BOLT: A UDP-Based Bulk File Transfer Protocol with Application-Aware Rate Control
BOLT is a UDP-based file transfer protocol designed for single-flow bulk transfer between authenticated peers over high-latency, lossy links. The motivating problem is the well-known underperformance of single-flow TCP under exogenous packet loss: on a 50 Mbps, 227 ms, 11%-loss satellite link, an 89 GB transfer over single-flow TCP requires more than forty days; BOLT completes the same transfer in 4.5 hours. The contributions are a state-machine rate controller ("Hose") with explicit memory of the last loss cliff, eliminating a sawtooth pathology that loss-blind probing produces under exogenous loss; per-packet authenticated encryption with derived non-transmitted nonces, saving twelve bytes per encrypted packet versus DTLS-style record protection; a nine-byte data header; lock-free bitmap-based deduplication and resume; and platform-aware send paths exploiting Linux UDP segmentation offload where available and a Windows spin pacer where it is not. The protocol is evaluated against three TCP regimes (classic, modern with BBR and RACK-TLP, and PEP-accelerated) on the operating conditions of a deployed Patagonian meteorological station and recovers throughput comparable to PEP-accelerated TCP without requiring operator infrastructure or terminating end-to-end semantics. The reference implementation is in production use.