How to apply a fault injector to evaluate the dependability of a network protocol implementation is the main focus of this paper. In the last years, we have been developing FIRMAMENT, a tool to inject faults directly into messages that pass through the kernel protocol stack. SCTP is a promising new protocol over IP that, due its enhanced reliability, is competing with TCP where dependability has to be guaranteed. Using FIRMAMENT we evaluate the error coverage and the performance degradation of SCTP under faults. Performing a complete fault injection campaign over a third party software give us a deep insight about the additional test strategies that are needed to reach significant dependability measures.