video-setup/architecture.md

Video Routing System — Architecture

Concept

A graph-based multi-peer video routing system where nodes are media processes and edges are transport connections. The graph carries video streams between sources, relay nodes, and sinks, with priority as a first-class property on paths — so that a low-latency monitoring feed and a high-quality archival feed can coexist and be treated differently by the system.

---

Design Rationale

Get It on the Wire First

A key principle driving the architecture is that capture devices should not be burdened with processing.

A Raspberry Pi attached to a camera (V4L2 source) is capable of pulling raw or MJPEG frames off the device, but it is likely too resource-constrained to also transcode, mux, or perform any non-trivial stream manipulation. Doing so would add latency and compete with the capture process itself.

The preferred model is:

1. Pi captures and transmits raw — reads frames directly from V4L2 (MJPEG or raw Bayer/YUV) and puts them on the wire over TCP as fast as possible, with no local transcoding
2. A more capable machine receives and defines the stream — a downstream node with proper CPU/GPU resources receives the raw feed and produces well-formed, containerized, or re-encoded output appropriate for the intended consumers (display, archive, relay)

This separation means the Pi's job is purely ingestion and forwarding. It keeps the capture loop tight and latency minimal. The downstream node then becomes the "source" of record for the rest of the graph.

This is also why the V4L2 remote control protocol is useful — the Pi doesn't need to run any control logic locally. It exposes its camera parameters over TCP, and the controlling machine adjusts exposure, white balance, codec settings, etc. remotely. The Pi just acts on the commands.

---

Graph Model

Nodes

Each node is a named process instance, identified by a namespace and name (e.g. v4l2:microscope, ffmpeg:ingest1, mpv:preview, archiver:main).

Node types:

Type	Role
Source	Produces video — V4L2 camera, screen grab, file, test signal
Relay	Receives one or more input streams and distributes to one or more outputs, each with its own delivery mode and buffer; never blocks upstream
Sink	Consumes video — display window, archiver, encoder output

A relay with multiple inputs is what would traditionally be called a mux — it combines streams from several sources and forwards them, possibly over a single transport. The dispatch and buffering logic is the same regardless of input count.

Edges

An edge is a transport connection between two nodes. Edges carry:

• The video stream itself (TCP, pipe, or other transport)
• A priority value
• A transport mode — opaque or encapsulated (see Transport Protocol)

Priority

Priority governs how the system allocates resources and makes trade-offs when paths compete:

• High priority (low latency) — frames are forwarded immediately; buffering is minimized; if a downstream node is slow it gets dropped frames, not delayed ones; quality may be lower
• Low priority (archival) — frames may be buffered, quality should be maximized; latency is acceptable; dropped frames are undesirable

Priority is a property of the path, not of the source. The same source can feed a high-priority monitoring path and a low-priority archival path simultaneously.

---

Control Plane

There is no central hub or broker. Nodes communicate directly with each other over the binary transport. Any node can hold the controller role (function_flags bit 3) — this means it has a user-facing interface (such as the web UI) through which the user can inspect the network, load a topology configuration, and establish or tear down connections between nodes.

The controller role is a capability, not a singleton. Multiple nodes could hold it simultaneously; which one a user interacts with is a matter of which they connect to. A node that is purely a source or relay with no UI holds no controller bits.

The practical flow is: a user starts a node with the controller role and a web interface, discovers the other nodes on the network via the multicast announcement layer, and uses the UI to configure how streams are routed between them. The controller node issues connection instructions directly to the relevant peers over the binary protocol — there is no intermediary.

V4L2 device control and enumeration are carried as control messages within the encapsulated transport on the same connection as video — see Transport Protocol.

---

Ingestion Pipeline (Raspberry Pi Example)

graph LR
    CAM[V4L2 Camera<br>dev/video0] -->|raw MJPEG| PI[Pi: ingest node]
    PI -->|encapsulated stream| RELAY[Relay]
    RELAY -->|high priority| DISPLAY[Display / Preview<br>low latency]
    RELAY -->|low priority| ARCHIVE[Archiver<br>high quality]
    CTRL[Controller node<br>web UI] -.->|V4L2 control<br>via transport| PI
    CTRL -.->|connection config| RELAY

The Pi runs a node process that dequeues V4L2 buffers and forwards each buffer as an encapsulated frame over TCP. It also exposes the V4L2 control endpoint for remote parameter adjustment.

Everything else happens on machines with adequate resources.

V4L2 Buffer Dequeuing

When a V4L2 device is configured for V4L2_PIX_FMT_MJPEG, the driver delivers one complete MJPEG frame per dequeued buffer — frame boundaries are guaranteed at the source. The ingest module dequeues these buffers and emits each one as an encapsulated frame directly into the transport. No scanning or frame boundary detection is needed.

This is the primary capture path. It is clean, well-defined, and relies on standard V4L2 kernel behaviour rather than heuristics.

Misbehaving Hardware: mjpeg_scan (Future)

Some hardware does not honour the per-buffer framing contract — cheap USB webcams or cameras with unusual firmware may concatenate multiple partial frames into a single buffer, or split one frame across multiple buffers. For these cases a separate optional mjpeg_scan module provides a fallback: it scans the incoming byte stream for JPEG SOI (0xFF 0xD8) and EOI (0xFF 0xD9) markers to recover frame boundaries heuristically.

This module is explicitly a workaround for non-compliant hardware. It is not part of the primary pipeline and will be implemented only if a specific device requires it. For sources with unusual container formats (AVI-wrapped MJPEG, HTTP multipart, RTSP with quirky packetisation), the preferred approach is to route through ffmpeg rather than write a custom parser.

---

Transport Protocol

Transport between nodes operates in one of two modes. The choice is per-edge and has direct implications for what the relay on that edge can do.

Opaque Binary Stream

The transport forwards bytes as they arrive with no understanding of frame boundaries. The relay acts as a pure byte pipe.

• Zero framing overhead
• Cannot drop frames (frame boundaries are unknown)
• Cannot multiplex multiple streams (no way to distinguish them)
• Cannot do per-frame accounting (byte budgets become byte-rate estimates only)
• Low-latency output is not available — the relay cannot discard a partial frame

This mode is appropriate for simple point-to-point forwarding where the consumer handles all framing, and where the relay has no need for frame-level intelligence.

Frame-Encapsulated Stream

Each message is prefixed with a small fixed-size header. This applies to both video frames and control messages — the transport is unified.

Header fields:

Field	Size	Purpose
message_type	2 bytes	Determines how the payload is interpreted
payload_length	4 bytes	Byte length of the following payload

The header is intentionally minimal. Any node — including a relay that does not recognise a message type — can skip or forward the frame by reading exactly payload_length bytes without needing to understand the payload. All message-specific identifiers (stream ID, correlation ID, etc.) live inside the payload and are handled by the relevant message type handler.

Message types and their payload structure:

Value	Type	Payload starts with
0x0001	Video frame	stream_id (u16), then compressed frame data
0x0002	Control request	request_id (u16), then command-specific fields
0x0003	Control response	request_id (u16), then result-specific fields
0x0004	Stream event	stream_id (u16), event_code (u8), then event-specific fields

Node-level messages (not tied to any stream or request) have no prefix beyond the header — the payload begins with the message-specific fields directly.

Control payloads are binary-serialized structures — see Protocol Serialization. Stream events carry lifecycle signals — see Device Resilience.

Unified Control and Video on One Connection

By carrying control messages on the same transport as video frames, the system avoids managing separate connections per peer. A node that receives a video stream can be queried or commanded over the same socket.

This directly enables remote device enumeration: a connecting node can issue a control request asking what V4L2 devices the remote host exposes, and receive the list in a control response — before any video streams are established. Discovery and streaming share the same channel.

The V4L2 control operations map naturally to control request/response pairs:

Operation	Direction
Enumerate devices	request → response
Get device controls (parameters, ranges, menus)	request → response
Get control values	request → response
Set control values	request → response (ack/fail)

Control messages are low-volume and can be interleaved with the video frame stream without meaningful overhead.

Capability Implications

Feature	Opaque	Encapsulated
Simple forwarding	yes	yes
Low-latency drop	no	yes
Per-frame byte accounting	no	yes
Multi-stream over one transport	no	yes
Sequence numbers / timestamps	no	yes (via extension)
Control / command channel	no	yes
Remote device enumeration	no	yes
Stream lifecycle signals	no	yes

The most important forcing function is low-latency relay: to drop a pending frame when a newer one arrives, the relay must know where frames begin and end. An opaque stream cannot support this, so any edge that requires low-latency output must use encapsulation.

Opaque streams are a valid optimization for leaf edges where the downstream consumer (e.g. an archiver writing raw bytes to disk) does its own framing, requires no relay intelligence, and has no need for remote control.

---

Relay Design

A relay receives frames from one or more upstream sources and distributes them to any number of outputs. Each output is independently configured with a delivery mode that determines how it handles the tension between latency and completeness.

Output Delivery Modes

Low-latency mode — minimize delay, accept loss

The output holds at most one pending frame. When a new frame arrives:

• If the slot is empty, the frame occupies it and is sent as soon as the transport allows
• If the slot is already occupied (transport not ready), the incoming frame is dropped — the pending frame is already stale enough

The consumer always receives the most recent frame the transport could deliver. Frame loss is expected and acceptable.

Completeness mode — minimize loss, accept delay

The output maintains a queue. When a new frame arrives it is enqueued. The transport drains the queue in order. When the queue is full, a drop policy is applied — either drop the oldest frame (preserve recency) or drop the newest (preserve continuity). Which policy fits depends on the consumer: an archiver may prefer continuity; a scrubber may prefer recency.

Memory Model

Compressed frames have variable sizes (I-frames vs P-frames, quality settings, scene complexity), so fixed-slot buffers waste memory unpredictably. The preferred model is per-frame allocation with explicit bookkeeping.

Each allocated frame is tracked with at minimum:

• Byte size
• Sequence number or timestamp
• Which outputs still hold a reference

Limits are enforced per output independently — not as a shared pool — so a slow completeness output cannot starve a low-latency output or exhaust global memory. Per-output limits have two axes:

• Frame count — cap on number of queued frames
• Byte budget — cap on total bytes in flight for that output

Both limits should be configurable. Either limit being reached triggers the drop policy.

Congestion: Two Sides

Congestion can arise at both ends of the relay and must be handled explicitly on each.

Inbound congestion (upstream → relay)

If the upstream source produces frames faster than any output can dispatch them:

• Low-latency outputs are unaffected by design — they always hold at most one frame
• Completeness outputs will see their queues grow; limits and drop policy absorb the excess

The relay never signals backpressure to the upstream. It is the upstream's concern to produce frames at a sustainable rate; the relay's concern is only to handle whatever arrives without blocking.

Outbound congestion (relay → downstream transport)

If the transport layer cannot accept a frame immediately:

• Low-latency mode: the pending frame is dropped when the next frame arrives; the transport sends the newest frame it can when it becomes ready
• Completeness mode: the frame stays in the queue; the queue grows until the transport catches up or limits are reached

The interaction between outbound congestion and the byte budget is important: a transport that is consistently slow will fill the completeness queue to its byte budget limit, at which point the drop policy engages. This is the intended safety valve — the budget defines the maximum acceptable latency inflation before the system reverts to dropping.

Congestion Signals

Even though the relay does not apply backpressure, it should emit observable congestion signals — drop counts, queue depth, byte utilization — on the control plane so that the controller can make decisions: reduce upstream quality, reroute, alert, or adjust budgets dynamically.

Multi-Input Scheduling

When a relay has multiple input sources feeding the same output, it needs a policy for which source's frame to forward next when the link is under pressure or when frames from multiple sources are ready simultaneously. This policy is the scheduler.

The scheduler is a separate concern from delivery mode (low-latency vs completeness) — delivery mode governs buffering and drop behaviour per output; the scheduler governs which input is served when multiple compete.

Candidate policies (not exhaustive — the design should keep the scheduler pluggable):

Policy	Behaviour
Strict priority	Always prefer the highest-priority source; lower-priority sources are only forwarded when no higher-priority frame is pending
Round-robin	Cycle evenly across all active inputs — one frame from each in turn
Weighted round-robin	Each input has a weight; forwarding interleaves at the given ratio (e.g. 1:3 means one frame from source A per three from source B)
Deficit round-robin	Byte-fair rather than frame-fair variant of weighted round-robin; useful when sources have very different frame sizes
Source suppression	A congested or degraded link simply stops forwarding from a given input entirely until conditions improve

Priority remains a property of the path (set at connection time). The scheduler uses those priorities plus runtime state (queue depths, drop rates) to make per-frame decisions.

The relay module should expose a scheduler interface so policies are interchangeable without touching routing logic. Which policies to implement first is an open question — see Open Questions.

graph TD
    UP1[Upstream Source A] -->|encapsulated stream| RELAY[Relay]
    UP2[Upstream Source B] -->|encapsulated stream| RELAY

    RELAY --> LS[Low-latency Output<br>single-slot<br>drop on collision]
    RELAY --> CS[Completeness Output<br>queued<br>drop on budget exceeded]
    RELAY --> OB[Opaque Output<br>byte pipe<br>no frame awareness]

    LS -->|encapsulated| LC[Low-latency Consumer<br>eg. preview display]
    CS -->|encapsulated| CC[Completeness Consumer<br>eg. archiver]
    OB -->|opaque| RAW[Raw Consumer<br>eg. disk writer]

    RELAY -.->|drop count<br>queue depth<br>byte utilization| CTRL[Controller node]

---

Codec Module

A codec module provides per-frame encode and decode operations for pixel data. It sits between raw pixel buffers and the transport — sources call encode before sending, sinks call decode after receiving. The relay and transport layers never need to understand pixel formats; they carry opaque payloads.

Stream Metadata

Receivers must know what format a frame payload is in before they can decode it. This is communicated once at stream setup via a stream_open control message rather than tagging every frame header. The message carries three fields:

format (u16) — the wire format of the payload bytes; determines how the receiver decodes the frame:

Value	Format
0x0001	MJPEG
0x0002	H.264
0x0003	H.265 / HEVC
0x0004	AV1
0x0005	FFV1
0x0006	ProRes
0x0007	QOI
0x0008	Raw pixels (see pixel_format)
0x0009	Raw pixels + ZSTD (see pixel_format)

pixel_format (u16) — pixel layout for raw formats; zero and ignored for compressed formats:

Value	Layout
0x0001	BGRA 8:8:8:8
0x0002	RGBA 8:8:8:8
0x0003	BGR 8:8:8
0x0004	YUV 4:2:0 planar
0x0005	YUV 4:2:2 packed

origin (u16) — how the frame was produced; informational only, does not affect decoding; useful for diagnostics, quality inference, and routing decisions:

Value	Origin
0x0001	Device native — camera or capture card encoded it directly
0x0002	libjpeg-turbo
0x0003	ffmpeg (libavcodec)
0x0004	ffmpeg (subprocess)
0x0005	VA-API direct
0x0006	NVENC direct
0x0007	Software (other)

A V4L2 camera outputting MJPEG has format=MJPEG, origin=device_native. The same format re-encoded in process has format=MJPEG, origin=libjpeg-turbo. The receiver decodes both identically; the distinction is available for logging and diagnostics without polluting the format identifier.

Format Negotiation

When a source node opens a stream channel it sends a stream_open control message that includes the codec identifier. The receiver can reject the codec if it has no decoder for it. This keeps codec knowledge at the edges — relay nodes are unaffected.

libjpeg-turbo

JPEG is the natural first codec: libjpeg-turbo provides SIMD-accelerated encode on both x86 and ARM, the output format is identical to what V4L2 cameras already produce (so the ingest and archive paths treat them the same), and it is universally decodable including in browsers via <img> or createImageBitmap. Lossy, but quality is configurable.

QOI

QOI (Quite OK Image Format) is a strong candidate for lossless screen grabs: it encodes and decodes in a single pass with no external dependencies, performs well on content with large uniform regions (UIs, text, diagrams), and the reference implementation is a single .h file. Output is larger than JPEG but decode is simpler and there is no quality loss. Worth benchmarking against JPEG at high quality settings for screen content.

ZSTD over Raw Pixels

ZSTD at compression level 1 is extremely fast and can achieve meaningful ratios on screen content (which tends to be repetitive). No pixel format conversion is needed — capture raw, compress raw, decompress raw, display raw. This avoids any colour space or chroma subsampling decisions and is entirely lossless. The downside is that even compressed, the payload is larger than JPEG for photographic content; for UI-heavy screens it can be competitive.

VA-API (Hardware H.264 Intra)

Intra-only H.264 via VA-API gives very high compression with GPU offload. This is the most complex option to set up and introduces a GPU dependency, but may be worthwhile for high-resolution grabs over constrained links. Deferred until simpler codecs are validated.

ffmpeg Backend

ffmpeg (via libavcodec or subprocess) is a practical escape hatch that gives access to a large number of codecs, container formats, and hardware acceleration paths without implementing them from scratch. It is particularly useful for archival formats where the encode latency of a more complex codec is acceptable.

Integration options:

• libavcodec — link directly against the library; programmatic API, tight integration, same process; introduces a large build dependency but gives full control over codec parameters and hardware acceleration (NVENC, VA-API, VideoToolbox, etc.)
• subprocess pipe — spawn ffmpeg, pipe raw frames to stdin, read encoded output from stdout; simpler, no build dependency, more isolated from the rest of the node process; latency is higher due to process overhead but acceptable for archival paths where real-time delivery is not required

The subprocess approach fits naturally into the completeness output path of the relay: frames arrive in order, there is no real-time drop pressure, and the ffmpeg process can be restarted independently if it crashes without taking down the node. libavcodec is the better fit for low-latency encoding (e.g. screen grab over a constrained link).

Archival formats of interest:

Format	Notes
H.265 / HEVC	~50% better compression than H.264 at same quality; NVENC and VA-API hardware support widely available
AV1	Best open-format compression; software encode is slow, hardware encode (AV1 NVENC on RTX 30+) is fast
FFV1	Lossless, designed for archival; good compression for video content; the format used by film archives
ProRes	Near-lossless, widely accepted in post-production toolchains; large files but easy to edit downstream

The encoder backend is recorded in the origin field of stream_open — the receiver cares only about format, not how the bytes were produced. Switching from a subprocess encode to libavcodec, or from software to hardware, requires no protocol change.

---

X11 / Xorg Integration

An xorg module provides two capabilities that complement the V4L2 camera pipeline: screen geometry queries and an X11-based video feed viewer. Both operate as first-class node roles.

Screen Geometry Queries (XRandR)

Using the XRandR extension, the module can enumerate connected outputs and retrieve their geometry — resolution, position within the desktop coordinate space, physical size, and refresh rate. This is useful for:

• Routing decisions: knowing the resolution of the target display before deciding how to scale or crop an incoming stream
• Screen grab source: determining the exact rectangle to capture for a given monitor
• Multi-monitor layouts: placing viewer windows correctly in a multi-head setup without guessing offsets

Queries are exposed as control request/response pairs on the standard transport, so a remote node can ask "what monitors does this machine have?" and receive structured geometry data without any X11 code on the asking side.

Screen Grab Source

The module can act as a video source by capturing the contents of a screen region using XShmGetImage (MIT-SHM extension) for zero-copy capture within the same machine. The captured region is a configurable rectangle — typically one full monitor by its XRandR geometry, but can be any sub-region.

Raw captured pixels are uncompressed — 1920×1080 at 32 bpp is ~8 MB per frame. Before the frame enters the transport it must be encoded. The grab loop calls the codec module to compress each frame, then encapsulates the result. The codec is configured per stream; see Codec Module.

The grab loop produces frames at a configured rate, encapsulates them, and feeds them into the transport like any other video source. Combined with geometry queries, a remote controller can enumerate monitors, select one, and start a screen grab stream without manual coordinate configuration.

Frame Viewer Sink

The module can act as a video sink by creating a window and rendering the latest received frame into it. The window:

• Geometry (size and monitor placement) is specified at stream open time, using XRandR data when targeting a specific output
• Can be made fullscreen on a chosen output
• Displays the most recently received frame — driven by the low-latency output mode of the relay; never buffers for completeness
• Forwards keyboard and mouse events back upstream as INPUT_EVENT protocol messages, enabling remote control use cases

Scale and crop are applied in the renderer — the incoming frame is stretched or letterboxed to fill the window. This allows a high-resolution source (Pi camera, screen grab) to be displayed scaled-down on a different machine.

This makes it the display-side counterpart of the V4L2 capture source: a frame grabbed from a camera on a Pi can be viewed on any machine in the network running a viewer sink node, with the relay handling the path and delivery mode.

Renderer: GLFW + OpenGL

The initial implementation uses GLFW for window and input management and OpenGL for rendering.

GLFW handles window creation, the event loop, resize, and input callbacks — it also supports Vulkan surface creation using the same API, which makes a future renderer swap straightforward. Input events (keyboard, mouse) are normalised by GLFW before being encoded as protocol messages.

The OpenGL renderer:

1. Receives a decoded frame as a pixel buffer (libjpeg-turbo for MJPEG, raw for uncompressed formats)
2. Uploads it as a 2D texture
3. Runs a fragment shader that handles YUV→RGB conversion (where needed) and scaling/filtering
4. Presents via GLFW's swap-buffers call

This keeps CPU load low — chroma conversion and scaling happen on the GPU — while keeping the implementation simple relative to a full Vulkan pipeline.

Renderer: Vulkan (future alternative)

A Vulkan renderer is planned as an alternative to the OpenGL one. GLFW's surface creation API is renderer-agnostic, so the window management and input handling code is shared. Only the renderer backend changes.

Vulkan offers more explicit control over presentation timing, multi-queue workloads, and compute shaders (e.g. on-GPU MJPEG decode via a compute pass if a suitable library is available). It is not needed for the initial viewer but worth having for high-frame-rate or multi-stream display scenarios.

The renderer selection should be a compile-time or runtime option — both implementations conform to the same internal interface (render_frame(pixel_buffer, width, height, format)).

---

Audio (Future)

Audio streams are not in scope for the initial implementation but the transport is designed to accommodate them without structural changes.

A future audio stream is just another message type on an existing transport connection — no new connection type or header field is needed. stream_id in the payload already handles multiplexing. The message type table has room for an audio_frame type alongside video_frame.

The main open question is codec and container: raw PCM is trivial to handle but large; compressed formats (Opus, AAC) need framing conventions. This is deferred until video is solid.

The frame allocator, relay, and archive modules should not assume that a frame implies video — they operate on opaque byte payloads with a message type and length, so audio frames will pass through the same infrastructure unchanged.

---

Device Resilience

Nodes that read from hardware devices (V4L2 cameras, media devices) must handle transient device loss — a USB camera that disconnects and reconnects, a device node that briefly disappears during a mode switch, or a stream that errors out and can be retried. This is not an early implementation concern but has structural implications that should be respected from the start.

The Problem by Layer

Source node / device reader

A device is opened by fd. On a transient disconnect, the fd becomes invalid — reads return errors or short counts. The device may reappear under the same path after some time. Recovery requires closing the bad fd, waiting or polling for the device to reappear, reopening, and restarting the capture loop. Any state tied to the old fd (ioctl configuration, stream-on status) must be re-established.

Opaque stream edge

The downstream receiver sees bytes stop. There is no mechanism in an opaque stream to distinguish "slow source", "dead source", or "recovered source". A reconnection produces a new byte stream that appears continuous to the receiver — but contains a hard discontinuity. The receiver has no way to know it should reset state. This is a known limitation of opaque mode. If the downstream consumer is sensitive to stream discontinuities (e.g. a frame parser), it must use encapsulated mode on that edge.

Encapsulated stream edge

The source node sends a stream_event message (0x0004) on the affected channel_id before the bytes stop (if possible) or as the first message when stream resumes. The payload carries an event code:

Code	Meaning
0x01	Stream interrupted — device lost, bytes will stop
0x02	Stream resumed — device recovered, frames will follow

On receiving stream_interrupted, downstream nodes know to discard any partial frame being assembled and reset parser state. On stream_resumed, they know a clean frame boundary follows and can restart cleanly.

Ingest module (MJPEG parser)

The two-pass EOI state machine is stateful per stream. It must expose an explicit reset operation that discards any partial frame in progress and returns the parser to a clean initial state. This reset is triggered by a stream_interrupted event, or by any read error from the device. Any frame allocation begun for the discarded partial frame must be released before the reset completes.

Frame allocator

A partial frame that was being assembled when the device dropped must be explicitly abandoned. The allocator must support an abandon operation distinct from a normal release — abandon means the allocation is invalid and any reference tracking for it should be unwound immediately. This prevents a partial allocation from sitting in the accounting tables and consuming budget.

Source Node Recovery Loop

The general structure for a resilient device reader (not yet implemented, for design awareness):

1. Open device, configure, start capture
2. On read error: emit stream_interrupted on the transport, close fd, enter retry loop
3. Poll for device reappearance (inotify on /dev, or timed retry)
4. On device back: reopen, reconfigure (ioctl state is lost), emit stream_resumed, resume capture
5. Log reconnection events to the control plane as observable signals

The retry loop must be bounded — a device that never returns should eventually cause the node to report a permanent failure rather than loop indefinitely.

Implications for Opaque Streams

If a source node is producing an opaque stream and the device drops, the TCP connection itself may remain open while bytes stop flowing. The downstream node only learns something is wrong via a timeout or its own read error. For this reason, opaque streams should only be used on edges where the downstream consumer either does not care about discontinuities or has its own out-of-band mechanism to detect them. Edges into an ingest node must use encapsulated mode.

---

Implementation Approach

The system is built module by module in C11. Each translation unit is developed and validated independently before being integrated. See planning.md for current status and module order, and conventions.md for code and project conventions.

The final deliverable is a single configurable node binary. During development, each module is exercised through small driver programs that live in the development tree, not in the module directories.

---

Protocol Serialization

Control message payloads use a compact binary format. The wire encoding is little-endian throughout — all target platforms (Raspberry Pi ARM, x86 laptop) are little-endian, and little-endian is the convention of most modern protocols (USB, Bluetooth LE, etc.).

Serialization Layer

A serial module provides the primitive read/write operations on byte buffers:

• put_u8, put_u16, put_u32, put_i32, put_u64 — write a value at a position in a buffer
• get_u8, get_u16, get_u32, get_i32, get_u64 — read a value from a position in a buffer

These are pure buffer operations with no I/O. Fields are never written by casting a struct to bytes — each field is placed explicitly, which eliminates struct padding and alignment assumptions.

Protocol Layer

A protocol module builds on serial and the transport to provide typed message functions:

write_v4l2_set_control(stream, id, value);
write_v4l2_get_control(stream, id);
write_v4l2_enumerate_controls(stream);

Each write_* function knows the exact wire layout of its message, packs the full frame (header + payload) into a stack buffer using put_*, then issues a single write to the stream. The corresponding read_* functions unpack responses using get_*.

This gives a clean two-layer separation: serial handles byte layout, protocol handles message semantics and I/O.

Web Interface as a Protocol Peer

The web interface (Node.js/Express) participates in the graph as a first-class protocol peer — it speaks the same binary protocol as any C node. There is no JSON bridge or special C code to serve the web layer. The boundary is:

• Socket side: binary protocol, framed messages, little-endian fields read with DataView (dataView.getUint32(offset, true) maps directly to get_u32)
• Browser side: HTTP/WebSocket, JSON, standard web APIs

A protocol.mjs module in the web layer mirrors the C protocol module — same message types, same wire layout, different language. This lets the web interface connect to any video node, send control requests (V4L2 enumeration, parameter get/set, device discovery), and receive structured responses.

Treating the web node as a peer also means it exercises the real protocol, which surfaces bugs that a JSON bridge would hide.

Future: Single Source of Truth via Preprocessor

The C protocol module and the JavaScript protocol.mjs currently encode the same wire format in two languages. This duplication is a drift risk — a change to a message layout must be applied in both places.

A future preprocessor will eliminate this. Protocol messages will be defined once in a language-agnostic schema, and the preprocessor will emit both:

• C source — put_*/get_* calls, struct definitions, write_*/read_* functions
• ESM JavaScript — DataView-based encode/decode, typed constants

The preprocessor is the same tool planned for generating error location codes (see common/error). The protocol schema becomes a single source of truth, and both the C and JavaScript implementations are derived artifacts.

---

Node Discovery

Standard mDNS (RFC 6762) uses UDP multicast over 224.0.0.251:5353 with DNS-SD service records. The wire protocol is well-defined and the multicast group is already in active use on most LANs. The standard service discovery stack (Avahi, Bonjour, nss-mdns) provides that transport but brings significant overhead: persistent daemons, D-Bus dependencies, complex configuration surface, and substantial resident memory. None of that is needed here.

The approach: reuse the multicast transport, define our own wire format.

Rather than DNS wire format, node announcements are encoded as binary frames using the same serialization layer (serial) and frame header used for video transport. A node joins the multicast group, broadcasts periodic announcements, and listens for announcements from peers.

Announcement Frame

Field	Size	Purpose
message_type	2 bytes	Discovery message type (e.g. 0x0010 for node announcement)
channel_id	2 bytes	Reserved / zero
payload_length	4 bytes	Byte length of payload
Payload	variable	Encoded node identity and capabilities

Payload fields:

Field	Type	Purpose
protocol_version	u8	Wire format version
site_id	u16	Site this node belongs to (0 = local / unassigned)
tcp_port	u16	Port where this node accepts transport connections
function_flags	u16	Bitfield declaring node capabilities (see below)
name_len	u8	Length of name string
name	bytes	Node name (namespace:instance, e.g. v4l2:microscope)

function_flags bits:

Bit	Mask	Meaning
0	0x0001	Source — produces video
1	0x0002	Relay — receives and distributes streams
2	0x0004	Sink — consumes video (display, archiver, etc.)
3	0x0008	Controller — participates in control plane coordination

A node may set multiple bits — a relay that also archives sets both RELAY and SINK.

Behaviour

• Nodes send announcements periodically (e.g. every 5 s) and immediately on startup
• No daemon — the node process itself sends and listens; no background service required
• On receiving an announcement, the control plane records the peer (address, port, name, function) and can initiate a transport connection if needed
• A node going silent for a configured number of announcement intervals is considered offline
• Announcements are informational only — the hub validates identity at connection time

No Avahi/Bonjour Dependency

The system does not link against, depend on, or interact with Avahi or Bonjour. It opens a raw UDP multicast socket directly, which requires only standard POSIX socket APIs. This keeps the runtime dependency footprint minimal and the behaviour predictable.

---

Multi-Site (Forward Compatibility)

The immediate use case is a single LAN. A planned future use case is site-to-site linking — two independent networks (e.g. a lab and a remote location) connected by a tunnel (SSH port-forward, WireGuard, etc.), where nodes on both sites are reachable from either side.

Site Identity

Every node carries a site_id (u16) in its announcement. In a single-site deployment this is always 0. When sites are joined, each site is assigned a distinct non-zero ID; nodes retain their IDs across the join and are fully addressable by (site_id, name) from anywhere in the combined network.

This field is reserved from day one so that multi-site never requires a wire format change or a rename of existing identifiers.

Site Gateway Node

A site gateway is a node that participates in both networks simultaneously — it has a connection on the local transport and a connection over the inter-site tunnel. It:

• Bridges discovery announcements between sites (rewriting site_id appropriately)
• Forwards encapsulated transport frames across the tunnel on behalf of cross-site edges
• Is itself a named node, so the control plane can see and reason about it

The tunnel transport is out of scope for now. The gateway is a node type, not a special infrastructure component — it uses the same wire protocol as everything else.

Site ID Translation

Both sides of a site-to-site link will independently default to site_id = 0. A gateway cannot simply forward announcements across the boundary — every node on both sides would appear as site 0 and be indistinguishable.

The gateway is responsible for site ID translation: it assigns a distinct non-zero site_id to each side of the link and rewrites the site_id field in all announcements and any protocol messages that carry a site_id as they cross the boundary. From each side's perspective, remote nodes appear with the translated ID assigned by the gateway; local nodes retain their own IDs.

This means site_id = 0 should be treated as "local / unassigned" and never forwarded across a site boundary without translation. A node that receives an announcement with site_id = 0 on a cross-site link should treat it as a protocol error from the gateway.

Addressing

A fully-qualified node address is site_id:namespace:instance. Within a single site, site_id is implicit and can be omitted. The control plane and discovery layer must store site_id alongside every peer record from the start, even if it is always 0, so that the upgrade to multi-site addressing requires only configuration and a gateway node — not code changes.

---

Open Questions

• What is the graph's representation format — in-memory object graph, serialized config, or both?
• How are connections established — does the controller push connection instructions to nodes, or do nodes pull from a known address?
• Drop policy for completeness queues: drop oldest (recency) or drop newest (continuity)? Should be per-output configurable.
• When a relay has multiple inputs on an encapsulated transport, how are streams tagged on the outbound side — same stream_id passthrough, or remapped?
• What transport is used for relay edges — TCP, UDP, shared memory for local hops?
• Should per-output byte budgets be hard limits or soft limits with hysteresis?
• Which relay scheduler policies should be implemented first, and what is the right interface for plugging in a custom policy? Minimum viable set is probably strict priority + weighted round-robin.