In this paper we first establish a general performance upper bound on the diversity-multiplexing tradeoff (DMT) for communication protocols for a half-duplex cooperative network where every node has a single antenna. This bound shows that the dynamic-decode-and-forward (DDF) protocol achieves optimal DMT among all variations of decode-and-forward based protocols when multiplexing gain r ≤ 1 over 2. Secondly, when r > 1 over 2 it shows that the diversity gain must be less than 1 for all protocols and for any number of relay nodes. Our bound also implies that the well-known cut-set-based DMT upper bound is not tight except for the single-relay network and that there are rooms to improve the performance of DDF protocol, thereby settling a long-time open question raised by Azarian et al.. In light of this breakthrough, a novel protocol based on a rate-split is proposed in this paper. Two variations, dynamic and non-dynamic, of the rate-split-based protocol are also presented. It is shown that the dynamic version achieves a DMT better than the DDF protocol, and in particular, meets the cut-set bound in the case of a single-relay network. The other variation, i.e., the non-dynamic one, has a much simpler implementation and reaches its optimal performance in only two channel uses. Moreover, it meets the cut-set bound in the case of a single-relay network.