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

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

170 West Tasman Drive San Jose CA 95134 United States of America snandaku@cisco.com

ART MMUSIC 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 .

Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents () in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

. Introduction
. Terminology
. Motivation
. SDP Attribute Analysis Framework
- . Category: NORMAL
- . Category: CAUTION
- . Category: IDENTICAL
- . Category: SUM
- . Category: TRANSPORT
- . Category: INHERIT
- . Category: IDENTICAL-PER-PT
- . Category: SPECIAL
- . Category: TBD
. Analysis of Existing Attributes
- . RFC 4566: SDP
- . RFC 4585: RTP/AVPF
- . RFC 5761: Multiplexing RTP and RTCP
- . RFC 3312: Integration of Resource Management and SIP
- . RFC 4574: SDP "label" Attribute
- . RFC 5432: QoS Mechanism Selection in SDP
- . RFC 4568: SDP Security Descriptions
- . RFC 5762: RTP over DCCP
- . RFC 6773: DCCP-UDP Encapsulation
- . RFC 5506: Reduced-Size RTCP in RTP Profile
- . RFC 6787: Media Resource Control Protocol Version 2
- . RFC 8445: ICE
- . RFC 5285: RTP Header Extensions
- . RFC 3605: RTCP Attribute in SDP
- . RFC 5576: Source-Specific SDP Attributes
- . RFC 7273: RTP Clock Source Signaling
- . RFC 6236: Image Attributes in SDP
- . RFC 7197: Duplication Delay Attribute in SDP
- . RFC 7266: RTCP XR Blocks for MOS Metric Reporting
- . RFC 6285: Rapid Acquisition of Multicast RTP Sessions
- . RFC 6230: Media Control Channel Framework
- . RFC 6364: SDP Elements for FEC Framework
- . RFC 4796: "content" Attribute
- . RFC 3407: SDP Simple Capability Declaration
- . RFC 6284: Port Mapping between Unicast and Multicast RTP Sessions
- . RFC 6714: MSRP-CEMA
- . RFC 4583: SDP Format for BFCP Streams
- . RFC 5547: SDP Offer/Answer for File Transfer
- . RFC 6849: SDP and RTP Media Loopback Extension
- . RFC 5760: RTCP with Unicast Feedback
- . RFC 3611: RTCP XR
- . RFC 5939: SDP Capability Negotiation
- . RFC 6871: SDP Media Capabilities Negotiation
- . RFC 7006: Miscellaneous Capabilities Negotiation in SDP
- . RFC 4567: Key Management Extensions for SDP and RTSP
- . RFC 4572: Comedia over TLS in SDP
- . RFC 4570: SDP Source Filters
- . RFC 6128: RTCP Port for Multicast Sessions
- . RFC 6189: ZRTP
- . RFC 4145: Connection-Oriented Media
- . RFC 6947: The SDP "altc" Attribute
- . RFC 7195: SDP Extension for Circuit-Switched Bearers in PSTN
- . RFC 7272: IDMS Using the RTP Control Protocol (RTCP)
- . RFC 5159: Open Mobile Alliance (OMA) Broadcast (BCAST) SDP Attributes
- . RFC 6193: Media Description for IKE in SDP
- . RFC 2326: Real Time Streaming Protocol
- . RFC 7826: Real-Time Streaming Protocol
- . RFC 6064: SDP and RTSP Extensions for 3GPP
- . RFC 3108: ATM SDP
- . 3GPP TS 183.063
- . 3GPP TS 24.229
- . ITU T.38
- . ITU-T Q.1970
- . ITU-T H.248.15
- . RFC 4975: The Message Session Relay Protocol
- . Historical Attributes
. bwtype Attribute Analysis
- . RFC 4566: SDP
- . RFC 3556: SDP Bandwidth Modifiers for RTCP Bandwidth
- . RFC 3890: Bandwidth Modifier for SDP
. rtcp-fb Attribute Analysis
- . RFC 4585: RTP/AVPF
- . RFC 5104: Codec Control Messages in AVPF
- . RFC 6285: Unicast-Based Rapid Acquisition of Multicast RTP Sessions (RAMS)
- . RFC 6679: ECN for RTP over UDP/IP
- . RFC 6642: Third-Party Loss Report
- . RFC 5104: Codec Control Messages in AVPF
. group Attribute Analysis
- . RFC 5888: SDP Grouping Framework
- . RFC 3524: Mapping Media Streams to Resource Reservation Flows
- . RFC 4091: ANAT Semantics
- . RFC 5956: FEC Grouping Semantics in SDP
- . RFC 5583: Signaling Media Decoding Dependency in SDP
- . RFC 7104: Duplication Grouping Semantics in the SDP
. ssrc-group Attribute Analysis
- . RFC 5576: Source-Specific SDP Attributes
- . RFC 7104: Duplication Grouping Semantics in the SDP
. QoS Mechanism Token Analysis
- . RFC 5432: QoS Mechanism Selection in SDP
. k= Attribute Analysis
- . RFC 4566: SDP
. content Attribute Analysis
- . RFC 4796
- . 3GPP TS 24.182
- . 3GPP TS 24.183
. Payload Formats
- . RFC 5109: RTP Payload Format for Generic FEC
. Multiplexing Considerations for Encapsulating Attributes
- . RFC 3407: cpar Attribute Analysis
- . RFC 5939 Analysis
  - . Recommendation: Procedures for Potential Configuration Pairing
- . RFC 6871 Analysis
  - . Recommendation: Dealing with Payload Type Numbers
  - . Recommendation: Dealing with Latent Configurations
. IANA Considerations
- . New "Multiplexing Categories" Subregistry
- . "Mux Category" Column for Subregistries
  - . Table: SDP bwtype
  - . Table: attribute-name
  - . Table: content SDP Parameters
  - . Table: Semantics for the "group" SDP Attribute
  - . Table: "rtcp-fb" Attribute Values
  - . Table: "ack" and "nack" Attribute Values
  - . Table: "depend" SDP Attribute Values
  - . Table: "cs-correlation" Attribute Values
  - . Table: Semantics for the "ssrc-group" SDP Attribute
  - . Table: SDP/RTSP Key Management Protocol Identifiers
  - . Table: Codec Control Messages
  - . Table: QoS Mechanism Tokens
  - . Table: SDP Capability Negotiation Option Tags
  - . Table: Timestamp Reference Clock Source Parameters
  - . Table: Media Clock Source Parameters
. Security Considerations
. References
- . Normative References
- . Informative References
Acknowledgements
Author's Address

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, 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) 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 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 .

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 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 when, and only when, they appear in all capitals, as shown here.

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) transports for use by RTP based on ICE 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 , 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 , 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.

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 .

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.

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

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 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

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. defines rules for when attributes and their values are implicitly applied to media description.

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

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 , 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

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 , , and 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 , 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 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.

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 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

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" , the specification defining the extension needs to be consulted to understand the multiplexing implications.

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

Analysis of Existing Attributes This section analyzes attributes listed in , 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)

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.

RFC 4566: SDP defines SDP that is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. RFC 4566 Attribute Analysis

Name	Notes	Level	Mux Category
sendrecv	Not impacted	B	NORMAL
sendonly	Not impacted	B	NORMAL
recvonly	Not impacted	B	NORMAL
inactive	Not impacted	B	NORMAL
cat	Not impacted	S	NORMAL
ptime	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
maxptime	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
orient	Not impacted	M	NORMAL
framerate	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
quality	Not impacted	M	NORMAL
rtpmap	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
fmtp	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
keywds	Not impacted	S	NORMAL
type	Not impacted	S	NORMAL
type:broadcast	Not impacted	S	NORMAL
type:H332	Not impacted	S	NORMAL
type:meeting	Not impacted	S	NORMAL
type:moderated	Not impacted	S	NORMAL
type:test	Not impacted	S	NORMAL
tool	Not impacted	S	NORMAL
charset	Not impacted	S	NORMAL
sdplang	Not impacted	B	NORMAL
lang	Not impacted	B	NORMAL

RFC 4585: RTP/AVPF 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. RFC 4585 Attribute Analysis

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

RFC 5761: Multiplexing RTP and RTCP 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. RFC 5761 Attribute Analysis

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

RFC 3312: Integration of Resource Management and SIP 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. RFC 3312 Attribute Analysis

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

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

RFC 4574: SDP "label" Attribute 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. RFC 4574 Attribute Analysis

Name	Notes	Level	Mux Category
label	Not impacted	M	NORMAL

RFC 5432: QoS Mechanism Selection in SDP defines procedures for negotiating QoS mechanisms using the SDP offer/answer model. RFC 5432 Attribute Analysis

Name	Notes	Level	Mux Category
qos-mech-send	Refer to .	B	TRANSPORT
qos-mech-recv	Refer to .	B	TRANSPORT

RFC 4568: SDP Security Descriptions 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. RFC 4568 Attribute Analysis

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

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. 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. RFC 5762 Attribute Analysis

Name	Notes	Current	Mux Category
dccp-service-code	If RFC 6773 is not 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.	M	CAUTION

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.

RFC 6773: DCCP-UDP Encapsulation 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. RFC 6773 Attribute Analysis

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

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.

RFC 5506: Reduced-Size RTCP in RTP Profile discusses benefits and issues that arise when allowing RTCP packets to be transmitted with reduced size. RFC 5506 Attribute Analysis

Name	Notes	Level	Mux Category
rtcp-rsize	Reduced-size RTCP affects the entire RTP session.	M	IDENTICAL

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. defines attributes for this purpose. RFC 6787 Attribute Analysis

Name	Notes	Level	Mux Category
resource	Not impacted	M	NORMAL
channel	Not impacted	M	NORMAL
cmid	Not impacted	M	NORMAL

RFC 8445: ICE 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. RFC 8445 Attribute Analysis

Name	Notes	Level	Mux Category
ice-lite	Not impacted	S	NORMAL
ice-options	Not impacted	S	NORMAL
ice-mismatch	Not impacted	S	NORMAL
ice-pwd	ice-pwd MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	B	TRANSPORT
ice-ufrag	ice-ufrag MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	B	TRANSPORT
candidate	ice candidate MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	M	TRANSPORT
remote-candidates	ice remote candidate MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	M	TRANSPORT
ice2	Not impacted	S	NORMAL

RFC 5285: RTP Header Extensions provides a general mechanism for using the header-extension feature of RTP. (Note: has been obsoleted by .) 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. RFC 5285 Attribute Analysis

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

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. specifies an early mechanism for indicating the RTCP port. RFC 3605 Attribute Analysis

Name	Notes	Level	Mux Category
rtcp	RTCP port MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	M	TRANSPORT

RFC 5576: Source-Specific SDP Attributes 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. RFC 5576 Attribute Analysis

Name	Notes	Level	Mux Category
ssrc	Refer to notes below.	M	NORMAL
ssrc-group	Refer to for specific analysis of the grouping semantics.	M	NORMAL
cname	Not impacted	SR	NORMAL
previous-ssrc	Refer to notes below	SR	NORMAL
fmtp	The attribute value MUST be the same for a given codec configuration.	SR	IDENTICAL-PER-PT

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

RFC 7273: RTP Clock Source Signaling specifies SDP signaling that identifies timestamp reference clock sources and SDP signaling that identifies the media clock sources in a multimedia session. RFC 7273 Attribute Analysis

Name	Notes	Level	Mux Category
ts-refclk	Not impacted	B	NORMAL
mediaclk	Not impacted	B	NORMAL
ts-refclk:ntp	Not impacted	B	NORMAL
ts-refclk:ptp	Not impacted	B	NORMAL
ts-refclk:gps	Not impacted	B	NORMAL
ts-refclk:gal	Not impacted	B	NORMAL
ts-refclk:glonass	Not impacted	B	NORMAL
ts-refclk:local	Not impacted	B	NORMAL
ts-refclk:private	Not impacted	B	NORMAL
mediaclk:sender	Not impacted	B	NORMAL
mediaclk:direct	Not impacted	B	NORMAL
mediaclk:IEEE1722	Not impacted	B	NORMAL

RFC 6236: Image Attributes in SDP 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. RFC 6236 Attribute Analysis

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

RFC 7197: Duplication Delay Attribute in SDP defines an attribute to indicate the presence of temporally redundant media streams and the duplication delay in SDP. RFC 7197 Attribute Analysis

Name	Notes	Level	Mux Category
duplication-delay	Not impacted	B	NORMAL

RFC 7266: RTCP XR Blocks for MOS Metric Reporting 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. RFC 7266 Attribute Analysis

Name	Notes	Level	Mux Category
calgextmap	Not impacted	B	NORMAL

RFC 6285: Rapid Acquisition of Multicast RTP Sessions 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. RFC 6285 Attribute Analysis

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

RFC 6230: Media Control Channel Framework 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. RFC 6230 Attribute Analysis

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

RFC 6364: SDP Elements for FEC Framework 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. RFC 6364 Attribute Analysis

Name	Notes	Level	Mux Category
fec-source-flow	Refer to the document defining specific FEC scheme.	M	SPECIAL
fec-repair-flow	Refer to the document defining specific FEC scheme.	M	SPECIAL
repair-window	Refer to the document defining specific FEC scheme.	M	SPECIAL

RFC 4796: "content" Attribute 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. RFC 4796 Attribute Analysis

Name	Notes	Level	Mux Category
content	Not impacted	M	NORMAL

RFC 3407: SDP Simple Capability Declaration defines a set of SDP attributes that enables SDP to provide a minimal and backwards-compatible capability declaration mechanism. RFC 3407 Attribute Analysis

Name	Notes	Level	Mux Category
sqn	Not impacted	B	NORMAL
cdsc	Not impacted	B	NORMAL
cpar	Refer to	B	INHERIT
cparmin	Refer to notes below	B	SPECIAL
cparmax	Refer to notes below	B	SPECIAL

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.

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

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

RFC 6714: MSRP-CEMA 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. RFC 6714 Attribute Analysis

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

NOTE: As per , 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.

RFC 4583: SDP Format for BFCP Streams 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. RFC 4583 Attribute Analysis

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

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

RFC 5547: SDP Offer/Answer for File Transfer provides a mechanism to negotiate the transfer of one or more files between two endpoints by using the SDP offer/answer model specified in . RFC 5547 Attribute Analysis

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

RFC 6849: SDP and RTP Media Loopback Extension 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. RFC 6849 Analysis

Name	Notes	Level	Mux Category
loopback rtp-pkt-loopback	The attribute value MUST be same for a given codec configuration.	M	IDENTICAL-PER-PT
loopback rtp-media-loopback	The attribute value MUST be same for a given codec configuration.	M	IDENTICAL-PER-PT
loopback-source	Not impacted	M	NORMAL
loopback-mirror	Not impacted	M	NORMAL

RFC 5760: RTCP with Unicast Feedback 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. RFC 5760 Attribute Analysis

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

RFC 3611: RTCP XR 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). RFC 3611 Attribute Analysis

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

RFC 5939: SDP Capability Negotiation 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. RFC 5939 Attribute Analysis

Name	Notes	Level	Mux Category
pcfg	Refer to	M	SPECIAL
acfg	Refer to	M	SPECIAL
csup	Not impacted	B	NORMAL
creq	Not impacted	B	NORMAL
acap	Refer to	B	INHERIT
tcap	Refer to	B	INHERIT
cap-v0	Not impacted	B	NORMAL

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. extends the framework by defining media capabilities that can be used to negotiate media types and their associated parameters. RFC 6871 Attribute Analysis

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

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 ) as opposed to within a media description only (per ). As described in , the use of identical configuration numbers between multiplexed (bundled) media descriptions is the default way of indicating compatible configurations in a bundle.

RFC 7006: Miscellaneous Capabilities Negotiation in SDP 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). RFC 7006 Attribute Analysis

Name	Notes	Level	Mux Category
bcap	Inherit the category SUM as applicable to the "b=" attribute	B	INHERIT
bcap-v0	Not impacted	B	NORMAL
ccap	The connection address type MUST be identical across all the multiplexed "m=" lines.	B	IDENTICAL
ccap-v0	Not impacted	B	NORMAL
icap	Not impacted	B	NORMAL
icap-v0	Not impacted	B	NORMAL

RFC 4567: Key Management Extensions for SDP and RTSP 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. RFC 4567 Attribute Analysis

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

RFC 4572: Comedia over TLS in SDP specifies how to establish secure connection-oriented media transport sessions over the Transport Layer Security (TLS) protocol using SDP. (Note: has been obsoleted by .) 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. RFC 4572 Attribute Analysis

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

RFC 4570: SDP Source Filters 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. RFC 4570 Attribute Analysis

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

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. removes this restriction by introducing a new SDP attribute. RFC 6128 Attribute Analysis

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

RFC 6189: ZRTP 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. RFC 6189 Attribute Analysis

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

RFC 4145: Connection-Oriented Media 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. RFC 4145 Attribute Analysis

Name	Notes	Level	Mux Category
setup	The setup attribute MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	B	TRANSPORT
connection	The connection attribute MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	B	TRANSPORT

RFC 6947: The SDP "altc" Attribute 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. RFC 6947 Attribute Analysis

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

RFC 7195: SDP Extension for Circuit-Switched Bearers in PSTN 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). RFC 7195 Attribute Analysis

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

NOTE: defines SDP attributes for establishing audio and video media streams over circuit-switched bearers by defining a new nettype value, "PSTN". However, 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.

RFC 7272: IDMS Using the RTP Control Protocol (RTCP) defines a new RTCP packet type and an RTCP Extended Report (XR) Block Type to be used for achieving Inter-Destination Media Synchronization (IDMS). RFC 7272 Attribute Analysis

Name	Notes	Level	Mux Category
rtcp-idms	Not impacted	M	NORMAL

RFC 5159: Open Mobile Alliance (OMA) Broadcast (BCAST) SDP Attributes provides descriptions of SDP attributes used by the Open Mobile Alliance's "Service and Content Protection for Mobile Broadcast Services" specification. RFC 5159 Attribute Analysis

Name	Notes	Level	Mux Category
bcastversion	Not impacted	S	NORMAL
stkmstream	Not impacted	B	NORMAL
SRTPAuthentication	Needs further analysis	M	TBD
SRTPROCTxRate	Needs further analysis	M	TBD

RFC 6193: Media Description for IKE in SDP 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. RFC 6193 Attribute Analysis

Name	Notes	Level	Mux Category
ike-setup	Unlikely to use IKE in the context of multiplexing	B	CAUTION
psk-fingerprint	Unlikely to use IKE in the context of multiplexing	B	CAUTION
ike-esp	Unlikely to use IKE in the context of multiplexing	B	CAUTION
ike-esp-udpencap	Unlikely to use IKE in the context of multiplexing	B	CAUTION

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. RFC 2326 Attribute Analysis

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

NOTE: 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 .

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. RFC 7826 Attribute Analysis

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

NOTE: 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 .

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. provides information about these extensions and registers the RTSP and SDP extensions with IANA. RFC 6064 Attribute Analysis

Name	Notes	Level	Mux Category
X-predecbufsize	Refer to notes below	M	CAUTION
X-initpredecbufperiod	Refer to notes below	M	CAUTION
X-initpostdecbufperiod	Refer to notes below	M	CAUTION
X-decbyterate	Refer to notes below	M	CAUTION
3gpp-videopostdecbufsize	Refer to notes below	M	CAUTION
framesize	Refer to notes below	M	CAUTION
3GPP-Integrity-Key	Refer to notes below	S	CAUTION
3GPP-SDP-Auth	Refer to notes below	S	CAUTION
3GPP-SRTP-Config	Refer to notes below	M	CAUTION
alt	Refer to notes below	M	CAUTION
alt-default-id	Refer to notes below	M	CAUTION
alt-group	Refer to notes below	S	CAUTION
3GPP-Adaptation-Support	Refer to notes below	M	CAUTION
3GPP-Asset-Information	Refer to notes below	B	CAUTION
mbms-mode	Refer to notes below	B	CAUTION
mbms-flowid	Refer to notes below	M	CAUTION
mbms-repair	Refer to notes below	B	CAUTION
3GPP-QoE-Metrics	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Corruption duration	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Rebuffering duration	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Initial buffering duration	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Successive loss of RTP packets	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Frame rate deviation	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Jitter duration	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Content Switch Time	Refer to notes below	B	CAUTION
3GPP-QoE-Metrics:Average Codec Bitrate	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Codec Information	Refer to notes below	M	CAUTION
3GPP-QoE-Metrics:Buffer Status	Refer to notes below	M	CAUTION

NOTE: 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 .

RFC 3108: ATM SDP describes conventions for using SDP described for controlling ATM bearer connections and any associated ATM Adaptation Layer (AAL). RFC 3108 Attribute Analysis

Name	Notes	Level	Mux Category
aalType	Refer to notes below	B	CAUTION
eecid	Refer to notes below	B	CAUTION
capability	Refer to notes below	B	CAUTION
qosClass	Refer to notes below	B	CAUTION
bcob	Refer to notes below	B	CAUTION
stc	Refer to notes below	B	CAUTION
upcc	Refer to notes below	B	CAUTION
atmQOSparms	Refer to notes below	B	CAUTION
atmTrfcDesc	Refer to notes below	B	CAUTION
abrParms	Refer to notes below	B	CAUTION
abrSetup	Refer to notes below	B	CAUTION
bearerType	Refer to notes below	B	CAUTION
lij	Refer to notes below	B	CAUTION
anycast	Refer to notes below	B	CAUTION
cache	Refer to notes below	B	CAUTION
bearerSigIE	Refer to notes below	B	CAUTION
aalApp	Refer to notes below	B	CAUTION
cbrRate	Refer to notes below	B	CAUTION
sbc	Refer to notes below	B	CAUTION
clkrec	Refer to notes below	B	CAUTION
fec	Refer to notes below	B	CAUTION
prtfl	Refer to notes below	B	CAUTION
structure	Refer to notes below	B	CAUTION
cpsSDUsize	Refer to notes below	B	CAUTION
aal2CPS	Refer to notes below	B	CAUTION
aal2CPSSDUrate	Refer to notes below	B	CAUTION
aal2sscs3661unassured	Refer to notes below	B	CAUTION
aal2sscs3661assured	Refer to notes below	B	CAUTION
aal2sscs3662	Refer to notes below	B	CAUTION
aal5sscop	Refer to notes below	B	CAUTION
atmmap	Refer to notes below	B	CAUTION
silenceSupp	Refer to notes below	B	CAUTION
ecan	Refer to notes below	B	CAUTION
gc	Refer to notes below	B	CAUTION
profileDesc	Refer to notes below	B	CAUTION
vsel	Refer to notes below	B	CAUTION
dsel	Refer to notes below	B	CAUTION
fsel	Refer to notes below	B	CAUTION
onewaySel	Refer to notes below	B	CAUTION
codecconfig	Refer to notes below	B	CAUTION
isup_usi	Refer to notes below	B	CAUTION
uiLayer1_Prot	Refer to notes below	B	CAUTION
chain	Refer to notes below	B	CAUTION

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.

3GPP TS 183.063 describes Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN); 3GPP TS 183.063 Attribute Analysis

Name	Notes	Level	Mux Category
PSCid	Not impacted	S	NORMAL
bc_service	Not impacted	S	NORMAL
bc_program	Not impacted	S	NORMAL
bc_service_package	Not impacted	S	NORMAL

3GPP TS 24.229 specifies an IP multimedia call control protocol based on Session Initial protocol and Session Description Protocol. 3GPP TS 24.229 Attribute Analysis

Name	Notes	Level	Mux Category
secondary-realm	secondary-realm MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	M	TRANSPORT
visited-realm	visited-realm MUST be the one that corresponds to the "m=" line chosen for setting up the underlying transport flow.	M	TRANSPORT
omr-m-cksum	Not impacted	M	NORMAL
omr-s-cksum	Not impacted	M	NORMAL
omr-m-att	Not impacted	M	NORMAL
omr-s-att	Not impacted	M	NORMAL
omr-m-bw	Not impacted	M	NORMAL
omr-s-bw	Not impacted	M	NORMAL
omr-codecs	Not impacted	M	NORMAL

ITU T.38 defines procedures for real-time Group 3 facsimile communications over IP networks. ITU T.38 Attribute Analysis

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

NOTE: As per , 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.

ITU-T Q.1970 defines Bearer Independent Call Control (BICC) IP bearer control protocol. ITU-T Q.1970 Attribute Analysis

Name	Notes	Level	Mux Category
ipbcp	ipbcp version identifies the types of IP bearer control protocol (IPBCP) message used in BICC (ITU-T Q.1901) environment that are limited to single-media payload. Refer to the pertinent ITU-T specifications while multiplexing.	S	SPECIAL

ITU-T H.248.15 ITU-T H.248.15 defines the Gateway Control Protocol SDP H.248 package attribute. ITU-T H.248.15 Attribute Analysis

Name	Notes	Level	Mux Category
h248item	It is only applicable for signaling the inclusion of H.248 extension packages to a gateway via the local and remote descriptors. The attribute itself is unaffected by multiplexing, but the package referenced in a specific use of the attribute can be impacted. Further analysis of each package is needed to determine if there is an issue. This is only a concern in environments using a decomposed server/gateway with H.248 signaled between them. The ITU-T will need to do further analysis of various packages when they specify how to signal the use of multiplexing to a gateway.	B	SPECIAL

RFC 4975: The Message Session Relay Protocol 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. RFC 4975 Attribute Analysis

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

NOTE: As per , 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.

Historical Attributes This section specifies analysis for the attributes that are included for historic usage alone by the . Historical Attribute Analysis

Name	Notes	Level	Mux Category
rtpred1	Historic attributes	M	CAUTION
rtpred2	Historic attributes	M	CAUTION

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

Name	Notes	Level	Mux Category
bwtype:CT	Not impacted	S	NORMAL
bwtype:AS	For media-level usage, the aggregate of individual bandwidth values is considered.	B	SUM

RFC 3556: SDP Bandwidth Modifiers for RTCP Bandwidth 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. RFC 3556 bwtype Analysis

Name	Notes	Level	Mux Category
bwtype:RS	Session-level usage represents session aggregate, and media-level usage indicates SUM of the individual values while multiplexing.	B	SUM
bwtype:RR	Session-level usage represents session aggregate, and media-level usage indicates SUM of the individual values while multiplexing.	B	SUM

RFC 3890: Bandwidth Modifier for SDP 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. RFC 3890 bwtype Analysis

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

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.

rtcp-fb Attribute Analysis This section analyzes rtcp-fb SDP attributes.

RFC 4585: RTP/AVPF 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. RFC 4585 Attribute Analysis

Name	Notes	Level	Mux Category
ack rpsi	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
ack app	Feedback parameters MUST be handled in the app-specific way when multiplexed.	M	SPECIAL
nack	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
nack pli	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
nack sli	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
nack rpsi	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
nack app	Feedback parameters MUST be handled in the app specific way when multiplexed.	M	SPECIAL
trr-int	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT

RFC 5104: Codec Control Messages in AVPF 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. RFC 5104 Attribute Analysis

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

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

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

RFC 6679: ECN for RTP over UDP/IP 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. RFC 6679 Attribute Analysis

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

RFC 6642: Third-Party Loss Report In a large RTP session using the RTCP feedback mechanism defined in , 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. 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. RFC 6642 Attribute Analysis

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

Name	Notes	Level	Mux Category
ccm fir	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
ccm tmmbr	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
ccm tstr	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
ccm vbcm	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT

group Attribute Analysis This section analyzes SDP "group" attribute semantics .

RFC 5888: SDP Grouping Framework defines a framework to group "m=" lines in SDP for different purposes. RFC 5888 Attribute Analysis

Name	Notes	Level	Mux Category
group:LS	Not impacted	S	NORMAL
group:FID	Not impacted	S	NORMAL

RFC 3524: Mapping Media Streams to Resource Reservation Flows 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). RFC 3524 Attribute Analysis

Name	Notes	Level	Mux Category
group:SRF	Not impacted	S	NORMAL

RFC 4091: ANAT Semantics defines ANAT semantics for the SDP grouping framework. (Note: has been obsoleted by .) The ANAT semantics allow alternative types of network addresses to establish a particular media stream. RFC 4091 Attribute Analysis

Name	Notes	Level	Mux Category
group:ANAT	ANAT semantics is obsoleted.	S	CAUTION

RFC 5956: FEC Grouping Semantics in SDP 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 . 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. RFC 5956 Attribute Analysis

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

RFC 5583: Signaling Media Decoding Dependency in SDP 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. RFC 5583 Attribute Analysis

Name	Notes	Level	Mux Category
group:DDP	Not impacted	S	NORMAL
depend lay	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT
depend mdc	The attribute value MUST be the same for a given codec configuration.	M	IDENTICAL-PER-PT

RFC 7104: Duplication Grouping Semantics in the SDP 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. RFC 7104 Attribute Analysis

Name	Notes	Level	Mux Category
group:DUP	Not impacted	S	NORMAL

ssrc-group Attribute Analysis This section analyzes "ssrc-group" semantics.

Name	Notes	Level	Mux Category
ssrc-group:FID	Not impacted	SR	NORMAL
ssrc-group:FEC	Not impacted	SR	NORMAL
ssrc-group:FEC-FR	Not impacted	SR	NORMAL

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

QoS Mechanism Token Analysis This section analyzes QoS tokes specified with SDP.

RFC 5432: QoS Mechanism Selection in SDP defines procedures to negotiate QoS mechanisms using the SDP offer/answer model. RFC 5432 Attribute Analysis

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

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 need to be taken into consideration.

k= Attribute Analysis

Name	Notes	Level	Mux Category
k=	It is not recommended to use this attribute under multiplexing.	S	CAUTION

content Attribute Analysis

RFC 4796 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. RFC 4796 Attribute Analysis

Name	Notes	Level	Mux Category
content:slides	Not impacted	M	NORMAL
content:speaker	Not impacted	M	NORMAL
content:main	Not impacted	M	NORMAL
content:sl	Not impacted	M	NORMAL
content:alt	Not impacted	M	NORMAL

3GPP TS 24.182 specifies an IP multimedia subsystem for customized alerting tones. 3GPP TS 24.182 Attribute Analysis

Name	Notes	Level	Mux Category
g.3gpp.cat	Usage defined for the IP multimedia subsystem	M	NORMAL

3GPP TS 24.183 specifies an IP multimedia subsystem for customized ringing signal. 3GPP TS 24.183 Attribute Analysis

Name	Notes	Level	Mux Category
g.3gpp.crs	Usage defined for the IP multimedia subsystem	M	NORMAL

Payload Formats

RFC 5109: RTP Payload Format for Generic FEC 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. RFC 5109 Payload Format Analysis

Name	Notes	Level	Mux Category
audio/ulpfec	Not recommended for multiplexing due to reuse of SSRCs.	M	CAUTION
video/ulpfec	Not recommended for multiplexing due to reuse of SSRCs.	M	CAUTION
text/ulpfec	Not recommended for multiplexing due to reuse of SSRCs.	M	CAUTION
application/ulpfec	Not recommended for multiplexing due to reuse of SSRCs.	M	CAUTION

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 , and 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.

RFC 3407: cpar Attribute Analysis The 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.

RFC 5939 Analysis 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, 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 , the negotiation process is done independently for each media description.

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 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 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, provides analysis of media-capability-extension attributes as defined in .

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.

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.

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.

RFC 6871 Analysis extends the capability negotiation framework described in 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.

Recommendation: Dealing with Payload Type Numbers 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 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 , which must be followed here, as well. As described in , 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.

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.

Recommendation: Dealing with Latent Configurations 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 . 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.

IANA Considerations 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 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.

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

Further entries can be registered using Standard Actions policies outlined in , 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.

"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 .

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: 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-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: content SDP Parameters The following values have been added to the "content SDP 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.

SDP Name	Mux Category
slides	NORMAL
speaker	NORMAL
sl	NORMAL
main	NORMAL
alt	NORMAL
g.3gpp.cat	NORMAL
g.3gpp.crs	NORMAL

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: "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: "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: "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: "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: 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: 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: Codec Control Messages The following values have been added to the "Codec Control Messages" 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
fir	IDENTICAL-PER-PT
tmmbr	IDENTICAL-PER-PT
tstr	IDENTICAL-PER-PT
vbcm	IDENTICAL-PER-PT

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: SDP Capability Negotiation Option Tags The following values have been added to the "SDP Capability Negotiation Option Tags" 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.

Option Tag	Mux Category
cap-v0	NORMAL
med-v0	NORMAL
bcap-v0	NORMAL
ccap-v0	NORMAL
icap-v0	NORMAL

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: Media Clock Source Parameters The following values have been added to the "Media 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
sender	NORMAL
direct	NORMAL
IEEE1722	NORMAL

Security Considerations The primary security considerations for RTP, including the way it is used here, are described in and . When multiplexing SDP attributes with the category "CAUTION", the implementations should be aware of possible issues described in this specification.

References Normative References Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. SDP: Session Description Protocol This memo defines the Session Description Protocol (SDP). SDP is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. [STANDARDS-TRACK] Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Negotiating Media Multiplexing Using the Session Description Protocol (SDP) Informative References SDP Descriptors for FLUTE This document specifies the use of SDP to describe the parameters required to begin, join, receive data from, and/or end FLUTE sessions. It also provides a Composite Session SDP media grouping semantic for grouping media streams into protocol-specific sessions, such as multiple-channel FLUTE sessions. Work in Progress Gateway control protocol: SDP ITU-T H.248 package attribute ITU-T Session Description Protocol (SDP) Parameters IANA IP Multimedia Subsystem (IMS) Customized Alerting Tones (CAT); Protocol specification 3GPP IP Multimedia Subsystem (IMS) Customized Ringing Signal (CRS); Protocol specification 3GPP IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 3GPP Q.1970: BICC IP bearer control protocol ITU-T Real Time Streaming Protocol (RTSP) 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. [STANDARDS-TRACK] Conventions for the use of the Session Description Protocol (SDP) for ATM Bearer Connections This document describes conventions for using the Session Description Protocol (SDP) described in RFC 2327 for controlling ATM Bearer Connections, and any associated ATM Adaptation Layer (AAL). The AALs addressed are Type 1, Type 2 and Type 5. [STANDARDS-TRACK] An Offer/Answer Model with Session Description Protocol (SDP) This document defines a mechanism by which two entities can make use of the Session Description Protocol (SDP) to arrive at a common view of a multimedia session between them. In the model, one participant offers the other a description of the desired session from their perspective, and the other participant answers with the desired session from their perspective. This offer/answer model is most useful in unicast sessions where information from both participants is needed for the complete view of the session. The offer/answer model is used by protocols like the Session Initiation Protocol (SIP). [STANDARDS-TRACK] Integration of Resource Management and Session Initiation Protocol (SIP) Session Description Protocol (SDP) Simple Capability Declaration This document defines a set of Session Description Protocol (SDP) attributes that enables SDP to provide a minimal and backwards compatible capability declaration mechanism. Such capability declarations can be used as input to a subsequent session negotiation, which is done by means outside the scope of this document. This provides a simple and limited solution to the general capability negotiation problem being addressed by the next generation of SDP, also known as SDPng. [STANDARDS-TRACK] Mapping of Media Streams to Resource Reservation Flows This document defines an extension to the Session Description Protocol (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). [STANDARDS-TRACK] RTP: A Transport Protocol for Real-Time Applications This memorandum describes RTP, the real-time transport protocol. RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services. RTP does not address resource reservation and does not guarantee quality-of- service for real-time services. The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality. RTP and RTCP are designed to be independent of the underlying transport and network layers. The protocol supports the use of RTP-level translators and mixers. Most of the text in this memorandum is identical to RFC 1889 which it obsoletes. There are no changes in the packet formats on the wire, only changes to the rules and algorithms governing how the protocol is used. The biggest change is an enhancement to the scalable timer algorithm for calculating when to send RTCP packets in order to minimize transmission in excess of the intended rate when many participants join a session simultaneously. [STANDARDS-TRACK] Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth This document defines an extension to the Session Description Protocol (SDP) to specify two additional modifiers for the bandwidth attribute. These modifiers may be used to specify the bandwidth allowed for RTP Control Protocol (RTCP) packets in a Real-time Transport Protocol (RTP) session. [STANDARDS-TRACK] Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP) The Session Description Protocol (SDP) is used to describe the parameters of media streams used in multimedia sessions. When a session requires multiple ports, SDP assumes that these ports have consecutive numbers. However, when the session crosses a network address translation device that also uses port mapping, the ordering of ports can be destroyed by the translation. To handle this, we propose an extension attribute to SDP. RTP Control Protocol Extended Reports (RTCP XR) This document defines the Extended Report (XR) packet type for the RTP Control Protocol (RTCP), and defines how the use of XR packets can be signaled by an application if it employs the Session Description Protocol (SDP). XR packets are composed of report blocks, and seven block types are defined here. The purpose of the extended reporting format is to convey information that supplements the six statistics that are contained in the report blocks used by RTCP's Sender Report (SR) and Receiver Report (RR) packets. Some applications, such as multicast inference of network characteristics (MINC) or voice over IP (VoIP) monitoring, require other and more detailed statistics. In addition to the block types defined here, additional block types may be defined in the future by adhering to the framework that this document provides. The Secure Real-time Transport Protocol (SRTP) This document describes the Secure Real-time Transport Protocol (SRTP), a profile of the Real-time Transport Protocol (RTP), which can provide confidentiality, message authentication, and replay protection to the RTP traffic and to the control traffic for RTP, the Real-time Transport Control Protocol (RTCP). [STANDARDS-TRACK] A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP) This document defines a Session Description Protocol (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 bit-rate value together with the maximum packet rate can then be used to calculate the real bit-rate over the transport actually used. The existing SDP bandwidth modifiers and their values include the bandwidth needed for the transport and IP layers. When using SDP with protocols like the Session Announcement Protocol (SAP), the Session Initiation Protocol (SIP), and the Real-Time Streaming Protocol (RTSP), and when the involved hosts has different transport overhead, for example due to different IP versions, the interpretation of what lower layer bandwidths are included is not clear. [STANDARDS-TRACK] The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework This document defines the Alternative Network Address Types (ANAT) semantics for the Session Description Protocol (SDP) grouping framework. The ANAT semantics allow alternative types of network addresses to establish a particular media stream. [STANDARDS-TRACK] TCP-Based Media Transport in the Session Description Protocol (SDP) This document describes how to express media transport over TCP using the Session Description Protocol (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 reestablishment. [STANDARDS-TRACK] Key Management Extensions for Session Description Protocol (SDP) and Real Time Streaming Protocol (RTSP) This document defines general extensions for Session Description Protocol (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. General guidelines are also given on how the framework should be used together with SIP and RTSP. The usage with the Multimedia Internet KEYing (MIKEY) key management protocol is also defined. [STANDARDS-TRACK] Session Description Protocol (SDP) Security Descriptions for Media Streams This document defines a Session Description Protocol (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. The attribute can be used with a variety of SDP media transports, and this document defines how to use it for the Secure Real-time Transport Protocol (SRTP) unicast media streams. The SDP crypto attribute requires the services of a data security protocol to secure the SDP message. [STANDARDS-TRACK] Session Description Protocol (SDP) Source Filters This document describes how to adapt the Session Description Protocol (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. [STANDARDS-TRACK] Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP) This document specifies how to establish secure connection-oriented media transport sessions over the Transport Layer Security (TLS) protocol using the Session Description Protocol (SDP). 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. This document extends and updates RFC 4145. [STANDARDS-TRACK] The Session Description Protocol (SDP) Label Attribute This document defines a new Session Description Protocol (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. [STANDARDS-TRACK] Session Description Protocol (SDP) Format for Binary Floor Control Protocol (BFCP) Streams This document specifies how to describe Binary Floor Control Protocol (BFCP) streams in Session Description Protocol (SDP) descriptions. User agents using the offer/answer model to establish BFCP streams use this format in their offers and answers. [STANDARDS-TRACK] Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF) Real-time media streams that use RTP are, to some degree, resilient against packet losses. Receivers may use the base mechanisms of the Real-time Transport Control Protocol (RTCP) to report packet reception statistics and thus allow a sender to adapt its transmission behavior in the mid-term. This is the sole means for feedback and feedback-based error repair (besides a few codec-specific mechanisms). This document 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. This early feedback profile (AVPF) maintains the AVP bandwidth constraints for RTCP and preserves scalability to large groups. [STANDARDS-TRACK] The Session Description Protocol (SDP) Content Attribute This document defines a new Session Description Protocol (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. [STANDARDS-TRACK] The Message Session Relay Protocol (MSRP) This document 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. [STANDARDS-TRACK] Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF) This document 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. The extensions discussed are messages related to the ITU-T Rec. H.271 Video Back Channel, Full Intra Request, Temporary Maximum Media Stream Bit Rate, and Temporal-Spatial Trade-off. [STANDARDS-TRACK] RTP Payload Format for Generic Forward Error Correction This document specifies 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 described in this document 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. This scheme is completely compatible with non-FEC-capable hosts, so the receivers in a multicast group that do not implement FEC can still work by simply ignoring the protection data. This specification obsoletes RFC 2733 and RFC 3009. The FEC specified in this document is not backward compatible with RFC 2733 and RFC 3009. [STANDARDS-TRACK] Session Description Protocol (SDP) Attributes for Open Mobile Alliance (OMA) Broadcast (BCAST) Service and Content Protection This document provides descriptions of Session Description Protocol (SDP) attributes used by the Open Mobile Alliance's Broadcast Service and Content Protection specification. This memo provides information for the Internet community. A General Mechanism for RTP Header Extensions This document provides a general mechanism to use the header extension feature of RTP (the Real-Time Transport Protocol). 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 de-centralized. The actual extensions in use in a session are signaled in the setup information for that session. [STANDARDS-TRACK] Quality of Service (QoS) Mechanism Selection in the Session Description Protocol (SDP) The offer/answer model for the Session Description Protocol (SDP) assumes that endpoints somehow establish the Quality of Service (QoS) required for the media streams they establish. Endpoints in closed environments typically agree out-of-band (e.g., using configuration information) regarding which QoS mechanism to use. However, on the Internet, there is more than one QoS service available. Consequently, there is a need for a mechanism to negotiate which QoS mechanism to use for a particular media stream. This document defines such a mechanism. [STANDARDS-TRACK] Support for Reduced-Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences This memo discusses benefits and issues that arise when allowing Real-time Transport Protocol (RTCP) packets to be transmitted with reduced size. The size can be reduced if the rules on how to create compound packets outlined in RFC 3550 are removed or changed. Based on that analysis, this memo defines certain changes to the rules to allow feedback messages to be sent as Reduced-Size RTCP packets under certain conditions when using the RTP/AVPF (Real-time Transport Protocol / Audio-Visual Profile with Feedback) profile (RFC 4585). This document updates RFC 3550, RFC 3711, and RFC 4585. [STANDARDS-TRACK] A Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File Transfer This document provides a mechanism to negotiate the transfer of one or more files between two endpoints by using the Session Description Protocol (SDP) offer/answer model specified in RFC 3264. SDP is extended to describe the attributes of the files to be transferred. The offerer can describe either the files it wants to send or the files it would like to receive. The answerer can either accept or reject the offer separately for each individual file. The transfer of one or more files is initiated after a successful negotiation. The Message Session Relay Protocol (MSRP) is defined as the default mechanism to actually carry the files between the endpoints. The conventions on how to use MSRP for file transfer are also provided in this document. [STANDARDS-TRACK] Source-Specific Media Attributes in the Session Description Protocol (SDP) The Session Description Protocol (SDP) provides mechanisms to describe attributes of multimedia sessions and of individual media streams (e.g., Real-time Transport Protocol (RTP) sessions) within a multimedia session, but does not provide any mechanism to describe individual media sources within a media stream. This document defines a mechanism to describe RTP media sources, which are identified by their synchronization source (SSRC) identifiers, in SDP, to associate attributes with these sources, and to express relationships among sources. It also defines several source-level attributes that can be used to describe properties of media sources. [STANDARDS-TRACK] Signaling Media Decoding Dependency in the Session Description Protocol (SDP) This memo defines semantics that allow for signaling the decoding dependency of different media descriptions with the same media type in the Session Description Protocol (SDP). This is required, for example, if media data is separated and transported in different network streams as a result of the use of a layered or multiple descriptive media coding process. A new grouping type "DDP" -- decoding dependency -- is defined, to be used in conjunction with RFC 3388 entitled "Grouping of Media Lines in the Session Description Protocol". In addition, an attribute is specified describing the relationship of the media streams in a "DDP" group indicated by media identification attribute(s) and media format description(s). [STANDARDS-TRACK] RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback This document specifies an extension to the Real-time Transport Control Protocol (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. In addition, it can be applied to any group that might benefit from a sender-controlled summarized reporting mechanism. [STANDARDS-TRACK] Multiplexing RTP Data and Control Packets on a Single Port This memo discusses issues that arise when multiplexing RTP data packets and RTP Control Protocol (RTCP) packets on a single UDP port. It updates RFC 3550 and RFC 3551 to describe when such multiplexing is and is not appropriate, and it explains how the Session Description Protocol (SDP) can be used to signal multiplexed sessions. [STANDARDS-TRACK] RTP and the Datagram Congestion Control Protocol (DCCP) The Real-time Transport Protocol (RTP) is a widely used transport for real-time multimedia on IP networks. The Datagram Congestion Control Protocol (DCCP) is a transport protocol that provides desirable services for real-time applications. This memo specifies a mapping of RTP onto DCCP, along with associated signalling, such that real- time applications can make use of the services provided by DCCP. [STANDARDS-TRACK] Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS) This document specifies how to use the Session Initiation Protocol (SIP) to establish a Secure Real-time Transport Protocol (SRTP) security context using the Datagram Transport Layer Security (DTLS) protocol. It describes a mechanism of transporting a fingerprint attribute in the Session Description Protocol (SDP) that identifies the key that will be presented during the DTLS handshake. The key exchange travels along the media path as opposed to the signaling path. The SIP Identity mechanism can be used to protect the integrity of the fingerprint attribute from modification by intermediate proxies. [STANDARDS-TRACK] The Session Description Protocol (SDP) Grouping Framework In this specification, we define a framework to group "m" lines in the Session Description Protocol (SDP) for different purposes. This framework uses the "group" and "mid" SDP attributes, both of which are defined in this specification. Additionally, we specify how to use the framework for two different purposes: for lip synchronization and for receiving a media flow consisting of several media streams on different transport addresses. This document obsoletes RFC 3388. [STANDARDS-TRACK] Session Description Protocol (SDP) Capability Negotiation The Session Description Protocol (SDP) was intended to describe multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. SDP was not intended to provide capability indication or capability negotiation; however, over the years, SDP has seen widespread adoption and as a result it has been gradually extended to provide limited support for these, notably in the form of the offer/answer model defined in RFC 3264. SDP does not define how to negotiate one or more alternative transport protocols (e.g., RTP profiles) or attributes. This makes it difficult to deploy new RTP profiles such as Secure RTP or RTP with RTCP-based feedback, negotiate use of different security keying mechanisms, etc. It also presents problems for some forms of media negotiation. The purpose of this document is to address these shortcomings by extending SDP with capability negotiation parameters and associated offer/answer procedures to use those parameters in a backwards compatible manner. The document 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. [STANDARDS-TRACK] Forward Error Correction Grouping Semantics in the Session Description Protocol This document defines the semantics for grouping the associated source and FEC-based (Forward Error Correction) repair flows in the Session Description Protocol (SDP). The semantics defined in this document are to be used with the SDP Grouping Framework (RFC 5888). 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, and they provide support for additive repair flows. SSRC-level (Synchronization Source) grouping semantics are also defined in this document for Real-time Transport Protocol (RTP) streams using SSRC multiplexing. [STANDARDS-TRACK] SDP and RTSP Extensions Defined for 3GPP Packet-Switched Streaming Service and Multimedia Broadcast/Multicast Service The Packet-switched Streaming Service (PSS) and the Multimedia Broadcast/Multicast Service (MBMS) defined by 3GPP use the Session Description Protocol (SDP) and Real Time Streaming Protocol (RTSP) with some extensions. This document provides information about these extensions and registers the RTSP and SDP extensions with IANA. This document is not an Internet Standards Track specification; it is published for informational purposes. RTP Control Protocol (RTCP) Port for Source-Specific Multicast (SSM) Sessions The Session Description Protocol (SDP) has an attribute that allows RTP applications to specify an address and a port associated with the RTP Control Protocol (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. This document removes this restriction by introducing a new SDP attribute. [STANDARDS-TRACK] ZRTP: Media Path Key Agreement for Unicast Secure RTP This document defines ZRTP, a protocol for media path Diffie-Hellman exchange to agree on a session key and parameters for establishing unicast Secure Real-time Transport Protocol (SRTP) sessions for Voice over IP (VoIP) applications. The ZRTP protocol is media path keying because it is multiplexed on the same port as RTP and does not require support in the signaling protocol. ZRTP does not assume a Public Key Infrastructure (PKI) or require the complexity of certificates in end devices. For the media session, ZRTP provides confidentiality, protection against man-in-the-middle (MiTM) attacks, and, in cases where the signaling protocol provides end-to-end integrity protection, authentication. ZRTP can utilize a Session Description Protocol (SDP) attribute to provide discovery and authentication through the signaling channel. To provide best effort SRTP, ZRTP utilizes normal RTP/AVP (Audio-Visual Profile) profiles. ZRTP secures media sessions that include a voice media stream and can also secure media sessions that do not include voice by using an optional digital signature. This document is not an Internet Standards Track specification; it is published for informational purposes. Media Description for the Internet Key Exchange Protocol (IKE) in the Session Description Protocol (SDP) This document 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 the Session Description Protocol (SDP) so that it can negotiate use of the Internet Key Exchange Protocol (IKE) for media sessions in the SDP offer/answer model. It also specifies a method to boot up IKE and generate IPsec security associations using a self-signed certificate. This document is not an Internet Standards Track specification; it is published for informational purposes. Media Control Channel Framework This document 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 the Session Initiation Protocol (SIP) to establish an application-level control mechanism between application servers and associated external servers such as media servers. The motivation for the creation of this framework is to provide an interface suitable to meet the requirements of a centralized conference system, where the conference system can be distributed, as defined by the XCON working group in the IETF. It is not, however, limited to this scope. [STANDARDS-TRACK] Negotiation of Generic Image Attributes in the Session Description Protocol (SDP) This document 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 \%hand- held 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. [STANDARDS-TRACK] Port Mapping between Unicast and Multicast RTP Sessions This document 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. [STANDARDS-TRACK] Unicast-Based Rapid Acquisition of Multicast RTP Sessions When an RTP receiver joins a multicast session, it may need to acquire and parse certain Reference Information before it can process any data sent in the multicast session. Depending on the join time, length of the Reference Information repetition (or appearance) interval, size of the Reference Information, and the application and transport properties, the time lag before an RTP receiver can usefully consume the multicast data, which we refer to as the Acquisition Delay, varies and can be large. This is an undesirable phenomenon for receivers that frequently switch among different multicast sessions, such as video broadcasts. In this document, we describe a method using the existing RTP and RTP Control Protocol (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. However, this method can also be used in other types of multicast applications where the acquisition delay is long enough to be a problem. [STANDARDS-TRACK] Session Description Protocol Elements for the Forward Error Correction (FEC) Framework This document specifies the use of the Session Description Protocol (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. [STANDARDS-TRACK] RTP Control Protocol (RTCP) Extension for a Third-Party Loss Report In a large RTP session using the RTP Control Protocol (RTCP) feedback mechanism defined in RFC 4585, 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. This memo 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 Session Description Protocol (SDP) signaling is also defined. [STANDARDS-TRACK] Explicit Congestion Notification (ECN) for RTP over UDP This memo specifies how Explicit Congestion Notification (ECN) can be used with the Real-time Transport Protocol (RTP) running over UDP, using the RTP Control Protocol (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 Session Traversal Utilities for NAT (STUN) extension used in the optional initialisation method using Interactive Connectivity Establishment (ICE). Signalling and procedures for negotiation of capabilities and initialisation methods are also defined. [STANDARDS-TRACK] Connection Establishment for Media Anchoring (CEMA) for the Message Session Relay Protocol (MSRP) This document 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 a Session Description Protocol (SDP) attribute, 'msrp-cema', that MSRP endpoints use to indicate support of the CEMA extension. [STANDARDS-TRACK] DCCP-UDP: A Datagram Congestion Control Protocol UDP Encapsulation for NAT Traversal This document specifies an alternative encapsulation of the Datagram Congestion Control Protocol (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. This document also updates the Session Description Protocol (SDP) information for DCCP defined in RFC 5762. [STANDARDS-TRACK] Media Resource Control Protocol Version 2 (MRCPv2) 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 Session Initiation Protocol (SIP), to coordinate MRCPv2 clients and servers and manage sessions between them, and the Session Description Protocol (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. [STANDARDS-TRACK] An Extension to the Session Description Protocol (SDP) and Real-time Transport Protocol (RTP) for Media Loopback The wide deployment of Voice over IP (VoIP), real-time text, and Video over IP services has introduced new challenges in managing and maintaining real-time voice/text/video quality, reliability, and overall performance. In particular, media delivery is an area that needs attention. One method of meeting these challenges is monitoring the media delivery performance by looping media back to the transmitter. This is typically referred to as "active monitoring" of services. Media loopback is especially popular in ensuring the quality of transport to the edge of a given VoIP, real-time text, or Video over IP service. Today, in networks that deliver real-time media, short of running 'ping' and 'traceroute' to the edge, administrators are left without the necessary tools to actively monitor, manage, and diagnose quality issues with their service. The extension defined herein adds new Session Description Protocol (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 VoIP, real-time text, and Video over IP performance metrics. Session Description Protocol (SDP) Media Capabilities Negotiation Session Description Protocol (SDP) capability negotiation provides a general framework for indicating and negotiating capabilities in SDP. The base framework defines only capabilities for negotiating transport protocols and attributes. This documents extends the framework by defining media capabilities that can be used to negotiate media types and their associated parameters. This document updates the IANA Considerations of RFC 5939. The Session Description Protocol (SDP) Alternate Connectivity (ALTC) Attribute This document 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 attribute, the "altc" attribute, solves the backward-compatibility problem that plagued Alternative Network Address Types (ANAT) due to their syntax. The proposed solution is applicable to scenarios where connectivity checks are not required. If connectivity checks are required, Interactive Connectivity Establishment (ICE), as specified in RFC 5245, provides such a solution. Miscellaneous Capabilities Negotiation in the Session Description Protocol (SDP) The Session Description Protocol (SDP) has been extended with a capability negotiation mechanism framework that allows the endpoints to negotiate transport protocols and attributes. This framework has been extended with a media capabilities negotiation mechanism that allows endpoints to negotiate additional media-related capabilities. This negotiation is embedded into the widely used SDP offer/answer procedures. This memo 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). Duplication Grouping Semantics in the Session Description Protocol Packet loss is undesirable for real-time multimedia sessions, but it can occur due to congestion or other unplanned network outages. This is especially true for IP multicast networks, where packet loss patterns can vary greatly between receivers. One technique that can be used to recover from packet loss without incurring unbounded delay for all the receivers is to duplicate the packets and send them in separate redundant streams. This document defines the semantics for grouping redundant streams in the Session Description Protocol (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. Session Description Protocol (SDP) Extension for Setting Audio and Video Media Streams over Circuit-Switched Bearers in the Public Switched Telephone Network (PSTN) This memo describes use cases, requirements, and protocol extensions for using the Session Description Protocol (SDP) offer/answer model for establishing audio and video media streams over circuit-switched bearers in the Public Switched Telephone Network (PSTN). Duplication Delay Attribute in the Session Description Protocol A straightforward approach to provide protection against packet losses due to network outages with a longest duration of T time units is to duplicate the original packets and send each copy separated in time by at least T time units. This approach is commonly referred to as "time-shifted redundancy", "temporal redundancy", or simply "delayed duplication". This document defines an attribute to indicate the presence of temporally redundant media streams and the duplication delay in the Session Description Protocol. RTP Control Protocol (RTCP) Extended Report (XR) Blocks for Mean Opinion Score (MOS) Metric Reporting This document defines an RTP Control Protocol (RTCP) Extended Report (XR) Block including two new segment types and associated Session Description Protocol (SDP) parameters that allow the reporting of mean opinion score (MOS) Metrics for use in a range of RTP applications. Inter-Destination Media Synchronization (IDMS) Using the RTP Control Protocol (RTCP) This document defines a new RTP Control Protocol (RTCP) Packet Type and an RTCP Extended Report (XR) Block Type to be used for achieving Inter-Destination Media Synchronization (IDMS). IDMS is the process of synchronizing playout across multiple media receivers. Using the RTCP XR IDMS Report Block defined in this document, media playout information from participants in a synchronization group can be collected. Based on the collected information, an RTCP IDMS Settings Packet can then be sent to distribute a common target playout point to which all the distributed receivers, sharing a media experience, can synchronize. Typical use cases in which IDMS is useful are social TV, shared service control (i.e., applications where two or more geographically separated users are watching a media stream together), distance learning, networked video walls, networked loudspeakers, etc. RTP Clock Source Signalling NTP format timestamps are used by several RTP protocols for synchronisation and statistical measurements. This memo specifies Session Description Protocol (SDP) signalling that identifies timestamp reference clock sources and SDP signalling that identifies the media clock sources in a multimedia session. Differentiated Services (Diffserv) and Real-Time Communication This memo describes the interaction between Differentiated Services (Diffserv) network quality-of-service (QoS) functionality and real- time network communication, including communication based on the Real-time Transport Protocol (RTP). Diffserv is based on network nodes applying different forwarding treatments to packets whose IP headers are marked with different Diffserv Codepoints (DSCPs). WebRTC applications, as well as some conferencing applications, have begun using the Session Description Protocol (SDP) bundle negotiation mechanism to send multiple traffic streams with different QoS requirements using the same network 5-tuple. The results of using multiple DSCPs to obtain different QoS treatments within a single network 5-tuple have transport protocol interactions, particularly with congestion control functionality (e.g., reordering). In addition, DSCP markings may be changed or removed between the traffic source and destination. This memo covers the implications of these Diffserv aspects for real-time network communication, including WebRTC. Real-Time Streaming Protocol Version 2.0 This memorandum defines the Real-Time Streaming Protocol (RTSP) version 2.0, which obsoletes RTSP version 1.0 defined in RFC 2326. RTSP is an application-layer protocol for the setup and control of 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. Sources of data can include both live data feeds and stored clips. This protocol is intended to control multiple data delivery sessions; provide a means for choosing delivery channels such as UDP, multicast UDP, and TCP; and provide a means for choosing delivery mechanisms based upon RTP (RFC 3550). Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP) This document specifies how to establish secure connection-oriented media transport sessions over the Transport Layer Security (TLS) protocol using the Session Description Protocol (SDP). It defines the 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. This document obsoletes RFC 4572 by clarifying the usage of multiple fingerprints. A General Mechanism for RTP Header Extensions This document provides a general mechanism to use the header extension feature of RTP (the Real-time Transport Protocol). 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. This document obsoletes RFC 5285. Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal This document describes a protocol for Network Address Translator (NAT) traversal for UDP-based communication. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN). This document obsoletes RFC 5245. Session Description Protocol (SDP) Format for Binary Floor Control Protocol (BFCP) Streams SDP: Session Description Protocol Procedures for real-time Group 3 facsimile communication over IP networks ITU-T Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN); IMS-based IPTV stage 3 specification ETSI

Acknowledgements I would like to thank and for suggesting the categories, contributing text, and reviewing the draft of this document. I would also like to thank , , , , , and 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:

(5.24, 5.32, 5.33, 14),
(5.54),
(5.26),
(5.14),
(5.21, 5.26, 12.2, 12.3),
(5.44, 5.50, 5.51),
(5.7, 5.8, 5.9, 5.55),
(5.2, 5.3, 5.9, 5.27, 5.47, 6.1 - 6.3, 8.3, 7),
(5.12, 5.27, 8.4),
(5.6, 10),
(5.7, 5.12, 5.35, 5.39, 5.45),
(5.40),
(5.1, 5.20, 5.22, 5.25, 5.38, 7.3, 8.2, 8.4, 8.6, 9.2, 13.1),
(7.2, 7.6),
(5.15, 5.27, 5.28, 9.1, 8.5),
(5.24),
(5.15),
(5.11),
(5.30),
(5.48),
(5.36, 5, 15, 9.1, 11.1),
(5.4, 5.5, 5.23, 8.1, 8.3, 8.5, 12.1),
(5.14),
(5.47, PM-Dir review),
(5.43, 5.16),
(5.48, 5.52), and
.

I would like to thank for the SECDIR review, for the Gen-ART review, and for the IANA review. Thanks to for Area Director review suggestions. Thanks to , , , , and the entire IESG for their reviews.

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170 West Tasman Drive San Jose CA 95134 United States of America snandaku@cisco.com