[Vortex] Any way to interrupt a reply in BEEP?

[Jens Alfke]

Is there any way in the BEEP protocol to interrupt a lengthy reply?  
For example, the straightforward way to download a file would be to  
send a MSG requesting the file, and receive an RPY or multiple ANS  
frames containing the file contents. But a large file might take a  
long time to download, and the recipient should be able to cancel  
partway through. How could that be done?

The only mechanisms I can think of are:

(1) Have the file-receiving peer send a message over a different  
channel, which the file-sending peer will interpret to mean that it  
should stop sending. (This only works with multiple ANS frames, of  
course.)

(2) Have the file-receiving peer close the channel it it wants to stop  
the download.

(3) Send the download in chunks, with a separate MSG/RPY cycle to  
fetch each chunk (sort of like a byte-range GET in HTTP.)

Any thoughts? Did I overlook something? Of these, #2 is easiest to  
implement and doesn't require defining an additional profile for the  
control messages; but is it OK to close a channel while a reply is  
still incomplete?

—Jens
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