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

[Juergen Stoetzer-Bradler <[e-mail]>]

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.