package mimic-happy-eyeballs

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A happy-eyeballs integration into mimic


Dune Dependency






A happy-eyeballs integration into mimic for MirageOS

Published: 30 Nov 2022


Mimic, a full-abstract way to instantiate a transmission protocol

mimic is a small project which gives you the opportunity to instantiate a transmission protocol - such as a TCP/IP connection - from dynamic values. A simple tutorial is available here. It explains to implement a ping-pong protocol and upgrade it to TLS.

Some examples

git or paf are examples where they use mimic as the only transmission protocol implementation available. It permits to be compatible with MirageOS without the complexity of functors (commonly used with functoria to unlock the possibility to abstract anything).


mimic is pretty-small (~ 700 lines) and the API wants to fit into several different contexts (HTTP, TLS or SSH). It's possible to make helpers from it such as some derivations for unix or mirage - as we commonly designed for [conduit][conduit]. However, with a big retro-spective, such piece of code should not include these derivations.

Indeed, they give an opportunity to the user to assert a non-compaibility with MirageOS if you use the unix derivation for example.

mimic wants to be abstract and "simple". Then, the user is able to construct something more complex and easy to use at his level - and it's what paf does for example or git-unix.

The goal of mimic

In the context of MirageOS which has a first stage which decides implementation of protocols according to arguments let us to provide a client which can work on many contexts:

  • as a simple executable which can use the host TCP/IP stack

  • as a full operating system which integrate its own TCP/IP stack

  • as something else which wants to use something else than the TCP/IP stack

That mostly means that, de facto, we can not assert a certain implementation of the underlying transmission protocol used by a protocol such as HTTP or SMTP. This required abstraction becomes more complexe when we start to think about composition of protocols (such as TCP/IP and TLS for instance).

This abstraction, in the context of a client, is not only determined by a static application of functors with our implementations. It depends on an user's input value which will choose the right transmission protocol. For instance:

  • expects TCP/IP + SSH

  • expects TCP/IP + HTTP

  • git:// expects TCP/IP

  • expects TCP/IP + TLS + HTTP

mimic gives the opportunity to provide a full implementation of the Mirage_flow.S interface and require a function to instantiate the given transmission protocol (which respects our interface). By this way and according to user's input values, mimic is able to choose an try to instantiate a certain transmission protocol and hide it into an not-fully abstracted type Mimic.flow.

It unlock the ability to implement a protocol such as the Git protocol - or something else such as the HTTP protocol. By this way, this implementation is, de facto compatible with MirageOS in any contexts. In the case of MirageOS, a simple registration of available transmission protocols via functoria is enough. For a more concrete usage such as the Unix usage, a derivation of your protocol with unix and a registration by default of some transmission protocols is enough too. The main difference is:

  • one is leaded by arguments of the user (and functoria)

  • the second is established by the developer

The result of the mimic's usage

More practically, in the MirageOS world, a device can not provide via its interface the connect function but it must implement it let write the functoria glue to to let it to call the connect function with available arguments (from the command-line).

For instance, a device can be described with this interface:

module type S = sig
  type t

  val read : t -> buffer
  val write : t -> buffer -> unit

And its implementation can be described with:

module TCP : sig
  include S

  val connect : ipv4 -> t

That mostly mean that, inside the which is your application, you don't have an access to the connect function:

module Make (My_device : Device.S) = struct
  let start (t : My_device.t) =

A hot-connect can not be available into the interface for a specific reason: the abstraction. Arguments required to connect/allocate a resource which represents our device depend on the implementation. As we said earlier, ocaml-tls expects a Tls.Config.client where Lwt_ssl expects an Ssl.context. It can be difficult to shape these values into an ultimate type (which is, of course, non-exhaustive from possible TLS implementations).

Mimic wants to provide this hot-connect function into your application (inside the without a static dependency to ocaml-tls or lwt_ssl à priori. Then, the functoria/mirage tool will choose right dependency according to the command-line invokation and produce the glue needed to be able to hot-connect a TLS connection over TCP/IP.

Reverse dependencies

mimic must be thought according to who use it. The API is not designed to be canonic and usable as is. It has been thought to unlock the full abstraction and the compatibility with MirageOS for others projects.

If you think that you can have an usage of mimic and something is missing, you should implement what you want outside mimic.

The Mirage_flow.S interface

Finally, the only assumption about design of protocols, transmission protocols, etc. is Mirage_flow.S. Several issues exist about this interface but the cost to upgrade the interface (to be unix-friendly for example) is huge when several MirageOS projects trust on this specific interface.


mimic can be hard to explain when we don't know all details about the MirageOS eco-system. The existence of this project can be critized when we don't really understand all details and how this project fits in.

The documentation is not very clear and does not explain the big-picture of mimic. So it's a real issue and the tutorial wants to fix it but my lack of English does not help me.

Dependencies (4)

  1. happy-eyeballs-mirage >= "0.3.0" & < "1.0.0"
  2. mimic = version
  3. dune >= "2.8"
  4. ocaml >= "4.08.0"

Dev Dependencies


Used by (2)

  1. git-mirage = "3.7.1" | >= "3.8.1"
  2. http-mirage-client < "0.0.6"




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