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

Magnus Westerlund <> Wed, 10 April 2019 08:18 UTC

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From: Magnus Westerlund <>
To: Adam Roach <>, The IESG <>
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Thread-Topic: [MMUSIC] Magnus Westerlund's Discuss on draft-ietf-mmusic-data-channel-sdpneg-25: (with DISCUSS and COMMENT)
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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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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:


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

  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 as an USVSsting
( 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.


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:<>