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.¶

5.1. RFC 4566: SDP

[RFC4566] defines SDP that is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation.¶

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.¶

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.¶

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.¶

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.¶

5.6. RFC 5432: QoS Mechanism Selection in SDP

[RFC5432] defines procedures for negotiating QoS mechanisms using the SDP offer/answer model.¶

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.¶

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.¶

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.¶

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

[RFC5506] discusses benefits and issues that arise when allowing RTCP packets to be transmitted with reduced size.¶

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.¶

5.12. RFC 8445: ICE

[RFC8445] describes a protocol for NAT traversal for UDP-based multimedia sessions established with the offer/answer model. ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN). ICE can be used by any protocol utilizing the offer/answer model, such as the SIP.¶

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.¶

5.14. RFC 3605: RTCP Attribute in SDP

Originally, SDP assumed that RTP and RTCP were carried on consecutive ports. However, this is not always true when NATs are involved. [RFC3605] specifies an early mechanism for indicating the RTCP port.¶

5.15. RFC 5576: Source-Specific SDP Attributes

[RFC5576] defines a mechanism for describing RTP media sources -- which are identified by their synchronization source (SSRC) identifiers -- in SDP, to associate attributes with these sources and express relationships among sources. It also defines several source-level attributes that can be used to describe properties of media sources.¶

NOTE: If SSRCs are repeated across "m=" lines being multiplexed, they MUST all represent the same underlying RTP Source.¶

5.16. RFC 7273: RTP Clock Source Signaling

[RFC7273] specifies SDP signaling that identifies timestamp reference clock sources and SDP signaling that identifies the media clock sources in a multimedia session.¶

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.¶

5.18. RFC 7197: Duplication Delay Attribute in SDP

[RFC7197] defines an attribute to indicate the presence of temporally redundant media streams and the duplication delay 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.¶

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.¶

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.¶

5.22. RFC 6364: SDP Elements for FEC Framework

[RFC6364] specifies the use of SDP to describe the parameters required to signal the Forward Error Correction (FEC) Framework Configuration Information between the sender(s) and receiver(s). This document also provides examples that show the semantics for grouping multiple source and repair flows together for the applications that simultaneously use multiple instances of the FEC Framework.¶

5.23. RFC 4796: "content" Attribute

[RFC4796] defines a new SDP media-level attribute, "content". The "content" attribute defines the content of the media stream to a more detailed level than the media description line. The sender of an SDP session description can attach the "content" attribute to one or more media streams. The receiving application can then treat each media stream differently (e.g., show it on a big or small screen) based on its content.¶

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.¶

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.¶

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.¶

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.¶

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].¶

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

[RFC6849] adds new SDP media types and attributes that enable establishment of media sessions where the media is looped back to the transmitter. Such media sessions will serve as monitoring and troubleshooting tools by providing the means for measurement of more advanced Voice over IP (VoIP), real-time text, and Video over IP performance metrics.¶

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.¶

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).¶

5.32. RFC 5939: SDP Capability Negotiation

[RFC5939] defines a general SDP Capability Negotiation framework. It also specifies how to provide attributes and transport protocols as capabilities and negotiate them using the framework. Extensions for other types of capabilities (e.g., media types and media formats) may be provided in other documents.¶

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.¶

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

[RFC7006] extends the SDP Capability Negotiation framework to allow endpoints to negotiate three additional SDP capabilities. In particular, this memo provides a mechanism to negotiate bandwidth ("b=" line), connection data ("c=" line), and session or media titles ("i=" line for each session or media).¶

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.¶

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.¶

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.¶

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.¶

5.39. RFC 6189: ZRTP

[RFC6189] defines ZRTP, a protocol for media path Diffie-Hellman exchange to agree on a session key and parameters for establishing unicast SRTP sessions for VoIP applications.¶

5.40. RFC 4145: Connection-Oriented Media

[RFC4145] describes how to express media transport over TCP using SDP. It defines the SDP "TCP" protocol identifier, the SDP "setup" attribute, which describes the connection setup procedure, and the SDP "connection" attribute, which handles connection re-establishment.¶

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.¶

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).¶

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)

[RFC7272] defines a new RTCP packet type and an RTCP Extended Report (XR) Block Type to be used for achieving Inter-Destination Media Synchronization (IDMS).¶

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

[RFC5159] provides descriptions of SDP attributes used by the Open Mobile Alliance's "Service and Content Protection for Mobile Broadcast Services" specification.¶

5.45. RFC 6193: Media Description for IKE in SDP

[RFC6193] specifies how to establish a media session that represents a virtual private network using the Session Initiation Protocol for the purpose of on-demand media/application sharing between peers. It extends the protocol identifier of SDP so that it can negotiate use of the Internet Key Exchange Protocol (IKE) for media sessions in the SDP offer/answer model.¶

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.¶

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.¶

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

The Packet-switched Streaming Service (PSS) and the Multimedia Broadcast/Multicast Service (MBMS) defined by 3GPP use SDP and RTSP with some extensions. [RFC6064] provides information about these extensions and registers the RTSP and SDP extensions with IANA.¶

NOTE: [RFC6064] 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].¶

5.49. RFC 3108: ATM SDP

[RFC3108] describes conventions for using SDP described for controlling ATM bearer connections and any associated ATM Adaptation Layer (AAL).¶

NOTE: RFC 3108 describes conventions for using SDP for characterizing ATM bearer connections using an AAL1, AAL2, or AAL5 adaptation layer. For AAL1, AAL2, and AAL5, bearer connections can be used to transport single media streams. In addition, for AAL1 and AAL2, multiple media streams can be multiplexed into a bearer connection. For all adaptation types (AAL1, AAL2, and AAL5), bearer connections can be bundled into a single media group. In all cases addressed by RFC 3108, a real-time media stream (voice, video, voiceband data, pseudowire, and others) or a multiplex of media streams is mapped directly into an ATM connection. RFC 3108 does not address cases where ATM serves as a low-level transport pipe for IP packets that can, in turn, carry one or more real-time (e.g., VoIP) media sessions with a life cycle different from that of the underlying ATM transport.¶

5.50. 3GPP TS 183.063

[TISPAN] describes Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN);¶

5.51. 3GPP TS 24.229

[IP-CALL] specifies an IP multimedia call control protocol based on Session Initial protocol and Session Description Protocol.¶

5.52. ITU T.38

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

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

[Q.1970] defines Bearer Independent Call Control (BICC) IP bearer control protocol.¶

5.54. ITU-T H.248.15

ITU-T H.248.15 [H.248.15] defines the Gateway Control Protocol SDP H.248 package attribute.¶

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.¶

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

This section specifies analysis for the attributes that are included for historic usage alone by the [IANA].¶

6.1. RFC 4566: SDP

[RFC4566] defines SDP that is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation.¶

6.2. RFC 3556: SDP Bandwidth Modifiers for RTCP Bandwidth

[RFC3556] defines an extension to SDP to specify two additional modifiers for the bandwidth attribute. These modifiers may be used to specify the bandwidth allowed for RTCP packets in an RTP session.¶

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.¶

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.¶

7.1. 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; it thus allows for short-term adaptation and implementation of efficient feedback-based repair mechanisms.¶

7.2. RFC 5104: Codec Control Messages in AVPF

[RFC5104] specifies a few extensions to the messages defined in the Audio-Visual Profile with Feedback (AVPF). They are helpful primarily in conversational multimedia scenarios where centralized multipoint functionalities are in use. However, some are also usable in smaller multicast environments and point-to-point calls.¶

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.¶

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.¶

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.¶

7.6. RFC 5104: Codec Control Messages in AVPF

[RFC5104] specifies a few extensions to the messages defined in the Audio-Visual Profile with Feedback (AVPF). They are helpful primarily in conversational multimedia scenarios where centralized multipoint functionalities are in use. However, some are also usable in smaller multicast environments and point-to-point calls.¶

8.1. RFC 5888: SDP Grouping Framework

[RFC5888] defines a framework to group "m=" lines in SDP for different purposes.¶

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).¶

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.¶

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.¶

8.5. RFC 5583: Signaling Media Decoding Dependency in SDP

[RFC5583] defines semantics that allow for signaling the decoding dependency of different media descriptions with the same media type in SDP. This is required, for example, if media data is separated and transported in different network streams as a result of using a layered or multiple descriptive media coding process.¶

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.¶

9.1. RFC 5576: Source-Specific SDP Attributes

[RFC5576] defines a mechanism for describing RTP media sources -- which are identified by their synchronization source (SSRC) identifiers -- in SDP, to associate attributes with these sources and express relationships among sources. It also defines several source-level attributes that can be used to describe properties of media sources.¶

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.¶

10.1. RFC 5432: QoS Mechanism Selection in SDP

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

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.¶

11.1. RFC 4566: SDP

[RFC4566] defines SDP that is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation.¶

12.1. RFC 4796

[RFC4796] defines a new SDP media-level attribute, "content". The "content" attribute defines the content of the media stream to a more detailed level than the media description line. The sender of an SDP session description can attach the "content" attribute to one or more media streams. The receiving application can then treat each media stream differently (e.g., show it on a big or small screen) based on its content.¶

12.2. 3GPP TS 24.182

[IMS-CAT] specifies an IP multimedia subsystem for customized alerting tones.¶

12.3. 3GPP TS 24.183

[IMS-CRS] specifies an IP multimedia subsystem for customized ringing signal.¶

13.1. RFC 5109: RTP Payload Format for Generic FEC

[RFC5109] describes a payload format for generic Forward Error Correction (FEC) for media data encapsulated in RTP. It is based on the exclusive-or (parity) operation. The payload format allows end systems to apply protection using various protection lengths and levels, in addition to using various protection group sizes to adapt to different media and channel characteristics. It enables complete recovery of the protected packets or partial recovery of the critical parts of the payload, depending on the packet loss situation.¶

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.¶

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

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