Internet Engineering Task Force (IETF) R. Even
Internet-Draft Huawei Technologies
Request for Comments: 8849
Category: Standards Track J. Lennox
Expires: August 31, 2017 Vidyo
February 27, 2017
ISSN: 2070-1721 8x8 / Jitsi
Mapping RTP streams Streams to CLUE Controlling Multiple Streams for Telepresence
(CLUE) Media Captures
This document describes how the Real Time transport Real-time Transport Protocol (RTP) is
used in the context of the CLUE protocol (ControLling mUltiple
streams Controlling Multiple Streams for tElepresence).
Telepresence (CLUE) protocol. It also describes the mechanisms and
recommended practice for mapping RTP media streams streams, as defined in the
Session Description Protocol (SDP) (SDP), to CLUE Media Captures and
defines a new RTP header extension (CaptureId). (CaptureID).
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. RTP topologies Topologies for CLUE . . . . . . . . . . . . . . . . . . . 3
4. Mapping CLUE Capture Encodings to RTP streams . . . . . . . . 4 Streams
5. MCC Constituent CaptureID definition . . . . . . . . . . . . 5 Definition
5.1. RTCP CaptureID SDES Item . . . . . . . . . . . . . . . . 5
5.2. RTP Header Extension . . . . . . . . . . . . . . . . . . 6
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Communication Security . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
Telepresence systems can send and receive multiple media streams.
The CLUE framework [I-D.ietf-clue-framework] Framework [RFC8845] defines Media Captures
(MC) (MCs) as a source
of Media, from one or more Capture Devices. A Media Capture may also
be constructed from other Media streams. A middle
box middlebox can express
conceptual Media Captures that it constructs from Media streams it
receives. A Multiple Content Capture (MCC) is a special Media
Capture composed of multiple Media Captures.
SIP Offer/Answer [RFC3264] uses SDP [RFC4566] to describe the
media streams. streams [RFC3550]. Each RTP stream has a unique
Synchronization Source (SSRC) within its RTP session. The content of
the RTP stream is created by an encoder in the endpoint. This may be
an original content from a camera or a content created by an
intermediary device like an MCU (Multipoint a Multipoint Control Unit). Unit (MCU).
This document makes recommendations for the CLUE architecture about
how RTP and RTCP RTP Control Protocol (RTCP) streams should be encoded and transmitted,
transmitted and how their relation to CLUE Media Captures should be
communicated. The proposed solution supports multiple RTP topologies
With regards to the media (audio, video video, and timed text), systems
that support CLUE use RTP for the media, SDP for codec and media
transport negotiation (CLUE individual encodings) encodings), and the CLUE
protocol for Media Capture description and selection. In order to
associate the media in the different protocols protocols, there are three mapping
mappings that need to be specified:
1. CLUE individual encodings to SDP
2. RTP streams to SDP (this is not a CLUE specific CLUE-specific mapping)
3. RTP streams to MC to map the received RTP steam stream to the current
MC in the MCC.
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 RFC2119[RFC2119]
BCP 14 [RFC2119] [RFC8174] when, and
indicate requirement levels for RTP processing only when, they appear in compliant CLUE
The definitions all
capitals, as shown here.
Definitions from the CLUE framework document
[I-D.ietf-clue-framework] section Framework (see Section 3 of [RFC8845]) are
used by this document as well.
3. RTP topologies Topologies for CLUE
The typical RTP topologies used by CLUE Telepresence telepresence systems specify
different behaviors for RTP and RTCP distribution. A number of RTP
topologies are described in [RFC7667]. For CLUE telepresence, the
relevant topologies include Point-to-Point, as well as Media-Mixing
mixers, Media- Switching mixers,
Mixers, Media-Switching Mixers, and Selective Forwarding Middleboxs. Middleboxes.
In the Point-to-Point topology, one peer communicates directly with a
single peer over unicast. There can be one or more RTP sessions,
each sent on a separate 5-tuple, and having that have a separate SSRC space,
with each RTP session carrying multiple RTP streams identified by
their SSRC. All SSRCs are recognized by the peers based on the
information in the RTCP Source description (SDES) report that
includes the CNAME Canonical Name (CNAME) and SSRC of the sent RTP streams.
There are different Point-to-Point use cases as specified in the CLUE
use case [RFC7205]. In some cases, a CLUE session which, that, at a high-level, high
level, is point-to-point may nonetheless have an RTP stream which that is
best described by one of the mixer topologies. For example, a CLUE
endpoint can produce composite or switched captures for use by a
receiving system with fewer displays than the sender has cameras.
The Media Capture may be described using an MCC.
For the Media Mixer media mixer topology [RFC7667], the peers communicate only
with the mixer. The mixer provides mixed or composited media
streams, using its own SSRC for the sent streams. If needed by the
CLUE endpoint, the conference roster information including conference
participants, endpoints, media media, and media-id (SSRC) can be determined
using the conference event package [RFC4575] element.
Media-Switching Mixers and Selective Forwarding Middleboxes behave as
described in [RFC7667] [RFC7667].
4. Mapping CLUE Capture Encodings to RTP streams Streams
The different topologies described in Section 3 create different SSRC
distribution models and RTP stream multiplexing points.
Most video conferencing systems today can separate multiple RTP
sources by placing them into RTP sessions using the SDP description;
the video conferencing application can also have some knowledge about
the purpose of each RTP session. For example, video conferencing
applications that have a primary video source and a slides video
source can send each media source in a separate RTP session with a
content attribute [RFC4796] [RFC4796], enabling different application behavior
for each received RTP media source. Demultiplexing is
straightforward because each media capture is sent as a single RTP
stream, with each RTP stream being sent in a separate RTP session, on
a distinct UDP 5-tuple. This will also be true for mapping the RTP
streams to Media Captures Encodings Capture Encodings, if each Media Capture Encodings Encoding uses a
separate RTP session, session and the consumer can identify it based on the
receiving RTP port. In this case, SDP only needs to label the RTP
session with an identifier that can be used to identify the Media
Capture in the CLUE description. The SDP label attribute serves as
Each Capture Encoding MUST be sent as a separate RTP stream. CLUE
endpoints MUST support sending each such RTP stream in a separate RTP
session signalled signaled by an SDP m= "m=" line. They MAY also support sending
some or all of the RTP streams in a single RTP session, using the
mechanism described in [I-D.ietf-mmusic-sdp-bundle-negotiation] [RFC8843] to relate RTP streams to SDP m= "m="
MCCs bring another mapping issue, in that an MCC represents multiple
Media Captures that can be sent as part of this the MCC if configured by
the consumer. When receiving an RTP stream which that is mapped to the
MCC, the consumer needs to know which original MC it is in order to
get the MC parameters from the advertisement. If a consumer
requested a MCC, the original MC does not have a capture encoding, Capture Encoding, so
it cannot be associated with an m-line "m=" line using a label as described
CLUE signaling [I-D.ietf-clue-signaling]. This "CLUE Signaling" [RFC8848]. It is important, for example, to get
correct scaling information for the original MC, which may be
different for the various MCs that are contributing to the MCC.
5. MCC Constituent CaptureID definition Definition
For a an MCC which that can represent multiple switched MCs MCs, there is a need
to know which MC is represented in the current RTP stream at any
given time. This requires a mapping from the SSRC of the RTP stream
conveying a particular MCC to the constituent MC. In order to
address this mapping mapping, this document defines an RTP header extension
and SDES item that includes the captureID of the original MC,
allowing the consumer to use the MC's original source MC's attributes like
the spatial information.
This mapping temporarily associates the SSRC of the RTP stream
conveying a particular MCC with the captureID of the single original
MC that is currently switched into the MCC. This mapping cannot be
used for the a composed case where more than one original MC is composed
into the MCC simultaneously.
If there is only one MC in the MCC MCC, then the media provider MUST send
the captureID of the current constituent MC in the RTP Header
Extension and as a an RTCP CaptureID SDES item. When the media
provider switches the MC it sends within an MCC, it MUST send the
captureID value for the MC that just switched into the MCC in an RTP
Header Extension and as a an RTCP CaptureID SDES item as specified in [RFC7941]
If there is more than one MC composed into the MCC MCC, then the media
provider MUST NOT send any of the MCs' captureIDs using this
mechanism. However, if an MCC is sending contributing source Contributing Source (CSRC)
information in the RTP header for a composed capture, it MAY send the
captureID values in the RTCP SDES packets giving source information
for the SSRC values sent as contributing sources (CSRCs). CSRCs.
If the media provider sends the captureID of a single MC switched
into an MCC, then later sends one composed stream of multiple MCs in
the same MCC, it MUST send the special value "-", a single dash single-dash
character, as the captureID RTP Header Extension and RTCP CaptureID
SDES item. The single dash single-dash character indicates there is no
applicable value for the MCC constituent CaptureID. The media
consumer interprets this as meaning that any previous CaptureID value
associated with this SSRC no longer applies. As
[I-D.ietf-clue-data-model-schema] [RFC8846] defines
the captureID syntax as "xs:ID", the single dash single-dash character is not a
legal captureID value, so there is no possibility of confusing it
with an actual captureID.
5.1. RTCP CaptureID SDES Item
This document specifies a new RTCP SDES item.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| CaptId=TBA CaptId=14 | length | CaptureID |
| .... |
Note to the RFC Editor: Please replace TBA with the value assigned by
This CaptureID is a variable-length UTF-8 string corresponding either to
either a CaptureID negotiated in the CLUE protocol, protocol or the single
This SDES item MUST be sent in an SDES packet within a compound RTCP
packet unless support for Reduced-size Reduced-Size RTCP has been negotiated as
specified in RFC 5506 [RFC5506], in which case it can be sent as an
SDES packet in a non-compound noncompound RTCP packet.
5.2. RTP Header Extension
The CaptureID is also carried in an RTP header extension [RFC5285], [RFC8285],
using the mechanism defined in [RFC7941].
Support is negotiated within SDP using the URN "urn:ietf:params:rtp-
The CaptureID is sent in a an RTP Header Extension because for switched
captures, receivers need to know which original MC corresponds to the
media being sent for an MCC, in order to correctly apply geometric
adjustments to the received media.
As discussed in [RFC7941], there is no need to send the CaptId Header
Extension with all RTP packets. Senders MAY choose to send it only
when a new MC is sent. If such a mode is being used, the header
extension SHOULD be sent in the first few RTP packets to reduce the
risk of losing it due to packet loss. See [RFC7941] for more
discussion of this. further
In this partial advertisement advertisement, the Media Provider advertises a
composed capture VC7 made of a big picture representing the current
speaker (VC3) and two picture-in-picture boxes representing the
previous speakers (the previous one -VC5- -- VC5 -- and the oldest one -VC6). --
<ns2:description lang="en">big picture of the current
speaker pips about previous speakers</ns2:description>
In this case case, the media provider will send capture IDs VC3, VC5 VC5, or
VC6 as an RTP header extension and RTCP SDES message for the RTP
stream associated with the MC.
Note that this is part of the full advertisement message example from
the CLUE data model[I-D.ietf-clue-data-model-schema] model example [RFC8846] and is not a valid xml XML
7. Communication Security
CLUE endpoints MUST support RTP/SAVPF profile profiles and SRTP the Secure Real-
time Transport Protocol (SRTP) [RFC3711]. CLUE endpoints MUST
support DTLS [RFC6347] and DTLS-SRTP [RFC5763] [RFC5764] for SRTP
All media channels SHOULD be secure via SRTP and the RTP/SAVPF
profile unless the RTP media and its associated RTCP are secure by
other means (see [RFC7201] and [RFC7202]).
All CLUE implementations MUST implement DTLS 1.0, 1.0 with the
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA and with the the P-256
curve [FIPS186]. The DTLS-SRTP protection profile
SRTP_AES128_CM_HMAC_SHA1_80 MUST be supported for SRTP.Encrypted SRTP. Encrypted
SRTP Header extensions [RFC6904] MUST be supported.
Implementations SHOULD implement DTLS 1.2 with the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite.
Implementations MUST favor cipher suites which that support PFS Perfect Forward
Secrecy (PFS) over non- PFS cipher suites and SHOULD favor AEAD
Authenticated Encryption with Associated Data (AEAD) over non-AEAD
NULL Protection profiles MUST NOT be used for RTP or RTCP.
CLUE endpoint endpoints MUST generate short-term persistent RTCP CNAMES, CNAMEs, as
specified in [RFC7022], and thus can't be used for long term long-term tracking
of the users.
The authors would like to thanks Allyn Romanow and Paul Witty for
contributing text to this work. Magnus Westerlund helped drafting
the security section.
9. IANA Considerations
This document defines a new extension URI in the RTP "RTP SDES Compact
Header Extensions Extensions" subregistry of the Real-Time "Real-Time Transport Protocol
(RTP) Parameters Parameters" registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:sdes:CaptId
Description: CLUE CaptId
Contact: email@example.com Roni Even <firstname.lastname@example.org>
Reference: RFC XXXX 8849
The IANA is requested to register has registered one new RTCP SDES items in the "RTCP SDES
Item Types" registry, as follows:
| Value | Abbrev | Name | Reference
| 14 | CCID | CLUE CaptId [RFCXXXX]
Note to the RFC Editor: Please replace RFCXXXX with this | RFC number.
10. 8849 |
9. Security Considerations
The security considerations of the RTP specification, the RTP/SAVPF
profile, and the various RTP/RTCP extensions and RTP payload formats
that form the complete protocol suite described in this memo apply.
It is not believed that there are any no new security considerations
resulting from the combination of these various protocol extensions.
The Extended "Extended Secure RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/SAVPF)" document [RFC5124] (RTP/SAVPF)
provides the handling of fundamental issues by offering
confidentiality, integrity integrity, and partial source authentication. A mandatory to implement
mandatory-to-implement and use media security solution is created by
combining this secured RTP profile and DTLS-SRTP keying [RFC5764] as
defined in the communication security section of this memo Section 7
RTCP packets convey a Canonical Name (CNAME) CNAME identifier that is used to associate RTP
packet streams that need to be synchronised synchronized across related RTP
sessions. Inappropriate choice of CNAME values can be a privacy
concern, since long-term persistent CNAME identifiers can be used to
track users across multiple calls. The communication security
section of this memo Section 7 (Section 7) mandates the generation of short-
term persistent RTCP CNAMES, CNAMEs, as specified in [RFC7022] [RFC7022], so they can't
be used for long term long-term tracking of the users.
Some potential denial of service denial-of-service attacks exist if the RTCP reporting
interval is configured to an inappropriate value. This could be done
by configuring the RTCP bandwidth fraction to an excessively large or
small value using the SDP "b=RR:" or "b=RS:" lines [RFC3556], or some
similar mechanism, or by choosing an excessively large or small value
for the RTP/AVPF minimal receiver report interval (if using SDP, this
is the "a=rtcp-fb:... trr-int" parameter) [RFC4585] [RFC4585]. The risks are
1. the The RTCP bandwidth could be configured to make the regular
reporting interval so large that effective congestion control
cannot be maintained, potentially leading to denial of service
due to congestion caused by the media traffic;
2. the The RTCP interval could be configured to a very small value,
causing endpoints to generate high rate high-rate RTCP traffic, which
leading leads to denial of service due to the non-congestion non-congestion-
controlled RTCP traffic; and
3. RTCP parameters could be configured differently for each
endpoint, with some of the endpoints using a large reporting
interval and some using a smaller interval, leading to denial of
service due to premature participant timeouts timeouts, which are due to
mismatched timeout periods which that are based on the reporting
interval (this is a particular concern if endpoints use a small
but non-zero value for the RTP/AVPF minimal receiver report
interval (trr-int) [RFC4585], as discussed in [I-D.ietf-avtcore-rtp-multi-stream]). [RFC8108]).
Premature participant timeout can be avoided by using the fixed (non-
reduced) minimum interval when calculating the participant timeout
[RFC8108]. To address the other concerns, endpoints SHOULD ignore
parameters that configure the RTCP reporting interval to be
significantly longer than the default five
second five-second interval specified
in [RFC3550] (unless the media data rate is so low that the longer
reporting interval roughly corresponds to 5% of the media data rate), rate)
or that configure the RTCP reporting interval small enough that the
RTCP bandwidth would exceed the media bandwidth.
The guidelines in [RFC6562] apply when using variable bit rate (VBR)
audio codecs such as Opus.
The use of the encryption
Encryption of the header extensions are is RECOMMENDED, unless there are
known reasons, like RTP middleboxes performing voice
activity based voice-activity-based
source selection or third party third-party monitoring that will greatly benefit
from the information, and this has been expressed using API or signalling.
signaling. If further evidence are is produced to show that information
leakage is significant from audio level indications, then the use of
encryption needs to be mandated at that time.
In multi-party communication scenarios using RTP Middleboxes; this middleboxes, the
middleboxes are REQUIRED, by this protocol, to not weaken the
sessions' security. The middlebox SHOULD maintain the confidentiality, integrity
maintain integrity, and perform source authentication. The middlebox
MAY perform checks that prevents prevent any endpoint participating in a
conference to impersonate another. Some additional security
considerations regarding multi-party topologies can be found in
The CaptureID is created as part of the CLUE protocol. The CaptId
SDES item is used to convey the same CaptureID value in the SDES
item. When sending the SDES item item, the security consideration considerations
specified in the security section Section 6 of [RFC7941] and in the communication security
section of this memo (see Section 7 7) are applicable. Note that since
the CaptureID is carried also carried in CLUE protocol
messages messages, it is
RECOMMENDED that this SDES item use at least similar protection
profiles as the CLUE protocol messages carried in the CLUE data
10.1. Normative References
Presta, R. and S. Romano, "An XML Schema for the CLUE data
model", draft-ietf-clue-data-model-schema-17 (work in
progress), August 2016.
Duckworth, M., Pepperell, A., and S. Wenger, "Framework
for Telepresence Multi-Streams", draft-ietf-clue-
framework-25 (work in progress), January 2016.
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-36 (work in progress), October 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <http://www.rfc-editor.org/info/rfc5763>. <https://www.rfc-editor.org/info/rfc5763>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>. <https://www.rfc-editor.org/info/rfc6347>.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
[RFC7941] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP
Header Extension for the RTP Control Protocol (RTCP)
Source Description Items", RFC 7941, DOI 10.17487/RFC7941,
August 2016, <http://www.rfc-editor.org/info/rfc7941>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8843] Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", RFC 8843,
DOI 10.17487/RFC8843, July 2020,
[RFC8845] Duckworth, M., Ed., Pepperell, A., and S. Wenger,
"Framework for Telepresence Multi-Streams", RFC 8845,
DOI 10.17487/RFC8845, July 2020,
[RFC8846] Presta, R. and S P. Romano, "An XML Schema for the
Controlling Multiple Streams for Telepresence (CLUE) Data
Model", DOI 10.17487/RFC8846, RFC 8846, July 2020,
10.2. Informative References
[FIPS186] National Institute of Standards and Technology, Technology (NIST),
"Digital Signature Standard", FIPS Standard (DSS)", FIPS, PUB 186-4,
DOI 10.6028/NIST.FIPS.186-4, July 2013.
Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-11 (work in progress),
Kyzivat, P., Xiao, L., Groves, C., and R. Hansen, "CLUE
Signaling", draft-ietf-clue-signaling-10 (work in
progress), January 2017. 2013,
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>. <https://www.rfc-editor.org/info/rfc3550>.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth",
RFC 3556, DOI 10.17487/RFC3556, July 2003,
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>. <https://www.rfc-editor.org/info/rfc4566>.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, DOI 10.17487/RFC4575, August
2006, <http://www.rfc-editor.org/info/rfc4575>. <https://www.rfc-editor.org/info/rfc4575>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
[RFC4796] Hautakorpi, J. and G. Camarillo, "The Session Description
Protocol (SDP) Content Attribute", RFC 4796,
DOI 10.17487/RFC4796, February 2007,
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>. <https://www.rfc-editor.org/info/rfc5124>.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
2009, <http://www.rfc-editor.org/info/rfc5506>. <https://www.rfc-editor.org/info/rfc5506>.
[RFC6562] Perkins, C. and JM. Valin, "Guidelines for the Use of
Variable Bit Rate Audio with Secure RTP", RFC 6562,
DOI 10.17487/RFC6562, March 2012,
[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022,
September 2013, <http://www.rfc-editor.org/info/rfc7022>. <https://www.rfc-editor.org/info/rfc7022>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
[RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
2014, <http://www.rfc-editor.org/info/rfc7202>. <https://www.rfc-editor.org/info/rfc7202>.
[RFC7205] Romanow, A., Botzko, S., Duckworth, M., and R. Even, Ed.,
"Use Cases for Telepresence Multistreams", RFC 7205,
DOI 10.17487/RFC7205, April 2014,
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
[RFC8108] Lennox, J., Westerlund, M., Wu, Q., and C. Perkins,
"Sending Multiple RTP Streams in a Single RTP Session",
RFC 8108, DOI 10.17487/RFC8108, March 2017,
[RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General
Mechanism for RTP Header Extensions", RFC 8285,
DOI 10.17487/RFC8285, October 2017,
[RFC8848] Hanton, R., Kyzivat, P., Xiao, L., and C. Groves, "Session
Signaling for Controlling Multiple Streams for
Telepresence (CLUE)", RFC 8848, DOI 10.17487/RFC8848, July
The authors would like to thank Allyn Romanow and Paul Witty for
contributing text to this work. Magnus Westerlund helped draft the
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