Packet Layer Erasure Coding in Interplanetary Links: The LTP Erasure Coding Link Service Adapter

Abstract

Interplanetary networks are affected by long propagation delays, intermittent connectivity, possible packet losses due to residual errors, and other impairments. To cope with these challenges, the delay-/disruption-tolerant networking (DTN) architecture utilizes the Licklider transmission protocol (LTP) as convergence layer on space links. The LTP reliable service (red) relies on Automatic Repeat reQuest, but very long propagation delays make packet layer forward error correcting (PL-FEC) codes very appealing to protect LTP segments from losses. The key advantage of FEC is that LTP retransmissions would be limited to the unlikely case of decoding failures. To this end, a new FEC-based protocol, to be inserted immediately below LTP, the erasure coding link service adapter (ECLSA), is presented here. ECLSA is completely transparent to LTP, relies on two alternative external libraries for coding/decoding, LibecDLR and OpenFEC, both using low density parity check codes and it is fully integrated with the ION DTN software package of NASA-JPL. This paper aims to provide a solid description of ECLSA, including features functional in a real deployment (such as the dynamic selection of codes). Performance is evaluated at the end of the paper, with nearly ideal results. ECLSA is released as free software and is already included in the “contrib” section of ION.