fix: Address packet overflow issues in the UTP adapter (#1403)

* Remove maximum packet size setting

It doesn't mean anything now. Anything sent on fragmented pipelines will
be broken up at 1400 bytes boundaries, and everything sent on a
non-fragmented pipeline is limited to a maximum size of 1400 bytes
(including overhead from the underlying protocols). Allowing to set that
limit can only lead to confusion among users.

* Send all reliable data on a fragmented pipeline

This is not the cleanest of solution, but until we figure out something
better in UTP, this can at least unblock Boss Room.

The core issue being fixed here is that we'd allow sending payloads up
to the MTU in size on a non-fragmented pipeline. But that's incorrect
since UTP must add its own overhead to the payload. If sending a payload
close to the MTU in size, the overhead would bring the total packet size
over the limit of the MTU.

Since UTP lacks a good way of figuring out what's the actual maximum
payload size that can be sent, for now we patch over the issue by
sending everything on a fragmented pipeline.

The exception is the unreliable messages. The fragmentation pipeline
doesn't cleanly handle having multiple fragmented payloads in flight
(and possibly arriving out of order). The reliable pipeline stage fixes
that, but for unreliable traffic it's better not to fragment it.

* Update adapter's CHANGELOG to reflect latest fixes

* Set baselib's maximum payload size to its default

Which is 2000 bytes. We won't need to hardcode that default value when
the new NetworkSettings API lands. So having a magical number floating
like that in the code is a temporary thing.

Author: simon-lemay-unity

com.unity.netcode.adapter.utp/CHANGELOG.md

@@ -1,6 +1,16 @@
 # Changelog
 All notable changes to this package will be documented in this file. The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 
+## [Unreleased]
+
+### Changed
+
+- Removed 'Maximum Packet Size' configuration field in the inspector. This would cause confusion since the maximum packet size is in effect always the MTU (1400 bytes on most platforms).
+
+### Fixed
+
+- Fixed packet overflow errors when sending payloads too close to the MTU (was mostly visible when using Relay).
+
 ## [1.0.0-pre.3] - 2021-10-22
 
 #### Added 

com.unity.netcode.adapter.utp/Runtime/UnityTransport.cs

@@ -16,7 +16,7 @@ namespace Unity.Netcode
     /// </summary>
     public interface INetworkStreamDriverConstructor
     {
-        void CreateDriver(UnityTransport transport, out NetworkDriver driver, out NetworkPipeline unreliableSequencedPipeline, out NetworkPipeline reliableSequencedPipeline, out NetworkPipeline reliableSequencedFragmentedPipeline);
+        void CreateDriver(UnityTransport transport, out NetworkDriver driver, out NetworkPipeline unreliableSequencedPipeline, out NetworkPipeline reliableSequencedFragmentedPipeline);
     }
 
     public static class ErrorUtilities
@@ -78,7 +78,6 @@ private enum State
         }
 
         public const int InitialBatchQueueSize = 6 * 1024;
-        public const int InitialMaxPacketSize = NetworkParameterConstants.MTU;
 
         private static ConnectionAddressData s_DefaultConnectionAddressData = new ConnectionAddressData()
         { Address = "127.0.0.1", Port = 7777 };
@@ -91,9 +90,6 @@ private enum State
         [Tooltip("Which protocol should be selected Relay/Non-Relay")]
         [SerializeField] private ProtocolType m_ProtocolType;
 
-        [Tooltip("Maximum size in bytes for a given packet")]
-        [SerializeField] private int m_MaximumPacketSize = InitialMaxPacketSize;
-
         [Tooltip("The maximum amount of packets that can be in the send/recv queues")]
         [SerializeField] private int m_MaxPacketQueueSize = 128;
 
@@ -146,7 +142,6 @@ public static implicit operator ConnectionAddressData(NetworkEndPoint d) =>
         private ulong m_ServerClientId;
 
         private NetworkPipeline m_UnreliableSequencedPipeline;
-        private NetworkPipeline m_ReliableSequencedPipeline;
         private NetworkPipeline m_ReliableSequencedFragmentedPipeline;
 
         public override ulong ServerClientId => m_ServerClientId;
@@ -199,7 +194,7 @@ public SimulatorUtility.Parameters ClientSimulatorParameters
 
         private void InitDriver()
         {
-            DriverConstructor.CreateDriver(this, out m_Driver, out m_UnreliableSequencedPipeline, out m_ReliableSequencedPipeline, out m_ReliableSequencedFragmentedPipeline);
+            DriverConstructor.CreateDriver(this, out m_Driver, out m_UnreliableSequencedPipeline, out m_ReliableSequencedFragmentedPipeline);
         }
 
         private void DisposeDriver()
@@ -222,15 +217,7 @@ private NetworkPipeline SelectSendPipeline(NetworkDelivery delivery, int size)
 
                 case NetworkDelivery.Reliable:
                 case NetworkDelivery.ReliableSequenced:
-                    return m_ReliableSequencedPipeline;
-
                 case NetworkDelivery.ReliableFragmentedSequenced:
-                    // No need to send on the fragmented pipeline if data is smaller than MTU.
-                    if (size < NetworkParameterConstants.MTU)
-                    {
-                        return m_ReliableSequencedPipeline;
-                    }
-
                     return m_ReliableSequencedFragmentedPipeline;
 
                 default:
@@ -607,7 +594,6 @@ public override void Initialize()
         {
             Debug.Assert(sizeof(ulong) == UnsafeUtility.SizeOf<NetworkConnection>(),
                 "Netcode connection id size does not match UTP connection id size");
-            Debug.Assert(m_MaximumPacketSize > 5, "Message buffer size must be greater than 5");
 
             m_NetworkParameters = new List<INetworkParameter>();
 
@@ -620,7 +606,7 @@ public override void Initialize()
 
             m_NetworkParameters.Add(new BaselibNetworkParameter()
             {
-                maximumPayloadSize = (uint)m_MaximumPacketSize,
+                maximumPayloadSize = 2000, // Default value in UTP.
                 receiveQueueCapacity = m_MaxPacketQueueSize,
                 sendQueueCapacity = m_MaxPacketQueueSize
             });
@@ -743,7 +729,7 @@ private void SendBatchedMessageAndClearQueue(SendTarget sendTarget, SendQueue se
             var payloadSize = sendQueue.Count + 1; // 1 extra byte to tell whether the message is batched or not
             if (payloadSize > NetworkParameterConstants.MTU) // If this is bigger than MTU then force it to be sent via the FragmentedReliableSequencedPipeline
             {
-                pipeline = SelectSendPipeline(NetworkDelivery.ReliableFragmentedSequenced, payloadSize);
+                pipeline = m_ReliableSequencedFragmentedPipeline;
             }
 
             var sendBuffer = sendQueue.GetData();
@@ -805,7 +791,7 @@ public override void Shutdown()
             m_ServerClientId = 0;
         }
 
-        public void CreateDriver(UnityTransport transport, out NetworkDriver driver, out NetworkPipeline unreliableSequencedPipeline, out NetworkPipeline reliableSequencedPipeline, out NetworkPipeline reliableSequencedFragmentedPipeline)
+        public void CreateDriver(UnityTransport transport, out NetworkDriver driver, out NetworkPipeline unreliableSequencedPipeline, out NetworkPipeline reliableSequencedFragmentedPipeline)
         {
             var netParams = new NetworkConfigParameter
             {
@@ -839,10 +825,6 @@ public void CreateDriver(UnityTransport transport, out NetworkDriver driver, out
                     typeof(UnreliableSequencedPipelineStage),
                     typeof(SimulatorPipelineStage),
                     typeof(SimulatorPipelineStageInSend));
-                reliableSequencedPipeline = driver.CreatePipeline(
-                    typeof(ReliableSequencedPipelineStage),
-                    typeof(SimulatorPipelineStage),
-                    typeof(SimulatorPipelineStageInSend));
                 reliableSequencedFragmentedPipeline = driver.CreatePipeline(
                     typeof(FragmentationPipelineStage),
                     typeof(ReliableSequencedPipelineStage),
@@ -853,7 +835,6 @@ public void CreateDriver(UnityTransport transport, out NetworkDriver driver, out
 #endif
             {
                 unreliableSequencedPipeline = driver.CreatePipeline(typeof(UnreliableSequencedPipelineStage));
-                reliableSequencedPipeline = driver.CreatePipeline(typeof(ReliableSequencedPipelineStage));
                 reliableSequencedFragmentedPipeline = driver.CreatePipeline(
                     typeof(FragmentationPipelineStage), typeof(ReliableSequencedPipelineStage)
                 );