Patch to add resume feature to scp

[Chris Rapier]

> Sorry, but I agree: scp is pretty much unmaintainable as it is - 
> without throwing the complexity of multiple incompatible
> implementations into the mix.

 > If you want to implement new feature, please base them on sftp. It
 > is a far saner protocol and many improvements only need changes on
 > the client. It also offers a clean protocol extension mechanism, so
 > changes that do require server support can be made without breaking
 > older clients.

Based on the interactions I have with the people that make use of our 
facilities most people still prefer SCP especially when it comes to 
automated file transfer processes. Since SCP, as such, is unmaintainable 
maybe it makes sense to actually write a new SCP that incorporates the 
best points of the old version (ease of use, ease of scripting, etc) 
with more advanced features. That way backward compatibility is 
maintained without abandoning a useful and widely used file transfer tool.

Something that makes use of the multiplex channels to handle file 
control information would allow concatenation of multiple files (the 
drop back into slow start with each file is a killer). Something that 
could do an auto resume would be great. Even better would be parallel 
file transfer, multiple streams for large (GB+) files, etc etc etc. Many 
of these features are available in things like GridFTP and bbFTP for 
example. Unfortunately, they are more difficult to use and users want 
the simplicity of SCP.

Obviously you can make use of other tools in association with ssh (the 
concatenation can be done with tar for example) but I've always felt 
that isn't the right answer. Requiring a user to have X and Y in order 
to do Z when it makes sense for Z to be part of the application just 
seems counter intuitive. This is especially true when you are dealing 
with minimal installs (embedded systems, XP, etc).

Without a doubt, this is just my opinion on things but it is based on 
dealing with a lot of users who ask for things like this.


Chris