v2.2.0 — Parallel TCP Streams

Summary

• Parallel TCP streams for file transfers — auto-enabled for files > 5 MiB
• 1.74x speedup on 50MB files over high-RTT relay links (77ms RTT)
• 1.21x faster than scp for 50MB files

What changed

• Client opens N WebSocket connections (sendFileParallel), each sending interleaved chunks
• Daemon uses seek-based writes to place each chunk at the correct offset
• Relay uses atomic header+binary forwarding to prevent interleaving
• Auto-parallel: files > 5 MiB get 2 streams by default
• Override with RCMD_PARALLEL_STREAMS=N (0 = disabled)

Benchmark (50MB random → rbm21, 77ms RTT relay)

Method	Time	Throughput	vs scp
scp	87.0s	600 KB/s	1.0x
rcmd 1 stream	124.9s	421 KB/s	0.70x
rcmd 2 streams	86.6s	608 KB/s	1.00x
rcmd 3 streams	72.0s	734 KB/s	1.21x

All transfers verified with sha256sum.

Test plan

• Unit tests pass
• 10MB transfer with 2 streams — integrity verified
• 50MB transfer with 3 streams — integrity verified
• Small file (1MB) uses single stream
• Auto-parallel for files > 5MB

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