The lack of integrated support for security has been a major shortcoming of Precision Time Protocol version 2 (PTPv2) for a long time. The upcoming PTPv2.1 aims at addressing this shortcoming in a variety of ways, including the introduction of lightweight message authentication. In this paper we provide an overview of the planned security features, and report results based on an implementation of the proposed integrated security mechanism based on the open source Linux PTP, including support for hardware timestamping. Our implementation includes an extension of Linux PTP to support transparent clocks. We provide results from an experimental testbed including a transparent clock, which illustrate that the extensions can be implemented in software at a low computational overhead, while supporting hardware timestamping. We also provide a discussion of the remaining vulnerabilities of PTP time synchronization, propose countermeasures, and discuss options for key management, which is not covered by the standard.