mmusic                                                       R. Abhishek
Internet-Draft                                                 S. Wenger
Intended status: Standards Track                                 Tencent
Expires: August 5, 2021                                 February 1, 2021

                 SDP Superimposition Grouping framework


   This document defines semantics that allow for signaling a new SDP
   group "supim" for superimposed media in an SDP session.  The "supim"
   attribute can be used by the application to relate all the
   superimposed visual media streams enabling them to be added as an
   overlay on top of any visual media stream.  The superimposition
   grouping semantics is helpful, if the media data is separate and
   transported via different sessions.

Status of This Memo

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   This Internet-Draft will expire on August 5, 2021.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Superimposition Group Identification Attribute  . . . . . . .   4
   4.  Use of group and mid  . . . . . . . . . . . . . . . . . . . .   5
   5.  "superposition" Attribute for Superimposition Group
       Identification Attribute  . . . . . . . . . . . . . . . . . .   5
   6.  Example of Supim  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Relationship with CLUE (informative)  . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Media superimposition herein is defined to be a visual media
   (video/image/text) which is superimposed on top of an already
   existing visual media such that the resulting foreground and
   background media can be displayed simultaneously.  Superimposition
   can be recursive in that visual media that is superimposed against
   its background can, in turn, be the background of another
   superimposed visual media.  The superimposed visual media displayed
   over a background media content may be anywhere between opaque and
   transparent.  Examples of applications for video superimposition
   include real-time multi-party gaming, where these superimposed media
   maybe used to provide additional details or stats about each player,
   or multi-party teleconferencing where visual media from users in the
   teleconference may be superimposed on a background media or over each
   other.  An example is shown in the figure below, where three
   foreground media have been superimposed over a background media, with
   one foreground media being partly superimposed over another
   foreground media.

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                  | Background media           |
                  |   _________                |
                  |  | Media A |               |
                  |  |_________|               |
                  |              __________    |
                  |       ______|__ Media B|   |
                  |      |Media |__|_______|   |
                  |      |_C_______|           |

               Figure 1: A example of media superimposition

   SDP is predominantly used for describing the format for multimedia
   communication sessions.  Many SDP-based systems use open standards
   such as RTP [RFC3550] for media transport and SIP [RFC3261] for
   session setup and control.  An SDP session may contain more than one
   media description with each media description identified by "m"=line.
   Each line denotes a single media stream.  If multiple visual media
   lines are present in a session, at present, their superimposition
   (foreground/background) relationship at the rendering device is
   undefined.  This memo introduces a mechanism in which certain
   rendering information becomes available.  The rendering information
   herein is limited to the foreground/background relationship of each
   grouped media vis-a-vis each other through a layer order value, and
   optionally a transparency value.  Where, spatially, the media is
   rendered is not covered by this memo, and is in many application
   scenarios a function of the user interface.  The CLUE framework
   [RFC8845] is available when the application requires defining capture
   (camera ports), and their geo-spatial relationship to each other is
   needed.  The superimposition grouping as described below enables a
   compliant receiver/renderer implementation to know the relative
   relevance of the visual media as coded by the sender(s) and, in a
   compliant implementation, observed by the renderer through
   superimposition when needed.  Of course, assuming sufficient screen
   real-estate, a renderer may not have to rely on superimposition
   mechanisms at all--when there is enough screen real-estate available,
   a valid display strategy may well be to show all media without
   overlapping and hence without superimposition.  However, when the
   screen real-estate becomes insufficient, then the information
   provided by the mechanisms defined in this memo can be used to order
   (in the sense of foreground to background) the visual media according
   to a hierarchy chosen by the sender or a middlebox, and based on
   their application knowledge.

   When multiple superimposed streams are transmitted within a session,
   the receiver needs to be able to relate the media streams to each

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   other.  This is achieved by the SDP grouping framework [RFC5888] by
   using the "group" attribute that groups different "m" lines in a
   session.  By using a new superimpose group semantic defined in this
   memo, a group's media streams can be uniquely identified across
   multiple SDP descriptions exchanged with different receivers, thereby
   identifying the streams in terms of their role in the session
   irrespective of its media type and transport protocol.  These
   superimposed streams within the group may be multiplexed based on the
   guidelines defined in [draft-ietf-avtcore-multiplex-guidelines-12].

   This document describes a new SDP group semantics for grouping the
   superimposition in an SDP session.  An SDP session description
   consists of one or multiple media lines known as "m" lines which can
   be identified by a token carried in a "mid" attribute.  The SDP
   session describes a session-level group level attribute that groups
   different media lines using a defined group semantics.  The semantics
   defined in this memo is to be used in conjunction with "The Session
   Description Protocol (SDP) Grouping Framework"[RFC5888].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Superimposition Group Identification Attribute

   The "superimposition media stream identification" attribute is used
   to identify the relationship of superimposed media streams within a
   session description.  In a superimposition group, the media lines MAY
   have different media formats but, to be meaningful, SHOULD be visual
   media.  There is no defined behavior for the rendering of non-visual
   media being grouped in a superimposition group.  Its formatting
   follows [RFC5888] in the use of 'mid' attribute to identify the media
   line to be included in the superimposition.

   This document defines a new group semantics "supim" identification
   media attribute, which is used to identify superimposition group
   media streams within a session description.  It is used for grouping
   the foreground and the background media streams intended for the
   purpose of composition with foreground media to be superimposed over
   the background media stream.  An application that chooses to
   implement the extension, receives a session description that contains
   "m" lines grouped together using "supim" semantics MUST superimpose
   the foreground media streams on top of the background media stream in
   case there is overlap.  For non-supporting devices, these media
   streams are treated as independent media streams.

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4.  Use of group and mid

   All group and mid attributes MUST follow the rules defined in
   [RFC5888].  The "mid" attribute MUST be used for all "m" lines
   covering visual media within a session description for which a
   foreground/background relationship is to be defined.  The foreground/
   background relationship of visual media within a session description
   that is not covered in a group is undefined.  No more than one group
   MUST be used within one session.  If the identification-tags
   associated with "a=group" lines do not map to any "m" lines, it MUST
   be ignored.

       semantics = "supim" /; semantics extension
                             as defined in RFC5888

5.  "superposition" Attribute for Superimposition Group Identification

   This memo defines a new media-level attribute, "superposition", with
   the following ABNF [RFC5234].  The identification-tag is defined in

     superimposition-attribute =
          "a=superposition:" "transparency:" transparency-tag,
                                            "layer:" layer-tag
     transparency-tag =transparency-value *("," transparency-value) CRLF
     transparency-value= alpha
     layer-tag =layering-order *("," layering-order) CRLF
     layering-order = beta

   Alpha describes the transparency for the media stream.  It is
   identified by its transparency-tag values in the transparency-
   attribute.  The transparency value must be an ASCII representation of
   an 8 bit signed integer with values between "-128" and "127", and
   linear weighting between the two extremes.  A value of -128 means
   media stream is opaque and the highest value of 127 means it is
   transparent.  Beta represents the layering order value for the media
   stream.  The layering order value is an integer value between 0 to n,
   where the value 0 represents the most background layer.  For each k
   within 0..n, a reconstructed sample of the k-th media is superimposed
   (while perhaps applying an alpha transparency value) on the 0 to k-th
   reconstructed samples in the same spatial position.  The transparency
   attribute MUST be omitted for layer with order 0, and the default
   transparency value for background media stream of -128 is applied.

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6.  Example of Supim

   The following example shows a session description for superimposed
   media stream in an SDP session.  The "group" line indicates that the
   "m" lines with tokens 1, 2 and 3 are grouped for the purpose of

   In the example shown below, three media streams are being transmitted
   for superimposition.  The background media stream along with the
   foreground media streams are grouped together using "supim".  All
   media streams are video with "superposition" attribute.  Media stream
   with layer order value 0 is intended for background.

       o=Alice 292742730 29277831 IN IP4
       c=IN IP4
       t=0 0
       a=group:supim 1 2 3
       m=video 30000 RTP/AVP 31
       a= superposition:transparency= -128, layer=0
       m=video 30002 RTP/AVP 31
       a= superposition:transparency=35, layer=1
       m=video 30003 RTP/AVP 31
       a= superposition:transparency=75, layer=2

   The transparency value is used for composing the foreground with the
   background media [Wiki.Alpha-compositing].  The "layer" value is
   relevant when two or more media streams are to be composed.  When the
   transparency value of the foreground is -128, the composed image will
   be the foreground image, as it is being displayed as opaque.
   Similarly, if the transparency value for the foreground media is 127,
   the resulting image will be the background media, as the foreground
   media stream is being presented fully transparent, hence invisible.
   The details of the weighting of foreground and background sample
   values based on a given alpha value is left undefined herein, beyond
   the abstract definition that alpha equal to -128 means opaque, and
   alpha equal to 127 means transparent, and the weighting is to be
   implemented such that it is visually linear for the values in
   between.  We do not define a weighting formula as these formulae
   would depend on many factors such as the colorspace and the sampling
   structure of the media.

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7.  Relationship with CLUE (informative)

   Edt. Note: maybe we remove this section later once there is a general
   understanding why CLUE in its current form is unsuitable.  The CLUE
   framework [RFC8845] and its associated suite of I-Ds and RFCs
   describe a telepresence framework that, at the first glance seems to
   have a lot in common with the technology proposed herein.  CLUE
   defines captures (camera ports), and their geo-spatial relationship
   to each other.  A render can use this information to put the
   reconstructed samples of the streams from the various captures into a
   suitable arrangement such that visually pleasant rendering can be
   achieved.  However, CLUE does not describe the relative relevance of
   the captures.  For that reason, CLUE would need to be extended in a
   spirit very similar to the one described in this memo to achieve the
   desired functionality.  CLUE has not seen wide deployment outside its
   intended key application (large room, multiple camera telepresence
   systems).  It's not reasonable to assume that small systems would
   willingly implement the overhead the (comparatively complex) CLUE
   protocols require when a simple SDP extension can serve the same

8.  Security Considerations

   All security considerations as defined in [RFC5888] apply:

   Using the "group" parameter with FID semantics, an entity that
   managed to modify the session descriptions exchanged between the
   participants to establish a multimedia session could force the
   participants to send a copy of the media to any destination of its

   Integrity mechanisms provided by protocols used to exchange session
   descriptions and media encryption can be used to prevent this attack.
   In SIP, Secure/Multipurpose Internet Mail Extensions (S/MIME)
   [RFC8550] and Transport Layer Security (TLS) [RFC8446] can be used to
   protect session description exchanges in an end-to-end and a hop-
   byhop fashion, respectively.

9.  IANA Considerations

   The following contact information shall be used for all registrations
   included here:

       Rohit Abhishek  <>
       Stephan Wenger <>
       The IETF MMUSIC working group <> or its successor
                                              as designated by the IESG.

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   This document defines a new SDP group semantics for media
   superimposition for a SDP session.  This attribute can be used by the
   application to group the foreground and the background media streams
   to be superimposed together in a session.  Semantics values to be
   used with this framework should be registered by the IANA following
   the Standards Action policy [RFC8126].  This document adds a new
   group semantics and follows the registry group defined in [RFC5888].

   The following semantics needs to be registered by IANA in Semantics
   for the "group" SDP Attribute under SDP Parameters.

   Semantics             Token          Reference
   Superimposition       supim          RFCXXXX

   The "supim" attribute is used to group different media streams to be
   superimposed together with one backgorund media stream and rest
   foreground streams.  Its format is defined in Section 3.

   The SDP media-level attribute "superposition" needs to be registered
   by IANA Semantics for "att-field (media-level only)".  The
   registration procedure in [RFC8866] applies.

   SDP Attribute ("att-field (media level only)"):

         Attribute name: superposition: transparency, layer
         Long form: superimposition transparency, superimposition layer
         Type of name: att-field
         Type of attribute: media level only
         Subject to charset: no
         Purpose: RFC 5583
         Reference: RFC 5583
         Values: alpha, beta

   The IANA Considerations section of the RFC MUST include the following
   information, which appears in the IANA registry along with the RFC
   number of the publication.

   o  A brief description of the semantics.

   o  Token to be used within the "group" attribute.  This token may be
      of any length, but SHOULD be no more than four characters long.

   o  Reference to a standards track RFC.

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10.  Acknowledgements

   The authors would like to thank Christer Holmberg and Paul Kyzivat
   for reviewing the draft and providing key ideas.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8550]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Certificate Handling", RFC 8550, DOI 10.17487/RFC8550,
              April 2019, <>.

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   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,

11.2.  Informative References

              Westerlund, M., Burman, B., Perkins, C., Alvestrand, H.,
              and R. Even, "Guidelines for using the Multiplexing
              Features of RTP to Support Multiple Media Streams", draft-
              ietf-avtcore-multiplex-guidelines-12 (work in progress),
              June 2020.

   [RFC8845]  Duckworth, M., Ed., Pepperell, A., and S. Wenger,
              "Framework for Telepresence Multi-Streams", RFC 8845,
              DOI 10.17487/RFC8845, January 2021,

              "Alpha compositing",

Authors' Addresses

   Rohit Abhishek
   2747 Park Blvd
   Palo Alto  94588


   Stephan Wenger
   2747 Park Blvd
   Palo Alto  94588


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