Add documentation on state of upstreaming of TLS 1.3 prototype
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/docs/architecture/tls13-experimental.md b/docs/architecture/tls13-experimental.md
new file mode 100644
index 0000000..ee8452b
--- /dev/null
+++ b/docs/architecture/tls13-experimental.md
@@ -0,0 +1,36 @@
+Experimental TLS 1.3 develpoments
+=================================
+
+Overview
+--------
+
+Mbed TLS doesn't support the TLS 1.3 protocol yet, but a prototype is in development.
+Stable parts of this prototype that can be independently tested are being successively
+upstreamed under the guard of the following macro:
+
+```
+MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
+```
+
+This macro will likely be renamed to `MBEDTLS_SSL_PROTO_TLS1_3` once a minimal viable
+implementation of the TLS 1.3 protocol is available.
+
+See the [documentation of `MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL`](../../include/mbedtls/config.h)
+for more information.
+
+Status
+------
+
+The following lists which parts of the TLS 1.3 prototype have already been upstreamed
+together with their level of testing:
+
+* TLS 1.3 record protection mechanisms
+
+ The record protection routines `mbedtls_ssl_{encrypt|decrypt}_buf()` have been extended
+ to support the modified TLS 1.3 record protection mechanism, including modified computation
+ of AAD, IV, and the introduction of a flexible padding.
+
+ Those record protection routines have unit tests in `test_suite_ssl` alongside the
+ tests for the other record protection routines.
+
+ TODO: Add some test vectors from RFC 8448.