I/O tracing is implemented in a system in which an I/O device is configured for protocol offload. A data unit having headers and payload is replicated and the replicated unit sent to the end node along with the payload data. In an alternative embodiment, the I/O device keeps track of the one or more protocol headers layers separately from the application payload. Information defining the relationship between the headers and payload is sent to the end node to enable it to reconstruct the data unit.