Re: [MMUSIC] Magnus Westerlund's Discuss on draft-ietf-mmusic-data-channel-sdpneg-25: (with DISCUSS and COMMENT)

"Roni Even (A)" <roni.even@huawei.com> Thu, 11 April 2019 08:01 UTC

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From: "Roni Even (A)" <roni.even@huawei.com>
To: Magnus Westerlund <magnus.westerlund@ericsson.com>, Adam Roach <adam@nostrum.com>, The IESG <iesg@ietf.org>
CC: "mmusic-chairs@ietf.org" <mmusic-chairs@ietf.org>, "draft-ietf-mmusic-data-channel-sdpneg@ietf.org" <draft-ietf-mmusic-data-channel-sdpneg@ietf.org>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] Magnus Westerlund's Discuss on draft-ietf-mmusic-data-channel-sdpneg-25: (with DISCUSS and COMMENT)
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Date: Thu, 11 Apr 2019 08:01:05 +0000
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Subject: Re: [MMUSIC] Magnus Westerlund's Discuss on draft-ietf-mmusic-data-channel-sdpneg-25: (with DISCUSS and COMMENT)
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Hi,
I looked at https://tools.ietf.org/html/draft-ietf-rtcweb-data-protocol-09 there label is defined

Label: Variable Length (sequence of characters)
      The name of the Data Channel as a UTF-8 encoded string as
      specified in [RFC3629<https://tools.ietf.org/html/rfc3629>]9>].  This may be an empty string.

Also protocol is specified there as
  Protocol: Variable Length (sequence of characters)
      If this is an empty string the protocol is unspecified.  If it is
      a non-empty string, it specifies a protocol registered in the
      'WebSocket Subprotocol Name Registry' created in [RFC6455<https://tools.ietf.org/html/rfc6455>]5>].  This
      string is UTF-8 encoded as specified in [RFC3629<https://tools.ietf.org/html/rfc3629>]9>].

This will map to w3c https://www.w3.org/TR/webrtc/  USVString defined for both protocol and label.  This made me wonder since the sub-protocol identifier in RFC6455 defines as a token (RFC2616) which in my understanding is ASCII.

draft-ietf-mmusic-data-channel-sdpneg-25 defies both as quoted string . Note that the mmusic document is referenced by CLUE data channel using SDP.

I think that taking Adam's suggestion make sense. I also think that specifying SDP values to only use UTF-8 may have issues with registration in IANA

So we can add to section 5.1.3

"In order to communicate with WEbRTC API the label attribute should

  *   Serialize the WebRTC label as a UTF-8 string
  *   Treat the UTF-8 serialization as a series of bytes
  *   For each byte in the serialization:
     *   If the byte can be expressed as a `quoted-char`, do so
     *   Otherwise, express the byte as an `escaped-char`.

"


Is this OK?


Roni



From: Magnus Westerlund [mailto:magnus.westerlund@ericsson.com]
Sent: Wednesday, April 10, 2019 11:18 AM
To: Adam Roach; The IESG
Cc: mmusic-chairs@ietf.org; draft-ietf-mmusic-data-channel-sdpneg@ietf.org; mmusic@ietf.org
Subject: Re: [MMUSIC] Magnus Westerlund's Discuss on draft-ietf-mmusic-data-channel-sdpneg-25: (with DISCUSS and COMMENT)

On 2019-04-09 18:51, Adam Roach wrote:
On 4/9/19 4:00 AM, Magnus Westerlund wrote:
On 2019-04-08 17:55, Adam Roach wrote:
On 4/5/19 7:04 AM, Magnus Westerlund via Datatracker wrote:

----------------------------------------------------------------------

DISCUSS:

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There might be a serious issue in label definition.



5.1.3.  Label Parameter



   The 'label' parameter indicates the name of the channel.  It

   represents a label that can be used to distinguish, in the context of

   the WebRTC API [WebRtcAPI], an RTCDataChannel object from other

   RTCDataChannel objects.  This parameter maps to the 'Label' parameter

   defined in [I-D.ietf-rtcweb-data-protocol].  The 'label' parameter is

   optional.  If it is not present, then its value defaults to the empty

   string.



  label-opt       = "label=" quoted-string

  quoted-string   = DQUOTE *(quoted-char / escaped-char) DQUOTE

  quoted-char     = SP / quoted-visible

  quoted-visible  = %x21 / %x23-24 / %x26-7E ; VCHAR without " or %

  escaped-char    = "%" HEXDIG HEXDIG



I interpret that as the intention is to enable the SDP Attribute to carry the

label as defined in W3C API. That value is in the current candidatate

specification an https://www.w3.org/TR/webrtc/ as an USVSsting

(https://heycam.github.io/webidl/#idl-USVString). And in the reference version

of the WebRTC API as an DOMstring. Both are not limited to ASCII and may

contain any Unicode characters. Thus the escaping mechanism defined appear to

be insufficient.



I think the "quoted-string" need a definition of what type of string this truly

are so that it is clear what a character in the string is.



In addition the specification of escaping is undersspecified. I would recommend

at least adding discussion of the need and how to escape DQUOTE and % that can

be relatively common operations.



I can see that this could benefit from some explicit detail about how to convert the cited USVString string into the "quoted-string" construct defined here. I think this roughly consists of:

  *   Serialize the WebRTC label as a UTF-8 string
  *   Treat the UTF-8 serialization as a series of bytes
  *   For each byte in the serialization:

     *   If the byte can be expressed as a `quoted-char`, do so
     *   Otherwise, express the byte as an `escaped-char`.

Would including text to that effect satisfy your concern?

Yes it would be clear what is expected. However, I fail to understand why one need to ASCIIify and escape the UTF-8 encoded characters where one could simply define the label value as an UTF-8 string something that is compatible with SDP specification. Wouldn't that reduce the set of characters needing escaping to a much smaller set.



That's a little tricky. RFC 4566: "SDP also allows other character sets such as ISO 8859-1 to be used when desired."

Even if we just constrain this to SIP, RFC 3261: "When no explicit charset parameter is provided by the sender, media subtypes of the 'text' type are defined to have a default charset value of 'UTF-8'."

So there's a bias towards UTF-8, but no strict requirement.

I'd support a short document that deprecated non-UTF-8 encodings of SDP so that specifications like this one could simply assume that the full character set can be used directly, but that's not the current state of affairs.

Is this something that actually should be addressed in4566bis?

Anyway, I am fine with the previous outline for solution as my issue was that it was not clear how to get the intended labels into the SDP when desired in a way that is interoperable. So I will remove my discuss when the document has been addressed.

cheers



Magnus Westerlund



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Network Architecture & Protocols, Ericsson Research

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Torshamnsgatan 23           | Mobile +46 73 0949079

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