4 This file provides some guidance on the thinking behind the design of the
5 state machine code to aid future maintenance.
7 The state machine code replaces an older state machine present in OpenSSL
8 versions 1.0.2 and below. The new state machine has the following objectives:
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
19 - Separate message flow state from handshake state (in order to better
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
28 The message flow state machine is divided into a reading sub-state machine and a
29 writing sub-state machine. See the source comments in statem.c for a more
30 detailed description of the various states and transitions possible.
32 Conceptually the state machine component is designed as follows:
36 -------------------------|-----statem.h------------------------------------
38 _______V____________________
42 | Core state machine code |
43 |____________________________|
47 _____________|____________ _____________|____________
49 | statem_clnt.c | | statem_srvr.c |
51 | TLS/DTLS client specific | | TLS/DTLS server specific |
52 | state machine code | | state machine code |
53 |__________________________| |__________________________|
54 | |_______________|__ |
55 | ________________| | |
57 ____________V_______V________ ________V______V_______________
59 | statem_lib.c | | statem_dtls.c |
61 | Non core functions common | | Non core functions common to |
62 | to both servers and clients | | both DTLS servers and clients |
63 |_____________________________| |_______________________________|