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

Paul Kyzivat <pkyzivat@alum.mit.edu> Wed, 21 October 2015 15:43 UTC

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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Wed, 21 Oct 2015 11:43:46 -0400
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Subject: Re: [MMUSIC] draft-ietf-mmusic-msrp-usage-data-channel - "a=setup" versus "a=dcsa:x setup"
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On 10/9/15 10:03 AM, Juergen Stoetzer-Bradler wrote:
> 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.

IMO it makes perfect sense to separate out the negotiation of the msrp 
role via a=dcsa setup.

That still leaves a=setup overloaded, for both SCTP setup and DTLS 
setup. There really ought to be separate negotiation of those two.

	Thanks,
	Paul

> 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.
>
>
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