diff --git a/core/sender/state.go b/core/sender/state.go
new file mode 100644
index 0000000..339ba86
--- /dev/null
+++ b/core/sender/state.go
@@ -0,0 +1,64 @@
+package sender
+
+import "fmt"
+
+type TransferState int
+
+const (
+	Initial TransferState = iota
+	WaitForFileRequest
+	SendingData
+	WaitForFileAck
+	WaitForCloseMessage
+	WaitForCloseAck
+)
+
+// UIUpdate is a struct that is continously communicated to the UI (if sender has attached a UI)
+type UIUpdate struct {
+	State    TransferState
+	Progress float32
+}
+
+// WrongStateError is a custom error for the Transfer sequence
+type WrongStateError struct {
+	expected TransferState
+	got      TransferState
+}
+
+// WrongStateError constructor
+func NewWrongStateError(expected, got TransferState) *WrongStateError {
+	return &WrongStateError{
+		expected: expected,
+		got:      got,
+	}
+}
+
+func (e *WrongStateError) Error() string {
+	return fmt.Sprintf("wrong message type, expected type: %d(%s), got: %d(%s)", e.expected, e.expected.Name(), e.got, e.got.Name())
+}
+
+// Name returns the associated to the state enum.
+func (s TransferState) Name() string {
+	switch s {
+		case Initial:
+			return "Initial"
+
+		case WaitForFileRequest:
+			return "WaitForFileRequest"
+
+		case SendingData:
+			return "SendingData"
+
+		case WaitForFileAck:
+			return "WaitForFileAck"
+
+		case WaitForCloseMessage:
+			return "WaitForCloseMessage"
+
+		case WaitForCloseAck:
+			return "WaitForCloseAck"
+
+		default:
+			return ""
+	}
+}