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Phase 4: L0 Transport & Queueing (UTCP + OPQ)

Status: ⏳ IN PREPARATION Target: L0 Transport Layer (l0-transport/) RFCs: RFC-0004 (UTCP), RFC-0005 (OPQ)

Overview

Phase 4 moves the project from static "Wire Frames" (LWF) to an active Transport Layer. It introduces the ability to send/receive packets over the network and manage offline persistence for asynchronous communication.

Scope

1. UTCP: Unreliable Transport Protocol (UDP)

Component: l0-transport/utcp.zig
Function: Fast-path UDP wrapper for LWF frames.
Key Features:
- Non-blocking UDP socket abstraction.
- Zero-copy frame ingestion (points directly into receive buffer).
- Rapid entropy validation (L1 check) before full frame parsing.
- Path MTU discovery (basic) for LWF FrameClass selection.

2. OPQ: Offline Packet Queue

Component: l0-transport/opq.zig
Function: High-resilience store-and-forward mechanism.
Key Features:
- Node Personas:
  - Client: Outbox only (Retention: <1hr, Buffer: <5MB).
  - Relay: Store-and-Forward (Retention: 72-96hr, Buffer: Quota-driven).
- Segmented WAL Storage: Persistent storage using 4MB segments for corruption isolation and atomic rotation.
- Queue Manifests: Merkle-committed summaries of currently stored frames for selective fetch.
- Quota Management: Hard disk-space limits and priority-based eviction (Least Trusted First/Expired First).
- Automatic Pruning: TTL-driven segment removal.

3. Frame Pipeline Integration

Component: l0_transport.zig (Index)
Function: Orchestrating the flow: UDP -> Ingestion -> OPQ -> Application.

Architecture

[ PEER ] <--- UDP ---> [ UTCP Socket ]
                            |
                     [ Frame Validator ] (Signature/Entropy/Timestamp)
                            |
                     [ OPQ (Persistent) ] <--- [ Storage ]
                            |
                     [ L1 State Machine ]

Readiness Checklist

Phase 3 PQXDH Handshake complete.
LWF Framing stable and tested.
UDP Socket abstraction prototyped.
Persistent storage engine selected (Simple WAL or Direct Filesystem).

Success Metrics

Performance: <5ms from UDP packet arrival to OPQ persistence.
Resilience: Lossless storage during 72-hour offline periods.
Security: Zero frame processing for invalid entropy stamps (DoS protection).

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