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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: April 30, 2015                              Huawei Technologies
                                                           M. Westerlund
                                                                Ericsson
                                                        October 27, 2014


                      RTP Stream Pause and Resume
                 draft-ietf-avtext-rtp-stream-pause-05

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 April 30, 2015.








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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Requirements Language . . . . . . . . . . . . . . . . . .   7
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Point to Point  . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  RTP Mixer to Media Sender . . . . . . . . . . . . . . . .   8
     3.3.  RTP Mixer to Media Sender in Point-to-Multipoint  . . . .   9
     3.4.  Media Receiver to RTP Mixer . . . . . . . . . . . . . . .  10
     3.5.  Media Receiver to Media Sender Across RTP Mixer . . . . .  10
   4.  Design Considerations . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Real-time Nature  . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Message Direction . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Apply to Individual Sources . . . . . . . . . . . . . . .  12
     4.4.  Consensus . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.5.  Message Acknowledgments . . . . . . . . . . . . . . . . .  12
     4.6.  Request Retransmission  . . . . . . . . . . . . . . . . .  13
     4.7.  Sequence Numbering  . . . . . . . . . . . . . . . . . . .  13



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     4.8.  Relation to Other Solutions . . . . . . . . . . . . . . .  13
   5.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Expressing Capability . . . . . . . . . . . . . . . . . .  15
     5.2.  Requesting to Pause . . . . . . . . . . . . . . . . . . .  15
     5.3.  Media Sender Pausing  . . . . . . . . . . . . . . . . . .  16
     5.4.  Requesting to Resume  . . . . . . . . . . . . . . . . . .  18
     5.5.  TMMBR/TMMBN Considerations  . . . . . . . . . . . . . . .  19
   6.  Participant States  . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Playing State . . . . . . . . . . . . . . . . . . . . . .  20
     6.2.  Pausing State . . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  Paused State  . . . . . . . . . . . . . . . . . . . . . .  21
       6.3.1.  RTCP BYE Message  . . . . . . . . . . . . . . . . . .  21
       6.3.2.  SSRC Time-out . . . . . . . . . . . . . . . . . . . .  22
     6.4.  Local Paused State  . . . . . . . . . . . . . . . . . . .  22
   7.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .  23
   8.  Message Details . . . . . . . . . . . . . . . . . . . . . . .  25
     8.1.  PAUSE . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.2.  PAUSED  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     8.3.  RESUME  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     8.4.  REFUSED . . . . . . . . . . . . . . . . . . . . . . . . .  28
     8.5.  Transmission Rules  . . . . . . . . . . . . . . . . . . .  28
   9.  Signaling . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     9.1.  Offer-Answer Use  . . . . . . . . . . . . . . . . . . . .  32
     9.2.  Declarative Use . . . . . . . . . . . . . . . . . . . . .  33
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  33
     10.1.  Offer-Answer . . . . . . . . . . . . . . . . . . . . . .  34
     10.2.  Point-to-Point Session . . . . . . . . . . . . . . . . .  35
     10.3.  Point-to-Multipoint using Mixer  . . . . . . . . . . . .  39
     10.4.  Point-to-Multipoint using Translator . . . . . . . . . .  41
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  44
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  45
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  45
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  45
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  46
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  46
     15.2.  Informative References . . . . . . . . . . . . . . . . .  46
   Appendix A.  Changes From Earlier Versions  . . . . . . . . . . .  47
     A.1.  Modifications Between Version -04 and -05 . . . . . . . .  47
     A.2.  Modifications Between Version -03 and -04 . . . . . . . .  47
     A.3.  Modifications Between Version -02 and -03 . . . . . . . .  48
     A.4.  Modifications Between Version -01 and -02 . . . . . . . .  48
     A.5.  Modifications Between Version -00 and -01 . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  49








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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 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.westerlund-avtcore-rtp-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, for example due to temporarily limited network or
   processing resources, and a centralized node may choose to pause such
   RTP streams without being requested to do so, but anyway send an
   explicit indication that the stream is paused.

   As the RTP streams required at any given point in time is highly
   dynamic in such scenarios, using the out-of-band signaling channel
   for pausing, and even more importantly resuming, an RTP stream is
   difficult due to the performance requirements.  Instead, the pause
   and resume signaling should be in the media plane and go directly
   between the affected nodes.  When using RTP [RFC3550] for media
   transport, using Extended RTP Profile for Real-time Transport Control
   Protocol (RTCP)-Based Feedback (RTP/AVPF) [RFC4585] appears
   appropriate.  No currently existing RTCP feedback message explicitly
   supports pausing and resuming an incoming RTP stream.  As this
   affects the generation of packets and may even allow the encoding



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   process to be paused, the functionality appears to match Codec
   Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)
   [RFC5104] and it is proposed to define 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  Indication 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 end points, 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", "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.

3.1.  Point to Point

   This is the most basic use case with an RTP session containing two
   End Points.  Each End Point 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




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   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.

   RTCWEB WG's use case and requirements document
   [I-D.ietf-rtcweb-use-cases-and-requirements] 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 memo provides means to optimize transport usage by stop sending
   muted streams and start sending again when unmuting.

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 SSRC,
   and any RTP streams sent to the participants will be sent using those
   SSRCs.  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].

                    +---+      +-----------+      +---+
                    | A |<---->|           |<---->| B |
                    +---+      |           |      +---+
                               |   Mixer   |
                    +---+      |           |      +---+
                    | C |<---->|           |<---->| D |
                    +---+      +-----------+      +---+

                    Figure 2: RTP Mixer in Unicast-only

   Which streams that are delivered to a given receiver, A, can depend
   on several things.  It can either be the RTP Mixer's own logic and



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   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.westerlund-avtcore-rtp-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 to pause a particular stream from being delivered to it.  It
   also needs to be able 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
   to the Mixer to pause some of its sent RTP streams, even without
   being explicitly asked to do so by the receiving client.  Such
   situations can for example be caused by a temporary lack of available
   Mixer network or processing resources.  An RTP stream receiver that
   no longer receives an RTP stream could interpret this as an error
   condition and try to take action to re-establish the RTP stream.
   Such action would likely be undesirable if the RTP stream was in fact
   deliberately paused by the Mixer.  Undesirable RTP stream receiver
   actions could be avoided if the Mixer is able to explicitly indicate
   that an RTP stream is deliberately paused.

   Just as for point-to-point (Section 3.1), there is only a single
   receiver of the stream, the RTP Mixer, and pausing or resuming a
   stream does not affect anyone else 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.






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                        +-----+
             +---+     /       \     +-----------+      +---+
             | 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

   An End Point 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.

   However, when the End Point 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 End Point, 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:



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   1.  The Mixer might not include CSRC in it's stream indications.

   2.  An End Point 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 End Point 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
   real-time in order for them to be purposeful.  The pause operation is
   arguably not very time critical since it mainly provides a reduction
   of resource usage.  Timely handling of the resume operation is
   however likely to directly impact the end-user's perceived quality
   experience, since it affects the availability of media that the user
   expects to receive more or less instantly.  It may also be highly
   desirable for a receiver to quickly learn that an RTP stream is
   intentionally paused on the RTP sender's own behalf.

4.2.  Message Direction

   It is the responsibility of an RTP stream receiver, who wants to
   pause or resume a stream from the sender(s), to transmit PAUSE and
   RESUME messages.  An RTP stream sender who likes 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.






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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, independent of it being multicast, a mesh with
   joint RTP session or a transport Translator based, 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.  A consequence of this is that a newly joining receiver,
   for example indicated by an RTCP Receiver Report containing both a
   new SSRC and a CNAME that does not already occur in the session,
   firstly needs to learn the existence of paused streams, and secondly
   should be able to resume any paused stream.  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.

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 End Point 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.  This




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   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
   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 [RFC4566] 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 End Point
   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



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   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.

   It is proposed to re-use CCM TMMBR and TMMBN [RFC5104] to the extent
   possible, and to define 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 response, or an
   extension to this specification.  This structure allows a single
   feedback message to handle pause functionality on a number of
   streams.

   The PAUSED functionality is also defined in such a way that it can be
   used 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 5.5.  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.






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   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 End Point 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).

   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.  Requesting to Pause

   An RTP stream receiver can choose to request PAUSE at any time,
   subject to AVPF timing rules.

   The PAUSE request contains a PauseID, which is incremented by one (in
   modulo arithmetic) with each PAUSE request that is not a re-
   transmission.  The PauseID is scoped by and thus a property of the
   targeted RTP stream (SSRC).

   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 5.4) for the
   same stream and with the same PauseID as in the disapproved PAUSE.
   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
   available 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 5.3) 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)



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   and may instead think that the PAUSE was lost.  In both cases, a
   PAUSE request can be re-transmitted using the same 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 PauseID from the disapproved PAUSE is invalidated by the RESUME
   and any new PAUSE must use an incremented PauseID (in modulo
   arithmetic) to be effective.

   An RTP stream sender receiving a PAUSE not using the available
   PauseID informs the RTP stream receiver sending the ineffective PAUSE
   of this condition by sending a REFUSED response that contains the
   next available PauseID value.  This REFUSED also informs the RTP
   stream receiver that it is probably not feasible to send another
   PAUSE for some time, not even with the available PauseID, since there
   are other RTP stream receivers that wish to receive the stream.

   A similar 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 available PauseID to use.  The
   REFUSED response 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 PauseID would otherwise have
   been effective, REFUSED contains the same PauseID as in the PAUSE
   request, and the PauseID is kept as available.  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.

   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.

5.3.  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 available PauseID.  Note that PauseID is



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   incremented when sending an unsolicited PAUSED (without having
   received a PAUSE).  It also sends the PAUSED indication in the next
   two regular RTCP reports, given that the pause condition is then
   still effective.

   There is no reply to a PAUSED indication; it is simply an explicit
   indication of the fact that an RTP stream is paused.  This can be
   helpful for the RTP stream receiver, for example to quickly
   understand that transmission is deliberately and temporarily
   suspended and no specific corrective action is needed.

   The RTP stream sender may want to apply some local consideration to
   exactly when the RTP stream is paused, for example completing some
   media unit or a forward error correction block, before pausing the
   stream.

   The PAUSED indication also contains information about the RTP
   extended highest sequence number when the pause became effective.
   This provides RTP stream receivers with 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 available
   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 next available
   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 End Point 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.








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5.4.  Requesting to Resume

   An RTP stream receiver can request to resume a stream with a RESUME
   request at any time, subject to AVPF timing rules.  The RTP stream
   receiver must include the available PauseID in the RESUME request for
   it to be effective.

   A pausing RTP stream sender that receives a RESUME including the
   correct available 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 MAY then respond with a REFUSED containing the same
   PauseID as in the RESUME.  When receiving such REFUSED with a PauseID
   identical to the one in the sent RESUME, RTP stream receivers SHOULD
   then avoid sending further RESUME requests for some reasonable amount
   of time, to allow the condition to clear.

   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 MAY
   resume a paused RTP stream at any time.  If TMMBR 0 was used to pause
   the RTP stream, it cannot be resumed due to local considerations,
   unless the RTP stream is paused only due to local considerations
   (Section 5.3) and thus no RTP stream receiver has requested to pause
   the stream with TMMBR 0.

   When resuming a paused stream, especially for media that makes use of
   temporal redundancy between samples such as video, the temporal
   dependency between samples taken before the pause and at the time
   instant the stream is resumed may not be appropriate to use in the
   encoding.  Should such temporal dependency between before and after
   the media was paused be used by the RTP stream sender, it requires
   the RTP stream receiver to have saved the sample from before the
   pause for successful continued decoding when resuming.  The use of
   this temporal dependency is left up to the RTP stream sender.  If
   temporal dependency is not used when the RTP stream is resumed, the
   first encoded sample after the pause will not contain any temporal
   dependency to samples before the pause (for video it may be a so-
   called intra picture).  If temporal dependency to before the pause is
   used by the RTP stream sender when resuming, and if the RTP stream



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   receiver did not save any sample from before the pause, the RTP
   stream receiver can use a FIR request [RFC5104] to explicitly ask for
   a sample without temporal dependency (for video a so-called intra
   picture), even at the same time as sending the RESUME.

5.5.  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
      indication when a stream is requested to be resumed with TMMBR >0.

   TMMBN >0:  Cannot be used as REFUSED indication when a stream is
      requested to be paused with TMMBR 0, for reasons stated in
      Section 5.2.

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 5.5) 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 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, for example by RTCP RR,
   it MUST change the hold-off to instead be based on the above formula.

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.

   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.

   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
   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.







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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 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.  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, and repeat it a sufficient number
   of times to reach a high probability that the message is correctly
   delivered, unless the stream was already in paused state
   (Section 6.3).

      Editor's note: Consider specifying an explicit PAUSED ACK message
      that stops this message retransmission.

   When using TMMBN 0 as PAUSED indication, being in paused state, and
   entering local paused state, the RTP stream sender SHALL send TMMBN 0
   with itself included in the TMMBN bounding set.

   As indicated in Figure 4, this 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.

   Pausing an RTP stream MUST NOT affect the sending of RTP keepalive
   [RFC6263][RFC5245] applicable to that RTP stream.

   When leaving the state, the stream state SHALL become Playing,
   regardless whether or not there were any RTP stream receivers that
   sent PAUSE for that stream, effectively clearing the RTP stream
   sender's memory for that stream.  This does however not apply when
   the stream was paused by a TMMBR 0, either before entering or during
   the Local Paused State, in which case leaving Local Paused State just
   removes the RTP sender from the TMMBN bounding set, and a new TMMBN
   with the updated bounding set MUST be sent accordingly.  The stream
   state can become Playing only when there is no entry with a bitrate
   value of 0 in the stream's bounding set.







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7.  Message Format

   Section 6 of AVPF [RFC4585] defines three types of low-delay RTCP
   feedback messages, i.e.  Transport layer, Payload-specific, and
   Application layer feedback messages.  This document defines a new
   Transport layer feedback message, this message is either a PAUSE
   request, a RESUME request, or one of four different types of
   acknowledgments in response to either PAUSE or RESUME requests.

   The Transport layer feedback messages are identified by having the
   RTCP payload type be RTPFB (205) as defined by AVPF [RFC4585].  The
   PAUSE and RESUME messages are identified by Feedback Message Type
   (FMT) value in common packet header for feedback message defined in
   section 6.1 of AVPF [RFC4585].  The PAUSE and RESUME transport
   feedback message is identified by the 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)                 :
     :                                                               :

   For the PAUSE and RESUME messages, the following interpretation of
   the packet fields will be:

   FMT:  The FMT value identifying the PAUSE and RESUME message: 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 End Points 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 consist 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 5: Syntax of FCI Entry in the PAUSE and RESUME message

   The FCI fields have the following definitions:

   Target SSRC (32 bits):  For a PAUSE and RESUME messages, 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 indication message

      4-15:  Reserved for future use

   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.

   Parameter Len: (8 bits):  Length of the Type Specific field in 32-bit
      words.  MAY be 0.




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   PauseID (16 bits):  Message sequence identification.  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.

8.  Message Details

   This section contains detailed explanations of each message defined
   in this specification.  All transmissions of request 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 5.5) 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, 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.

8.1.  PAUSE

   An RTP stream receiver MAY schedule PAUSE for transmission at any
   time.

   PAUSE has no defined Type Specific parameters and Parameter Len MUST
   be set to 0.

   PauseID SHOULD be the available PauseID, as indicated by PAUSED
   (Section 8.2) 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 PauseID information, in which
   case the PAUSE will either succeed, or the correct PauseID can be
   found in the returned REFUSED (Section 8.4).  A PauseID that is
   matching the available PauseID is henceforth also called a valid
   PauseID.

   PauseID needs to be incremented by one, in modulo arithmetic, for
   each PAUSE request that is not a retransmission, compared to what was



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   used in the last PAUSED indication sent by the media sender.  This is
   to ensure that the PauseID matches what is the current available
   PauseID at the RTP stream sender.  The RTP stream sender increments
   what it considers to be the available PauseID when entering Playing
   State (Section 6.1).

   For the scope of this specification, a PauseID larger than the
   current one is defined as having a value between and including
   (PauseID + 1) MOD 2^16 and (PauseID + 2^14) MOD 2^16, where "MOD" is
   the modulo operator.  Similarly, a PauseID smaller than the current
   one is defined as having a value between and including (PauseID -
   2^15) MOD 2^16 and (PauseID - 1) MOD 2^16.

   If an RTP stream receiver that sent a PAUSE with a certain PauseID
   receives a RESUME with the same PauseID, it is RECOMMENDED that it
   refrains from sending further PAUSE requests for some appropriate
   time since the RESUME indicates that there are other receivers that
   still wishes to receive the stream.

   If the targeted RTP stream does not pause, if no PAUSED indication
   with a larger PauseID than the one used in PAUSE, and if no REFUSED
   is received within 2 * RTT + T_dither_max, the PAUSE MAY be scheduled
   for retransmission, using the same 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
   valid PAUSE, and unless local considerations currently makes it
   impossible to pause the stream, it SHALL enter Pausing State
   (Section 6.2) when reaching an appropriate place to pause in the
   stream, and act accordingly.

   If an RTP stream sender receives a valid PAUSE while in Pausing,
   Paused (Section 6.3) or Local Paused (Section 6.4) States, the
   received PAUSE SHALL be ignored.

8.2.  PAUSED

   The PAUSED indication MUST be sent whenever entering Paused State
   (Section 6.3) as a result of receiving a valid PAUSE (Section 8.1)
   request, or when entering Local Paused State (Section 6.4) based on a
   RTP stream sender local decision.

   PauseID MUST contain the available, valid value to be included in a
   subsequent RESUME (Section 8.3).






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   PAUSED SHALL contain a 32 bit parameter with the RTP extended highest
   sequence number valid when the RTP stream was paused.  Parameter Len
   MUST be set to 1.

   After having entered Paused or Local Paused State and thus having
   sent PAUSED once, PAUSED MUST also be included in the next two
   regular RTCP reports, given that the pause condition is then still
   effective.

   While remaining in Paused or Local Paused States, PAUSED MAY be
   included in all regular RTCP reports.

   When in Paused or Local Paused States, It is RECOMMENDED to send
   PAUSED at the earliest opportunity and also to include it in the next
   two regular RTCP reports, whenever the RTP stream sender learns that
   there are End Points 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.

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 valid PauseID, as indicated by PAUSED
   (Section 8.2) or implicitly determined by previously received PAUSE
   (Section 8.1) or RESUME requests.  A randomly chosen PauseID MAY be
   used if it was not possible to retrieve PauseID information, in which
   case the RESUME will either succeed, or the correct PauseID can be
   found in a returned REFUSED (Section 8.4).

   RESUME has no defined Type Specific parameters and Parameter Len MUST
   be set to 0.

   When an RTP stream sender in Pausing (Section 6.2), Paused
   (Section 6.3) or Local Paused State (Section 6.4) receives a valid
   RESUME, 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
   the RESUME request with a valid PauseID, or receives a RESUME request
   with an invalid PauseID while in Paused or Pausing state, the RTP
   stream sender sends a REFUSED message as specified below.

   If an RTP stream sender in Playing State receives a RESUME containing
   either a valid PauseID or a PauseID that is less than the valid
   PauseID, the received RESUME SHALL be ignored.




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8.4.  REFUSED

   REFUSED has no defined Type Specific parameters and Parameter Len
   MUST be set to 0.

   If an RTP stream sender receives a valid PAUSE (Section 8.1) or
   RESUME (Section 8.3) request that cannot be fulfilled by the sender
   due to some local consideration, it SHALL schedule transmission of a
   REFUSED indication containing the valid PauseID from the rejected
   request.

   If an RTP stream sender receives PAUSE or RESUME requests with a non-
   valid PauseID it SHALL schedule a REFUSED response containing the
   available, valid PauseID, except if the RTP stream sender is in
   Playing State and receives a RESUME with a PauseID less than the
   valid one, in which case the RESUME SHALL be ignored.

   If several PAUSE or RESUME that would render identical REFUSED
   responses 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 invalid 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, and unless the PauseID
   number space has wrapped since REFUSED was last sent with that
   PauseID, further REFUSED messages with that PauseID SHOULD be sent in
   regular RTCP reports.

   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.

   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
   indication.

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.






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   All messages defined in this specification, as well as TMMBR/TMMBN
   used for pause/resume purposes (Section 5.5), 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.

   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.

         Editor's note: Consider specifying a PAUSED ACK message as
         explicit indication of reception.

   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":



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   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 paused
      after receiving a PAUSE request.  This condition occurs when there
      will be only 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 also indicate that it would be possible
      to use CCM TMMBR/TMMBN as pause/resume signaling.

   o  "config" allows for partial implementation of this specification
      according to the different roles in the use cases section
      (Section 3), and takes a value that describes what 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



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         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.

   When signaling a config value other than 1, an implementation MAY
   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 6: Supported messages for different config values

   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] [SP "nowait"] [SP byte-string]
   pause-config       = "config=" pause-config-value
   pause-config-value = %x31-38
   ; byte-string as defined in RFC 4566, for future extensions


                              Figure 7: ABNF

   An endpoint implementing this specification and using SDP to signal
   capability SHOULD indicate the new "pause" parameter with ccm
   signaling, but MAY use existing ccm tmmbr signaling [RFC5104] if the
   limitations in functionality as described in this specification
   coming from such usage are considered acceptable.  The messages from




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   this specification SHOULD NOT be used towards receivers that did not
   declare capability to receive those messages.

   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 does not have an
   explicit "pause" specification.

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 parameter 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 8: Config values in Offer/Answer

   An offer or answer omitting the config attribute, MUST be interpreted
   as equivalent to config=1.  In all cases the answerer MAY also
   completely remove any "pause" CCM parameter to indicate that it does
   not understand or desire to use any pause functionality for the
   affected payload types.





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   If the offerer believes that itself and the intended answerer are
   likely the only End Points in the RTP session, it MAY include the
   "nowait" sub-parameter on the "pause" line in the offer.  If an
   answerer receives the "nowait" sub-parameter on the "pause" line in
   the SDP, and if it has information that the offerer and itself are
   not the only End Points in the RTP session, it MUST NOT include any
   "nowait" sub-parameter 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.

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.

   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.

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 Translator






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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 9: 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:

   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 10: SDP Answer With Pause and Resume Capability

   The answerer will not allow its sent streams to be paused or resumed
   and thus restricts the answer to indicate config=2.  It also supports
   pausing its own RTP streams due to local considerations, which is why
   config=2 is chosen rather than config=4.  The answerer somehow knows
   that it will not be a point-to-point RTP session and has therefore
   removed "nowait" from the "pause" line, meaning that the offerer must
   use a non-zero hold-off period when being requested to pause the
   stream.



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

           +---------------+                   +---------------+
           |  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 >        |
                  :                                  :


          Figure 11: Pause and Resume Operation in Point-to-Point

   Figure 11 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).







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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 >        |
                  :                                  :


            Figure 12: TMMBR Pause and Resume in Point-to-Point

   Figure 12 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 13: Unsolicited PAUSED using TMMBN

   Figure 13 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 14: Pause and Resume Operation With Messages Lost

   Figure 14 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 the RESUME PauseID is
   less than the valid one and will be ignored by the RTP stream sender.
   It will also not cause any unwanted resume even if the stream was
   paused based on a PAUSE from some other receiver before receiving the
   RESUME, since the valid PauseID is now larger than the one in the
   stray RESUME and will only cause a REFUSED containing the new valid
   PauseID from 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 15: Pause Request is Refused in Point-to-Point

   In Figure 15, 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 16: 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 Translator in an RTP session forwards the message from
   one peer to all the others.  Unlike Mixer, the Translator 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 Translator case.
   The parentheses in the figures contains (Target SSRC, PauseID)
   information for the messages defined in this specification.

         +-------------+     +-------------+     +--------------+
         |  Sender(S)  |     | Translator  |     | 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 17: Pause and Resume Operation Between Two Participants Using
                               a Translator




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   Figure 17 describes how a Translator can help the receiver in pausing
   and resuming the sender.  The sender S sends RTP data to the receiver
   R through Translator, 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 Translator.  Since this describes only a single pause
   operation for a single RTP stream sender, all messages uses a single
   PauseID, in this example 3.







































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     +-----+            +-----+            +-----+            +-----+
     |  S  |            |  T  |            | 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 18: Pause and Resume Operation Between One Sender and Two
                       Receivers Through Translator

   Figure 18 explains the pause and resume operations when a transport
   Translator 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
   Translator, which is forwarded to R1 and R2 through the Translator,
   T.  R1 and R2 receives RTP data from Translator at t2.  At this




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   point, both R1 and R2 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 translator T.  The sender S
   sees the RESUME at time t6 and continues to send data to T 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.  The RTP sender
   immediately resumes the stream.

   Consider also an RTP session which includes one or more receivers,
   paused sender(s), and a Translator.  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 and RESUME.  The exchange of these new messages MAY have
   some security implications, which need to be addressed by the user.
   Following are some important implications,

   1.  Identity spoofing - An attacker can spoof him/herself as an
       authenticated user and can falsely pause or resume any source
       transmission.  In order to prevent this type of attack, a strong
       authentication and integrity protection mechanism is needed.

   2.  Denial of Service (DoS) - An attacker can falsely pause all
       source streams which MAY result in Denial of Service (DoS).  An
       Authentication protocol may prevent this attack.

   3.  Man-in-Middle Attack (MiMT) - The pausing and resuming of an RTP
       source is prone to a Man-in-Middle attack.  Public key
       authentication may be used to prevent MiMT.

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.  Emil Ivov, Christian Groves and Bernard
   Aboba provided valuable review comments.




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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.

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

   [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-topologies-update]
              Westerlund, M. and S. Wenger, "RTP Topologies", draft-
              ietf-avtcore-rtp-topologies-update-04 (work in progress),
              August 2014.

   [I-D.ietf-avtext-rtp-grouping-taxonomy]
              Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
              "A Taxonomy of Grouping Semantics and Mechanisms for Real-
              Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
              rtp-grouping-taxonomy-02 (work in progress), June 2014.







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   [I-D.ietf-rtcweb-use-cases-and-requirements]
              Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
              Time Communication Use-cases and Requirements", draft-
              ietf-rtcweb-use-cases-and-requirements-14 (work in
              progress), February 2014.

   [I-D.westerlund-avtcore-rtp-simulcast]
              Westerlund, M. and S. Nandakumar, "Using Simulcast in RTP
              Sessions", draft-westerlund-avtcore-rtp-simulcast-04 (work
              in progress), July 2014.

   [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.

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

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

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

Appendix A.  Changes From Earlier Versions

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

A.1.  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.2.  Modifications Between Version -03 and -04

   o  Change of Copyright boilerplate





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A.3.  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.4.  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.5.  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

   Phone: +46107141311
   Email: bo.burman@ericsson.com
   URI:   www.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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