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 “Supported MIB Objects” 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 “MIB Architecture” 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 “Simple Object Identifier”.

Figure 1-2 Simple Object Identifier

Under the internet subtree there are four subtrees.

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

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:

System Group. Contains generic configuration information.
Interface Group. Contains generic information on the entities at the interface layer.
Address Translation Group (AT). Contains address resolution information.
Internet Protocol Group (IP). Contains information used to keep track of the IP layer on the managed node.
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.
Transmission Control Protocol Group (TCP). Contains information used to keep track of the application entities using TCP.
User Datagram Protocol Group (UDP). Contains information used to keep track of the application entities using UDP.
Exterior Gateway Protocol Group (EGP). Contains information about the EGP, if it is implemented.

MIB II adds the following two groups to the list.

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.
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.