Added support for TCP Fast Open in server when using Linux.

server_http.hpp

@@ -351,6 +351,8 @@ namespace SimpleWeb {
       std::string address;
       /// Set to false to avoid binding the socket to an address that is already in use. Defaults to true.
       bool reuse_address = true;
+      /// Makes use of RFC 7413 or TCP Fast Open (TFO)
+      bool fast_open = false;
     };
     /// Set before calling start().
     Config config;
@@ -399,6 +401,14 @@ namespace SimpleWeb {
         acceptor = std::unique_ptr<asio::ip::tcp::acceptor>(new asio::ip::tcp::acceptor(*io_service));
       acceptor->open(endpoint.protocol());
       acceptor->set_option(asio::socket_base::reuse_address(config.reuse_address));
+      if (config.fast_open && is_tcp_fast_open_supported(connection_mode::server))
+      {
+#if defined(__linux__) && defined(TCP_FASTOPEN)
+        const int qlen = 5; // This seems to be the value that is used in other examples.
+        boost::system::error_code ec;
+        acceptor->set_option(boost::asio::detail::socket_option::integer<IPPROTO_TCP, TCP_FASTOPEN>(qlen), ec);
+#endif // End Linux
+      }
       acceptor->bind(endpoint);
 
       after_bind();

utility.hpp

@@ -8,6 +8,10 @@
 #include <memory>
 #include <string>
 #include <unordered_map>
+#if defined(__linux__)
+#include <fstream>
+#include <sstream>
+#endif
 
 #if __cplusplus > 201402L || _MSVC_LANG > 201402L
 #include <string_view>
@@ -26,6 +30,35 @@ namespace SimpleWeb {
 #endif
 
 namespace SimpleWeb {
+  enum class connection_mode { client = 1, server = 2 };
+  inline bool is_tcp_fast_open_supported(const connection_mode mode)
+  {
+#if defined(__linux__)
+    std::ifstream ifs("/proc/sys/net/ipv4/tcp_fastopen");
+    if (!ifs.is_open()) { return false; }
+    std::string line;
+    if (!std::getline(ifs, line)) { return false; }
+    std::istringstream iss(line);
+    uint8_t value = 0;
+    iss >> value;
+    if (iss.fail()) { return false; } // Should never happen in theory.
+    auto mode_matches_request = [](const connection_mode mode, const uint8_t value)
+    {
+      const auto m = static_cast<uint8_t>(mode);
+      if (mode == connection_mode::client ||
+          mode == connection_mode::server)
+      {
+        return (m & value) != 0;
+      }
+      return false;
+    };
+    return mode_matches_request(mode, value);
+#else
+    (void)mode;
+    return false;
+#endif
+  }
+
   inline bool case_insensitive_equal(const std::string &str1, const std::string &str2) noexcept {
     return str1.size() == str2.size() &&
            std::equal(str1.begin(), str1.end(), str2.begin(), [](char a, char b) {