attributeName

html,
body,
svg {
  height: 100%;
}

<svg viewBox="0 0 250 250" xmlns="http://www.w3.org/2000/svg">
  <rect x="50" y="50" width="100" height="100">
    <animate
      attributeType="XML"
      attributeName="y"
      from="0"
      to="50"
      dur="5s"
      repeatCount="indefinite" />
  </rect>
</svg>