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

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

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.