1State Machine Design 2==================== 3 4This file provides some guidance on the thinking behind the design of the 5state machine code to aid future maintenance. 6 7The state machine code replaces an older state machine present in OpenSSL 8versions 1.0.2 and below. The new state machine has the following objectives: 9 10 - Remove duplication of state code between client and server 11 - Remove duplication of state code between TLS and DTLS 12 - Simplify transitions and bring the logic together in a single location 13 so that it is easier to validate 14 - Remove duplication of code between each of the message handling functions 15 - Receive a message first and then work out whether that is a valid 16 transition - not the other way around (the other way causes lots of issues 17 where we are expecting one type of message next but actually get something 18 else) 19 - Separate message flow state from handshake state (in order to better 20 understand each) 21 * message flow state = when to flush buffers; handling restarts in the 22 event of NBIO events; handling the common flow of steps for reading a 23 message and the common flow of steps for writing a message etc 24 * handshake state = what handshake message are we working on now 25 - Control complexity: only the state machine can change state: keep all 26 the state changes local to the state machine component 27 28The message flow state machine is divided into a reading sub-state machine and a 29writing sub-state machine. See the source comments in statem.c for a more 30detailed description of the various states and transitions possible. 31 32Conceptually the state machine component is designed as follows: 33 34 libssl 35 | 36 -------------------------|-----statem.h------------------------------------ 37 | 38 _______V____________________ 39 | | 40 | statem.c | 41 | | 42 | Core state machine code | 43 |____________________________| 44 statem_local.h ^ ^ 45 _________| |_______ 46 | | 47 _____________|____________ _____________|____________ 48 | | | | 49 | statem_clnt.c | | statem_srvr.c | 50 | | | | 51 | TLS/DTLS client specific | | TLS/DTLS server specific | 52 | state machine code | | state machine code | 53 |__________________________| |__________________________| 54 | |_______________|__ | 55 | ________________| | | 56 | | | | 57 ____________V_______V________ ________V______V_______________ 58 | | | | 59 | statem_lib.c | | statem_dtls.c | 60 | | | | 61 | Non core functions common | | Non core functions common to | 62 | to both servers and clients | | both DTLS servers and clients | 63 |_____________________________| |_______________________________| 64