SNMP

SNMP (Simple Network Management Protocol) is a framework that provides facilities for managing and monitoring network resources on the Network.



SNMP is a standard produced by working groups within the IETF (Internet Engineering Task Force). It is published as a set of RFCs.
SNMP is more than a protocol. It is a management framework, including an architecture, an information model, management operations, etc.



Components of SNMP

• SNMP agents
• SNMP managers
• Management Information Bases (MIBs)
• SNMP protocol itself

Network Management Architecture

 

SNMP Components

• Follows the client server architecture
• Agents:
  • Fully implements the SNMP protocol
  • Stores and retrieves the management information’s as defined by MIB
  • Asynchronous signal events to the manager
  • Can be proxy for some other non-snmp device.
• Manager:
  • Implemented as NMS
  • Implements full SNMP protocol
• Able to
  • Query agents
  • Get response from agents
  • Set variables on agents
  • Acknowledge asynchronous events from agent

 

MIB – Management  Information Base

• Management Information is modeled as (managed) objects and relationships among them.
• A MIB (Management Information Bases) is a collection of objects, grouped for a specific management purpose.
• All objects are organized in the global MIB tree.
• Each MIB represents a sub tree of this global MIB tree.
• The leaf objects of the tree contain object instances with the state and control variables of the managed system.
• MIB-II is the most popular MIB. It is implemented in most SNMP-managed devices.
• Device manufacturers often define their own device specific MIBs.

What is Managed Objects ?

• Each managed object is assigned an object identifier (OID)
• The OID is specified in a MIB file.
• An OID can be represented as a sequence of integers separated by decimal points or by a text string:

Example:
– 1.3.6.1.2.1.4.6.
– iso.org.dod.internet.mgmt.mib-2.ip.ipForwDatagrams

• When an SNMP manager requests an object, it sends the OID to the SNMP agent.
• Managed objects are organized in a tree-like hierarchy and the OIDs reflect the structure of the hierarchy.
• Each OID represents a node in the tree.
• The OID 1.3.6.1.2.1 (iso.org.dod.internet.mgmt.mib-2) is at the top of the hierarchy for all managed objects of the MIB-II.
• Manufacturers of networking equipment can add product specific objects to the hierarchy.

Management  Information Base Tree

 

 

SMI, ASN.1 and BER

• Structure of Management Information (SMI)
  • Defines set of rules used to specify the format for defining managed objects or the devices that are accessed using SNMP
• Abstract Syntax Notation One (ASN.1)
  • The MIB modules and the SMI are expressed using a text based data description notation called as ASN.1
  • ASCII text format
• Basic Encoding rules (BER)
  • Encoding technique for SNMP MIB messages conveyed across the network
  • Encoded in binary format

SMIv1 – Structure of Management Information

• SMIv1 is described in RFCs 1155, 1212, 1215
• These RFC Describes:
  • How MIB modules are defined with CCITT X.208 ASN.1 data description language
  • The subset of the ASN.1 language that is used in MIBs
  • The addition of the APPLICATION data type to ASN.1, specifically for use with SNMP MIBs
  • All ASN.1 constructs are serialized using the CCITT X.209 BER for transmission across the wire
  • Definition of the high-level structure of the Internet branch (iso(1).org(3).dod(6).internet(1)) of the MIB naming tree
  • The definition and description of an SNMP managed object

SMIv2 – Structure of Management Information

• SMIv2 is described in RFCs 1442, 1443, 1444
• These RFCs describes:
  • SMIv2 is a backward compatible update to SMIv1
  • The only exception is the Counter64 type defined by SMIv2
  • Counter64 cannot be created in SMIv2
  • RFC 2089 defines how bilingual (SMIv1 & SMIv2) agents handle the Counter64 data type
  • IETF requires that new and revised RFCs specify MIB modules using SMIv2

SMI Data Types

 

	SMI V1	SMI V2
Simple Types	Integer Octet String Object Identifier	Integer Octet String Object Identifier Integer32
Application Types	– Gauge Counter – Time Ticks IpAddress Opaque NetworkAddress	Unassigned32 Gauge32 Counter32 Counter64 Time Ticks IpAddress Opaque –
Pseudo Types	–	Bits

 

ASN.1 – Abstract Syntax Notation One

• ASN.1 is nothing more than a language definition. It is similar to C/C++ and other programming languages.
• Defining Objects
  • Syntax
  • Access
  • Status
  • Description
  • Index
  • value

BER – Basic Encoding Rules

• The relationship between ASN.1 and BER parallels that of source code and machine code.
• CCITT X.209 specifies the Basic Encoding Rules
• All SNMP messages are converted / serialized from ASN.1 notation into smaller, binary data (BER)
• Each ASN.1 value is encoded as octet string

• This encoding results into a sequence of tag, length & value

SNMP & OSI MODEL

7	Application Layer	SNMP Management and Agent APIs
6	Presentation Layer	ASN 1.1 and BER
5	Session Layer	RPC and NetBIOS
4	Transport Layer	TCP and UDP
3	Network Layer	IP and IPX
2	Data Link Layer	Ethernet, Token Ring, FDDI
1	Physical Layer

 

Basic Message Format

 

 

SNMP Versions

Three versions are in use today:

• SNMPv1 (1990)
• SNMPv2c (1996)
  • Adds “GetBulk” function and some new types
  • Adds RMON (remote monitoring) capability
• SNMPv3 (2002)
  • SNMPv3 started from SNMPv1 (and not SNMPv2c)
  • Addresses security
• All versions are still used today
• Many SNMP agents and managers support all three versions of the protocol.

SNMP Standards

• SMI
  • Structure Of Management Information
  • SMIv1 is defined by RFC1155,RFC1212 and RFC1215
  • SMIv2 RFC1902, RFC1903 and RFC1904
• MIB-I and MIB-II
  • Management Information Base
  • MIB-I was the first version issued as 1156
  • MIB-II RFC 1213
  • A large number of additional MIBs exist
• SNMP V1
  • Simple Network Management Protocol
  • RFC 1155,1157
  • Name is used in a more general sense
• SNMPv2c
  • RFC 1901, 1906
• & SNMPv3
  • RFC 1905, 1906, 2271, 2272, 2274 and RFC 2275

 

For Reference

http://www.ietf.org
http://searchnetworking.techtarget.com
http://docwiki.cisco.com/wiki/Simple_Network_Management Protocol
Network Management: Principles and Practice By Mani Subramanian
Essential SNMP, 2nd Edition by Douglas Mauro, Kevin Schmidt