A VPN can be an implementation problem due to the interactions possible over a heterogeneous network. Authentication via IPSec is currently limited to shared secrets or RSA signatures, but in the future it will need to migrate to a workable PKI certificate system.

Many commercial VPN solutions exist, as well as open source implementations. The lack of support and documentation for the open source efforts might limit their usefulness in mission-critical systems, but might not affect systems that have small numbers of users and defined hardware.