Joe Touch wrote:
Liberal means that if it's possibly valid, you should accept it as such.
That necessitates the protocol designer explicitly flagging some things as
invalid. Obvious example is a should be signed message lacking a signature. If
taking the most liberal view (as above) the protocol needs to say something
like "if the signature is missing or invalid, then the message must be
rejected". I don't think that's anything new, I've seen it done.
I can see at least the following cases where making intent clear is, in my
opinion at least, a good idea:
- Security and other sensitive cases of failure. Need to say explicitly reject.
- Mechanisms designed for extensions. While the Postel principle makes it
unnecessary to say so, it really doesn't hurt saying that a message shouldn't
be rejected just for this reason.
- Where what you receive is a container of multiple things (messages in a
packet, TLVs in a message). Making the assumed dependence/independence clear
doesn't hurt (if rejecting/ignoring one, does this impact on the others?).
That's not something that spirals out of control in size, a couple of sentences
would cover most cases.
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