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TCP/IP Network Administration

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Troubleshooting TCP/IP
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11.9 Simple Network Management Protocol

Troubleshooting is necessary to recover from problems, but the ultimate goal of the network administrator is to avoid problems. That is also the goal of network management software. The network management software used on TCP/IP networks is based on the Simple Network Management Protocol (SNMP).

SNMP is a client/server protocol. In SNMP terminology, it is described as a manager/agent protocol. The agent (the server) runs on the device being managed, which is called the Managed Network Entity. The agent monitors the status of the device and reports that status to the manager.

The manager (the client) runs on the Network Management Station (NMS). The NMS collects information from all of the different devices that are being managed, consolidates it, and presents it to the network administrator. This design places all of the data manipulation tools and most of the human interaction on the NMS. Concentrating the bulk of the work on the manager means that the agent software is small and easy to implement. Correspondingly, most TCP/IP network equipment comes with an SNMP management agent.

SNMP is a request/response protocol. UDP port 161 is its well-known port. SNMP uses UDP as its transport protocol because it has no need for the overhead of TCP. "Reliability" is not required because each request generates a response. If the SNMP application does not receive a response, it simply re-issues the request. "Sequencing" is not needed because each request and each response travels as a single datagram.

The request and response messages that SNMP sends in the datagrams are called Protocol Data Units (PDU). The five PDUs used by SNMP are listed in Table 11.3 These message types allow the manager to request management information, and when appropriate, to modify that information. The messages also allow the agent to respond to manager requests and to notify the manager of unusual situations.

Table 11.3: SNMP Protocol Data Units
GetRequestManager requests an update.
GetNextRequestManager requests the next entry in a table.
GetResponseAgent answers a manager request.
SetRequestManager modifies data on the managed device.
TrapAgent alerts manager of an unusual event.

The NMS periodically requests the status of each managed device (GetRequest) and each agent responds with the status of its device (GetResponse). Making periodic requests is called polling. Polling reduces the burden on the agent because the NMS decides when polls are needed, and the agent simply responds. Polling also reduces the burden on the network because the polls originate from a single system at a predictable rate. The shortcoming of polling is that it does not allow for real-time updates. If a problem occurs on a managed device, the manager does not find out until the agent is polled. To handle this, SNMP uses a modified polling system called trap-directed polling.

A trap is an interrupt signaled by a predefined event. When a trap event occurs, the SNMP agent does not wait for the manager to poll; instead it immediately sends information to the manager. Traps allow the agent to inform the manager of unusual events while allowing the manager to maintain control of polling. SNMP traps are sent on UDP port 162. The manager sends polls on port 161 and listens for traps on port 162. Table 11.4 lists the trap events defined in the RFCs.

Table 11.4: Generic Traps Defined in the RFCs
coldStartAgent restarted; possible configuration changes
warmStartAgent reinitialized without configuration changes
enterpriseSpecificAn event significant to this hardware or software
authenticationFailureAgent received an unauthenticated message
linkDownAgent detected a network link failure
linkUpAgent detected a network link coming up
egpNeighborLossThe device's EGP neighbor is down

The last three entries in this table show the roots of SNMP in Simple Gateway Management Protocol (SGMP), which was a tool for tracking the status of network routers. Routers are generally the only devices that have multiple network links to keep track of and are the only devices that run Exterior Gateway Protocol (EGP). [12] These traps are not significant for most systems.

[12] EGP is covered in Chapter 7.

The most important trap may be the enterpriseSpecific trap. The events that signal this trap are defined differently by every vendor's SNMP agent software. Therefore it is possible for the trap to be tuned to events that are significant for that system. SNMP uses the term "enterprise" to refer to something that is privately defined by a vendor or organization as opposed to something that is globally defined by an RFC.

SNMP has twice as much jargon as the rest of networking - and that's saying something! Managed Network Entity, NMS, PDU, trap, polling, enterprise - that's just the beginning! We also need to mention (below) what SMI is, what a MIB is, and what ANS.1 is used for. Why this bewildering array of acronyms and buzzwords? I think there are two main reasons:

Now that you know why you have to suffer through all of this jargon, let's define a few more important terms.

The Structure of Management Information (SMI) defines how data should be presented in an SNMP environment. The SMI is documented in RFC 1155 and RFC 1065, Structure and Identification of Management Information for TCP/IP-based Internets. The SMI defines how managed objects are named, the syntax in which they are defined, and how they are encoded for transmission over the network. The SMI is based on previous ISO work.

Each managed object is given a globally unique name called an object identifier. The object identifier is part of a hierarchical name space that is managed by the ISO. The hierarchical name structure is used, just like it is in DNS, to guarantee that each name is globally unique. In an object identifier, each level of the hierarchy is identified by a number.

Objects are defined just as formally as they are named. The syntax used to define managed objects is Abstract Syntax Notation One (ASN.1). ASN.1 is ISO Standard 8824, Specification of Abstract Syntax Notation One (ASN.1). It is a very formal set of language rules for defining data. It makes the data definition independent of incompatibilities between systems and character sets. ASN.1 also includes a set of rules for encoding data for transfer over a network. These rules are defined in ISO Standard 8825, Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1). The Basic Encoding Rules (BER) define that bit 8 of an octet is sent first, that 2's complement is used for signed integers, and other nitty-gritty details of data transmission.

Every object managed by SNMP has a unique object identifier defined by the ASN.1 syntax and encoding defined by BER. When all of these unique objects are grouped together, they are called the Management Information Base (MIB). The MIB refers to all information that is managed by SNMP. However, we usually refer to "a MIB" or "the MIBs" (plural), meaning the individual databases of management information formally defined by an RFC or privately defined by a vendor.

MIBI and MIBII are standards defined by RFCs. MIBII is a superset of MIBI, and is the standard MIB for monitoring TCP/IP. It provides such information as the number of packets transmitted into and out of an interface, and the number of errors that occurred sending and receiving those packets - useful information for spotting usage trends and potential trouble spots. Every agent supports MIBI or MIBII.

Some systems also provide a private MIB in addition to the standard MIBII. Private MIBs add to the monitoring capability by providing system-specific information. Most UNIX systems do not provide private MIBs. Private MIBs are most common on network hardware like routers, hubs, and switches.

No matter what MIBs are provided by the agents, it is the monitoring software that displays the information for the system administrator. A private MIB won't do you any good unless your network monitoring software also supports that MIB. For this reason, most administrators prefer to purchase a monitor from the vendor that supplies the bulk of their network equipment. Another possibility is to select a monitor that includes a MIB compiler, which gives you the most flexibility. A MIB compiler reads in the ASN.1 description of a MIB and adds the MIB to the monitor. A MIB compiler makes the monitor extensible because if you can get the ASN.1 source from the network equipment vendor, you can add the vendor's private MIB to your monitor.

MIB compilers are only part of the advanced features offered by some monitors. Some of the features offered are:

Network maps

Some monitors automatically draw a map of the network. Colors are used to indicate the state (up, down, etc.) of the devices on the network. At a glance, the network manager sees the overall state of the network.

Tabular data displays

Data displayed in tables or rendered into charts is used to make comparisons between different devices. Some monitors output data that can then be read into a standard spreadsheet or graphing program.


Filters sift the data coming in from the agents in order to detect certain conditions.


Alarms indicate when "thresholds" are exceeded or special events occur. For example, you may want an alarm to trigger when your server exceeds some specified number of transmit errors.

Don't be put off by the jargon. All of this detail is necessary to formally define a network management scheme that is independent of the managed systems, but you don't need to memorize it. You need to know that a MIB is a collection of management information, that an NMS is the network management station, and that an agent runs in each managed device in order to make intelligent decisions when selecting an SNMP monitor. This information provides that necessary background. The features available in network monitors vary widely; so does the price. Select an SNMP monitor that is suitable for the complexity of your network and the size of your budget.

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