diff --git a/docs/architecture/tls13-experimental.md b/docs/architecture/tls13-experimental.md
index 3db16e0..10cbfa1 100644
--- a/docs/architecture/tls13-experimental.md
+++ b/docs/architecture/tls13-experimental.md
@@ -47,3 +47,22 @@
   Those functions are implemented in `library/ssl_tls13_keys.c` and
   tested in `test_suite_ssl` using test vectors from RFC 8448 and
   https://tls13.ulfheim.net/.
+
+- New TLS Message Processing Stack (MPS)
+
+  The TLS 1.3 prototype is developed alongside a rewrite of the TLS messaging layer,
+  encompassing low-level details such as record parsing, handshake reassembly, and
+  DTLS retransmission state machine.
+
+  MPS has the following components:
+  - Layer 1 (Datagram handling)
+  - Layer 2 (Record handling)
+  - Layer 3 (Message handling)
+  - Layer 4 (Retransmission State Machine)
+  - Reader  (Abstracted pointer arithmetic and reassembly logic for incoming data)
+  - Writer  (Abstracted pointer arithmetic and fragmentation logic for outgoing data)
+
+  Of those components, the following have been upstreamed
+  as part of `MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL`:
+
+  - Reader ([`library/mps_reader.h`](../../library/mps_reader.h))