Phase 6.3: TCP window management — track guest window, advertise host kernel rcv-space#75
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dpsoft wants to merge 10 commits intosmoltcp-passt-port-phase6.2-async-connectfrom
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10 bite-sized tasks covering proper TCP windowing: - TcpNatEntry tracks guest_window (u32) + guest_window_scale (u8) - handle_tcp_frame parses tcp.window_scale() on guest SYN, stores per-flow; updates guest_window on every incoming frame - build_tcp_packet_static signature changes to take (window_len, window_scale) — caller decides - SYN-ACK negotiates OUR_WINDOW_SCALE = 7 (passt's default; 128x) - New host_recv_window helper queries TCP_INFO.tcpi_rcv_space and scales it for the advertised window on outgoing frames - relay_tcp_nat_data gates host→guest sends on entry.guest_window to honor real backpressure - Three new pins: tcp_advertised_window_tracks_guest_buffer (BROKEN_ON_PURPOSE → flips at Task 7), tcp_window_scale_negotiated_in_synack, plus tcp_bulk_throughput_constrained_window parametric bench Severity: MEDIUM — perf gap. Hardcoded window_len: 65535 caps throughput at 64 KB / RTT regardless of bandwidth, and inject_to_guest can grow unbounded if the guest is slow.
Adds tcp_bulk_throughput_constrained_window bench that exercises the Task 7 window-gating path under three guest-window sizes (4096, 16384, 65536 bytes). Mirrors tcp_bulk_throughput_1mb with a parametric window so regressions in window-constrained relay show up numerically.
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Status: DRAFT. Stacked on PR #74 (Phase 6.2 async connect).
What this branch does
Stops ignoring the guest's advertised window and stops hardcoding our own.
Severity
Medium — perf gap. Today the relay emits `window_len: 65535, window_scale: None` on every outgoing frame, ignores the guest's advertised window entirely, and never honors guest backpressure. Throughput is capped at 64 KB / RTT regardless of bandwidth, and `inject_to_guest` could grow unbounded if the guest is slow.
Bench evidence (`bench-compare.sh --baseline 47868f0 --skip-vm`)
Wall-clock vs master
Commits (10 + plan)
Validation