We propose a new scheme for reliable transport, both for unicast and multicast flows, in Delay Tolerant Networks (DTNs). Reliability is ensured through the use of Global Selective ACKnowledgements (G-SACKs) which contain detailed (and potentially global) information about the receipt of packets at all the destinations. The motivation for using G-SACKs comes from the observation that one should take the maximum advantage of the contact opportunities which occur quite infrequently in DTNs. We also propose sharing of "packet header space" with G-SACK information and allow for random linear coding at the relay nodes. Our results from extensive simulations of the proposed scheme quantify the gains due to each new feature.