HP 3000 Manuals

HP SNMP/XL User's Guide

Concepts and Definitions 

Following is information on some concepts related to SNMP.

Community Name 

Requests for information on an agent are accompanied by a community name.
A community name is similar to a password preventing unauthorized network
managers from accessing agent information (MIB values).  The community
name sent with a request must match the community name expected by the
target SNMP node (agent) and is case-sensitive.  Some SNMP nodes support
multiple levels of access based on the community name (e.g.  read-only
versus read-write).  Most nodes support the community name "public,"
generally considered the default.

Traps 

An agent can send information to the manager without a request from the
manager.  Such an operation is called a trap (or an event).  Traps inform
the manager of changes that occur on the agent system, such as a reboot,
without being asked.  The agent knows which manager system to send traps
to via its trap destination.  A trap destination identifies a manager
system that is to receive the agent's traps.

Protocol Data Units (PDUs) 

The Protocol Data Unit (PDU) is the format of data exchanged between the
SNMP manager and agent.  This data object is exchanged by peers and
contains protocol control information and user data.

SMI 

The Structure of Management Information (SMI) is a document where the
SNMP network management information structure and language for organizing
that information is defined.

MIB 

The information on the agent that the manager requests resides in the
Management Information Base (MIB). The MIB is a virtual database of
managed objects contained within the SNMP agent.  The MIB is not a
physically distinct database, but rather it is a concept that includes
configuration and status values normally available on the agent system.

The MIB defines the set of objects to be managed by SNMP and defines the
syntax by which the data is represented.  It allows you to get statistics
and tabular data relevant to an internet protocol layer such as TCP. It
also allows you to reset some of the internet protocol values.  The MIB
actually outlines the set of objects (statistics and tables) that are
meaningful for each internet protocol it covers.

The MIB stores information needed to manage devices on a network.  It
contains a list of network objects and their attributes, such as the
number of packets sent to a network interface, routing table entries, and
protocol-specific variables for IP routing.  MIB I includes objects
dealing with IP internetworking routing variables.  MIB II, now an
Internet standard, adds new objects to the MIB I groups and also adds two
new groups.  The new groups add media devices and network devices to the
SNMP capabilities.  See appendix B for a list of supported MIB objects.

The MIB conforms to the encoding rules determined by the American
National Standards Institute.  These rules are called Abstract Syntax
Notation One (ASN.1).  ASN.1 is used:

   *   To define the formats of the PDUs (Protocol Data Unit) exchanged
       by the management protocol.

   *   As a means of defining the managed objects.

These objects (Object Types) are given an object identifier (name),
syntax, definition, and access information.

Object Identifier (Name).   

Each object type has a unique name which is called an Object Identifier.
An Object Identifier is a sequence of integers that lead you to a certain
node in the MIB architecture.  Object Identifiers are organized in a
hierarchical tree-like structure.  Figure 1-1 shows the MIB architecture
with some of the defined Object Identifiers.


          Figure 1-1.  MIB Architecture 

For example the sysDescr Object Identifier is represented as
1.3.6.1.2.1.1.1.0.  (The 0 added at the end is an instance
sub-identifier.  This means the one and only instance of sysDescr.)  This
Object Identifier structure is shown in figure 1-2.


          Figure 1-2.  Sample Object Identifier 

Under the internet subtree there are four subtrees.

   1.  The directory subtree(1) is reserved for the OSI directory in the
       Internet.

   2.  The mgmt subtree(2) is used to identify objects which are defined
       in documents approved by the Internet Activities Board (IAB). At
       present, this includes only one subtree, mib.

   3.  The experimental subtree(3) is used to identify objects used in
       Internet experiments.  If an experiment is successful, it will be
       moved over into the Internet-standard MIB.

   4.  The private subtree(4) is used to identify objects that are
       defined privately and not standardized.  At present, this includes
       only one subtree, enterprises, where vendor-specific objects are
       registered.

Syntax.   

The syntax of an object defines the abstract data structure for that
object type.  It determines how a value is displayed according to ASN.1.

Definition.   

The definition describes the meaning of the object type.

Access Infomation.   

Once SNMP acknowledges the SNMP entity as a member of a community, the
managed node determines the level of access that is allowed.  Access
modes can be:  Read-only, Read-Write, Write-only, or Not accessible.

MIB Groups 

The MIB is divided into groups allowing the SNMP manager to poll the SNMP
agents for information.  MIB I divides the objects into the following
eight groups:

   1.  System Group.  Contains generic configuration information.

   2.  Interface Group.  Contains generic information on the entities at
       the interface layer.

   3.  Address Translation Group (AT). Contains address resolution
       information.

   4.  Internet Protocol Group (IP). Contains information used to keep
       track of the IP layer on the managed node.

   5.  Internet Control Message Protocol Group (ICMP). Contains 26
       counters, counting how many times this message type was generated
       by the local IP entity and how many times this message type was
       received by the local IP entity.  It also counts the total number
       of ICMP messages received, sent, received in error, or not sent
       due to error.

   6.  Transmission Control Protocol Group (TCP). Contains information
       used to keep track of the application entities using TCP.

   7.  User Datagram Protocol Group (UDP). Contains information used to
       keep track of the application entities using UDP.

   8.  Exterior Gateway Protocol Group (EGP). Contains information about
       the EGP, if it is implemented.

MIB II adds the following two groups to the list.

   1.  Transmission Group.  Holds a place for media-specific MIBS. They
       start out in the experimental branch and may eventually be placed
       in the Internet-standard MIB.

   2.  SNMP Group.  Contains information used to keep track of SNMP
       application entities.  It provides statistical information about
       the SNMP protocol entity and tracks the amount of management
       traffic that a device responds to.

MPE/iX 5.0 Documentation