Introduce TCP_FASTOPEN support

For best results, use Linux 4.11+ and a server modified to set the
TCP_FASTOPEN flag.

This should in particular benefit users of legacy forwarding.

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -372,6 +372,10 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     return advanced.isProxyProtocol();
   }
 
+  public boolean useTcpFastOpen() {
+    return advanced.tcpFastOpen;
+  }
+
   public Metrics getMetrics() {
     return metrics;
   }
@@ -608,6 +612,10 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     @ConfigKey("proxy-protocol")
     private boolean proxyProtocol = false;
 
+    @Comment("Enables TCP fast open support on the proxy. Requires the proxy to run on Linux.")
+    @ConfigKey("tcp-fast-open")
+    private boolean tcpFastOpen = false;
+
     private Advanced() {
     }
 
@@ -619,6 +627,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
         this.connectionTimeout = toml.getLong("connection-timeout", 5000L).intValue();
         this.readTimeout = toml.getLong("read-timeout", 30000L).intValue();
         this.proxyProtocol = toml.getBoolean("proxy-protocol", false);
+        this.tcpFastOpen = toml.getBoolean("tcp-fast-open", false);
       }
     }
 
@@ -646,6 +655,10 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
       return proxyProtocol;
     }
 
+    public boolean isTcpFastOpen() {
+      return tcpFastOpen;
+    }
+
     @Override
     public String toString() {
       return "Advanced{"
@@ -655,6 +668,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
           + ", connectionTimeout=" + connectionTimeout
           + ", readTimeout=" + readTimeout
           + ", proxyProtocol=" + proxyProtocol
+          + ", tcpFastOpen=" + tcpFastOpen
           + '}';
     }
   }

proxy/src/main/java/com/velocitypowered/proxy/network/ConnectionManager.java

@@ -15,6 +15,7 @@ import io.netty.channel.ChannelFutureListener;
 import io.netty.channel.ChannelOption;
 import io.netty.channel.EventLoopGroup;
 import io.netty.channel.WriteBufferWaterMark;
+import io.netty.channel.epoll.EpollChannelOption;
 import io.netty.resolver.dns.DnsAddressResolverGroup;
 import io.netty.resolver.dns.DnsNameResolverBuilder;
 import io.netty.util.concurrent.EventExecutor;
@@ -102,6 +103,11 @@ public final class ConnectionManager {
         .childOption(ChannelOption.TCP_NODELAY, true)
         .childOption(ChannelOption.IP_TOS, 0x18)
         .localAddress(address);
+
+    if (server.getConfiguration().useTcpFastOpen()) {
+      bootstrap.option(EpollChannelOption.TCP_FASTOPEN, 3);
+    }
+
     bootstrap.bind()
         .addListener((ChannelFutureListener) future -> {
           final Channel channel = future.channel();
@@ -151,13 +157,17 @@ public final class ConnectionManager {
    * @return a new {@link Bootstrap}
    */
   public Bootstrap createWorker(@Nullable EventLoopGroup group) {
-    return new Bootstrap()
+    Bootstrap bootstrap = new Bootstrap()
         .channel(this.transportType.socketChannelClass)
         .option(ChannelOption.TCP_NODELAY, true)
         .option(ChannelOption.CONNECT_TIMEOUT_MILLIS,
             this.server.getConfiguration().getConnectTimeout())
         .group(group == null ? this.workerGroup : group)
         .resolver(this.resolverGroup);
+    if (server.getConfiguration().useTcpFastOpen()) {
+      bootstrap.option(EpollChannelOption.TCP_FASTOPEN_CONNECT, true);
+    }
+    return bootstrap;
   }
 
   /**