Delay Tolerant Networking (DTN) has been proposed as a global overlay architecture to provide network connectivity in challenged environments such as deep-space communications field. Bundle Protocol along with its convergence layer protocols transform end-to-end reliability into questionable hop-by-hop reliable communication and imposes a two-tier routing policy that allows flexible routes only within regions. Following the OSI model, in this paper we discuss the advantages and disadvantages of having a transport protocol relying on DTN for providing reliable and transparent transfer of data between end systems and offering common transport layer services such as end-to-end packet-oriented retransmission, flow control, path diversity and redundant transmission. In that context, we highlight features from DS-TP and DTTP that correspond to the requirements of transport protocols for space. We present space communication scenarios, which cannot be adequately addressed by the current DTN architecture.