Lagback

Lagback refers to getting teleported back to a previous position. It can be abused to clip inside walls, or jump higher in a similar fashion as Blips. This practice may be a bannable offense on multiplayer servers. In this article, we solely focus on singleplayer 1.8.

Known causes for lagback

• Colliding with a boat
• Falling and sneaking at the last moment
• Moving inside a cobweb too quickly
• Stepping up with an obscene amount of speed (Speed 100, for example)

Explanation

The lagback itself is caused by this portion of code (in class NetHandlerPlayServer):

public void processPlayer(PacketPlayer packetIn)
{
    //heavily simplified, ignores exceptional cases where the player is considered inactive (sleeping, teleporting, watching end credits, noclip, flying, mounted)
    double posX = this.playerEntity.posX;
    double posY = this.playerEntity.posY;
    double posZ = this.playerEntity.posZ;
    double norm_squared = 0.0D;
    double distX = packetIn.getPositionX() - this.lastPosX;
    double distY = packetIn.getPositionY() - this.lastPosY;
    double distZ = packetIn.getPositionZ() - this.lastPosZ;

    double posY_original = this.playerEntity.posY;
    this.lastPosX = this.playerEntity.posX;
    this.lastPosY = this.playerEntity.posY;
    this.lastPosZ = this.playerEntity.posZ;
    double e_posX = packetIn.getPositionX();
    double e_posY = packetIn.getPositionY();
    double e_posZ = packetIn.getPositionZ();
    
    float yaw = this.playerEntity.rotationYaw;
    float pitch = this.playerEntity.rotationPitch;
    if (packetIn.getRotating()) {yaw = packetIn.getYaw(); pitch = packetIn.getPitch();}

    this.playerEntity.onUpdateEntity();
    this.playerEntity.setPositionAndRotation(this.lastPosX, this.lastPosY, this.lastPosZ, yaw, pitch);

    //the code may stop here under certain circumstances, to be determined.
    
    double deltaX = e_posX - this.playerEntity.posX;
    double deltaY = e_posY - this.playerEntity.posY;
    double deltaZ = e_posZ - this.playerEntity.posZ;

    boolean noCollisionInside = worldserver.getCollidingBoundingBoxes(this.playerEntity, this.playerEntity.getEntityBoundingBox().contract(0.0625, 0.0625, 0.0625).isEmpty();

    if (this.playerEntity.onGround && !packetIn.isOnGround() && deltaY > 0.0D)
        this.playerEntity.jump();

    this.playerEntity.moveEntity(deltaX, deltaY, deltaZ);
    this.playerEntity.onGround = packetIn.isOnGround();
                    
    double errorX = e_posX - this.playerEntity.posX;
    double errorZ = e_posZ - this.playerEntity.posZ;
    double error_squared = deltaX * deltaX + deltaZ * deltaZ;
    boolean movedWrongly = (delta_squared > 0.0625D && !this.playerEntity.theItemInWorldManager.isCreative());

    this.playerEntity.setPositionAndRotation(e_posX, e_posY, e_posZ, yaw, pitch);
    
    boolean noCollisionInside_updated = worldserver.getCollidingBoundingBoxes(this.playerEntity, this.playerEntity.getEntityBoundingBox().contract(0.0625, 0.0625, 0.0625).isEmpty();

    if (noCollisionInside && (movedWrongly || !noCollisionInside_updated) && !this.playerEntity.isPlayerSleeping())
    {
        this.setPlayerLocation(this.lastPosX, this.lastPosY, this.lastPosZ, yaw, pitch);
        return;
    }
    
    AxisAlignedBB axisalignedbb = this.playerEntity.getEntityBoundingBox().expand(0.0625, 0.0625, 0.0625.addCoord(0.0, -0.55, 0.0);

    if (!this.serverController.isFlightAllowed() && !this.playerEntity.capabilities.allowFlying && !worldserver.checkBlockCollision(axisalignedbb))
    {
        if (deltaY >= -0.03125D)
        {
            ++this.floatingTickCount;
            if (this.floatingTickCount > 80)
            {
                this.kickPlayerFromServer("Flying is not enabled on this server");
                return;
            }
        }
    }
    else
    {
        this.floatingTickCount = 0;
    }

    this.playerEntity.onGround = packetIn.isOnGround();
    this.playerEntity.handleFalling(this.playerEntity.posY - posY_original, packetIn.isOnGround());
}

The reason for the desynchronization is still being determined.