more practicial framework now

src/forwarder.hpp

 #ifndef UDP_FORWARDER_DYN_FORWARDER_HPP_
 #define UDP_FORWARDER_DYN_FORWARDER_HPP_
 
-#include <unordered_map>
 #include <rlib/sys/sio.hpp>
 
 #include "utils.hpp"
@@ -10,14 +9,7 @@
 
 using std::string;
 
-struct ConnectionMapping {
-    std::unordered_map<string, fd_t> client2server;
-    std::unordered_multimap<fd_t, string> server2client;
-    static string clientInfoAsKey(string ip, uint16_t port) {
-        // Also works for ipv6. We just want to eliminate duplication, rather than make it easy to read. 
-        return ip + '@' + port;
-    }
-};
+
 
 class Forwarder {
 public:

src/protocols/plain.hpp

@@ -34,11 +34,36 @@ namespace Protocols {
 			epoll_event events[MAX_EVENTS];
 			char buffer[DGRAM_BUFFER_SIZE];
 			// WARN: If you want to modify this program to work for both TCP and UDP, PLEASE use rlib::sockIO::recv instead of fixed buffer.
-			
+
+			auto onEvent = [&](auto activeFd) {
+				if (activeFd == ipcPipeInboundEnd) {
+					// Outbound gave me a message to forward! Send it. 
+					auto targetClientId = rlib::sockIO::recv_msg(activeFd);
+					auto msg = rlib::sockIO::recv_msg(activeFd);
+
+					auto [clientAddr, clientPort] = ConnectionMapping::parseClientId(targetClientId);
+
+
+				}
+				else if (activeFd == listenFd) {
+					SockAddr clientAddr;
+					auto ret = recvfrom(activeFd, buffer, sizeof(buffer), 0, &clientAddr.addr, &clientAddr.len);
+					if (ret == -1) throw std::runtime_error("recvfrom failed. ");
+
+
+				}
+
+			};
+
 			rlog.info("PlainListener listening InboundPort [{}]:{} ...", listenAddr, listenPort);
 			while (true) {
-				// ...
-				// epoll
+				auto nfds = epoll_wait(epollFd, events, MAX_EVENTS, -1);
+				if (nfds == -1)
+					throw std::runtime_error("epoll_wait failed.");
+				
+				for (auto cter = 0; cter < nfds; ++cter) {
+					onEvent(events[cter].data.fd);
+				}
 			}
 
 		}

src/utils.hpp

@@ -4,6 +4,7 @@
 #include <rlib/sys/os.hpp>
 #include <rlib/sys/sio.hpp>
 #include <thread>
+#include <unordered_map>
 #include "common.hpp"
 
 #if RLIB_OS_ID == OS_LINUX
@@ -12,6 +13,32 @@
 #include <wepoll.h>
 #endif
 
+struct SockAddr {
+    union {
+        sockaddr_storage addr_storage;
+        sockaddr addr;
+        sockaddr_in in4;
+        sockaddr_in6 in6;
+    };
+    int len;
+};
+
+struct ConnectionMapping {
+    std::unordered_map<string, fd_t> client2server;
+    std::unordered_multimap<fd_t, string> server2client;
+    static string makeClientId(const SockAddr &osStrust) const {
+        // ClientId is a binary string.
+        string result(sizeof(osStruct), '\0');
+        std::memcpy(result.data(), &osStruct, sizeof(osStrust));
+        return result;
+    }
+    static void parseClientId(const string &clientId, SockAddr &output) const {
+        static_assert(sizeof(output) == sizeof(SockAddr), "error: programming error detected.");
+        if (clientId.size() != sizeof(output))
+            throw std::invalid_argument("parseClientId, invalid input binary string length.");
+        std::memcpy(&output, clientId.data(), sizeof(output));
+    }
+};
 
 inline void epoll_add_fd(fd_t epollFd, fd_t fd) {
     epoll_event event {