improvment:

when HPAI is 0 until now the stack could not answer (sendbyte unicast fail) due to addr = 0.
not the remoteIP and remotePort are used, when addr = 0.

This may also fix OpenKNX/OAM-IP-Router#6

src/rp2040_arduino_platform.cpp

@@ -337,6 +337,8 @@ int RP2040ArduinoPlatform::readBytesMultiCast(uint8_t* buffer, uint16_t maxLen)
     }
 
     _udp.read(buffer, len);
+    _remoteIP = _udp.remoteIP();
+    _remotePort = _udp.remotePort();
 
     // print("Remote IP: ");
     // print(_udp.remoteIP().toString().c_str());
@@ -349,7 +351,12 @@ int RP2040ArduinoPlatform::readBytesMultiCast(uint8_t* buffer, uint16_t maxLen)
 bool RP2040ArduinoPlatform::sendBytesUniCast(uint32_t addr, uint16_t port, uint8_t* buffer, uint16_t len)
 {
     IPAddress ucastaddr(htonl(addr));
+
+    if(!addr)
+        ucastaddr = _remoteIP;
 
+    if(!port)
+        port = _remotePort;
     // print("sendBytesUniCast to:");
     // println(ucastaddr.toString().c_str());
 

src/rp2040_arduino_platform.h

@@ -131,6 +131,9 @@ class RP2040ArduinoPlatform : public ArduinoPlatform
     #endif
     protected: pin_size_t _rxPin = UART_PIN_NOT_DEFINED; 
     protected: pin_size_t _txPin = UART_PIN_NOT_DEFINED;
+
+    protected: IPAddress _remoteIP = 0;
+    protected: uint16_t _remotePort = 0;
 };
 
 #endif