diff --git a/include/rawrtcdc/external.h b/include/rawrtcdc/external.h
new file mode 100644
index 0000000..7e79063
--- /dev/null
+++ b/include/rawrtcdc/external.h
@@ -0,0 +1,99 @@
+#pragma once
+#include <rawrtcc/code.h>
+#include <re.h>
+
+/**
+ * External DTLS role.
+ */
+enum rawrtc_external_dtls_role {
+    RAWRTC_EXTERNAL_DTLS_ROLE_CLIENT,
+    RAWRTC_EXTERNAL_DTLS_ROLE_SERVER,
+};
+
+/**
+ * External DTLS transport state.
+ */
+enum rawrtc_external_dtls_transport_state {
+    RAWRTC_EXTERNAL_DTLS_TRANSPORT_STATE_NEW_OR_CONNECTING,
+    RAWRTC_EXTERNAL_DTLS_TRANSPORT_STATE_CONNECTED,
+    RAWRTC_EXTERNAL_DTLS_TRANSPORT_STATE_CLOSED_OR_FAILED,
+};
+
+/**
+ * DTLS role getter.
+ *
+ * `*rolep` will contain the current external DTLS role.
+ * `arg` is the argument passed to the SCTP transport context.
+ *
+ * Return `RAWRTC_CODE_SUCCESS` in case the role has been set or any
+ * other code in case of an error.
+ */
+typedef enum rawrtc_code (*rawrtc_dtls_role_getter)(
+    enum rawrtc_external_dtls_role* const rolep,  // de-referenced
+    void* const arg);
+
+/**
+ * DTLS transport state getter.
+ *
+ * `*statep` will contain the current external DTLS transport state.
+ * `arg` is the argument passed to the SCTP transport context.
+ *
+ * Return `RAWRTC_CODE_SUCCESS` in case the state has been set or any
+ * other code in case of an error.
+ */
+typedef enum rawrtc_code (*rawrtc_dtls_transport_state_getter)(
+    enum rawrtc_external_dtls_transport_state* const statep,  // de-referenced
+    void* const arg);
+
+/**
+ * SCTP transport outbound data handler.
+ *
+ * `buffer` contains the data to be fed to the DTLS transport.
+ * Note that the `mbuf` structure shall not be `mem_ref`ed or
+ * `mem_deref`ed since it hasn't been allocated properly for
+ * optimisation purposes. This has been done since we expect you to
+ * either send this data directly or drop it. There's no need to hold
+ * data back. If you for any reason need the data after the callback
+ * returned, you are required to copy it.
+ * `tos` contains the type of service field as reported by usrsctp.
+ * `set_df` TODO: Probably don't fragment bit? Dunno...
+ *
+ * Return `RAWRTC_CODE_SUCCESS` in case the packet has been sent (or
+ * dropped) or any other code in case of an error.
+ */
+typedef enum rawrtc_code (*rawrtc_sctp_transport_outbound_handler)(
+    struct mbuf* const buffer, uint8_t const tos, uint8_t const set_df, void* const arg);
+
+/**
+ * SCTP transport detach handler.
+ * Will be called when the SCTP transport is about to be closed and
+ * should be detached from the underlying DTLS transport. At this
+ * point, no further data should be fed to the SCTP transport.
+ *
+ * `arg` is the argument passed to the SCTP transport context.
+ */
+typedef void (*rawrtc_sctp_transport_detach_handler)(void* const arg);
+
+/**
+ * SCTP transport destroyed handler.
+ * Will be called when the SCTP transport is about to be free'd.
+ *
+ * Note: This handler only exists for cleanup purposes. You may not use
+ *       any of the transport's functions at this point.
+ *
+ * `arg` is the argument passed to the SCTP transport context.
+ */
+typedef void (*rawrtc_sctp_transport_destroyed_handler)(void* const arg);
+
+/**
+ * SCTP transport context.
+ */
+struct rawrtc_sctp_transport_context {
+    rawrtc_dtls_role_getter role_getter;
+    rawrtc_dtls_transport_state_getter state_getter;
+    rawrtc_sctp_transport_outbound_handler outbound_handler;
+    rawrtc_sctp_transport_detach_handler detach_handler;  // nullable
+    rawrtc_sctp_transport_destroyed_handler destroyed_handler;  // nullable
+    bool trace_packets;
+    void* arg;  // nullable
+};