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

In singleplayer, the lagback itself is caused by this portion of code (in class NetHandlerPlayServer):

public void processPlayer(PacketPlayer packetIn)
{
    //heavily simplified, ignores exceptional cases (flying, sleeping, teleporting, , noclip, mounted, respawning...)
    double posY_original = this.playerEntity.posY;
    this.lastPosX = this.playerEntity.posX;
    this.lastPosY = this.playerEntity.posY;
    this.lastPosZ = this.playerEntity.posZ;
    double posX = packetIn.getPositionX();
    double posY = packetIn.getPositionY();
    double 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);
    
    double deltaX = posX - this.playerEntity.posX;
    double deltaY = posY - this.playerEntity.posY;
    double deltaZ = 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 = posX - this.playerEntity.posX;
    double errorZ = posZ - this.playerEntity.posZ;
    double error_squared = errorX * errorX + errorZ * errorZ;
    boolean movedWrongly = (delta_squared > 0.0625D && !this.playerEntity.theItemInWorldManager.isCreative());

    this.playerEntity.setPositionAndRotation(posX, posY, posZ, yaw, pitch);
    
    boolean noCollisionInside_updated = worldserver.getCollidingBoundingBoxes(this.playerEntity, this.playerEntity.getEntityBoundingBox().contract(0.0625, 0.0625, 0.0625).isEmpty();

    //detects a desync - "lagbacks" the player to their previous position
    if (noCollisionInside && (movedWrongly || !noCollisionInside_updated))
    {
        this.setPlayerLocation(this.lastPosX, this.lastPosY, this.lastPosZ, yaw, pitch);
        return;
    }

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

There are essentially two ways to trigger a lagback:

• Moving "wrongly".
• Moving into a collision box.

The latter only seems to happen when colliding with boats. The next section shall be dedicated to detailing how the player can move "wrongly".