diff --git a/docs/architecture/tls13-experimental.md b/docs/architecture/tls13-experimental.md
index 0e3c7cc..bcf3e34 100644
--- a/docs/architecture/tls13-experimental.md
+++ b/docs/architecture/tls13-experimental.md
@@ -34,3 +34,7 @@
   tests for the other record protection routines.
 
   TODO: Add some test vectors from RFC 8448.
+
+- The HKDF key derivation function on which the TLS 1.3 key schedule is based,
+  is already present as an independent module controlled by `MBEDTLS_HKDF_C`
+  independently of the development of the TLS 1.3 prototype.