Add Record Layer documentation

Record Layer Design

===================

This file provides some guidance on the thinking behind the design of the

record layer code to aid future maintenance.

The record layer is divided into a number of components. At the time of writing

there are four: SSL3_RECORD, SSL3_BUFFER, DLTS1_BITMAP and RECORD_LAYER. Each

of these components is defined by:

1) A struct definition of the same name as the component

2) A set of source files that define the functions for that component

3) A set of accessor macros

All struct definitions are in record.h. The functions and macros are either

defined in record.h or record_locl.h dependent on whether they are intended to

be private to the record layer, or whether they form part of the API to the rest

of libssl.

The source files map to components as follows:

dtls1_bitmap.c                                   -> DTLS1_BITMAP component

ssl3_buffer.c                                    -> SSL3_BUFFER component

ssl3_record.c                                    -> SSL3_RECORD component

rec_layer_s23.c, rec_layer_s3.c, rec_layer_d1.c  -> RECORD_LAYER component

The RECORD_LAYER component is a facade pattern, i.e. it provides a simplified

interface to the record layer for the rest of libssl. The other 3 components are

entirely private to the record layer and therefore should never be accessed

directly by libssl.

Any component can directly access its own members - they are private to that

component, e.g. ssl3_buffer.c can access members of the SSL3_BUFFER struct

without using a macro. No component can directly access the members of another

component, e.g. ssl3_buffer cannot reach inside the RECORD_LAYER component to

directly access its members. Instead components use accessor macros, so if code

in ssl3_buffer.c wants to access the members of the RECORD_LAYER it uses the

RECORD_LAYER_* macros.

Conceptually it looks like this:

                        libssl

                           |

---------------------------|-----record.h--------------------------------------

                           |

                    _______V______________

                   |                      |

                   |    RECORD_LAYER      |

                   |                      |

                   |    rec_layer_s23.c   |

                   |          ^           |

                   |          |           |

                   |    rec_layer_s3.c    |

                   |          ^           |

                   | _________|__________ |

                   ||                    ||

                   || DTLS1_RECORD_LAYER ||

                   ||                    ||

                   || rec_layer_d1.c     ||

                   ||____________________||

                   |______________________|

        record_locl.h     ^   ^   ^

         _________________|   |   |_________________

        |                     |                     |

   _____V_________      ______V________      _______V________

  |               |    |               |    |                |

  | SSL3_BUFFER   |    | SSL3_RECORD   |    | DTLS1_BITMAP   |

  |               |--->|               |    |                |

  | ssl3_buffer.c |    | ssl3_record.c |    | dtls1_bitmap.c |

  |_______________|    |_______________|    |________________|

The three RECORD_LAYER source files build progressively on each other, i.e.

the simplest is rec_layer_s23.c. This provides the most basic functions used

for version negotiation. Next rec_layer_s3.c adds the SSL/TLS layer. Finally

rec_layer_d1.c builds off of the SSL/TLS code to provide DTLS specific

capabilities. It uses some DTLS specific RECORD_LAYER component members which

should only be accessed from rec_layer_d1.c. These are held in the

DTLS1_RECORD_LAYER struct.