Data Comm for Business, Inc.
|Your Complete Data Communications Solution Provider|
THE LINK TO EAST EARLOBE IS DOWN
THE LINK HAS BEEN RESTORED USING DIAL BACKUP
MR. JONES FROM THE TELEPHONE COMPANY
IS HOLDING FOR YOU ON LINE 2
The primary goal was if it's broke, what broke, who is responsible for fixing it, and how can we operate until it is fixed? A secondary goal was if it's working , how well is it working?
During that period and for many years afterward an absolute plethora of "Network Management Systems" were introduced. There was only one problem - they did not manage the network. They were designed with the primary goal of being a vehicle to accelerate the sale of a particular vendor's products, and incidentally provide a diagnostic tool for that vendor's products. Since the vendor community could see no benefit to providing an open network management capability this goal could only be achieved by pressure from the user community. This is in fact what happened.
This is simply written for the person who uttered those famous words:
One would think that the Simple Network Management Protocol would define a protocol. As it turns out the protocol is only one of three elements that make up SNMP. The elements of SNMP are the following:
SNMP relies on the concept of a central management station(s), the manager, communicating with a number of managed devices throughout the network, the agents. The communications used is UDP/IP again reflecting the simplicity required. The manager either requests information from the agent (GET), or directs the agent to change the value of a variable (SET). The result of the SET may result in an action within the node such as disconnecting a port. The basic concept is that SNMP deals with managed objects (variables) in the node which are mapped by the MIB. The MIB is a virtual database reflecting the resources of the agent, and the complete definition of each object. This definition includes the name, syntax, definition, description, and access privileges.
SNMP supports three basic operations on the MIB objects:
The SMI (Structure of Management Information) is a set of rules that defines the naming relationship of managed objects (variables). The SMI also defines generic data types such Integer, Counter, etc. The SMI organizes the managed objects into a hierarchy for identification purposes such that each managed object is absolutely identified.
For a variety of reasons, or non-reasons depending on the point of view taken, all of the elements of SNMP: the protocol, the SMI, and the MIB, are formally defined using a subset of the ISO defined language known as Abstract Syntax Notation One (ASN.1). While ASN.1 is not the simplest of concepts to deal with (a generous supply of Aspirin or Scotch will help) it does provide a tool to define data and data structures in a machine independent manner, and the Basic Encoding Rules (BER) define the transfer syntax. ASN.1 is used to define the format of the data being exchanged and to define the objects being managed. Prior to discussing the BER it is important to understand the basic encoding unit, the octet, is eight bits long. Since a byte is not necessarily a byte as data is moved from machine to machine (to say nothing of a word) it is necessary to have a formal definition of this unit. As data is passed to the network the most significant bit is bit 8 and the least significant is bit 1.
Each ASN.1 type is encoded as three fields, tag, length, and value. The simple type of tags are Integer, Octet String, and Object Identifier. Constructor types are Sequence and Sequence Of. A Sequence may be thought of as a data structure or an array of mixed type of variables, while a Sequence Of is an array of homogenous variables. The Object Identifier uniquely identifies a named object (variable, managed object). This is done by transversing the MIB starting at the most basic element until the object in question is reached. For example the PC that is a part of the PA2000 would be identified as 188.8.131.52.184.108.40.206.2 where:
The SNMP operations are packaged in protocol data units (PDU) that contain the operation (GET, SET, etc.) along with some information and authentication items. The SNMP packet is represented as a Sequence of the following items.
|Community Name||Octet String|
|PDU||A Sequence of the following|
|Tag||Defines PDU as GET, SET, etc.|
|Length||Length of the PDU|
|Request ID||Used for correlation between agent and manager|
|Error Status||Non-zero represents type of error|
|Error Index||Indicates which Varbind caused the error|
Basically there are seven generic trap definitions as follows:
|Length||Length of the PDU|
|Object Identifier||This is the sysObjectID which identifies the type agent|
|Network Address||The IP address of the agent sending the trap|
|Generic Trap Type||Defined above|
|Specific Type of Trap||This will be zero unless the Generic Trap is Enterprise Specific then it will be enterprise defined.|
|TimeTicks||Time in tens of milliseconds since the agent was last initialized - probably of great value.|
**** If the Generic trap type is not Enterprise Specific the PDU ends here***
Sequence of Varbinds termed "interesting information" and will be unique to the node with as many variables as the writer of the agent deems necessary to convey his message.
Douglas Steedman, Abstract Syntax Notation One (ASN.1): the Tutorial and
Technology Apprasials Ltd., 1990 ISBN 1-871802-06-7
RFC 1155 Structure of Management Information
RFC 1157 Simple Network Managment Protocol
RFC 1212 Concise MIB Definitions
RFC 1213 Management Information Base II