Internet Engineering Task Force (IETF)                     S. Nandakumar
Request for Comments: 8859                                         Cisco
Category: Standards Track                                   January 2021
ISSN: 2070-1721

A Framework for Session Description Protocol (SDP) Attributes When
Multiplexing

Abstract

   The purpose of this specification is to provide a framework for
   analyzing the multiplexing characteristics of Session Description
   Protocol (SDP) attributes when SDP is used to negotiate the usage of
   a single 5-tuple for sending and receiving media associated with
   multiple media descriptions.

   This specification also categorizes the existing SDP attributes based
   on the framework described herein.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8859.

Copyright Notice

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

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   described in the Simplified BSD License.

   1.  Introduction
   2.  Terminology
   3.  Motivation
   4.  SDP Attribute Analysis Framework
     4.1.  Category: NORMAL
     4.2.  Category: CAUTION
     4.3.  Category: IDENTICAL
     4.4.  Category: SUM
     4.5.  Category: TRANSPORT
     4.6.  Category: INHERIT
     4.7.  Category: IDENTICAL-PER-PT
     4.8.  Category: SPECIAL
     4.9.  Category: TBD
   5.  Analysis of Existing Attributes
     5.1.  RFC 4566: SDP
     5.2.  RFC 4585: RTP/AVPF
     5.3.  RFC 5761: Multiplexing RTP and RTCP
     5.4.  RFC 3312: Integration of Resource Management and SIP
     5.5.  RFC 4574: SDP "label" Attribute
     5.6.  RFC 5432: QoS Mechanism Selection in SDP
     5.7.  RFC 4568: SDP Security Descriptions
     5.8.  RFC 5762: RTP over DCCP
     5.9.  RFC 6773: DCCP-UDP Encapsulation
     5.10. RFC 5506: Reduced-Size RTCP in RTP Profile
     5.11. RFC 6787: Media Resource Control Protocol Version 2
     5.12. RFC 8445: ICE
     5.13. RFC 5285: RTP Header Extensions
     5.14. RFC 3605: RTCP Attribute in SDP
     5.15. RFC 5576: Source-Specific SDP Attributes
     5.16. RFC 7273: RTP Clock Source Signaling
     5.17. RFC 6236: Image Attributes in SDP
     5.18. RFC 7197: Duplication Delay Attribute in SDP
     5.19. RFC 7266: RTCP XR Blocks for MOS Metric Reporting
     5.20. RFC 6285: Rapid Acquisition of Multicast RTP Sessions
     5.21. RFC 6230: Media Control Channel Framework
     5.22. RFC 6364: SDP Elements for FEC Framework
     5.23. RFC 4796: "content" Attribute
     5.24. RFC 3407: SDP Simple Capability Declaration
     5.25. RFC 6284: Port Mapping between Unicast and Multicast RTP
            Sessions
     5.26. RFC 6714: MSRP-CEMA
     5.27. RFC 4583: SDP Format for BFCP Streams
     5.28. RFC 5547: SDP Offer/Answer for File Transfer
     5.29. RFC 6849: SDP and RTP Media Loopback Extension
     5.30. RFC 5760: RTCP with Unicast Feedback
     5.31. RFC 3611: RTCP XR
     5.32. RFC 5939: SDP Capability Negotiation
     5.33. RFC 6871: SDP Media Capabilities Negotiation
     5.34. RFC 7006: Miscellaneous Capabilities Negotiation in SDP
     5.35. RFC 4567: Key Management Extensions for SDP and RTSP
     5.36. RFC 4572: Comedia over TLS in SDP
     5.37. RFC 4570: SDP Source Filters
     5.38. RFC 6128: RTCP Port for Multicast Sessions
     5.39. RFC 6189: ZRTP
     5.40. RFC 4145: Connection-Oriented Media
     5.41. RFC 6947: The SDP "altc" Attribute
     5.42. RFC 7195: SDP Extension for Circuit-Switched Bearers in
            PSTN
     5.43. RFC 7272: IDMS Using the RTP Control Protocol (RTCP)
     5.44. RFC 5159: Open Mobile Alliance (OMA) Broadcast (BCAST) SDP
            Attributes
     5.45. RFC 6193: Media Description for IKE in SDP
     5.46. RFC 2326: Real Time Streaming Protocol
     5.47. RFC 7826: Real-Time Streaming Protocol
     5.48. RFC 6064: SDP and RTSP Extensions for 3GPP
     5.49. RFC 3108: ATM SDP
     5.50. 3GPP TS 183.063
     5.51. 3GPP TS 24.229
     5.52. ITU T.38
     5.53. ITU-T Q.1970
     5.54. ITU-T H.248.15
     5.55. RFC 4975: The Message Session Relay Protocol
     5.56. Historical Attributes
   6.  bwtype Attribute Analysis
     6.1.  RFC 4566: SDP
     6.2.  RFC 3556: SDP Bandwidth Modifiers for RTCP Bandwidth
     6.3.  RFC 3890: Bandwidth Modifier for SDP
   7.  rtcp-fb Attribute Analysis
     7.1.  RFC 4585: RTP/AVPF
     7.2.  RFC 5104: Codec Control Messages in AVPF
     7.3.  RFC 6285: Unicast-Based Rapid Acquisition of Multicast RTP
           Sessions (RAMS)
     7.4.  RFC 6679: ECN for RTP over UDP/IP
     7.5.  RFC 6642: Third-Party Loss Report
     7.6.  RFC 5104: Codec Control Messages in AVPF
   8.  group Attribute Analysis
     8.1.  RFC 5888: SDP Grouping Framework
     8.2.  RFC 3524: Mapping Media Streams to Resource Reservation
           Flows
     8.3.  RFC 4091: ANAT Semantics
     8.4.  RFC 5956: FEC Grouping Semantics in SDP
     8.5.  RFC 5583: Signaling Media Decoding Dependency in SDP
     8.6.  RFC 7104: Duplication Grouping Semantics in the SDP
   9.  ssrc-group Attribute Analysis
     9.1.  RFC 5576: Source-Specific SDP Attributes
     9.2.  RFC 7104: Duplication Grouping Semantics in the SDP
   10. QoS Mechanism Token Analysis
     10.1.  RFC 5432: QoS Mechanism Selection in SDP
   11. k= Attribute Analysis
     11.1.  RFC 4566: SDP
   12. content Attribute Analysis
     12.1.  RFC 4796
     12.2.  3GPP TS 24.182
     12.3.  3GPP TS 24.183
   13. Payload Formats
     13.1.  RFC 5109: RTP Payload Format for Generic FEC
   14. Multiplexing Considerations for Encapsulating Attributes
     14.1.  RFC 3407: cpar Attribute Analysis
     14.2.  RFC 5939 Analysis
       14.2.1.  Recommendation: Procedures for Potential Configuration
               Pairing
     14.3.  RFC 6871 Analysis
       14.3.1.  Recommendation: Dealing with Payload Type Numbers
       14.3.2.  Recommendation: Dealing with Latent Configurations
   15. IANA Considerations
     15.1.  New "Multiplexing Categories" Subregistry
     15.2.  "Mux Category" Column for Subregistries
       15.2.1.  Table: SDP bwtype
       15.2.2.  Table: attribute-name
       15.2.3.  Table: content SDP Parameters
       15.2.4.  Table: Semantics for the "group" SDP Attribute
       15.2.5.  Table: "rtcp-fb" Attribute Values
       15.2.6.  Table: "ack" and "nack" Attribute Values
       15.2.7.  Table: "depend" SDP Attribute Values
       15.2.8.  Table: "cs-correlation" Attribute Values
       15.2.9.  Table: Semantics for the "ssrc-group" SDP Attribute
       15.2.10. Table: SDP/RTSP Key Management Protocol Identifiers
       15.2.11. Table: Codec Control Messages
       15.2.12. Table: QoS Mechanism Tokens
       15.2.13. Table: SDP Capability Negotiation Option Tags
       15.2.14. Table: Timestamp Reference Clock Source Parameters
       15.2.15. Table: Media Clock Source Parameters
   16. Security Considerations
   17. References
     17.1.  Normative References
     17.2.  Informative References
   Acknowledgements

Author's Address

1. Introduction

   SDP defines several attributes for capturing characteristics that
   apply to the individual media descriptions (described by "m=" lines)
   and the overall multimedia session.  Typically, different media types
   (audio, video, etc.) described using different media descriptions
   represent separate RTP sessions that are carried over individual
   transport-layer flows.  However, [RFC8843] defines a way to use a
   single address:port combination (BUNDLE address) for receiving media
   associated with multiple SDP media descriptions.  This would, for
   example, allow the usage of a single set of Interactive Connectivity
   Establishment (ICE) [RFC8445] candidates for multiple media
   descriptions.  This, in turn, has made it necessary to understand the
   interpretation and usage of the SDP attributes defined for the
   multiplexed media descriptions.

   Given the number of SDP attributes registered with the [IANA] and the
   possibility of new attributes being defined in the future, there is
   need for a framework to analyze these attributes for their
   applicability in the transport multiplexing use cases.

   The document starts with providing the motivation for requiring such
   a framework.  This is followed by introduction to the SDP attribute
   analysis framework and procedures, following which several sections
   apply the framework to the SDP attributes registered with the [IANA].

2. Terminology

   5-tuple:  A collection of the following values: source address,
      source port, destination address, destination port, and transport-
      layer protocol.

   3GPP:  Third Generation Partnership Project; see
      <https://www.3gpp.org> for more information about this
      organization.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3. Motivation

   An effort to reduce the number of necessary transport-level flows is
   required because of the time and complications involved in setting up
   Secure Real-time Transport Protocol (SRTP) [RFC5763] transports for
   use by RTP based on ICE [RFC8445] and Datagram Transport Layer
   Security (DTLS).  These procedures motivate conservation of ports
   bindings on the Network Address Translators (NATs).  This necessity
   has resulted in the definition of ways, such as that described in
   [RFC8843], to multiplex RTP over a single transport flow in order to
   preserve network resources such as port numbers.  This imposes
   further restrictions on applicability of the SDP attributes as they
   are defined today.

   The specific problem is that there are attribute combinations that
   make sense when specified on independent "m=" lines -- as with
   classical SDP -- that do not make sense when those "m=" lines are
   then multiplexed over the same transport.  To give an obvious
   example, ICE permits each "m=" line to have an independently
   specified "ice-ufrag" attribute.  However, if the media from multiple
   "m=" lines is multiplexed over the same ICE component, then the
   meaning of media-level "ice-ufrag" attributes becomes muddled.

   At the time of writing this document, there are close to 250 SDP
   attributes registered with the [IANA], and more will be added in the
   future.  There is no clearly defined procedure to establish the
   validity/applicability of these attributes when used with transport
   multiplexing.

4. SDP Attribute Analysis Framework

   Attributes in an SDP session description can be defined at the
   session level, media level, or source level.  Informally, there are
   various semantic groupings for these attributes.  One such grouping
   could be as follows:

   *  Attributes related to media content such as media type, encoding
      schemes, and payload types.

   *  Attributes specifying media transport characteristics such as RTP/
      RTP Control Protocol (RTCP) port numbers, network addresses, and
      QoS.

   *  Metadata description attributes capturing session timing and
      origin information.

   *  Attributes establishing relationships between media descriptions,
      such as grouping framework [RFC5888].

   The proposed framework analyzes the SDP attributes usage under
   multiplexing and assigns each SDP attribute to an appropriate
   multiplexing category.  Since the multiplexing categories defined in
   this specification are independent of any informal semantic groupings
   of the SDP attributes, the categorizations assigned are normative.

4.1. Category: NORMAL

   The attributes in the NORMAL category can be independently specified
   when multiplexed, and they retain their original semantics.

   In the example given below, the direction and label attributes are
   independently specified for audio and video "m=" lines.  These
   attributes are not impacted by multiplexing these media streams over
   a single transport-layer flow.

        v=0
        o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
        s=
        c=IN IP4 host.atlanta.example.com
        t=0 0
        m=audio 49172 RTP/AVP 99
        a=sendonly
        a=label:1
        a=rtpmap:99 iLBC/8000
        m=video 49172 RTP/AVP 31
        a=recvonly
        a=label:2
        a=rtpmap:31 H261/90000

4.2. Category: CAUTION

   It is not advisable to multiplex with the attributes in the CAUTION
   category, since their usage under multiplexing might lead to
   incorrect behavior.

   Example: Multiplexing media descriptions over a single Datagram
   Congestion Control Protocol (DCCP) transport [RFC5762] is not
   recommended, since DCCP is a connection-oriented protocol and
   therefore doesn't allow multiple connections on the same 5-tuple.

        v=0
        o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
        s=
        c=IN IP4 client.biloxi.example.com
        t=0 0
        m=video 5004 DCCP/RTP/AVP 99
        a=rtpmap:99 h261/9000
        a=dccp-service-code:SC=x52545056
        a=setup:passive
        a=connection:new
        m=video 5004 DCCP/RTP/AVP 100
        a=rtpmap:100 h261/9000
        a=dccp-service-code:SC=x5254504f
        a=setup:passive
        a=connection:new

4.3. Category: IDENTICAL

   The attributes and their associated values (if any) in the IDENTICAL
   category MUST be repeated across all the media descriptions under
   multiplexing.

   Attributes such as rtcp-mux fall into this category.  Since RTCP
   reporting is done per RTP session, RTCP multiplexing MUST be enabled
   for both the audio and video "m=" lines if they are transported over
   a single 5-tuple.

        v=0
        o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
        s=
        c=IN IP4 client.biloxi.example.com
        t=0 0
        m=audio 34567 RTP/AVP 97
        a=rtcp-mux
        m=video 34567 RTP/AVP 31
        a=rtpmap:31 H261/90000
        a=rtcp-mux

   Note: Even though IDENTICAL attributes must be repeated across all
   media descriptions under multiplexing, they might not always be
   explicitly encoded across all media descriptions.  [RFC8843] defines
   rules for when attributes and their values are implicitly applied to
   media description.

4.4. Category: SUM

   The attributes in the SUM category can be set as they are normally
   used, but software using them in the multiplexing scenario MUST apply
   the sum of all the attributes being multiplexed instead of trying to
   use them independently.  This is typically used for bandwidth or
   other rate-limiting attributes to the underlying transport.

   The software parsing the SDP sample below should use the aggregate
   Application Specific (AS) bandwidth value from the individual media
   descriptions to determine the AS value for the multiplexed session.
   Thus the calculated AS value would be 256+64 kilobits per second for
   the given example.

         v=0
         o=test 2890844526 2890842807 IN IP4 client.biloxi.example.com
         c=IN IP4 client.biloxi.example.com
         t=0 0
         m=audio 49170 RTP/AVP 0
         b=AS:64
         m=video 51372 RTP/AVP 31
         b=AS:256

4.5. Category: TRANSPORT

   The attributes in the TRANSPORT category can be set normally for
   multiple items in a multiplexed group, but the software MUST pick the
   one that's associated with the "m=" line whose information is used
   for setting up the underlying transport.

   In the example below, the "a=crypto" attribute is defined for both
   the audio and video "m=" lines.  The video media line's "a=crypto"
   attribute is chosen since its MID value (bar) appears first in the
   "a=group:BUNDLE" line.  This is due to the BUNDLE grouping semantic
   [RFC8843], which mandates that the values from the "m=" line
   corresponding to the mid appearing first on the "a=group:BUNDLE" line
   be considered for setting up the RTP transport.

        v=0
        o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
        s=
        c=IN IP4 host.atlanta.example.com
        t=0 0
        a=group:BUNDLE bar foo
        m=audio 49172 RTP/AVP 99
        a=mid:foo
        a=crypto:1 AES_CM_128_HMAC_SHA1_80
          inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
        a=rtpmap:99 iLBC/8000
        m=video 51374 RTP/AVP 31
        a=mid:bar
        a=crypto:1 AES_CM_128_HMAC_SHA1_80
          inline:EcGZiNWpFJhQXdspcl1ekcmVCNWpVLcfHAwJSoj|2^20|1:32
        a=rtpmap:96 H261/90000

4.6. Category: INHERIT

   The attributes in the INHERIT category encapsulate other SDP
   attributes or parameters.  These attributes inherit their
   multiplexing characteristics from the attributes or parameters they
   encapsulate.  Such attributes are defined in [RFC3407], [RFC5939],
   and [RFC6871] as part of a generic framework for indicating and
   negotiating capabilities in the SDP related to transport, media, and
   media format.

   The inheritance manifests itself when the encapsulated attribute or
   parameter is being leveraged.  In the case of SDP Capability
   Negotiation [RFC5939], for example, this occurs when a capability
   (encapsulating attribute) is used as part of a configuration; the
   configuration inherits the multiplexing category of each of its
   constituent (encapsulated) attributes and parameters.  The inherited
   attributes MUST be coherent in order to form a valid configuration
   from a multiplexing point of view (see Section 14 for further
   details).

          v=0
          o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
          s=
          c=IN IP4 host.atlanta.example.com
          t=0 0
          m=video 3456 RTP/AVP 100
          a=rtpmap:100 VP8/90000
          a=fmtp:100 max-fr=30;max-fs=8040
          a=sqn: 0
          a=cdsc: 1 video RTP/AVP 100
          a=cpar: a=rtcp-mux
          m=video 3456 RTP/AVP 101
          a=rtpmap:101 VP8/90000
          a=fmtp:100 max-fr=15;max-fs=1200
          a=cdsc: 2 video RTP/AVP 101
          a=cpar: a=rtcp-mux

   In this example, the category IDENTICAL is inherited by the cpar-
   encapsulated "rtcp-mux" attribute.

4.7. Category: IDENTICAL-PER-PT

   The attributes in the IDENTICAL-PER-PT category define the RTP
   payload configuration on the basis of the payload type, and they MUST
   have identical values across all the media descriptions for a given
   RTP payload type when repeated.  These payload types identify the
   same codec configuration as defined in Section 9.1 of [RFC8843] under
   this context.

   In the SDP example below, Payload Types 96 and 97 are repeated across
   all the video "m=" lines, and all the payload-specific parameters
   (for example, rtpmap and fmtp) are identical.  (Note: some line
   breaks are due to formatting only.)

        v=0
        o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
        s=
        c=IN IP4 host.atlanta.example.com
        t=0 0
        a=group:BUNDLE cam1 cam2
        m=video 96 97
        a=mid:cam1
        a=rtpmap:96 H264/90000
        a=fmtp:96 profile-level-id=42400d; max-fs=3600; max-fps=3000;
        max-mbps=108000; max-br=1000
        a=rtpmap:97 H264/90000
        a=fmtp:97 profile-level-id=42400a; max-fs=240; max-fps=3000;
        max-mbps=7200; max-br=200
        m=video  96 97
        a=mid:cam2
        a=rtpmap:96 H264/90000
        a=fmtp:96 profile-level-id=42400d; max-fs=3600; max-fps=3000;
        max-mbps=108000; max-br=1000
        a=rtpmap:97 H264/90000
        a=fmtp:97 profile-level-id=42400a; max-fs=240; max-fps=3000;
        max-mbps=7200; max-br=200

4.8. Category: SPECIAL

   For the attributes in the SPECIAL category, the text in the
   specification defining the attribute MUST be consulted for further
   handling when multiplexed.

   As an example, for the attribute "extmap" [RFC5285], the
   specification defining the extension needs to be consulted to
   understand the multiplexing implications.

4.9. Category: TBD

The attributes in the TBD category have not been analyzed under the
proposed multiplexing framework and SHOULD NOT be multiplexed.

5. Analysis of Existing Attributes

   This section analyzes attributes listed in [IANA], grouped under the
   IETF document that defines them.

   The "Level" column indicates whether the attribute is currently
   specified as:

   *  S -- Session level

   *  M -- Media level

   *  B -- Both (Implies either a session level or a media level
      attribute)

   *  SR -- Source-level (for a single SSRC) [RFC5576]

   The "Mux Category" column identifies the multiplexing category
   assigned to each attribute, and the "Notes" column captures
   additional informative details regarding the assigned category,
   wherever necessary.

5.1. RFC 4566: SDP

5.2. RFC 4585: RTP/AVPF

   [RFC4585] defines an extension to the Audio-visual Profile (AVP) that
   enables receivers to provide, statistically, more immediate feedback
   to the senders and thus allows for short-term adaptation and
   efficient feedback-based repair mechanisms to be implemented.

    +=========+===========================+=======+==================+
    | Name    | Notes                     | Level | Mux Category     |
    +=========+===========================+=======+==================+
    | rtcp-fb | Since RTCP feedback       | M     | IDENTICAL-PER-PT |
    |         | attributes are scoped by  |       |                  |
    |         | payload type (PT), their  |       |                  |
    |         | values MUST be identical  |       |                  |
    |         | for a given PT across the |       |                  |
    |         | multiplexed "m=" lines.   |       |                  |
    +---------+---------------------------+-------+------------------+

                   Table 2: RFC 4585 Attribute Analysis

5.3. RFC 5761: Multiplexing RTP and RTCP

   [RFC5761] discusses issues that arise when multiplexing RTP data
   packets and RTP Control Protocol (RTCP) packets on a single UDP port.
   It describes when such multiplexing is and is not appropriate, and it
   explains how the SDP can be used to signal multiplexed sessions.

   +==========+=================================+=======+==============+
   | Name     | Notes                           | Level | Mux          |
   |          |                                 |       | Category     |
   +==========+=================================+=======+==============+
   | rtcp-mux | RTP and RTCP multiplexing       | M     | IDENTICAL    |
   |          | affects the entire RTP session. |       |              |
   +----------+---------------------------------+-------+--------------+

                    Table 3: RFC 5761 Attribute Analysis

5.4. RFC 3312: Integration of Resource Management and SIP

   [RFC3312] defines a generic framework for preconditions, which are
   extensible through IANA registration.  This document also discusses
   how network quality of service can be made a precondition for
   establishment of sessions initiated by the Session Initiation
   Protocol (SIP).  These preconditions require that the participant
   reserve network resources before continuing with the session.

          +======+======================+=======+==============+
          | Name | Notes                | Level | Mux Category |
          +======+======================+=======+==============+
          | des  | Refer to notes below | M     | CAUTION      |
          +------+----------------------+-------+--------------+
          | conf | Refer to notes below | M     | CAUTION      |
          +------+----------------------+-------+--------------+
          | curr | Refer to notes below | M     | CAUTION      |
          +------+----------------------+-------+--------------+

                   Table 4: RFC 3312 Attribute Analysis

   NOTE: A mismatched set of preconditions across media descriptions
   results in session establishment failures due to inability to meet
   the requested resource reservations.

5.5. RFC 4574: SDP "label" Attribute

   [RFC4574] defines a new SDP media-level attribute: "label".  The
   "label" attribute carries a pointer to a media stream in the context
   of an arbitrary network application that uses SDP.  The sender of the
   SDP document can attach the "label" attribute to a particular media
   stream or streams.  The application can then use the provided pointer
   to refer to each particular media stream in its context.

              +=======+==============+=======+==============+
              | Name  | Notes        | Level | Mux Category |
              +=======+==============+=======+==============+
              | label | Not impacted | M     | NORMAL       |
              +-------+--------------+-------+--------------+

                    Table 5: RFC 4574 Attribute Analysis

5.6. RFC 5432: QoS Mechanism Selection in SDP

5.7. RFC 4568: SDP Security Descriptions

   [RFC4568] defines an SDP cryptographic attribute for unicast media
   streams.  The attribute describes a cryptographic key and other
   parameters that serve to configure security for a unicast media
   stream in either a single message or a roundtrip exchange.

     +========+=================================+=======+===========+
     | Name   | Notes                           | Level | Mux       |
     |        |                                 |       | Category  |
     +========+=================================+=======+===========+
     | crypto | crypto attribute MUST be the    | M     | TRANSPORT |
     |        | one that corresponds to the     |       |           |
     |        | "m=" line chosen for setting up |       |           |
     |        | the underlying transport flow.  |       |           |
     +--------+---------------------------------+-------+-----------+

                   Table 7: RFC 4568 Attribute Analysis

5.8. RFC 5762: RTP over DCCP

   RTP is a widely used transport for real-time multimedia on IP
   networks.  DCCP is a transport protocol that provides desirable
   services for real-time applications.  [RFC5762] specifies a mapping
   of RTP onto DCCP, along with associated signaling, such that real-
   time applications can make use of the services provided by DCCP.

     +===================+======================+=========+==========+
     | Name              | Notes                | Current | Mux      |
     |                   |                      |         | Category |
     +===================+======================+=========+==========+
     | dccp-service-code | If RFC 6773 is not   | M       | CAUTION  |
     |                   | being used in        |         |          |
     |                   | addition to RFC5762, |         |          |
     |                   | the port in the "m=" |         |          |
     |                   | line is a DCCP port. |         |          |
     |                   | Being a connection-  |         |          |
     |                   | oriented protocol,   |         |          |
     |                   | DCCP does not allow  |         |          |
     |                   | multiple connections |         |          |
     |                   | on the same 5-tuple. |         |          |
     +-------------------+----------------------+---------+----------+

                    Table 8: RFC 5762 Attribute Analysis

   NOTE: If RFC 6773 is being used in addition to RFC 5762, and the
   DCCP-in-UDP layer has additional demultiplexing, then it may be
   possible to use different DCCP service codes for each DCCP flow,
   given each uses a different DCCP port.  However, doing so might
   conflict with the media type of the "m=" line.  None of this is
   standardized yet, and it wouldn't work as explained.  Hence
   performing multiplexing is not recommended even in this alternate
   scenario.

5.9. RFC 6773: DCCP-UDP Encapsulation

   [RFC6773] specifies an alternative encapsulation of DCCP, referred to
   as DCCP-UDP.  This encapsulation allows DCCP to be carried through
   the current generation of Network Address Translation (NAT)
   middleboxes without modification of those middleboxes.

      +===========+==============================+=======+==========+
      | Name      | Notes                        | Level | Mux      |
      |           |                              |       | Category |
      +===========+==============================+=======+==========+
      | dccp-port | Multiplexing is not          | M     | CAUTION  |
      |           | recommended due to potential |       |          |
      |           | conflict between the port    |       |          |
      |           | used for DCCP encapsulation/ |       |          |
      |           | decapsulation and the RTP.   |       |          |
      +-----------+------------------------------+-------+----------+

                    Table 9: RFC 6773 Attribute Analysis

   NOTE: RFC 6773 allows DCCP-UDP encapsulation, with the UDP port being
   the port of the DCCP encapsulation/decapsulation service.  This
   encapsulation allows arbitrary DCCP packets to be encapsulated, and
   the DCCP port chosen can conflict with the port chosen for the RTP
   traffic.  Multiplexing several DCCP-in-UDP encapsulations on the same
   UDP port with no RTP traffic on the same port implies collapsing
   several DCCP port spaces together.  Whether or not this works depends
   on the nature of DCCP encapsulation and ports choices; it is thus
   very application dependent.

5.10. RFC 5506: Reduced-Size RTCP in RTP Profile

5.11. RFC 6787: Media Resource Control Protocol Version 2

   The Media Resource Control Protocol Version 2 (MRCPv2) allows client
   hosts to control media service resources such as speech synthesizers,
   recognizers, verifiers, and identifiers residing in servers on the
   network.  MRCPv2 is not a "stand-alone" protocol; it relies on other
   protocols, such as the SIP, to coordinate MRCPv2 clients and servers
   and manage session between them, and SDP to describe, discover, and
   exchange capabilities.  It also depends on SIP and SDP to establish
   the media sessions and associated parameters between the media source
   or sink and the media server.  Once this is done, the MRCPv2 exchange
   operates over the control session established above, allowing the
   client to control the media-processing resources on the speech
   resource server.  [RFC6787] defines attributes for this purpose.

            +==========+==============+=======+==============+
            | Name     | Notes        | Level | Mux Category |
            +==========+==============+=======+==============+
            | resource | Not impacted | M     | NORMAL       |
            +----------+--------------+-------+--------------+
            | channel  | Not impacted | M     | NORMAL       |
            +----------+--------------+-------+--------------+
            | cmid     | Not impacted | M     | NORMAL       |
            +----------+--------------+-------+--------------+

                  Table 11: RFC 6787 Attribute Analysis

5.12. RFC 8445: ICE

5.13. RFC 5285: RTP Header Extensions

   [RFC5285] provides a general mechanism for using the header-extension
   feature of RTP.  (Note: [RFC5285] has been obsoleted by [RFC8285].)
   It provides the option to use a small number of small extensions in
   each RTP packet, where the universe of possible extensions is large
   and registration is decentralized.  The actual extensions in use in a
   session are signaled in the setup information for that session.

    +========+================================+=======+==============+
    | Name   | Notes                          | Level | Mux Category |
    +========+================================+=======+==============+
    | extmap | Refer to the document defining | B     | SPECIAL      |
    |        | the specific RTP extension.    |       |              |
    +--------+--------------------------------+-------+--------------+

                  Table 13: RFC 5285 Attribute Analysis

5.14. RFC 3605: RTCP Attribute in SDP

5.15. RFC 5576: Source-Specific SDP Attributes

5.16. RFC 7273: RTP Clock Source Signaling

5.17. RFC 6236: Image Attributes in SDP

   [RFC6236] proposes a new generic session setup attribute to make it
   possible to negotiate different image attributes, such as image size.
   A possible use case is to make it possible for a low-end handheld
   terminal to display video without the need to rescale the image,
   something that may consume large amounts of memory and processing
   power.  The document also helps to maintain an optimal bitrate for
   video as only the image size that is desired by the receiver is
   transmitted.

    +===========+==========================+=======+==================+
    | Name      | Notes                    | Level | Mux Category     |
    +===========+==========================+=======+==================+
    | imageattr | The attribute value MUST | M     | IDENTICAL-PER-PT |
    |           | be the same for a given  |       |                  |
    |           | codec configuration.     |       |                  |
    +-----------+--------------------------+-------+------------------+

                   Table 17: RFC 6236 Attribute Analysis

5.18. RFC 7197: Duplication Delay Attribute in SDP

5.19. RFC 7266: RTCP XR Blocks for MOS Metric Reporting

   [RFC7266] defines an RTCP Extended Report (XR) Block that includes
   two new segment types and associated SDP parameters that allow the
   reporting of mean opinion score (MOS) metrics for use in a range of
   RTP applications.

           +============+==============+=======+==============+
           | Name       | Notes        | Level | Mux Category |
           +============+==============+=======+==============+
           | calgextmap | Not impacted | B     | NORMAL       |
           +------------+--------------+-------+--------------+

                  Table 19: RFC 7266 Attribute Analysis

5.20. RFC 6285: Rapid Acquisition of Multicast RTP Sessions

   [RFC6285] describes a method of using the existing RTP and RTCP
   machinery that reduces the acquisition delay.  In this method, an
   auxiliary unicast RTP session carrying the reference information to
   the receiver precedes or accompanies the multicast stream.  This
   unicast RTP flow can be transmitted at a faster-than-natural bitrate
   to further accelerate the acquisition.  The motivating use case for
   this capability is multicast applications that carry real-time
   compressed audio and video.

         +==============+=================+=======+==============+
         | Name         | Notes           | Level | Mux Category |
         +==============+=================+=======+==============+
         | rams-updates | Not recommended | M     | CAUTION      |
         +--------------+-----------------+-------+--------------+

                   Table 20: RFC 6285 Attribute Analysis

5.21. RFC 6230: Media Control Channel Framework

   [RFC6230] describes a framework and protocol for application
   deployment where the application programming logic and media
   processing are distributed.  This implies that application
   programming logic can seamlessly gain access to appropriate resources
   that are not co-located on the same physical network entity.  The
   framework uses SIP to establish an application-level control
   mechanism between application servers and associated external servers
   such as media servers.

             +========+==============+=======+==============+
             | Name   | Notes        | Level | Mux Category |
             +========+==============+=======+==============+
             | cfw-id | Not impacted | M     | NORMAL       |
             +--------+--------------+-------+--------------+

                  Table 21: RFC 6230 Attribute Analysis

5.22. RFC 6364: SDP Elements for FEC Framework

5.23. RFC 4796: "content" Attribute

5.24. RFC 3407: SDP Simple Capability Declaration

   [RFC3407] defines a set of SDP attributes that enables SDP to provide
   a minimal and backwards-compatible capability declaration mechanism.

         +=========+======================+=======+==============+
         | Name    | Notes                | Level | Mux Category |
         +=========+======================+=======+==============+
         | sqn     | Not impacted         | B     | NORMAL       |
         +---------+----------------------+-------+--------------+
         | cdsc    | Not impacted         | B     | NORMAL       |
         +---------+----------------------+-------+--------------+
         | cpar    | Refer to Section 14  | B     | INHERIT      |
         +---------+----------------------+-------+--------------+
         | cparmin | Refer to notes below | B     | SPECIAL      |
         +---------+----------------------+-------+--------------+
         | cparmax | Refer to notes below | B     | SPECIAL      |
         +---------+----------------------+-------+--------------+

                   Table 24: RFC 3407 Attribute Analysis

   NOTE: The attributes "a=cparmin" and "a=cparmax" define minimum and
   maximum numerical values associated with the attributes described in
   "a=cpar".

   Since the cpar attribute can either define a "b=" attribute or any
   "a=" attribute, the multiplexing category depends on the actual
   attribute being encapsulated and the implications of the numerical
   values assigned.  Hence it is recommended to consult the
   specification defining attributes "cparmin" and "cparmax" to further
   analyze their behavior under multiplexing.

5.25. RFC 6284: Port Mapping between Unicast and Multicast RTP Sessions

   [RFC6284] presents a port-mapping solution that allows RTP receivers
   to choose their own ports for an auxiliary unicast session in RTP
   applications using both unicast and multicast services.  The solution
   provides protection against denial-of-service or packet amplification
   attacks that could be used to cause one or more RTP packets to be
   sent to a victim client.

     +=================+=========================+=======+==========+
     | Name            | Notes                   | Level | Mux      |
     |                 |                         |       | Category |
     +=================+=========================+=======+==========+
     | portmapping-req | Not recommended if port | M     | CAUTION  |
     |                 | mapping is required by  |       |          |
     |                 | the application         |       |          |
     +-----------------+-------------------------+-------+----------+

                  Table 25: RFC 6284 Attribute Analysis

5.26. RFC 6714: MSRP-CEMA

   [RFC6714] defines a Message Session Relay Protocol (MSRP) extension,
   Connection Establishment for Media Anchoring (CEMA).  Support of this
   extension is optional.  The extension allows middleboxes to anchor
   the MSRP connection without the need for middleboxes to modify the
   MSRP messages; thus, it also enables secure end-to-end MSRP
   communication in networks where such middleboxes are deployed.  This
   document also defines an SDP attribute, "msrp-cema", that MSRP
   endpoints use to indicate support of the CEMA extension.

        +===========+======================+=======+==============+
        | Name      | Notes                | Level | Mux Category |
        +===========+======================+=======+==============+
        | msrp-cema | Refer to notes below | M     | TBD          |
        +-----------+----------------------+-------+--------------+

                   Table 26: RFC 6714 Attribute Analysis

   NOTE: As per Section 9 of [RFC8843], there exists no publicly
   available specification that defines procedures for multiplexing/
   demultiplexing MSRP flows over a single 5-tuple.  Once such a
   specification is available, the assignments of multiplexing
   categories for the attributes in this section could be revisited.

5.27. RFC 4583: SDP Format for BFCP Streams

   [RFC4583] specifies how to describe Binary Floor Control Protocol
   (BFCP) streams in SDP descriptions.  User agents using the offer/
   answer model to establish BFCP streams use this format in their
   offers and answers.

        +===========+======================+=======+==============+
        | Name      | Notes                | Level | Mux Category |
        +===========+======================+=======+==============+
        | floorctrl | Refer to notes below | M     | TBD          |
        +-----------+----------------------+-------+--------------+
        | confid    | Refer to notes below | M     | TBD          |
        +-----------+----------------------+-------+--------------+
        | userid    | Refer to notes below | M     | TBD          |
        +-----------+----------------------+-------+--------------+
        | floorid   | Refer to notes below | M     | TBD          |
        +-----------+----------------------+-------+--------------+

                   Table 27: RFC 4583 Attribute Analysis

   NOTE: [RFC4583] has been obsoleted by [RFC8856], which redefines the
   SDP attributes listed in this section, including the "Mux Category"
   values.  However, [RFC8856] does not change the "Mux Category" values
   of the attributes.

   NOTE: As per Section 9 of [RFC8843], there exists no publicly
   available specification that defines procedures for multiplexing/
   demultiplexing BFCP streams over a single 5-tuple.  Once such a
   specification is available, the assignments of multiplexing
   categories for the attributes in this section could be revisited.

5.28. RFC 5547: SDP Offer/Answer for File Transfer

   [RFC5547] provides a mechanism to negotiate the transfer of one or
   more files between two endpoints by using the SDP offer/answer model
   specified in [RFC3264].

    +==================+======================+=======+==============+
    | Name             | Notes                | Level | Mux Category |
    +==================+======================+=======+==============+
    | file-selector    | Refer to notes below | M     | TBD          |
    +------------------+----------------------+-------+--------------+
    | file-transfer-id | Refer to notes below | M     | TBD          |
    +------------------+----------------------+-------+--------------+
    | file-disposition | Refer to notes below | M     | TBD          |
    +------------------+----------------------+-------+--------------+
    | file-date        | Refer to notes below | M     | TBD          |
    +------------------+----------------------+-------+--------------+
    | file-icon        | Refer to notes below | M     | TBD          |
    +------------------+----------------------+-------+--------------+
    | file-range       | Refer to notes below | M     | TBD          |
    +------------------+----------------------+-------+--------------+

                  Table 28: RFC 5547 Attribute Analysis

   NOTE: As per Section 9 of [RFC8843], there exists no publicly
   available specification that defines procedures for multiplexing/
   demultiplexing MSRP flows over a single 5-tuple.  Once such a
   specification is available, the assignments of multiplexing
   categories for attributes in this section could be revisited.

5.29. RFC 6849: SDP and RTP Media Loopback Extension

5.30. RFC 5760: RTCP with Unicast Feedback

   [RFC5760] specifies an extension to RTCP to use unicast feedback to a
   multicast sender.  The proposed extension is useful for single-source
   multicast sessions such as source-specific multicast (SSM)
   communication where the traditional model of many-to-many group
   communication is either not available or not desired.

      +==============+=========================+=======+===========+
      | Name         | Notes                   | Level | Mux       |
      |              |                         |       | Category  |
      +==============+=========================+=======+===========+
      | rtcp-unicast | The attribute MUST be   | M     | IDENTICAL |
      |              | reported across all     |       |           |
      |              | multiplexed "m=" lines. |       |           |
      +--------------+-------------------------+-------+-----------+

                  Table 30: RFC 5760 Attribute Analysis

5.31. RFC 3611: RTCP XR

   [RFC3611] defines the Extended Report (XR) packet type for RTCP and
   defines how the use of XR packets can be signaled by an application
   if it employs the Session Description Protocol (SDP).

             +=========+==============+=======+==============+
             | Name    | Notes        | Level | Mux Category |
             +=========+==============+=======+==============+
             | rtcp-xr | Not impacted | B     | NORMAL       |
             +---------+--------------+-------+--------------+

                   Table 31: RFC 3611 Attribute Analysis

5.32. RFC 5939: SDP Capability Negotiation

5.33. RFC 6871: SDP Media Capabilities Negotiation

   SDP capability negotiation provides a general framework for
   indicating and negotiating capabilities in SDP.  The base framework
   only defines capabilities for negotiating transport protocols and
   attributes.  [RFC6871] extends the framework by defining media
   capabilities that can be used to negotiate media types and their
   associated parameters.

       +========+======================+=======+==================+
       | Name   | Notes                | Level | Mux Category     |
       +========+======================+=======+==================+
       | rmcap  | Refer to Section 14  | B     | IDENTICAL-PER-PT |
       +--------+----------------------+-------+------------------+
       | omcap  | Not impacted         | B     | NORMAL           |
       +--------+----------------------+-------+------------------+
       | mfcap  | Refer to Section 14  | B     | IDENTICAL-PER-PT |
       +--------+----------------------+-------+------------------+
       | mscap  | Refer to Section 14  | B     | INHERIT          |
       +--------+----------------------+-------+------------------+
       | lcfg   | Refer to Section 14  | B     | SPECIAL          |
       +--------+----------------------+-------+------------------+
       | sescap | Refer to notes below | S     | CAUTION          |
       +--------+----------------------+-------+------------------+
       | med-v0 | Not impacted         | S     | NORMAL           |
       +--------+----------------------+-------+------------------+

                  Table 33: RFC 6871 Attribute Analysis

   NOTE: The "sescap" attribute is not recommended for use with
   multiplexing.  The reason is that it requires the use of unique
   configuration numbers across the entire SDP (per [RFC6871]) as
   opposed to within a media description only (per [RFC5939]).  As
   described in Section 14, the use of identical configuration numbers
   between multiplexed (bundled) media descriptions is the default way
   of indicating compatible configurations in a bundle.

5.34. RFC 7006: Miscellaneous Capabilities Negotiation in SDP

5.35. RFC 4567: Key Management Extensions for SDP and RTSP

   [RFC4567] defines general extensions for SDP and Real-Time Streaming
   Protocol (RTSP) to carry messages, as specified by a key management
   protocol, in order to secure the media.  These extensions are
   presented as a framework to be used by one or more key management
   protocols.  As such, their use is meaningful only when complemented
   by an appropriate key management protocol.

        +==========+==========================+=======+===========+
        | Name     | Notes                    | Level | Mux       |
        |          |                          |       | Category  |
        +==========+==========================+=======+===========+
        | key-mgmt | Key management protocol  | B     | IDENTICAL |
        |          | MUST be identical across |       |           |
        |          | all the "m=" lines.      |       |           |
        +----------+--------------------------+-------+-----------+
        | mikey    | Key management protocol  | B     | IDENTICAL |
        |          | MUST be identical across |       |           |
        |          | all the "m=" lines.      |       |           |
        +----------+--------------------------+-------+-----------+

                   Table 35: RFC 4567 Attribute Analysis

5.36. RFC 4572: Comedia over TLS in SDP

   [RFC4572] specifies how to establish secure connection-oriented media
   transport sessions over the Transport Layer Security (TLS) protocol
   using SDP.  (Note: [RFC4572] has been obsoleted by [RFC8122].)  It
   defines a new SDP protocol identifier, "TCP/TLS".  It also defines
   the syntax and semantics for an SDP "fingerprint" attribute that
   identifies the certificate that will be presented for the TLS
   session.  This mechanism allows media transport over TLS connections
   to be established securely, so long as the integrity of session
   descriptions is assured.

   +=============+=================================+=======+===========+
   | Name        | Notes                           | Level | Mux       |
   |             |                                 |       | Category  |
   +=============+=================================+=======+===========+
   | fingerprint | fingerprint value MUST be       | B     | TRANSPORT |
   |             | the one that corresponds        |       |           |
   |             | to the "m=" line chosen         |       |           |
   |             | for setting up the              |       |           |
   |             | underlying transport flow.      |       |           |
   +-------------+---------------------------------+-------+-----------+

                   Table 36: RFC 4572 Attribute Analysis

5.37. RFC 4570: SDP Source Filters

   [RFC4570] describes how to adapt SDP to express one or more source
   addresses as a source filter for one or more destination "connection"
   addresses.  It defines the syntax and semantics for an SDP "source-
   filter" attribute that may reference either IPv4 or IPv6 address(es)
   as either an inclusive or exclusive source list for either multicast
   or unicast destinations.  In particular, an inclusive source filter
   can be used to specify a source-specific multicast (SSM) session.

      +===============+=========================+=======+===========+
      | Name          | Notes                   | Level | Mux       |
      |               |                         |       | Category  |
      +===============+=========================+=======+===========+
      | source-filter | The attribute MUST be   | B     | IDENTICAL |
      |               | repeated across all     |       |           |
      |               | multiplexed "m=" lines. |       |           |
      +---------------+-------------------------+-------+-----------+

                   Table 37: RFC 4570 Attribute Analysis

5.38. RFC 6128: RTCP Port for Multicast Sessions

   SDP has an attribute that allows RTP applications to specify an
   address and a port associated with the RTCP traffic.  In RTP-based
   source-specific multicast (SSM) sessions, the same attribute is used
   to designate the address and the RTCP port of the Feedback Target in
   the SDP description.  However, the RTCP port associated with the SSM
   session itself cannot be specified by the same attribute to avoid
   ambiguity and thus is required to be derived from the "m=" line of
   the media description.  Deriving the RTCP port from the "m=" line
   imposes an unnecessary restriction.  [RFC6128] removes this
   restriction by introducing a new SDP attribute.

     +================+==========================+=======+===========+
     | Name           | Notes                    | Level | Mux       |
     |                |                          |       | Category  |
     +================+==========================+=======+===========+
     | multicast-rtcp | Multicast RTCP port MUST | B     | IDENTICAL |
     |                | be identical across all  |       |           |
     |                | the "m=" lines.          |       |           |
     +----------------+--------------------------+-------+-----------+

                   Table 38: RFC 6128 Attribute Analysis

5.39. RFC 6189: ZRTP

5.40. RFC 4145: Connection-Oriented Media

5.41. RFC 6947: The SDP "altc" Attribute

   [RFC6947] proposes a mechanism that allows the same SDP offer to
   carry multiple IP addresses of different address families (e.g., IPv4
   and IPv6).  The proposed "altc" attribute solves the backward-
   compatibility problem that plagued Alternative Network Address Types
   (ANAT) due to their syntax.

      +======+=================================+=======+===========+
      | Name | Notes                           | Level | Mux       |
      |      |                                 |       | Category  |
      +======+=================================+=======+===========+
      | altc | The IP address and port MUST be | M     | TRANSPORT |
      |      | the ones that correspond to the |       |           |
      |      | "m=" line chosen for setting up |       |           |
      |      | the underlying transport flow.  |       |           |
      +------+---------------------------------+-------+-----------+

                  Table 41: RFC 6947 Attribute Analysis

5.42. RFC 7195: SDP Extension for Circuit-Switched Bearers in PSTN

   [RFC7195] describes use cases, requirements, and protocol extensions
   for using the SDP offer/answer model for establishing audio and video
   media streams over circuit-switched bearers in the Public Switched
   Telephone Network (PSTN).

     +=========================+=============+=======+==============+
     | Name                    | Notes       | Level | Mux Category |
     +=========================+=============+=======+==============+
     | cs-correlation:callerid | Refer to    | M     | TBD          |
     |                         | notes below |       |              |
     +-------------------------+-------------+-------+--------------+
     | cs-correlation:uuie     | Refer to    | M     | TBD          |
     |                         | notes below |       |              |
     +-------------------------+-------------+-------+--------------+
     | cs-correlation:dtmf     | Refer to    | M     | TBD          |
     |                         | notes below |       |              |
     +-------------------------+-------------+-------+--------------+
     | cs-correlation:external | Refer to    | M     | TBD          |
     |                         | notes below |       |              |
     +-------------------------+-------------+-------+--------------+

                  Table 42: RFC 7195 Attribute Analysis

   NOTE: [RFC7195] defines SDP attributes for establishing audio and
   video media streams over circuit-switched bearers by defining a new
   nettype value, "PSTN".  However, Section 7.2 of [RFC8843] requires
   the "c=" line nettype value to be "IN".  If there exists in future a
   specification that defines procedures to multiplex media streams over
   nettype "PSTN", the multiplexing categories for attributes in this
   section could be revisited.

5.43. RFC 7272: IDMS Using the RTP Control Protocol (RTCP)

5.44. RFC 5159: Open Mobile Alliance (OMA) Broadcast (BCAST) SDP
Attributes

5.45. RFC 6193: Media Description for IKE in SDP

5.46. RFC 2326: Real Time Streaming Protocol

   The Real Time Streaming Protocol, or RTSP, is an application-level
   protocol for control over the delivery of data with real-time
   properties.  RTSP provides an extensible framework to enable
   controlled, on-demand delivery of real-time data, such as audio and
   video.

        +=========+=======================+=======+==============+
        | Name    | Notes                 | Level | Mux Category |
        +=========+=======================+=======+==============+
        | etag    | RFC 2326 is obsolete. | B     | CAUTION      |
        +---------+-----------------------+-------+--------------+
        | range   | RFC 2326 is obsolete. | B     | CAUTION      |
        +---------+-----------------------+-------+--------------+
        | control | RFC 2326 is obsolete. | B     | CAUTION      |
        +---------+-----------------------+-------+--------------+
        | mtag    | RFC 2326 is obsolete. | B     | CAUTION      |
        +---------+-----------------------+-------+--------------+

                  Table 46: RFC 2326 Attribute Analysis

   NOTE: [RFC2326] defines SDP attributes that are applicable in the
   declarative usage of SDP alone.  For the purposes of this document,
   only the offer/answer usage of SDP is considered to be mandated by
   [RFC8843].

5.47. RFC 7826: Real-Time Streaming Protocol

   The Real-Time Streaming Protocol, or RTSP, is an application-level
   protocol for control over the delivery of data with real-time
   properties.  RTSP provides an extensible framework to enable
   controlled, on-demand delivery of real-time data, such as audio and
   video.

      +=========+===========================+=======+==============+
      | Name    | Notes                     | Level | Mux Category |
      +=========+===========================+=======+==============+
      | range   | RTSP is not supported for | B     | CAUTION      |
      |         | RTP stream multiplexing.  |       |              |
      +---------+---------------------------+-------+--------------+
      | control | RTSP is not supported for | B     | CAUTION      |
      |         | RTP stream multiplexing.  |       |              |
      +---------+---------------------------+-------+--------------+
      | mtag    | RTSP is not supported for | B     | CAUTION      |
      |         | RTP stream multiplexing.  |       |              |
      +---------+---------------------------+-------+--------------+

                  Table 47: RFC 7826 Attribute Analysis

   NOTE: [RFC7826] defines SDP attributes that are applicable in the
   declarative usage of SDP alone.  For the purposes of this document,
   only the offer/answer usage of SDP is considered to be mandated by
   [RFC8843].

5.48. RFC 6064: SDP and RTSP Extensions for 3GPP

5.49. RFC 3108: ATM SDP

5.50. 3GPP TS 183.063

5.51. 3GPP TS 24.229

5.52. ITU T.38

   [T.38] defines procedures for real-time Group 3 facsimile
   communications over IP networks.

      +=======================+=============+=======+==============+
      | Name                  | Notes       | Level | Mux Category |
      +=======================+=============+=======+==============+
      | T38FaxVersion         | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38MaxBitRate         | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxFillBitRemoval  | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxTranscodingMMR  | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxTranscodingJBIG | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxRateManagement  | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxMaxBuffer       | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxMaxDatagram     | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxUdpEC           | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxMaxIFP          | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxUdpECDepth      | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38FaxUdpFECMaxSpan   | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38ModemType          | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+
      | T38VendorInfo         | Refer to    | M     | TBD          |
      |                       | notes below |       |              |
      +-----------------------+-------------+-------+--------------+

                  Table 52: ITU T.38 Attribute Analysis

   NOTE: As per Section 9 of [RFC8843], there exists no publicly
   available specification that defines procedures for multiplexing/
   demultiplexing fax protocol flows over a single 5-tuple.  Once such a
   specification is available, the multiplexing category assignments for
   the attributes in this section could be revisited.

5.53. ITU-T Q.1970

5.54. ITU-T H.248.15

5.55. RFC 4975: The Message Session Relay Protocol

   [RFC4975] describes the Message Session Relay Protocol, a protocol
   for transmitting a series of related instant messages in the context
   of a session.  Message sessions are treated like any other media
   stream when set up via a rendezvous or session-creation protocol such
   as the Session Initiation Protocol.

       +======================+=============+=======+==============+
       | Name                 | Notes       | Level | Mux Category |
       +======================+=============+=======+==============+
       | accept-types         | Refer to    | M     | TBD          |
       |                      | notes below |       |              |
       +----------------------+-------------+-------+--------------+
       | accept-wrapped-types | Refer to    | M     | TBD          |
       |                      | notes below |       |              |
       +----------------------+-------------+-------+--------------+
       | max-size             | Refer to    | M     | TBD          |
       |                      | notes below |       |              |
       +----------------------+-------------+-------+--------------+
       | path                 | Refer to    | M     | TBD          |
       |                      | notes below |       |              |
       +----------------------+-------------+-------+--------------+

                   Table 55: RFC 4975 Attribute Analysis

   NOTE: As per Section 9 of [RFC8843], there exists no publicly
   available specification that defines procedures for multiplexing/
   demultiplexing MSRP flows over a single 5-tuple.  Once such a
   specification is available, the multiplexing categories assignments
   for the attributes in this section could be revisited.

5.56. Historical Attributes

6. bwtype Attribute Analysis

This section specifies handling of specific bandwidth attributes when
used in multiplexing scenarios.

6.1. RFC 4566: SDP

6.2. RFC 3556: SDP Bandwidth Modifiers for RTCP Bandwidth

6.3. RFC 3890: Bandwidth Modifier for SDP

   [RFC3890] defines SDP Transport Independent Application Specific
   Maximum (TIAS) bandwidth modifier that does not include transport
   overhead; instead, an additional packet-rate attribute is defined.
   The transport-independent bitrate value together with the maximum
   packet rate can then be used to calculate the real bitrate over the
   transport actually used.

   +=============+==================================+=======+==========+
   | Name        | Notes                            | Level | Mux      |
   |             |                                  |       | Category |
   +=============+==================================+=======+==========+
   | bwtype:TIAS | The usage of TIAS is not         | B     | SPECIAL  |
   |             | defined under offer/answer       |       |          |
   |             | usage.                           |       |          |
   +-------------+----------------------------------+-------+----------+
   | maxprate    | The usage of TIAS and            | B     | SPECIAL  |
   |             | maxprate is not well             |       |          |
   |             | defined under multiplexing.      |       |          |
   +-------------+----------------------------------+-------+----------+

                     Table 59: RFC 3890 bwtype Analysis

   NOTE: The intention of TIAS is that the media-level bitrate is
   multiplied with the known per-packet overhead for the selected
   transport and the maxprate value to determine the worst-case bitrate
   from the transport to more accurately capture the required usage.
   Summing TIAS values independently across "m=" lines and multiplying
   the computed sum with maxprate and the per-packet overhead would
   inflate the value significantly.  Instead, performing multiplication
   and adding the individual values is a more appropriate usage.

7. rtcp-fb Attribute Analysis

This section analyzes rtcp-fb SDP attributes.

7.1. RFC 4585: RTP/AVPF

7.2. RFC 5104: Codec Control Messages in AVPF

7.3. RFC 6285: Unicast-Based Rapid Acquisition of Multicast RTP
Sessions (RAMS)

   [RFC6285] describes a method of using the existing RTP and RTCP
   machinery that reduces the acquisition delay.  In this method, an
   auxiliary unicast RTP session carrying the Reference Information to
   the receiver precedes or accompanies the multicast stream.  This
   unicast RTP flow can be transmitted at a faster-than-natural bitrate
   to further accelerate the acquisition.  The motivating use case for
   this capability is multicast applications that carry real-time
   compressed audio and video.

      +======+==========================+=======+==================+
      | Name | Notes                    | Level | Mux Category     |
      +======+==========================+=======+==================+
      | nack | The attribute value MUST | M     | IDENTICAL-PER-PT |
      | rai  | be the same for a given  |       |                  |
      |      | codec configuration.     |       |                  |
      +------+--------------------------+-------+------------------+

                  Table 62: RFC 6285 Attribute Analysis

7.4. RFC 6679: ECN for RTP over UDP/IP

   [RFC6679] specifies how Explicit Congestion Notification (ECN) can be
   used with the RTP running over UDP, using the RTCP as a feedback
   mechanism.  It defines a new RTCP Extended Report (XR) block for
   periodic ECN feedback, a new RTCP transport feedback message for
   timely reporting of congestion events, and a STUN extension used in
   the optional initialization method using ICE.

   +=================+============================+=======+===========+
   | Name            | Notes                      | Level | Mux       |
   |                 |                            |       | Category  |
   +=================+============================+=======+===========+
   | ecn-capable-rtp | ECN markup is enabled at   | M     | IDENTICAL |
   |                 | the RTP session level.     |       |           |
   +-----------------+----------------------------+-------+-----------+
   | nack ecn        | This attribute enables ECN | M     | IDENTICAL |
   |                 | at the RTP session level.  |       |           |
   +-----------------+----------------------------+-------+-----------+

                  Table 63: RFC 6679 Attribute Analysis

7.5. RFC 6642: Third-Party Loss Report

   In a large RTP session using the RTCP feedback mechanism defined in
   [RFC4585], a feedback target may experience transient overload if
   some event causes a large number of receivers to send feedback at
   once.  This overload is usually avoided by ensuring that feedback
   reports are forwarded to all receivers, allowing them to avoid
   sending duplicate feedback reports.  However, there are cases where
   it is not recommended to forward feedback reports, and this may allow
   feedback implosion.  [RFC6642] discusses these cases and defines a
   new RTCP Third-Party Loss Report that can be used to inform receivers
   that the feedback target is aware of some loss event, allowing them
   to suppress feedback.  Associated SDP signaling is also defined.

      +=======+==========================+=======+==================+
      | Name  | Notes                    | Level | Mux Category     |
      +=======+==========================+=======+==================+
      | nack  | The attribute value MUST | M     | IDENTICAL-PER-PT |
      | tllei | be the same for a given  |       |                  |
      |       | codec configuration.     |       |                  |
      +-------+--------------------------+-------+------------------+
      | nack  | The attribute value MUST | M     | IDENTICAL-PER-PT |
      | pslei | be the same for a given  |       |                  |
      |       | codec configuration.     |       |                  |
      +-------+--------------------------+-------+------------------+

                   Table 64: RFC 6642 Attribute Analysis

7.6. RFC 5104: Codec Control Messages in AVPF

8. group Attribute Analysis

This section analyzes SDP "group" attribute semantics [RFC5888].

8.1. RFC 5888: SDP Grouping Framework

8.2. RFC 3524: Mapping Media Streams to Resource Reservation Flows

   [RFC3524] defines an extension to the SDP grouping framework.  It
   allows requesting a group of media streams to be mapped into a single
   resource reservation flow.  The SDP syntax needed is defined, as well
   as a new "semantics" attribute called Single Reservation Flow (SRF).

            +===========+==============+=======+==============+
            | Name      | Notes        | Level | Mux Category |
            +===========+==============+=======+==============+
            | group:SRF | Not impacted | S     | NORMAL       |
            +-----------+--------------+-------+--------------+

                   Table 67: RFC 3524 Attribute Analysis

8.3. RFC 4091: ANAT Semantics

   [RFC4091] defines ANAT semantics for the SDP grouping framework.
   (Note: [RFC4091] has been obsoleted by [RFC8445].)  The ANAT
   semantics allow alternative types of network addresses to establish a
   particular media stream.

   +============+==============================+=======+==============+
   | Name       | Notes                        | Level | Mux Category |
   +============+==============================+=======+==============+
   | group:ANAT | ANAT semantics is obsoleted. | S     | CAUTION      |
   +------------+------------------------------+-------+--------------+

                  Table 68: RFC 4091 Attribute Analysis

8.4. RFC 5956: FEC Grouping Semantics in SDP

   [RFC5956] defines the semantics for grouping the associated source
   and FEC-based repair flows in SDP.  The semantics defined in the
   document are to be used with the SDP Grouping Framework [RFC5888].
   These semantics allow the description of grouping relationships
   between the source and repair flows when one or more source and/or
   repair flows are associated in the same group; they also provide
   support for additive repair flows.  SSRC-level grouping semantics are
   also defined in this document for RTP streams using SSRC
   multiplexing.

          +==============+==============+=======+==============+
          | Name         | Notes        | Level | Mux Category |
          +==============+==============+=======+==============+
          | group:FEC-FR | Not impacted | S     | NORMAL       |
          +--------------+--------------+-------+--------------+

                  Table 69: RFC 5956 Attribute Analysis

8.5. RFC 5583: Signaling Media Decoding Dependency in SDP

8.6. RFC 7104: Duplication Grouping Semantics in the SDP

   [RFC7104] defines the semantics for grouping redundant streams in
   SDP.  The semantics defined in this document are to be used with the
   SDP Grouping Framework.  Grouping semantics at the synchronization
   source (SSRC) level are also defined in this document for RTP streams
   using SSRC multiplexing.

            +===========+==============+=======+==============+
            | Name      | Notes        | Level | Mux Category |
            +===========+==============+=======+==============+
            | group:DUP | Not impacted | S     | NORMAL       |
            +-----------+--------------+-------+--------------+

                   Table 71: RFC 7104 Attribute Analysis

9. ssrc-group Attribute Analysis

This section analyzes "ssrc-group" semantics.

9.1. RFC 5576: Source-Specific SDP Attributes

9.2. RFC 7104: Duplication Grouping Semantics in the SDP

   [RFC7104] defines the semantics for grouping redundant streams in
   SDP.  The semantics defined in this document are to be used with the
   SDP Grouping Framework.  Grouping semantics at the synchronization
   source (SSRC) level are also defined in this document for RTP streams
   using SSRC multiplexing.

         +================+==============+=======+==============+
         | Name           | Notes        | Level | Mux Category |
         +================+==============+=======+==============+
         | ssrc-group:DUP | Not impacted | SR    | NORMAL       |
         +----------------+--------------+-------+--------------+

                  Table 73: RFC 7104 Attribute Analysis

10. QoS Mechanism Token Analysis

This section analyzes QoS tokes specified with SDP.

10.1. RFC 5432: QoS Mechanism Selection in SDP

   [RFC5432] defines procedures to negotiate QoS mechanisms using the
   SDP offer/answer model.

       +======+================================+=======+===========+
       | Name | Notes                          | Level | Mux       |
       |      |                                |       | Category  |
       +======+================================+=======+===========+
       | rsvp | rsvp attribute MUST be the one | B     | TRANSPORT |
       |      | that corresponds to the "m="   |       |           |
       |      | line chosen for setting up the |       |           |
       |      | underlying transport flow.     |       |           |
       +------+--------------------------------+-------+-----------+
       | nsis | rsvp attribute MUST be the one | B     | TRANSPORT |
       |      | that corresponds to the "m="   |       |           |
       |      | line chosen for setting up the |       |           |
       |      | underlying transport.          |       |           |
       +------+--------------------------------+-------+-----------+

                   Table 74: RFC 5432 Attribute Analysis

   NOTE: A single Differentiated Services Code Point (DSCP) for each
   flow being multiplexed doesn't impact multiplexing, since QoS
   mechanisms are signaled/scoped per flow.  For scenarios that involve
   having different DSCP code points for packets being transmitted over
   the same 5-tuple, issues as discussed in [RFC7657] need to be taken
   into consideration.

11. k= Attribute Analysis

11.1. RFC 4566: SDP

12. content Attribute Analysis

12.1. RFC 4796

12.2. 3GPP TS 24.182

12.3. 3GPP TS 24.183

13. Payload Formats

13.1. RFC 5109: RTP Payload Format for Generic FEC

14. Multiplexing Considerations for Encapsulating Attributes

   This section deals with recommendations for defining the multiplexing
   characteristics of the SDP attributes that encapsulate other SDP
   attributes/parameters.  As of today, such attributes, for example,
   are defined in [RFC3407], [RFC5939] and [RFC6871] as part of a
   generic framework for indicating and negotiating transport-, media-,
   and media-format-related capabilities in the SDP.

   The behavior of such attributes under multiplexing is, in turn,
   defined by the multiplexing behavior of the attributes they
   encapsulate, which are made known once the offer/answer negotiation
   process is completed.

14.1. RFC 3407: cpar Attribute Analysis

   The [RFC3407] capability parameter attribute "a=cpar" encapsulates a
   "b=" (bandwidth) or an "a=" attribute.  For bandwidth attribute
   encapsulation, the category SUM is inherited.  For the case of "a="
   attribute, the category corresponding to the SDP attribute being
   encapsulated is inherited.

    v=0
    o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
    s=
    c=IN IP4 host.atlanta.example.com
    t=0 0
    m=video 3456 RTP/AVP 100
    a=rtpmap:100 VP8/90000
    a=sqn: 0
    a=cdsc: 1 video RTP/AVP 100
    a=cpar: a=rtcp-mux
    m=video 3456 RTP/AVP 101
    a=rtpmap:101 VP8/90000
    a=fmtp:100 max-fr=15;max-fs=1200
    a=cdsc: 2 video RTP/AVP 101
    a=cpar: a=rtcp-mux

   In this example, the category IDENTICAL is inherited for the cpar-
   encapsulated "rtcp-mux" attribute.

14.2. RFC 5939 Analysis

   [RFC5939] defines a general SDP capability negotiation framework.  It
   also specifies how to provide transport protocols and SDP attributes
   as capabilities and negotiate them using the framework.

   For this purpose, [RFC5939] defines the following:

   *  A set of capabilities for the session and its associated media-
      stream components, supported by each side.  The attribute "a=acap"
      defines how to list an attribute name and its associated value (if
      any) as a capability.  The attribute "a=tcap" defines how to list
      transport protocols (e.g., "RTP/AVP") as capabilities.

   *  A set of potential configurations ("a=pcfg") provided by the
      offerer to indicate which combinations of those capabilities can
      be used for the session and its associated media stream
      components.  Potential configurations are not ready for use until
      fully negotiated.  They provide an alternative that MAY be used,
      subject to SDP capability-negotiation procedures.  In particular,
      the answerer MAY choose one of the potential configurations for
      use as part of the current offer/answer exchange.

   *  An actual configuration ("a=acfg") for the session and its
      associated media stream components.  The actual configuration
      identifies the potential configuration that was negotiated for
      use.  Use of an actual configuration does not require any further
      negotiation.

   *  A negotiation process that takes the current actual and the set of
      potential configurations (combinations of capabilities) as input
      and provides the negotiated actual configurations as output.  In
      [RFC5939], the negotiation process is done independently for each
      media description.

14.2.1. Recommendation: Procedures for Potential Configuration Pairing

   This section provides recommendations for entities generating and
   processing SDP under the generic capability-negotiation framework as
   defined in [RFC5939] under the context of media-stream multiplexing.

   These recommendations are provided for the purposes of enabling the
   offerer to make sure that the generated potential configurations
   between the multiplexed streams can (easily) be negotiated to be
   consistent between those streams.  In particular, the procedures aim
   to simplify the answerer's procedure for choosing potential
   configurations that are consistent across all the multiplexed media
   descriptions.

   A potential configuration selects a set of attributes and parameters
   that become part of the media description when negotiated.  When
   multiplexing media descriptions with potential configurations
   specified, there MAY be a need for coordinating this selection
   between multiplexed media descriptions to ensure the right
   multiplexing behavior.

   Although it is possible to analyze the various potential
   configurations in multiplexed media descriptions to find combinations
   that satisfy such constraints, it can quickly become complicated to
   do so.

   The procedures defined in [RFC5939] state that each potential
   configuration in the SDP has a unique configuration number; however,
   the scope of uniqueness is limited to each media description.  To
   make it simple for the answerer to chose valid combinations of
   potential configurations across media descriptions in a given BUNDLE
   group, we provide a simple rule for constructing potential
   configurations:

   *  Let m-bundle be the set of media descriptions that form a given
      bundle.

   *  Let m-bundle-pcfg be the set of media descriptions in m-bundle
      that include one or more potential configurations.

   *  Each media description in m-bundle-pcfg MUST have at least one
      potential configuration with the same configuration number (e.g.,
      "1").

   *  For each potential configuration with configuration number x in m-
      bundle-pcfg, the offerer MUST ensure that if the answerer chooses
      configuration number x in each of the media descriptions in m-
      bundle-pcfg, then the resulting SDP will have all multiplexing
      constraints satisfied for those media descriptions.

   *  Since it is nearly impossible to define a generic mechanism for
      various capability extensions, this document doesn't provide
      procedures for dealing with the capability-extension attributes.
      However, Section 14.3 provides analysis of media-capability-
      extension attributes as defined in [RFC6871].

   The above allows the answerer to easily find multiplexing-compatible
   combinations of potential configurations.  The answerer simply
   chooses a potential configuration (number) that is present in all of
   the media descriptions with potential configurations in the bundle.

   Note that it is still possible for the offerer to provide additional
   potential configurations with independent configuration numbers.  The
   answerer will have to perform more complicated analysis to determine
   valid multiplexed combinations of those.

14.2.1.1. Example: Transport-Capability Multiplexing

   v=0
   o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
   s=
   c=IN IP4 host.atlanta.example.com
   t=0 0
   a=tcap:1 RTP/SAVPF
   a=tcap:2 RTP/SAVP
   a=group:BUNDLE audio video
   m=audio
   a=mid:audio
   a=pcfg:1 t=1
   a=pcfg:2
   m=video
   a=mid:video
   a=pcfg:1 t=1
   a=pcfg:2 t=2

   In this example, the potential configurations that offer transport-
   protocol capability of RTP/SAVPF have the same configuration number
   "1" in both the audio and video media descriptions.

14.2.1.2. Example: Attribute-Capability Multiplexing

   v=0
   o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
   s=
   c=IN IP4 host.atlanta.example.com
   t=0 0
   a=acap:1 a=rtcp-mux
   a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_80
     inline:EcGZiNWpFJhQXdspcl1ekcmVCNWpVLcfHAwJSoj|2^20|1:32
   a=group:BUNDLE audio video
   m=audio 49172 RTP/AVP 99
   a=mid:audio
   a=pcfg:1 a=1
   a=pcfg:2
   m=video 560024 RTP/AVP 100
   a=mid:video
   a=pcfg:1 a=1
   a=pcfg:2 a=2

   In this example, the potential configuration number "1" is repeated
   while referring to attribute capability a=rtcp-mux, since the
   behavior is IDENTICAL for the attribute a=rtcp-mux under
   multiplexing.

14.3. RFC 6871 Analysis

   [RFC6871] extends the capability negotiation framework described in
   [RFC5939] by defining media capabilities that can be used to indicate
   and negotiate media types and their associated format parameters.  It
   also allows indication of latent configurations and session
   capabilities.

14.3.1. Recommendation: Dealing with Payload Type Numbers

   [RFC6871] defines a new payload type parameter ("pt") to be used with
   the potential, actual, and latent configuration parameters.  The
   parameter associates RTP payload type numbers with the referenced
   RTP-based media-format capabilities ("a=rmcap") defined in [RFC6871]
   and is appropriate only when the transport protocol uses RTP.  This
   means that the same payload type number can be assigned as part of
   potential or actual configurations in different media descriptions in
   a bundle.  There are rules for the usage of identical payload type
   values across multiplexed "m=" lines, described in [RFC8843], which
   must be followed here, as well.  As described in Section 14.2.1, the
   use of identical configuration numbers for compatible configurations
   in different media descriptions that are part of the bundle provides
   a way to ensure that the answerer can easily pick compatible
   configurations here, as well.

14.3.1.1. Example: Attribute Capability under Shared Payload Type

   The attributes "a=rmcap" and "a=mfcap" follow the above
   recommendations under multiplexing.

   v=0
   o=- 25678 753849 IN IP4 192.0.2.1
   s=
   c=IN IP4 192.0.2.1
   t=0 0
   a=creq:med-v0
   m=audio 54322 RTP/AVP 96
   a=rtpmap:96 AMR-WB/16000/1
   a=fmtp:96 mode-change-capability=1; max-red=220;
   mode-set=0,2,4,7
   a=rmcap:1,3 audio AMR-WB/16000/1
   a=rmcap:2 audio AMR/8000/1
   a=mfcap:1,2 mode-change-capability=1
   a=mfcap:3 mode-change-capability=2
   a=pcfg:1 m=1 pt=1:96
   a=pcfg:2 m=2 pt=2:97
   a=pcfg:3 m=3 pt=3:98
   m=audio 54322 RTP/AVP 96
   a=rtpmap:96 AMR-WB/16000/1
   a=fmtp:96 mode-change-capability=1; max-red=220;
   mode-set=0,2,4,7
   a=rmcap:4 audio AMR/8000/1
   a=rmcap:5 audio OPUS/48000/2
   a=mfcap:5 minptime=40
   a=mfcap:4 mode-change-capability=1
   a=pcfg:1 m=4 pt=4:97
   a=pcfg:4 m=5 pt=5:101

   In this example, the potential configuration number "1" is repeated
   when referring to media and media-format capability used for the
   Payload Type 96.  This implies that both media capabilities 2 and 4,
   along with their media-format capabilities, MUST refer to the same
   codec configuration, as per the definition of IDENTICAL-PER-PT.

14.3.2. Recommendation: Dealing with Latent Configurations

   [RFC6871] adds the notion of a latent configuration that provides
   configuration information that may be used to guide a subsequent
   offer/exchange -- e.g., by adding another media stream or using
   alternative codec combinations not currently offered.  Latent
   configurations have configuration numbers that cannot overlap with
   the potential configuration numbers [RFC6871].  Supported
   combinations of potential and latent configurations are indicated by
   use of the "a=sescap" attribute; however, use of this attribute is
   not recommended with multiplexed media, since it requires the use of
   unique configuration numbers across the SDP.  Taken together, this
   means there is no well-defined way to indicate supported combinations
   of latent configurations, or combinations of latent and potential
   configurations with multiplexed media.  It is still allowed to use
   the latent configuration attribute; however, the limitations above
   will apply.  To determine valid combinations, actual negotiation will
   have to be attempted subsequently instead.

15. IANA Considerations

   Section 15.1 defines a new subregistry, which has been added by the
   IANA, for identifying the initial registrations for various
   multiplexing categories applicable, as described in this document.

   IANA has added a new column named "Mux Category" to several of the
   subregistries in the "Session Description Protocol (SDP) Parameters"
   registry.  The tables in Section 15.2 identify the names of entries
   in the existing subregistry and specify the value to be put in the
   new "Mux Category" column of the associated IANA registry for each.

15.1. New "Multiplexing Categories" Subregistry

   A new subregistry has been created.  It is called "Multiplexing
   Categories" and has the following registrations initially:

   +=========================+===========+
   | Multiplexing Categories | Reference |
   +=========================+===========+
   | NORMAL                  | RFC 8859  |
   +-------------------------+-----------+
   | CAUTION                 | RFC 8859  |
   +-------------------------+-----------+
   | IDENTICAL               | RFC 8859  |
   +-------------------------+-----------+
   | TRANSPORT               | RFC 8859  |
   +-------------------------+-----------+
   | SUM                     | RFC 8859  |
   +-------------------------+-----------+
   | INHERIT                 | RFC 8859  |
   +-------------------------+-----------+
   | IDENTICAL-PER-PT        | RFC 8859  |
   +-------------------------+-----------+
   | SPECIAL                 | RFC 8859  |
   +-------------------------+-----------+
   | TBD                     | RFC 8859  |
   +-------------------------+-----------+

                   Table 80

   Further entries can be registered using Standard Actions policies
   outlined in [RFC8126], which requires IESG review and approval and
   Standards Track IETF RFC publication.

   Each registration needs to indicate the multiplexing category value
   to be added to the "Multiplexing Categories" subregistry, as defined
   in this section.

   Such a registration MUST also indicate the applicability of the newly
   defined multiplexing category value to various subregistries defined
   in the "Session Description Protocol (SDP) Parameters" registry.

15.2. "Mux Category" Column for Subregistries

   Each subsection identifies a subregistry of the "Session Description
   Protocol (SDP) Parameters" registry.  The tables list the column that
   identifies the SDP attribute name/Token/Value from the corresponding
   subregistries and the values to be used for the new "Mux Category"
   column to be added.

   Entries in the existing subregistries of the "Session Description
   Protocol (SDP) Parameters" registry that lack a value for the "Mux
   Category" in this specification will get a value of "TBD".

   The registration policy for updates to the "Mux Category" column
   values for existing parameters, or when registering new parameters,
   is beyond the scope of this document.  The registration policy for
   the affected table is defined in [RFC8866].

15.2.1. Table: SDP bwtype

   The following values have been added to the "bwtype" subregistry of
   the "Session Description Protocol (SDP) Parameters" registry.  The
   references have been updated to point to this RFC as well as the
   previous references.

   +==========+==============+
   | SDP Name | Mux Category |
   +==========+==============+
   | CT       | NORMAL       |
   +----------+--------------+
   | AS       | SUM          |
   +----------+--------------+
   | RS       | SUM          |
   +----------+--------------+
   | RR       | SUM          |
   +----------+--------------+
   | TIAS     | SPECIAL      |
   +----------+--------------+

             Table 81

15.2.2. Table: attribute-name

   The following values have been added to the "attribute-name"
   (formerly "att-field") subregistry of the "Session Description
   Protocol (SDP) Parameters" registry.  The references have been
   updated to point to this RFC as well as the previous references.

   NOTE: The attributes from [FLUTE] ("flute-tsi", "flute-ch", "FEC-
   declaration", "FEC-OTI-extension", "content-desc") were not analyzed
   for their multiplexing behavior, due to the expired status of the
   draft.  For the purposes of this specification, the multiplexing
   category of "TBD" is assigned.

   +==========================+==================+
   | SDP Name                 | Mux Category     |
   +==========================+==================+
   | cat                      | NORMAL           |
   +--------------------------+------------------+
   | keywds                   | NORMAL           |
   +--------------------------+------------------+
   | type                     | NORMAL           |
   +--------------------------+------------------+
   | type:broadcast           | NORMAL           |
   +--------------------------+------------------+
   | type:H332                | NORMAL           |
   +--------------------------+------------------+
   | type:meeting             | NORMAL           |
   +--------------------------+------------------+
   | type:moderated           | NORMAL           |
   +--------------------------+------------------+
   | type:test                | NORMAL           |
   +--------------------------+------------------+
   | charset                  | NORMAL           |
   +--------------------------+------------------+
   | charset:iso8895-1        | NORMAL           |
   +--------------------------+------------------+
   | tool                     | NORMAL           |
   +--------------------------+------------------+
   | ipbcp                    | SPECIAL          |
   +--------------------------+------------------+
   | group                    | NORMAL           |
   +--------------------------+------------------+
   | ice-lite                 | NORMAL           |
   +--------------------------+------------------+
   | ice-options              | NORMAL           |
   +--------------------------+------------------+
   | bcastversion             | NORMAL           |
   +--------------------------+------------------+
   | 3GPP-Integrity-Key       | CAUTION          |
   +--------------------------+------------------+
   | 3GPP-SDP-Auth            | CAUTION          |
   +--------------------------+------------------+
   | alt-group                | CAUTION          |
   +--------------------------+------------------+
   | PSCid                    | NORMAL           |
   +--------------------------+------------------+
   | bc_service               | NORMAL           |
   +--------------------------+------------------+
   | bc_program               | NORMAL           |
   +--------------------------+------------------+
   | bc_service_package       | NORMAL           |
   +--------------------------+------------------+
   | sescap                   | CAUTION          |
   +--------------------------+------------------+
   | rtsp-ice-d-m             | TBD              |
   +--------------------------+------------------+
   | recvonly                 | NORMAL           |
   +--------------------------+------------------+
   | sendrecv                 | NORMAL           |
   +--------------------------+------------------+
   | sendonly                 | NORMAL           |
   +--------------------------+------------------+
   | sdplang                  | NORMAL           |
   +--------------------------+------------------+
   | lang                     | NORMAL           |
   +--------------------------+------------------+
   | h248item                 | SPECIAL          |
   +--------------------------+------------------+
   | sqn                      | NORMAL           |
   +--------------------------+------------------+
   | cdsc                     | NORMAL           |
   +--------------------------+------------------+
   | cpar                     | INHERIT          |
   +--------------------------+------------------+
   | cparmin                  | SPECIAL          |
   +--------------------------+------------------+
   | cparmax                  | SPECIAL          |
   +--------------------------+------------------+
   | rtcp-xr                  | NORMAL           |
   +--------------------------+------------------+
   | maxprate                 | SPECIAL          |
   +--------------------------+------------------+
   | setup                    | TRANSPORT        |
   +--------------------------+------------------+
   | connection               | TRANSPORT        |
   +--------------------------+------------------+
   | key-mgmt                 | IDENTICAL        |
   +--------------------------+------------------+
   | source-filter            | IDENTICAL        |
   +--------------------------+------------------+
   | inactive                 | NORMAL           |
   +--------------------------+------------------+
   | fingerprint              | TRANSPORT        |
   +--------------------------+------------------+
   | flute-tsi                | TBD              |
   +--------------------------+------------------+
   | flute-ch                 | TBD              |
   +--------------------------+------------------+
   | FEC-declaration          | TBD              |
   +--------------------------+------------------+
   | FEC-OTI-extension        | TBD              |
   +--------------------------+------------------+
   | content-desc             | TBD              |
   +--------------------------+------------------+
   | ice-pwd                  | TRANSPORT        |
   +--------------------------+------------------+
   | ice-ufrag                | TRANSPORT        |
   +--------------------------+------------------+
   | stkmstream               | NORMAL           |
   +--------------------------+------------------+
   | extmap                   | SPECIAL          |
   +--------------------------+------------------+
   | qos-mech-send            | TRANSPORT        |
   +--------------------------+------------------+
   | qos-mech-recv            | TRANSPORT        |
   +--------------------------+------------------+
   | csup                     | NORMAL           |
   +--------------------------+------------------+
   | creq                     | NORMAL           |
   +--------------------------+------------------+
   | acap                     | INHERIT          |
   +--------------------------+------------------+
   | tcap                     | INHERIT          |
   +--------------------------+------------------+
   | 3GPP-QoE-Metrics         | CAUTION          |
   +--------------------------+------------------+
   | 3GPP-Asset-Information   | CAUTION          |
   +--------------------------+------------------+
   | mbms-mode                | CAUTION          |
   +--------------------------+------------------+
   | mbms-repair              | CAUTION          |
   +--------------------------+------------------+
   | ike-setup                | IDENTICAL        |
   +--------------------------+------------------+
   | psk-fingerprint          | IDENTICAL        |
   +--------------------------+------------------+
   | multicast-rtcp           | IDENTICAL        |
   +--------------------------+------------------+
   | rmcap                    | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | omcap                    | NORMAL           |
   +--------------------------+------------------+
   | mfcap                    | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | mscap                    | INHERIT          |
   +--------------------------+------------------+
   | 3gpp.iut.replication     | TBD              |
   +--------------------------+------------------+
   | bcap                     | INHERIT          |
   +--------------------------+------------------+
   | ccap                     | IDENTICAL        |
   +--------------------------+------------------+
   | icap                     | NORMAL           |
   +--------------------------+------------------+
   | etag                     | CAUTION          |
   +--------------------------+------------------+
   | duplication-delay        | NORMAL           |
   +--------------------------+------------------+
   | range                    | CAUTION          |
   +--------------------------+------------------+
   | control                  | CAUTION          |
   +--------------------------+------------------+
   | mtag                     | CAUTION          |
   +--------------------------+------------------+
   | ts-refclk                | NORMAL           |
   +--------------------------+------------------+
   | mediaclk                 | NORMAL           |
   +--------------------------+------------------+
   | calgextmap               | NORMAL           |
   +--------------------------+------------------+
   | ptime                    | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | orient                   | NORMAL           |
   +--------------------------+------------------+
   | orient:portrait          | NORMAL           |
   +--------------------------+------------------+
   | orient:landscape         | NORMAL           |
   +--------------------------+------------------+
   | orient:seascape          | NORMAL           |
   +--------------------------+------------------+
   | framerate                | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | quality                  | NORMAL           |
   +--------------------------+------------------+
   | rtpmap                   | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | fmtp                     | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | rtpred1                  | CAUTION          |
   +--------------------------+------------------+
   | rtpred2                  | CAUTION          |
   +--------------------------+------------------+
   | T38FaxVersion            | TBD              |
   +--------------------------+------------------+
   | T38MaxBitRate            | TBD              |
   +--------------------------+------------------+
   | T38FaxFillBitRemoval     | TBD              |
   +--------------------------+------------------+
   | T38FaxTranscodingMMR     | TBD              |
   +--------------------------+------------------+
   | T38FaxTranscodingJBIG    | TBD              |
   +--------------------------+------------------+
   | T38FaxRateManagement     | TBD              |
   +--------------------------+------------------+
   | T38FaxMaxBuffer          | TBD              |
   +--------------------------+------------------+
   | T38FaxMaxDatagram        | TBD              |
   +--------------------------+------------------+
   | T38FaxUdpEC              | TBD              |
   +--------------------------+------------------+
   | maxptime                 | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | des                      | CAUTION          |
   +--------------------------+------------------+
   | curr                     | CAUTION          |
   +--------------------------+------------------+
   | conf                     | CAUTION          |
   +--------------------------+------------------+
   | mid                      | NORMAL           |
   +--------------------------+------------------+
   | rtcp                     | TRANSPORT        |
   +--------------------------+------------------+
   | rtcp-fb                  | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | label                    | NORMAL           |
   +--------------------------+------------------+
   | T38VendorInfo            | TBD              |
   +--------------------------+------------------+
   | crypto                   | TRANSPORT        |
   +--------------------------+------------------+
   | eecid                    | CAUTION          |
   +--------------------------+------------------+
   | aalType                  | CAUTION          |
   +--------------------------+------------------+
   | capability               | CAUTION          |
   +--------------------------+------------------+
   | qosClass                 | CAUTION          |
   +--------------------------+------------------+
   | bcob                     | CAUTION          |
   +--------------------------+------------------+
   | stc                      | CAUTION          |
   +--------------------------+------------------+
   | upcc                     | CAUTION          |
   +--------------------------+------------------+
   | atmQOSparms              | CAUTION          |
   +--------------------------+------------------+
   | atmTrfcDesc              | CAUTION          |
   +--------------------------+------------------+
   | abrParms                 | CAUTION          |
   +--------------------------+------------------+
   | abrSetup                 | CAUTION          |
   +--------------------------+------------------+
   | bearerType               | CAUTION          |
   +--------------------------+------------------+
   | lij                      | CAUTION          |
   +--------------------------+------------------+
   | anycast                  | CAUTION          |
   +--------------------------+------------------+
   | cache                    | CAUTION          |
   +--------------------------+------------------+
   | bearerSigIE              | CAUTION          |
   +--------------------------+------------------+
   | aalApp                   | CAUTION          |
   +--------------------------+------------------+
   | cbrRate                  | CAUTION          |
   +--------------------------+------------------+
   | sbc                      | CAUTION          |
   +--------------------------+------------------+
   | clkrec                   | CAUTION          |
   +--------------------------+------------------+
   | fec                      | CAUTION          |
   +--------------------------+------------------+
   | prtfl                    | CAUTION          |
   +--------------------------+------------------+
   | structure                | CAUTION          |
   +--------------------------+------------------+
   | cpsSDUsize               | CAUTION          |
   +--------------------------+------------------+
   | aal2CPS                  | CAUTION          |
   +--------------------------+------------------+
   | aal2CPSSDUrate           | CAUTION          |
   +--------------------------+------------------+
   | aal2sscs3661unassured    | CAUTION          |
   +--------------------------+------------------+
   | aal2sscs3661assured      | CAUTION          |
   +--------------------------+------------------+
   | aal2sscs3662             | CAUTION          |
   +--------------------------+------------------+
   | aal5sscop                | CAUTION          |
   +--------------------------+------------------+
   | atmmap                   | CAUTION          |
   +--------------------------+------------------+
   | silenceSupp              | CAUTION          |
   +--------------------------+------------------+
   | ecan                     | CAUTION          |
   +--------------------------+------------------+
   | gc                       | CAUTION          |
   +--------------------------+------------------+
   | profileDesc              | CAUTION          |
   +--------------------------+------------------+
   | vsel                     | CAUTION          |
   +--------------------------+------------------+
   | dsel                     | CAUTION          |
   +--------------------------+------------------+
   | fsel                     | CAUTION          |
   +--------------------------+------------------+
   | onewaySel                | CAUTION          |
   +--------------------------+------------------+
   | codecconfig              | CAUTION          |
   +--------------------------+------------------+
   | isup_usi                 | CAUTION          |
   +--------------------------+------------------+
   | uiLayer1_Prot            | CAUTION          |
   +--------------------------+------------------+
   | chain                    | CAUTION          |
   +--------------------------+------------------+
   | floorctrl                | TBD              |
   +--------------------------+------------------+
   | confid                   | TBD              |
   +--------------------------+------------------+
   | userid                   | TBD              |
   +--------------------------+------------------+
   | floorid                  | TBD              |
   +--------------------------+------------------+
   | FEC                      | NORMAL           |
   +--------------------------+------------------+
   | accept-types             | TBD              |
   +--------------------------+------------------+
   | accept-wrapped-types     | TBD              |
   +--------------------------+------------------+
   | max-size                 | TBD              |
   +--------------------------+------------------+
   | path                     | TBD              |
   +--------------------------+------------------+
   | dccp-service-code        | CAUTION          |
   +--------------------------+------------------+
   | rtcp-mux                 | IDENTICAL        |
   +--------------------------+------------------+
   | candidate                | TRANSPORT        |
   +--------------------------+------------------+
   | ice-mismatch             | NORMAL           |
   +--------------------------+------------------+
   | remote-candidates        | TRANSPORT        |
   +--------------------------+------------------+
   | SRTPAuthentication       | TBD              |
   +--------------------------+------------------+
   | SRTPROCTxRate            | TBD              |
   +--------------------------+------------------+
   | rtcp-rsize               | IDENTICAL        |
   +--------------------------+------------------+
   | file-selector            | TBD              |
   +--------------------------+------------------+
   | file-transfer-id         | TBD              |
   +--------------------------+------------------+
   | file-disposition         | TBD              |
   +--------------------------+------------------+
   | file-date                | TBD              |
   +--------------------------+------------------+
   | file-icon                | TBD              |
   +--------------------------+------------------+
   | file-range               | TBD              |
   +--------------------------+------------------+
   | depend                   | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | ssrc                     | NORMAL           |
   +--------------------------+------------------+
   | ssrc-group               | NORMAL           |
   +--------------------------+------------------+
   | rtcp-unicast             | IDENTICAL        |
   +--------------------------+------------------+
   | pcfg                     | SPECIAL          |
   +--------------------------+------------------+
   | acfg                     | SPECIAL          |
   +--------------------------+------------------+
   | zrtp-hash                | TRANSPORT        |
   +--------------------------+------------------+
   | X-predecbufsize          | CAUTION          |
   +--------------------------+------------------+
   | X-initpredecbufperiod    | CAUTION          |
   +--------------------------+------------------+
   | X-initpostdecbufperiod   | CAUTION          |
   +--------------------------+------------------+
   | X-decbyterate            | CAUTION          |
   +--------------------------+------------------+
   | 3gpp-videopostdecbufsize | CAUTION          |
   +--------------------------+------------------+
   | framesize                | CAUTION          |
   +--------------------------+------------------+
   | 3GPP-SRTP-Config         | CAUTION          |
   +--------------------------+------------------+
   | alt                      | CAUTION          |
   +--------------------------+------------------+
   | alt-default-id           | CAUTION          |
   +--------------------------+------------------+
   | 3GPP-Adaption-Support    | CAUTION          |
   +--------------------------+------------------+
   | mbms-flowid              | CAUTION          |
   +--------------------------+------------------+
   | fec-source-flow          | SPECIAL          |
   +--------------------------+------------------+
   | fec-repair-flow          | SPECIAL          |
   +--------------------------+------------------+
   | repair-window            | SPECIAL          |
   +--------------------------+------------------+
   | rams-updates             | CAUTION          |
   +--------------------------+------------------+
   | imageattr                | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | cfw-id                   | NORMAL           |
   +--------------------------+------------------+
   | portmapping-req          | CAUTION          |
   +--------------------------+------------------+
   | ecn-capable-rtp          | IDENTICAL        |
   +--------------------------+------------------+
   | visited-realm            | TRANSPORT        |
   +--------------------------+------------------+
   | secondary-realm          | TRANSPORT        |
   +--------------------------+------------------+
   | omr-s-cksum              | NORMAL           |
   +--------------------------+------------------+
   | omr-m-cksum              | NORMAL           |
   +--------------------------+------------------+
   | omr-codecs               | NORMAL           |
   +--------------------------+------------------+
   | omr-m-att                | NORMAL           |
   +--------------------------+------------------+
   | omr-s-att                | NORMAL           |
   +--------------------------+------------------+
   | omr-m-bw                 | NORMAL           |
   +--------------------------+------------------+
   | omr-s-bw                 | NORMAL           |
   +--------------------------+------------------+
   | msrp-cema                | TBD              |
   +--------------------------+------------------+
   | dccp-port                | CAUTION          |
   +--------------------------+------------------+
   | resource                 | NORMAL           |
   +--------------------------+------------------+
   | channel                  | NORMAL           |
   +--------------------------+------------------+
   | cmid                     | NORMAL           |
   +--------------------------+------------------+
   | content                  | NORMAL           |
   +--------------------------+------------------+
   | lcfg                     | SPECIAL          |
   +--------------------------+------------------+
   | loopback                 | NORMAL           |
   +--------------------------+------------------+
   | loopback-source          | NORMAL           |
   +--------------------------+------------------+
   | loopback-mirror          | NORMAL           |
   +--------------------------+------------------+
   | chatroom                 | TBD              |
   +--------------------------+------------------+
   | altc                     | TRANSPORT        |
   +--------------------------+------------------+
   | T38FaxMaxIFP             | TBD              |
   +--------------------------+------------------+
   | T38FaxUdpECDepth         | TBD              |
   +--------------------------+------------------+
   | T38FaxUdpFECMaxSpan      | TBD              |
   +--------------------------+------------------+
   | T38ModemType             | TBD              |
   +--------------------------+------------------+
   | cs-correlation           | TBD              |
   +--------------------------+------------------+
   | rtcp-idms                | NORMAL           |
   +--------------------------+------------------+
   | cname                    | NORMAL           |
   +--------------------------+------------------+
   | previous-ssrc            | NORMAL           |
   +--------------------------+------------------+
   | fmtp                     | IDENTICAL-PER-PT |
   +--------------------------+------------------+
   | ts-refclk                | NORMAL           |
   +--------------------------+------------------+
   | mediaclk                 | NORMAL           |
   +--------------------------+------------------+

                       Table 82

15.2.3. Table: content SDP Parameters

15.2.4. Table: Semantics for the "group" SDP Attribute

   The following values have been added to the "Semantics for the
   'group' SDP Attribute" subregistry of the "Session Description
   Protocol (SDP) Parameters" registry.  The references have been
   updated to point to this RFC as well as the previous references.

   +========+==============+
   | Token  | Mux Category |
   +========+==============+
   | LS     | NORMAL       |
   +--------+--------------+
   | FID    | NORMAL       |
   +--------+--------------+
   | SRF    | NORMAL       |
   +--------+--------------+
   | ANAT   | CAUTION      |
   +--------+--------------+
   | FEC    | NORMAL       |
   +--------+--------------+
   | FEC-FR | NORMAL       |
   +--------+--------------+
   | CS     | NORMAL       |
   +--------+--------------+
   | DDP    | NORMAL       |
   +--------+--------------+
   | DUP    | NORMAL       |
   +--------+--------------+

            Table 84

15.2.5. Table: "rtcp-fb" Attribute Values

   The following values have been added to the "'rtcp-fb' Attribute
   Values" subregistry of the "Session Description Protocol (SDP)
   Parameters" registry.  The references have been updated to point to
   this RFC as well as the previous references.

   +============+==================+
   | Value Name | Mux Category     |
   +============+==================+
   | ack        | IDENTICAL-PER-PT |
   +------------+------------------+
   | app        | SPECIAL          |
   +------------+------------------+
   | ccm        | IDENTICAL-PER-PT |
   +------------+------------------+
   | nack       | IDENTICAL-PER-PT |
   +------------+------------------+
   | trr-int    | IDENTICAL-PER-PT |
   +------------+------------------+

                Table 85

15.2.6. Table: "ack" and "nack" Attribute Values

   The following values have been added to the "'ack' and 'nack'
   Attribute Values" subregistry of the "Session Description Protocol
   (SDP) Parameters" registry.  The references have been updated to
   point to this RFC as well as the previous references.

   +============+==================+
   | Value Name | Mux Category     |
   +============+==================+
   | sli        | IDENTICAL-PER-PT |
   +------------+------------------+
   | pli        | IDENTICAL-PER-PT |
   +------------+------------------+
   | rpsi       | IDENTICAL-PER-PT |
   +------------+------------------+
   | app        | SPECIAL          |
   +------------+------------------+
   | rai        | IDENTICAL-PER-PT |
   +------------+------------------+
   | tllei      | IDENTICAL-PER-PT |
   +------------+------------------+
   | pslei      | IDENTICAL-PER-PT |
   +------------+------------------+
   | ecn        | IDENTICAL        |
   +------------+------------------+

                Table 86

15.2.7. Table: "depend" SDP Attribute Values

   The following values have been added to the "'depend' SDP Attribute
   Values" subregistry of the "Session Description Protocol (SDP)
   Parameters" registry.  The references have been updated to point to
   this RFC as well as the previous references.

   +=======+==================+
   | Token | Mux Category     |
   +=======+==================+
   | lay   | IDENTICAL-PER-PT |
   +-------+------------------+
   | mdc   | IDENTICAL-PER-PT |
   +-------+------------------+

             Table 87

15.2.8. Table: "cs-correlation" Attribute Values

   The following values have been added to the "'cs-correlation'
   Attribute Values" subregistry of the "Session Description Protocol
   (SDP) Parameters" registry.  The references have been updated to
   point to this RFC as well as the previous references.

   +==========+==============+
   | Value    | Mux Category |
   +==========+==============+
   | callerid | TBD          |
   +----------+--------------+
   | uuie     | TBD          |
   +----------+--------------+
   | dtmf     | TBD          |
   +----------+--------------+
   | external | TBD          |
   +----------+--------------+

             Table 88

15.2.9. Table: Semantics for the "ssrc-group" SDP Attribute

   The following values have been added to the "Semantics for the 'ssrc-
   group' SDP Attribute" subregistry of the "Session Description
   Protocol (SDP) Parameters" registry.  The references have been
   updated to point to this RFC as well as the previous references.

   +========+==============+
   | Token  | Mux Category |
   +========+==============+
   | FID    | NORMAL       |
   +--------+--------------+
   | FEC    | NORMAL       |
   +--------+--------------+
   | FEC-FR | NORMAL       |
   +--------+--------------+
   | DUP    | NORMAL       |
   +--------+--------------+

            Table 89

15.2.10. Table: SDP/RTSP Key Management Protocol Identifiers

   The following values have been added to the "SDP/RTSP key management
   protocol identifiers" subregistry of the "Session Description
   Protocol (SDP) Parameters" registry.  The references have been
   updated to point to this RFC as well as the previous references.

   +============+==============+
   | Value Name | Mux Category |
   +============+==============+
   | mikey      | IDENTICAL    |
   +------------+--------------+

              Table 90

15.2.11. Table: Codec Control Messages

15.2.12. Table: QoS Mechanism Tokens

   The following values have been added to the "QoS Mechanism Tokens"
   subregistry of the "Session Description Protocol (SDP) Parameters"
   registry.  The references have been updated to point to this RFC as
   well as the previous references.

   +===============+==============+
   | QoS Mechanism | Mux Category |
   +===============+==============+
   | rsvp          | TRANSPORT    |
   +---------------+--------------+
   | nsis          | TRANSPORT    |
   +---------------+--------------+

               Table 92

15.2.13. Table: SDP Capability Negotiation Option Tags

15.2.14. Table: Timestamp Reference Clock Source Parameters

   The following values have been added to the "Timestamp Reference
   Clock Source Parameters" subregistry of the "Session Description
   Protocol (SDP) Parameters" registry.  The references have been
   updated to point to this RFC as well as the previous references.

   +=========+==============+
   | Name    | Mux Category |
   +=========+==============+
   | ntp     | NORMAL       |
   +---------+--------------+
   | ptp     | NORMAL       |
   +---------+--------------+
   | gps     | NORMAL       |
   +---------+--------------+
   | gal     | NORMAL       |
   +---------+--------------+
   | glonass | NORMAL       |
   +---------+--------------+
   | local   | NORMAL       |
   +---------+--------------+
   | private | NORMAL       |
   +---------+--------------+

            Table 94

15.2.15. Table: Media Clock Source Parameters

16. Security Considerations

   The primary security considerations for RTP, including the way it is
   used here, are described in [RFC3550] and [RFC3711].

   When multiplexing SDP attributes with the category "CAUTION", the
   implementations should be aware of possible issues described in this
   specification.

17. References

17.1. 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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [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,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8843]  Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", RFC 8843,
              DOI 10.17487/RFC8843, January 2021,
              <https://www.rfc-editor.org/info/rfc8843>.

17.2. Informative References

   [FLUTE]    Walsh, R., Peltotalo, J., Peltotalo, S., Curcio, I. D.,
              and H. Mehta, "SDP Descriptors for FLUTE", Work in
              Progress, Internet-Draft, draft-ietf-rmt-flute-sdp-03, 12
              September 2012,
              <https://tools.ietf.org/html/draft-ietf-rmt-flute-sdp-03>.

   [H.248.15] ITU-T, "Gateway control protocol: SDP ITU-T H.248 package
              attribute", ITU-T Recommendation H.248.15, March 2013,
              <https://www.itu.int/rec/T-REC-H.248.15>.

   [IANA]     IANA, "Session Description Protocol (SDP) Parameters",
              <https://www.iana.org/assignments/sdp-parameters>.

   [IMS-CAT]  3GPP, "IP Multimedia Subsystem (IMS) Customized Alerting
              Tones (CAT); Protocol specification",
              Specification 24.182, Specification 24.182, January 2015,
              <https://www.3gpp.org/ftp/Specs/html-info/24182.htm>.

   [IMS-CRS]  3GPP, "IP Multimedia Subsystem (IMS) Customized Ringing
              Signal (CRS); Protocol specification",
              Specification 24.183, September 2016,
              <https://www.3gpp.org/ftp/Specs/html-info/24183.htm>.

   [IP-CALL]  3GPP, "IP multimedia call control protocol based on
              Session Initiation Protocol (SIP) and Session Description
              Protocol (SDP); Stage 3", Specification 24.229, September
              2016,
              <https://www.3gpp.org/ftp/Specs/html-info/24229.htm>.

   [Q.1970]   ITU-T, "Q.1970: BICC IP bearer control protocol", ITU-T
              Recommendation Q.1970, September 2006,
              <https://www.itu.int/rec/T-REC-Q.1970-200609-I/en>.

   [RFC2326]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
              Streaming Protocol (RTSP)", RFC 2326,
              DOI 10.17487/RFC2326, April 1998,
              <https://www.rfc-editor.org/info/rfc2326>.

   [RFC3108]  Kumar, R. and M. Mostafa, "Conventions for the use of the
              Session Description Protocol (SDP) for ATM Bearer
              Connections", RFC 3108, DOI 10.17487/RFC3108, May 2001,
              <https://www.rfc-editor.org/info/rfc3108>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC3312]  Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
              "Integration of Resource Management and Session Initiation
              Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October
              2002, <https://www.rfc-editor.org/info/rfc3312>.

   [RFC3407]  Andreasen, F., "Session Description Protocol (SDP) Simple
              Capability Declaration", RFC 3407, DOI 10.17487/RFC3407,
              October 2002, <https://www.rfc-editor.org/info/rfc3407>.

   [RFC3524]  Camarillo, G. and A. Monrad, "Mapping of Media Streams to
              Resource Reservation Flows", RFC 3524,
              DOI 10.17487/RFC3524, April 2003,
              <https://www.rfc-editor.org/info/rfc3524>.

   [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, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3556]  Casner, S., "Session Description Protocol (SDP) Bandwidth
              Modifiers for RTP Control Protocol (RTCP) Bandwidth",
              RFC 3556, DOI 10.17487/RFC3556, July 2003,
              <https://www.rfc-editor.org/info/rfc3556>.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              DOI 10.17487/RFC3605, October 2003,
              <https://www.rfc-editor.org/info/rfc3605>.

   [RFC3611]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
              "RTP Control Protocol Extended Reports (RTCP XR)",
              RFC 3611, DOI 10.17487/RFC3611, November 2003,
              <https://www.rfc-editor.org/info/rfc3611>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC3890]  Westerlund, M., "A Transport Independent Bandwidth
              Modifier for the Session Description Protocol (SDP)",
              RFC 3890, DOI 10.17487/RFC3890, September 2004,
              <https://www.rfc-editor.org/info/rfc3890>.

   [RFC4091]  Camarillo, G. and J. Rosenberg, "The Alternative Network
              Address Types (ANAT) Semantics for the Session Description
              Protocol (SDP) Grouping Framework", RFC 4091,
              DOI 10.17487/RFC4091, June 2005,
              <https://www.rfc-editor.org/info/rfc4091>.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              DOI 10.17487/RFC4145, September 2005,
              <https://www.rfc-editor.org/info/rfc4145>.

   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, DOI 10.17487/RFC4567, July
              2006, <https://www.rfc-editor.org/info/rfc4567>.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
              <https://www.rfc-editor.org/info/rfc4568>.

   [RFC4570]  Quinn, B. and R. Finlayson, "Session Description Protocol
              (SDP) Source Filters", RFC 4570, DOI 10.17487/RFC4570,
              July 2006, <https://www.rfc-editor.org/info/rfc4570>.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572,
              DOI 10.17487/RFC4572, July 2006,
              <https://www.rfc-editor.org/info/rfc4572>.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574,
              DOI 10.17487/RFC4574, August 2006,
              <https://www.rfc-editor.org/info/rfc4574>.

   [RFC4583]  Camarillo, G., "Session Description Protocol (SDP) Format
              for Binary Floor Control Protocol (BFCP) Streams",
              RFC 4583, DOI 10.17487/RFC4583, November 2006,
              <https://www.rfc-editor.org/info/rfc4583>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC4796]  Hautakorpi, J. and G. Camarillo, "The Session Description
              Protocol (SDP) Content Attribute", RFC 4796,
              DOI 10.17487/RFC4796, February 2007,
              <https://www.rfc-editor.org/info/rfc4796>.

   [RFC4975]  Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
              "The Message Session Relay Protocol (MSRP)", RFC 4975,
              DOI 10.17487/RFC4975, September 2007,
              <https://www.rfc-editor.org/info/rfc4975>.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
              February 2008, <https://www.rfc-editor.org/info/rfc5104>.

   [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, DOI 10.17487/RFC5109, December
              2007, <https://www.rfc-editor.org/info/rfc5109>.

   [RFC5159]  Dondeti, L., Ed. and A. Jerichow, "Session Description
              Protocol (SDP) Attributes for Open Mobile Alliance (OMA)
              Broadcast (BCAST) Service and Content Protection",
              RFC 5159, DOI 10.17487/RFC5159, March 2008,
              <https://www.rfc-editor.org/info/rfc5159>.

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
              2008, <https://www.rfc-editor.org/info/rfc5285>.

   [RFC5432]  Polk, J., Dhesikan, S., and G. Camarillo, "Quality of
              Service (QoS) Mechanism Selection in the Session
              Description Protocol (SDP)", RFC 5432,
              DOI 10.17487/RFC5432, March 2009,
              <https://www.rfc-editor.org/info/rfc5432>.

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
              2009, <https://www.rfc-editor.org/info/rfc5506>.

   [RFC5547]  Garcia-Martin, M., Isomaki, M., Camarillo, G., Loreto, S.,
              and P. Kyzivat, "A Session Description Protocol (SDP)
              Offer/Answer Mechanism to Enable File Transfer", RFC 5547,
              DOI 10.17487/RFC5547, May 2009,
              <https://www.rfc-editor.org/info/rfc5547>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <https://www.rfc-editor.org/info/rfc5576>.

   [RFC5583]  Schierl, T. and S. Wenger, "Signaling Media Decoding
              Dependency in the Session Description Protocol (SDP)",
              RFC 5583, DOI 10.17487/RFC5583, July 2009,
              <https://www.rfc-editor.org/info/rfc5583>.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760,
              DOI 10.17487/RFC5760, February 2010,
              <https://www.rfc-editor.org/info/rfc5760>.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <https://www.rfc-editor.org/info/rfc5761>.

   [RFC5762]  Perkins, C., "RTP and the Datagram Congestion Control
              Protocol (DCCP)", RFC 5762, DOI 10.17487/RFC5762, April
              2010, <https://www.rfc-editor.org/info/rfc5762>.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <https://www.rfc-editor.org/info/rfc5763>.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <https://www.rfc-editor.org/info/rfc5888>.

   [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
              Capability Negotiation", RFC 5939, DOI 10.17487/RFC5939,
              September 2010, <https://www.rfc-editor.org/info/rfc5939>.

   [RFC5956]  Begen, A., "Forward Error Correction Grouping Semantics in
              the Session Description Protocol", RFC 5956,
              DOI 10.17487/RFC5956, September 2010,
              <https://www.rfc-editor.org/info/rfc5956>.

   [RFC6064]  Westerlund, M. and P. Frojdh, "SDP and RTSP Extensions
              Defined for 3GPP Packet-Switched Streaming Service and
              Multimedia Broadcast/Multicast Service", RFC 6064,
              DOI 10.17487/RFC6064, January 2011,
              <https://www.rfc-editor.org/info/rfc6064>.

   [RFC6128]  Begen, A., "RTP Control Protocol (RTCP) Port for Source-
              Specific Multicast (SSM) Sessions", RFC 6128,
              DOI 10.17487/RFC6128, February 2011,
              <https://www.rfc-editor.org/info/rfc6128>.

   [RFC6189]  Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
              Media Path Key Agreement for Unicast Secure RTP",
              RFC 6189, DOI 10.17487/RFC6189, April 2011,
              <https://www.rfc-editor.org/info/rfc6189>.

   [RFC6193]  Saito, M., Wing, D., and M. Toyama, "Media Description for
              the Internet Key Exchange Protocol (IKE) in the Session
              Description Protocol (SDP)", RFC 6193,
              DOI 10.17487/RFC6193, April 2011,
              <https://www.rfc-editor.org/info/rfc6193>.

   [RFC6230]  Boulton, C., Melanchuk, T., and S. McGlashan, "Media
              Control Channel Framework", RFC 6230,
              DOI 10.17487/RFC6230, May 2011,
              <https://www.rfc-editor.org/info/rfc6230>.

   [RFC6236]  Johansson, I. and K. Jung, "Negotiation of Generic Image
              Attributes in the Session Description Protocol (SDP)",
              RFC 6236, DOI 10.17487/RFC6236, May 2011,
              <https://www.rfc-editor.org/info/rfc6236>.

   [RFC6284]  Begen, A., Wing, D., and T. Van Caenegem, "Port Mapping
              between Unicast and Multicast RTP Sessions", RFC 6284,
              DOI 10.17487/RFC6284, June 2011,
              <https://www.rfc-editor.org/info/rfc6284>.

   [RFC6285]  Ver Steeg, B., Begen, A., Van Caenegem, T., and Z. Vax,
              "Unicast-Based Rapid Acquisition of Multicast RTP
              Sessions", RFC 6285, DOI 10.17487/RFC6285, June 2011,
              <https://www.rfc-editor.org/info/rfc6285>.

   [RFC6364]  Begen, A., "Session Description Protocol Elements for the
              Forward Error Correction (FEC) Framework", RFC 6364,
              DOI 10.17487/RFC6364, October 2011,
              <https://www.rfc-editor.org/info/rfc6364>.

   [RFC6642]  Wu, Q., Ed., Xia, F., and R. Even, "RTP Control Protocol
              (RTCP) Extension for a Third-Party Loss Report", RFC 6642,
              DOI 10.17487/RFC6642, June 2012,
              <https://www.rfc-editor.org/info/rfc6642>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <https://www.rfc-editor.org/info/rfc6679>.

   [RFC6714]  Holmberg, C., Blau, S., and E. Burger, "Connection
              Establishment for Media Anchoring (CEMA) for the Message
              Session Relay Protocol (MSRP)", RFC 6714,
              DOI 10.17487/RFC6714, August 2012,
              <https://www.rfc-editor.org/info/rfc6714>.

   [RFC6773]  Phelan, T., Fairhurst, G., and C. Perkins, "DCCP-UDP: A
              Datagram Congestion Control Protocol UDP Encapsulation for
              NAT Traversal", RFC 6773, DOI 10.17487/RFC6773, November
              2012, <https://www.rfc-editor.org/info/rfc6773>.

   [RFC6787]  Burnett, D. and S. Shanmugham, "Media Resource Control
              Protocol Version 2 (MRCPv2)", RFC 6787,
              DOI 10.17487/RFC6787, November 2012,
              <https://www.rfc-editor.org/info/rfc6787>.

   [RFC6849]  Kaplan, H., Ed., Hedayat, K., Venna, N., Jones, P., and N.
              Stratton, "An Extension to the Session Description
              Protocol (SDP) and Real-time Transport Protocol (RTP) for
              Media Loopback", RFC 6849, DOI 10.17487/RFC6849, February
              2013, <https://www.rfc-editor.org/info/rfc6849>.

   [RFC6871]  Gilman, R., Even, R., and F. Andreasen, "Session
              Description Protocol (SDP) Media Capabilities
              Negotiation", RFC 6871, DOI 10.17487/RFC6871, February
              2013, <https://www.rfc-editor.org/info/rfc6871>.

   [RFC6947]  Boucadair, M., Kaplan, H., Gilman, R., and S.
              Veikkolainen, "The Session Description Protocol (SDP)
              Alternate Connectivity (ALTC) Attribute", RFC 6947,
              DOI 10.17487/RFC6947, May 2013,
              <https://www.rfc-editor.org/info/rfc6947>.

   [RFC7006]  Garcia-Martin, M., Veikkolainen, S., and R. Gilman,
              "Miscellaneous Capabilities Negotiation in the Session
              Description Protocol (SDP)", RFC 7006,
              DOI 10.17487/RFC7006, September 2013,
              <https://www.rfc-editor.org/info/rfc7006>.

   [RFC7104]  Begen, A., Cai, Y., and H. Ou, "Duplication Grouping
              Semantics in the Session Description Protocol", RFC 7104,
              DOI 10.17487/RFC7104, January 2014,
              <https://www.rfc-editor.org/info/rfc7104>.

   [RFC7195]  Garcia-Martin, M. and S. Veikkolainen, "Session
              Description Protocol (SDP) Extension for Setting Audio and
              Video Media Streams over Circuit-Switched Bearers in the
              Public Switched Telephone Network (PSTN)", RFC 7195,
              DOI 10.17487/RFC7195, May 2014,
              <https://www.rfc-editor.org/info/rfc7195>.

   [RFC7197]  Begen, A., Cai, Y., and H. Ou, "Duplication Delay
              Attribute in the Session Description Protocol", RFC 7197,
              DOI 10.17487/RFC7197, April 2014,
              <https://www.rfc-editor.org/info/rfc7197>.

   [RFC7266]  Clark, A., Wu, Q., Schott, R., and G. Zorn, "RTP Control
              Protocol (RTCP) Extended Report (XR) Blocks for Mean
              Opinion Score (MOS) Metric Reporting", RFC 7266,
              DOI 10.17487/RFC7266, June 2014,
              <https://www.rfc-editor.org/info/rfc7266>.

   [RFC7272]  van Brandenburg, R., Stokking, H., van Deventer, O.,
              Boronat, F., Montagud, M., and K. Gross, "Inter-
              Destination Media Synchronization (IDMS) Using the RTP
              Control Protocol (RTCP)", RFC 7272, DOI 10.17487/RFC7272,
              June 2014, <https://www.rfc-editor.org/info/rfc7272>.

   [RFC7273]  Williams, A., Gross, K., van Brandenburg, R., and H.
              Stokking, "RTP Clock Source Signalling", RFC 7273,
              DOI 10.17487/RFC7273, June 2014,
              <https://www.rfc-editor.org/info/rfc7273>.

   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,
              <https://www.rfc-editor.org/info/rfc7657>.

   [RFC7826]  Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, Ed., "Real-Time Streaming Protocol
              Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December
              2016, <https://www.rfc-editor.org/info/rfc7826>.

   [RFC8122]  Lennox, J. and C. Holmberg, "Connection-Oriented Media
              Transport over the Transport Layer Security (TLS) Protocol
              in the Session Description Protocol (SDP)", RFC 8122,
              DOI 10.17487/RFC8122, March 2017,
              <https://www.rfc-editor.org/info/rfc8122>.

   [RFC8285]  Singer, D., Desineni, H., and R. Even, Ed., "A General
              Mechanism for RTP Header Extensions", RFC 8285,
              DOI 10.17487/RFC8285, October 2017,
              <https://www.rfc-editor.org/info/rfc8285>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8856]  Camarillo, G., Kristensen, T., and C. Holmberg, "Session
              Description Protocol (SDP) Format for Binary Floor Control
              Protocol (BFCP) Streams", RFC 8856, DOI 10.17487/RFC8856,
              January 2021, <https://www.rfc-editor.org/info/rfc8856>.

   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,
              <https://www.rfc-editor.org/info/rfc8866>.

   [T.38]     ITU-T, "Procedures for real-time Group 3 facsimile
              communication over IP networks", ITU-T
              Recommendation T.38, November 2015,
              <https://www.itu.int/rec/T-REC-T.38/e>.

   [TISPAN]   ETSI, "Telecommunications and Internet converged Services
              and Protocols for Advanced Networking (TISPAN); IMS-based
              IPTV stage 3 specification", Technical Specification 183
              063 V2.1.0, June 2008, <https://www.etsi.org/deliver/
              etsi_ts/183000_183099/183063/02.01.00_60/
              ts_183063v020100p.pdf>.

Acknowledgements

   I would like to thank Cullen Jennings and Flemming Andreasen for
   suggesting the categories, contributing text, and reviewing the draft
   of this document.  I would also like to thank Magnus Westerlund,
   Christer Holmberg, Jonathan Lennox, Bo Burman, Ari Keränen, and Dan
   Wing for suggesting structural changes that improved the document's
   readability.

   I would like also to thank the following experts for their inputs and
   reviews as listed:

      Flemming Andreasen (5.24, 5.32, 5.33, 14),
      Rohan Mahy (5.54),
      Eric Burger (5.26),
      Christian Huitema (5.14),
      Christer Holmberg (5.21, 5.26, 12.2, 12.3),
      Richard Ejzak (5.44, 5.50, 5.51),
      Colin Perkins (5.7, 5.8, 5.9, 5.55),
      Magnus Westerlund (5.2, 5.3, 5.9, 5.27, 5.47, 6.1 - 6.3, 8.3, 7),
      Roni Even (5.12, 5.27, 8.4),
      Subha Dhesikan (5.6, 10),
      Dan Wing (5.7, 5.12, 5.35, 5.39, 5.45),
      Cullen Jennings (5.40),
      Ali C Begen (5.1, 5.20, 5.22, 5.25, 5.38, 7.3, 8.2, 8.4, 8.6, 9.2,
      13.1),
      Bo Burman (7.2, 7.6),
      Charles Eckel (5.15, 5.27, 5.28, 9.1, 8.5),
      Paul Kyzivat (5.24),
      Ian Johansson (5.15),
      Saravanan Shanmugham (5.11),
      Paul E Jones (5.30),
      Rajesh Kumar (5.48),
      Jonathan Lennox (5.36, 5, 15, 9.1, 11.1),
      Mo Zanaty (5.4, 5.5, 5.23, 8.1, 8.3, 8.5, 12.1),
      Christian Huitema (5.14),
      Qin Wu (5.47, PM-Dir review),
      Hans Stokking (5.43, 5.16),
      Christian Groves (5.48, 5.52), and
      Thomas Stach.

   I would like to thank Chris Lonvick for the SECDIR review, Dan
   Romascanu for the Gen-ART review, and Sabrina Tanamal for the IANA
   review.

   Thanks to Ben Campbell for Area Director review suggestions.  Thanks
   to Spencer Dawkins, Stephen Farrel, Alissa Cooper, Mirja Kühlewind,
   and the entire IESG for their reviews.

Author's Address

   Suhas Nandakumar
   Cisco
   170 West Tasman Drive
   San Jose, CA 95134
   United States of America

Abstract

Status of This Memo

Copyright Notice

Table of Contents

Author's Address

1. Introduction

2. Terminology

3. Motivation

4. SDP Attribute Analysis Framework

4.1. Category: NORMAL

4.2. Category: CAUTION

4.3. Category: IDENTICAL

4.4. Category: SUM

4.5. Category: TRANSPORT

4.6. Category: INHERIT

4.7. Category: IDENTICAL-PER-PT

4.8. Category: SPECIAL

4.9. Category: TBD

5. Analysis of Existing Attributes

5.1. RFC 4566: SDP

5.2. RFC 4585: RTP/AVPF

5.3. RFC 5761: Multiplexing RTP and RTCP

5.4. RFC 3312: Integration of Resource Management and SIP

5.5. RFC 4574: SDP "label" Attribute

5.6. RFC 5432: QoS Mechanism Selection in SDP

5.7. RFC 4568: SDP Security Descriptions

5.8. RFC 5762: RTP over DCCP

5.9. RFC 6773: DCCP-UDP Encapsulation

5.10. RFC 5506: Reduced-Size RTCP in RTP Profile

5.11. RFC 6787: Media Resource Control Protocol Version 2

5.12. RFC 8445: ICE

5.13. RFC 5285: RTP Header Extensions

5.14. RFC 3605: RTCP Attribute in SDP

5.15. RFC 5576: Source-Specific SDP Attributes

5.16. RFC 7273: RTP Clock Source Signaling

5.17. RFC 6236: Image Attributes in SDP

5.18. RFC 7197: Duplication Delay Attribute in SDP

5.19. RFC 7266: RTCP XR Blocks for MOS Metric Reporting

5.20. RFC 6285: Rapid Acquisition of Multicast RTP Sessions

5.21. RFC 6230: Media Control Channel Framework

5.22. RFC 6364: SDP Elements for FEC Framework

5.23. RFC 4796: "content" Attribute

5.24. RFC 3407: SDP Simple Capability Declaration

5.25. RFC 6284: Port Mapping between Unicast and Multicast RTP Sessions

5.26. RFC 6714: MSRP-CEMA

5.27. RFC 4583: SDP Format for BFCP Streams

5.28. RFC 5547: SDP Offer/Answer for File Transfer

5.29. RFC 6849: SDP and RTP Media Loopback Extension

5.30. RFC 5760: RTCP with Unicast Feedback

5.31. RFC 3611: RTCP XR

5.32. RFC 5939: SDP Capability Negotiation

5.33. RFC 6871: SDP Media Capabilities Negotiation

5.34. RFC 7006: Miscellaneous Capabilities Negotiation in SDP

5.35. RFC 4567: Key Management Extensions for SDP and RTSP

5.36. RFC 4572: Comedia over TLS in SDP

5.37. RFC 4570: SDP Source Filters

5.38. RFC 6128: RTCP Port for Multicast Sessions

5.39. RFC 6189: ZRTP

5.40. RFC 4145: Connection-Oriented Media

5.41. RFC 6947: The SDP "altc" Attribute

5.42. RFC 7195: SDP Extension for Circuit-Switched Bearers in PSTN

5.43. RFC 7272: IDMS Using the RTP Control Protocol (RTCP)

5.44. RFC 5159: Open Mobile Alliance (OMA) Broadcast (BCAST) SDP Attributes

5.45. RFC 6193: Media Description for IKE in SDP

5.46. RFC 2326: Real Time Streaming Protocol

5.47. RFC 7826: Real-Time Streaming Protocol

5.48. RFC 6064: SDP and RTSP Extensions for 3GPP

5.49. RFC 3108: ATM SDP

5.50. 3GPP TS 183.063

5.51. 3GPP TS 24.229

5.52. ITU T.38

5.53. ITU-T Q.1970

5.54. ITU-T H.248.15

5.55. RFC 4975: The Message Session Relay Protocol

5.56. Historical Attributes

6. bwtype Attribute Analysis

6.1. RFC 4566: SDP

6.2. RFC 3556: SDP Bandwidth Modifiers for RTCP Bandwidth

6.3. RFC 3890: Bandwidth Modifier for SDP

7. rtcp-fb Attribute Analysis

5.44. RFC 5159: Open Mobile Alliance (OMA) Broadcast (BCAST) SDP
Attributes

7.3. RFC 6285: Unicast-Based Rapid Acquisition of Multicast RTP
Sessions (RAMS)