>1997-11-01</date>
<rev>A</rev>
<file>oam_intro.xml</file>
</header>
<marker id="oam principles"></marker>
<p>The Operation and Maintenance (OAM) support in OTP consists of a
generic model for management subsystems in OTP, and some components
to be used in these subsystems. This section describes the model.</p>
<p>The main idea in the model is that it is not tied to any specific
management protocol. An Application Programming Interface (API) is
defined, which can be used to write adaptations for specific
management protocols.</p>
<p>Each OAM component in OTP is implemented as one sub-application, which
can be included in a management application for the system. Notice that
such a complete management application is not in the scope of this
generic functionality. However, this section includes examples
illustrating how such an application can be built.</p>
<section>
<title>Terminology</title>
<p>The protocol-independent architectural model on the network level
is the well-known client-server model for management operations. This
model is based on the client-server principle, where the manager
(client) sends a request from a manager to an agent (server) when it
accesses management information. The agent sends a reply back to the
manager. There are two main differences to the normal
client-server model:</p>
<list type="bulleted">
<item><p>Usually a few managers communicate with many agents.</p></item>
<item><p>The agent can spontaneously send a notification, for example,
an alarm, to the manager.</p></item>
</list>
<p>The following picture illustrates the idea:</p>
<image file="../oam/terminology.gif">
<icaption>Terminology</icaption>
</image>
<p>The manager is often referred to as the <em>Network Management
System (NMS)</em>, to emphasize that it usually is realized as a
program that presents data to an operator.</p>
<p>The agent is an entity that executes within a <em>Network
Element (NE)</em>. In OTP, the NE can be a distributed system,
meaning that the distributed system is managed as one entity.
Of course, the agent can be configured to be able to run on one
of several nodes, making it a distributed OTP application.</p>
<p>The management information is defined in a <em>Management
Information Base (MIB)</em>. It is a formal definition of which
information the agent makes available to the manager. The
manager accesses the MIB through a management protocol, such
as SNMP, CMIP, HTTP, or CORBA. Each protocol has its own MIB
definition language. In SNMP, it is a subset of ASN.1, in CMIP
it is GDMO, in HTTP it is implicit, and using CORBA, it is IDL.</p>
<p>Usually, the entities defined in the MIB are
called <em>Managed Objects (MOs)</em>, although they do not
have to be objects in the object-oriented way. For example,
a simple scalar variable defined in a MIB is called an MO. The
MOs are logical objects, not necessarily with a one-to-one
mapping to the resources.</p>
</section>
<section>
<title>Model</title>
<p>This section presents the generic protocol-independent model
for use within an OTP-based NE. This model is used by
all OAM components and can be used by the applications. The
advantage of the model is that it clearly separates the
resources from the management protocol. The resources do not
need to be aware of which management protocol is used to manage
the system. The same resources can therefore be managed with
different protocols.</p>
<p>The entities involved in this model are the agent, which
terminates the management protocol, and the resources, which
is to be managed, that is, the actual application entities.
The resources should in general have no knowledge of the
management protocol used, and the agent should have no
knowledge of the managed resources. This implies that a
translation mechanism is needed, to translate the management
operations to operations on the resources. This translation
mechanism is usually called <em>instrumentation</em> and the
function that implements it is called <em>instrumentation
function</em>. The instrumentation functions are written for
each combination of management protocol and resource to be
managed. For example, if an application is to be managed by
SNMP and HTTP, two sets of instrumentation functions are
defined; one that maps SNMP requests to the resources, and
one that, for example, generates an HTML page for some
resources.</p>
<p>When a manager makes a request to the agent, the following
illustrates the situation:</p>
<image file="../oam/snmp_model_1.gif">
<icaption>Request to An Agent by a Manager</icaption>
</image>
<p>The mapping between an instrumentation function and a
resource is not necessarily 1-1. It is also possible to write
one instrumentation function for each resource, and use that
function from different protocols.</p>
<p>The agent receives a request and maps it to calls to one or
more instrumentation functions. These functions perform
operations on the resources to implement the semantics
associated with the MO.</p>
<p>For example, a system that is managed with SNMP and HTTP
can be structured as follows:</p>
<image file="../oam/snmp_model_2.gif">
<icaption>Structure of a System Managed with SNMP and HTTP</icaption>
</image>
<p>The resources can send notifications to the manager as well.
Examples of notifications are events and alarms. The resource
needs to generate protocol-independent notifications.
The following picture illustrates how this is achieved:</p>
<image file="../oam/snmp_model_3.gif">
<icaption>Notification Handling</icaption>
</image>
<p>The main idea is that the resource sends the notifications as
Erlang terms to a dedicated <c>gen_event</c> process. Into this
process, handlers for the different management protocols are
installed. When an event is received by this process, it is
forwarded to each installed handler. The handlers are
responsible for translating the event into a notification to be
sent over the management protocol. For example, a handler for
SNMP translates each event into an SNMP trap.</p>
</section>
<section>
<title>SNMP-Based OAM</title>
<p>For all OAM components, SNMP adaptations are provided. Other
adaptations might be defined in the future.</p>
<p>The OAM components, and some other OTP applications, define
SNMP MIBs. These MIBs are written in SNMPv2 SMI syntax, as
defined in RFC 1902. For convenience we also deliver the SNMPv1
SMI equivalent. All MIBs are designed to be v1/v2 compatible,
that is, the v2 MIBs do not use any construct not available in
v1.</p>
<section>
<title>MIB Structure</title>
<p>The top-level OTP MIB is called <c>OTP-REG</c> and it is
included in the SNMP application. All other OTP MIBs
import some objects from this MIB.</p>
<p>Each MIB is contained in one application. The MIB text
files are stored under <c><![CDATA[mibs/<MIB>.mib]]></c> in
the application directory. The generated <c>.hrl</c> files
with constant declarations are stored under
<c><![CDATA[include/<MIB>.hrl]]></c>, and the compiled MIBs
are stored under <c><![CDATA[priv/mibs/<MIB>.bin]]></c>. </p>
<p>An application that needs to import an MIB into another
MIB is to use the <c>il</c> option to the SNMP MIB compiler:</p>
<code type="none">
snmp:c("MY-MIB", [{il, ["snmp/priv/mibs"]}]).</code>
<p>If the application needs to include a generated
<c>.hrl</c> file, it is to use the <c>-include_lib</c>
directive to the Erlang compiler:</p>
<code type="none">
-module(my_mib).
-include_lib("snmp/include/OTP-REG.hrl").</code>
<p>Here is a list of some of the MIBs defined in the OTP system:</p>
<list type="bulleted">
<item><p><c>OTP-REG</c> (in SNMP) contains the top-level
OTP registration objects, used by all other MIBs.</p></item>
<item><p><c>OTP-TC</c> (in SNMP) contains the general
Textual Conventions, which can be used by any other MIB.</p></item>
<item><p><c>OTP-SNMPEA-MIB</c> (in <c>snmp</c>)
contains objects for instrumentation and control of the extensible
SNMP agent itself. The agent also implements the standard SNMPv2-MIB
(or v1 part of MIB-II, if SNMPv1 is used).</p></item>
</list>
<p>The different applications use different strategies for
loading the MIBs into the agent. Some MIB implementations are
code-only, while others need a server. One way, used by the
code-only MIB implementations, is for the user to call a
function such as
<c>snmpa:unload_mibs(Agent, [Mib])</c>
to load the MIB, and
<c>snmpa:unload_mibs(Agent, [Mib])</c>
to unload the MIB. See the manual page for each application for
a description of how to load each MIB.</p>
</section>
</section>
</chapter>
