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rfc:rfc7728

Internet Engineering Task Force (IETF) B. Burman Request for Comments: 7728 A. Akram Updates: 5104 Ericsson Category: Standards Track R. Even ISSN: 2070-1721 Huawei Technologies

                                                         M. Westerlund
                                                              Ericsson
                                                         February 2016
                    RTP Stream Pause and Resume

Abstract

 With the increased popularity of real-time multimedia applications,
 it is desirable to provide good control of resource usage, and users
 also demand more control over communication sessions.  This document
 describes how a receiver in a multimedia conversation can pause and
 resume incoming data from a sender by sending real-time feedback
 messages when using the Real-time Transport Protocol (RTP) for real-
 time data transport.  This document extends the Codec Control Message
 (CCM) RTP Control Protocol (RTCP) feedback package by explicitly
 allowing and describing specific use of existing CCMs and adding a
 group of new real-time feedback messages used to pause and resume RTP
 data streams.  This document updates RFC 5104.

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 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7728.

Burman, et al. Standards Track [Page 1] RFC 7728 RTP Stream Pause February 2016

Copyright Notice

 Copyright (c) 2016 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
 (http://trustee.ietf.org/license-info) 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Burman, et al. Standards Track [Page 2] RFC 7728 RTP Stream Pause February 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
 2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.1.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   5
   2.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.3.  Requirements Language . . . . . . . . . . . . . . . . . .   7
 3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.1.  Point to Point  . . . . . . . . . . . . . . . . . . . . .   8
   3.2.  RTP Mixer to Media Sender . . . . . . . . . . . . . . . .   9
   3.3.  RTP Mixer to Media Sender in Point to Multipoint  . . . .  10
   3.4.  Media Receiver to RTP Mixer . . . . . . . . . . . . . . .  11
   3.5.  Media Receiver to Media Sender across RTP Mixer . . . . .  11
 4.  Design Considerations . . . . . . . . . . . . . . . . . . . .  12
   4.1.  Real-Time Nature  . . . . . . . . . . . . . . . . . . . .  12
   4.2.  Message Direction . . . . . . . . . . . . . . . . . . . .  12
   4.3.  Apply to Individual Sources . . . . . . . . . . . . . . .  12
   4.4.  Consensus . . . . . . . . . . . . . . . . . . . . . . . .  13
   4.5.  Message Acknowledgments . . . . . . . . . . . . . . . . .  13
   4.6.  Request Retransmission  . . . . . . . . . . . . . . . . .  14
   4.7.  Sequence Numbering  . . . . . . . . . . . . . . . . . . .  14
   4.8.  Relation to Other Solutions . . . . . . . . . . . . . . .  14
 5.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .  15
   5.1.  Expressing Capability . . . . . . . . . . . . . . . . . .  16
   5.2.  PauseID . . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.3.  Requesting to Pause . . . . . . . . . . . . . . . . . . .  17
   5.4.  Media Sender Pausing  . . . . . . . . . . . . . . . . . .  18
   5.5.  Requesting to Resume  . . . . . . . . . . . . . . . . . .  19
   5.6.  TMMBR/TMMBN Considerations  . . . . . . . . . . . . . . .  20
 6.  Participant States  . . . . . . . . . . . . . . . . . . . . .  22
   6.1.  Playing State . . . . . . . . . . . . . . . . . . . . . .  22
   6.2.  Pausing State . . . . . . . . . . . . . . . . . . . . . .  22
   6.3.  Paused State  . . . . . . . . . . . . . . . . . . . . . .  23
     6.3.1.  RTCP BYE Message  . . . . . . . . . . . . . . . . . .  24
     6.3.2.  SSRC Time-Out . . . . . . . . . . . . . . . . . . . .  24
   6.4.  Local Paused State  . . . . . . . . . . . . . . . . . . .  24
 7.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .  26
 8.  Message Details . . . . . . . . . . . . . . . . . . . . . . .  28
   8.1.  PAUSE . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   8.2.  PAUSED  . . . . . . . . . . . . . . . . . . . . . . . . .  30
   8.3.  RESUME  . . . . . . . . . . . . . . . . . . . . . . . . .  31
   8.4.  REFUSED . . . . . . . . . . . . . . . . . . . . . . . . .  32
   8.5.  Transmission Rules  . . . . . . . . . . . . . . . . . . .  32
 9.  Signaling . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   9.1.  Offer/Answer Use  . . . . . . . . . . . . . . . . . . . .  37
   9.2.  Declarative Use . . . . . . . . . . . . . . . . . . . . .  39

Burman, et al. Standards Track [Page 3] RFC 7728 RTP Stream Pause February 2016

 10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  39
   10.1.  Offer/Answer . . . . . . . . . . . . . . . . . . . . . .  40
   10.2.  Point-to-Point Session . . . . . . . . . . . . . . . . .  41
   10.3.  Point to Multipoint Using Mixer  . . . . . . . . . . . .  45
   10.4.  Point to Multipoint Using Translator . . . . . . . . . .  47
 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  50
 12. Security Considerations . . . . . . . . . . . . . . . . . . .  50
 13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
   13.1.  Normative References . . . . . . . . . . . . . . . . . .  52
   13.2.  Informative References . . . . . . . . . . . . . . . . .  53
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  54
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  54
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  55

1. Introduction

 As real-time communication attracts more people, more applications
 are created; multimedia conversation applications is one example.
 Multimedia conversation further exists in many forms, for example,
 peer-to-peer chat application and multiparty video conferencing
 controlled by central media nodes, such as RTP Mixers.
 Multimedia conferencing may involve many participants; each has its
 own preferences for the communication session, not only at the start
 but also during the session.  This document describes several
 scenarios in multimedia communication where a conferencing node or
 participant chooses to temporarily pause an incoming RTP [RFC3550]
 stream and later resume it when needed.  The receiver does not need
 to terminate or inactivate the RTP session and start all over again
 by negotiating the session parameters, for example, using SIP
 [RFC3261] with the Session Description Protocol (SDP) [RFC4566]
 offer/answer [RFC3264].
 Centralized nodes, like RTP Mixers or Multipoint Control Units (MCUs)
 that use either logic based on voice activity, other measurements, or
 user input could reduce the resources consumed in both the sender and
 the network by temporarily pausing the RTP streams that aren't
 required by the RTP Mixer.  If the number of conference participants
 are greater than what the conference logic has chosen to present
 simultaneously to receiving participants, some participant RTP
 streams sent to the RTP Mixer may not need to be forwarded to any
 other participant.  Those RTP streams could then be temporarily
 paused.  This becomes especially useful when the media sources are
 provided in multiple encoding versions (Simulcast) [SDP-SIMULCAST] or
 with Multi-Session Transmission (MST) of scalable encoding such as
 Scalable Video Coding (SVC) [RFC6190].  There may be some of the

Burman, et al. Standards Track [Page 4] RFC 7728 RTP Stream Pause February 2016

 defined encodings or a combination of scalable layers that are not
 used or cannot be used all of the time.  As an example, a centralized
 node may choose to pause such unused RTP streams without being
 explicitly requested to do so, maybe due to temporarily limited
 network or processing resources.  It may then also send an explicit
 indication that the streams are paused.
 As the set of RTP streams required at any given point in time is
 highly dynamic in such scenarios, using the out-of-band signaling
 channel for pausing, and even more importantly resuming, an RTP
 stream is difficult due to the performance requirements.  Instead,
 the pause and resume signaling should be in the media plane and go
 directly between the affected nodes.  When using RTP [RFC3550] for
 media transport, using "Extended RTP Profile for Real-time Transport
 Control Protocol (RTCP)-Based Feedback (RTP/AVPF)" [RFC4585] appears
 appropriate.  No currently existing RTCP feedback message explicitly
 supports pausing and resuming an incoming RTP stream.  As this
 affects the generation of packets and may even allow the encoding
 process to be paused, the functionality appears to match Codec
 Control Messages (CCMs) in the RTP Audio-Visual Profile with Feedback
 (AVPF) [RFC5104].  This document defines the solution as a CCM
 extension.
 The Temporary Maximum Media Bitrate Request (TMMBR) message of CCM is
 used by video conferencing systems for flow control.  It is desirable
 to be able to use that method with a bitrate value of zero for pause,
 whenever possible.  This specification updates RFC 5104 by adding the
 new pause and resume semantics to the TMMBR and Temporary Maximum
 Media Bitrate Notification (TMMBN) messages.

2. Definitions

2.1. Abbreviations

 AVPF:     Audio-Visual Profile with Feedback (RFC 4585)
 CCM:      Codec Control Message (RFC 5104)
 CNAME:    Canonical Name (RTCP Source Description)
 CSRC:     Contributing Source (RTP)
 FCI:      Feedback Control Information (AVPF)
 FIR:      Full Intra Refresh (CCM)
 FMT:      Feedback Message Type (AVPF)

Burman, et al. Standards Track [Page 5] RFC 7728 RTP Stream Pause February 2016

 MCU:      Multipoint Control Unit
 MTU:      Maximum Transfer Unit
 PT:       Payload Type (RTP)
 RTP:      Real-time Transport Protocol (RFC 3550)
 RTCP:     RTP Control Protocol (RFC 3550)
 RTCP RR:  RTCP Receiver Report
 RTCP SR:  RTCP Sender Report
 SDP:      Session Description Protocol (RFC 4566)
 SIP:      Session Initiation Protocol (RFC 3261)
 SSRC:     Synchronization Source (RTP)
 SVC:      Scalable Video Coding
 TMMBR:    Temporary Maximum Media Bitrate Request (CCM)
 TMMBN:    Temporary Maximum Media Bitrate Notification (CCM)
 UA:       User Agent (SIP)
 UDP:      User Datagram Protocol (RFC 768)

2.2. Terminology

 In addition to the following, the definitions from RTP [RFC3550],
 AVPF [RFC4585], CCM [RFC5104], and RTP Taxonomy [RFC7656] also apply
 in this document.
 Feedback Messages:  CCM [RFC5104] categorized different RTCP feedback
    messages into four types: Request, Command, Indication, and
    Notification.  This document places the PAUSE and RESUME messages
    into the Request category, PAUSED as an Indication, and REFUSED as
    a Notification.
    PAUSE:    Request from an RTP stream receiver to pause a stream
    RESUME:   Request from an RTP stream receiver to resume a paused
              stream

Burman, et al. Standards Track [Page 6] RFC 7728 RTP Stream Pause February 2016

    PAUSED:   Indication from an RTP stream sender that a stream is
              paused
    REFUSED:  Notification from an RTP stream sender that a PAUSE or
              RESUME request will not be honored
 Mixer:  The intermediate RTP node that receives an RTP stream from
    different endpoints, combines them to make one RTP stream, and
    forwards them to destinations, in the sense described for Topo-
    Mixer in "RTP Topologies" [RFC7667].
 Participant:  A member that is part of an RTP session, acting as the
    receiver, sender, or both.
 Paused sender:  An RTP stream sender that has stopped its
    transmission, i.e., no other participant receives its RTP
    transmission, based on having received either a PAUSE request,
    defined in this specification, or a local decision.
 Pausing receiver:  An RTP stream receiver that sends a PAUSE request,
    defined in this specification, to another participant(s).
 Stream:  Used as a short term for RTP stream, unless otherwise noted.
 Stream receiver:  Short for RTP stream receiver; the RTP entity
    responsible for receiving an RTP stream, usually a Media
    Depacketizer.
 Stream sender:  Short for RTP stream sender; the RTP entity
    responsible for creating an RTP stream, usually a Media
    Packetizer.

2.3. Requirements Language

 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 RFC
 2119 [RFC2119].

Burman, et al. Standards Track [Page 7] RFC 7728 RTP Stream Pause February 2016

3. Use Cases

 This section discusses the main use cases for RTP stream pause and
 resume.
 The RTCWEB WG's use case and requirements document [RFC7478] defines
 the following API requirements in Appendix A, which is also used by
 the W3C WebRTC WG:
 A8  The web API must provide means for the web application to mute/
     unmute a stream or stream component(s).  When a stream is sent to
     a peer, mute status must be preserved in the stream received by
     the peer.
 A9  The web API must provide means for the web application to cease
     the sending of a stream to a peer.
 This document provides means to optimize transport usage by stopping
 the sending of muted streams and starting the sending of streams
 again when unmuted.  Here, it is assumed that "mute" above can be
 taken to apply also to media other than audio.  At the time of
 publication for this specification, the RTCWEB WG did not specify any
 pause/resume functionality.

3.1. Point to Point

 This is the most basic use case with an RTP session containing two
 endpoints.  Each endpoint sends one or more streams.
                          +---+         +---+
                          | A |<------->| B |
                          +---+         +---+
                       Figure 1: Point to Point
 The usage of RTP stream pause in this use case is to temporarily halt
 delivery of streams that the sender provides but the receiver does
 not currently use.  This can, for example, be due to minimized
 applications where the video stream is not actually shown on any
 display, or it is not used in any other way, such as being recorded.
 In this case, since there is only a single receiver of the stream,
 pausing or resuming a stream does not impact anyone else other than
 the sender and the single receiver of that stream.

Burman, et al. Standards Track [Page 8] RFC 7728 RTP Stream Pause February 2016

3.2. RTP Mixer to Media Sender

 One of the most commonly used topologies in centralized conferencing
 is based on the RTP Mixer [RFC7667].  The main reason for this is
 that it provides a very consistent view of the RTP session towards
 each participant.  That is accomplished through the Mixer originating
 its own streams, identified by distinct SSRC values, and any RTP
 streams sent to the participants will be sent using those SSRC
 values.  If the Mixer wants to identify the underlying media sources
 for its conceptual streams, it can identify them using CSRC.  The
 stream the Mixer provides can be an actual mix of multiple media
 sources, but it might also be switching received streams as described
 in Sections 3.6 - 3.8 of "RTP Topologies" [RFC7667].
                  +---+      +-----------+      +---+
                  | A |<---->|           |<---->| B |
                  +---+      |           |      +---+
                             |   Mixer   |
                  +---+      |           |      +---+
                  | C |<---->|           |<---->| D |
                  +---+      +-----------+      +---+
                  Figure 2: RTP Mixer in Unicast Only
 Which streams from clients B, C, and D that are delivered to a given
 receiver, A, can depend on several things:
 o  The RTP Mixer's own logic and measurements, such as voice activity
    on the incoming audio streams.
 o  The number of sent media sources exceed what is reasonable to
    present simultaneously at any given receiver.
 o  A human controlling the conference that determines how the media
    should be mixed.  This would be more common in lecture or similar
    applications where regular listeners may be prevented from
    breaking into the session unless approved by the moderator.
 o  The streams may also be part of a Simulcast [SDP-SIMULCAST] or
    scalable encoded (for Multi-Session Transmission) [RFC6190], thus
    providing multiple versions that can be delivered by the RTP
    stream sender.
 These examples indicate that there are numerous reasons why a
 particular stream would not currently be in use but must be available
 for use at very short notice if any dynamic event occurs that causes
 a different stream selection to be done in the Mixer.

Burman, et al. Standards Track [Page 9] RFC 7728 RTP Stream Pause February 2016

 Because of this, it would be highly beneficial if the Mixer could
 request the RTP stream sender to pause a particular stream.  The
 Mixer also needs to be able to request the RTP stream sender to
 resume delivery with minimal delay.
 In some cases, especially when the Mixer sends multiple RTP streams
 per receiving client, there may be situations that make it desirable
 for the Mixer to pause some of its sent RTP streams, even without
 being explicitly asked to do so by the receiving client.  Such
 situations can, for example, be caused by a temporary lack of
 available Mixer network or processing resources.  An RTP stream
 receiver that no longer receives an RTP stream could interpret this
 as an error condition and try to take action to re-establish the RTP
 stream.  Such action would likely be undesirable if the RTP stream
 was in fact deliberately paused by the Mixer.  Undesirable RTP stream
 receiver actions could be avoided if the Mixer is able to explicitly
 indicate that an RTP stream is deliberately paused.
 Just as for point to point (Section 3.1), there is only a single
 receiver of the stream, the RTP Mixer, and pausing or resuming a
 stream does not affect anyone else other than the sender and single
 receiver of that stream.

3.3. RTP Mixer to Media Sender in Point to Multipoint

 This use case is similar to the previous section; however, the RTP
 Mixer is involved in three domains that need to be separated: the
 Multicast Network (including participants A and C), participant B,
 and participant D.  The difference from above is that A and C share a
 multicast domain, which is depicted below.
                      +-----+
           +---+     /       \     +-----------+      +---+
           | A |<---/         \    |           |<---->| B |
           +---+   /   Multi-  \   |           |      +---+
                  +    cast     +->|   Mixer   |
           +---+   \  Network  /   |           |      +---+
           | C |<---\         /    |           |<---->| D |
           +---+     \       /     +-----------+      +---+
                      +-----+
              Figure 3: RTP Mixer in Point to Multipoint
 If the RTP Mixer pauses a stream from A, it will not only pause the
 stream towards itself but will also stop the stream from arriving to
 C, which C is heavily impacted by, might not approve of, and should
 thus have a say on.

Burman, et al. Standards Track [Page 10] RFC 7728 RTP Stream Pause February 2016

 If the Mixer resumes a paused stream from A, it will be resumed also
 towards C.  In this case, if C is not interested, it can simply
 ignore the stream and is not impacted as much as above.
 In this use case, there are several receivers of a stream, and the
 Mixer must take special care so as not to pause a stream that is
 still wanted by some receivers.

3.4. Media Receiver to RTP Mixer

 In this use case, the direction of the request to pause is the
 opposite compared to the two previous use cases.  An endpoint in
 Figure 2 could potentially request to pause the delivery of a given
 stream.  Possible reasons include those in the point-to-point case
 (Section 3.1) above.
 When the RTP Mixer is only connected to individual unicast paths, the
 use case and any considerations are identical to the point-to-point
 use case.
 However, when the endpoint requesting stream pause is connected to
 the RTP Mixer through a multicast network, such as A or C in
 Figure 3, the use case instead becomes identical to the one in
 Section 3.3, only with reverse direction of the streams and pause/
 resume requests.

3.5. Media Receiver to Media Sender across RTP Mixer

 An endpoint, like A in Figure 2, could potentially request to pause
 the delivery of a given stream, like one of B's, over any of the
 SSRCs used by the Mixer by sending a pause request for the CSRC
 identifying the stream.  However, the authors are of the opinion that
 this is not a suitable solution for several reasons:
 1.  The Mixer might not include CSRC in its stream indications.
 2.  An endpoint cannot rely on the CSRC to correctly identify the
     stream to be paused when the delivered media is some type of mix.
     A more elaborate stream identification solution is needed to
     support this in the general case.
 3.  The endpoint cannot determine if a given stream is still needed
     by the RTP Mixer to deliver to another session participant.
 Due to the above reasons, we exclude this use case from further
 consideration.

Burman, et al. Standards Track [Page 11] RFC 7728 RTP Stream Pause February 2016

4. Design Considerations

 This section describes the requirements that this specification needs
 to meet.

4.1. Real-Time Nature

 The first section (Section 1) of this specification describes some
 possible reasons why a receiver may pause an RTP sender.  Pausing and
 resuming is time dependent, i.e., a receiver may choose to pause an
 RTP stream for a certain duration, after which the receiver may want
 the sender to resume.  This time dependency means that the messages
 related to pause and resume must be transmitted to the sender in a
 timely fashion in order for them to be purposeful.  The pause
 operation is arguably not as time critical as the resume operation,
 since it mainly provides a reduction of resource usage.  Timely
 handling of the resume operation is, however, likely to directly
 impact the end-user's perceived quality experience, since it affects
 the availability of media that the user expects to receive more or
 less instantly.  It may also be highly desirable for a receiver to
 quickly learn that an RTP stream is intentionally paused on the RTP
 sender's own behalf.

4.2. Message Direction

 It is the responsibility of an RTP stream receiver that wants to
 pause or resume a stream from the sender(s) to transmit PAUSE and
 RESUME messages.  An RTP stream sender that wants to pause itself can
 often simply do it, but sometimes this will adversely affect the
 receiver and an explicit indication that the RTP stream is paused may
 then help.  Any indication that an RTP stream is paused is the
 responsibility of the RTP stream sender and may in some cases not
 even be needed by the stream receiver.

4.3. Apply to Individual Sources

 The PAUSE and RESUME messages apply to single RTP streams identified
 by their SSRC, which means the receiver targets the sender's SSRC in
 the PAUSE and RESUME requests.  If a paused sender starts sending
 with a new SSRC, the receivers will need to send a new PAUSE request
 in order to pause it.  PAUSED indications refer to a single one of
 the sender's own paused SSRC.

Burman, et al. Standards Track [Page 12] RFC 7728 RTP Stream Pause February 2016

4.4. Consensus

 An RTP stream sender should not pause an SSRC that some receiver
 still wishes to receive.
 The reason is that in RTP topologies where the stream is shared
 between multiple receivers, a single receiver on that shared network
 must not single-handedly cause the stream to be paused without
 letting all other receivers voice their opinions on whether or not
 the stream should be paused.  Such shared networks can, for example,
 be multicast, a mesh with a joint RTP session, or a transport
 Translator-based network.  A consequence of this is that a newly
 joining receiver first needs to learn the existence of paused streams
 and secondly should be able to resume any paused stream.  A newly
 joining receiver can, for example, be detected through an RTCP
 Receiver Report containing both a new SSRC and a CNAME that does not
 already occur in the session.  Any single receiver wanting to resume
 a stream should also cause it to be resumed.  An important exception
 to this is when the RTP stream sender is aware of conditions that
 make it desirable or even necessary to pause the RTP stream on its
 own behalf, without being explicitly asked to do so.  Such local
 consideration in the RTP sender takes precedence over RTP receiver
 wishes to receive the stream.

4.5. Message Acknowledgments

 RTP and RTCP does not guarantee reliable data transmission.  It uses
 whatever assurance the lower-layer transport protocol can provide.
 However, this is commonly UDP that provides no reliability
 guarantees.  Thus, it is possible that a PAUSE and/or RESUME message
 transmitted from an RTP endpoint does not reach its destination,
 i.e., the targeted RTP stream sender.  When PAUSE or RESUME reaches
 the RTP stream sender and is effective, i.e., an active RTP stream
 sender pauses or a resuming RTP stream sender has media data to
 transmit, it is immediately seen from the arrival or non-arrival of
 RTP packets for that RTP stream.  Thus, no explicit acknowledgments
 are required in this case.
 In some cases, when a PAUSE or RESUME message reaches the RTP stream
 sender, it will not be able to pause or resume the stream due to some
 local consideration, for example, lack of data to transmit.  In this
 error condition, a negative acknowledgment may be needed to avoid
 unnecessary retransmission of requests (Section 4.6).

Burman, et al. Standards Track [Page 13] RFC 7728 RTP Stream Pause February 2016

4.6. Request Retransmission

 When the stream is not affected as expected by a PAUSE or RESUME
 request, the request may have been lost and the sender of the request
 will need to retransmit it.  The retransmission should take the
 round-trip time into account, and will also need to take the normal
 RTCP bandwidth and timing rules applicable to the RTP session into
 account, when scheduling retransmission of feedback.
 When it comes to resume requests or unsolicited paused indications
 that are more time critical, the best performance may be achieved by
 repeating the message as often as possible until a sufficient number
 have been sent to reach a high probability of message delivery or at
 an explicit indication that the message was delivered.  For resume
 requests, such explicit indication can be delivery of the RTP stream
 being requested to be resumed.

4.7. Sequence Numbering

 A PAUSE request message will need to have a sequence number to
 separate retransmissions from new requests.  A retransmission keeps
 the sequence number unchanged, while it is incremented every time a
 new PAUSE request is transmitted that is not a retransmission of a
 previous request.
 Since RESUME always takes precedence over PAUSE and is even allowed
 to avoid pausing a stream, there is a need to keep strict ordering of
 PAUSE and RESUME.  Thus, RESUME needs to share sequence number space
 with PAUSE and implicitly reference which PAUSE it refers to.  For
 the same reasons, the explicit PAUSED indication also needs to share
 sequence number space with PAUSE and RESUME.

4.8. Relation to Other Solutions

 A performance comparison between SIP/SDP and RTCP signaling
 technologies was made and included in draft versions of this
 specification.  Using SIP and SDP to carry pause and resume
 information means that they will need to traverse the entire
 signaling path to reach the signaling destination (either the remote
 endpoint or the entity controlling the RTP Mixer) across any
 signaling proxies that potentially also have to process the SDP
 content to determine if they are expected to act on it.  The amount
 of bandwidth required for a signaling solution based on SIP/SDP is in
 the order of at least 10 times more than an RTCP-based solution.
 Especially for a UA sitting on mobile wireless access, this will risk
 introducing delays that are too long (Section 4.1) to provide a good
 user experience, and the bandwidth cost may also be considered
 infeasible compared to an RTCP-based solution.  RTCP data sent

Burman, et al. Standards Track [Page 14] RFC 7728 RTP Stream Pause February 2016

 through the media path, which is likely shorter (contains fewer
 intermediate nodes) than the signaling path, may have to traverse a
 few intermediate nodes anyway.  The amount of processing and
 buffering required in intermediate nodes to forward those RTCP
 messages is, however, believed to be significantly less than for
 intermediate nodes in the signaling path.  Based on those
 considerations, RTCP is chosen as the signaling protocol for the
 pause and resume functionality.

5. Solution Overview

 The proposed solution implements pause and resume functionality based
 on sending AVPF RTCP feedback messages from any RTP session
 participant that wants to pause or resume a stream targeted at the
 stream sender, as identified by the sender SSRC.
 This solution reuses CCM TMMBR and TMMBN [RFC5104] to the extent
 possible and defines a small set of new RTCP feedback messages where
 new semantics is needed.
 A single feedback message specification is used to implement the new
 messages.  The message consists of a number of Feedback Control
 Information (FCI) blocks, where each block can be a PAUSE request, a
 RESUME request, a PAUSED indication, a REFUSED notification, or an
 extension to this specification.  This structure allows a single
 feedback message to handle pause functionality on a number of
 streams.
 The PAUSED functionality is also defined in such a way that it can be
 used as a standalone by the RTP stream sender to indicate a local
 decision to pause, and it can inform any receiver of the fact that
 halting media delivery is deliberate and which RTP packet was the
 last transmitted.
 Special considerations that apply when using TMMBR/TMMBN for pause
 and resume purposes are described in Section 5.6.  This specification
 applies to both the new messages defined herein as well as their
 TMMBR/TMMBN counterparts, except when explicitly stated otherwise.
 An obvious exception is any reference to the message parameters that
 are only available in the messages defined here.  For example, any
 reference to PAUSE in the text below is equally applicable to
 TMMBR 0, and any reference to PAUSED is equally applicable to TMMBN
 0.  Therefore, and for brevity, TMMBR/TMMBN will not be mentioned in
 the text, unless there is specific reason to do so.
 This section is intended to be explanatory and therefore
 intentionally contains no mandatory statements.  Such statements can
 instead be found in other parts of this specification.

Burman, et al. Standards Track [Page 15] RFC 7728 RTP Stream Pause February 2016

5.1. Expressing Capability

 An endpoint can use an extension to CCM SDP signaling to declare
 capability to understand the messages defined in this specification.
 Capability to understand only a subset of messages is possible, to
 support partial implementation, which is specifically believed to be
 feasible for the 'RTP Mixer to Media Sender' use case (Section 3.2).
 In that use case, only the RTP Mixer has capability to request the
 media sender to pause or resume.  Consequently, in that same use
 case, only the media sender has capability to pause and resume its
 sent streams based on requests from the RTP Mixer.  Allowing for
 partial implementation of this specification is not believed to
 hamper interoperability, as long as the subsets are well defined and
 describe a consistent functionality, including a description of how a
 more capable implementation must perform fallback.
 For the case when TMMBR/TMMBN are used for pause and resume purposes,
 it is possible to explicitly express joint support for TMMBR and
 TMMBN, but not for TMMBN only.

5.2. PauseID

 All messages defined in this specification (Section 8) contain a
 PauseID, satisfying the design consideration on sequence numbering
 (Section 4.7).  This PauseID is scoped by and thus a property of the
 targeted RTP stream (SSRC) and is not only a sequence number for
 individual messages.  Instead, it numbers an entire "pause and resume
 operation" for the RTP stream, typically keeping PauseID constant for
 multiple, related messages.  The PauseID value used during such
 operation is called the current PauseID.  A new "pause and resume
 operation" is defined to start when the RTP stream sender resumes the
 RTP stream after it was being paused.  The current PauseID is then
 incremented by one in modulo arithmetic.  In the subsequent
 descriptions below, it is sometimes necessary to refer to PauseID
 values that were already used as the current PauseID, which is
 denoted as the past PauseID.  It should be noted that since PauseID
 uses modulo arithmetic, a past PauseID may have a larger value than
 the current PauseID.  Since PauseID uses modulo arithmetic, it is
 also useful to define what PauseID values are considered "past" to
 clearly separate it from what could be considered "future" PauseID
 values.  Half of the entire PauseID value range is chosen to
 represent a past PauseID, while a quarter of the PauseID value range
 is chosen to represent future values.  The remaining quarter of the
 PauseID value range is intentionally left undefined in that respect.

Burman, et al. Standards Track [Page 16] RFC 7728 RTP Stream Pause February 2016

5.3. Requesting to Pause

 An RTP stream receiver can choose to send a PAUSE request at any
 time, subject to AVPF timing rules.
 The PAUSE request contains the current PauseID (Section 5.2).
 When a non-paused RTP stream sender receives the PAUSE request, it
 continues to send the RTP stream while waiting for some time to allow
 other RTP stream receivers in the same RTP session that saw this
 PAUSE request to disapprove by sending a RESUME (Section 5.5) for the
 same stream and with the same current PauseID as in the PAUSE being
 disapproved.  If such a disapproving RESUME arrives at the RTP stream
 sender during the hold-off period before the stream is paused, the
 pause is not performed.  In point-to-point configurations, the hold-
 off period may be set to zero.  Using a hold-off period of zero is
 also appropriate when using TMMBR 0 and is in line with the semantics
 for that message.
 If the RTP stream sender receives further PAUSE requests with the
 current PauseID while waiting as described above, those additional
 requests are ignored.
 If the PAUSE request is lost before it reaches the RTP stream sender,
 it will be discovered by the RTP stream receiver because it continues
 to receive the RTP stream.  It will also not see any PAUSED
 indication (Section 5.4) for the stream.  The same condition can be
 caused by the RTP stream sender having received a disapproving RESUME
 from stream receiver A for a PAUSE request sent by stream sender B,
 except that the PAUSE sender (B) did not receive the RESUME (from A)
 and may instead think that the PAUSE was lost.  In both cases, a
 PAUSE request can be retransmitted using the same current PauseID.
 If using TMMBR 0, the request MAY be retransmitted when the requester
 fails to receive a TMMBN 0 confirmation.
 If the pending stream pause is aborted due to a disapproving RESUME,
 the pause and resume operation for that PauseID is concluded, the
 current PauseID is updated, and any new PAUSE must therefore use the
 new current PauseID to be effective.
 An RTP stream sender receiving a PAUSE not using the current PauseID
 informs the RTP stream receiver sending the ineffective PAUSE of this
 condition by sending a REFUSED notification that contains the current
 PauseID value.
 A situation where an ineffective PauseID is chosen can appear when a
 new RTP stream receiver joins a session and wants to PAUSE a stream
 but does not yet know the current PauseID to use.  The REFUSED

Burman, et al. Standards Track [Page 17] RFC 7728 RTP Stream Pause February 2016

 notification will then provide sufficient information to create a
 valid PAUSE.  The required extra signaling round trip is not
 considered harmful, since it is assumed that pausing a stream is not
 time critical (Section 4.1).
 There may be local considerations making it impossible or infeasible
 to pause the stream, and the RTP stream sender can then respond with
 a REFUSED.  In this case, if the used current PauseID would otherwise
 have been effective, REFUSED contains the same current PauseID as in
 the PAUSE request.  Note that when using TMMBR 0 as PAUSE, that
 request cannot be refused (TMMBN > 0) due to the existing restriction
 in Section 4.2.2.2 of [RFC5104] that TMMBN shall contain the current
 bounding set, and the fact that a TMMBR 0 will always be the most
 restrictive point in any bounding set, regardless of the bounding set
 overhead value.
 If the RTP stream sender receives several identical PAUSE requests
 for an RTP stream that was already responded to at least once with
 REFUSED and the condition causing REFUSED remains, those additional
 REFUSED notifications should be sent with regular RTCP timing.  A
 single REFUSED can respond to several identical PAUSE requests.

5.4. Media Sender Pausing

 An RTP stream sender can choose to pause the stream at any time.
 This can be either a result of receiving a PAUSE or based on some
 local sender consideration.  When it does, it sends a PAUSED
 indication, containing the current PauseID.  Note that the current
 PauseID in an unsolicited PAUSED (without having received a PAUSE) is
 incremented compared to a previously sent PAUSED.  It also sends the
 PAUSED indication in the next two regular RTCP reports, given that
 the pause condition is then still effective.
 There is no reply to a PAUSED indication; it is simply an explicit
 indication of the fact that an RTP stream is paused.  This can be
 helpful for the RTP stream receiver, for example, to quickly
 understand that transmission is deliberately and temporarily
 suspended and no specific corrective action is needed.
 The RTP stream sender may want to apply some local consideration to
 exactly when the RTP stream is paused, for example, completing some
 media unit or a forward error correction block, before pausing the
 stream.
 The PAUSED indication also contains information about the RTP
 extended highest sequence number when the pause became effective.
 This provides RTP stream receivers with firsthand information that
 allows them to know whether they lost any packets just before the

Burman, et al. Standards Track [Page 18] RFC 7728 RTP Stream Pause February 2016

 stream paused or when the stream is resumed again.  This allows RTP
 stream receivers to quickly and safely take into account that the
 stream is paused in, for example, retransmission or congestion
 control algorithms.
 If the RTP stream sender receives PAUSE requests with the current
 PauseID while the stream is already paused, those requests are
 ignored.
 As long as the stream is being paused, the PAUSED indication MAY be
 sent together with any regular RTCP Sender Report (SR) or Receiver
 Report (RR).  Including PAUSED in this way allows RTP stream
 receivers to join while the stream is paused and to quickly know that
 there is a paused stream, what the last sent extended RTP sequence
 number is, and what the current PauseID is, which enables them to
 construct valid PAUSE and RESUME requests at a later stage.
 When the RTP stream sender learns that a new endpoint has joined the
 RTP session, for example, by a new SSRC and a CNAME that was not
 previously seen in the RTP session, it should send PAUSED indications
 for all its paused streams at its earliest opportunity.  In addition,
 it should continue to include PAUSED indications in at least two
 regular RTCP reports.

5.5. Requesting to Resume

 An RTP stream receiver can request the RTP stream sender to resume a
 stream with a RESUME request at any time, subject to AVPF timing
 rules.  The RTP stream receiver must include the current PauseID in
 the RESUME request for it to be effective.
 A pausing RTP stream sender that receives a RESUME including the
 current PauseID resumes the stream at the earliest opportunity.
 Receiving RESUME requests for a stream that is not paused does not
 require any action and can be ignored.
 There may be local considerations at the RTP stream sender, for
 example, that the media device is not ready, making it temporarily
 impossible to resume the stream at that point in time, and the RTP
 stream sender can then respond with a REFUSED containing the current
 PauseID.  When receiving such REFUSED with a current PauseID
 identical to the one in the sent RESUME, RTP stream receivers should
 avoid sending further RESUME requests for some reasonable amount of
 time to allow the condition to clear.  An RTP stream sender having
 sent a REFUSED SHOULD resume the stream through local considerations
 (see below) when the condition that caused the REFUSED is no longer
 true.

Burman, et al. Standards Track [Page 19] RFC 7728 RTP Stream Pause February 2016

 If the RTP stream sender receives several identical RESUME requests
 for an RTP stream that was already at least once responded to with
 REFUSED and the condition causing REFUSED remains, those additional
 REFUSED notifications should be sent with regular RTCP timing.  A
 single REFUSED can respond to several identical RESUME requests.
 A pausing RTP stream sender can apply local considerations and can
 resume a paused RTP stream at any time.  If TMMBR 0 was used to pause
 the RTP stream, resumption is prevented by protocol, even if the RTP
 sender would like to resume due to local considerations.  If TMMBR/
 TMMBN signaling is used, the RTP stream is paused due to local
 considerations (Section 5.4), and the RTP stream sender thus owns the
 TMMBN bounding set, the RTP stream can be resumed due to local
 considerations.
 When resuming a paused stream, especially for media that makes use of
 temporal redundancy between samples such as video, it may not be
 appropriate to use such temporal dependency in the encoding between
 samples taken before the pause and at the time instant the stream is
 resumed.  Should such temporal dependency between media samples
 before and after the media was paused be used by the RTP stream
 sender, it requires the RTP stream receiver to have saved the samples
 from before the pause for successful continued decoding when
 resuming.  The use of this temporal dependency of media samples from
 before the pause is left up to the RTP stream sender.  If temporal
 dependency on samples from before the pause is not used when the RTP
 stream is resumed, the first encoded sample after the pause will not
 contain any temporal dependency on samples before the pause (for
 video it may be a so-called intra picture).  If temporal dependency
 on samples from before the pause is used by the RTP stream sender
 when resuming, and if the RTP stream receiver did not save any sample
 from before the pause, the RTP stream receiver can use a FIR request
 [RFC5104] to explicitly ask for a sample without temporal dependency
 (for video a so-called intra picture), even at the same time as
 sending the RESUME.

5.6. TMMBR/TMMBN Considerations

 As stated above, TMMBR/TMMBN may be used to provide pause and resume
 functionality for the point-to-point case.  If the topology is not
 point to point, TMMBR/TMMBN cannot safely be used for pause or
 resume.  This use is expected to be mainly for interworking with
 implementations that don't support the messages defined in this
 specification (Section 8) but make use of TMMBR/TMMBN to achieve a
 similar effect.

Burman, et al. Standards Track [Page 20] RFC 7728 RTP Stream Pause February 2016

 This is a brief summary of what functionality is provided when using
 TMMBR/TMMBN:
 TMMBR 0:  Corresponds to PAUSE, without the requirement for any hold-
    off period to wait for RESUME before pausing the RTP stream.
 TMMBR > 0:  Corresponds to RESUME when the RTP stream was previously
    paused with TMMBR 0.  Since there is only a single RTP stream
    receiver, there is no need for the RTP stream sender to delay
    resuming the stream until after sending TMMBN > 0 or to apply the
    hold-off period specified in [RFC5104] before increasing the
    bitrate from zero.  The bitrate value used when resuming after
    pausing with TMMBR 0 is either according to known limitations or
    based on starting a stream with the configured maximum for the
    stream or session, for example, given by "b=" line in SDP.
 TMMBN 0:  Corresponds to PAUSED when the RTP stream was paused with
    TMMBR 0 but may, just as PAUSED, also be used unsolicited.  An
    unsolicited RTP stream pause based on local sender considerations
    uses the RTP stream's own SSRC as the TMMBR restriction owner in
    the TMMBN message bounding set.  It also corresponds to a REFUSED
    notification when a stream is requested to be resumed with
    TMMBR > 0, thus resulting in the stream sender becoming the owner
    of the bounding set in the TMMBN message.
 TMMBN > 0:  Cannot be used as a REFUSED notification when a stream is
    requested to be paused with TMMBR 0, for reasons stated in
    Section 5.3.

Burman, et al. Standards Track [Page 21] RFC 7728 RTP Stream Pause February 2016

6. Participant States

 This document introduces three new states for a stream in an RTP
 sender, according to the figure and subsections below.  Any
 references to PAUSE, PAUSED, RESUME, and REFUSED in this section
 SHALL be taken to apply to the extent possible also when TMMBR/TMMBN
 are used (Section 5.6) for this functionality.
       +------------------------------------------------------+
       |                     Received RESUME                  |
       v                                                      |
  +---------+ Received PAUSE  +---------+ Hold-off period +--------+
  | Playing |---------------->| Pausing |---------------->| Paused |
  |         |<----------------|         |                 |        |
  +---------+ Received RESUME +---------+                 +--------+
    ^     |                        | PAUSE decision           |
    |     |                        v                          |
    |     |  PAUSE decision   +---------+    PAUSE decision   |
    |     +------------------>| Local   |<--------------------+
    +-------------------------| Paused  |
            RESUME decision   +---------+
                 Figure 4: RTP Pause States in Sender

6.1. Playing State

 This state is not new but is the normal media sending state from
 [RFC3550].  When entering the state, the current PauseID MUST be
 incremented by one in modulo arithmetic.  The RTP sequence number for
 the first packet sent after a pause SHALL be incremented by one
 compared to the highest RTP sequence number sent before the pause.
 The first RTP timestamp for the first packet sent after a pause
 SHOULD be set according to capture times at the source, meaning the
 RTP timestamp difference compared to before the pause reflects the
 time the RTP stream was paused.

6.2. Pausing State

 In this state, the RTP stream sender has received at least one PAUSE
 message for the stream in question.  The RTP stream sender SHALL wait
 during a hold-off period for the possible reception of RESUME
 messages for the RTP stream being paused before actually pausing RTP
 stream transmission.  The hold-off period to wait SHALL be long
 enough to allow another RTP stream receiver to respond to the PAUSE
 with a RESUME, if it determines that it would not like to see the
 stream paused.  This hold-off period is determined by the formula:
    2 * RTT + T_dither_max,

Burman, et al. Standards Track [Page 22] RFC 7728 RTP Stream Pause February 2016

 where RTT is the longest round trip known to the RTP stream sender
 and T_dither_max is defined in Section 3.4 of [RFC4585].  The hold-
 off period MAY be set to 0 by some signaling (Section 9) means when
 it can be determined that there is only a single receiver, for
 example, in point to point or some unicast situations.
 If the RTP stream sender has set the hold-off period to 0 and
 receives information that it was an incorrect decision and that there
 are in fact several receivers of the stream, it MUST change the hold-
 off period to be based on the above formula instead.
 An RTP stream sender SHOULD use the following criteria to determine
 if there is only a single receiver, unless it has explicit and more
 reliable information:
 o  Observing only a single CNAME across all received SSRCs (CNAMEs
    for received CSRCs are insignificant), or
 o  If RTCP reporting groups [MULTI-STREAM-OPT] is used, observing
    only a single, endpoint external RTCP reporting group.

6.3. Paused State

 An RTP stream is in paused state when the sender pauses its
 transmission after receiving at least one PAUSE message and the hold-
 off period has passed without receiving any RESUME message for that
 stream.  Pausing transmission SHOULD only be done when reaching an
 appropriate place to pause in the stream, like a media boundary that
 avoids a media receiver to trigger repair or concealment actions.
 When entering the state, the RTP stream sender SHALL send a PAUSED
 indication to all known RTP stream receivers, and SHALL also repeat
 PAUSED in the next two regular RTCP reports, as long as it is then
 still in paused state.
 Pausing an RTP stream MUST NOT affect the sending of RTP keepalive
 [RFC6263][RFC5245] applicable to that RTP stream.
 The following subsections discuss some potential issues when an RTP
 sender goes into paused state.  These conditions are also valid if an
 RTP Translator is used in the communication.  When an RTP Mixer
 implementing this specification is involved between the participants
 (which forwards the stream by marking the RTP data with its own
 SSRC), it SHALL be a responsibility of the Mixer to control sending
 PAUSE and RESUME requests to the sender.  The below conditions also
 apply to the sender and receiver parts of the RTP Mixer,
 respectively.

Burman, et al. Standards Track [Page 23] RFC 7728 RTP Stream Pause February 2016

6.3.1. RTCP BYE Message

 When a participant leaves the RTP session, it sends an RTCP BYE
 message.  In addition to the semantics described in Sections 6.3.4
 and 6.3.7 of RTP [RFC3550], the following two conditions MUST also be
 considered when an RTP participant sends an RTCP BYE message:
 o  If a paused sender sends an RTCP BYE message, receivers observing
    this SHALL NOT send further PAUSE or RESUME requests to it.
 o  Since a sender pauses its transmission on receiving the PAUSE
    requests from any receiver in a session, the sender MUST keep
    record of which receiver caused the RTP stream to pause.  If that
    receiver sends an RTCP BYE message observed by the sender, the
    sender SHALL resume the RTP stream.  No receivers that were in the
    RTP session when the stream was paused objected that the stream
    was paused, but if there were so far undetected receivers added to
    the session during pause, those may not have learned about the
    existence of the paused stream because either there was no PAUSED
    sent for the paused RTP stream or those receivers did not support
    PAUSED.  Resuming the stream when the pausing party leaves the RTP
    session allows those potentially undetected receivers to learn
    that the stream exists.

6.3.2. SSRC Time-Out

 Section 6.3.5 in RTP [RFC3550] describes the SSRC time-out of an RTP
 participant.  Every RTP participant maintains a sender and receiver
 list in a session.  If a participant does not get any RTP or RTCP
 packets from some other participant for the last five RTCP reporting
 intervals, it removes that participant from the receiver list.  Any
 streams that were paused by that removed participant SSRC SHALL be
 resumed.

6.4. Local Paused State

 This state can be entered at any time, based on local decision from
 the RTP stream sender.  Pausing transmission SHOULD only be done when
 reaching an appropriate place to pause in the stream, like a media
 boundary that avoids a media receiver to trigger repair or
 concealment actions.
 As with paused state (Section 6.3), the RTP stream sender SHALL send
 a PAUSED indication to all known RTP stream receivers, when entering
 the state, unless the stream was already in paused state
 (Section 6.3).  Such PAUSED indication SHALL be repeated a sufficient

Burman, et al. Standards Track [Page 24] RFC 7728 RTP Stream Pause February 2016

 number of times to reach a high probability that the message is
 correctly delivered, stopping such repetition whenever leaving the
 state.
 When using TMMBN 0 as a PAUSED indication and when already in paused
 state, the actions when entering local paused state depends on the
 bounding set overhead value in the received TMMBR 0 that caused the
 paused state and the bounding set overhead value used in (the RTP
 stream sender's own) TMMBN 0:
 TMMBN 0 overhead <= TMMBR 0 overhead:  The RTP stream sender SHALL
    NOT send any new TMMBN 0 replacing that active (more restrictive)
    bounding set, even if entering local paused state.
 TMMBN 0 overhead > TMMBR 0 overhead:  The RTP stream sender SHALL
    send TMMBN 0 with itself in the TMMBN bounding set when entering
    local paused state.
 The case above, when using TMMBN 0 as a PAUSED indication, being in
 local paused state, and having received a TMMBR 0 with a bounding set
 overhead value greater than the value the RTP stream sender would
 itself use in a TMMBN 0, requires further consideration and is for
 clarity henceforth referred to as "restricted local paused state".
 As indicated in Figure 4, local paused state has higher precedence
 than paused state (Section 6.3), and RESUME messages alone cannot
 resume a paused RTP stream as long as the local decision still
 applies.  An RTP stream sender in local paused state is responsible
 for leaving the state whenever the conditions that caused the
 decision to enter the state no longer apply.
 If the RTP stream sender is in restricted local paused state, it
 cannot leave that state until the TMMBR 0 limit causing the state is
 removed by a TMMBR > 0 (RESUME).  If the RTP stream sender then needs
 to stay in local paused state due to local considerations, it MAY
 continue pausing the RTP stream by entering local paused state and
 MUST then act accordingly, including sending a TMMBN 0 with itself in
 the bounding set.
 Pausing an RTP stream MUST NOT affect the sending of RTP keepalive
 [RFC6263][RFC5245] applicable to that RTP stream.
 When leaving the local paused state, the stream state SHALL become
 Playing, regardless of whether or not there were any RTP stream
 receivers that sent PAUSE for that stream during the local paused
 state, effectively clearing the RTP stream sender's memory for that
 stream.

Burman, et al. Standards Track [Page 25] RFC 7728 RTP Stream Pause February 2016

7. Message Format

 Section 6 of AVPF [RFC4585] defines three types of low-delay RTCP
 feedback messages, i.e., transport-layer, payload-specific, and
 application-layer feedback messages.  This document defines a new
 transport-layer feedback message, which is further subtyped into
 either a PAUSE request, a RESUME request, a PAUSED indication, or a
 REFUSED notification.
 The transport-layer feedback messages are identified by having the
 RTCP payload type be RTPFB (205) as defined by AVPF [RFC4585].  This
 transport-layer feedback message, containing one or more of the
 subtyped messages, is henceforth referred to as the PAUSE-RESUME
 message.  The specific FCI format is identified by a Feedback Message
 Type (FMT) value in a common packet header for the feedback message
 defined in Section 6.1 of AVPF [RFC4585].  The PAUSE-RESUME
 transport-layer feedback message FCI is identified by FMT value = 9.
 The Common Packet Format for feedback messages defined by AVPF
 [RFC4585] is:
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|   FMT   |       PT      |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  SSRC of packet sender                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  SSRC of media source                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :            Feedback Control Information (FCI)                 :
   :                                                               :
         Figure 5: AVPF Common Feedback Message Packet Format
 For the PAUSE-RESUME message defined in this memo, the following
 interpretations of the packet fields apply:
 FMT:  The FMT value identifying the PAUSE-RESUME FCI: 9
 PT:  Payload Type = 205 (RTPFB)
 Length:  As defined by AVPF, i.e., the length of this packet in
    32-bit words minus one, including the header and any padding.

Burman, et al. Standards Track [Page 26] RFC 7728 RTP Stream Pause February 2016

 SSRC of packet sender:  The SSRC of the RTP session participant
    sending the messages in the FCI.  Note, for endpoints that have
    multiple SSRCs in an RTP session, any of its SSRCs MAY be used to
    send any of the pause message types.
 SSRC of media source:  Not used; SHALL be set to 0.  The FCI
    identifies the SSRC the message is targeted for.
 The FCI field consists of one or more PAUSE, RESUME, PAUSED, or
 REFUSED messages or any future extension.  These messages have the
 following FCI format:
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Target SSRC                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  |  Res  | Parameter Len |           PauseID             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                         Type Specific                         :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 6: Syntax of FCI Entry in the PAUSE and RESUME Message
 The FCI fields have the following definitions:
 Target SSRC (32 bits):  For a PAUSE-RESUME message, this value is the
    SSRC that the request is intended for.  For PAUSED, it MUST be the
    SSRC being paused.  If pausing is the result of a PAUSE request,
    the value in PAUSED is effectively the same as Target SSRC in a
    related PAUSE request.  For REFUSED, it MUST be the Target SSRC of
    the PAUSE or RESUME request that cannot change state.  A CSRC MUST
    NOT be used as a target as the interpretation of such a request is
    unclear.
 Type (4 bits):  The pause feedback type.  The values defined in this
    specification are as follows:
    0: PAUSE request message.
    1: RESUME request message.
    2: PAUSED indication message.
    3: REFUSED notification message.

Burman, et al. Standards Track [Page 27] RFC 7728 RTP Stream Pause February 2016

    4-15:  Reserved for future use.  FCI fields with these Type values
       SHALL be ignored on reception by receivers and MUST NOT be used
       by senders implementing this specification.
 Res: (4 bits):  Type Specific reserved.  It SHALL be ignored by
    receivers implementing this specification and MUST be set to 0 by
    senders implementing this specification.
 Parameter Len (8 bits):  Length of the Type Specific field in 32-bit
    words.  MAY be 0.
 PauseID (16 bits):  Message sequence identification, as described in
    Section 5.2.  SHALL be incremented by one modulo 2^16 for each new
    PAUSE message, unless the message is retransmitted.  The initial
    value SHOULD be 0.  The PauseID is scoped by the Target SSRC,
    meaning that PAUSE, RESUME, and PAUSED messages therefore share
    the same PauseID space for a specific Target SSRC.
 Type Specific (variable):  Defined per pause feedback type.  MAY be
    empty.  A receiver implementing this specification MUST be able to
    skip and ignore any unknown Type Specific data, even for Type
    values defined in this specification.

8. Message Details

 This section contains detailed explanations of each message defined
 in this specification.  All transmissions of requests and indications
 are governed by the transmission rules as defined by Section 8.5.
 Any references to PAUSE, PAUSED, RESUME, and REFUSED in this section
 SHALL be taken to apply to the extent possible and also when TMMBR/
 TMMBN are used (Section 5.6) for this functionality.  TMMBR/TMMBN MAY
 be used instead of the messages defined in this specification when
 the effective topology is point to point.  This use is expected to be
 mainly for interworking with implementations that don't support the
 messages defined in this specification but make use of TMMBR/TMMBN to
 achieve a similar effect.  If either sender or receiver learns that
 the topology is not point to point, TMMBR/TMMBN MUST NOT be used for
 pause/resume functionality.  If the messages defined in this
 specification are supported in addition to TMMBR/TMMBN by all
 involved parties, pause/resume signaling MUST use messages from this
 specification.  If the topology is not point to point and the
 messages defined in this specification are not supported, pause/
 resume functionality with TMMBR/TMMBN MUST NOT be used.
 For the scope of this specification, a past PauseID (Section 5.2) is
 defined as having a value between and including (PauseID - 2^15) MOD
 2^16 and (PauseID - 1) MOD 2^16, where "MOD" is the modulo operator.

Burman, et al. Standards Track [Page 28] RFC 7728 RTP Stream Pause February 2016

 Similarly, a future PauseID is defined as having a value between and
 including (PauseID + 1) MOD 2^16 and (PauseID + 2^14) MOD 2^16.  It
 is intentional that future PauseID is not defined as the entire range
 outside that of past PauseID.  The remaining range of PauseID is
 simply "not current".

8.1. PAUSE

 An RTP stream receiver MAY schedule PAUSE for transmission at any
 time.
 PAUSE has no defined Type Specific parameters.
 PauseID SHOULD be the current PauseID, as indicated by PAUSED
 (Section 8.2), REFUSED (Section 8.4), or implicitly determined by
 previously received PAUSE or RESUME (Section 8.3) requests.  A
 randomly chosen PauseID MAY be used if it was not possible to
 retrieve current PauseID information, in which case the PAUSE will
 either succeed or the current PauseID can be found in the returned
 REFUSED (Section 8.4).
 It can be noted that as a result of what is described in Section 6.1,
 PauseID is incremented by one, in modulo arithmetic, for each PAUSE
 request that is not a retransmission, compared to what was used in
 the last PAUSED indication sent by the media sender.  PauseID in the
 message is supposed to match current PauseID at the RTP stream
 sender.
 If an RTP stream receiver that sent a PAUSE with a certain PauseID
 for a Target SSRC receives a RESUME or a REFUSED with the same
 PauseID for the same Target SSRC, it is RECOMMENDED that it refrains
 from scheduling further PAUSE requests for some appropriate time.
 This is because the RESUME indicates that there are other receivers
 that still wish to receive the stream, and the REFUSED indicates that
 the RTP stream sender is currently not able to pause the stream.
 What is an appropriate time can vary from application to application
 and will also depend on the importance of achieving the bandwidth
 saving, but 2-5 regular RTCP intervals is expected to be appropriate.
 If the targeted RTP stream does not pause, if no PAUSED indication
 with a future PauseID compared to the one used in PAUSE is received,
 and if no REFUSED with the current or a future PauseID is received
 within 2 * RTT + T_dither_max, the PAUSE MAY be scheduled for
 retransmission, using the same current PauseID.  RTT is the observed
 round trip to the RTP stream sender, and T_dither_max is defined in
 Section 3.4 of [RFC4585].  An RTP stream receiver in a bi-directional
 RTP communication will generally have an RTT estimate to the RTP
 stream sender, e.g., from RTCP SR/RR as described in Section 6.4 of

Burman, et al. Standards Track [Page 29] RFC 7728 RTP Stream Pause February 2016

 [RFC3550].  However, RTP stream receivers that don't send any RTP
 streams will lack an RTT estimate unless they use additional
 mechanisms, such as the "Receiver Reference Time Report Block" part
 of RTCP XR [RFC3611].  RTP stream receivers that lack an RTT estimate
 to the sender SHOULD use 500 ms as the default value.
 When an RTP stream sender in playing state (Section 6.1) receives a
 PAUSE with the current PauseID, and unless local considerations
 currently make it impossible to pause the stream, it SHALL enter
 pausing state (Section 6.2) and act accordingly.
 If an RTP stream sender receives a PAUSE with the current PauseID
 while in pausing, paused (Section 6.3), or local paused (Section 6.4)
 states, the received PAUSE SHALL be ignored.

8.2. PAUSED

 The PAUSED indication, if supported, MUST be sent whenever entering
 paused state (Section 6.3) or local paused state (Section 6.4).
 PauseID in the PAUSED message MUST contain the current PauseID that
 can be included in a subsequent RESUME (Section 8.3).  For local
 paused state, this means that PauseID in the message is the current
 PauseID, just as if the RTP stream sender had sent a PAUSE to itself.
 PAUSED SHALL contain a fixed-length 32-bit parameter at the start of
 the Type Specific field with the extended RTP sequence number of the
 last RTP packet sent before the RTP stream was paused, in the same
 format as the extended highest sequence number received
 (Section 6.4.1 of [RFC3550]).
 After having entered paused or local paused state and thus having
 sent PAUSED once, PAUSED MUST also be included in (at least) the next
 two regular RTCP reports, given that the pause condition is then
 still effective.
 PAUSED indications MAY be retransmitted, subject to transmission
 rules (Section 8.5), to increase the probability that the message
 reaches the receiver in a timely fashion.  This can be especially
 important when entering local paused state.  The number of
 repetitions to use could be tuned to observed loss rate and desired
 loss probability, for example, based on RTCP reports received from
 the intended message target.
 While remaining in paused or local paused states, PAUSED MAY be
 included in all compound RTCP reports, as long as the negotiated RTCP
 bandwidth is not exceeded.

Burman, et al. Standards Track [Page 30] RFC 7728 RTP Stream Pause February 2016

 When in paused or local paused states, whenever the RTP stream sender
 learns that there are endpoints that did not previously receive the
 stream, for example, by RTCP reports with an SSRC and a CNAME that
 were not previously seen in the RTP session, it is RECOMMENDED to
 send PAUSED at the earliest opportunity and also to include it in (at
 least) the next two regular RTCP reports, given that the pause
 condition is then still effective.

8.3. RESUME

 An RTP stream receiver MAY schedule RESUME for transmission whenever
 it wishes to resume a paused stream or disapprove a stream from being
 paused.
 PauseID SHOULD be the current PauseID, as indicated by PAUSED
 (Section 8.2) or implicitly determined by previously received PAUSE
 (Section 8.1) or RESUME requests.  A randomly chosen PauseID MAY be
 used if it was not possible to retrieve current PauseID information,
 in which case the RESUME will either succeed or the current PauseID
 can be found in a returned REFUSED (Section 8.4).
 If an RTP stream receiver that sent a RESUME with a certain PauseID
 receives a REFUSED with the same PauseID, it is RECOMMENDED that it
 refrains from scheduling further RESUME requests for some appropriate
 time since the REFUSE indicates that it is currently not possible to
 resume the stream.  What is an appropriate time can vary from
 application to application and will also depend on the importance of
 resuming the stream, but 1-2 regular RTCP intervals is expected to be
 appropriate.
 RESUME requests MAY be retransmitted, subject to transmission rules
 (Section 8.5), to increase the probability that the message reaches
 the receiver in a timely fashion.  The number of repetitions to use
 could be tuned to observed loss rate and desired loss probability,
 for example, based on RTCP reports received from the intended message
 target.  Such retransmission SHOULD stop as soon as RTP packets from
 the targeted stream are received or when a REFUSED with the current
 PauseID for the targeted RTP stream is received.
 RESUME has no defined Type Specific parameters.
 When an RTP stream sender in pausing (Section 6.2), paused
 (Section 6.3), or local paused state (Section 6.4) receives a RESUME
 with the current PauseID, and unless local considerations currently
 make it impossible to resume the stream, it SHALL enter playing state
 (Section 6.1) and act accordingly.  If the RTP stream sender is
 incapable of honoring a RESUME request with the current PauseID, or
 if it receives a RESUME request with a PauseID that is not the

Burman, et al. Standards Track [Page 31] RFC 7728 RTP Stream Pause February 2016

 current PauseID while in paused or pausing state, the RTP stream
 sender SHALL schedule a REFUSED message for transmission as specified
 below.
 If an RTP stream sender in playing state receives a RESUME containing
 either the current PauseID or a past PauseID, the received RESUME
 SHALL be ignored.

8.4. REFUSED

 If an RTP stream sender receives a PAUSE (Section 8.1) or RESUME
 (Section 8.3) request containing the current PauseID, where the
 requested action cannot be fulfilled by the RTP stream sender due to
 some local consideration, it SHALL schedule transmission of a REFUSED
 notification containing the current PauseID from the rejected
 request.
 REFUSED has no defined Type Specific parameters.
 If an RTP stream sender receives a PAUSE or RESUME request with a
 PauseID that is not the current PauseID, it SHALL schedule a REFUSED
 notification containing the current PauseID, except if the RTP stream
 sender is in playing state and receives a RESUME with a past PauseID,
 in which case the RESUME SHALL be ignored.
 If several PAUSE or RESUME requests that would render identical
 REFUSED notifications are received before the scheduled REFUSED is
 sent, duplicate REFUSED notifications MUST NOT be scheduled for
 transmission.  This effectively lets a single REFUSED respond to
 several ineffective PAUSE or RESUME requests.
 An RTP stream receiver that sent a PAUSE or RESUME request and
 receives a REFUSED containing the same PauseID as in the request
 SHOULD refrain from sending an identical request for some appropriate
 time to allow the condition that caused REFUSED to clear.  For PAUSE,
 an appropriate time is suggested in Section 8.1.  For RESUME, an
 appropriate time is suggested in Section 8.3.
 An RTP stream receiver that sent a PAUSE or RESUME request and
 receives a REFUSED containing a PauseID different from the request
 MAY schedule another request using the PauseID from the REFUSED
 notification.

8.5. Transmission Rules

 The transmission of any RTCP feedback messages defined in this
 specification MUST follow the normal AVPF-defined timing rules and
 depend on the session's mode of operation.

Burman, et al. Standards Track [Page 32] RFC 7728 RTP Stream Pause February 2016

 All messages defined in this specification, as well as TMMBR/TMMBN
 used for pause/resume purposes (Section 5.6), can use either Regular,
 Early, or Immediate timings but should make a trade-off between
 timely transmission (Section 4.1) and RTCP bandwidth consumption.
 This can be achieved by taking the following into consideration:
 o  It is recommended that PAUSE use Early or Immediate timing, except
    for retransmissions where RTCP bandwidth can motivate the use of
    Regular timing.
 o  The first transmission of PAUSED for each (non-wrapped) PauseID is
    recommended to be sent with Immediate or Early timing to stop
    unnecessary repetitions of PAUSE.  It is recommended that
    subsequent transmissions of PAUSED for that PauseID use Regular
    timing to avoid excessive PAUSED RTCP bandwidth caused by multiple
    PAUSE requests.
 o  It is recommended that unsolicited PAUSED (sent when entering
    local paused state (Section 6.4)) always use Immediate or Early
    timing, until PAUSED for that PauseID is considered delivered at
    least once to all receivers of the paused RTP stream, to avoid RTP
    stream receivers that take unnecessary corrective action when the
    RTP stream is no longer received, after which it is recommended
    that PAUSE uses Regular timing (as for PAUSED triggered by PAUSE
    above).
 o  RESUME is often time critical, and it is recommended that it
    always uses Immediate or Early timing.
 o  The first transmission of REFUSED for each (non-wrapped) PauseID
    is recommended to be sent with Immediate or Early timing to stop
    unnecessary repetitions of PAUSE or RESUME.  It is recommended
    that subsequent REFUSED notifications for that PauseID use Regular
    timing to avoid excessive REFUSED RTCP bandwidth caused by
    multiple unreasonable requests.

9. Signaling

 The capability of handling messages defined in this specification MAY
 be exchanged at a higher layer such as SDP.  This document extends
 the "rtcp-fb" attribute defined in Section 4 of AVPF [RFC4585] to
 include the request for pause and resume.  This specification follows
 all the rules defined in AVPF [RFC4585] and CCM [RFC5104] for an
 "rtcp-fb" attribute relating to the payload type in a session
 description.

Burman, et al. Standards Track [Page 33] RFC 7728 RTP Stream Pause February 2016

 This specification defines a new parameter "pause" to the "ccm"
 feedback value defined in CCM [RFC5104], representing the capability
 to understand the RTCP feedback message and all of the defined FCIs
 of PAUSE, RESUME, PAUSED, and REFUSED.
    Note: When TMMBR 0 / TMMBN 0 are used to implement pause and
    resume functionality (with the restrictions described in this
    specification), signaling the "rtcp-fb" attribute with the "ccm"
    and "tmmbr" parameters is sufficient and no further signaling is
    necessary.  There is, however, no guarantee that TMMBR/TMMBN
    implementations predating this specification work exactly as
    described here when used with a bitrate value of 0.
 The "pause" parameter has two optional attributes, which are "nowait"
 and "config":
 o  "nowait" indicates that the hold-off period defined in Section 6.2
    can be set to 0, reducing the latency before the stream can be
    paused after receiving a PAUSE request.  This condition occurs
    when there will only be a single receiver per direction in the RTP
    session, for example, in point-to-point sessions.  It is also
    possible to use in scenarios using unidirectional media.  The
    conditions that allow "nowait" to be set (Section 6.2) also
    indicate that it would be possible to use CCM TMMBR/TMMBN as
    pause/resume signaling.
 o  "config" allows for partial implementation of this specification
    according to the different roles in the use-cases section
    (Section 3) and takes a value that describes what subset is
    implemented:
    1  Full implementation of this specification.  This is the default
       configuration.  A missing "config" pause attribute MUST be
       treated equivalent to providing a "config" value of 1.
    2  The implementation intends to send PAUSE and RESUME requests
       for received RTP streams and is thus also capable of receiving
       PAUSED and REFUSED.  It does not support receiving PAUSE and
       RESUME requests, but it may pause sent RTP streams due to local
       considerations and then intend to send PAUSED for them.
    3  The implementation supports receiving PAUSE and RESUME requests
       targeted for RTP streams it sends.  It will send PAUSED and
       REFUSED as needed.  The node will not send any PAUSE and RESUME
       requests but supports and desires receiving PAUSED if received
       RTP streams are paused.

Burman, et al. Standards Track [Page 34] RFC 7728 RTP Stream Pause February 2016

    4  The implementation intends to send PAUSE and RESUME requests
       for received RTP streams and is thus also capable of receiving
       PAUSED and REFUSED.  It cannot pause any RTP streams it sends,
       and thus does not support receiving PAUSE and RESUME requests,
       and it also does not support sending PAUSED indications.
    5  The implementation supports receiving PAUSE and RESUME requests
       targeted for RTP streams it sends.  It will send PAUSED and
       REFUSED as needed.  It does not support sending PAUSE and
       RESUME requests to pause received RTP streams, and it also does
       not support receiving PAUSED indications.
    6  The implementation supports sent and received RTP streams being
       paused due to local considerations and thus supports sending
       and receiving PAUSED indications.
    7  The implementation supports and desires to receive PAUSED
       indications for received RTP streams but does not pause or send
       PAUSED indications for sent RTP streams.  It does not support
       any other messages defined in this specification.
    8  The implementation supports pausing sent RTP streams and
       sending PAUSED indications for them but does not support
       receiving PAUSED indications for received RTP streams.  It does
       not support any other messages defined in this specification.
 All implementers of this specification are encouraged to include full
 support for all messages ("config=1"), but it is recognized that this
 is sometimes not meaningful for implementations operating in an
 environment where only parts of the functionality provided by this
 specification are needed.  The above defined "config" functionality
 subsets provide a trade-off between completeness and the need for
 implementation interoperability, achieving at least a level of
 functionality corresponding to what is desired by the least-capable
 party when used as specified here.  Implementing any functionality
 subsets other than those defined above is NOT RECOMMENDED.
 When signaling a "config" value other than 1, an implementation MUST
 ignore non-supported messages on reception and SHOULD omit sending
 messages not supported by the remote peer.  One example where it can
 be motivated to send messages that some receivers do not support is
 when there are multiple message receivers with different message
 support (different "config" values).  That approach avoids letting
 the least-capable receiver limit the functionality provided to
 others.  The below table summarizes per-message send and receive
 support for the different "config" pause attribute values ("X"
 indicating support and "-" indicating non-support):

Burman, et al. Standards Track [Page 35] RFC 7728 RTP Stream Pause February 2016

   +---+-----------------------------+-----------------------------+
   | # | Send                        | Receive                     |
   |   | PAUSE RESUME PAUSED REFUSED | PAUSE RESUME PAUSED REFUSED |
   +---+-----------------------------+-----------------------------+
   | 1 |   X      X      X      X    |   X      X      X      X    |
   | 2 |   X      X      X      -    |   -      -      X      X    |
   | 3 |   -      -      X      X    |   X      X      X      -    |
   | 4 |   X      X      -      -    |   -      -      X      X    |
   | 5 |   -      -      X      X    |   X      X      -      -    |
   | 6 |   -      -      X      -    |   -      -      X      -    |
   | 7 |   -      -      -      -    |   -      -      X      -    |
   | 8 |   -      -      X      -    |   -      -      -      -    |
   +---+-----------------------------+-----------------------------+
      Figure 7: Supported Messages for Different "config" Values
 In the above description of partial implementations, "config" values
 2 and 4 correspond to the RTP Mixer in the 'RTP Mixer to Media
 Sender' use case (Section 3.2), and "config" values 3 and 5
 correspond to the media sender in that same use case.  For that use
 case, it should be clear that an RTP Mixer implementing only "config"
 values 3 or 5 will not provide a working solution.  Similarly, for
 that use case, a media sender implementing only "config" values 2 or
 4 will not provide a working solution.  Both the RTP Mixer and the
 media sender will of course work when implementing the full set of
 messages, corresponding to "config=1".
 A partial implementation is not suitable for pause/resume support
 between cascaded RTP Mixers, but it would require support
 corresponding to "config=1" between such RTP Mixers.  This is because
 an RTP Mixer is then also a media sender towards the other RTP Mixer,
 requiring support for the union of "config" values 2 and 3 or
 "config" values 4 and 5, which effectively becomes "config=1".
 As can be seen from Figure 7 above, "config" values 2 and 3 differ
 from "config" values 4 and 5 only in that in the latter, the PAUSE/
 RESUME message sender (e.g., the RTP Mixer side) does not support
 local pause (Section 6.4) for any of its own streams and therefore
 also does not support sending PAUSED.
 Partial implementations that only support local pause functionality
 can declare this capability through "config" values 6-8.
 Viable fallback rules between different "config" values are described
 in Section 9.1 and Figure 9.

Burman, et al. Standards Track [Page 36] RFC 7728 RTP Stream Pause February 2016

 This is the resulting ABNF [RFC5234], extending the existing ABNF in
 Section 7.1 of CCM [RFC5104]:
 rtcp-fb-ccm-param  =/ SP "pause" *(SP pause-attr)
 pause-attr         = pause-config ; partial message support
                    / "nowait"     ; no hold-off period
                    / byte-string  ; for future extensions
 pause-config       = "config=" pause-config-value
 pause-config-value = 1*2DIGIT
 ; byte-string as defined in RFC 4566
                            Figure 8: ABNF
 An endpoint implementing this specification and using SDP to signal
 capability SHOULD indicate the new "pause" parameter with "ccm"
 signaling but MAY instead use existing "ccm tmmbr" signaling
 [RFC5104] if the limitations in functionality when using TMMBR/TMMBN
 as described in this specification (Section 5.6) are considered
 acceptable.  In that case, no partial message support is possible.
 The messages from this specification (Section 8) SHOULD NOT be used
 towards receivers that did not declare capability to receive those
 messages.
 The pause functionality can normally be expected to work
 independently of the payload type.  However, there might exist
 situations where an endpoint needs to restrict or at least configure
 the capabilities differently depending on the payload type carrying
 the media stream.  Reasons for this might relate to capabilities to
 correctly handle media boundaries and avoid any pause or resume
 operation to occur where it would leave a receiver or decoder with no
 choice than to attempt to repair or discard the media received just
 prior to or at the point of resuming.
 There MUST NOT be more than one "a=rtcp-fb" line with "pause"
 applicable to a single payload type in the SDP, unless the additional
 line uses "*" as the payload type, in which case "*" SHALL be
 interpreted as applicable to all listed payload types that do not
 have an explicit "pause" specification.  The "config" pause attribute
 MUST NOT appear more than once for each "pause" CCM parameter.  The
 "nowait" pause attribute MUST NOT appear more than once for each
 "pause" CCM parameter.

9.1. Offer/Answer Use

 An offerer implementing this specification needs to include the
 "pause" CCM parameter with a suitable configuration attribute
 ("config") in the SDP, according to what messages it intends to send
 and desires to receive in the session.

Burman, et al. Standards Track [Page 37] RFC 7728 RTP Stream Pause February 2016

 In SDP offer/answer, the "config" pause attribute and its message
 directions are interpreted based on the agent providing the SDP.  The
 offerer is described in an offer, and the answerer is described in an
 answer.
 An answerer receiving an offer with a "pause" CCM line and a "config"
 pause attribute with a certain value, describing a certain capability
 to send and receive messages, MAY change the "config" pause attribute
 value in the answer to another configuration.  The permitted answers
 are listed in the below table.
    SDP Offer "config" value | Permitted SDP Answer "config" values
    -------------------------+-------------------------------------
                 1           | 1, 2, 3, 4, 5, 6, 7, 8
                 2           | 3, 4, 5, 6, 7, 8
                 3           | 2, 4, 5, 6, 7, 8
                 4           | 5, 6, 7, 8
                 5           | 4, 6, 7, 8
                 6           | 6, 7, 8
                 7           | 8
                 8           | 7
               Figure 9: "config" Values in Offer/Answer
 An offer or answer omitting the "config" pause attribute MUST be
 interpreted as equivalent to "config=1".  Implementations of this
 specification MUST NOT use any "config" values other than those
 defined above in an offer or answer and MUST remove the "pause" CCM
 line in the answer when receiving an offer with a "config" value it
 does not understand.  In all cases, the answerer MAY also completely
 remove any "pause" CCM line to indicate that it does not understand
 or desire to use any pause functionality for the affected payload
 types.
 If the offerer believes that itself and the intended answerer are
 likely the only endpoints in the RTP session, it MAY include the
 "nowait" pause attribute on the "pause" line in the offer.  If an
 answerer receives the "nowait" pause attribute on the "pause" line in
 the SDP, and if it has information that the offerer and itself are
 not the only endpoints in the RTP session, it MUST NOT include any
 "nowait" pause attribute on its "pause" line in the SDP answer.  The
 answerer MUST NOT add "nowait" on the "pause" line in the answer
 unless it is present on the "pause" line in the offer.  If both offer
 and answer contain a "nowait" pause attribute, then the hold-off
 period is configured to 0 at both the offerer and answerer.
 Unknown pause attributes MUST be ignored in the offer and MUST then
 be omitted from the answer.

Burman, et al. Standards Track [Page 38] RFC 7728 RTP Stream Pause February 2016

 If both "pause" and "tmmbr" are present in the offer, both MAY be
 included also in the answer, in which case TMMBR/TMMBN MUST NOT be
 used for pause/resume purposes (with a bitrate value of 0), to avoid
 signaling ambiguity.

9.2. Declarative Use

 In declarative use, the SDP is used to configure the node receiving
 the SDP.  This has implications on the interpretation of the SDP
 signaling extensions defined in this specification.
 First, the "config" pause attribute and its message directions are
 interpreted based on the node receiving the SDP, and it describes the
 RECOMMENDED level of operation.  If the joining client does not
 support the indicated "config" value, some RTP session stream
 optimizations may not be possible in that some RTP streams will not
 be paused by the joining client, and/or the joining client may not be
 able to resume and receive wanted streams because they are paused.
 Second, the "nowait" pause attribute, if included, is followed as
 specified.  It is the responsibility of the declarative SDP sender to
 determine if a configured node will participate in a session that
 will be point to point, based on the usage.  For example, a
 conference client being configured for an any source multicast
 session using the Session Announcement Protocol (SAP) [RFC2974] will
 not be in a point-to-point session, thus "nowait" cannot be included.
 A Real-Time Streaming Protocol (RTSP) [RFC2326] client receiving a
 declarative SDP may very well be in a point-to-point session,
 although it is highly doubtful that an RTSP client would need to
 support this specification, considering the inherent PAUSE support in
 RTSP.
 Unknown pause attributes MUST be ignored.
 If both "pause" and "tmmbr" are present in the SDP, TMMBR/TMMBN MUST
 NOT be used for pause/resume purposes (with a bitrate value of 0) to
 avoid signaling ambiguity.

10. Examples

 The following examples show use of PAUSE and RESUME messages,
 including use of offer/answer:
 1.  Offer/Answer
 2.  Point-to-Point Session
 3.  Point to Multipoint using Mixer

Burman, et al. Standards Track [Page 39] RFC 7728 RTP Stream Pause February 2016

 4.  Point to Multipoint using Relay

10.1. Offer/Answer

 The below figures contain an example of how to show support for
 pausing and resuming the streams, as well as indicating whether or
 not the hold-off period can be set to 0.
 v=0
 o=alice 3203093520 3203093520 IN IP4 alice.example.com
 s=Pausing Media
 t=0 0
 c=IN IP4 alice.example.com
 m=audio 49170 RTP/AVPF 98 99
 a=rtpmap:98 G719/48000
 a=rtpmap:99 PCMA/8000
 a=rtcp-fb:* ccm pause nowait
         Figure 10: SDP Offer with Pause and Resume Capability
 The offerer supports all of the messages defined in this
 specification, leaving out the optional "config" pause attribute.
 The offerer also believes that it will be the sole receiver of the
 answerer's stream as well as that the answerer will be the sole
 receiver of the offerer's stream and thus includes the "nowait" pause
 attribute for the "pause" parameter.
 This is the SDP answer:
 v=0
 o=bob 293847192 293847192 IN IP4 bob.example.com
 s=-
 t=0 0
 c=IN IP4 bob.example.com
 m=audio 49202 RTP/AVPF 98
 a=rtpmap:98 G719/48000
 a=rtcp-fb:98 ccm pause config=2
        Figure 11: SDP Answer with Pause and Resume Capability
 The answerer will not allow its sent streams to be paused or resumed
 and thus restricts the answer to indicate "config=2".  It also
 supports pausing its own RTP streams due to local considerations,
 which is why "config=2" is chosen rather than "config=4".  The
 answerer somehow knows that it will not be a point-to-point RTP
 session and has therefore removed "nowait" from the "pause" line,
 meaning that the offerer must use a non-zero hold-off period when
 being requested to pause the stream.

Burman, et al. Standards Track [Page 40] RFC 7728 RTP Stream Pause February 2016

 When using TMMBR 0 / TMMBN 0 to achieve pause and resume
 functionality, there are no differences in SDP compared to CCM
 [RFC5104]; therefore, no such examples are included here.

10.2. Point-to-Point Session

 This is the most basic scenario, which involves two participants,
 each acting as a sender and/or receiver.  Any RTP data receiver sends
 PAUSE or RESUME messages to the sender, which pauses or resumes
 transmission accordingly.  The hold-off period before pausing a
 stream is 0.
         +---------------+                   +---------------+
         |  RTP Sender   |                   | RTP Receiver  |
         +---------------+                   +---------------+
                :           t1: RTP data           :
                | -------------------------------> |
                |           t2: PAUSE(3)           |
                | <------------------------------- |
                |       < RTP data paused >        |
                |           t3: PAUSED(3)          |
                | -------------------------------> |
                :       < Some time passes >       :
                |           t4: RESUME(3)          |
                | <------------------------------- |
                |           t5: RTP data           |
                | -------------------------------> |
                :       < Some time passes >       :
                |           t6: PAUSE(4)           |
                | <------------------------------- |
                |       < RTP data paused >        |
                |           t7: PAUSED(4)          |
                | -------------------------------> |
                :                                  :
        Figure 12: Pause and Resume Operation in Point to Point
 Figure 12 shows the basic pause and resume operation in a
 point-to-point scenario.  At time t1, an RTP sender sends data to a
 receiver.  At time t2, the RTP receiver requests the sender to pause
 the stream, using PauseID 3 (which it knew since before in this
 example).  The sender pauses the data and replies with a PAUSED
 containing the same PauseID.  Some time later (at time t4), the
 receiver requests the sender to resume, which resumes its
 transmission.  The next PAUSE, sent at time t6, contains an updated
 PauseID (4), with a corresponding PAUSED being sent at time t7.

Burman, et al. Standards Track [Page 41] RFC 7728 RTP Stream Pause February 2016

         +---------------+                   +---------------+
         |  RTP Sender   |                   | RTP Receiver  |
         +---------------+                   +---------------+
                :           t1: RTP data           :
                | -------------------------------> |
                |           t2: TMMBR 0            |
                | <------------------------------- |
                |       < RTP data paused >        |
                |           t3: TMMBN 0            |
                | -------------------------------> |
                :       < Some time passes >       :
                |           t4: TMMBR 150000       |
                | <------------------------------- |
                |           t5: RTP data           |
                | -------------------------------> |
                :       < Some time passes >       :
                |           t6: TMMBR 0            |
                | <------------------------------- |
                |       < RTP data paused >        |
                |           t7: TMMBN 0            |
                | -------------------------------> |
                :                                  :
          Figure 13: TMMBR Pause and Resume in Point to Point
 Figure 13 describes the same point-to-point scenario as above, but
 using TMMBR/TMMBN signaling.

Burman, et al. Standards Track [Page 42] RFC 7728 RTP Stream Pause February 2016

         +---------------+                 +----------------+
         | RTP Sender A  |                 | RTP Receiver B |
         +---------------+                 +----------------+
                :           t1: RTP data           :
                | -------------------------------> |
                |       < RTP data paused >        |
                |           t2: TMMBN {A:0}        |
                | -------------------------------> |
                :       < Some time passes >       :
                |           t3: TMMBR 0            |
                | <------------------------------- |
                |           t4: TMMBN {A:0,B:0}    |
                | -------------------------------> |
                :       < Some time passes >       :
                |           t5: TMMBN {B:0}        |
                | -------------------------------> |
                :       < Some time passes >       :
                |           t6: TMMBR 80000        |
                | <------------------------------- |
                |           t7: RTP data           |
                | -------------------------------> |
                :                                  :
               Figure 14: Unsolicited PAUSED Using TMMBN
 Figure 14 describes the case when an RTP stream sender (A) chooses to
 pause an RTP stream due to local considerations.  Both A and the RTP
 stream receiver (B) use TMMBR/TMMBN signaling for pause/resume
 purposes.  A decides to pause the RTP stream at time t2 and uses
 TMMBN 0 to signal PAUSED, including itself in the TMMBN bounding set.
 At time t3, despite the fact that the RTP stream is still paused, B
 decides that it is no longer interested in receiving the RTP stream
 and signals PAUSE by sending a TMMBR 0.  As a result of that, the
 bounding set now contains both A and B, and A sends out a new TMMBN
 reflecting that.  After a while, at time t5, the local considerations
 that caused A to pause the RTP stream no longer apply, causing it to
 remove itself from the bounding set and to send a new TMMBN
 indicating this.  At time t6, B decides that it is now interested in
 receiving the RTP stream again and signals RESUME by sending a TMMBR
 containing a bitrate value greater than 0, causing A to resume
 sending RTP data.

Burman, et al. Standards Track [Page 43] RFC 7728 RTP Stream Pause February 2016

       +---------------+                       +---------------+
       |  RTP Sender   |                       | RTP Receiver  |
       +---------------+                       +---------------+
              :           t1: RTP data                :
              | ------------------------------------> |
              |                   t2: PAUSE(7), lost  |
              |                   <---X-------------- |
              |                                       |
              |           t3: RTP data                |
              | ------------------------------------> |
              :                                       :
              |   < Time-out, still receiving data >  |
              |           t4: PAUSE(7)                |
              | <------------------------------------ |
              |          < RTP data paused >          |
              |           t5: PAUSED(7)               |
              | ------------------------------------> |
              :          < Some time passes >         :
              |                   t6: RESUME(7), lost |
              |                   <---X-------------- |
              |           t7: RESUME(7)               |
              | <------------------------------------ |
              |           t8: RTP data                |
              | ------------------------------------> |
              |           t9: RESUME(7)               |
              | <------------------------------------ |
              :                                       :
       Figure 15: Pause and Resume Operation with Messages Lost
 Figure 15 describes what happens if a PAUSE message from an RTP
 stream receiver does not reach the RTP stream sender.  After sending
 a PAUSE message, the RTP stream receiver waits for a time-out to
 detect if the RTP stream sender has paused the data transmission or
 has sent a PAUSED indication according to the rules discussed in
 Section 6.3.  As the PAUSE message is lost on the way (at time t2),
 RTP data continues to reach to the RTP stream receiver.  When the
 timer expires, the RTP stream receiver schedules a retransmission of
 the PAUSE message, which is sent at time t4.  If the PAUSE message
 now reaches the RTP stream sender, it pauses the RTP stream and
 replies with PAUSED.
 At time t6, the RTP stream receiver wishes to resume the stream again
 and sends a RESUME, which is lost.  This does not cause any severe
 effect, since there is no requirement to wait until further RESUME
 requests are sent, and another RESUME is sent already at time t7,
 which now reaches the RTP stream sender that consequently resumes the
 stream at time t8.  The time interval between t6 and t7 can vary but

Burman, et al. Standards Track [Page 44] RFC 7728 RTP Stream Pause February 2016

 may, for example, be one RTCP feedback transmission interval as
 determined by the AVPF rules.
 The RTP stream receiver did not realize that the RTP stream was
 resumed in time to stop yet another scheduled RESUME from being sent
 at time t9.  This is, however, harmless since RESUME contains a past
 PauseID and will be ignored by the RTP stream sender.  It will also
 not cause the RTP stream to be resumed even if the stream was paused
 again based on a PAUSE from some other receiver before receiving the
 RESUME, since the current PauseID was updated compared to the one in
 the stray RESUME, which contains a past PauseID and will be ignored
 by the RTP stream sender.
          +---------------+                 +---------------+
          |  RTP Sender   |                 | RTP Receiver  |
          +---------------+                 +---------------+
                 :           t1: RTP data          :
                 | ------------------------------> |
                 |           t2: PAUSE(11)         |
                 | <------------------------------ |
                 |                                 |
                 |    < Cannot pause RTP data >    |
                 |           t3: REFUSED(11)       |
                 | ------------------------------> |
                 |                                 |
                 |           t4: RTP data          |
                 | ------------------------------> |
                 :                                 :
         Figure 16: Pause Request is Refused in Point to Point
 In Figure 16, the receiver requests to pause the sender, which
 refuses to pause due to some consideration local to the sender and
 responds with a REFUSED message.

10.3. Point to Multipoint Using Mixer

 An RTP Mixer is an intermediate node connecting different transport-
 level clouds.  The Mixer receives streams from different RTP sources,
 selects or combines them based on the application's needs, and
 forwards the generated stream(s) to the destination.  The Mixer
 typically puts its own SSRC(s) in RTP data packets instead of the
 original source(s).
 The Mixer keeps track of all the streams delivered to the Mixer and
 how they are currently used.  In this example, Mixer (M) selects the
 video stream to deliver to the RTP stream receiver (R) based on the
 voice activity of the RTP stream senders (S1 and S2).  The video

Burman, et al. Standards Track [Page 45] RFC 7728 RTP Stream Pause February 2016

 stream will be delivered to R using M's SSRC and with a CSRC
 indicating the original source.
 Note that PauseID is not of any significance for the example and is
 therefore omitted in the description.
   +-----+            +-----+            +-----+            +-----+
   |  R  |            |  M  |            | S1  |            | S2  |
   +-----+            +-----+            +-----+            +-----+
      :                  :   t1:RTP(S1)     :                  :
      |   t2:RTP(M:S1)   |<-----------------|                  |
      |<-----------------|                  |                  |
      |                  |   t3:RTP(S2)     |                  |
      |                  |<------------------------------------|
      |                  |   t4: PAUSE(S2)  |                  |
      |                  |------------------------------------>|
      |                  |                  |  t5: PAUSED(S2)  |
      |                  |<------------------------------------|
      |                  |                  | <S2:No RTP to M> |
      |                  |   t6: RESUME(S2) |                  |
      |                  |------------------------------------>|
      |                  |                  |  t7: RTP to M    |
      |                  |<------------------------------------|
      |   t8:RTP(M:S2)   |                  |                  |
      |<-----------------|                  |                  |
      |                  |   t9:PAUSE(S1)   |                  |
      |                  |----------------->|                  |
      |                  |   t10:PAUSED(S1) |                  |
      |                  |<-----------------|                  |
      |                  | <S1:No RTP to M> |                  |
      :                  :                  :                  :
   Figure 17: Pause and Resume Operation for a Voice-Activated Mixer
 The session starts at t1 with S1 being the most active speaker and
 thus being selected as the single video stream to be delivered to R
 (t2) using M's SSRC but with S1 as the CSRC (indicated after the
 colon in the figure).  Then S2 joins the session at t3 and starts
 delivering an RTP stream to M.  As S2 has less voice activity then
 S1, M decides to pause S2 at t4 by sending S2 a PAUSE request.  At
 t5, S2 acknowledges with PAUSED and at the same instant stops
 delivering RTP to M.  At t6, the user at S2 starts speaking and
 becomes the most active speaker and M decides to switch the video
 stream to S2 and therefore quickly sends a RESUME request to S2.  At
 t7, S2 has received the RESUME request and acts on it by resuming RTP
 stream delivery to M.  When the RTP stream from t7 arrives at M, it
 switches this RTP stream into its SSRC (M) at t8 and changes the CSRC
 to S2.  As S1 now becomes unused, M issues a PAUSE request to S1 at

Burman, et al. Standards Track [Page 46] RFC 7728 RTP Stream Pause February 2016

 t9, which is acknowledged at t10 with PAUSED, and the RTP stream from
 S1 stops being delivered.

10.4. Point to Multipoint Using Translator

 A transport Relay in an RTP session forwards the message from one
 peer to all the others.  Unlike Mixer, the Relay does not mix the
 streams or change the SSRC of the messages or RTP media.  These
 examples are to show that the messages defined in this specification
 can be safely used also in a transport Relay case.  The parentheses
 in the figures contains (Target SSRC, PauseID) information for the
 messages defined in this specification.
        +-------------+     +-------------+     +-------------+
        |  Sender(S)  |     |    Relay    |     | Receiver(R) |
        +-------------+     +-------------+     +-------------+
               : t1: RTP(S)        :                   :
               |------------------>|                   |
               |                   | t2: RTP (S)       |
               |                   |------------------>|
               |                   | t3: PAUSE(S,3)    |
               |                   |<------------------|
               | t4:PAUSE(S,3)     |                   |
               |<------------------|                   |
               : <Sender waiting for possible RESUME>  :
               |          < RTP data paused >          |
               | t5: PAUSED(S,3)   |                   |
               |------------------>|                   |
               |                   | t6: PAUSED(S,3)   |
               |                   |------------------>|
               :                   :                   :
               |                   | t7: RESUME(S,3)   |
               |                   |<------------------|
               | t8: RESUME(S,3)   |                   |
               |<------------------|                   |
               | t9: RTP (S)       |                   |
               |------------------>|                   |
               |                   | t10: RTP (S)      |
               |                   |------------------>|
               :                   :                   :
 Figure 18: Pause and Resume Operation between Two Participants Using
                                a Relay
 Figure 18 describes how a Relay can help the receiver (R) in pausing
 and resuming the sender (S).  S sends RTP data to R through the
 Relay, which just forwards the data without modifying the SSRCs.  R
 sends a PAUSE request to S which, in this example, knows that there

Burman, et al. Standards Track [Page 47] RFC 7728 RTP Stream Pause February 2016

 may be more receivers of the stream and waits a non-zero hold-off
 period to see if there is any other receiver that wants to receive
 the data, and when no disapproving RESUME messages are received, it
 pauses itself and replies with PAUSED.  Similarly R resumes S by
 sending a RESUME request through the Relay.  Since this describes
 only a single pause and resume operation for a single RTP stream
 sender, all messages use a single PauseID; in this example, it's
 three.
   +-----+            +-----+            +-----+            +-----+
   |  S  |            | Rel |            | R1  |            | R2  |
   +-----+            +-----+            +-----+            +-----+
      : t1:RTP(S)        :                  :                  :
      |----------------->|                  |                  |
      |                  | t2:RTP(S)        |                  |
      |                  |----------------->------------------>|
      |                  | t3:PAUSE(S,7)    |                  |
      |                  |<-----------------|                  |
      | t4:PAUSE(S,7)    |                  |                  |
      |<-----------------|------------------------------------>|
      |                  |                  |   t5:RESUME(S,7) |
      |                  |<------------------------------------|
      | t6:RESUME(S,7)   |                  |                  |
      |<-----------------|----------------->|                  |
      |                  | <RTP stream continues to R1 and R2> |
      |                  |                  |   t7: PAUSE(S,8) |
      |                  |<------------------------------------|
      | t8:PAUSE(S,8)    |                  |                  |
      |<-----------------|----------------->|                  |
      :                  :                  :                  :
      | < Pauses RTP stream >               |                  |
      | t9:PAUSED(S,8)   |                  |                  |
      |----------------->|                  |                  |
      |                  | t10:PAUSED(S,8)  |                  |
      |                  |----------------->------------------>|
      :                  :                  :                  :
      |                  | t11:RESUME(S,8)  |                  |
      |                  |<-----------------|                  |
      | t12:RESUME(S,8)  |                  |                  |
      |<-----------------|------------------------------------>|
      | t13:RTP(S)       |                  |                  |
      |----------------->|                  |                  |
      |                  | t14:RTP(S)       |                  |
      |                  |----------------->------------------>|
      :                  :                  :                  :
   Figure 19: Pause and Resume Operation between One Sender and Two
                        Receivers through Relay

Burman, et al. Standards Track [Page 48] RFC 7728 RTP Stream Pause February 2016

 Figure 19 explains the pause and resume operations when a transport
 Relay (Rel) is involved between a sender (S) and two receivers (R1
 and R2) in an RTP session.  Each message exchange is represented by
 the time it happens.  At time t1, S starts sending an RTP stream to
 Rel, which forwards it to R1 and R2.  R1 and R2 receives RTP data
 from Rel at t2.  At this point, both R1 and R2 will send RTCP
 Receiver Reports to S informing that they received S's stream.
 After some time (at t3), R1 chooses to pause the stream.  On
 receiving the PAUSE request from R1 at t4, S knows that there is at
 least one receiver that may still want to receive the data and uses a
 non-zero hold-off period to wait for possible RESUME messages.  R2
 did also receive the PAUSE request at time t4 and since it still
 wants to receive the stream, it sends a RESUME for it at time t5,
 which is forwarded to sender S by Rel.  S sees the RESUME at time t6
 and continues to send data to Rel, which forwards it to both R1 and
 R2.  At t7, R2 chooses to pause the stream by sending a PAUSE request
 with an updated PauseID.  S still knows that there is more than one
 receiver (R1 and R2) that may want the stream and again waits a non-
 zero hold-off period, after which, and not having received any
 disapproving RESUME messages, it concludes that the stream must be
 paused.  S now stops sending the stream and replies with PAUSED to R1
 and R2.  When any of the receivers (R1 or R2) choose to resume the
 stream from S, in this example R1, it sends a RESUME request to S
 (also seen by R2).  S immediately resumes the stream.
 Consider also an RTP session that includes one or more receivers,
 paused sender(s), and a Relay.  Further assume that a new participant
 joins the session, which is not aware of the paused sender(s).  On
 receiving knowledge about the newly joined participant, e.g., any RTP
 traffic or RTCP report (i.e., either SR or RR) from the newly joined
 participant, the paused sender(s) immediately sends PAUSED
 indications for the paused streams since there is now a receiver in
 the session that did not pause the sender(s) and may want to receive
 the streams.  Having this information, the newly joined participant
 has the same possibility as any other participant to resume the
 paused streams.

Burman, et al. Standards Track [Page 49] RFC 7728 RTP Stream Pause February 2016

11. IANA Considerations

 Per this specification, IANA has made the following registrations:
 1.  A new value for media stream pause/resume has been registered in
     the "FMT Values for RTPFB Payload Types" registry located at the
     time of publication at: <http://www.iana.org/assignments/rtp-
     parameters>
     Value:  9
     Name:  PAUSE-RESUME
     Long Name:  Media Pause/Resume
     Reference:  RFC 7728
 2.  A new value "pause" to be registered with IANA in the "Codec
     Control Messages" registry located at the time of publication at:
     <http://www.iana.org/assignments/sdp-parameters>
     Value Name:  pause
     Long Name:  Media Pause/Resume
     Usable with:  ccm
     Reference:  RFC 7728

12. Security Considerations

 This document extends CCM [RFC5104] and defines new messages, i.e.,
 PAUSE, RESUME, PAUSED, and REFUSED.  The exchange of these new
 messages has some security implications, which need to be addressed
 by the user.
 The messages defined in this specification can have a substantial
 impact on the perceived media quality if used in a malicious way.
 First of all, there is the risk for Denial of Service (DoS) on any
 RTP session that uses the PAUSE-RESUME functionality.  By injecting
 one or more PAUSE requests into the RTP session, an attacker can
 potentially prevent any media from flowing, especially when the hold-
 off period is zero.  The injection of PAUSE messages is quite simple,
 requiring knowledge of the SSRC and the PauseID.  This information is
 visible to an on-path attacker unless RTCP messages are encrypted.
 Even off-path attacks are possible as signaling messages often carry
 the SSRC value, while the 16-bit PauseID has to be guessed or tried.
 The way of protecting the RTP session from these injections is to

Burman, et al. Standards Track [Page 50] RFC 7728 RTP Stream Pause February 2016

 perform source authentication combined with message integrity to
 prevent other than intended session participants from sending these
 messages.  The security solution should provide replay protection.
 Otherwise, if a session is long lived enough for the PauseID value to
 wrap, an attacker could replay old messages at the appropriate time
 to influence the media sender state.  There exist several different
 choices for securing RTP sessions to prevent this type of attack.
 The Secure Real-time Transport Protocol (SRTP) is the most common,
 but also other methods exist as discussed in "Options for Securing
 RTP Sessions" [RFC7201].
 Most of the methods for securing RTP, however, do not provide source
 authentication of each individual participant in a multiparty use
 case.  In case one of the session participants is malicious, it can
 wreck significant havoc within the RTP session and similarly cause a
 DoS on the RTP session from within.  That damage can also be
 attempted to be obfuscated by having the attacker impersonate other
 endpoints within the session.  These attacks can be mitigated by
 using a solution that provides true source authentication of all
 participants' RTCP packets.  However, that has other implications.
 For multiparty sessions including a middlebox, that middlebox is
 RECOMMENDED to perform checks on all forwarded RTCP packets so that
 each participant only uses its set of SSRCs to prevent the attacker
 from utilizing another participant's SSRCs.  An attacker that can
 send a PAUSE request that does not reach any participants other than
 the media sender can cause a stream to be paused without providing
 opportunity for opposition.  This is mitigated in multiparty
 topologies that ensure that requests are seen by all or most of the
 RTP session participants, enabling these participants to send a
 RESUME.  In topologies with middleboxes that consume and process
 PAUSE requests, the middlebox can also mitigate such behavior as it
 will commonly not generate or forward a PAUSE message if it knows of
 another participant having use for the media stream.
 The above text has been focused on using the PAUSE message as the
 tool for malicious impact on the RTP session.  That is because of the
 greater impact from denying users access to RTP media streams.  In
 contrast, if an attacker attempts to use RESUME in a malicious
 purpose, it will result in the media streams being delivered.
 However, such an attack basically prevents the use of the pause and
 resume functionality.  Thus, it potentially forces a reduction of the
 media quality due to limitation in available resources, like
 bandwidth that must be shared.
 The session establishment signaling is also a potential venue of
 attack, as that can be used to prevent the enabling of pause and
 resume functionality by modifying the signaling messages.  The above
 mitigation of attacks based on source authentication also requires

Burman, et al. Standards Track [Page 51] RFC 7728 RTP Stream Pause February 2016

 the signaling system to securely handle identities and assert that
 only the intended identities are allowed into the RTP session and
 provided with the relevant security contexts.

13. References

13.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            DOI 10.17487/RFC3264, June 2002,
            <http://www.rfc-editor.org/info/rfc3264>.
 [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>.
 [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>.
 [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,
            <http://www.rfc-editor.org/info/rfc4585>.
 [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
            "Codec Control Messages in the RTP Audio-Visual Profile
            with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
            February 2008, <http://www.rfc-editor.org/info/rfc5104>.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.
 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245,
            DOI 10.17487/RFC5245, April 2010,
            <http://www.rfc-editor.org/info/rfc5245>.

Burman, et al. Standards Track [Page 52] RFC 7728 RTP Stream Pause February 2016

 [RFC6263]  Marjou, X. and A. Sollaud, "Application Mechanism for
            Keeping Alive the NAT Mappings Associated with RTP / RTP
            Control Protocol (RTCP) Flows", RFC 6263,
            DOI 10.17487/RFC6263, June 2011,
            <http://www.rfc-editor.org/info/rfc6263>.

13.2. Informative References

 [MULTI-STREAM-OPT]
            Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
            "Sending Multiple Media Streams in a Single RTP Session:
            Grouping RTCP Reception Statistics and Other Feedback",
            Work in Progress, draft-ietf-avtcore-rtp-multi-stream-
            optimisation-11, December 2015.
 [RFC2326]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
            Streaming Protocol (RTSP)", RFC 2326,
            DOI 10.17487/RFC2326, April 1998,
            <http://www.rfc-editor.org/info/rfc2326>.
 [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
            Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
            October 2000, <http://www.rfc-editor.org/info/rfc2974>.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            DOI 10.17487/RFC3261, June 2002,
            <http://www.rfc-editor.org/info/rfc3261>.
 [RFC3611]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
            "RTP Control Protocol Extended Reports (RTCP XR)",
            RFC 3611, DOI 10.17487/RFC3611, November 2003,
            <http://www.rfc-editor.org/info/rfc3611>.
 [RFC6190]  Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
            "RTP Payload Format for Scalable Video Coding", RFC 6190,
            DOI 10.17487/RFC6190, May 2011,
            <http://www.rfc-editor.org/info/rfc6190>.
 [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
            Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
            <http://www.rfc-editor.org/info/rfc7201>.
 [RFC7478]  Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
            Time Communication Use Cases and Requirements", RFC 7478,
            DOI 10.17487/RFC7478, March 2015,
            <http://www.rfc-editor.org/info/rfc7478>.

Burman, et al. Standards Track [Page 53] RFC 7728 RTP Stream Pause February 2016

 [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
            B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
            for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
            DOI 10.17487/RFC7656, November 2015,
            <http://www.rfc-editor.org/info/rfc7656>.
 [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
            DOI 10.17487/RFC7667, November 2015,
            <http://www.rfc-editor.org/info/rfc7667>.
 [SDP-SIMULCAST]
            Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
            "Using Simulcast in SDP and RTP Sessions", Work in
            Progress, draft-ietf-mmusic-sdp-simulcast-04, February
            2016.

Acknowledgments

 Daniel Grondal made valuable contributions during the initial
 versions of this document.  The authors would also like to thank Emil
 Ivov, Christian Groves, David Mandelberg, Meral Shirazipour, Spencer
 Dawkins, Bernard Aboba, and Ben Campbell, who provided valuable
 review comments.

Contributors

 Daniel Grondal contributed in the creation and writing of early
 versions of this specification.  Christian Groves contributed
 significantly to the SDP "config" pause attribute and its use in
 offer/answer.

Burman, et al. Standards Track [Page 54] RFC 7728 RTP Stream Pause February 2016

Authors' Addresses

 Bo Burman
 Ericsson
 Kistavagen 25
 SE - 164 80 Kista
 Sweden
 Email: bo.burman@ericsson.com
 Azam Akram
 Ericsson
 Farogatan 6
 SE - 164 80 Kista
 Sweden
 Phone: +46107142658
 Email: akram.muhammadazam@gmail.com
 URI:   www.ericsson.com
 Roni Even
 Huawei Technologies
 Tel Aviv
 Israel
 Email: roni.even@mail01.huawei.com
 Magnus Westerlund
 Ericsson
 Farogatan 6
 SE - 164 80 Kista
 Sweden
 Phone: +46107148287
 Email: magnus.westerlund@ericsson.com

Burman, et al. Standards Track [Page 55]

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