Use a lower default MTU for ENet hosts, and make it configurable.

This fixes packet loss issues on some VPN solutions.

Patch By: sera
Differential Revision: https://code.wildfiregames.com/D4967
This was SVN commit r27599.

binaries/data/config/default.cfg

@@ -525,6 +525,7 @@ lateobservers = everyone          ; Allow observers to join the game after it st
 observerlimit = 8                 ; Prevent further observer joins in running games if this limit is reached
 observermaxlag = -1               ; Make clients wait for observers if they lag more than X turns behind. -1 means "never wait for observers".
 autocatchup = true        ; Auto-accelerate the sim rate if lagging behind (as an observer).
+enetmtu = 1372            ; Lower ENet protocol MTU in case packets get further fragmented on the UDP layer which may cause drops.
 
 [overlay]
 fps = "false"                     ; Show frames per second in top right corner

source/network/NetClient.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2022 Wildfire Games.
+/* Copyright (C) 2023 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -20,6 +20,7 @@
 #include "NetClient.h"
 
 #include "NetClientTurnManager.h"
+#include "NetEnet.h"
 #include "NetMessage.h"
 #include "NetSession.h"
 
@@ -247,7 +248,7 @@ bool CNetClient::TryToConnect(const CStr& hostJID, bool localNetwork)
 	}
 
 	ENetAddress hostAddr{ ENET_HOST_ANY, ENET_PORT_ANY };
-	ENetHost* enetClient = enet_host_create(&hostAddr, 1, 1, 0, 0);
+	ENetHost* enetClient = PS::Enet::CreateHost(&hostAddr, 1, 1);
 
 	if (!enetClient)
 	{

source/network/NetEnet.cpp

@@ -0,0 +1,62 @@
+/* Copyright (C) 2023 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "precompiled.h"
+
+#include "NetEnet.h"
+
+#include "ps/ConfigDB.h"
+#include "ps/containers/Span.h"
+
+namespace PS
+{
+
+namespace Enet
+{
+
+// ENet protocol MTU
+// ENet by default uses 1400 as ENet packet fragment max size, adding ICMP and
+// IPv4 headers this may exceed the MTU for some VPN solutions [1], so provide
+// a lower default.
+// MTU negotiation server-side needs [2], which was merged after enet-1.3.17,
+// so the user configured value may be ignored on older versions.
+// [1] https://github.com/lsalzman/enet/issues/132
+// [2] https://github.com/lsalzman/enet/pull/222
+constexpr enet_uint32 HOST_DEFAULT_MTU = 1372;
+
+ENetHost* CreateHost(const ENetAddress* address, size_t peerCount, size_t channelLimit)
+{
+	// TODO: Maybe allow user to set rate limits?
+
+	ENetHost* host = enet_host_create(address, peerCount, channelLimit, 0, 0);
+	if (!host)
+		return nullptr;
+
+	// Public ENet API doesn't offer a means to change MTU, so do it in a
+	// way least likely to break with ENet updates.
+	enet_uint32 mtu = HOST_DEFAULT_MTU;
+	CFG_GET_VAL("network.enetmtu", mtu);
+	host->mtu = mtu;
+	for (ENetPeer& p : PS::span{host->peers, host->peerCount})
+		enet_peer_reset(&p);
+
+	return host;
+}
+
+} // namespace Enet
+
+} // namespace PS

source/network/NetEnet.h

@@ -0,0 +1,44 @@
+/* Copyright (C) 2023 Wildfire Games.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+ * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+ * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+ * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+ * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#ifndef INCLUDED_NETENET
+#define INCLUDED_NETENET
+
+#include "lib/external_libraries/enet.h"
+
+namespace PS
+{
+
+namespace Enet
+{
+
+/**
+ * Wrapper for enet_host_create setting default values and custom mtu, taking
+ * care of user configuration.
+ */
+ENetHost* CreateHost(const ENetAddress* address, size_t peerCount, size_t channelLimit);
+
+} // namespace Enet
+
+} // namespace PS
+
+#endif	// #ifndef INCLUDED_NETENET

source/network/NetServer.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2022 Wildfire Games.
+/* Copyright (C) 2023 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -20,6 +20,7 @@
 #include "NetServer.h"
 
 #include "NetClient.h"
+#include "NetEnet.h"
 #include "NetMessage.h"
 #include "NetSession.h"
 #include "NetServerTurnManager.h"
@@ -223,7 +224,7 @@ bool CNetServerWorker::SetupConnection(const u16 port)
 	addr.port = port;
 
 	// Create ENet server
-	m_Host = enet_host_create(&addr, MAX_CLIENTS, CHANNEL_COUNT, 0, 0);
+	m_Host = PS::Enet::CreateHost(&addr, MAX_CLIENTS, CHANNEL_COUNT);
 	if (!m_Host)
 	{
 		LOGERROR("Net server: enet_host_create failed");

source/network/NetSession.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2021 Wildfire Games.
+/* Copyright (C) 2023 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -20,6 +20,7 @@
 #include "NetSession.h"
 
 #include "NetClient.h"
+#include "NetEnet.h"
 #include "NetMessage.h"
 #include "NetServer.h"
 #include "NetStats.h"
@@ -60,12 +61,8 @@ bool CNetClientSession::Connect(const CStr& server, const u16 port, ENetHost* en
 	ENSURE(!m_Host);
 	ENSURE(!m_Server);
 
-	// Create ENet host
-	ENetHost* host;
-	if (enetClient != nullptr)
-		host = enetClient;
-	else
-		host = enet_host_create(NULL, 1, CHANNEL_COUNT, 0, 0);
+	// Create ENet host if necessary.
+	ENetHost* host = enetClient != nullptr ? enetClient : PS::Enet::CreateHost(nullptr, 1, CHANNEL_COUNT);
 
 	if (!host)
 		return false;