# MSRT Protocol Standard
MSRT, MSRT, is a protocol standard for message-oriented realtime serial links.
This document is intentionally high level. The current goal is to keep protocol ownership and crate boundaries clear while the first no-std draft is hardened.
The current version is a v1 no-std protocol foundation. It is suitable for internal testing and boundary review, but v1 is not complete until reliable transport behavior is implemented and verified.
## Scope
The MSRT standard owns:
- core protocol primitive types
- packet and protocol frame boundaries
- wire envelope boundaries
- channel identity and routing semantics
- acknowledgement and partial-reliability concepts
- ordering, deduplication, timeout, and sliding-window contracts
- protocol engine behavior for send, receive, response, and progress
- error surfaces shared by all implementations
The MSRT standard does not own:
- UART drivers
- DMA drivers
- embedded-hal adapters
- operating-system serial APIs
- tokio, async-std, or any other executor integration
- CLI tools
- simulator transport backends
## Implementation Model
MSRT is designed to be implemented in at least two implementation environments.
MCU implementations are expected to use `no_std` and may avoid allocation entirely.
Host implementations are expected to use normal Rust with operating-system support.
Both implementation environments must implement the same protocol. Environment-specific crates may be added later, but they should depend on the standard protocol crates rather than becoming part of the standard itself.
## Current Crates
- `msrt`: portable facade crate for the protocol standard.
- `msrt::core`: shared primitive types and packet/frame boundaries.
- `msrt::error`: shared protocol error types and result alias.
- `msrt::reliability`: partial-reliability module boundaries.
- `msrt::engine`: protocol engine boundary that coordinates send, receive, response, and progress.
- `msrt::endpoint`: connection lifecycle helpers that map peers to engine sessions.
- `msrt::integrity`: packet integrity backends configured by `EngineConfig`.
- `msrt::wire`: byte-stream wire envelope boundaries.
The crate enables `std` by default, but it can be built with `default-features = false` so the standard remains portable to MCU environments.
Packet integrity is selected when an `Engine` is initialized. The default `EngineConfig` uses CRC-16/XMODEM. Other backends, such as CRC-32, CRC-64, or lightweight keyed validation, are selected in code through `EngineConfig::integrity`; no runtime configuration file is required by the protocol core.
See [MSRT v1 Stable Protocol Draft](architectures/v1/srt-stable-protocol-draft.md) for the current v1 draft wire and packet model.
See [MSRT v1 Reliable Transport Plan](architectures/v1/srt-reliable-transport-plan.md) for the remaining v1 reliability work.