We present a new scheme for active, or programmable, packets based upon a new packet language, SNAP (Safe Networking with Active Packets). SNAP's semantics permit us to prove that all SNAP programs are safe with respect to network resource usage and evaluation isolation. Furthermore, we describe an implementation of a SNAP interpreter, snapd, which achieves high performance for standard networking tasks. This work represents the first active packet system that is demonstrated to be both safe and efficient.