| 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_locl.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 | |
| |_____________________________| |_______________________________| |
| |
