Re: [MMUSIC] WGLC on draft-ietf-mmusic-msrp-usage-data-channel-16

Paul Kyzivat <pkyzivat@alum.mit.edu> Wed, 13 May 2020 14:24 UTC

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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Wed, 13 May 2020 10:24:15 -0400
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Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-msrp-usage-data-channel-16
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Christer,

Comment at end.

On 5/12/20 9:58 AM, Christer Holmberg wrote:
> Hi,
>      >>>- Added text to make more clear the scope of path and MSRP URI:
>      >>
>      >>>Data channels are negotiated independently of the value of the "path"
>      >> attribute. This means that when MSRP messages are transported on a data
>      >> channel, the "path" attribute
>      >>
>      >>>is not used for transport negotiation or routing of the messages. MSRP
>      >> endpoints using data channels can set the value of the "path" attribute,
>      >> including the MSRP URI, and use it as described in RFC4975
>      >>
>      >> I think we need to be a little more explicit, so I suggest something like:
>      >>
>      >> "When MSRP messages are transported on a data channel, the "path"
>      >> attribute is not used for routing of the messages. The MSRP data channel
>      >> is established using the SDP offer/answer procedures defined in
>      >> [I-D.ietf-rtcweb-data-channel], and the MSRP messages are then
>      >> transported on that data channel. Because of that an MSRP endpoint can
>      >> set the MSRP-URI authority value of the "path" attribute at its own
>      >> discretion, following the syntax and rules in [RFC4975]."
>      >
>      > I'm concerned that this might imply that an msrp(s) URI is not
>      > sufficient information to establish communication. (That some additional
>      > information, received out-of-band, might be a prerequisite.)
>      >
>      > Do you intend that to be a possible situation?
>      >
>      > I would hope that it is not - that an msrp(s) URI is sufficient. In that
>      > case, both the transport setup and the path attribute would be derived
>      > from the same source.
> 
>      I am not sure I understand....
> 
>      The MSRP URI is *not* enough to establish the SCTP association and the data channel. The procedures for establishing a data channel are defined in [I-D.ietf-rtcweb-data-channel], and then each usage have some specific attribute values etc.
> 
>     We are not modifying the procedures how to establish a data channel - we are defining how to send MSRP messages over a data channel.

[I-D.ietf-rtcweb-data-channel] defines *procedures* for establishing a 
data channel, just as SIP provides procedures for establishing a sip 
session. In both cases, those procedures must be parameterized. In the 
case of SIP a sip URI is generally sufficient. (In some environments you 
might need some extra environmental information, such as how to get 
yourself registered first and the need to use an access proxy.)

In this case, the msrp(s) uri should provide all the parameterizing 
information I need to send msrp messages to this destination, over and 
above info about my environment and basic standards. The "dc" parameter 
in the URI tells me that I need use a data channel.

I shouldn't need to supply any extra information *about the destination*.

	Thanks,
	Paul