When some ASP.NET applications, all on dedicated server, need to communicate which security option is appropriate ?
Data to be transferred is small in amount; SSL, X509 certificate or any option is available.
The service should remain private between application servers. New server should be added to the cluster easily.
EDIT: All servers are communicating over internet (no VPN, no secure physical connection…)
For your particular scenario I would say Transport Security with Certificate Authentication
However, it might be a good idea to peruse the different scenarios and options available.