Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc4566bis-34.txt> (SDP: Session Description Protocol) to Proposed Standard
Magnus Westerlund <magnus.westerlund@ericsson.com> Tue, 16 April 2019 08:47 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: Tue, 16 Apr 2019 08:47:25 +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 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 >>> > _______________________________________________ > mmusic mailing list > mmusic@ietf.org > https://www.ietf.org/mailman/listinfo/mmusic > -- Magnus Westerlund ---------------------------------------------------------------------- Network Architecture & Protocols, Ericsson Research ---------------------------------------------------------------------- Ericsson AB | Phone +46 10 7148287 Torshamnsgatan 23 | Mobile +46 73 0949079 SE-164 80 Stockholm, Sweden | mailto: magnus.westerlund@ericsson.com ----------------------------------------------------------------------
- [MMUSIC] Last Call: <draft-ietf-mmusic-rfc4566bis… The IESG
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Magnus Westerlund
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Roni Even (A)
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Paul Kyzivat
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Magnus Westerlund
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Paul Kyzivat
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Magnus Westerlund
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Paul Kyzivat
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Magnus Westerlund
- Re: [MMUSIC] Last Call: <draft-ietf-mmusic-rfc456… Magnus Westerlund