I am in the intial stage of architecting and building an application in
which each client running the application will occasionally send a set of
binary files to other "peers" running the same application.   The
applications will be managed centrally via a server which will handle
registration and authorization/authentication.   The cient app will be a
.NET Windows Forms application, and the server will be accessed via a .NET
web service.

This is my first significant experience with web services.  It is clear to
me how to wire the clients to access the server and trade information and
files with the server.  My question is how to enable clients to send objects
directly between one another?   Can .NET web services be utilized for this
kind of peer-to-peer communication?

As I understand it, there is a potential problem with the use of web
services for peer-to-peer communication:  Each client would have to have a
.asmx web service running on IIS.  But many (if not most) clients will not
have IIS available for security reasons.   Perhaps I'm asking a niave
question here, but can SOAP messages be exchanged between clients(preferable
via http to help overcome firewall challenges) without utilizing IIS on the
clients?

Finally, if anyone can recommend a good article on peer-to-peer
communication between clients (without the use of IIS on the clients) I
would appreciate that.  I prefer samples in C#.

Thanks,
-- Bruce