mikael-lovqvist/pico-w-experiments

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Mikael Lövqvist c02ec01f75 Initial commit

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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.