This document describes a bit of history and functionality of the SNMP protocol for monitoring.
SNMP Monitoring and Basic Architecture:
SNMP(Simple Network Management Protocol) was developed in the 80s by the IETF (Internet Engineer Task Force) as part of the IP(Internet Protocol Suite). There have been a few competitors developed by organizations, but over time SNMP has become the de-facto standard for monitoring network-connected devices. In addition to providing monitoring capabilities SNMP, also provides the ability for remote management to a variety of devices.
SNMP-based monitoring systems consist of Agents, which are installed on devices connected to networks and Network Management Systems/Stations(NMS), which are used to actively collect information from Agents as well as run tests against network-connected devices—usually for the purpose of determining health.
In addition to an SNMP agent serving out information about its device, it may also push notifications (known as traps) across the network to an NMS. This would be used for such information as that the device just started (known as coldstart).
In cases where SNMP is being utilized for remote management of devices, users would push changes across networks to Agents installed on devices, which then enact the prescribed changes.
Every piece of information an SNMP Agent serves out, and every type of trap that it supports, is associated with a unique identifier known as an OID (Object Identifier).
OID is a system created by the ITU and ISO organizations for international standardization of Data. It isn’t only used for network monitoring or SNMP, it is a standard used for all sorts of data. Examples of this are Hl7, which hass to do with electronic medical record taxonomy or Lightweight Directory Access Protocol(LDAP) protocol attributes and object classes. Think of it as an international standard for indexing data.
The OID system is heiarchical, which is to say, it is a tree structure. For instance, the OID .1.3.6, 6 would be part of .1.3 heiarchy, .3.6 would be part of the .1 hearchy etc… The root of the OID tree is only 2 values: .1 for ISO (which is where SNMP lives) and .0 for the ITU. Certain sections of the subtree for .1 are free-form, other sections are delegated to particular companies to use and yet other sections of three require that you register with ISO or the ITU. This includes goverments, large buisiness etc…
SNMP OIDs can start being found at OID .22.214.171.124
.1 - ISO assigned. .1.3 - ISO identified organizations .1.3.6 - The US Department of Defense .126.96.36.199 - The Internet
Most OIDs for business network devices and systems then fall under
.188.8.131.52.4.1 - IANA (Internet Assigned Numbers Authority) registered private enterprises.
Under this hierarchy, you will see OIDs for all companies setting up custom OIDs for equipment. Here are some examples:
.184.108.40.206.4.1.9 - Cisco Systems (They embraced SNMP pretty early as you can see by their low number) .220.127.116.11.4.1.2636 - Juniper .18.104.22.168.4.1.311 - Microsoft.
Now, say an NMS station wants to pull interface Output Queue drops from a router made by Cisco; it would make a request for the value of OID: .22.214.171.124.126.96.36.199.2188.8.131.52.1.11
Which you can see is part of their tree at 184.108.40.206.4.9. Any custom statistic they develop should be registered under that portion of the OID tree.
MIBs and Enterprise Non-specific OIDs:
A MIB (Management Information Base), contains descriptions for every OID registered with either ISO or ITU. An MIB is an ASCII file, which is human readable and formatted in such a way as to work with what is called a MIB compiler. A MIB compiler simply builds a browsable tree of OIDs based descriptions of individual OIDs contained therein. The descriptions of OIDs contained in MIBs provide a short-name syntax for each OID used, the type of data allowed to be stored, and information on how that OID is used.
It is a common misconception that MIBs are necessary to enable functionality on SNMP Agents or devices. This is not true; loading MIBs into an agent simply allows that agent or client to provide more detailed information on the OIDs that it serves out or queries.
For instance, here is an SNMP client querying a server for all of it’s OIDs when the client has only the ISO MIB loaded:
iso.220.127.116.11.18.104.22.168 = STRING: "Linux PEI-HQ-NMS1 4.2.0-27-generic #32~14.04.1-Ubuntu SMP Fri Jan 22 15:32:26 UTC 2016 x86_64" iso.22.214.171.124.126.96.36.199 = OID: iso.188.8.131.52.1.8072.3.2.10 iso.184.108.40.206.220.127.116.11 = Timeticks: (1223) 0:00:12.23 iso.18.104.22.168.22.214.171.124 = STRING: "root" iso.126.96.36.199.188.8.131.52 = STRING: "PEI-HQ-NMS1" iso.184.108.40.206.220.127.116.11 = STRING: "Unknown" iso.18.104.22.168.22.214.171.124 = Timeticks: (0) 0:00:00.00 iso.126.96.36.199.188.8.131.52.2.1 = OID: iso.184.108.40.206.220.127.116.11.1 iso.18.104.22.168.22.214.171.124.2.2 = OID: iso.126.96.36.199.188.8.131.52.1 iso.184.108.40.206.220.127.116.11.2.3 = OID: iso.18.104.22.168.22.214.171.124.1 iso.126.96.36.199.188.8.131.52.2.4 = OID: iso.184.108.40.206.3.1 iso.220.127.116.11.18.104.22.168.2.5 = OID: iso.22.214.171.124.1.49 iso.126.96.36.199.188.8.131.52.2.6 = OID: iso.184.108.40.206.1.4 iso.220.127.116.11.18.104.22.168.2.7 = OID: iso.22.214.171.124.1.50 iso.126.96.36.199.188.8.131.52.2.8 = OID: iso.184.108.40.206.220.127.116.11.1 iso.18.104.22.168.22.214.171.124.2.9 = OID: iso.126.96.36.199.188.8.131.52.3
Here is a similar query from a Sun system with the SNMPv2 MIB loaded on the client:
SNMPv2-MIB::sysDescr.0 = STRING: ILOM machine custom description SNMPv2-MIB::sysObjectID.0 = OID: SUN-ILOM-SMI-MIB::sunILOMSystems DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (16439826) 1 day, 21:39:58.26 SNMPv2-MIB::sysContact.0 = STRING: set via snmp test SNMPv2-MIB::sysName.0 = STRING: SUNSPHOSTNAME SNMPv2-MIB::sysLocation.0 = STRING: SNMPv2-MIB::sysServices.0 = INTEGER: 72 SNMPv2-MIB::sysORLastChange.0 = Timeticks: (14) 0:00:00.14 SNMPv2-MIB::sysORID.1 = OID: IF-MIB::ifMIB SNMPv2-MIB::sysORID.2 = OID: SNMPv2-MIB::snmpMIB SNMPv2-MIB::sysORID.3 = OID: TCP-MIB::tcpMIB SNMPv2-MIB::sysORID.4 = OID: RFC1213-MIB::ip SNMPv2-MIB::sysORID.5 = OID: UDP-MIB::udpMIB SNMPv2-MIB::sysORID.6 = OID: SNMP-VIEW-BASED-ACM-MIB::vacmBasicGroup SNMPv2-MIB::sysORID.7 = OID: SNMP-FRAMEWORK-MIB::snmpFrameworkMIBCompliance SNMPv2-MIB::sysORID.8 = OID: SNMP-MPD-MIB::snmpMPDCompliance SNMPv2-MIB::sysORID.9 = OID: SNMP-USER-BASED-SM-MIB::usmMIBCompliance
As you can see, the MIB is simply translating pieces of the OID into a more user-readable format. Instead of 184.108.40.206.220.127.116.11.0, we see SNMPv2-MIB::sysDescr.0, which for many things is easier to discern.
Mainly, MIBs contain lists of all OIDs by purpose.
Vendor Non-Specific OIDs/MIBs
Certain statistics aren’t vendor specific and have been around for a long time. Most NMS systems don’t poll for enterprise specific OIDs, but go after these long-hanging fruit that are common to all network connected devices. CPU Utilization, Disk drive utilization, and Network Utilization are all examples of types of stats that aren’t unique to a particular vendor. Most of these MIBS were created/maintained by the Internet Engineering Taskforce and are contained under:
.18.104.22.168.2.1 -> SNMP MIB2
with .22.214.171.124.2 being the IETF management tree.
Here are some really handy MIBs under this heiarchy:
126.96.36.199.2.1.1 - SNMP MIB2 System: Contains system description, contact information etc... 188.8.131.52.2.1.2 - the IF-MIB, which contains a wide variety of network statistics information. Everything from number of Octets coming into an interface to a description of the interface. 184.108.40.206.2.1.25 - The Host-resources MIB, everything from disk utilization to CPU and Memory. 220.127.116.11.2.1.43 - The Printer MIB, which contains all general printer statistics.