extmod/modtls_mbedtls: Wire in support for DTLS.

This commit enables support for DTLS, i.e. TLS over datagram transport
protocols like UDP.  While support for DTLS is absent in CPython, it is
worth supporting it in MicroPython because it is the basis of the
ubiquitous CoAP protocol, used in many IoT projects.

To select DTLS, a new set of "protocols" are added to SSLContext:
- ssl.PROTOCOL_DTLS_CLIENT
- ssl.PROTOCOL_DTLS_SERVER

If one of these is set, the library assumes that the underlying socket is a
datagram-like socket (i.e. UDP or similar).

Our own timer callbacks are implemented because the out of the box
implementation relies on `gettimeofday()`.

This new DTLS feature is enabled on all ports that use mbedTLS.

This commit is an update to a previous PR #10062.

Addresses issue #5270 which requested DTLS support.

Signed-off-by: Keenan Johnson <keenan.johnson@gmail.com>

2 files changed, 54 insertions, 0 deletions

diff --git a/tests/extmod/tls_dtls.py b/tests/extmod/tls_dtls.py
new file mode 100644
index 000000000..b2d716769
--- /dev/null
+++ b/tests/extmod/tls_dtls.py
@@ -0,0 +1,51 @@
+# Test DTLS functionality including timeout handling
+
+try:
+    from tls import PROTOCOL_DTLS_CLIENT, PROTOCOL_DTLS_SERVER, SSLContext, CERT_NONE
+    import io
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+class DummySocket(io.IOBase):
+    def __init__(self):
+        self.write_buffer = bytearray()
+        self.read_buffer = bytearray()
+
+    def write(self, data):
+        return len(data)
+
+    def readinto(self, buf):
+        # This is a placeholder socket that doesn't actually read anything
+        # so the read buffer is always empty.
+        return None
+
+    def ioctl(self, req, arg):
+        if req == 4:  # MP_STREAM_CLOSE
+            return 0
+        return -1
+
+
+# Create dummy sockets for testing
+server_socket = DummySocket()
+client_socket = DummySocket()
+
+# Wrap the DTLS Server
+dtls_server_ctx = SSLContext(PROTOCOL_DTLS_SERVER)
+dtls_server_ctx.verify_mode = CERT_NONE
+dtls_server = dtls_server_ctx.wrap_socket(server_socket, do_handshake_on_connect=False)
+print("Wrapped DTLS Server")
+
+# Wrap the DTLS Client
+dtls_client_ctx = SSLContext(PROTOCOL_DTLS_CLIENT)
+dtls_client_ctx.verify_mode = CERT_NONE
+dtls_client = dtls_client_ctx.wrap_socket(client_socket, do_handshake_on_connect=False)
+print("Wrapped DTLS Client")
+
+# Trigger the timing check multiple times with different elapsed times
+for i in range(10):  # Try multiple iterations to hit the timing window
+    dtls_client.write(b"test")
+    data = dtls_server.read(1024)  # This should eventually hit the timing condition
+
+print("OK")
diff --git a/tests/extmod/tls_dtls.py.exp b/tests/extmod/tls_dtls.py.exp
new file mode 100644
index 000000000..78d72bff1
--- /dev/null
+++ b/tests/extmod/tls_dtls.py.exp
@@ -0,0 +1,3 @@
+Wrapped DTLS Server
+Wrapped DTLS Client
+OK