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

Paul Kyzivat <pkyzivat@alum.mit.edu> Thu, 14 May 2020 15:57 UTC

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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Thu, 14 May 2020 11:57:28 -0400
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Archived-At: <https://mailarchive.ietf.org/arch/msg/mmusic/-Oq40I2_hW7Btbdxvjm7JQaJ4-Y>
Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-msrp-usage-data-channel-16
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On 5/13/20 2:37 PM, Christer Holmberg wrote:
> Hi,
> 
>      >> One more thing: if you by "destination" mean the IP address and port of the remote peer, you get that from the c/m- line.
>      >
>      > And where does *the* m line get the fqdn/ip from???
>      >
>      > Assume you gave me your msrp address once, and I have it in my address
>      > book. Today I decide I want to chat with you. I need to set up a session
>      > with you, and your msrp url is what I have to work with.
>      >
>      > Because the URL has the "dc" parameter I can infer that I will need a
>      > data channel. I must set up the the DTLS session and then create a data
>      > channel to do that. How do I know what to put into the c/m- line? Are
>      > you saying that I need some *other* information about you before I can
>      > do that? Or can I derive the c/m- line data from it?
> 
>      I don't think an MSRP-URI is something you would put in your address book. If you use SIP to negotiate the MSRP session, then you would have a SIP-URI in your address book, and you would use that to establish a SIP session in order to negotiate the MSRP session.
> 
>      Even in RFC 4975, you use SIP and offer/answer to negotiate the MSRP session. You don't just start sending MSRP messages without exchanging the MSPR session parameters (including path) first.
> 
>       But, maybe I completely misunderstand your issue.

I don't know. I'm trying to understand the intended usage and how it 
plays out in the example. If what I have in my address book for you is 
just your sip URI, then how do I figure out how to establish a chat 
session? How do I decide to use a data channel rather than MSRP over 
TLS? And do I just somehow derive the msrp URL from your sip URL?

I guess if I am running in a browser and using WebRTC then I try a data 
channel because there is no alternative I can use. But that isn't this 
example. Must I have something else in my address book to tell me what 
you support? Or should I be trying both in my invite and seeing what you 
accept? (That also isn't this example.)

I would hope that the draft doesn't require magical thinking for the 
example to work.

	Thanks,
	Paul