Re: [abnf-discuss] Target audience for ABNF

Carsten Bormann <> Mon, 20 November 2017 17:45 UTC

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From: Carsten Bormann <>
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Date: Mon, 20 Nov 2017 18:45:18 +0100
Cc: Dave Crocker <>,
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To: Paul Kyzivat <>
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Subject: Re: [abnf-discuss] Target audience for ABNF
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Hi Paul,

I think these are good observations for a future FDT (ABNFplus :-) or CDDL 2.0).

> Yes, and I gather that is what is being planned for the future. This will work ok if the ABNF is complete and self contained. But there are large numbers of important RFCs where it isn't, and in a practical sense can't be. For instance RFC6665 is an extension to RFC3261. It includes the following as a prelude to the ABNF it includes:
>   The Augmented BNF [RFC5234] definitions for the various new and
>   modified syntax elements follows.  The notation is as used in
>   [RFC3261], and any elements not defined in this section are as
>   defined in SIP and the documents to which it refers.

Of course, YANG solves this by defining an import/export interface.
ABNF could grow a convention that makes something like

> 	foo = <defined in RFCxxxx>

actually well-defined.
(We have discussed this for CDDL and didn’t want to include this in this round.)

> Another issue is that there is ambiguity about the status of the Core Rules defined in RFC5234. Can anyone writing an ABNF grammar assume that the core rules are predefined for their use? Or must the grammar indicate specifically (how?) that it is implicitly including their definitions? If they are automatically present, then they are reserved rule names, and cannot be defined differently in any ABNF grammar.

Right.  In CDDL, we have a defined “prelude” that is added to all specifications, effectively becoming a set of reserved names.  ABNF could use the above convention with RFC 5234 as the source, or grow a wholesale “import RFC5234” style statement.

Grüße, Carsten