NetworkTransform sync can fail when Interpolation is off due to network conditions

[zachstronaut]

Description

It is possible due to network conditions (jitter and packet loss non-zero) for NetworkTransform to receive in TransformStateUpdate() two incoming NetworkTransformState values in the same Unity frame.

If NetworkTransformState A contains an updated rotation but NetworkTransformState B only contains an updated position, then the m_LocalAuthoritativeNetworkState in ApplyAuthoritativeState() on the next OnUpdate() will only contain the data for state B, so the rotation change will never happen and the two clients will now be out of sync (different rotations).

I'm not exactly sure why Interpolation negates this problem, but I would suppose it has something to do with the fact OnNetworkStateChanged() is called on both A and B when they are received, so the data isn't lost to the interpolaters?

The problem will occur with real world network problems, but if you simulate something terrible like Delay=250, Jitter=40, Loss=20 it becomes a lot easier to reproduce.

Environment

OS: Windows 11
Unity Version: 6000.1.3f1
Netcode Version: 2.4.2
Netcode Topology: Client-Server

[zachstronaut]

You can also cause position to lose sync between clients this way.

Imagine NetworkTransformState A had a X position change but NetworkTransformState B had a Z position change, and both of those arrive on the same frame. ApplyAuthoritativeState() will not have access to the X coordinate data.

[zachstronaut]

The NetworkObject and NetworkTransform settings:

--

Here is a simple character controller that can be used to desync rotation. Hold D until walking due East and then hold A until walking due West. The rotation won't always sync on the non-host client by the time the position only data is being sent.

using UnityEngine;
using Unity.Netcode;

public class FakePlayer2 : NetworkBehaviour
{
    public float speed = 10f;
    public float turnSpeed = 500f;

    void Update()
    {
        if (IsOwner)
        {
            bool isMoving = false;
            Vector3 lookTarget = Vector3.zero;

            if (Input.GetKey(KeyCode.W))
            {
                isMoving = true;
                lookTarget += Vector3.forward;
            }
            if (Input.GetKey(KeyCode.S))
            {
                isMoving = true;
                lookTarget += -Vector3.forward;
            }
            if (Input.GetKey(KeyCode.A))
            {
                isMoving = true;
                lookTarget += -Vector3.right;
            }
            if (Input.GetKey(KeyCode.D))
            {
                isMoving = true;
                lookTarget += Vector3.right;
            }

            if (isMoving)
            {
                transform.rotation = Quaternion.RotateTowards(transform.rotation, Quaternion.LookRotation(lookTarget, Vector3.up), Time.deltaTime * turnSpeed);
                transform.position += transform.forward * Time.deltaTime * speed;
            }
        }
    }
}

--

And here is a simple character controller that can cause position to get out of sync. Simply hold any single direction (WASD) a bit and then switch to holding another different direction. Then, for example, the X changes won't always sync before the exclusively Z changes start arriving.

using UnityEngine;
using Unity.Netcode;

public class FakePlayer : NetworkBehaviour
{
    public float speed = 10f;

    void Update()
    {
        if (IsOwner)
        {
            if (Input.GetKey(KeyCode.W))
            {
                transform.position += transform.forward * Time.deltaTime * speed;
            }
            if (Input.GetKey(KeyCode.S))
            {
                transform.position -= transform.forward * Time.deltaTime * speed;
            }
            if (Input.GetKey(KeyCode.A))
            {
                transform.position -= transform.right * Time.deltaTime * speed;
            }
            if (Input.GetKey(KeyCode.D))
            {
                transform.position += transform.right * Time.deltaTime * speed;
            }
        }
    }
}

[zachstronaut]

Incidentally, is it possible that re-parenting and teleportation both potentially also suffer from bugs caused by network conditions -- multiple re-parenting or multiple teleporting messages arriving on the same Unity frame?

[zachstronaut]

Potential solution:

if an unconsumed (by OnUpdate) m_LocalAuthoritativeNetworkState exists when you receive new state, the previous and the new one could be combined.

For example, if the prev unconsumed m_LocalAuthoritativeNetworkState had rotational changes or an X position change, but the new state only has Z position change, you could add the new Z position change to the pending local authoritative state.