While I know of no careful study I think there is considerable
anecdotal evidence (well at least folklore) that holding the
connection open is a bad idea. It may reduce network traffic but
increases the load on the server. For example, I think one of the big
Mac software archives started requesting that people download from
them using gopher instead of ftp because they could serve many times
the number of gopher clients as ftp clients with the same hardware.
The only significant difference is that ftp holds open the connection
and thus consumes server resources while the client user browses the
archives. This turns out to be costly.
An alternate proposal that would gain most of the network benefits
would be an MGET method in HTTP. This would require two connections
to download a document containing, say 20 images which is not as
good as 1, I know, but it would reduce load on the server rather
than increase it. It would also be vey easy to implement
in servers and browsers.
Another proposal which has been discussed is to use MIME multi-part
document format to make an HTML document with 20 images into a
single document. The problem with this is that it represents
substantial effort on the part of browser and server writers and
is, for that reason, not likely to get implemented.
--John Franks Dept of Math. Northwestern University john@math.nwu.edu