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63 lines
3.3 KiB
Text
63 lines
3.3 KiB
Text
State Machine Design
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====================
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This file provides some guidance on the thinking behind the design of the
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state machine code to aid future maintenance.
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The state machine code replaces an older state machine present in OpenSSL
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versions 1.0.2 and below. The new state machine has the following objectives:
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- Remove duplication of state code between client and server
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- Remove duplication of state code between TLS and DTLS
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- Simplify transitions and bring the logic together in a single location
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so that it is easier to validate
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- Remove duplication of code between each of the message handling functions
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- Receive a message first and then work out whether that is a valid
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transition - not the other way around (the other way causes lots of issues
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where we are expecting one type of message next but actually get something
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else)
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- Separate message flow state from handshake state (in order to better
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understand each)
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- message flow state = when to flush buffers; handling restarts in the
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event of NBIO events; handling the common flow of steps for reading a
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message and the common flow of steps for writing a message etc
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- handshake state = what handshake message are we working on now
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- Control complexity: only the state machine can change state: keep all
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the state changes local to the state machine component
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The message flow state machine is divided into a reading sub-state machine and a
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writing sub-state machine. See the source comments in statem.c for a more
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detailed description of the various states and transitions possible.
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Conceptually the state machine component is designed as follows:
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libssl
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---------------------------|-----statem.h--------------------------------------
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_______V____________________
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| statem.c |
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| Core state machine code |
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|____________________________|
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statem_local.h ^ ^
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_________| |_______
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_____________|____________ _____________|____________
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| statem_clnt.c | | statem_srvr.c |
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| TLS/DTLS client specific | | TLS/DTLS server specific |
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| state machine code | | state machine code |
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|__________________________| |__________________________|
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| |_______________|__ |
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| ________________| | |
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____________V_______V________ ________V______V_______________
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| statem_both.c | | statem_dtls.c |
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| Non core functions common | | Non core functions common to |
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| to both servers and clients | | both DTLS servers and clients |
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|_____________________________| |_______________________________|
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