Custody Transfer and Compressed Status Reporting for Bundle Protocol Version 7

As space missions increase, there is a growing need to replace point-to-point communication with an efficient and reliable network-centric communication approach. Disruption/Delay Tolerant Networking (DTN) with the Bundle Protocol (BP) has been selected as an interoperable network protocol in the LunaNet Interoperability Specification. It is also considered for future Earth Observation and Mars communication scenarios. In a DTN, the "bundle" -- the fundamental data unit of BP -- requires dedicated mechanisms to ensure reliability due to the challenges posed by intermittent connectivity and long delays. The previous version of BP, BPv6, contained a mechanism for reliable transfer between "custodial nodes" called "custody transfer". However, this approach has been removed from the core protocol specification for BPv7, which requires a corresponding BP reliability extension to be defined separately. This paper introduces a new custody transfer process for BPv7 (expected to be published by CCSDS as an experimental specification in 2025). The core features of this new custody transfer method for BPv7 are: (1) A strategy to efficiently identify sets of bundles by sequence numbering (2) A new Custody Transfer Extension Block and a corresponding administrative record, Compressed Custody Signal, to efficiently report on the acceptance or rejection of custody using sequence numbering (3) A new Compressed Reporting Extension Block requesting reporting on bundle processing steps using a corresponding administrative record with sequence numbering for efficiency. The paper will describe those concepts and their design, specification, and implementation in detail. These mechanisms have been prototyped in the ESA BP implementation and tested in Earth Observation and Lunar communication simulation scenarios. The results will be presented, as will an outlook on future work in the DTN reliable transfer domain.