Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc4566bis-34.txt> (SDP: Session Description Protocol) to Proposed Standard

Magnus Westerlund <magnus.westerlund@ericsson.com> Fri, 03 May 2019 09:59 UTC

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From: Magnus Westerlund <magnus.westerlund@ericsson.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc4566bis-34.txt> (SDP: Session Description Protocol) to Proposed Standard
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Date: Fri, 3 May 2019 09:59:44 +0000
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Subject: Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc4566bis-34.txt> (SDP: Session Description Protocol) to Proposed Standard
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Hi,

There appear that there are not much interest in this issue. I think if
there no one else that have input into this then there are not a problem
that affects anyone, but it also leaves things in an uncertain state,
but I guess that is what we will do.

Cheers

Magnus

On 2019-04-16 10:48, Magnus Westerlund wrote:
> Hi Paul,
>
> I think this needs input from more people.
>
> I did make a mistake in my thinking about the charset encodings. With
> UTF-8 there are only US-ASCII that are a pure sub-set of UTF-8 when
> encoded. So you are right about the implications.
>
> Cheers
>
> Magnus
>
> On 2019-04-16 01:17, Paul Kyzivat wrote:
>> On 4/15/19 4:02 AM, Magnus Westerlund wrote:
>>> Hi Paul,
>>>
>>> If I attempt to interpret what you are saying as conclusion it has
>>> several implications, or am I missunderstanding you?
>>>
>>> 1. Obsolete a=charset attribute. The only other options would be to
>>> define a small set of attribute to which it applies like "i=".
>> That is what I am suggesting. I can't see how it ever could have worked 
>> *in general* for arbitrary charset values. And at this point I don't see 
>> a need.
>>
>> Alternatively, restrict the charsets which can work to those which have 
>> some benign values.
>>
>> IMO the most troublesome charsets are those that are wider than one 
>> byte, like UTF-16 or UTF-32, since they would have to coexist with the 
>> rest of the sdp being the ascii subset of UTF-8. Also troublesome are 
>> charsets not vaguely related to ascii, like the variants of EBCDIC.
>>
>>> 2. Clarify section 5 that using UTF-8 characters without NUL, CR, LF in
>>> attribute values and textual fields are okay.
>> That would amount to limiting charset to UTF-8, which is same as 
>> deprecating it. 
>
>>> 3. Define how attribute values that may contain NUL, CR and LF use a
>>> specified escaping mechanism
>> Right now it says that the charset must define an escaping mechanism, 
>> without saying how that might happen. AFAIK *none* of the charsets 
>> define an escaping mechanism.
>>
>> But I also don't see how one could define such a mechanism independent 
>> of the charset since it must presumably reserve some character(s) from 
>> that charset to introduce the escape.
>>
>>> 4. Define an escaping mechanism that applies to SDP and UTF-8 strings
>>>
>>> 5. Legacy warning in attempting to use escaping mechanism in old
>>> attributes.
>> Not sure what to say about 4 & 5.
>>
>>> Is this a fair summary?
>> Whatever.
>>
>> I'm hoping I'm missing something about how this was intended to work in 
>> the first place.
>>
>> 	Thanks,
>> 	Paul
>>
>>> Cheers
>>>
>>> Magnus
>>>
>>>
>>>
>>> On 2019-04-12 17:45, Paul Kyzivat wrote:
>>>> On 4/12/19 4:59 AM, Magnus Westerlund wrote:
>>>>> Hi,
>>>>>
>>>>> To be clear, I am not proposing that any existing attribute would be
>>>>> forced to suddenly accept UTF-8 strings with no charset restriction.
>>>>> Simply that new attribute's values can be defined to be UTF-8 in
>>>>> general. I think the important distinction here is that a parser must be
>>>>> ready to accept any UTF-8 character as the sender can't know if a
>>>>> particular charset limitation applies to a specific consumer. However,
>>>>> in form I don't see a problem of the SDP itself informing the consumer
>>>>> that there is a charset limitation applied in this document.
>>>>>
>>>>> My interpretation of the current situation is that it we hesitate to use
>>>>> UTF-8 fields due to the uncertainty in the requirements on consumers of
>>>>> SDP.
>>>> After re-reviewing the text, I think we may have a can of worms here
>>>> that has existed a long time, perhaps from the beginning. Let me explain:
>>>>
>>>> Section 5 says:
>>>>
>>>>      An SDP description is entirely textual.  SDP field names and
>>>>      attribute names use only the US-ASCII subset of UTF-8, but textual
>>>>      fields and attribute values MAY use the full ISO 10646 character set
>>>>      in UTF-8 encoding, or some other character set defined by the
>>>>      "a=charset:" attribute.
>>>>
>>>> Section 6.10 (charset attribute) says:
>>>>
>>>>      The charset specified MUST be one of those registered in the IANA
>>>>      Character Sets registry (http://www.iana.org/assignments/character-
>>>>      sets), such as ISO-8859-1.
>>>>      ...
>>>>      Note that a character set specified MUST still prohibit the use of
>>>>      bytes 0x00 (Nul), 0x0A (LF), and 0x0d (CR).  Character sets requiring
>>>>      the use of these characters MUST define a quoting mechanism that
>>>>      prevents these bytes from appearing within text fields.
>>>>
>>>> The charsets registry has a *lot* of entries, including a lot of ancient
>>>> obsolete ones not vaguely related to ascii. (E.g., EBCDIC-related ones.)
>>>> Many of these refer to RFC1345, which seems to be a pre-unicode attempt
>>>> to rationalize character sets. The charsets registry also includes
>>>> UTF-16 and UTF-32. It is really hard to understand how certain parts of
>>>> an SDP body might be in UTF-8 while other parts are in UTF-32.
>>>>
>>>> I can't make any sense of:
>>>>
>>>>      Character sets requiring
>>>>      the use of these characters MUST define a quoting mechanism that
>>>>      prevents these bytes from appearing within text fields.
>>>>
>>>> AFAIK character sets don't define quoting mechanisms. Also, I don't know
>>>> what it means for a charset to require use of particular characters. And
>>>> this text is very sloppy in muddling the use of "character" and "byte".
>>>>
>>>> The bottom line is that this is a mess. I'm not sure if it ever could
>>>> have worked. Nor do I understand what usage it was trying to
>>>> accommodate. (Note that most of this stuff is largely unchanged since
>>>> RFC2327.)
>>>>
>>>> Perhaps all the charset-dependent stuff should be obsoleted.
>>>>
>>>> 	Thanks,
>>>> 	Paul
>>>>
>>>> 	
>>>>
>>>> _______________________________________________
>>>> mmusic mailing list
>>>> mmusic@ietf.org
>>>> https://www.ietf.org/mailman/listinfo/mmusic
>>>>
>> _______________________________________________
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>> mmusic@ietf.org
>> https://www.ietf.org/mailman/listinfo/mmusic
>>

-- 

Magnus Westerlund 

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