Re: [MMUSIC] draft-ietf-mmusic-msrp-usage-data-channel - "a=setup" versus "a=dcsa:x setup"

[Christian Groves <[e-mail]>]

Hello Albrecht and Christer,

The "rule of thumb" I had is covered by Juergen's text. It was basically 
that an endpoint only includes a=dsca:setup if it wanted a role 
different than what is indicated in a=setup. However I think Christer 
has a separate view.

I think the original source of the problem is that RFC6135 links the 
MSRP establishment role to the a=setup. I.e. the active side is the one 
that sends the first MSRP message. So these are linked. To unlink it for 
MSRP over DC its proposed to use a=setup (which was defined for 
transport connection setup) in the DC (via dsca) to control the MSRP 
protocol setup.

I think at the end of the day the decision is whether we want every 
WebRTC endpoint to send "a=setup" and "a=dsca:setup" every time it wants 
to set up a MSRP datachannel or whether it only has to send a=dsca:setup 
if wants a different role from the a=setup.

Regards, Christian

On 16/10/2015 9:20 AM, Christer Holmberg wrote:
> Albrecht, I can't parse what you said.
>
> If a piece of information is required in order for something to work, 
> then it must be mandatory to include that piece of information. Of 
> course, you may define an implicit default value, but the information 
> must be there.
>
> Regards,
>
> Christer
>
> Sent from my Windows Phone
> ------------------------------------------------------------------------
> From: Schwarz, Albrecht (Albrecht) 
> <mailto:[e-mail]>
> Sent: ‎14/‎10/‎2015 20:09
> To: Christer Holmberg <mailto:[e-mail]>; 
> Stoetzer-Bradler, Juergen (Juergen) 
> <mailto:[e-mail]>; [e-mail] 
> <mailto:[e-mail]>; Christian Groves 
> ([e-mail]) <mailto:[e-mail]>
> Subject: RE: [MMUSIC] draft-ietf-mmusic-msrp-usage-data-channel - 
> "a=setup" versus "a=dcsa:x setup"
>
> Hello Christer,
>
> such a prescriptive sentence
>
> />//If a setup value is required for the MSRP session establishment, 
> then the dcsa setup attribute must be mandatory./
>
> relates to a “*conditionally ‘mandatory’*”, i.e., an *optional* 
> tagging in general, right?
>
> @Christian, you had a good rule of thumb proposed, covering the 
> conditionality.
>
> What’s your view?
>
> Regards,
>
> Albrecht
>
> *From:*mmusic [mailto:[e-mail]] *On Behalf Of *Christer 
> Holmberg
> *Sent:* Dienstag, 13. Oktober 2015 15:01
> *To:* Stoetzer-Bradler, Juergen (Juergen); [e-mail]
> *Subject:* Re: [MMUSIC] draft-ietf-mmusic-msrp-usage-data-channel - 
> "a=setup" versus "a=dcsa:x setup"
>
> Hi,
>
> If a setup value is required for the MSRP session establishment, then 
> the dcsa setup attribute must be mandatory.
>
> Regards,
>
> Christer
>
> *From:*Juergen Stoetzer-Bradler 
> [mailto:[e-mail]]
> *Sent:* 13. lokakuuta 2015 15:55
> *To:* Christer Holmberg; [e-mail] <mailto:[e-mail]>
> *Subject:* Re: [MMUSIC] draft-ietf-mmusic-msrp-usage-data-channel - 
> "a=setup" versus "a=dcsa:x setup"
>
> Hello Christer,
>
> Thank you for your comments.
>
> A dcsa embedded setup attribute does not have any MSRP transport 
> protocol related establishment role semantics.
> But an MSRP over data channel endpoint still needs to know its MSRP 
> session establishment role.
> In some data channel to TCP (or TLS) transport interworking cases, 
> which are described in section 6,
> this MSRP session establishment role cannot always be derived from the 
> DTLS/SCTP establishment role.
> In such cases a dcsa embedded setup attribute for MSRP seems to be 
> required.
> However, in some other interworking cases, and especially in 
> end-to-end data channel cases the MSRP session
> establishment role could be tightly coupled with the DTLS/SCTP 
> establishment roles and hence with the "a=setup" attribute.
> If we don't allow the "a=setup" attribute to also describe the MSRP 
> session establishment role (if no "a=dcsa:x setup"
> attribute is present), then we should probably specify that a dcsa 
> embedded setup attribute is mandatory
> for MSRP over data channel sessions.
>
> For MSRP as sub-protocol, would a mandatory "a=dcsa:x setup:<role>" 
> attribute be agreeable?
>
> Thanks,
> Juergen
>
> On 10.10.2015 06:26, Christer Holmberg wrote:
>
>     Hi.
>
>     First, I think you suggest far too much text, but that is editorial.
>
>     Second, as you say, the media level 'setup' attribute is used to
>     negotiate the SCTPoDTLS association establishment. But, I don't
>     think it should be used to negotiate the MSRP roles etc in case
>     the 'dcsa:x setup' is not present. The 'setup' attribute has
>     nothing to do with the protocol carried within the data channel -
>     the reason we defined the 'dsca' attribute was to be able to carry
>     such information.
>
>     Regards,
>
>     Christer
>
>     Sent from my Windows Phone
>
>     ------------------------------------------------------------------------
>
>     *From: *Juergen Stoetzer-Bradler
>     <mailto:[e-mail]>
>     *Sent: *‎09/‎10/‎2015 17:03
>     *To: *[e-mail] <mailto:[e-mail]>
>     *Subject: *[MMUSIC] draft-ietf-mmusic-msrp-usage-data-channel -
>     "a=setup" versus "a=dcsa:x setup"
>
>     Hello,
>
>     An MSRP over data channel related SDP media description will have
>     a media level "a=setup:<role>" attribute and may additionally
>     contain an "a=dcsa" encapsulated setup attribute "a=dcsa:x
>     setup:<role>". Current version
>     draft-ietf-mmusic-msrp-usage-data-channel-02
>     does not yet have any text related to the setup attribute except
>     for listing it as potential dcsa encapsulated MSRP specific attribute.
>     As the media level "a=setup" attribute is used to negotiation the
>     DTLS and SCTP establishment roles, and as the setup attribute is
>     also used
>     in MSRP over TCP cases to negotiate the "active" role of MSRP
>     session endpoints (RFCs 6135, 6714) we propose to explicitly describe
>     the semantics of dcsa encapsulated setup attributes for MSRP over
>     data channel in draft-ietf-mmusic-msrp-usage-data-channel.
>
>     We propose to add a new section 5.1.1.3 "Media Description Level
>     setup Attribute Versus Data Channel Specific setup Attribute"
>     after section 5.1.1.2 ("Use of dcsa Attribute") and to extend
>     existing section 6 ("Gateway Configuration") with setup attribute
>     related procedures.
>
>     Would the following new section 5.1.1.3 and modified section 6 be
>     agreeable?
>
>     Thanks,
>     Juergen
>
>     Proposed new Section 5.1.1.3:
>
>     5.1.1.3.  Media Description Level setup Attribute Versus Data
>     Channel Specific setup Attribute
>
>        The SDP setup attribute, as introduced in [RFC4145], can be used in
>        WebRTC data channel related SDP media descriptions as a media level
>        attribute, which is directly associated with the corresponding
>     DTLS/
>        SCTP "m" line.  In this case the setup attribute is of the form
>        "a=setup:<role>", where <role> assumes values as defined in
>        [RFC4145].  Such a setup attribute is then used as specified in
>        [I-D.ietf-mmusic-sctp-sdp] in order to negotiate the establishment
>        roles of the DTLS connection and the SCTP association.
>
>        Additionally, the setup attribute can be embedded in a dcsa
>     attribute
>        and hence can explicitly be associated with an MSRP session over a
>        specific data channel.  In such a case it is of the form "a=dcsa:x
>        setup:<role>", with x being the data channel's SCTP stream
>        identifier.  Such a dcsa attribute embedded setup attribute has no
>        relationship with the DTLS connection and SCTP association
>        establishment roles.
>
>        dcsa attribute embedded setup attributes are OPTIONAL for MSRP
>        sessions over data channels.
>
>        If an MSRP over data channel description contains a dcsa embedded
>        setup attribute, then this embedded setup attribute is used to
>        negotiate, which MSRP session endpoint assumes the active role
>     as per
>        Section 4.2.2 of [RFC6135] and Section 5.4 of [RFC4975].
>
>        If an MSRP over data channel description does not contain a dcsa
>        embedded setup attribute, then the media description level
>     "a=setup"
>        attribute, which is associated with the data channel's DTLS
>        connection and SCTP association, is used to negotiate, which MSRP
>        session endpoint assumes the active role.
>
>        If an MSRP over data channel endpoint wants to negotiate an
>     active or
>        non-active MSRP session establishment role, which is different from
>        its DTLS connection and SCTP association establishment role,
>     then it
>        MUST include a dcsa embedded setup attribute for this MSRP session.
>        In this case this dcsa embedded setup attribute's value is
>     different
>        from the value of the DTLS/SCTP "m" line's "a=setup" attribute.
>
>
>     Proposed modified section 6:
>
>     6.  Gateway Configuration
>
>        This section describes the network configuration where one MSRP
>        endpoint uses data channels as MSRP transport, the other MSRP
>        endpoint uses TLS/TCP connections as MSRP transport, and the
>     two MSRP
>        endpoints interwork via an MSRP gateway.
>
>        Specifically, a gateway can be configured to interwork an MSRP
>        session over a data channel with a peer that does not support data
>        channel transport in one of two ways.  In one model, the gateway
>        performs as a MSRP B2BUA to interwork all the procedures as
>     necessary
>        between the endpoints.  No further specification is needed for this
>        model.
>
>        Alternately, the gateway can use CEMA procedures to provide
>     transport
>        level interworking between MSRP endpoints using different transport
>        protocols as follows.
>
>        When the gateway performs transport level interworking between MSRP
>        endpoints, all of the procedures in Section 5 apply to each peer,
>        with the following additions:
>
>        o  The endpoint establishing an MSRP session using data channel
>           transport SHALL NOT request inclusion of any relays, although it
>           MAY interoperate with a peer that signals the use of relays.
>
>        o  The gateway receiving an SDP offer that includes a request to
>           negotiate an MSRP session on a data channel can provide
>     transport
>           level interworking in the same manner as a CEMA SBC by
>     forwarding
>           TCP or TLS transport parameters in a new "m" line with the
>           appropriate attributes within the forwarded SDP offer.
>
>           *  If the received data channel side SDP offer contains a dcsa
>              embedded setup attribute for the to be negotiated MSRP
>     session,
>              then the gateway interworks this dcsa embedded setup
>     attribute
>              with the media description level "a=setup" attribute of this
>              MSRP session's "m" line in its forwarded SDP offer.
>
>           *  If the received data channel side SDP offer does not
>     contain a
>              dcsa embedded setup attribute for the to be negotiated MSRP
>              session, then the gateway interworks the media description
>              level "a=setup" attribute, which is directly associated with
>              the data channel's DTLS/SCTP "m" line, with the media
>              description level "a=setup" attribute of this MSRP session's
>              "m" line in its forwarded SDP offer.
>
>        o  Similarly, a gateway receiving an SDP offer to negotiate an MSRP
>           session using TCP or TLS transport with an endpoint that only
>           supports data channel transport for MSRP can provide transport
>           level interworking in the same manner as a CEMA SBC by
>           establishing a new data channel for the MSRP session with the
>           target endpoint.
>
>           *  In this case the gateway interworks the TCP or TLS associated
>              media description level "a=setup" attribute of the to be
>              negotiated MSRP session's received "m" line either with the
>              media description level "a=setup" attribute of the data
>              channel's DTLS/SCTP "m" line, or with the dcsa embedded setup
>              attribute of this MSRP session's new data channel.
>
>           *  If the gateway's DTLS connection and SCTP association
>              establishment role on its data channel side is equal to the
>              "a=setup" attribute's value of the received MSRP over TCP or
>              TLS "m" line, then the gateway MAY add a dcsa embedded setup
>              attribute to the description of the MSRP session's new data
>              channel in its generated data channel side offer.
>              Alternatively, in this case the gateway MAY omit adding
>     such a
>              dcsa embedded setup attribute to the description of the MSRP
>              session's new data channel in its generated data channel side
>              offer.  Otherwise, if the gateway's DTLS connection and SCTP
>              association establishment role is different from the MSRP
>     over
>              TCP or TLS associated received "a=setup" attribute's value,
>              then the gateway MUST add a dcsa embedded setup attribute to
>              the description of the MSRP session's new data channel in its
>              generated data channel side offer.
>
>           *  If the gateway adds a dcsa embedded setup attribute
>     associated
>              with this MSRP session to its data channel side SDP
>     offer, then
>              the value of this embedded setup attribute MUST be equal
>     to the
>              value of the "a=setup" attribute, which is associated
>     with this
>              MSRP session's "m" line in the received TCP or TLS side SDP
>              offer.
>