diff --git a/acn/pdu.h b/acn/pdu.h
index 62008b8..4c94c5d 100644
--- a/acn/pdu.h
+++ b/acn/pdu.h
@@ -62,52 +62,65 @@ struct pdu_flags
  * @brief The pdu_header struct
  */
 struct pdu_header : public pdu_stream_object {};
-
+// C++20
+//template <class T>
+//concept header = std::is_base_of<pdu_header, T>::value;
 
 /**
  * @brief The pdu_data struct
  */
 struct pdu_data : public pdu_stream_object {};
+// C++20
+//template <class T>
+//concept data = std::is_base_of<pdu_data, T>::value;
 
+// C++20
+//template <class T>
+//concept pdu = std::is_base_of<Pdu, T>::value;
 
 /**
  * @brief PDU::pdu_data subclass that encapsulates other PDU.
  * @tparam T PDU decendant subclass
  */
-template <typename T>
+//template<pdu T> // C++20
+template<class T>
 struct Block
     : public pdu_data
 {
-    std::shared_ptr<std::vector<std::shared_ptr<T>>> pdu
-        = std::shared_ptr<std::vector<std::shared_ptr<T>>>
+  Block() {
+    static_assert(std::is_base_of<Pdu, T>::value,
+        "type parameter of ACN::PDU::Block must derive from ACN::PDU::Pdu");
+  }
+  std::shared_ptr<std::vector<std::shared_ptr<T>>> pdu
+                             = std::shared_ptr<std::vector<std::shared_ptr<T>>>
                                           (new std::vector<std::shared_ptr<T>>);
-    void setParent(std::shared_ptr<Pdu> parent) {
-      for (auto p : *pdu)
-        p->setParent(parent);
-    }
-    size_t streamSize() const override {
-      size_t s = 0;
-      for (auto &child : *pdu)
-        s += child->streamSize();
-      return s;
-    }
-    void iStream(Stream s) override {
-      while(s->good()) {
-          std::shared_ptr<T> p(new T());
-          p->iStream(s);
-          if (s->fail())              // stream failed during pdu read
-            break;
-          if (p->stream()->fail())    // pdu buffer failed
-            continue;
-          if (!pdu->empty())          // set inheritee
-            p->setInherit(pdu->back());
-          pdu->push_back(p);          // add to block
-        }
-    }
-    void oStream(Stream s) const override {
-      for ( const auto & child : *pdu )
-        child->oStream(s);
-    };
+  void setParent(std::shared_ptr<Pdu> parent) {
+    for (auto p : *pdu)
+      p->setParent(parent);
+  }
+  size_t streamSize() const override {
+    size_t s = 0;
+    for (auto &child : *pdu)
+      s += child->streamSize();
+    return s;
+  }
+  void iStream(Stream s) override {
+    while(s->good()) {
+        std::shared_ptr<T> p(new T());
+        p->iStream(s);
+        if (s->fail())              // stream failed during pdu read
+          break;
+        if (p->stream()->fail())    // pdu buffer failed
+          continue;
+        if (!pdu->empty())          // set inheritee
+          p->setInherit(pdu->back());
+        pdu->push_back(p);          // add to block
+      }
+  }
+  void oStream(Stream s) const override {
+    for ( const auto & child : *pdu )
+      child->oStream(s);
+  };
 };
 
 
@@ -149,6 +162,7 @@ public:
   void setInherit(std::shared_ptr<Pdu> pdu) {inherit_ = pdu;}
 
   // protocol payloads
+//template<header T> // C++20
   template<class T>
   void createHeader()
   {
@@ -159,6 +173,8 @@ public:
           header_->iStream(stream_);
       }
   }
+
+//template<data T> // C++20
   template<class T>
   void createData()
   {
@@ -169,6 +185,7 @@ public:
           data_->iStream(stream_);
       }
   }
+//template<pdu T> // C++20
   template<class T>
   void createDataBlock() {
     auto block = new PDU::Block<T>();
@@ -194,6 +211,7 @@ protected:
  * @brief Callback that understands how to proccess a PDU type.
  * @tparam T PDU decendant subclass
  */
+//template<pdu T> // C++20
 template <class T>
 using Handler = std::function<void(std::shared_ptr<T>)>;