glm-5.2
2313c51f12
Phase 1 architecture for alknet-channels (multiplexing proxy on alknet/channels). Grounded in the completed de-risk POC (28 tests) and the landed ADR-070 (BidiStreamSource trait + Connection::from_source). ADRs: - 071: 9-byte chunk wire format (generalizes TTY's 5-byte) - 072: channel 0 pre-negotiated as alknet/call (no special control plane) - 073: channel lifecycle operations on the call protocol — channel/open, close, control, resources/subscribe; direction field pinned; subscribe from day one (not poll-for-v1 — StreamingHandler machinery exists) - 074: ChannelBidiStreamSource implements BidiStreamSource (ADR-070); into_sub_streams() typed accessor for TTY; accept_bi() generic path - 075: ChannelsAdapter + ChannelManager; REQ-CH-01..04 wire invariants - 076: bounded-buffer backpressure (1 MiB), 256-channel cap, monotonic IDs - 077: TTY inside channels uses sub-streams, not own wire format; amends ADR-052 scope to direct-connect TTY; channels feature on tty - 078: two-pump shutdown-on-completion contract (handler-level) - 079: hub relay translates channel 0, byte-forwards data channels - 080: ChannelClient (QUIC-only); AlknetClient core extraction deferred (OQ-55) Spec docs: overview, channels-wire, channels-connection, channels-adapter, channel-operations, channel-client. OQ-56 (full windowing) and OQ-57 (two-pump helper extraction) are genuine deferred(scope) deferrals with concrete blocking conditions; the contracts are decided, only the extensions are deferred. Hedging audit converted three research hedges into decisions: resources/subscribe (not poll), server-assigned IDs (not if-zero-RTT), bounded-buffer (not if-HOL-becomes-a-problem).
34 lines
2.1 KiB
Markdown
34 lines
2.1 KiB
Markdown
# OQ-56: Full Channel-Level Flow-Control Windowing
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- **Origin**: `docs/research/alknet-channels/phase-0-findings.md` §DP-5,
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§OQ-CH-03; `docs/architecture/decisions/076-backpressure-channel-limits-id-reuse.md`
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- **Status**: deferred(scope)
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- **Door type**: two-way (additive — per-channel window tracking does not
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change the wire format)
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- **Priority**: low
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- **Blocked on**: a real deployment observes head-of-line blocking on a
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saturated channel where the bounded-buffer's stop-reading mitigation is
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insufficient. The trigger is specific: a channel whose consumer is
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persistently slower than its producer, causing the demux to stall that
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channel's reads frequently enough that other channels' throughput is
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measurably affected. The intended use cases (TTY, SSH, tunnels) are not
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high-throughput in the HOL-blocking sense; the trigger requires a
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high-throughput use case (e.g., file transfer over a tunnel) that
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saturates a channel.
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- **Resolution**: Not yet decidable. The bounded-buffer backpressure
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(ADR-076, default 1 MiB per `(channel_id, stream_type)`) is the decided
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v1 mechanism — validated by the POC's 1 MiB `tunnel_large_payload` test
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with no deadlock and no cross-channel blocking. Full channel-level
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windowing (SSH-style sliding-window per channel) is an additive extension
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that does not change the wire format; it adds per-channel window tracking
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to the demux/mux. The decision to add it depends on whether the
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bounded-buffer mitigation is sufficient in practice, which can only be
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determined by a deployment that hits the limitation.
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- **What does NOT block on this**: the bounded-buffer mechanism is decided
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and is the v1 implementation. Full windowing is an extension, not a
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prerequisite. The channels crate ships with bounded-buffer backpressure;
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full windowing is added if and only if the trigger condition is observed.
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- **Cross-references**: ADR-076 (bounded-buffer decision), ADR-071 (wire
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format — unchanged by windowing extension),
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`docs/research/alknet-channels/poc-summary.md` §POC Target 1
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(backpressure validation) |