Re: [Cbor] MIME tag 257 vs 36

Carsten Bormann <cabo@tzi.org> Sat, 26 September 2020 06:57 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Sat, 26 Sep 2020 08:57:00 +0200
Cc: cbor@ietf.org, Jim Schaad <ietf@augustcellars.com>
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To: Laurence Lundblade <lgl@island-resort.com>
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Subject: Re: [Cbor] MIME tag 257 vs 36
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Hi Laurence,

> On 2020-09-25, at 19:11, Laurence Lundblade <lgl@island-resort.com> wrote:
> 
> Hi Carsten,
> 
> Thanks for the detailed explanation.
> 
> Is this a good summary of the upshot?
> 
> Text lines in Internet protocols (on the wire) are delimited by either a CRLF or just an LF. Officially many protocols specify CRLF, but implementations often work with either.

Indeed.

> CBOR type 3 text can be either line ending, even a mixture of both.

CBOR text strings can contain any UTF-8, so they are agnostic to line endings.
(“CBOR type 3 text” is a pleonasm, but also leaving out the word “major”.)

> Operating systems usually have a line end convention. Windows uses CRLF. Linux and MacOS use LF. Some applications on a given OS may work with either and some may prefer the OS’s line ending convention.

Right.

> The majority of use cases and CBOR protocols using type 3 text will work with either line ending. However, some use cases or protocols may not work with either in which case translation to and/or from the local line end convention, typically that of the OS, is necessary.

I think the important observation is that most of the limitations to one type of line ending stem from using line endings as part of the encoding/framing of a TCP-based protocol.  E.g., SMTP is less tolerant of modern line endings because it needs to react to CRLF dot CRLF (unless some SMTP framing is used).  That is not a problem with CBOR-based protocols, as the framing is coming from the self-delimiting CBOR encoding, not from parsing text streams.

> Are you proposing that CDDL be able to specify a line end convention for type 3 items?

I think that might be useful.  Most text strings in CBOR protocols are “1-D”, i.e., they don’t employ line endings.  Where “2-D” text is intended, indicating a preference for a form of line ending, or a restriction to a form of line ending, might be useful.  Of course, restricting text content can today be done with regexes (and soon with ABNF); there is no .feature in regex/ABNF though…

Grüße, Carsten