SNMP Terminology

This document details some of the terminology that might be new to those who are not familiar with SNMP.

It's not very comprehensive right now, but we hope to flush it out real soon now.

Basic SNMP Terminology

• SNMP : Simple Network Management Protocol.

  The SNMP specificatins define two things: a protocol and a data definition language.
  The SNMP protocol defines a few simple operations: GET, GET-NEXT, SET and TRAP. Versions 2c and 3 define a few more: GET-BULK and INFORM. These are the requests that are sent between clients and servers.
  The data definition language is the Structure of Management Information, or SMI. It is based on ASN.1, and is used to create text documents that define Managed Information Bases, or MIBs. A MIB defines a set of data objects and the format for those objects. Each data object is identified by a unique Object Identifier, or OID.

• OID : An Object Identifier

  An OID is a series of integers that uniquely identify a data object. OIDs can be represented in two ways. The first is using raw numbers, like so: .1.3.6.1.2.1.1.5.0

  While the raw numbers are easy for a computer to understand, it is a little harder for us humans. A MIB can be used to convert the number to the names used in the MIB. The above OID translates to: .iso.org.dod.internet.mgmt.mib-2.system.sysName.0
  

• PDU : A Protocol Data Unit

  A PDU is simply a data packet. The SNMP specifications define the format for each supported PDU type. SNMP PDUs contain various header fields (which depend on the protocol version being used and the PDU type), and variable bindings (varbinds).

• varbinds : Variable bindings

  A Variable binding, or varbind, is an OID for an object, and the data value associated with that object.

• GET request

  A GET request PDU ...

• GETNEXT request

  ...

• SET request

  ...

• Notifications

  A notification is an unsolicted message from a SNMP agent or device that is sent to a notification receiver (aka Network Managment System (NMS) or Element Management System (EMS)). Notifications generally indicate a status change on the device. It could be information about the change, or a message to indicate an alarm condition. SNMP defines two types of notifications: traps and informs.

• Traps

  A SNMP trap is an unsolicited message sent to a notification receiver. Unlike messages sent to a SNMP agent, traps are not confirmed by the receiver. If it is critical that a Network Management System (NMS) be able to detect when a message is lost, then the device must provide a method for detecting lost messages, such as a log table or sequence numbers. If the device supports informs, critical notifications should be sent using informs.

• Informs

  A SNMP inform is an unsolicited message sent to a notification receiver. It was not introduced into the protocol until SNMP version 2c, so it is not availabe in devices that only support SNMP version 1. Unlike SNMP traps, informs are confirmed messages, and thus are more appropriate for critical notifications.

SNMP Related Terminology

• A SNMP subagent is a process which uses a protocol other than SNMP to communicate with a master agent. The subagent does not directly communicate with SNMP client, instead it relies on the master agent to handle the protocol details of SNMP. One of the primary advantages of subagents is that they can be added or removed dynamically, and new agents can be developed with any changes to the master agent. The two most commonly used protocols for subagents are AgentX and SMUX.

• AgentX is an agent extension protocol. An AgentX master agent acts as a central communication center for one or more AgentX sub-agents. The IETF document defining AgentX is RFC 2741.

• SMUX is an agent extension protocol. It started on the IETF standards track, but was superceded by later work (eg AgentX), and the document defining it (RFC 1227) was moved to historic status.

MIB Terminology

• MAX-ACCESS

• Augmentation table

Network/Socket Terminology

• UDP

  UDP, or User Datagram Protocol, is a network packet protocol, defined in RFC 768. UDP is a connectionless protocol, which means that the packet is sent, and no confirmation is received from the other side (like sending a letter in the mail). This makes it less resource intensive, but also less reliable, than TCP connections.

• TCP

  TCP, or Transmission Control Protocol, is a network packet protocol, defined in RFC 793. TCP is a connection oriented protocol, which means that the sender and receiver communicate to try and ensure the packet is delivered (like sending a fax). This makes it more resource intensive, but also more reliable than UDP packets.

SNMP Table Terminology

• Dependencies
  When one or more columns in a table depend on another object (in the same table, or in another table), a DEPENDENCY exists. For example, if you have a table that determine a color with three columns (red, green and blue) that define the percentage of each primary color, the total for the three columns must equal 100 percent. So, in addition to checking that each colums has a valid value between 0 and 100, the total of all three columns must equal 100.

• row creation

Net-SNMP Developer Terminology

Table attributes

• Transient data
  TRANSIENT data is data that may be overwritten by another funtion or process. For example, many OS functions return data in a static buffer that will be reused the next time the function is called. Because of this, we will assume that you will copy the raw data retrieved from these other sources to a generated structure for use within the Net-SNMP agent. (Don't worry, we'll help you)

• External data
  EXTERNAL data is data that is owned by some other process, device, file or mechanism. The agent must use some interface to read or modify the data. An external process may modify the data without the agent's knowledge. For example, the Net-SNMP agent implements the interface table (ifTable), which reports on network interfaces. The host operating system owns this data, and Net-SNMP must use system calls to report or manipulate the data. Examples of external data include data stored in kernel space, in files, in another non-memory shared process, and data stored in devices.

• Unsorted data
  UNSORTED data is data that is not kept in the same order as the way SNMP expects the index(es) for the table to be kept. [It could very well be sorted in some other order, but for the purpose of SNMP, the order is incorrect.] If you're not sure if your data is sorted in an SNMP compliant way, its likely not.

  Because the raw data is unsorted, to satisfy a particular request, the entire data set must be examined to find the apropriate index.

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rstory at freesnmp dot com
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