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