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SNMP Protocol Message Generation, Addressing, Transport and Retransmission
(Page 3 of 3)
Dealing with Lost Transmissions
The second issue with UDP is the
price we pay for its efficiency and simplicity: a lack of transport
features. UDP doesn't guarantee data delivery or handle retransmissions,
which means a request or reply could in theory be lost in transit. Only
the device that initially sends a request can know if there was a problem
with transport—it sends the request, and if it receives no reply
knows either the request or response got lost. This puts the responsibility
for retransmission on the part of the device that sends the request
message.
NMSes sending requests to SNMP agents
generally use a timer to keep track of how much time has elapsed since
a request was sent. If the response doesn't arrive within a certain
time interval, the request is sent again. Because of how SNMP works,
having a request be received more than once accidentally will normally
not cause any problems (a property known as idempotence). The
NMS does need to employ an algorithm to ensure that it does not generate
too many retransmissions and clog the network (especially since congestion
might be causing the loss of its messages in the first place.)
Handling Lost Trap Messages
Since traps are unconfirmed, there
is no way for the intended recipient of a trap PDU to know if did not
arrive, nor is there any way for the sender of the trap PDU to know.
This is just a weakness in the protocol; the overall reliability of
TCP/IP (and the underlying networks) ensures that these messages are
not lost very often.
 Key Concept: SNMP is designed with a separately-defined set of protocol operations and transport mappings, so it can be carried over many different internetworking technologies. The most common of these is transport mechanisms is TCP/IP, where SNMP makes use of UDP running over IP, for its efficient and simple communication. The lack of reliability features in UDP means that requests must be tracked by the device sending them and retransmitted if no reply is received. The limited size of UDP messages restricts the amount of information that can be sent in any SNMP PDU.
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