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f0de3959 MC |
1 | State Machine Design |
2 | ==================== | |
3 | ||
4 | This file provides some guidance on the thinking behind the design of the | |
5 | state machine code to aid future maintenance. | |
6 | ||
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: | |
1dc1ea18 DDO |
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 | |
f0de3959 MC |
27 | |
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. | |
31 | ||
32 | Conceptually the state machine component is designed as follows: | |
33 | ||
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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 | | | | | | |
8b6a7da3 | 59 | | statem_lib.c | | statem_dtls.c | |
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60 | | | | | |
61 | | Non core functions common | | Non core functions common to | | |
62 | | to both servers and clients | | both DTLS servers and clients | | |
63 | |_____________________________| |_______________________________| |