As networks become more complex, in terms of device population, topology and distances, it has been getting more and more important for network administrators to have some easy and convenient way for controlling all pieces of the whole network.
Basic features of a network management system include device information retrieval and device remote control. Former often takes shape of gathering device operation statistics, while latter can be seen in device remote configuration facilities.
For any information to be exchanged between entities, some agreement on information format and transmission procedure needs to be settled beforehand. This is what is conventionally called a Protocol.
Large networks nowdays, may host thousands of different devices. To benefit network manager's interoperability and simplicity, any device on the network should carry out most common and important management operations in a well known, unified way. Therefore, an important feature of a network management system would be a Convention on management information naming and presentation.
Sometimes, management operations should be performed on large number of managed devices. For a network manager to complete such a management round in a reasonably short period of time, an important feature of a network management software would be Performance.
Some of network devices may run on severely limited resources what invokes another property of a proper network management facility: Low resource consumption.
In practice, the latter requirement translates into low CPU cycles and memory footprint for management software aboard device being managed.
As networking becomes a more crucial part of our daily lives, security issues have become more apparent. As a side note, even Internet technologies, having military roots, did not pay much attention to security initially. So, the last key feature of network management appears to be Security.
Data passed back and forth through the course of management operations should be at least authentic and sometimes hidden from possible observers.
All these problems were approached many times through about three decades of networking history. Some solutions collapsed over time for one reason or another, while others, such as Simple Network Management Protocol (SNMP), evolve into an industry standard.
The SNMP management model includes three distinct entities -- Agent, Manager and Proxy talking to each other over network.
Agent entity is basically a software running somewhere in a networked device and having the following distinguishing properties:
The latter feature is a source of management information for Agent, as well as a target for remote control operations.
Modern SNMP standards suggest splitting Agent functionality on two parts. Such Agents may run SNMP for local processes called Subagents, which interface with managed devices internals. Communication between Master Agent and its Subagents is performed using a simplified version of original SNMP protocol, known as AgentX, which is designed to run only within a single host.
Manager entity is usually an application used by humans (or daemons) for performing various network management tasks, such as device statistics retrieval or remote control.
Sometimes, Agents and Managers may run peer-to-peer within a single entity that is called Proxy. Proxies can often be seen in application-level firewalling or may serve as SNMP protocol translators between otherwise SNMP version-incompatible Managers and Agents.
For Manager to request Agent for an operation on a particular part of managed device, some convention on device's components naming is needed. Once some components are identified, Manager and Agent would have to agree upon possible components' states and their semantics.
SNMP approach to both problems is to represent each component of a device as a named object, similar to named variables seen in programming languages, and state of a component maps to a value associated with this imaginary variable. These are called Managed Objects in SNMP.
For representing a group of similar components of a device, such as network interfaces, Managed Objects can be organized into a so-called conceptual table.
Manager talks to Agent by sending it messages of several types. Message type implies certain action to be taken. For example, GET message instructs Agent to report back values of Managed Objects whose names are indicated in message.
There's also a way for Agent to notify Manager of an event occurred to Agent. This is done through so-called Trap messages. Trap message also carries Managed Objects and possibly Values, but besides that it has an ID of event in form of integer number or a Managed Object.
For naming Managed Objects, SNMP uses the concept of Object Identifier. As an example of Managed Object, .iso.org.dod.internet.mgmt.mib-2.system.sysName.0 represents human-readable name of a device where Agent is running.
Managed Objects values are always instances of ASN.1 types (such as Integer) or SNMP-specific subtypes (such as IpAddress). As in programming languages, type has an effect of restricting possible set of states Managed Object may ever enter.
Whenever SNMP entities talk to each other, they refer to Managed Objects whose semantics (and value type) must be known in advance by both parties. SNMP Agent may be seen as a primary source of information on Managed Objects, as they are implemented by Agent. In this model, Manager should have a map of Managed Objects contained within each Agent to talk to.
SNMP standard introduces a set of ASN.1 language constructs (such as ASN.1 subtypes and MACROs) which is called Structure of Management Information (SMI). Collections of related Managed Objects described in terms of SMI comprise Management Information Base (MIB) modules.
Commonly used Managed Objects form core MIBs that become part of SNMP standard. The rest of MIBs are normally created by vendors who build SNMP Agents into their products.
More often then not, Manager implementations could parse MIB files and use Managed Objects information for names resolution, value type determination, pretty printing and so on. This feature is known as MIB parser support.
First SNMP version dates back to 1988 when a set of IETF RFC's were first published ( RFC1065, RFC1066, RFC1067 ). These documents describe protocol operations (in terms of message syntax and semantics), SMI and a few core MIBs. The first version appears to be lightweight and easy to implement. Although, its poor security became notorious over years (Security? Not My Problem!), because cleartext password used for authentication (AKA Community String) is extremely easy to eavesdrop and replay, even after almost 20 years, slightly refined standard ( RFC1155, RFC1157, RFC1212 ) still seems to be the most frequent encounter in modern SNMP devices.
In effort to fix security issues of SNMPv1 and to make protocol faster for operations on large number of Managed Objects, SNMP Working Group at IETF came up with SNMPv2. This new protocol offers bulk transfers of Managed Objects information (by means of new, GETBULK message payload), improved security and re-worked SMI. But its new party-based security system turned out to be too complicated. In the end, security part of SNMPv2 has been dropped in favor of community-based authentication system used in SNMPv1. The result of this compromise is known as SNMPv2c (where "c" stands for community) and is still widely supported without being a standard ( RFC1902, RFC1903, RFC1904, RFC1905, RFC1906, RFC1907, RFC1908 ).
The other compromise targeted at offering greater security than SNMPv1, without falling into complexities of SNMPv2, has been attempted by replacing SNMPv2 party-based security system with newly developed user-based security model. This variant of protocol is known as SNMPv2u. Although neither widely implemented nor standardized, User Based Security Model (USM) of SNMPv2u got eventually adopted as one of possibly many SNMPv3 security models.
As of this writing, SNMPv3 is current standard for SNMP. Although it's based heavily on previous SNMP specifications, SNMPv3 offers many innovations but also brings significant complexity. Additions to version 3 are mostly about protocol operations. SMI part of standard is inherited intact from SNMPv2.
SNMPv3 system is designed as a framework that consists of a core, known as Message and PDU Dispatcher, and several abstract subsystems: Message Processing Subsystem (MP), responsible for SNMP message handling, Transport Dispatcher, used for carrying over messages, and Security Subsystem, which deals with message authentication and encryption issues. The framework defines subsystems interfaces to let feature-specific modules to be plugged into SNMPv3 core thus forming particular feature-set of SNMP system. Typical use of this modularity feature could be seen in multiprotocol systems -- legacy SNMP protocols are implemented as version-specific MP and security modules. Native SNMPv3 functionality relies upon v3 message processing and User-Based Security modules.
Besides highly detailed SNMP system specification, SNMPv3 standard also defines a typical set of SNMP applications and their behavior. These applications are Manager, Agent and Proxy ( RFC3411, RFC3412, RFC3413, RFC3414, RFC3415, RFC3416, RFC3417, RFC3418 ).