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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  TCP/IP Network Management Framework and Protocols (SNMP and RMON)                 9  TCP/IP Simple Network Management Protocol (SNMP) Protocol                      9  SNMP Protocol Messaging and Message Formats

SNMP Protocol Messaging and Message Formats 1 2 3 SNMP Message Field Definitions, General Message Format and Message Sections

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.)

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.

SNMP Protocol Messaging and Message Formats 1 2 3 SNMP Message Field Definitions, General Message Format and Message Sections