In infrastructure-less wireless systems network-wise time and frequency synchronization can be achieved by exchanging mutual synchronization errors among neighboring nodes. Cooperative synchronization is based on the use of distributed digital locked loops (D-DLLs), as the extension to distributed systems of the classical concept of (analog or digital) locked loops. The convergence to a synchronized state depends ultimately on the degree of connectivity of the network. D-DLLs can be specialized for time or frequency synchronization by adopting an appropriate error detector, but preserving the same control loop. The focus of this paper is on distributed frequency synchronization for packet-based communication. A novel detector is proposed, which approximates the local mean frequency error from the uncoordinated transmission of packets by neighboring nodes. The performance of distributed frequency locked loops (D-FLLs) is evaluated for a wireless network employing packet-based cooperative relaying. Numerical validations are used to compare different frequency synchronization protocols in terms of speed of convergence and degradation of end-to-end performances.