Global Catalog Replication Paths

In a Win2000 tree structure, (Example One parent and 2 child domains with sites internal to the domain) do the Global Catalogs replicate between each other in a domain and or between each domain?s Global Catalog or do they rely on active directories?
If the Global Catalogs replicate between each other how do they replicate and what are the replication paths?
If the Global Catalogs replicate between each other, what method/s do they use.
I.E. UDP/TCP port (what port #), or through multicast or what?

Note: A Basic diagram of replication paths would be helpful.