In-Band Network Telemetry (INT) is a new technique in the area of Software-defined networking (SDN) for monitoring SDN enabled networks. INT monitoring provides fine-grained INT data with less load on the control plane since monitoring is done directly at the data plane. The collected INT data is added as packet headers "In-band" at each INT device along the flow path. The INT data is later composed into telemetry packets which are sent to a collector that is responsible for processing the INT data. The collector of the INT data needs to have good performance since there is a large amount of data that has to be processed quickly. INTCollector, a high performance collector of INT data, is a response to this challenge. The performance of INTCollector is optimized by implementing critical parts in eXpress Data Path (XDP), enabling fast packet processing. INTCollector is, moreover, able to reduce processing of INT data and the need for storage space since it employs a strategy where only important INT data is collected, decided by an internal event detection mechanism.The event detection mechanism in INTCollector can however be offloaded to the INT device itself, with possible perfomance benefits for the collector. Programming Protocol-Independent Packet Processors (P4) opens up this possibility by providing a language for programming network devices. This thesis presents an implementation of INT in P4 with offloaded event detection. We use a programmable P4 testbed to perform an experimental evaluation, which reveal that offloading does indeed benefit INTCollector in terms of performance. Offloading also comes with the advantage of  allowing parameters of the event detection logic at the data plane to be accessible to the control plane.