How do I play multiplayer?

I have bought DOOM 3 BFG edition, which includes The Ultimate DOOM and DOOM 2, in addition to the expansion packs for Doom 3. I know the original incarnations and many of the source ports of DOOM and DOOM 2 supported multiplayer, but I don't seem to be able to find the option to start or join a multiplayer game in the version of The Ultimate DOOM that is bundled with the DOOM 3 BFG edition.

How do I play multiplayer The Ultimate DOOM - BFG edition?