Because protocol software is difficult and expensive to implement and test, it is often ported between systems instead of rewritten from scratch. Unfortunately, porting protocol software can be as difficult as from-scratch development, due to inherent differences in subsystem design. Thus, protocol subsystems can have a profound effect on the portability of a protocol implementation. We propose an approach permitting the incorporation of new protocols into a subsystem other than their ``native'' one without the drawbacks or expense of porting and original development. Our approach is based on protocol module encapsulation, which allows unmodified protocol code developed for one protocol subsystem to be used within another. We relate our experiences designing, implementing, and measuring the performance of our protocol encapsulation modules, using an AppleTalk protocol stack as a baseline.