One document matched: draft-ietf-mmusic-msid-00.xml

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<rfc category="std" docName="draft-ietf-mmusic-msid-00" ipr="trust200902">
  <front>
    <title abbrev="MSID in SDP">Cross Session Stream Identification in the
    Session Description Protocol</title>

    <author fullname="Harald Alvestrand" initials="H. T." surname="Alvestrand">
      <organization>Google</organization>

      <address>
        <postal>
          <street>Kungsbron 2</street>

          <city>Stockholm</city>

          <region></region>

          <code>11122</code>

          <country>Sweden</country>
        </postal>

        <email>harald@alvestrand.no</email>
      </address>
    </author>

    <date day="10" month="February" year="2013" />

    <abstract>
      <t>This document specifies a grouping mechanism for RTP media streams
      that can be used to specify relations between media streams within
      different RTP sessions as well as within a single RTP session, and
      independently of whether these media streams are described within one
      SDP m-line or in multiple m-lines.</t>

      <t>This mechanism is used to signal the association between the RTP
      concept of SSRC and the WebRTC concept of "MediaStream" /
      "MediaStreamTrack" using SDP signaling.</t>

      <t>This document is a work item of the MMUSIC WG, whose discussion list
      is mmusic@ietf.org.</t>
    </abstract>

    <note title="Requirements Language">
      <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
      "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
      document are to be interpreted as described in <xref
      target="RFC2119">RFC 2119</xref>.</t>
    </note>
  </front>

  <middle>
    <section title="Introduction">
      <t></t>

      <section title="Structure Of This Document">
        <t>This document extends the SSRC grouping framework <xref
        target="RFC5888"></xref> by adding a new grouping relation that can
        cross RTP session boundaries if needed.</t>

        <t><xref target="sect-why"></xref> gives the background on why a new
        mechanism is needed.</t>

        <t><xref target="sect-definition"></xref> gives the definition of the
        new mechanism.</t>

        <t><xref target="sect-media-stream"></xref> gives the application of
        the new mechanism for providing necessary semantic information for the
        association of MediaStreamTracks to MediaStreams in the WebRTC API
        .</t>
      </section>

      <section anchor="sect-why" title="Why A New Mechanism Is Needed">
        <t>When media is carried by RTP <xref target="RFC3550"></xref>, each
        RTP media stream is distinguished inside an RTP session by its SSRC;
        each RTP session is distinguished from all other RTP sessions by being
        on a different transport association (strictly speaking, 2 transport
        associations, one used for RTP and one used for RTCP, unless RTCP
        multiplexing <xref target="RFC5761"></xref> is used).</t>

        <t>There exist cases where an application using RTP and SDP needs to
        signal some relationship between RTP media streams that may be carried
        in either the same RTP session or different RTP sessions. For
        instance, there may be a need to signal a relationship between a video
        track in one RTP session and an audio track in another RTP session. In
        traditional SDP, it is not possible to signal that these two tracks
        should be carried in one session, so they are carried in different RTP
        sessions.</t>

        <t>Traditionally, SDP was used to describe the RTP sessions, with one
        m-line being used to describe each RTP session. With the advent of
        extensions like BUNDLE <xref
        target="I-D.ietf-mmusic-sdp-bundle-negotiation"></xref>, this
        association may be more complex, with multiple m-lines being used to
        describe one RTP session; the rest of this document therefore talks
        about m-lines, not RTP sessions, when describing the signalling
        mechanism.</t>

        <t>The SSRC grouping mechanism ("a=ssrc-group") <xref
        target="RFC5576"></xref> can be used to associate RTP media streams
        when those RTP media streams are described by the same m-line. The
        semantics of this mechanism prevent the association of RTP media
        streams that are spread across different m-lines.</t>

        <t>The SDP grouping framework <xref target="RFC5888"></xref> can be
        used to group m-lines. When an m-line describes one and only one RTP
        media stream, it is possible to associate RTP media streams across
        different m-lines. However, if an m-line has multiple RTP media
        streams, using multiple SSRCs, the SDP grouping framework cannot be
        used for this purpose.</t>

        <t>There are use cases (some of which are discussed in <xref
        target="I-D.westerlund-avtcore-multiplex-architecture"></xref> ) where
        neither of these approaches is appropriate; In those cases, a new
        mechanism is needed.</t>

        <t>In addition, there is sometimes the need for an application to
        specify some application-level information about the association
        between the SSRC and the group. This is not possible using either of
        the frameworks above.</t>
      </section>

      <section title="Application to the WEBRTC MediaStream">
        <t>The W3C WebRTC API specification <xref
        target="W3C.WD-webrtc-20120209"></xref> specifies that communication
        between WebRTC entities is done via MediaStreams, which contain
        MediaStreamTracks. A MediaStreamTrack is generally carried using a
        single SSRC in an RTP session (forming an RTP media stream. The
        collision of terminology is unfortunate.) There might possibly be
        additional SSRCs, possibly within additional RTP sessions, in order to
        support functionality like forward error correction or simulcast. This
        complication is ignored below.</t>

        <t>In the RTP specification, media streams are identified using the
        SSRC field. Streams are grouped into RTP Sessions, and also carry a
        CNAME. Neither CNAME nor RTP session correspond to a MediaStream.
        Therefore, the association of an RTP media stream to MediaStreams need
        to be explicitly signaled.</t>

        <t>The marking needs to be on a per-SSRC basis, since one RTP session
        can carry media from multiple MediaStreams, and one MediaStream can
        have media in multiple RTP sessions. This means that the <xref
        target="RFC4574"></xref> "label" attribute, which is used to label
        m-lines, is not usable for this purpose.</t>

        <t>The marking needs to also carry the unique identifier of the RTP
        media stream as a MediaStreamTrack within the media stream; this is
        done using a single letter to identify whether it belongs in the video
        or audio track list, and the MediaStreamTrack's position within that
        array.</t>

        <t>This usage is described in <xref
        target="sect-media-stream"></xref>.</t>
      </section>
    </section>

    <section anchor="sect-definition" title="The Msid Mechanism">
      <t>This document extends the Source-Specific Media Attributes framework
      <xref target="RFC5576"></xref> by adding a new "msid" attribute that can
      be used with the "a=ssrc" SDP attribute. This new attribute allows
      endpoints to associate RTP media streams that are carried in the same or
      different m-lines, as well as allowing application-specific information
      to the association.</t>

      <t>The value of the "msid" attribute consists of an identifier and
      optional application-specific data, according to the following ABNF
      <xref target="RFC5234"></xref> grammar:</t>

      <t></t>

      <figure>
        <artwork><![CDATA[
  ; "attribute" is defined in RFC 4566.
  ; This attribute should be used with the ssrc-attr from RFC 5576.
  attribute =/ msid-attr
  msid-attr = "msid:" identifier [ " " appdata ]
  identifier = token
  appdata = token

]]></artwork>
      </figure>

      <t>An example MSID value for the SSRC 1234 might look like this:</t>

      <figure>
        <artwork><![CDATA[  a=ssrc:1234 msid:examplefoo v1

]]></artwork>
      </figure>

      <t>The identifier is a string of ASCII characters chosen from 0-9, a-z,
      A-Z and - (hyphen), consisting of between 1 and 64 characters. It MUST
      be unique among the identifier values used in the same SDP session. It
      is RECOMMENDED that is generated using a random-number generator.</t>

      <t>Application data is carried on the same line as the identifier,
      separated from the identifier by a space.</t>

      <t>The identifier uniquely identifies a group within the scope of an SDP
      description.</t>

      <t>There may be multiple msid attributes on a single SSRC. There may
      also be multiple SSRCs that have the same value for identifier and
      application data.</t>

      <t>Endpoints can update the associations between SSRCs as expressed by
      msid attributes at any time; the semantics and restrictions of such
      grouping and ungrouping are application dependent.</t>
    </section>

    <section title="The Msid-Semantic Attribute">
      <t>In order to fully reproduce the semantics of the SDP and SSRC
      grouping frameworks, a session-level attribute is defined for signaling
      the semantics associated with an msid grouping.</t>

      <t>This OPTIONAL attribute gives the group identifier and its group
      semantic; it carries the same meaning as the ssrc-group-attr of RFC 5576
      section 4.2, but uses the identifier of the group rather than a list of
      SSRC values.</t>

      <t>An empty list of identifiers is an indication that the sender
      understands the indicated semantic, but has no msid groupings of the
      given type in the present SDP.</t>

      <t>The ABNF of msid-semantic is:</t>

      <figure>
        <artwork><![CDATA[
  attribute =/ msid-semantic-attr
  msid-semantic-attr = "msid-semantic:" token (" " identifier)*
  token = <as defined in RFC 4566>
   ]]></artwork>
      </figure>

      <t>The semantic field may hold values from the IANA registries
      "Semantics for the "ssrc-group" SDP Attribute" and "Semantics for the
      "group" SDP Attribute".</t>

      <t>An example msid-semantic might look like this:</t>

      <figure>
        <artwork><![CDATA[  a=msid-semantic:LS xyzzy forolow

]]></artwork>
      </figure>

      <t>This means that the SDP description has two lip sync groups, with the
      group identifiers xyzzy and forolow, respectively.</t>
    </section>

    <section anchor="sect-media-stream"
             title="Applying Msid to WebRTC MediaStreams">
      <t>This section creates a new semantic for use with the framework
      defined in <xref target="sect-definition"></xref>, to be used for
      associating SSRCs representing MediaStreamTracks within MediaStreams as
      defined in <xref target="W3C.WD-webrtc-20120209"></xref>.</t>

      <t>The semantic token for this semantic is "WMS" (short for WebRTC Media
      Stream).</t>

      <t>The value of the msid corresponds to the "id" attribute of a
      MediaStream.</t>

      <t>In a WebRTC-compatible SDP description, all SSRCs intending to be
      sent from one peer will be identified in the SDP generated by that
      entity.</t>

      <t>The appdata for a WebRTC MediaStreamTrack consists of the "id"
      attribute of a MediaStreamTrack.</t>

      <t>If two different SSRCs have the same value for identifier and
      appdata, it means that these two SSRCs are both intended for the same
      MediaStreamTrack. This may occur if the sender wishes to use simulcast
      or forward error correction, or if the sender intends to switch between
      multiple codecs on the same MediaStreamTrack.</t>

      <t>When an SDP description is updated, a specific msid continues to
      refer to the same MediaStream. Once negotiation has completed on a
      session, there is no memory; an msid value that appears in a later
      negotiation will be taken to refer to a new MediaStream.</t>

      <t>The following are the rules for handling updates of the list of SSRCs
      and their msid values.</t>

      <t><list style="symbols">
          <t>When a new msid value occurs in the description, the recipient
          can signal to its application that a new MediaStream has been
          added.</t>

          <t>When a description is updated to have more SSRCs with the same
          msid value, but different appdata values, the recipient can signal
          to its application that new media stream tracks have been added to
          the media stream.</t>

          <t>When a description is updated to no longer list the msid value on
          a specific ssrc, the recipient can signal to its application that
          the corresponding media stream track has been closed.</t>

          <t>When a description is updated to no longer list the msid value on
          any ssrc, the recipient can signal to its application that the media
          stream has been closed.</t>
        </list>In addition to signaling that the track is closed when it
      disappears from the SDP, the track will also be signaled as being closed
      when the SSRC disappears by the rules of <xref target="RFC3550"></xref>
      section 6.3.4 (BYE packet received) and 6.3.5 (timeout).</t>

      <section anchor="s-nonsignal" title="Handling of non-signalled tracks">
        <t>Pre-WebRTC entities will not send msid. This means that there will
        be some incoming RTP packets with SSRCs where the recipient does not
        know about a corresponding MediaStream id.</t>

        <t>Handling will depend on whether or not any SSRCs are signaled in
        the relevant m-line(s). There are two cases:</t>

        <t><list style="symbols">
            <t>No SSRC is signaled with an msid attribute. The SDP session is
            assumed to be a backwards-compatible session. All incoming SSRCs,
            on all m-lines that are part of the SDP session, are assumed to
            belong to independent media streams, each with one track. The
            identifier of this media stream and of the media stream track is a
            randomly generated string; the label of this media stream will be
            set to "Non-WMS stream".</t>

            <t>Some SSRCs are signaled with an msid attribute. In this case,
            the session is WebRTC compatible, and the newly arrived SSRCs are
            either caused by a bug or by timing skew between the arrival of
            the media packets and the SDP description. These packets MAY be
            discarded, or they MAY be buffered for a while in order to allow
            immediate startup of the media stream when the SDP description is
            updated. The arrival of media packets MUST NOT cause a new
            MediaStreamTrack to be signaled.</t>
          </list></t>

        <t>If a WebRTC entity sends a description, it MUST include the
        msid-semantic:WMS attribute, even if no media streams are sent. This
        allows us to distinguish between the case of no media streams at the
        moment and the case of legacy SDP generation.</t>

        <t>It follows from the above that the WebRTC entity must have the SDP
        of the other party before it can decide correctly whether or not a
        "default" MediaStream should be created. RTP media packets that arrive
        before the remote party's SDP MUST be buffered or discarded, and MUST
        NOT cause a new MediaStreamTrack to be signalled.</t>

        <t>It follows from the above that media stream tracks in the "default"
        media stream cannot be closed by signaling; the application must
        instead signal these as closed when the SSRC disappears according to
        the rules of RFC 3550 section 6.3.4 and 6.3.5.</t>

        <t>NOTE IN DRAFT: Previous versions of this memo suggested adding all
        incoming SSRCs to a single MediaStream. This is problematic because we
        do not know if the SSRCs are synchronized or not before we learn the
        CNAME of the SSRCs, which only happens when an RTCP packet arrives.
        How to identify a non-WMS stream is still open for discussion -
        including whether it's necessary to do so. Using the stream label
        seems like an easy thing to do for debuggability - it's not signalled,
        and is intended for human consumption anyway.</t>
      </section>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>This document requests IANA to register the "msid" attribute in the
      "att-field (source level)" registry within the SDP parameters registry,
      according to the procedures of <xref target="RFC5576"></xref></t>

      <t>The required information is:</t>

      <t><list style="symbols">
          <t>Contact name, email: IETF, contacted via mmusic@ietf.org, or a
          successor address designated by IESG</t>

          <t>Attribute name: msid</t>

          <t>Long-form attribute name: Media stream group Identifier</t>

          <t>The attribute value contains only ASCII characters, and is
          therefore not subject to the charset attribute.</t>

          <t>The attribute gives an association over a set of SSRCs,
          potentially in different m-lines. It can be used to signal the
          relationship between a WebRTC MediaStream and a set of SSRCs.</t>

          <t>The details of appropriate values are given in RFC XXXX.</t>
        </list></t>

      <t>This document requests IANA to create a new registry called
      "Semantics for the msid-semantic SDP attribute", which should have
      exactly the same rules as for the "Semantics for the ssrc-group SDP
      attribute" registry (Expert Review), and to register the "WMS" semantic
      within this new registry.</t>

      <t>The required information is:</t>

      <t><list style="symbols">
          <t>Description: WebRTC Media Stream, as given in RFC XXXX.</t>

          <t>Token: WMS</t>

          <t>Standards track reference: RFC XXXX</t>
        </list>IANA is requested to replace "RFC XXXX" with the RFC number of
      this document upon publication.</t>
    </section>

    <section anchor="Security" title="Security Considerations">
      <t>An adversary with the ability to modify SDP descriptions has the
      ability to switch around tracks between media streams. This is a special
      case of the general security consideration that modification of SDP
      descriptions needs to be confined to entities trusted by the
      application.</t>

      <t>If implementing buffering as mentioned in section <xref
      target="s-nonsignal"></xref>, the amount of buffering should be limited
      to avoid memory exhaustion attacks.</t>

      <t>No other attacks that are relevant to the browser's security have
      been identified that depend on this mechanism.</t>
    </section>

    <section anchor="Acknowledgements" title="Acknowledgements">
      <t>This note is based on sketches from, among others, Justin Uberti and
      Cullen Jennings.</t>

      <t>Special thanks to Miguel Garcia and Paul Kyzivat for their work in
      reviewing this draft, with many specific language suggestions.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include="reference.RFC.2119"?>

      <?rfc include='reference.RFC.3550'?>

      <?rfc include='reference.RFC.5576'?>

      <?rfc include='reference.RFC.5234'?>

      <?rfc include='reference.W3C.WD-webrtc-20120209'?>
    </references>

    <references title="Informative References">
      <?rfc include='reference.RFC.4574'?>

      <?rfc include='reference.RFC.5761'?>

      <?rfc include='reference.RFC.5888'?>

      <?rfc include='reference.I-D.ietf-mmusic-sdp-bundle-negotiation'?>

      <?rfc include='reference.I-D.westerlund-avtcore-multiplex-architecture'?>
    </references>

    <section title="Design considerations, open questions and and alternatives">
      <t>This appendix should be deleted before publication as an RFC.</t>

      <t>One suggested mechanism has been to use CNAME instead of a new
      attribute. This was abandoned because CNAME identifies a synchronization
      context; one can imagine both wanting to have tracks from the same
      synchronization context in multiple MediaStreams and wanting to have
      tracks from multiple synchronization contexts within one MediaStream
      (but the latter is impossible, since a MediaStream is defined to impose
      synchronization on its members).</t>

      <t>Another suggestion has been to put the msid value within an attribute
      of RTCP SR (sender report) packets. This doesn't offer the ability to
      know that you have seen all the tracks currently configured for a media
      stream.</t>

      <t>There has been a suggestion that this mechanism could be used to mute
      tracks too. This is not done at the moment.</t>

      <t>Discarding of incoming data when the SDP description isn't updated
      yet (section 3) may cause clipping. However, the same issue exists when
      crypto keys aren't available. Input sought.</t>

      <t>There's been a suggestion that acceptable SSRCs should be signaled in
      a response, giving a recipient the ability to say "no" to certain SSRCs.
      This is not supported in the current version of this document.</t>
    </section>

    <section title="Change log">
      <t>This appendix should be deleted before publication as an RFC.</t>

      <section title="Changes from rtcweb-msid-00 to -01">
        <t>Added track identifier.</t>

        <t>Added inclusion-by-reference of
        draft-lennox-mmusic-source-selection for track muting.</t>

        <t>Some rewording.</t>
      </section>

      <section title="Changes from alvestrand-rtcweb-msid-01 to -02">
        <t>Split document into sections describing a generic grouping
        mechanism and sections describing the application of this grouping
        mechanism to the WebRTC MediaStream concept.</t>

        <t>Removed the mechanism for muting tracks, since this is not central
        to the MSID mechanism.</t>
      </section>

      <section title="Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00">
        <t>Changed the draft name according to the wishes of the MMUSIC group
        chairs.</t>

        <t>Added text indicting cases where it's appropriate to have the same
        appdata for multiple SSRCs.</t>

        <t>Minor textual updates.</t>
      </section>

      <section title="Changes from alvestrand-mmusic-msid-00 to -01">
        <t>Increased the amount of explanatory text, much based on a review by
        Miguel Garcia.</t>

        <t>Removed references to BUNDLE, since that spec is under active
        discussion.</t>

        <t>Removed distinguished values of the MSID identifier.</t>

        <t></t>
      </section>

      <section title="Changes from alvestrand-mmusic-msid-01 to -02">
        <t>Changed the order of the "msid-semantic: " attribute's value fields
        and allowed multiple identifiers. This makes the attribute useful as a
        marker for "I understand this semantic".</t>

        <t>Changed the syntax for "identifier" and "appdata" to be
        "token".</t>

        <t>Changed the registry for the "msid-semantic" attribute values to be
        a new registry, based on advice given in Atlanta.</t>
      </section>

      <section title="Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00">
        <t>Updated terminology to refer to m-lines rather than RTP sessions
        when discussing SDP formats and the ability of other linking
        mechanisms to refer to SSRCs.</t>

        <t>Changed the "default" mechanism to return independent streams after
        considering the synchronization problem.</t>

        <t>Removed the space from between "msid-semantic" and its value, to be
        consistent with RFC 5576.</t>
      </section>
    </section>
  </back>
</rfc>