# 33 — Transport

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We covered the standards-level view of HSMS and SECS-I in
chapters 11 and 12.  This chapter drops down into the
implementation: how `hsms::Connection` actually moves bytes,
the asio executor model, the single-threaded strand contract,
and why the transport layer doesn't need locks.

---

## The two transport modules

```
include/secsgem/hsms/
├── header.hpp        — Frame format primitives (length prefix, header, SType).
└── connection.hpp    — One-socket session manager + T-timers + S9 emission.

include/secsgem/secsi/
├── header.hpp        — 10-byte SECS-I block header.
├── block.hpp         — Block split / assemble (multi-block messages).
├── protocol.hpp      — IO-free line-turnaround FSM.
└── tcp_transport.hpp — asio TCP wrapper around the FSM (tunnel for testing).
```

Each module owns one TCP endpoint (or in the SECS-I case, a tunnel
endpoint).  Both are **single-threaded by design**.

---

## hsms::Connection — top to bottom

### Lifecycle

```cpp
// apps/secs_server.cpp — passive-equipment startup
asio::io_context io;

asio::ip::tcp::acceptor acc(io, asio::ip::tcp::endpoint{asio::ip::tcp::v4(), port});

acc.async_accept([&](std::error_code ec, asio::ip::tcp::socket sock) {
  auto conn = std::make_shared<hsms::Connection>(
      std::move(sock), Mode::Passive, /*device_id=*/0, timers);
  conn->set_message_handler(...);
  conn->set_closed_handler(...);
  conn->start();
});

io.run();   // blocks until all work is done
```

`Connection::start()` either:

- **Passive** — arms T7 (waiting for Select.req) and starts the
  read loop.
- **Active** — initiates the Select.req exchange, then starts the
  read loop.

### Read path

Three async steps repeated forever:

```
async_read(socket, 4 bytes) → on_length()
async_read(socket, length bytes) → on_payload()
handle_frame(decoded Frame) → dispatch
```

In code, [`src/hsms/connection.cpp`](../src/hsms/connection.cpp):

```cpp
void Connection::read_length() {
  asio::async_read(socket_, asio::buffer(len_buf_, 4),
      [self = shared_from_this()](std::error_code ec, std::size_t n) {
        self->on_length(ec, n);
      });
}

void Connection::on_length(std::error_code ec, std::size_t n) {
  if (ec) return close("read_length");
  uint32_t len = decode_be32(len_buf_);
  payload_.resize(len);
  asio::async_read(socket_, asio::buffer(payload_),
      [self = shared_from_this()](std::error_code ec, std::size_t n) {
        self->on_payload(ec, n);
      });
}
```

Each callback is on the socket's executor.  No locks because
nothing else can be touching the read state — by construction.

### Write path

A send queue + one outstanding `async_write`:

```cpp
void Connection::send_frame(Frame frame) {
  send_queue_.push_back(std::move(frame));
  if (send_queue_.size() == 1) write_next();
}

void Connection::write_next() {
  auto& frame = send_queue_.front();
  send_buf_ = frame.encode();
  asio::async_write(socket_, asio::buffer(send_buf_),
      [self = shared_from_this()](std::error_code ec, std::size_t) {
        self->send_queue_.pop_front();
        if (ec) return self->close("write");
        if (!self->send_queue_.empty()) self->write_next();
      });
}
```

Same single-threaded discipline — `send_queue_` is only touched
on the executor.  Callers from other threads must `asio::post`.

### Timers

Five `asio::steady_timer`s, one per HSMS T-timer:

```cpp
// src/hsms/connection.cpp:30
Connection::Connection(...)
  : socket_(std::move(sock)),
    t3_timer_(socket_.get_executor()),
    t6_timer_(socket_.get_executor()),
    t7_timer_(socket_.get_executor()),
    t8_timer_(socket_.get_executor()),
    linktest_timer_(socket_.get_executor()),
    timers_(timers) { }
```

All five share the socket's executor.  When a timer fires, its
handler runs on the same executor as the read/write loop — so
again no locks for timer-vs-IO interaction.

T3 is special: there's one T3 timer per in-flight W=1 message
(correlated by `system_bytes`).  These are short-lived
`steady_timer`s allocated when the request is sent and destroyed
when the reply arrives.  See `src/hsms/connection.cpp:447`:

```cpp
auto t3 = std::make_shared<asio::steady_timer>(socket_.get_executor());
t3->expires_after(timers_.t3);
in_flight_.insert({system_bytes, RequestState{std::move(cb), t3, ...}});
t3->async_wait([self, system_bytes](std::error_code ec) {
  if (ec) return;  // cancelled (reply arrived)
  self->on_t3_expire(system_bytes);
});
```

---

## The asio executor / strand model

All `Connection` state lives on **one executor**.  In simple cases
that's just `io_context.get_executor()` — a single-threaded loop.
In production, an EAP may run multiple `io_context::run()` threads
*per connection* by wrapping work in an `asio::strand`.

### What a strand is

A **strand** is an executor that guarantees mutual exclusion
between handlers it dispatches.  Multiple threads can call
`io.run()`; the strand picks one of them at a time to run the
next handler in its queue.

For HSMS purposes, `socket_.get_executor()` already gives strand
semantics if the underlying `io_context` is single-threaded.  For
multi-threaded `io_context`, the application wraps with
`asio::make_strand`:

```cpp
auto strand = asio::make_strand(io);
asio::ip::tcp::socket sock(strand);
auto conn = std::make_shared<hsms::Connection>(std::move(sock), ...);
```

Now `conn` has all its IO running on `strand`, while the
`io_context` can use 8 threads to handle 100 different connections.

### What this means for the caller

From any thread that isn't the strand's currently-running handler,
the caller MUST marshal onto the strand:

```cpp
// From a sensor-thread callback:
asio::post(conn->executor(), [conn, msg = std::move(msg)] {
  conn->send_data(std::move(msg));
});
```

Calling `conn->send_data` directly from another thread is **a
race**.  Same for any store mutation.  TSan catches this and the
test suite enforces it.

The contract is documented in detail in
[`docs/INTEGRATION.md`](INTEGRATION.md) §3.

---

## secsi::Protocol — the IO-free FSM

SECS-I's protocol layer is structured differently: the FSM has
**no IO at all**.  It takes events (bytes received, application
asked to send, timer fired) and produces a list of `Action`s
(transmit these bytes, arm a timer, deliver a block to the
application).

```cpp
// include/secsgem/secsi/protocol.hpp
struct ActionTransmit       { std::vector<uint8_t> bytes; };
struct ActionStartTimer     { Timer which; };
struct ActionCancelTimer    { Timer which; };
struct ActionDeliverBlock   { Block block; };
struct ActionRaiseError     { std::string reason; };
```

The wrapper (`secsi::TcpTransport`) drives the FSM:

```cpp
void TcpTransport::on_byte(uint8_t b) {
  auto actions = protocol_.handle(EventByte{b});
  for (const auto& a : actions) execute(a);
}

void TcpTransport::execute(const Action& a) {
  std::visit([this](auto&& v) {
    using T = std::decay_t<decltype(v)>;
    if constexpr (std::is_same_v<T, ActionTransmit>)    write_bytes(v.bytes);
    else if constexpr (std::is_same_v<T, ActionStartTimer>)   arm_timer(v.which);
    else if constexpr (std::is_same_v<T, ActionCancelTimer>)  cancel_timer(v.which);
    else if constexpr (std::is_same_v<T, ActionDeliverBlock>) deliver(v.block);
    else if constexpr (std::is_same_v<T, ActionRaiseError>)   raise(v.reason);
  }, a);
}
```

This design makes the **whole FSM** unit-testable.  No sockets,
no timers, just `Event` in → `Action` out.  Tests:

- [`tests/test_secsi.cpp`](../tests/test_secsi.cpp) — basic FSM
  state walks.
- [`tests/test_secsi_timers.cpp`](../tests/test_secsi_timers.cpp)
  — every timer scenario via synthetic `EventTimeout` injection.
- [`tests/test_secsi_tcp.cpp`](../tests/test_secsi_tcp.cpp) —
  end-to-end via `TcpTransport`.

Same pattern repeats for E84 (chapter 18) and the GEM
communication-state FSM (chapter 13): IO-free FSM + asio adapter
+ separate test suites for each layer.

---

## Why this is the right shape

### Pros of single-threaded + IO-free

- **No mutexes.**  Anywhere.
- **Trivial reasoning.**  When you read `Connection::send_frame`,
  you can be sure nothing else is mutating the queue.
- **Fast.**  No lock contention, no atomic round-trips.
- **Testable.**  IO-free FSM lets you exercise every transition
  without IO.

### Cons

- **Callers must know about the strand.**  Multi-threaded
  applications need `asio::post` boilerplate.
- **One slow handler blocks the rest.**  A 100 ms handler delays
  every other message until it returns.  Fix: don't write slow
  handlers; if you must, dispatch the slow work to another
  executor and return immediately.

For a SECS/GEM equipment runtime — where the natural shape is "one
TCP socket, one event loop" — the pros far outweigh the cons.

---

## Where to go next

You've now seen the bottom layer in detail: how bytes move,
how state machines drive transitions without IO, how the single-
threaded contract makes everything safe.  Next chapter goes back
up one level: **the codec** — the encoder/decoder that turns
the Item type from chapter 10 into wire bytes, plus the SML
human-readable form.

Next: [→ 34 Codec and SML](34_codec_and_sml.md)