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To summarize, I've shown that protocol evolution can be modeled similarly to format evolution and that the format evolution rules can be applied to protocol evolution but this has limited usability. I've suggested that underlying protocol evolution is different from format evolution because protocol state machines typically do not allow substitutability of a newer state machine with an older one, that the presence of a network results in desire for earlier failures. The result of this analysis is that protocol designers can and should provide format and protocol evolution but that they shouldn't have high hopes that they can regularly provide for compatible protocol evolution.

Dave Orchard's Blog: Protocol Extensibility and Versioning