Internet layer used TCP and IP protocols through the network. TCP operates at each end point of the path and manages end to end delivery of message segments. IP operates on all nodes on the path and routes message datagram. Internet routers as shown in Figure 2.18 do not need a transport layer for routing. They only implemented for maintenance purposes.
In a DTN, the protocol stacks of all nodes include both bundle and transport layers. DTN gateways have the same double stack layers as DTN routers as shown in Figure 2.19. DTN gateways can run different lower layer protocols on each side of their double stack. This allows DTN gateway to span two regions that use different lower layer protocols.