Re: [netmod] [Technical Errata Reported] RFC6020 (4911)

Hi Lada,

I don't feel strongly, but basically agree with Martin.

The YANG 1.0 spec is ambiguous, YANG 1.1 has fixed that.  I guess that 
most pragmatic YANG 1.0 implementations would treat a backslash followed 
by something other than n,t,", or / as just a backslash character.

This issue seems to only come up in the context of pattern statements.

The best pragmatic answer that I've seen to this is from rfc6087bis 
section 5.11.2: "Patterns and Ranges" which states that implementations 
SHOULD use single quotes rather than double quotes for patterns and 
hence carefully side steps the issue.

This is the solution that we used to fix the issue in our proprietary 
modules, and also the one that I recommended to OpenConfig for their 
models (but I don't know whether they will implement this).

Rob

On 18/01/2017 14:24, Ladislav Lhotka wrote:
>> On 18 Jan 2017, at 14:55, Martin Bjorklund <mbj@tail-f.com> wrote:
>>
>> RFC Errata System <rfc-editor@rfc-editor.org> wrote:
>>> The following errata report has been submitted for RFC6020,
>>> "YANG - A Data Modeling Language for the Network Configuration
>>> Protocol (NETCONF)".
>>>
>>> --------------------------------------
>>> You may review the report below and at:
>>> http://www.rfc-editor.org/errata_search.php?rfc=6020&eid=4911
>>>
>>> --------------------------------------
>>> Type: Technical
>>> Reported by: Ladislav Lhotka <lhotka@nic.cz>
>>>
>>> Section: 6.1.3
>>>
>>> Original Text
>>> -------------
>>> Within a double-quoted string (enclosed within " "), a backslash
>>> character introduces a special character, which depends on the
>>> character that immediately follows the backslash:
>>>
>>> \n      new line
>>> \t      a tab character
>>> \"      a double quote
>>> \      a single backslash
>>>
>>>
>>> Corrected Text
>>> --------------
>>> Within a double-quoted string (enclosed within " "), a backslash
>>> character introduces a special character, which depends on the
>>> character that immediately follows the backslash:
>>>
>>> \n      new line
>>> \t      a tab character
>>> \"      a double quote
>>> \      a single backslash
>>>
>>> The backslash MUST NOT be followed by any other character.
>>>
>>> Notes
>>> -----
>>> The text doesn't state whether other characters may follow the
>>> backslash, and if yes, what it means. Existing implementations have
>>> used three approaches:
>>>
>>> 1. report an error if another character follows the backslash
>>> 2. keep only the character following the backslash, i.e., for example,
>>> "\x" is the same as "x".
>>> 3. keep both the backslash and the character following it.
>>>
>>> This ambiguity is undesirable and YANG 1.1 [RFC 7950] explicitly
>>> adopted option #1. However, many modules are still being written using
>>> YANG version 1.0, so it is important to clarify this issue in RFC 6020
>>> as well.
>> I don't think this errata should be accepted.  As stated, the spec is
>> unclear, and YANG 1.1 has fixed this problem.  But it is not clear
>> that the original intention when RFC 6020 was written was #1.
>> Accepting this errata now would make existing implementations and
>> modules invalid.
> The problem is that the spec is clearly ambiguous and it is impossible to decide whether such a module is valid or not and, if it is, what the other backslash-escaped characters mean. Existing implementations can already reject such modules - the fact that pyang (and probably other tail-f tools) adopted one interpretation doesn't mean that everybody does the same.
>
>> The solution moving forward is to use YANG 1.1.
>>
> YANG 1.0 modules continue to be written, and I think it is important to stop this problem from spreading further. I think tools should at least issue a warning because otherwise future upgrades to YANG 1.1 may become a nightmare - modules will suddenly break in unexpected places.
>
> If this erratum is rejected, what is the basis for accepting erratum #4909 that started this discussion?
>
> Lada
>
>> /martin
> --
> Ladislav Lhotka, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
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