I need to add a WebSocket-to-TCP proxy to my Play 2.3 application, but while the outgoing TCP connection using Akka I/O supports back-pressure, I don't see anything for the WebSocket. There's clearly no support in the actor-based API, but [James Roper says](http://brikis98.blogspot.com/2013/11/play-scala-and-iteratees-vs-nodejs.html?showComment=1385511897410#c3240399653494822522):

> 
> Iteratees handle this by design, you can't feed a new element into an
>  iteratee until last future it returns has been redeemed, because you
>  don't have a reference to it until then.
> 
> 
> 

However, I don't see what he's referring to. `Iteratee.foreach`, as used in the examples, seems too simple. The only futures I see in the iteratee API are for completing the result of the computation. Should I be completing a `Future[Unit]` for each message or what?