Re: I-D Action: draft-thomson-postel-was-wrong-01.txt

[Joe Touch <[e-mail]>]

On 6/23/2017 2:25 AM, Dearlove, Christopher (UK) wrote:
> 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. 
That's quite typical. Many protocols clearly indicate explicit invalid
cases.

> 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.
When not specified, "silently ignore" is another option.

> - 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.
Agreed.
> - 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.
Right - the Postel Principle isn't a license to be lazy in either a
protocol spec or implementation.

Joe