diff --git a/openbsc/doc/control-interface.txt b/openbsc/doc/control-interface.txt
new file mode 100644
index 0000000..b43cafc
--- /dev/null
+++ b/openbsc/doc/control-interface.txt
@@ -0,0 +1,21 @@
+The protocol for the control interface is wrapped inside the ip.access header
+with the IPAC_PROTO_OSMO protocol ID (0xee). Inside the ip.access header is
+a struct ipaccess_head_ext with protocol ID 0x00 which indicates the control
+interface.
+
+After that the actual protocol is text based:
+
+* Getting the value of a variable
+-> GET <id> <var>
+<- GET_REPLY <id> <var> <val>
+or ERROR <id> <reason>
+
+* Setting the value of a variable
+-> SET <id> <var> <val>
+<- SET_REPLY <id> <var> <val>
+or ERROR <id> <reason>
+
+* A value changes which triggers a trap
+<- TRAP <var> <val>
+
+<id> needs to be unique within a connection. '0' is not allowed