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