IB also provides free full data (called True Data) with a different feed, but it's 5 seconds delayed. You need to add -TD to the regular symbol:
es-201209-globex-td
Live Data
There is no delay, but it comes in 100
ms packets (it's not a continuous data stream), so some data will inevitably be missing.
True Data
5 second delayed because it's transmitted once every 5 seconds, but it's complete and includes all data missed by Live Data.
Some charting software packages try to combine both Live and True feeds, with mixed results. You can check Sierra Chart user forums to learn more about this approach.