The successful architecture and supporting protocols of today's Internet operate poorly when faced with operating enviroranents chalacterized by very long delay padls and frequent network partitions.These problems ale exacerbated by end-nodes that have severe power or memory constraints.Because of lacking always-on infrastructure in mobile and extreme environments

many such networks have their ownspeciatized protocols

and do not utilize TCP/IP.To achieve interoperability between them

researchers had proposed a network architecture and application interface structured around optionally and reliable asynchronous message forwarding

with limited expectations of end-to-end connectivity and node resources.This architectnre is calleddelay-tolerant networks(DTN).It operates as an overlay networkabove the transport layer.In this paper

we study the state-of-the-art architecture and key techniques for DTN

discuss their merits and deficiencies

and point out potential issues.It is our purpose to stimulate more research in this new promising network.