2018-09-13 19:18:07 +00:00
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Record Layer 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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record layer code to aid future maintenance.
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The record layer is divided into a number of components. At the time of writing
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there are four: SSL3_RECORD, SSL3_BUFFER, DLTS1_BITMAP and RECORD_LAYER. Each
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of these components is defined by:
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1) A struct definition of the same name as the component
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2) A set of source files that define the functions for that component
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3) A set of accessor macros
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All struct definitions are in record.h. The functions and macros are either
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2020-03-17 21:27:57 +00:00
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defined in record.h or record_local.h dependent on whether they are intended to
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2018-09-13 19:18:07 +00:00
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be private to the record layer, or whether they form part of the API to the rest
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of libssl.
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The source files map to components as follows:
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dtls1_bitmap.c -> DTLS1_BITMAP component
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ssl3_buffer.c -> SSL3_BUFFER component
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ssl3_record.c -> SSL3_RECORD component
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rec_layer_s3.c, rec_layer_d1.c -> RECORD_LAYER component
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The RECORD_LAYER component is a facade pattern, i.e. it provides a simplified
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interface to the record layer for the rest of libssl. The other 3 components are
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entirely private to the record layer and therefore should never be accessed
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directly by libssl.
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Any component can directly access its own members - they are private to that
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component, e.g. ssl3_buffer.c can access members of the SSL3_BUFFER struct
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without using a macro. No component can directly access the members of another
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component, e.g. ssl3_buffer cannot reach inside the RECORD_LAYER component to
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directly access its members. Instead components use accessor macros, so if code
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in ssl3_buffer.c wants to access the members of the RECORD_LAYER it uses the
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RECORD_LAYER_* macros.
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Conceptually it looks like this:
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libssl
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---------------------------|-----record.h--------------------------------------
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_______V______________
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| RECORD_LAYER |
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| rec_layer_s3.c |
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| ^ |
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| _________|__________ |
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|| ||
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|| DTLS1_RECORD_LAYER ||
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|| ||
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|| rec_layer_d1.c ||
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||____________________||
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|______________________|
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2020-03-17 21:27:57 +00:00
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record_local.h ^ ^ ^
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2018-09-13 19:18:07 +00:00
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_________________| | |_________________
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_____V_________ ______V________ _______V________
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| | | | | |
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| SSL3_BUFFER | | SSL3_RECORD | | DTLS1_BITMAP |
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| |--->| | | |
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| ssl3_buffer.c | | ssl3_record.c | | dtls1_bitmap.c |
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|_______________| |_______________| |________________|
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The two RECORD_LAYER source files build on each other, i.e.
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the main one is rec_layer_s3.c which provides the core SSL/TLS layer. The second
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one is rec_layer_d1.c which builds off of the SSL/TLS code to provide DTLS
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specific capabilities. It uses some DTLS specific RECORD_LAYER component members
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which should only be accessed from rec_layer_d1.c. These are held in the
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DTLS1_RECORD_LAYER struct.
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