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_both.c               | | statem_dtls.c                 |

  |                             | |                               |

  | Non core functions common   | | Non core functions common to  |

  | to both servers and clients | | both DTLS servers and clients |

  |_____________________________| |_______________________________|