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rfc3416: Version 2 of the Protocol Operations for the Simple Network Management Protocol (SNMP)

However, in order to calculate the round trip time for transmission and processing of a request-response transaction, the application needs to use a different request-id value on a retransmitted request. The latter strategy is recommended for use in the majority of situations.

With a non-unique request ID in all requests and responses, how do you know the responses are not arriving out of order?

The value of the request-id field in a Response-PDU takes the value of the request-id field in the request PDU to which it is a response. By use of the request-id value, an application can distinguish the (potentially multiple) outstanding requests, and thereby correlate incoming responses with outstanding requests. In cases where an unreliable datagram service is used, the request-id also provides a simple means of identifying messages duplicated by the network. Use of the same request-id on a retransmission of a request allows the response to either the original transmission or the retransmission to satisfy the request.