To kick off the multiplayer improvements for version 2.1, I’ve started with adding access to data files over the network. For example, if a server is running a custom PWAD that you don’t have, the client will automatically download a copy before joining the game.
That pretty much sums it up for the user-facing portion of this feature. You will see a popup displaying download progress, and there is an option to cancel.
Much of this work was recently merged to the master branch (in the form of 83 commits) and is included now in unstable builds. Note that this kind of a larger influx of changes usually leads to new glitches also being introduced… I will be improving the code in the coming days/weeks.