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Versions: (draft-westerlund-avtext-rtp-stream-pause) 00 01 02 03 04 05 06 07 08 09 10 RFC 7728

Network Working Group                                          B. Burman
Internet-Draft                                                  A. Akram
Updates: 5104 (if approved)                                     Ericsson
Intended status: Standards Track                                 R. Even
Expires: January 4, 2016                             Huawei Technologies
                                                           M. Westerlund
                                                                Ericsson
                                                            July 3, 2015


                      RTP Stream Pause and Resume
                 draft-ietf-avtext-rtp-stream-pause-08

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 Real-time Transport Protocol (RTP) for real time
   data transport.  This document extends the Codec Control Messages
   (CCM) RTCP feedback package by explicitly allowing and describing
   specific use of existing CCM messages 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 4, 2016.








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Copyright Notice

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   than English.

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 . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Point to Point  . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  RTP Mixer to Media Sender . . . . . . . . . . . . . . . .   8
     3.3.  RTP Mixer to Media Sender in Point-to-Multipoint  . . . .  10
     3.4.  Media Receiver to RTP Mixer . . . . . . . . . . . . . . .  10
     3.5.  Media Receiver to Media Sender Across RTP Mixer . . . . .  11
   4.  Design Considerations . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Real-time Nature  . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Message Direction . . . . . . . . . . . . . . . . . . . .  12
     4.3.  Apply to Individual Sources . . . . . . . . . . . . . . .  12
     4.4.  Consensus . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.5.  Message Acknowledgments . . . . . . . . . . . . . . . . .  13
     4.6.  Request Retransmission  . . . . . . . . . . . . . . . . .  13
     4.7.  Sequence Numbering  . . . . . . . . . . . . . . . . . . .  13



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     4.8.  Relation to Other Solutions . . . . . . . . . . . . . . .  14
   5.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Expressing Capability . . . . . . . . . . . . . . . . . .  15
     5.2.  PauseID . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  Requesting to Pause . . . . . . . . . . . . . . . . . . .  16
     5.4.  Media Sender Pausing  . . . . . . . . . . . . . . . . . .  17
     5.5.  Requesting to Resume  . . . . . . . . . . . . . . . . . .  18
     5.6.  TMMBR/TMMBN Considerations  . . . . . . . . . . . . . . .  20
   6.  Participant States  . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  Playing State . . . . . . . . . . . . . . . . . . . . . .  21
     6.2.  Pausing State . . . . . . . . . . . . . . . . . . . . . .  21
     6.3.  Paused State  . . . . . . . . . . . . . . . . . . . . . .  22
       6.3.1.  RTCP BYE Message  . . . . . . . . . . . . . . . . . .  22
       6.3.2.  SSRC Time-out . . . . . . . . . . . . . . . . . . . .  23
     6.4.  Local Paused State  . . . . . . . . . . . . . . . . . . .  23
   7.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  Message Details . . . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  PAUSE . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     8.2.  PAUSED  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.3.  RESUME  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.4.  REFUSED . . . . . . . . . . . . . . . . . . . . . . . . .  30
     8.5.  Transmission Rules  . . . . . . . . . . . . . . . . . . .  31
   9.  Signaling . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     9.1.  Offer-Answer Use  . . . . . . . . . . . . . . . . . . . .  36
     9.2.  Declarative Use . . . . . . . . . . . . . . . . . . . . .  37
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.1.  Offer-Answer . . . . . . . . . . . . . . . . . . . . . .  38
     10.2.  Point-to-Point Session . . . . . . . . . . . . . . . . .  39
     10.3.  Point-to-Multipoint using Mixer  . . . . . . . . . . . .  44
     10.4.  Point-to-Multipoint using Translator . . . . . . . . . .  46
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  50
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  51
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  51
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     15.2.  Informative References . . . . . . . . . . . . . . . . .  52
   Appendix A.  Changes From Earlier Versions  . . . . . . . . . . .  53
     A.1.  Modifications Between Version -07 and -08 . . . . . . . .  53
     A.2.  Modifications Between Version -06 and -07 . . . . . . . .  55
     A.3.  Modifications Between Version -05 and -06 . . . . . . . .  56
     A.4.  Modifications Between Version -04 and -05 . . . . . . . .  56
     A.5.  Modifications Between Version -03 and -04 . . . . . . . .  56
     A.6.  Modifications Between Version -02 and -03 . . . . . . . .  57
     A.7.  Modifications Between Version -01 and -02 . . . . . . . .  57
     A.8.  Modifications Between Version -00 and -01 . . . . . . . .  57
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  58




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1.  Introduction

   As real-time communication attracts more people, more applications
   are created; multimedia conversation applications being 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 SDP [RFC4566] Offer/Answer [RFC3264].

   Centralized nodes, like RTP Mixers or MCUs, which either uses 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) [I-D.ietf-mmusic-sdp-simulcast] or with
   Multi-Session Transmission (MST) of scalable encoding such as SVC
   [RFC6190].  There may be some of the defined encodings or 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 the 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



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   encoding process to be paused, the functionality appears to match
   Codec Control Messages in the RTP Audio-Visual Profile with Feedback
   (AVPF) [RFC5104].  This document defines the solution as a Codec
   Control Message (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.

2.  Definitions

2.1.  Abbreviations

   3GPP:  3rd Generation Partnership Project

   AVPF:  Audio-Visual Profile with Feedback (RFC 4585)

   BGW:  Border Gateway

   CCM:  Codec Control Messages (RFC 5104)

   CNAME:  Canonical Name (RTCP SDES)

   CSRC:  Contributing Source (RTP)

   FB:  Feedback (AVPF)

   FCI:  Feedback Control Information (AVPF)

   FIR:  Full Intra Refresh (CCM)

   FMT:  Feedback Message Type (AVPF)

   LTE:  Long-Term Evolution (3GPP)

   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




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   SDP:  Session Description Protocol (RFC 4566)

   SGW:  Signaling Gateway

   SIP:  Session Initiation Protocol (RFC 3261)

   SSRC:  Synchronization Source (RTP)

   SVC:  Scalable Video Coding

   TCP:  Transmission Control Protocol (RFC 793)

   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
   [I-D.ietf-avtext-rtp-grouping-taxonomy] 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 Request category, PAUSED as Indication and REFUSED as
      Notification.

      PAUSE:  Request from an RTP stream receiver to pause a stream

      RESUME:  Request from an RTP stream receiver to resume a paused
         stream

      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 which receives an RTP stream from
      different endpoints, combines them to make one RTP stream and
      forwards to destinations, in the sense described in Topo-Mixer of
      RTP Topologies [I-D.ietf-avtcore-rtp-topologies-update].




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   Participant:  A member which is part of an RTP session, acting as
      receiver, sender or both.

   Paused sender:  An RTP stream sender that has stopped its
      transmission, i.e. no other participant receives its RTP
      transmission, either based on having received a PAUSE request,
      defined in this specification, or based on a local decision.

   Pausing receiver:  An RTP stream receiver which sends a PAUSE
      request, defined in this specification, to other 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].

3.  Use Cases

   This section discusses the main use cases for RTP stream pause and
   resume.

   RTCWEB WG's use case and requirements document [RFC7478] defines the
   following API requirements in Appendix A, used also by 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 stop
   sending muted streams and start sending again when unmuting.  It is
   here assumed that "mute" above can be taken to apply also to other




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   media than audio.  At the time of publication for this specification,
   RTCWEB 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, and neither is it 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 than the
   sender and the single receiver of that stream.

3.2.  RTP Mixer to Media Sender

   One of the most commonly used topologies in centralized conferencing
   is based on the RTP Mixer [I-D.ietf-avtcore-rtp-topologies-update].
   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
   [I-D.ietf-avtcore-rtp-topologies-update].












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                    +---+      +-----------+      +---+
                    | 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.  It can either be the RTP
   Mixer's own logic and measurements such as voice activity on the
   incoming audio streams.  It can be that the number of sent media
   sources exceed what is reasonable to present simultaneously at any
   given receiver.  It can also be 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.  The streams may also be part of a Simulcast
   [I-D.ietf-mmusic-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.

   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 makes 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



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   stream does not affect anyone else 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.

   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 special
   care must be taken 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 the ones 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.




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   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 it's 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.

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




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   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.

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 to 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 firstly 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 makes it desirable
   or even necessitates 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.





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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 are effective, i.e., an active RTP stream sender
   pauses, or a resuming RTP stream sender have 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).

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 are 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



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   with PAUSE and implicitly references 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 it 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 has to process the SDP content to
   determine if they are expected to act on it.  The amount of bandwidth
   required for a SIP/SDP-based signaling solution is in the order of at
   least 10 times more than an RTCP-based solution.  Especially for 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 is sent through the media path,
   which is likely shorter (contains fewer intermediate nodes) than the
   signaling path, may anyway have to traverse a few intermediate nodes.
   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 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 re-uses 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, 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.





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   The PAUSED functionality is also defined in such a way that it can be
   used standalone by the RTP stream sender to indicate a local decision
   to pause, and 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 in herein as well as their
   TMMBR/TMMBN counterparts, except when explicitly stated otherwise.
   An obvious exception are 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.

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), 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,



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   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 current PauseID, which is denoted as 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 that 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 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.

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 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 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 a stream receiver A for a PAUSE request sent by a stream sender
   B, but 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



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   PAUSE request can be re-transmitted using the same current PauseID.
   If using TMMBR 0, the request MAY be re-transmitted 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
   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 bounding set
   overhead value.

   If the RTP stream sender receives several identical PAUSE for an RTP
   stream that was already at least once responded with REFUSED and the
   condition causing REFUSED remains, those additional REFUSED 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 either be as a result of receiving a PAUSE, or be based on
   some local sender consideration.  When it does, it sends a PAUSED
   indication, containing the current PauseID.  Note that current
   PauseID in an unsolicited PAUSED (without having received a PAUSE),
   is incremented compared to previously sent PAUSED.  It also sends the




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   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 first hand information
   allowing them to know whether they lost any packets just before the
   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 SR or RR.  Including PAUSED in
   this way allows RTP stream receivers joining while the stream is
   paused to quickly know that there is a paused stream, what the last
   sent extended RTP sequence number was, and what the current PauseID
   is to be able 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.  It should in
   addition 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.



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   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.

   If the RTP stream sender receives several identical RESUME for an RTP
   stream that was already at least once responded with REFUSED and the
   condition causing REFUSED remains, those additional REFUSED 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, if 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



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   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 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-parameter 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 TMMBR restriction owner in the
      TMMBN message bounding set.  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 REFUSED notification when a stream is
      requested to be paused with TMMBR 0, for reasons stated in
      Section 5.3.

6.  Participant States

   This document introduces three new states for a stream in an RTP
   sender, according to the figure and sub-sections 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.



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

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 Time Stamp for the first packet sent after a pause
   SHOULD be set according to capture times at the source, meaning the
   RTP Time Stamp 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,

   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.




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   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 to instead be based on the above formula.

   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
      [I-D.ietf-avtcore-rtp-multi-stream-optimisation] 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.

   Following sub-sections discusses 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.

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 section 6.3.4 and




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   6.3.7 of RTP [RFC3550], 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 that 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, either because 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 for 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), and repeat it a sufficient number of times to reach a
   high probability that the message is correctly delivered, and
   stopping such repetition whenever leaving the state.

   When using TMMBN 0 as 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



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   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 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 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.

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 sub-typed into




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   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 sub-
   typed messages, is henceforth referred to as the PAUSE-RESUME
   message.  The specific FCI format is identified by a Feedback Message
   Type (FMT) value in common packet header for feedback message defined
   in section 6.1 of AVPF [RFC4585].  The PAUSE-RESUME transport
   feedback message FCI is identified by FMT value = TBA1.

   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: TBA1

   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.

   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.





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   The Feedback Control Information (FCI) field consists of one or more
   PAUSE, RESUME, PAUSED, REFUSED, 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.

      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.  SHALL be ignored by
      receivers implementing this specification and MUST be set to 0 by
      senders implementing this specification.





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   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 re-transmitted.  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 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.  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.
   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.
   Future PauseID is intentionally not defined as the entire range that
   was not already defined as 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.




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   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 wishes 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].

   When an RTP stream sender in Playing State (Section 6.1) receives a
   PAUSE with the current PauseID, and unless local considerations
   currently makes 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.







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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 RTP extended highest sequence number
   (Section 6.4.1 of [RFC3550]) valid when the RTP stream was paused.

   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.

   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 was
   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 to 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



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   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 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
   makes 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
   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.




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   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 that would render identical REFUSED
   notifications are received before the scheduled REFUSED is sent,
   duplicate REFUSED MUST NOT be scheduled for transmission.  This
   effectively lets a single REFUSED respond to several ineffective
   PAUSE or RESUME requests.

   If REFUSED containing a certain PauseID was already sent and yet more
   PAUSE or RESUME messages are received that require additional REFUSED
   with that specific PauseID to be scheduled, further REFUSED messages
   with that PauseID SHOULD be sent in regular RTCP reports.  An
   exception to the previous rule is when the stream was paused and
   resumed so many times that the PauseID number space has wrapped since
   REFUSED was last sent with that PauseID.

   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
   depends on the session's mode of operation.

   All messages defined in this specification, as well as TMMBR/TMMBN
   used for pause/resume purposes (Section 5.6), MAY use either Regular,
   Early or Immediate timings, taking the following into consideration:

   o  PAUSE SHOULD use Early or Immediate timing, except for
      retransmissions that SHOULD use Regular timing.





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   o  The first transmission of PAUSED for each (non-wrapped) PauseID
      SHOULD be sent with Immediate or Early timing, while subsequent
      PAUSED for that PauseID SHOULD use Regular timing.  Unsolicited
      PAUSED (sent when entering Local Paused State (Section 6.4))
      SHOULD 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, after which it SHOULD use Regular timing.

   o  RESUME SHOULD always use Immediate or Early timing.

   o  The first transmission of REFUSED for each (non-wrapped) PauseID
      SHOULD be sent with Immediate or Early timing, while subsequent
      REFUSED for that PauseID SHOULD use Regular timing.

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 payload type in a session description.

   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 rtcp-fb attribute with ccm tmmbr
      parameter is sufficient and no further signaling is necessary.
      There is however no guarantee that TMMBR/TMMBN implementations
      pre-dating this specification work exactly as described here when
      used with a bitrate value of 0.

   The "pause" parameter has two optional attributes, "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.



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   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 sub-set is
      implemented:

      1  Full implementation of this specification.  This is the default
         configuration.  A missing config 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 may pause sent RTP streams due to local
         considerations and then intends 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.

      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 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 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.





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   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
   sub-sets 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.  Implementation of any other
   functionality sub-sets of this specification than the above defined
   is NOT RECOMMENDED.

   When signaling a config value other than 1, an implementation MUST
   ignore non-supported messages on reception, and MAY omit sending non-
   supported messages.  The below table summarizes per-message send and
   receive support for the different config attribute values ("X"
   indicating support and "-" indicating non-support):

     +---+-----------------------------+-----------------------------+
     | # | 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=2 and 4
   correspond to the RTP Mixer in the RTP Mixer to Media Sender use case
   (Section 3.2), and config=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=3 or 5 will not provide a working
   solution.  Similarly, for that use case, a Media Sender implementing
   only config=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 would require support corresponding
   to config=1 between such RTP Mixers.  This is because an RTP Mixer is
   then also Media Sender towards the other RTP Mixer, requiring support



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   for the union of config=2 and 3 or config=4 and 5, which effectively
   becomes config=1.

   As can be seen from Figure 7 above, config=2 and 3 differ from
   config=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=6-8.

   Viable fallback rules between different config are described in
   Section 9.1 and Figure 9.

   This is the resulting ABNF [RFC5234], extending 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
                      / 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.  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.




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   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 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 "pause"
   CCM parameter with suitable configuration attribute ("config") in the
   SDP, according to what messages it intends to send and desires to
   receive in the session.

   In SDP offer/answer, the "config" 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
   attribute with a certain value, describing a certain capability to
   send and receive messages, MAY change the config 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 attribute, MUST be interpreted
   as equivalent to config=1.  Implementations of this specification
   MUST NOT use any other config values than the ones 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




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   "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" attribute on the "pause" line in the offer.  If an answerer
   receives the "nowait" 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"
   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
   contained a "nowait" parameter, then the hold-off period is
   configured to 0 at both offerer and answerer.

   Unknown pause attributes MUST be ignored in the offer and MUST then
   be omitted from the answer.

   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" attribute and its message directions are
   interpreted based on the node receiving the SDP, and 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" parameter, 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 SAP [RFC2974] will not be in a point to point session,
   thus "nowait" cannot be included.  An 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.



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   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 shows 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

   4.  Point-to-Multipoint using Relay

10.1.  Offer-Answer

   The below figures contains an example 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 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" sub-
   parameter for the "pause" parameter.

   This is the SDP answer:





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   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.

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



















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           +---------------+                   +---------------+
           |  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 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 the RTP stream
   sender (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 to receive
   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 to receive 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                |
                | ------------------------------------> |
                :                                       :
                |    <Timeout, 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 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
   are sent and another RESUME are sent already at time t7, which now
   reaches the RTP stream sender that consequently resumes the stream at



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   time t8.  The time interval between t6 and t7 can vary, but 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 any unwanted resume 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)         |
                   | <------------------------------ |
                   |                                 |
                   |  < Can not 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, it selects the video



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   stream to deliver to the receiver R based on the voice activity of
   the RTP stream senders.  The video stream will be delivered to R
   using M's SSRC and with an 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 the Mixer SSRC but with S1 as CSRC (indicated after the
   colon in the figure).  Then S2 joins the session at t3 and starts
   delivering an RTP stream to the Mixer.  As S2 has less voice activity
   then S1, the Mixer decides to pause S2 at t4 by sending S2 a PAUSE
   request.  At t5, S2 acknowledges with a PAUSED and at the same
   instant stops delivering RTP to the Mixer.  At t6, the user at S2
   starts speaking and becomes the most active speaker and the Mixer
   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



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   stream from t7 arrives at the Mixer, it switches this RTP stream into
   its SSRC (M) at t8 and changes the CSRC to S2.  As S1 now becomes
   unused, the Mixer issues a PAUSE request to S1 at t9, which is
   acknowledged at t10 with a 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




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   Figure 18 describes how a Relay can help the receiver in pausing and
   resuming the sender.  The sender S sends RTP data to the receiver R
   through Relay, which just forwards the data without modifying the
   SSRCs.  The receiver sends a PAUSE request to the sender, which in
   this example knows that there 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, does not receive any
   disapproving RESUME, hence pauses itself and replies with PAUSED.
   Similarly the receiver resumes the sender by sending RESUME request
   through Relay.  Since this describes only a single pause and resume
   operation for a single RTP stream sender, all messages uses a single
   PauseID, in this example 3.







































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

   Figure 19 explains the pause and resume operations when a transport
   Relay is involved between a sender and two receivers in an RTP
   session.  Each message exchange is represented by the time it
   happens.  At time t1, Sender (S) starts sending an RTP stream to the
   Relay, which is forwarded to R1 and R2 through the Relay, Rel. R1 and
   R2 receives RTP data from Relay at t2.  At this point, both R1 and R2




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   will send RTCP Receiver Reports to S informing that they receive 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 are 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 the sender S by the Relay.  The sender S sees
   the RESUME at time t6 and continues to send data to Relay which
   forwards to both R1 and R2.  At t7, the receiver R2 chooses to pause
   the stream by sending a PAUSE request with an updated PauseID.  The
   sender S still knows that there are 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,
   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) chooses to resume the stream from S, in this
   example R1, it sends a RESUME request to the sender (also seen by
   R2).  The RTP sender immediately resumes the stream.

   Consider also an RTP session which 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.

11.  IANA Considerations

   This specification requests the following registrations from IANA:

   1.  A new value for media stream pause / resume to be registered with
       IANA in the "FMT Values for RTPFB Payload Types" registry located
       at the time of publication at: http://www.iana.org/assignments/
       rtp-parameters/rtp-parameters.xhtml#rtp-parameters-8

       Value:  TBA1

       Name:  PAUSE-RESUME

       Long Name:  Media Pause / Resume



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       Reference:  This RFC

   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/sdp-
       parameters.xhtml#sdp-parameters-19

       Value Name:  pause

       Long Name:  Media Pause / Resume

       Usable with:  ccm

       Reference:  This RFC

12.  Security Considerations

   This document extends the CCM [RFC5104] and defines new messages,
   i.e. PAUSE, RESUME, PAUSED, and REFUSED.  The exchange of these new
   messages have some security implications, which need to be addressed
   by the user.

   The messages defined in this specification can have 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 signalling messages often carry
   the SSRC value, while the 16-bit PauseID have to be guessed or tried.
   The way of protecting the RTP session from these injections is to
   perform source authentication combined with message integrity, to
   prevent other than intended session participants from sending these
   messages.  There exist several different choices for securing RTP
   sessions to prevent this type of attack.  SRTP is the 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 multi-party 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



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   using a solution that provides true source authentication of all
   participants' RTCP packets.  However, that has other implications.
   For multi-party 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
   utilizing another participant's SSRCs.

   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 that the media streams are delivered.
   However, such an attack basically prevents the use the Pause and
   Resume functionality.  Thus, potentially forcing a reduction of the
   media quality due to limitation in available resources, like
   bandwidth that must be shared.

   The session establishment signalling is also a potential venue of
   attack, as that can be used to prevent the enabling of Pause and
   Resume functionality by modifying the signalling messages.  The above
   mitigation of attacks based on source authentication also requires
   the signalling system to securely handle identities, and assert that
   only the intended identities are allowed into the RTP session and
   provided the relevant security contexts.

13.  Contributors

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

14.  Acknowledgements

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

15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.






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   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June
              2002.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [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, July
              2006.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC6263]  Marjou, X. and A. Sollaud, "Application Mechanism for
              Keeping Alive the NAT Mappings Associated with RTP / RTP
              Control Protocol (RTCP) Flows", RFC 6263, June 2011.

15.2.  Informative References

   [I-D.ietf-avtcore-rtp-multi-stream-optimisation]
              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",
              draft-ietf-avtcore-rtp-multi-stream-optimisation-05 (work
              in progress), February 2015.

   [I-D.ietf-avtcore-rtp-topologies-update]
              Westerlund, M. and S. Wenger, "RTP Topologies", draft-
              ietf-avtcore-rtp-topologies-update-10 (work in progress),
              July 2015.






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   [I-D.ietf-avtext-rtp-grouping-taxonomy]
              Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, "A Taxonomy of Semantics and Mechanisms for
              Real-Time Transport Protocol (RTP) Sources", draft-ietf-
              avtext-rtp-grouping-taxonomy-07 (work in progress), June
              2015.

   [I-D.ietf-mmusic-sdp-simulcast]
              Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
              "Using Simulcast in SDP and RTP Sessions", draft-ietf-
              mmusic-sdp-simulcast-00 (work in progress), January 2015.

   [RFC2326]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
              Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC6190]  Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
              "RTP Payload Format for Scalable Video Coding", RFC 6190,
              May 2011.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, April 2014.

   [RFC7478]  Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
              Time Communication Use Cases and Requirements", RFC 7478,
              March 2015.

Appendix A.  Changes From Earlier Versions

   NOTE TO RFC EDITOR: Please remove this section prior to publication.

A.1.  Modifications Between Version -07 and -08

   Changes based on IESG AD Evaluation.

   o  Moved the mentioning of RTCWEB RFC7478 API requirements out from
      3.1 to section 3, adding a couple of clarifying sentences.

   o  Highlighted that the use case in section 3.4 deals with a
      different direction of the pause request than the previous use
      cases.



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   o  Added text on partial capability and interoperability to section
      5.1.

   o  Added an overview explanation of PauseID as a new section 5.2, and
      moved a few sentences on PauseID from other 5.x sections in there.

   o  Changed all occurrences of "available" and "valid" PauseID to the
      more clear "current" PauseID, and re-phrased sentences involving
      that to become more clear.

   o  Changed all occurrences of "smaller" and "larger" PauseID to
      "past" and "future", respectively, to better align with "current".

   o  Removed an incorrect sentence in 5.2 about when it is not feasible
      to send repeated PAUSE.

   o  Changed a few capitalized words that could be taken as normative
      text from section 5, which is intended to be a non-normative
      description.

   o  Added some explanatory text on why RTP stream is resumed when the
      stream receiver that paused the stream leaves the RTP session to
      last bullet in 6.3.1.

   o  Added caption to Figure 5.

   o  Moved the detailed description on what PauseID ranges are defined
      as "past" and "future" before section 8.1, instead of having it in
      section 8.1, and added a comment on the "not current" part of the
      value range.

   o  Added text in section 8.1 on appropriate time to wait between
      sending PAUSE, when the first PAUSE was rejected by a RESUME or a
      REFUSED.

   o  Added text in section 8.3 on appropriate time to wait between
      sending RESUME, when the first RESUME was rejected by a REFUSED.

   o  Added text in section 8.4 on time to wait before sending the
      REFUSED request again, referencing sections 8.1 and 8.3.

   o  Added a couple of paragraphs in section 9 on partial capability
      and interoperability, including a description on when different
      config values are expected to be useful, and when they are not.

   o  Added arrows in Figure 19 to highlight that the Relay sends out
      all received messages to all receivers, not only the first PAUSE
      message.



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   o  Changed references to RFC3264 and RFC4566 to be normative.

   o  Updated ietf-rtcweb-use-cases-and-requirements reference to be
      RFC7478.

   o  Editorial improvements and clarifications.

A.2.  Modifications Between Version -06 and -07

   o  Completely rewrote the Security Consideration section.

   o  Aligned text such that REFUSED is always referred to as a
      notification, not indication.

   o  Added and changed text in several places, clarifying the case when
      TMMBR/TMMBN bounding set overhead value matters, related to
      whether local RTP stream sender or remote RTP stream receiver owns
      the TMMBR 0 restriction, and the consequences this has on pause/
      resume logic.

   o  Moved text on when to stop media stream transmission from when
      receiving PAUSE and entering pausing state, to when entering
      paused or local paused states.

   o  Added text on how to determine if there is a single receiver or
      not, aligned with what is specified in draft-ietf-avtcore-multi-
      stream, adding a reference to draft-ietf-avtcore-multi-stream-
      optimisation to be able to use a single RTCP reporting group as
      one criteria.

   o  Added clarifying text on repeating PAUSED and RESUME messages only
      as long as remaining in the relevant state.

   o  Clarified that it is the RTP stream sender's responsibility to
      leave local paused state when the condition causing that state is
      no longer true.

   o  Added text to better allow for extensions to this specification,
      since there is already some text on extensions.

   o  Corrected and amended ABNF to make CCM pause parameters order-
      independent, allow for a larger config pause attribute value
      range, and added corresponding text to handle that additional
      flexibility.

   o  Added SDP rules on how to handle unknown pause attribute values.





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   o  Clarified how to handle an SDP with both "ccm pause" and "ccm
      tmmbr".

   o  Changed from "Translator" to "Relay" in examples, to make it
      clearer in relation to the updated topologies draft.

   o  Editorial improvements.

A.3.  Modifications Between Version -05 and -06

   o  Clarified in Message Details section for PAUSED that
      retransmission of the message can be used to increase the
      probability that the message reaches the receiver in a timely
      fashion, and also added text that says the number of repetitions
      can be tuned to observed loss rate and the desired loss
      probability.  Also removed Editor's notes on potential ACK for
      unsolicited PAUSED, since the issue is solved by the above.

   o  In the same section as above, added that PAUSED may be included in
      all compound RTCP reports, as long as the negotiated RTCP
      bandwidth is not exceeded.

   o  In Message Details section for RESUME, added text on
      retransmission similar to the one mentioned for PAUSED above.
      Also included text that says such retransmission SHOULD stop as
      soon as RTP packets or a REFUSED with a valid PauseID from the
      targeted stream are received.

   o  Changed simulcast reference, since that draft was moved from
      AVTCORE to MMUSIC and made WG draft.

   o  Changed End Point to Endpoint to reflect change in RTP Grouping
      Taxonomy draft.

   o  Editorial improvements.

A.4.  Modifications Between Version -04 and -05

   o  Added text in sections 4.1, 4.6, 6.4 and 8.5 on retransmission and
      timing of unsolicited PAUSED, to improve the message timeliness
      and probability of reception.

A.5.  Modifications Between Version -03 and -04

   o  Change of Copyright boilerplate






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A.6.  Modifications Between Version -02 and -03

   o  Changed the section on SDP signaling to be more explicit and clear
      in what is supported, replacing the 'paused' parameter and the
      'dir' attribute with a 'config' parameter that can take a value,
      and an explicit listing of what each value means.

   o  Added a sentence in section on paused state (Section 6.3) that
      pause must not affect RTP keepalive.

   o  Replaced REFUSE message name with REFUSED throughout, to better
      indicate that it is not a command but a notification.

   o  Added text in a few places, clarifying that PAUSED message may be
      used unsolicited due to RTP sender local considerations, and also
      clarified the interaction between this usage and an RTP stream
      receiver pausing the stream.  Also added an example describing
      this case.

   o  Clarified that when TMMBN 0 is used as PAUSED message, and when
      sent unsolicited due to RTP sender local considerations, the TMMBN
      message includes the RTP stream sender itself as part of the
      bounding set.

   o  Clarified that there is no reply to a PAUSED indication.

   o  Improved the IANA section.

   o  Editorial improvements.

A.7.  Modifications Between Version -01 and -02

   o  Replaced most text on relation with other signaling technologies
      in previous section 5 with a single, summarizing paragraph, as
      discussed at IETF 90 in Toronto, and placed it as the last sub-
      section of section 4 (design considerations).

   o  Removed unused references.

A.8.  Modifications Between Version -00 and -01

   o  Corrected text in section 6.5 and 6.2 to indicate that a PAUSE
      signaled via TMMBR 0 cannot be REFUSED using TMMBN > 0

   o  Improved alignment with RTP Taxonomy draft, including the change
      of Packet Stream to RTP Stream

   o  Editorial improvements



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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: muhammad.azam.akram@ericsson.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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