README
1Record Layer Design
2===================
3
4This file provides some guidance on the thinking behind the design of the
5record layer code to aid future maintenance.
6
7The record layer is divided into a number of components. At the time of writing
8there are four: SSL3_RECORD, SSL3_BUFFER, DLTS1_BITMAP and RECORD_LAYER. Each
9of these components is defined by:
101) A struct definition of the same name as the component
112) A set of source files that define the functions for that component
123) A set of accessor macros
13
14All struct definitions are in record.h. The functions and macros are either
15defined in record.h or record_local.h dependent on whether they are intended to
16be private to the record layer, or whether they form part of the API to the rest
17of libssl.
18
19The source files map to components as follows:
20
21dtls1_bitmap.c -> DTLS1_BITMAP component
22ssl3_buffer.c -> SSL3_BUFFER component
23ssl3_record.c -> SSL3_RECORD component
24rec_layer_s3.c, rec_layer_d1.c -> RECORD_LAYER component
25
26The RECORD_LAYER component is a facade pattern, i.e. it provides a simplified
27interface to the record layer for the rest of libssl. The other 3 components are
28entirely private to the record layer and therefore should never be accessed
29directly by libssl.
30
31Any component can directly access its own members - they are private to that
32component, e.g. ssl3_buffer.c can access members of the SSL3_BUFFER struct
33without using a macro. No component can directly access the members of another
34component, e.g. ssl3_buffer cannot reach inside the RECORD_LAYER component to
35directly access its members. Instead components use accessor macros, so if code
36in ssl3_buffer.c wants to access the members of the RECORD_LAYER it uses the
37RECORD_LAYER_* macros.
38
39Conceptually it looks like this:
40
41 libssl
42 |
43---------------------------|-----record.h--------------------------------------
44 |
45 _______V______________
46 | |
47 | RECORD_LAYER |
48 | |
49 | rec_layer_s3.c |
50 | ^ |
51 | _________|__________ |
52 || ||
53 || DTLS1_RECORD_LAYER ||
54 || ||
55 || rec_layer_d1.c ||
56 ||____________________||
57 |______________________|
58 record_local.h ^ ^ ^
59 _________________| | |_________________
60 | | |
61 _____V_________ ______V________ _______V________
62 | | | | | |
63 | SSL3_BUFFER | | SSL3_RECORD | | DTLS1_BITMAP |
64 | |--->| | | |
65 | ssl3_buffer.c | | ssl3_record.c | | dtls1_bitmap.c |
66 |_______________| |_______________| |________________|
67
68
69The two RECORD_LAYER source files build on each other, i.e.
70the main one is rec_layer_s3.c which provides the core SSL/TLS layer. The second
71one is rec_layer_d1.c which builds off of the SSL/TLS code to provide DTLS
72specific capabilities. It uses some DTLS specific RECORD_LAYER component members
73which should only be accessed from rec_layer_d1.c. These are held in the
74DTLS1_RECORD_LAYER struct.
75