Network: Use shorter UDP MTU where data can be split across multiple packets.

src/network/core/config.h

@@ -33,6 +33,7 @@ static const uint16 NETWORK_ADMIN_PORT            = 3977;         ///< The defau
 static const uint16 NETWORK_DEFAULT_DEBUGLOG_PORT = 3982;         ///< The default port debug-log is sent to (TCP)
 
 static const uint16 SEND_MTU                      = 1460;         ///< Number of bytes we can pack in a single packet
+static const uint16 SEND_MTU_SHORT                = 1400;         ///< Number of bytes we can pack in a single packet (conservative)
 
 static const byte NETWORK_GAME_ADMIN_VERSION      =    1;         ///< What version of the admin network do we use?
 static const byte NETWORK_GAME_INFO_VERSION       =    4;         ///< What version of game-info do we use?
@@ -59,6 +60,7 @@ static const uint NETWORK_GRF_NAME_LENGTH         =   80;         ///< Maximum l
  * This limit is reached when PACKET_UDP_SERVER_RESPONSE reaches the maximum size of SEND_MTU bytes.
  */
 static const uint NETWORK_MAX_GRF_COUNT           =   62;
+static const uint NETWORK_MAX_GRF_COUNT_SHORT     =   59;
 
 static const uint NETWORK_NUM_LANGUAGES           =   36; ///< Number of known languages (to the network protocol) + 1 for 'any'.
 

src/network/core/udp.cpp

@@ -84,15 +84,17 @@ NetworkRecvStatus NetworkUDPSocketHandler::CloseConnection(bool error)
  * @param all  send the packet using all sockets that can send it
  * @param broadcast whether to send a broadcast message
  */
-void NetworkUDPSocketHandler::SendPacket(Packet *p, NetworkAddress *recv, bool all, bool broadcast)
+void NetworkUDPSocketHandler::SendPacket(Packet *p, NetworkAddress *recv, bool all, bool broadcast, bool short_mtu)
 {
 	if (this->sockets.size() == 0) this->Listen();
 
-	if (p->size > SEND_MTU) {
+	const uint MTU = short_mtu ? SEND_MTU_SHORT : SEND_MTU;
+
+	if (p->size > MTU) {
 		p->PrepareToSend();
 
 		uint64 token = this->fragment_token++;
-		const uint PAYLOAD_MTU = SEND_MTU - (1 + 2 + 8 + 1 + 1 + 2);
+		const uint PAYLOAD_MTU = MTU - (1 + 2 + 8 + 1 + 1 + 2);
 
 		const uint8 frag_count = (p->size + PAYLOAD_MTU - 1) / PAYLOAD_MTU;
 
@@ -108,7 +110,7 @@ void NetworkUDPSocketHandler::SendPacket(Packet *p, NetworkAddress *recv, bool a
 			frag.Send_binary((const char *) p->buffer + offset, payload_size);
 			current_frag++;
 			offset += payload_size;
-			this->SendPacket(&frag, recv, all, broadcast);
+			this->SendPacket(&frag, recv, all, broadcast, short_mtu);
 			frag.ResetState(PACKET_UDP_EX_MULTI);
 		}
 		assert_msg(current_frag == frag_count, "%u, %u", current_frag, frag_count);

src/network/core/udp.h

@@ -258,7 +258,7 @@ public:
 	bool Listen();
 	void Close() override;
 
-	void SendPacket(Packet *p, NetworkAddress *recv, bool all = false, bool broadcast = false);
+	void SendPacket(Packet *p, NetworkAddress *recv, bool all = false, bool broadcast = false, bool short_mtu = false);
 	void ReceivePackets();
 
 	void SendNetworkGameInfo(Packet *p, const NetworkGameInfo *info);

src/network/network_content.cpp

@@ -219,7 +219,7 @@ void ClientNetworkContentSocketHandler::RequestContentList(uint count, const Con
 		 * A packet begins with the packet size and a byte for the type.
 		 * Then this packet adds a uint16 for the count in this packet.
 		 * The rest of the packet can be used for the IDs. */
-		uint p_count = min(count, (SEND_MTU - sizeof(PacketSize) - sizeof(byte) - sizeof(uint16)) / sizeof(uint32));
+		uint p_count = min(count, (SEND_MTU_SHORT - sizeof(PacketSize) - sizeof(byte) - sizeof(uint16)) / sizeof(uint32));
 
 		Packet *p = new Packet(PACKET_CONTENT_CLIENT_INFO_ID);
 		p->Send_uint16(p_count);
@@ -245,7 +245,7 @@ void ClientNetworkContentSocketHandler::RequestContentList(ContentVector *cv, bo
 
 	this->Connect();
 
-	const uint max_per_packet = min<uint>(255, (SEND_MTU - sizeof(PacketSize) - sizeof(byte) - sizeof(uint8)) /
+	const uint max_per_packet = min<uint>(255, (SEND_MTU_SHORT - sizeof(PacketSize) - sizeof(byte) - sizeof(uint8)) /
 			(sizeof(uint8) + sizeof(uint32) + (send_md5sum ? /*sizeof(ContentInfo::md5sum)*/16 : 0))) - 1;
 
 	uint offset = 0;
@@ -361,7 +361,7 @@ void ClientNetworkContentSocketHandler::DownloadSelectedContentFallback(const Co
 		 * A packet begins with the packet size and a byte for the type.
 		 * Then this packet adds a uint16 for the count in this packet.
 		 * The rest of the packet can be used for the IDs. */
-		uint p_count = min(count, (SEND_MTU - sizeof(PacketSize) - sizeof(byte) - sizeof(uint16)) / sizeof(uint32));
+		uint p_count = min(count, (SEND_MTU_SHORT - sizeof(PacketSize) - sizeof(byte) - sizeof(uint16)) / sizeof(uint32));
 
 		Packet *p = new Packet(PACKET_CONTENT_CLIENT_CONTENT);
 		p->Send_uint16(p_count);

src/network/network_udp.cpp

@@ -205,7 +205,7 @@ void ServerNetworkUDPSocketHandler::Reply_CLIENT_FIND_SERVER_extended(Packet *p,
 	this->SendNetworkGameInfoExtended(&packet, ngi, flags, version);
 
 	/* Let the client know that we are here */
-	this->SendPacket(&packet, client_addr);
+	this->SendPacket(&packet, client_addr, false, false, true);
 
 	DEBUG(net, 2, "[udp] queried (extended: %u) from %s", version, client_addr->GetHostname());
 }
@@ -341,7 +341,7 @@ void ServerNetworkUDPSocketHandler::Receive_CLIENT_GET_NEWGRFS(Packet *p, Networ
 		 * The name could be an empty string, if so take the filename. */
 		size_t required_length = sizeof(info.ident.grfid) + sizeof(info.ident.md5sum) +
 				min(strlen(info.name) + 1, (size_t)NETWORK_GRF_NAME_LENGTH);
-		if (packet_len + required_length > SEND_MTU - 4) { // 4 is 3 byte header + grf count in reply
+		if (packet_len + required_length > SEND_MTU_SHORT - 4) { // 4 is 3 byte header + grf count in reply
 			flush_response();
 		}
 		packet_len += required_length;
@@ -432,7 +432,7 @@ void ClientNetworkUDPSocketHandler::Receive_SERVER_RESPONSE_Common(Packet *p, Ne
 			if (c->status != GCS_NOT_FOUND || strcmp(c->GetName(), UNKNOWN_GRF_NAME_PLACEHOLDER) != 0) continue;
 			in_request[in_request_count] = c;
 			in_request_count++;
-			if (in_request_count == NETWORK_MAX_GRF_COUNT) flush_request();
+			if (in_request_count == NETWORK_MAX_GRF_COUNT_SHORT) flush_request();
 		}
 
 		if (in_request_count > 0) flush_request();