User's Manual

Network Support—Chapter 4
123700 Series Color Mobile Computer User’s Manual
SNMP Configuration
Simple Network Management Protocol (SNMP) was developed in the late
1980s to provide a general-purpose internetworking management protocol.
Its primary goal was to be simple so nothing would stand in the way of its
ubiquitous deployment. To this end, it has been very successful as it is cur-
rently deployed in almost every major internetworking product on the
market. However, like many achieved goals, the primary strength can also
become a weakness.
The Focus was “Simple”
An extreme example of simplicity versus power can be realized by compar-
ing SNMP against the Common Management Information Protocol
(CMIP), the ISO entry to the standard management protocol world.
CMIP has a very rich set of primitives and a core set of data elements.
However, to implement CMIP, a subset of the protocol must be selected.
Then, to achieve interoperability, this subset must be agreed upon with
other implementors. As SNMP was specified completely and with no op-
tions, one implemented what was there and interoperability was assured.
Returning to simplicity, SNMP was built simply for a number of reasons
other than time to market: robustness in the face of network failure, low
overhead in the devices running the protocol; and ease of debugging the
protocol itself (the last thing you want to debug is the management protocol
that is supposed t o be helping you debug your network). Thus, the SNMP
limited itself to the User Datagram Protocol (UDP). This gave the
implementor the ability and responsibility to manage lost packets and
perform any necessary retransmissions. As network debugging in the face
of changing routes will certainly mean losing packets, retaining this control
from the transport service (layer 4) was considered essential. Since a
network management protocol will run continuously, it is mandatory that
it consume as minimal a network resource as possible. UDP allows the
necessary control over packet transmissions, packet size and content
(packetization). It is a natural choice.
Using SNMP
SNMP has three control primitives that initiate data flow from the
requester (get, get-next, and set). There are two control primitives the
responder uses to reply. One is used in response to the requester’ s direct
query (get-response) and the other is an asynchronous response to obtain
the requester’ s attention (trap). All five of these primitives are carried by
UDP and are thus limited in size by the amount of data that can fit into a
single UDP packet. The relatively small message size was a goal of the
design but for some reasonable set of network management functions, it
imposes a limitation.
Often in network management, it is necessary to obtain bulk information
without knowing at first what is in that bulk. In one case, there is a set of
problems having to do with packets not going where they are supposed to,
due to device misconfiguration that prevents proper protocol operation
where one needs to view the entire set of data.