  Two major relay-selection strategies widely applied in cooperative
decode-and-forward (DF) relaying networks, namely, reactive relay selection
(RRS) and proactive relay selection (PRS), are generally looked upon as
independent and studied separately. In this paper, RRS and PRS are proven to be
equivalent with respect to the end-to-end outage probability from the first
principle, i.e. their respective relay-selection criteria. On the other hand,
RRS is shown to be superior to PRS with respect to the end-to-end symbol error
rate. Afterwards, a case study of a general DF relaying system, subject to
co-channel interferences and additive white Gaussian noise at both the relaying
nodes and the destination, is performed to explicitly illustrate the
aforementioned outage equivalence. These fundamental relations provide
intuitive yet insightful performance benchmarks for comparing various
applications of these two relay-selection strategies.
