# TCP Command Frame

## Authorship

This document was generated using ChatGPT (OpenAI), version 5.2.
It has not been independently reviewed or validated yet.

# Planned design

## Transport

* TCP (lwIP raw API).
* Single synchronous request/response per connection.
* TCP flow control is the only queue.

## Future goals

* HMAC with preshared key

## Framing


```
cmd (1 byte)
len (2 bytes, uint16)
payload (len bytes)
```

## Limits

* `MAX_PAYLOAD` compile-time constant.
* Fixed local frame buffer: `uint8_t frame[MAX_PAYLOAD]`.
* No dynamic allocation based on `len`.

## Parser

* Stateful stream parser (READ_CMD → READ_LEN → READ_PAYLOAD).
* Accumulate until full frame received.
* When `len > MAX_PAYLOAD` → `tcp_abort()` immediately.
* On complete frame → execute command → send response → reset parser.

## Memory safety

* Only copy up to `MAX_PAYLOAD`.
* Call `tcp_recved()` only for consumed bytes.
* `pbuf_free()` after processing.
* No resynchronization logic.
* Protocol violation ⇒ connection closed.

## Commands (initial)

* `SETUP_I2C`
* `I2C_XFER` (write + read with bounded `wlen`/`rlen`)

## Result

* Deterministic RAM usage.
* Clear failure behavior.
* Minimal parser complexity.
* No reliance on TCP message boundaries.