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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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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