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RFC 7728

Proposed STD
Pages: 55
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RTP Stream Pause and Resume

Part 1 of 3, p. 1 to 21
None       Next RFC Part

Updates:    5104


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Internet Engineering Task Force (IETF)                         B. Burman
Request for Comments: 7728                                      A. Akram
Updates: 5104                                                   Ericsson
Category: Standards Track                                        R. Even
ISSN: 2070-1721                                      Huawei Technologies
                                                           M. Westerlund
                                                                Ericsson
                                                           February 2016


                      RTP Stream Pause and Resume

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

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

Page 2 
Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

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Table of Contents

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

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

1.  Introduction

   As real-time communication attracts more people, more applications
   are created; multimedia conversation applications is one example.
   Multimedia conversation further exists in many forms, for example,
   peer-to-peer chat application and multiparty video conferencing
   controlled by central media nodes, such as RTP Mixers.

   Multimedia conferencing may involve many participants; each has its
   own preferences for the communication session, not only at the start
   but also during the session.  This document describes several
   scenarios in multimedia communication where a conferencing node or
   participant chooses to temporarily pause an incoming RTP [RFC3550]
   stream and later resume it when needed.  The receiver does not need
   to terminate or inactivate the RTP session and start all over again
   by negotiating the session parameters, for example, using SIP
   [RFC3261] with the Session Description Protocol (SDP) [RFC4566]
   offer/answer [RFC3264].

   Centralized nodes, like RTP Mixers or Multipoint Control Units (MCUs)
   that use either logic based on voice activity, other measurements, or
   user input could reduce the resources consumed in both the sender and
   the network by temporarily pausing the RTP streams that aren't
   required by the RTP Mixer.  If the number of conference participants
   are greater than what the conference logic has chosen to present
   simultaneously to receiving participants, some participant RTP
   streams sent to the RTP Mixer may not need to be forwarded to any
   other participant.  Those RTP streams could then be temporarily
   paused.  This becomes especially useful when the media sources are
   provided in multiple encoding versions (Simulcast) [SDP-SIMULCAST] or
   with Multi-Session Transmission (MST) of scalable encoding such as
   Scalable Video Coding (SVC) [RFC6190].  There may be some of the

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   defined encodings or a combination of scalable layers that are not
   used or cannot be used all of the time.  As an example, a centralized
   node may choose to pause such unused RTP streams without being
   explicitly requested to do so, maybe due to temporarily limited
   network or processing resources.  It may then also send an explicit
   indication that the streams are paused.

   As the set of RTP streams required at any given point in time is
   highly dynamic in such scenarios, using the out-of-band signaling
   channel for pausing, and even more importantly resuming, an RTP
   stream is difficult due to the performance requirements.  Instead,
   the pause and resume signaling should be in the media plane and go
   directly between the affected nodes.  When using RTP [RFC3550] for
   media transport, using "Extended RTP Profile for Real-time Transport
   Control Protocol (RTCP)-Based Feedback (RTP/AVPF)" [RFC4585] appears
   appropriate.  No currently existing RTCP feedback message explicitly
   supports pausing and resuming an incoming RTP stream.  As this
   affects the generation of packets and may even allow the encoding
   process to be paused, the functionality appears to match Codec
   Control Messages (CCMs) in the RTP Audio-Visual Profile with Feedback
   (AVPF) [RFC5104].  This document defines the solution as a CCM
   extension.

   The Temporary Maximum Media Bitrate Request (TMMBR) message of CCM is
   used by video conferencing systems for flow control.  It is desirable
   to be able to use that method with a bitrate value of zero for pause,
   whenever possible.  This specification updates RFC 5104 by adding the
   new pause and resume semantics to the TMMBR and Temporary Maximum
   Media Bitrate Notification (TMMBN) messages.

2.  Definitions

2.1.  Abbreviations

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

   CCM:      Codec Control Message (RFC 5104)

   CNAME:    Canonical Name (RTCP Source Description)

   CSRC:     Contributing Source (RTP)

   FCI:      Feedback Control Information (AVPF)

   FIR:      Full Intra Refresh (CCM)

   FMT:      Feedback Message Type (AVPF)

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   MCU:      Multipoint Control Unit

   MTU:      Maximum Transfer Unit

   PT:       Payload Type (RTP)

   RTP:      Real-time Transport Protocol (RFC 3550)

   RTCP:     RTP Control Protocol (RFC 3550)

   RTCP RR:  RTCP Receiver Report

   RTCP SR:  RTCP Sender Report

   SDP:      Session Description Protocol (RFC 4566)

   SIP:      Session Initiation Protocol (RFC 3261)

   SSRC:     Synchronization Source (RTP)

   SVC:      Scalable Video Coding

   TMMBR:    Temporary Maximum Media Bitrate Request (CCM)

   TMMBN:    Temporary Maximum Media Bitrate Notification (CCM)

   UA:       User Agent (SIP)

   UDP:      User Datagram Protocol (RFC 768)

2.2.  Terminology

   In addition to the following, the definitions from RTP [RFC3550],
   AVPF [RFC4585], CCM [RFC5104], and RTP Taxonomy [RFC7656] also apply
   in this document.

   Feedback Messages:  CCM [RFC5104] categorized different RTCP feedback
      messages into four types: Request, Command, Indication, and
      Notification.  This document places the PAUSE and RESUME messages
      into the Request category, PAUSED as an Indication, and REFUSED as
      a Notification.

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

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

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      PAUSED:   Indication from an RTP stream sender that a stream is
                paused

      REFUSED:  Notification from an RTP stream sender that a PAUSE or
                RESUME request will not be honored

   Mixer:  The intermediate RTP node that receives an RTP stream from
      different endpoints, combines them to make one RTP stream, and
      forwards them to destinations, in the sense described for Topo-
      Mixer in "RTP Topologies" [RFC7667].

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

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

   Pausing receiver:  An RTP stream receiver that sends a PAUSE request,
      defined in this specification, to another participant(s).

   Stream:  Used as a short term for RTP stream, unless otherwise noted.

   Stream receiver:  Short for RTP stream receiver; the RTP entity
      responsible for receiving an RTP stream, usually a Media
      Depacketizer.

   Stream sender:  Short for RTP stream sender; the RTP entity
      responsible for creating an RTP stream, usually a Media
      Packetizer.

2.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

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3.  Use Cases

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

   The RTCWEB WG's use case and requirements document [RFC7478] defines
   the following API requirements in Appendix A, which is also used by
   the W3C WebRTC WG:

   A8  The web API must provide means for the web application to mute/
       unmute a stream or stream component(s).  When a stream is sent to
       a peer, mute status must be preserved in the stream received by
       the peer.

   A9  The web API must provide means for the web application to cease
       the sending of a stream to a peer.

   This document provides means to optimize transport usage by stopping
   the sending of muted streams and starting the sending of streams
   again when unmuted.  Here, it is assumed that "mute" above can be
   taken to apply also to media other than audio.  At the time of
   publication for this specification, the RTCWEB WG did not specify any
   pause/resume functionality.

3.1.  Point to Point

   This is the most basic use case with an RTP session containing two
   endpoints.  Each endpoint sends one or more streams.

                            +---+         +---+
                            | A |<------->| B |
                            +---+         +---+

                         Figure 1: Point to Point

   The usage of RTP stream pause in this use case is to temporarily halt
   delivery of streams that the sender provides but the receiver does
   not currently use.  This can, for example, be due to minimized
   applications where the video stream is not actually shown on any
   display, or it is not used in any other way, such as being recorded.
   In this case, since there is only a single receiver of the stream,
   pausing or resuming a stream does not impact anyone else other than
   the sender and the single receiver of that stream.

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3.2.  RTP Mixer to Media Sender

   One of the most commonly used topologies in centralized conferencing
   is based on the RTP Mixer [RFC7667].  The main reason for this is
   that it provides a very consistent view of the RTP session towards
   each participant.  That is accomplished through the Mixer originating
   its own streams, identified by distinct SSRC values, and any RTP
   streams sent to the participants will be sent using those SSRC
   values.  If the Mixer wants to identify the underlying media sources
   for its conceptual streams, it can identify them using CSRC.  The
   stream the Mixer provides can be an actual mix of multiple media
   sources, but it might also be switching received streams as described
   in Sections 3.6 - 3.8 of "RTP Topologies" [RFC7667].

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

                    Figure 2: RTP Mixer in Unicast Only

   Which streams from clients B, C, and D that are delivered to a given
   receiver, A, can depend on several things:

   o  The RTP Mixer's own logic and measurements, such as voice activity
      on the incoming audio streams.

   o  The number of sent media sources exceed what is reasonable to
      present simultaneously at any given receiver.

   o  A human controlling the conference that determines how the media
      should be mixed.  This would be more common in lecture or similar
      applications where regular listeners may be prevented from
      breaking into the session unless approved by the moderator.

   o  The streams may also be part of a Simulcast [SDP-SIMULCAST] or
      scalable encoded (for Multi-Session Transmission) [RFC6190], thus
      providing multiple versions that can be delivered by the RTP
      stream sender.

   These examples indicate that there are numerous reasons why a
   particular stream would not currently be in use but must be available
   for use at very short notice if any dynamic event occurs that causes
   a different stream selection to be done in the Mixer.

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   Because of this, it would be highly beneficial if the Mixer could
   request the RTP stream sender to pause a particular stream.  The
   Mixer also needs to be able to request the RTP stream sender to
   resume delivery with minimal delay.

   In some cases, especially when the Mixer sends multiple RTP streams
   per receiving client, there may be situations that make it desirable
   for the Mixer to pause some of its sent RTP streams, even without
   being explicitly asked to do so by the receiving client.  Such
   situations can, for example, be caused by a temporary lack of
   available Mixer network or processing resources.  An RTP stream
   receiver that no longer receives an RTP stream could interpret this
   as an error condition and try to take action to re-establish the RTP
   stream.  Such action would likely be undesirable if the RTP stream
   was in fact deliberately paused by the Mixer.  Undesirable RTP stream
   receiver actions could be avoided if the Mixer is able to explicitly
   indicate that an RTP stream is deliberately paused.

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

3.3.  RTP Mixer to Media Sender in Point to Multipoint

   This use case is similar to the previous section; however, the RTP
   Mixer is involved in three domains that need to be separated: the
   Multicast Network (including participants A and C), participant B,
   and participant D.  The difference from above is that A and C share a
   multicast domain, which is depicted below.

                        +-----+
             +---+     /       \     +-----------+      +---+
             | A |<---/         \    |           |<---->| B |
             +---+   /   Multi-  \   |           |      +---+
                    +    cast     +->|   Mixer   |
             +---+   \  Network  /   |           |      +---+
             | C |<---\         /    |           |<---->| D |
             +---+     \       /     +-----------+      +---+
                        +-----+

                Figure 3: RTP Mixer in Point to Multipoint

   If the RTP Mixer pauses a stream from A, it will not only pause the
   stream towards itself but will also stop the stream from arriving to
   C, which C is heavily impacted by, might not approve of, and should
   thus have a say on.

Top      ToC       Page 11 
   If the Mixer resumes a paused stream from A, it will be resumed also
   towards C.  In this case, if C is not interested, it can simply
   ignore the stream and is not impacted as much as above.

   In this use case, there are several receivers of a stream, and the
   Mixer must take special care so as not to pause a stream that is
   still wanted by some receivers.

3.4.  Media Receiver to RTP Mixer

   In this use case, the direction of the request to pause is the
   opposite compared to the two previous use cases.  An endpoint in
   Figure 2 could potentially request to pause the delivery of a given
   stream.  Possible reasons include those in the point-to-point case
   (Section 3.1) above.

   When the RTP Mixer is only connected to individual unicast paths, the
   use case and any considerations are identical to the point-to-point
   use case.

   However, when the endpoint requesting stream pause is connected to
   the RTP Mixer through a multicast network, such as A or C in
   Figure 3, the use case instead becomes identical to the one in
   Section 3.3, only with reverse direction of the streams and pause/
   resume requests.

3.5.  Media Receiver to Media Sender across RTP Mixer

   An endpoint, like A in Figure 2, could potentially request to pause
   the delivery of a given stream, like one of B's, over any of the
   SSRCs used by the Mixer by sending a pause request for the CSRC
   identifying the stream.  However, the authors are of the opinion that
   this is not a suitable solution for several reasons:

   1.  The Mixer might not include CSRC in its stream indications.

   2.  An endpoint cannot rely on the CSRC to correctly identify the
       stream to be paused when the delivered media is some type of mix.
       A more elaborate stream identification solution is needed to
       support this in the general case.

   3.  The endpoint cannot determine if a given stream is still needed
       by the RTP Mixer to deliver to another session participant.

   Due to the above reasons, we exclude this use case from further
   consideration.

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4.  Design Considerations

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

4.1.  Real-Time Nature

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

4.2.  Message Direction

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

4.3.  Apply to Individual Sources

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

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4.4.  Consensus

   An RTP stream sender should not pause an SSRC that some receiver
   still wishes to receive.

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

4.5.  Message Acknowledgments

   RTP and RTCP does not guarantee reliable data transmission.  It uses
   whatever assurance the lower-layer transport protocol can provide.
   However, this is commonly UDP that provides no reliability
   guarantees.  Thus, it is possible that a PAUSE and/or RESUME message
   transmitted from an RTP endpoint does not reach its destination,
   i.e., the targeted RTP stream sender.  When PAUSE or RESUME reaches
   the RTP stream sender and is effective, i.e., an active RTP stream
   sender pauses or a resuming RTP stream sender has media data to
   transmit, it is immediately seen from the arrival or non-arrival of
   RTP packets for that RTP stream.  Thus, no explicit acknowledgments
   are required in this case.

   In some cases, when a PAUSE or RESUME message reaches the RTP stream
   sender, it will not be able to pause or resume the stream due to some
   local consideration, for example, lack of data to transmit.  In this
   error condition, a negative acknowledgment may be needed to avoid
   unnecessary retransmission of requests (Section 4.6).

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4.6.  Request Retransmission

   When the stream is not affected as expected by a PAUSE or RESUME
   request, the request may have been lost and the sender of the request
   will need to retransmit it.  The retransmission should take the
   round-trip time into account, and will also need to take the normal
   RTCP bandwidth and timing rules applicable to the RTP session into
   account, when scheduling retransmission of feedback.

   When it comes to resume requests or unsolicited paused indications
   that are more time critical, the best performance may be achieved by
   repeating the message as often as possible until a sufficient number
   have been sent to reach a high probability of message delivery or at
   an explicit indication that the message was delivered.  For resume
   requests, such explicit indication can be delivery of the RTP stream
   being requested to be resumed.

4.7.  Sequence Numbering

   A PAUSE request message will need to have a sequence number to
   separate retransmissions from new requests.  A retransmission keeps
   the sequence number unchanged, while it is incremented every time a
   new PAUSE request is transmitted that is not a retransmission of a
   previous request.

   Since RESUME always takes precedence over PAUSE and is even allowed
   to avoid pausing a stream, there is a need to keep strict ordering of
   PAUSE and RESUME.  Thus, RESUME needs to share sequence number space
   with PAUSE and implicitly reference which PAUSE it refers to.  For
   the same reasons, the explicit PAUSED indication also needs to share
   sequence number space with PAUSE and RESUME.

4.8.  Relation to Other Solutions

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

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

5.  Solution Overview

   The proposed solution implements pause and resume functionality based
   on sending AVPF RTCP feedback messages from any RTP session
   participant that wants to pause or resume a stream targeted at the
   stream sender, as identified by the sender SSRC.

   This solution reuses CCM TMMBR and TMMBN [RFC5104] to the extent
   possible and defines a small set of new RTCP feedback messages where
   new semantics is needed.

   A single feedback message specification is used to implement the new
   messages.  The message consists of a number of Feedback Control
   Information (FCI) blocks, where each block can be a PAUSE request, a
   RESUME request, a PAUSED indication, a REFUSED notification, or an
   extension to this specification.  This structure allows a single
   feedback message to handle pause functionality on a number of
   streams.

   The PAUSED functionality is also defined in such a way that it can be
   used as a standalone by the RTP stream sender to indicate a local
   decision to pause, and it can inform any receiver of the fact that
   halting media delivery is deliberate and which RTP packet was the
   last transmitted.

   Special considerations that apply when using TMMBR/TMMBN for pause
   and resume purposes are described in Section 5.6.  This specification
   applies to both the new messages defined herein as well as their
   TMMBR/TMMBN counterparts, except when explicitly stated otherwise.
   An obvious exception is any reference to the message parameters that
   are only available in the messages defined here.  For example, any
   reference to PAUSE in the text below is equally applicable to
   TMMBR 0, and any reference to PAUSED is equally applicable to TMMBN
   0.  Therefore, and for brevity, TMMBR/TMMBN will not be mentioned in
   the text, unless there is specific reason to do so.

   This section is intended to be explanatory and therefore
   intentionally contains no mandatory statements.  Such statements can
   instead be found in other parts of this specification.

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5.1.  Expressing Capability

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

   For the case when TMMBR/TMMBN are used for pause and resume purposes,
   it is possible to explicitly express joint support for TMMBR and
   TMMBN, but not for TMMBN only.

5.2.  PauseID

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

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5.3.  Requesting to Pause

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

   The PAUSE request contains the current PauseID (Section 5.2).

   When a non-paused RTP stream sender receives the PAUSE request, it
   continues to send the RTP stream while waiting for some time to allow
   other RTP stream receivers in the same RTP session that saw this
   PAUSE request to disapprove by sending a RESUME (Section 5.5) for the
   same stream and with the same current PauseID as in the PAUSE being
   disapproved.  If such a disapproving RESUME arrives at the RTP stream
   sender during the hold-off period before the stream is paused, the
   pause is not performed.  In point-to-point configurations, the hold-
   off period may be set to zero.  Using a hold-off period of zero is
   also appropriate when using TMMBR 0 and is in line with the semantics
   for that message.

   If the RTP stream sender receives further PAUSE requests with the
   current PauseID while waiting as described above, those additional
   requests are ignored.

   If the PAUSE request is lost before it reaches the RTP stream sender,
   it will be discovered by the RTP stream receiver because it continues
   to receive the RTP stream.  It will also not see any PAUSED
   indication (Section 5.4) for the stream.  The same condition can be
   caused by the RTP stream sender having received a disapproving RESUME
   from stream receiver A for a PAUSE request sent by stream sender B,
   except that the PAUSE sender (B) did not receive the RESUME (from A)
   and may instead think that the PAUSE was lost.  In both cases, a
   PAUSE request can be retransmitted using the same current PauseID.
   If using TMMBR 0, the request MAY be retransmitted when the requester
   fails to receive a TMMBN 0 confirmation.

   If the pending stream pause is aborted due to a disapproving RESUME,
   the pause and resume operation for that PauseID is concluded, the
   current PauseID is updated, and any new PAUSE must therefore use the
   new current PauseID to be effective.

   An RTP stream sender receiving a PAUSE not using the current PauseID
   informs the RTP stream receiver sending the ineffective PAUSE of this
   condition by sending a REFUSED notification that contains the current
   PauseID value.

   A situation where an ineffective PauseID is chosen can appear when a
   new RTP stream receiver joins a session and wants to PAUSE a stream
   but does not yet know the current PauseID to use.  The REFUSED

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   notification will then provide sufficient information to create a
   valid PAUSE.  The required extra signaling round trip is not
   considered harmful, since it is assumed that pausing a stream is not
   time critical (Section 4.1).

   There may be local considerations making it impossible or infeasible
   to pause the stream, and the RTP stream sender can then respond with
   a REFUSED.  In this case, if the used current PauseID would otherwise
   have been effective, REFUSED contains the same current PauseID as in
   the PAUSE request.  Note that when using TMMBR 0 as PAUSE, that
   request cannot be refused (TMMBN > 0) due to the existing restriction
   in Section 4.2.2.2 of [RFC5104] that TMMBN shall contain the current
   bounding set, and the fact that a TMMBR 0 will always be the most
   restrictive point in any bounding set, regardless of the bounding set
   overhead value.

   If the RTP stream sender receives several identical PAUSE requests
   for an RTP stream that was already responded to at least once with
   REFUSED and the condition causing REFUSED remains, those additional
   REFUSED notifications should be sent with regular RTCP timing.  A
   single REFUSED can respond to several identical PAUSE requests.

5.4.  Media Sender Pausing

   An RTP stream sender can choose to pause the stream at any time.
   This can be either a result of receiving a PAUSE or based on some
   local sender consideration.  When it does, it sends a PAUSED
   indication, containing the current PauseID.  Note that the current
   PauseID in an unsolicited PAUSED (without having received a PAUSE) is
   incremented compared to a previously sent PAUSED.  It also sends the
   PAUSED indication in the next two regular RTCP reports, given that
   the pause condition is then still effective.

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

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

   The PAUSED indication also contains information about the RTP
   extended highest sequence number when the pause became effective.
   This provides RTP stream receivers with firsthand information that
   allows them to know whether they lost any packets just before the

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   stream paused or when the stream is resumed again.  This allows RTP
   stream receivers to quickly and safely take into account that the
   stream is paused in, for example, retransmission or congestion
   control algorithms.

   If the RTP stream sender receives PAUSE requests with the current
   PauseID while the stream is already paused, those requests are
   ignored.

   As long as the stream is being paused, the PAUSED indication MAY be
   sent together with any regular RTCP Sender Report (SR) or Receiver
   Report (RR).  Including PAUSED in this way allows RTP stream
   receivers to join while the stream is paused and to quickly know that
   there is a paused stream, what the last sent extended RTP sequence
   number is, and what the current PauseID is, which enables them to
   construct valid PAUSE and RESUME requests at a later stage.

   When the RTP stream sender learns that a new endpoint has joined the
   RTP session, for example, by a new SSRC and a CNAME that was not
   previously seen in the RTP session, it should send PAUSED indications
   for all its paused streams at its earliest opportunity.  In addition,
   it should continue to include PAUSED indications in at least two
   regular RTCP reports.

5.5.  Requesting to Resume

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

   A pausing RTP stream sender that receives a RESUME including the
   current PauseID resumes the stream at the earliest opportunity.
   Receiving RESUME requests for a stream that is not paused does not
   require any action and can be ignored.

   There may be local considerations at the RTP stream sender, for
   example, that the media device is not ready, making it temporarily
   impossible to resume the stream at that point in time, and the RTP
   stream sender can then respond with a REFUSED containing the current
   PauseID.  When receiving such REFUSED with a current PauseID
   identical to the one in the sent RESUME, RTP stream receivers should
   avoid sending further RESUME requests for some reasonable amount of
   time to allow the condition to clear.  An RTP stream sender having
   sent a REFUSED SHOULD resume the stream through local considerations
   (see below) when the condition that caused the REFUSED is no longer
   true.

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   If the RTP stream sender receives several identical RESUME requests
   for an RTP stream that was already at least once responded to with
   REFUSED and the condition causing REFUSED remains, those additional
   REFUSED notifications should be sent with regular RTCP timing.  A
   single REFUSED can respond to several identical RESUME requests.

   A pausing RTP stream sender can apply local considerations and can
   resume a paused RTP stream at any time.  If TMMBR 0 was used to pause
   the RTP stream, resumption is prevented by protocol, even if the RTP
   sender would like to resume due to local considerations.  If TMMBR/
   TMMBN signaling is used, the RTP stream is paused due to local
   considerations (Section 5.4), and the RTP stream sender thus owns the
   TMMBN bounding set, the RTP stream can be resumed due to local
   considerations.

   When resuming a paused stream, especially for media that makes use of
   temporal redundancy between samples such as video, it may not be
   appropriate to use such temporal dependency in the encoding between
   samples taken before the pause and at the time instant the stream is
   resumed.  Should such temporal dependency between media samples
   before and after the media was paused be used by the RTP stream
   sender, it requires the RTP stream receiver to have saved the samples
   from before the pause for successful continued decoding when
   resuming.  The use of this temporal dependency of media samples from
   before the pause is left up to the RTP stream sender.  If temporal
   dependency on samples from before the pause is not used when the RTP
   stream is resumed, the first encoded sample after the pause will not
   contain any temporal dependency on samples before the pause (for
   video it may be a so-called intra picture).  If temporal dependency
   on samples from before the pause is used by the RTP stream sender
   when resuming, and if the RTP stream receiver did not save any sample
   from before the pause, the RTP stream receiver can use a FIR request
   [RFC5104] to explicitly ask for a sample without temporal dependency
   (for video a so-called intra picture), even at the same time as
   sending the RESUME.

5.6.  TMMBR/TMMBN Considerations

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

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   This is a brief summary of what functionality is provided when using
   TMMBR/TMMBN:

   TMMBR 0:  Corresponds to PAUSE, without the requirement for any hold-
      off period to wait for RESUME before pausing the RTP stream.

   TMMBR > 0:  Corresponds to RESUME when the RTP stream was previously
      paused with TMMBR 0.  Since there is only a single RTP stream
      receiver, there is no need for the RTP stream sender to delay
      resuming the stream until after sending TMMBN > 0 or to apply the
      hold-off period specified in [RFC5104] before increasing the
      bitrate from zero.  The bitrate value used when resuming after
      pausing with TMMBR 0 is either according to known limitations or
      based on starting a stream with the configured maximum for the
      stream or session, for example, given by "b=" line in SDP.

   TMMBN 0:  Corresponds to PAUSED when the RTP stream was paused with
      TMMBR 0 but may, just as PAUSED, also be used unsolicited.  An
      unsolicited RTP stream pause based on local sender considerations
      uses the RTP stream's own SSRC as the TMMBR restriction owner in
      the TMMBN message bounding set.  It also corresponds to a REFUSED
      notification when a stream is requested to be resumed with
      TMMBR > 0, thus resulting in the stream sender becoming the owner
      of the bounding set in the TMMBN message.

   TMMBN > 0:  Cannot be used as a REFUSED notification when a stream is
      requested to be paused with TMMBR 0, for reasons stated in
      Section 5.3.


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