# Improving Bundle Routing in a Space DTN by Approximating the Transmission Time of the Reliable LTP
## 1 Introduction  
Space networking is experiencing persistent growth due to the interest of both government agencies and the private sector in extending terrestrial networks to outer space and supporting diverse applications that span from complementing network coverage, reaching areas of difficult access, to deep space exploration missions. However, space networking involves crucial challenges compared with terrestrial communication systems. In particular, the lines of sight and signal-to-noise ratios (SNRs) of the communication links are impacted by the constant motion of the nodes. Signal degradation due to atmospheric factors and solar radiation can also affect some of the links, and the large physical dimensions of the network bring extreme propagation delays. The delay-tolerant networking (DTN) approach that originated from the Interplanetary Internet (IPN) concept [1] addresses this problem through a store-carry-and-forward scheme, whereby data pieces can be stored long term until they can be forwarded either to the destination node or to a relay. In NASA's DTN architecture, the bundle protocol (BP) [2] defines the basic transmission unit that can be routed to the intended destination over a network of intermittent link availability. The data forwarding can only occur during the times a node is within the communication reach of another (i.e., during contact), and therefore, a given source-to-sink communication may require the use of multiple intermediate nodes and contacts. The contact graph routing (CGR) [3] algorithm uses knowledge of the future contacts to determine the path for each bundle which can be acquired with reasonable accuracy from node position calculations using orbital equations or ephemeris. A link budget analysis can find the start time and duration of the contacts. However, the actual contact realization features may vary from the predictions, which may lead to transmission failures. To address this problem, the architecture defines a reliable convergence adapter (i.e., the Licklidertransmission protocol (LTP)) which can be activated to allow retransmissions below the BP. The BP also includes a reliable transfer mode (custody transfer) that can retransmit or reroute lost bundles along the path. To prevent saturating a given link, CGR takes into account the remaining capacity (volume) available for the selected contact. However, the current method does not take into account the possible retransmissions occurring at the LTP convergence adapter that can lengthen the transmission time and therefore impact the end-to-end bundle performance. To optimize the contact path calculations, accurate predictions of the one-hop bundle transmission time are needed. In focusing on but not limiting the discussion to space networking, this requirement may pose a problem, as it is difficult to accurately anticipate such values over lossy channels. With radio transmission errors, the LTP requires multiple transmission rounds to reliably deliver a bundle to the next node. Such retransmissions and the long propagation delay of the links contribute to amplifying the bundle transmission time. Moreover, the effective bit error rate (BER) of given links may not be consistent over time. Current methods assume that the physical layer is error-free or simply disregard the transmission time (i.e., find routes based solely on the link propagation delays). As an example, the reference implementation of the Interplanetary Overlay Network (ION) [4] defines a contact as a tuple containing the source and sink node numbers, start and end times, and the link rate, excluding the anticipated channel BER. Instead, a common and constant BER value is assumed for all links but not used in the bundle transmission time estimation. Another observation is that if a disruption occurs on the link while a bundle is being transmitted, then the transmission will either need to be resumed or restarted at a later time, which also contributes to increasing the effective bundle latency.