tracking branch for a rebase

I received an interesting question today.

If "git pull" knows what remote tracking brach to pull from, why doesn't "git rebase" automatically know I want to rebase on top of my remote tracking branch?

He had done a "git remote update", reviewed the changes, and now wants to rebase his changes on top of the new upstream.

I told him about git pull --rebase, and branch.name.rebase, but didn't have an easy answer to the "what is my remote tracking branch" question in this case.

All the information is in git config branch.name.{remote,merge}, but I can't find anything that just answers that question. Nor did I have a good answer for why "git rebase" doesn't just default to rebasing the current branch on top of the tracking branch.

Thoughts?

		-- Pete