IP Network Management

Simple Network Management Protocol (SNMP)

SNMP is a protocol for managing just about any sort of device that can be attached to a TCP/IP network. The requirements for the managed devices are kept minimal to ensure that even small devices can be managed. To "manage" means to monitor, configure, shutdown, restart, etc.

To manage an internet you must manage the routers, bridges, computers, printers, etc. Since these devices can be on different physical networks (e.g. token ring and Ethernet) in different places, the management protocol must be a higher level protocol (higher than data link level). SNMP sits on top of a transport protocol (UDP) for this reason.

Advantages to this scheme:

one protocol works for all devices that speak IP (uniformity)
devices may be anywhere on the net (no need for a direct connection with managing node)

Disadvantages to this scheme:

the lower layers must be working for SNMP to work (devices may be unreachable when in trouble)

Elements in the SNMP model

Managed nodes - anything with an IP connection
Management stations - where the software runs, almost always a general purpose workstation
Management information - what the nodes maintain
A management protocol - how the nodes and management stations talk to each other

Each device being managed runs a manager agent, or server. The person who manages uses a client. Nice clients are graphical. The complexity is in the management stations; the servers in the nodes are kept as simple as possible.

Management Data

The form of the data held by each device is specified in the MIB (management information base). The form of the MIB is standardized so that queries can be device independent.

Each piece of data held in the MIB is known as an object (but no particular relation to OO objects). Some examples of the objects kept in the MIB are:

system.sysUpTime

interfaces.ifNumber

interfaces.ifMtu

ip.ipRoutingTable

tcp.tcpRtoMin

The objects stored in a MIB are defined and referenced using the ISO ASN.1 (Abstract Syntax Notation). ASN.1 specifies a means of creating a global, absolute, hierarchical name space. Different organizations have control over different parts of the space. Each level has a number, so the name of a variable could be textual or a string of numbers.

<figure 7-32 Tanenbaum shows the name space>
    root
        ccitt
        iso
            standard
            registration authority
            member-body
            identified-organization
                dod
                    internet
                        directory
                        mgmt
                        experimental
                        private
                        security
                        snmpv2
        joint-iso-ccitt

The categories of information in iso.identified-organization.dod.internet.mgmt are:

Each node in the tree has a number, and you can describe an object with names, numbers or a mixture of the two. For example:

are both valid names for te ipInReceives variable in the MIB. See the file /etc/snmp.mib for the ASN.1 definition of the MIB.

ASN.1 (or a subset of it) is also used to transfer information between heterogeneous machines. An encoding standard is used called Basic Encoding Rules.

The groups of objects in the MIB (e.g. ip, tcp, system) are of various types:

Reporting

Most data is gathered by the management station polling the agents. However, agents may report interesting events to management stations which are registered with them. The information is kept to a minimum to reduce network traffic. If a management station wants to know more about the event it polls the agent to find out more. Such agent-initiated events are known as traps and include

coldStart, warmStart, linkDown, linkUp Some agents aren't smart enough to report on their own. A proxy agent may talk to them and report for them.

Protocol

The SNMP protocol is quite simple:

The SNMP protocol doesn't use the kinds of commands you might expect (add/delete routes, disable/enable interface, reboot, etc) but forces everything into a fetch/store paradigm. This promotes simplicity and stability of the protocol.

To reboot, you store a value into the MIB which is the timeToNextReboot variable.

Commands

snmpstat

Available on most UNIX platforms. A simple command line client that lets you query agents and manipulate MIBs.

Examples of use

This comes from Marking an interface down

Suppose that a PPP interface attached to a router named paris is reporting many errors. The administrator can mark the link as being "down'' while the phone company checks the line. SNMP can be used to do this remotely.

First, the error count can be obtained with snmpstat.

   # snmpstat -i paris public
   Interface statistics
                        Type      InOctet   InPckts   InErrs   IfMtu
   Name    Address      Speed     OutOctet OutPckts OutErrs OutQlen
   net1    paris        enetv2    18952186 18239     7036     1500
           02000000ec6c 10000000 4392148   4183      147      0
   lo0     localhost    loop      5832      64        0        2048
                        0         5832      64        0        0
   ppp0    paris        ppp       3682304   14384     4800     296
                        9600      2122240   8290      2138     0

Next, the interface can be marked as being ``down'' using setany.

   # setany paris public ifAdminStatus.3 -i 2
   Name: ifAdminStatus.3
   Value: 2

When the line is repaired, the interface can be reactivated in a similar fashion.