State Machine Design
====================

This file provides some guidance on the thinking behind the design of the
state machine code to aid future maintenance.

The state machine code replaces an older state machine present in OpenSSL
versions 1.0.2 and below. The new state machine has the following objectives:

  - Remove duplication of state code between client and server
  - Remove duplication of state code between TLS and DTLS
  - Simplify transitions and bring the logic together in a single location
    so that it is easier to validate
  - Remove duplication of code between each of the message handling functions
  - Receive a message first and then work out whether that is a valid
    transition - not the other way around (the other way causes lots of issues
    where we are expecting one type of message next but actually get something
    else)
  - Separate message flow state from handshake state (in order to better
    understand each)
    * message flow state = when to flush buffers; handling restarts in the
      event of NBIO events; handling the common flow of steps for reading a
      message and the common flow of steps for writing a message etc
    * handshake state = what handshake message are we working on now
  - Control complexity: only the state machine can change state: keep all
    the state changes local to the state machine component

The message flow state machine is divided into a reading sub-state machine and a
writing sub-state machine. See the source comments in statem.c for a more
detailed description of the various states and transitions possible.

Conceptually the state machine component is designed as follows:

                          libssl
                             |
    -------------------------|-----statem.h------------------------------------
                             |
                      _______V____________________
                     |                            |
                     |    statem.c                |
                     |                            |
                     |    Core state machine code |
                     |____________________________|
          statem_local.h     ^          ^
                   _________|          |_______
                  |                            |
     _____________|____________   _____________|____________
    |                          | |                          |
    | statem_clnt.c            | | statem_srvr.c            |
    |                          | |                          |
    | TLS/DTLS client specific | | TLS/DTLS server specific |
    | state machine code       | | state machine code       |
    |__________________________| |__________________________|
                 |        |_______________|__       |
                 |        ________________|  |      |
                 |       |                   |      |
     ____________V_______V________   ________V______V_______________
    |                             | |                               |
    | statem_lib.c                | | statem_dtls.c                 |
    |                             | |                               |
    | Non core functions common   | | Non core functions common to  |
    | to both servers and clients | | both DTLS servers and clients |
    |_____________________________| |_______________________________|