diff --git a/architecture.md b/architecture.md
index 37694b8..498a8a5 100644
--- a/architecture.md
+++ b/architecture.md
@@ -125,10 +125,10 @@ Header fields:
 | Value | Meaning |
 |---|---|
 | `0x0001` | Video frame |
-| `0x0002` | Control request (JSON) |
-| `0x0003` | Control response (JSON) |
+| `0x0002` | Control request |
+| `0x0003` | Control response |
 
-Video frame payloads are raw compressed frames. Control payloads are JSON — the same request/response structure as the V4L2 RPC protocol, but carried inline on the same connection rather than on a separate port.
+Video frame payloads are raw compressed frames. Control payloads are binary-serialized structures — see [Protocol Serialization](#protocol-serialization).
 
 ### Unified Control and Video on One Connection
 
@@ -248,6 +248,35 @@ The final deliverable is a single configurable node binary. During development,
 
 ---
 
+## 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:
+
+```c
+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.
+
+---
+
 ## Open Questions
 
 - What is the graph's representation format — in-memory object graph, serialized config, or both?
diff --git a/conventions.md b/conventions.md
index 0e837e4..5e9e437 100644
--- a/conventions.md
+++ b/conventions.md
@@ -4,6 +4,7 @@
 
 - **C11** throughout (`-std=c11`)
 - **`_GNU_SOURCE`** defined via `-D_GNU_SOURCE` in all Makefiles — enables the full GNU/Linux feature set (POSIX 2008, BSD, GNU and Linux-specific extensions); do not define this in source files
+- **LTO enabled** via `-flto` in all Makefiles — the compiler handles inlining across translation units at link time; do not use `static` on functions solely to enable inlining
 - Target platform: Linux (V4L2, epoll, etc. are Linux-specific)
 
 ---
diff --git a/dev/cli/Makefile b/dev/cli/Makefile
index 48ea0ca..5d74e5e 100644
--- a/dev/cli/Makefile
+++ b/dev/cli/Makefile
@@ -1,6 +1,6 @@
 ROOT   := $(abspath ../..)
 CC      = gcc
-CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -I$(ROOT)/include
+CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
 BUILD   = $(ROOT)/build
 
 COMMON_OBJ     = $(BUILD)/common/error.o
diff --git a/planning.md b/planning.md
index c0db72c..ee1e2dc 100644
--- a/planning.md
+++ b/planning.md
@@ -37,14 +37,16 @@ Modules are listed in intended build order. Each depends only on modules above i
 
 | # | Module | Status | Notes |
 |---|---|---|---|
-| 1 | `common` | not started | Error types, base definitions — no dependencies |
-| 2 | `media_ctrl` | not started | Media Controller API — device and topology enumeration, pad format config |
-| 3 | `v4l2_ctrl` | not started | V4L2 controls — enumerate, get, set camera parameters |
-| 4 | `transport` | not started | Encapsulated transport — header encode/decode, framed TCP read/write |
-| 5 | `frame_alloc` | not started | Per-frame allocation with bookkeeping (byte budget, ref counting) |
-| 6 | `relay` | not started | Input dispatch to output queues (low-latency and completeness modes) |
-| 7 | `ingest` | not started | MJPEG frame parser (two-pass EOI state machine, opaque stream → discrete frames) |
-| 8 | `archive` | not started | Write frames to disk, control messages to JSON log |
+| 1 | `common` | done | Error types, base definitions — no dependencies |
+| 2 | `media_ctrl` | done | Media Controller API — device and topology enumeration, pad format config |
+| 3 | `v4l2_ctrl` | done | V4L2 controls — enumerate, get, set camera parameters |
+| 4 | `serial` | not started | `put`/`get` primitives for little-endian binary serialization into byte buffers |
+| 5 | `transport` | not started | Encapsulated transport — frame header, TCP stream abstraction, single-write send |
+| 6 | `protocol` | not started | Typed `write_*`/`read_*` functions for all message types; builds on serial + transport |
+| 7 | `frame_alloc` | not started | Per-frame allocation with bookkeeping (byte budget, ref counting) |
+| 8 | `relay` | not started | Input dispatch to output queues (low-latency and completeness modes) |
+| 9 | `ingest` | not started | MJPEG frame parser (two-pass EOI state machine, opaque stream → discrete frames) |
+| 10 | `archive` | not started | Write frames to disk, control messages to binary log |
 
 ---
 
diff --git a/src/modules/common/Makefile b/src/modules/common/Makefile
index dd5fc61..d2c5ab0 100644
--- a/src/modules/common/Makefile
+++ b/src/modules/common/Makefile
@@ -1,6 +1,6 @@
 ROOT   := $(abspath ../../..)
 CC      = gcc
-CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -I$(ROOT)/include
+CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
 BUILD   = $(ROOT)/build/common
 
 .PHONY: all clean
diff --git a/src/modules/media_ctrl/Makefile b/src/modules/media_ctrl/Makefile
index c51ac12..91aca5c 100644
--- a/src/modules/media_ctrl/Makefile
+++ b/src/modules/media_ctrl/Makefile
@@ -1,6 +1,6 @@
 ROOT   := $(abspath ../../..)
 CC      = gcc
-CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -I$(ROOT)/include
+CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
 BUILD   = $(ROOT)/build/media_ctrl
 
 .PHONY: all clean
diff --git a/src/modules/v4l2_ctrl/Makefile b/src/modules/v4l2_ctrl/Makefile
index d321706..07150bb 100644
--- a/src/modules/v4l2_ctrl/Makefile
+++ b/src/modules/v4l2_ctrl/Makefile
@@ -1,6 +1,6 @@
 ROOT   := $(abspath ../../..)
 CC      = gcc
-CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -I$(ROOT)/include
+CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
 BUILD   = $(ROOT)/build/v4l2_ctrl
 
 .PHONY: all clean