Protocol for slipstreaming another cyclist? I have a nice 12 km commute where lots of different bicyclers travel. The entire stretch is only for pedestrians&amp;bicycles, no cars to worry about.  

I ride a regular old 5-speed bike (not a racing bicycle or mountain bike), but I'm a very fast biker compared to the general bike traffic I see (average 25-30 km/h). I'm only overtaken by people on electrobikes, and by fanatics (all the right gear, but no bell because it weighs 12 grams?).

It might be interesting to ride closely behind someone whose speed matches mine, but what's the protocol for doing this, apart from road safety concerns? Should I address people first, ask their permission? Or do I just approach from behind and stay close to them, without specifically indicating that I'm there? 

At my speed, I won't be stalking a granny -- it would probably only ever be a lone hobby race cyclist here or there. Note that I am steering clear of team bicyclists; they probably have their own patterns that I don't want to interfere with.

I've noted someone following me a few times (without any indication) but it doesn't bother me because people who can keep up with me usually overtake me after a while anyway, so at best I've given them a moment's pause. As long as they don't crash me, they're welcome.

Add-on question:
It's clearly more polite to take turns leading, but does it cost you more energy when someone is following you, compared to cycling alone?
I've found it courteous to come up beside the person and ask permission. I usually say "how far you going?" or "mind if I join you?". The real key is to take turns drafting. If the person says yes, I  usually take the first pull in front just to show them I'm not a wheel sucker. 

As an avid cyclist, I've found very few things more annoying than riding for a good chunk of time, only to look back and find someone just sitting on your wheel with no announcement or indication that they are there.