path: root/doc/rfc/rfc7728.txt

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.












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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
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   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.

























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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





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   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





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   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)




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   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





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      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].













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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.








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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.




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   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.




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   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.





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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.









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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).








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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



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   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.



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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.







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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



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   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



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   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.




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   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.






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   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.























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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,



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   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.




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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





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   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.




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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.







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   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.






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      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.



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   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



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   [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.




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   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



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   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.



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   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.






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   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.





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      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):




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     +---+-----------------------------+-----------------------------+
     | # | 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.






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   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.



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   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.



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   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



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   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.



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   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.





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           +---------------+                   +---------------+
           |  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.
























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           +---------------+                 +----------------+
           | 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.










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         +---------------+                       +---------------+
         |  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



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   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



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   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



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   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



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   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



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   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.














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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



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   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



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   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>.



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   [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>.



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   [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.





















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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














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