Re: [Cbor] [art] Artart early review of draft-ietf-cbor-file-magic-02

Michael Richardson <> Thu, 26 August 2021 18:27 UTC

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From: Michael Richardson <>
To: (Dale R. Worley), Bernard Aboba <>,,,
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Date: Thu, 26 Aug 2021 14:27:38 -0400
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Subject: Re: [Cbor] [art] Artart early review of draft-ietf-cbor-file-magic-02
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Dale R. Worley <> wrote:
    > There's an odd characteristic about this document, not in its content,
    > but in its style.  It's written as a proposal (a word which is used
    > several times), and yet it's intended to be a BCP, which tells people
    > what do to.

Yes, fair point.
1) It was written as an invididual draft originally, and sought feedback.
   (Request for Comments...)

2) It's a protocol for writing a protocol really.
   Each author that has something they want to mark up are told to think
   about the different parts, and then register something and document it in
   their document.
   So it's more of a framework for CBOR authors to follow.
   That's perhaps why it's not as authoritative as normal.

    > A magic number is ideally a unique fingerprint, present in the first
    > 4 or 8 bytes of the file, which does not change when the contents
    > change, and does not depend upon the length of the file.

    > (Though it would be more accurate to say, "... present at a defined
    > location in the first few bytes of the file ...".)  If this was a
    > specification, it would say essentially the same fact as "This scheme
    > has the desired property of magic numbers of having a unique fingerprint
    > at a fixed location near the beginning of the data."

Good comments.
I can re-edit to be more authoritative if you like.

    > I get the sense that the authors are ambivalent whether they are saying
    > what implementers should do, or just floating an idea.  Though I don't
    > know if that is accurate.

    > As far as content goes, I favor the "tag wrapped" version over the "tag
    > sequence", as it has assured upward-compatibility (if the implementation
    > follows the requirement to ignore unknown tags).

I also prefer it, although I implemented the tag sequence in the protocol
that made me write this document.

    > In particular, if the data consists of a sequence of CBOR items, each one could be tagged (if
    > they might differ) or only the first one be tagged (if they don't).  But
    > that's a different matter.

Michael Richardson <>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide