Some more words about channels:

channel primitives was designed to provide enough facilities to make
inter-image communication.
There was no intent in building a bidirectional, with delivery
notifications channels, because of speed reasons, and because some
communication schemes would not require a guaranteed delivery at all.
As analogy, you may consider channels as a low-level network
transport, on top of which you place a TCP/IP layer or, of course,
anything what you want.
Or, you may consider a real world example:
 - when someone telling something to you, he sending a message. But he
can't know if you heard it, understand it and even if so, willing to
say something in response.
He can _know_ that you received his message only if you say something
in response, or do something, which will confirm that message is
delivered and understood well :)

-- 
Best regards,
Igor Stasenko AKA sig.