The core of `ocaml-tls` (the opam package `tls`, available in the `lib`

subdirectory) is an library independent of schedulers and does not perform any

I/O operations. The library is designed so that a `Tls.Engine.state` state

informs you of when to write and when to feed more data. It does not use

mutation and is in a value-passing style (so, read data and state is the input,

and data to be sent or presented to the upper layer, and state is the output).

There are therefore `ocaml-tls` derivations with different schedulers that

perform read and write operations. These derivations offer an interface similar

to what an SSL socket (like [ssl][ssl]) can offer.

- [lwt](https://ocsigen.org/lwt/latest/manual/manual): `tls-lwt` proposes to

initiate a TLS flow with `Lwt_io.{input,output}_channel` from a Unix socket.

It can also propose an abstract type `Tls_lwt.Unix.t` (which can be created

from a Unix socket) associated with a `Tls_lwt.Unix` interface similar to a

Unix socket.

- [miou](https://github.com/robur-coop/miou): `tls-miou-unix` proposes a TLS

flow via an abstract type `Tls_miou_unix.t` and an interface similar to a Unix

socket from a `Miou_unix.file_descr` socket.

- [MirageOS](https://mirageos.org): `tls-mirage` proposes a composition

of a [`Mirage_flow.S`](https://github.com/mirage/mirage-flow/) module to

obtain a new `Mirage_flow.S` (corresponding to the TLS layer) which uses the

lwt scheduler.

- [eio](https://github.com/ocaml-multicore/eio): `tls-eio` proposes the creation

of an _eio flow_ from another _eio flow_.

- [async](https://github.com/janestreet/async): `tls-async` proposes a TLS flow

via `Async.{Reader,Writer}.t` from a `Async.Socket`.

Depending on the scheduler you choose, you should choose one of these

`ocaml-tls` derivations, distributed in the mentioned opam packages (tls-lwt,

tls-mirage, tls-eio, tls-async). Each one takes advantage of what the scheduler

used has to offer.

`ocaml-tls` can also be used as it is in order to be able to compose with other

protocols without choosing a scheduler. This is the case, for example, with

[sendmail.starttls][sendmail], which composes the SMTP and TLS protocols. The

user can also be more selective about the use of certificates involved in a TLS

connection, as [albatross][albatross] can offer in its transactions between

clients and the server.

When seen as OCaml values, the critical elements that enable instantiation of a

TLS connection can be very finely controlled.

ocaml-tls is currently used for [MirageOS unikernels](https://mirageos.org),

which makes it portable and available on many systems (even the most restricted

ones such as [Solo5](https://github.com/solo5/solo5) as long as OCaml is

available on them.

[sendmail]: https://github.com/mirage/colombe

[albatross]: https://github.com/robur-coop/albatross

[ssl]: https://github.com/savonet/ocaml-ssl