Overview#Protocol ossification happens when new Protocol features or changes in behavior that were not known before risks ending up considered bad or illegal by such systems or devices.
Often these boxes were deployed some time ago when the protocols had a feature set of that time. Introducing new features or changes in behavior that were not known before risks ending up considered bad or illegal by such boxes. Such traffic may well just be dropped or delayed to the degree that users really do not want to use those features.
Changes to TCP or TLS also suffer from Protocol ossification: some boxes between a client and the remote server will spot unknown new TCP options and block such connections since they do not know what the options are. If allowed to detect protocol details, systems learn how protocols typically behave and over time it becomes impossible to change them.