make the path MTU configurable

include/simulator/simulator.hpp

@@ -946,6 +946,8 @@ namespace sim
 		route const& get_incoming_route() const
 		{ return m_incoming_route; }
 
+		int get_path_mtu(asio::ip::address ip) const;
+
 	private:
 
 		sim::simulation& m_sim;
@@ -981,6 +983,12 @@ namespace sim
 		// return the hops an outgoing packet from ep need to traverse before
 		// reaching the network (for instance a DSL modem)
 		virtual route outgoing_route(asio::ip::address ip) = 0;
+
+		// return the path MTU between the two IP addresses
+		// For TCP sockets, this will be called once when the connection is
+		// established. For UDP sockets it's called for every burst of packets
+		// that are sent
+		virtual int path_mtu(asio::ip::address ip1, asio::ip::address ip2) = 0;
 	};
 
 	struct default_config : configuration
@@ -992,6 +1000,8 @@ namespace sim
 			, asio::ip::address dst) override final;
 		virtual route incoming_route(asio::ip::address ip) override final;
 		virtual route outgoing_route(asio::ip::address ip) override final;
+		virtual int path_mtu(asio::ip::address ip1, asio::ip::address ip2)
+			override final;
 
 	private:
 		std::shared_ptr<queue> m_network;

src/default_config.cpp

@@ -53,6 +53,11 @@ namespace sim {
 		return route(it->second);
 	}
 
+	int default_config::path_mtu(asio::ip::address ip1, asio::ip::address ip2)
+	{
+		return 1475;
+	}
+
 	// return the hops an outgoing packet from ep need to traverse before
 	// reaching the network (for instance a DSL modem)
 	route default_config::outgoing_route(asio::ip::address ip)

src/io_service.cpp

@@ -38,6 +38,11 @@ namespace sim { namespace asio {
 		assert(false);
 	}
 
+	int io_service::get_path_mtu(asio::ip::address ip) const
+	{
+		return m_sim.config().path_mtu(m_ip, ip);
+	}
+
 	void io_service::stop()
 	{
 		// TODO: cancel all outstanding handler associated with this io_service

src/tcp_socket.cpp

@@ -33,7 +33,7 @@ namespace ip {
 	tcp::socket::socket(io_service& ios)
 		: socket_base(ios)
 		, m_connect_timer(ios)
-		, m_mss(1475) // TODO: make configurable
+		, m_mss(1475)
 		, m_queue_size(0)
 		, m_recv_timer(ios)
 		, m_is_v4(true)
@@ -297,6 +297,8 @@ namespace ip {
 			m_bound_to = addr;
 		}
 		m_channel = m_io_service.internal_connect(this, target, ec);
+		m_mss = m_io_service.get_path_mtu(target.address());
+		m_cwnd = m_mss * 2;
 		if (ec)
 		{
 			m_channel.reset();

src/udp_socket.cpp

@@ -339,10 +339,10 @@ namespace ip {
 
 		time_point now = chrono::high_resolution_clock::now();
 
-		// this is outgoing bandwidth
+		// this is outgoing NIC bandwidth
 		// TODO: make this configurable
-		const int bandwidth = 1000000; // 1 MB/s
-		const int mtu = 1475;
+		const int bandwidth = 100000000; // 100 MB/s
+		const int mtu = m_io_service.get_path_mtu(dst.address());
 
 		if (ret > mtu)
 		{