RTP Stream Pause and Resume
draft-ietf-avtext-rtp-stream-pause-02
The information below is for an old version of the document.
| Document | Type |
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|
|---|---|---|---|
| Authors | Bo Burman , Azam Akram , Roni Even , Magnus Westerlund | ||
| Last updated | 2014-07-24 | ||
| Replaces | draft-westerlund-avtext-rtp-stream-pause | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-avtext-rtp-stream-pause-02
Network Working Group B. Burman
Internet-Draft A. Akram
Updates: 5104 (if approved) Ericsson
Intended status: Standards Track R. Even
Expires: January 25, 2015 Huawei Technologies
M. Westerlund
Ericsson
July 24, 2014
RTP Stream Pause and Resume
draft-ietf-avtext-rtp-stream-pause-02
Abstract
With the increased popularity of real-time multimedia applications,
it is desirable to provide good control of resource usage, and users
also demand more control over communication sessions. This document
describes how a receiver in a multimedia conversation can pause and
resume incoming data from a sender by sending real-time feedback
messages when using Real-time Transport Protocol (RTP) for real time
data transport. This document extends the Codec Control Messages
(CCM) RTCP feedback package by explicitly allowing and describing
specific use of existing CCM messages and adding a group of new real-
time feedback messages used to pause and resume RTP data streams.
This document updates RFC 5104.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 25, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . 9
3.5. Media Receiver to Media Sender Across RTP Mixer . . . . . 10
4. Design Considerations . . . . . . . . . . . . . . . . . . . . 10
4.1. Real-time Nature . . . . . . . . . . . . . . . . . . . . 10
4.2. Message Direction . . . . . . . . . . . . . . . . . . . . 11
4.3. Apply to Individual Sources . . . . . . . . . . . . . . . 11
4.4. Consensus . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 11
4.6. Retransmitting Requests . . . . . . . . . . . . . . . . . 12
4.7. Sequence Numbering . . . . . . . . . . . . . . . . . . . 12
4.8. Relation to Other Solutions . . . . . . . . . . . . . . . 12
5. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Expressing Capability . . . . . . . . . . . . . . . . . . 13
5.2. Requesting to Pause . . . . . . . . . . . . . . . . . . . 14
5.3. Media Sender Pausing . . . . . . . . . . . . . . . . . . 15
5.4. Requesting to Resume . . . . . . . . . . . . . . . . . . 16
5.5. TMMBR/TMMBN Considerations . . . . . . . . . . . . . . . 17
6. Participant States . . . . . . . . . . . . . . . . . . . . . 18
6.1. Playing State . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Pausing State . . . . . . . . . . . . . . . . . . . . . . 19
6.3. Paused State . . . . . . . . . . . . . . . . . . . . . . 19
6.3.1. RTCP BYE Message . . . . . . . . . . . . . . . . . . 20
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6.3.2. SSRC Time-out . . . . . . . . . . . . . . . . . . . . 20
6.4. Local Paused State . . . . . . . . . . . . . . . . . . . 20
7. Message Format . . . . . . . . . . . . . . . . . . . . . . . 20
8. Message Details . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. PAUSE . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.2. PAUSED . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.3. RESUME . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.4. REFUSE . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.5. Transmission Rules . . . . . . . . . . . . . . . . . . . 26
9. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1. Offer-Answer Use . . . . . . . . . . . . . . . . . . . . 29
9.2. Declarative Use . . . . . . . . . . . . . . . . . . . . . 30
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.1. Offer-Answer . . . . . . . . . . . . . . . . . . . . . . 31
10.2. Point-to-Point Session . . . . . . . . . . . . . . . . . 33
10.3. Point-to-Multipoint using Mixer . . . . . . . . . . . . 36
10.4. Point-to-Multipoint using Translator . . . . . . . . . . 38
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
12. Security Considerations . . . . . . . . . . . . . . . . . . . 42
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 42
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
15.1. Normative References . . . . . . . . . . . . . . . . . . 42
15.2. Informative References . . . . . . . . . . . . . . . . . 43
Appendix A. Changes From Earlier Versions . . . . . . . . . . . 44
A.1. Modifications Between Version -01 and -02 . . . . . . . . 44
A.2. Modifications Between Version -00 and -01 . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
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].
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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 all of the time.
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
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
and resume, 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)
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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
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)
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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 REFUSE 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
REFUSE Notification from an RTP stream sender that a PAUSE or
RESUME request will not be honored
Mixer: The intermediate RTP node which receives an RTP stream from
different 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].
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.
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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
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.
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3.2. RTP Mixer to Media Sender
One of the most commonly used topologies in centralized conferencing
is based on the RTP Mixer [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
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-Stream 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.
Just as for point-to-point (Section 3.1), there is only a single
receiver of the stream, the RTP Mixer, and pausing or resuming a
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stream does not affect anyone else than the sender and single
receiver of that stream.
3.3. RTP Mixer to Media Sender in Point-to-Multipoint
This use case is similar to the previous section, however the RTP
Mixer is involved in three domains that need to be separated; the
Multicast Network (including participants A and C), participant B,
and participant D. The difference from above is that A and C share a
multicast domain, which is depicted below.
+-----+
+---+ / \ +-----------+ +---+
| A |<---/ \ | |<---->| B |
+---+ / Multi- \ | | +---+
+ Cast +->| Mixer |
+---+ \ Network / | | +---+
| C |<---\ / | |<---->| D |
+---+ \ / +-----------+ +---+
+-----+
Figure 3: RTP Mixer in Point-to-Multipoint
If the RTP Mixer pauses a stream from A, it will not only pause the
stream towards itself, but will also stop the stream from arriving to
C, which C is heavily impacted by, might not approve of, and should
thus have a say on.
If the Mixer resumes a paused stream from A, it will be resumed also
towards C. In this case, if C is not interested it can simply ignore
the stream and is not impacted as much as above.
In this use case there are several receivers of a stream and special
care must be taken as not to pause a stream that is still wanted by
some receivers.
3.4. Media Receiver to RTP Mixer
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
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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:
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.
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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
simply do it. Any indication that an RTP stream is paused is the
responsibility of the RTP stream sender and may in some cases not
even be needed by the stream receiver.
4.3. Apply to Individual Sources
The PAUSE and RESUME messages apply to single RTP streams identified
by their SSRC, which means the receiver targets the sender's SSRC in
the PAUSE and RESUME requests. If a paused sender starts sending
with a new SSRC, the receivers will need to send a new PAUSE request
in order to pause it. PAUSED indications refer to a single one of
the sender's own, paused SSRC.
4.4. Consensus
An RTP stream sender should not pause an SSRC that some receiver
still wishes to receive. The reason is that in RTP topologies where
the stream is shared between multiple receivers, a single receiver on
that shared network, 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.
4.5. 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.
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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
error condition, a negative acknowledgment, may be needed to avoid
unnecessary retransmission of requests (Section 4.6).
4.6. Retransmitting Requests
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 that are more time critical, the
best resume performance may be achieved by repeating the request as
often as possible until a sufficient number have been sent to reach a
high probability of request delivery, or the stream gets delivered.
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. Considerations that apply when using
TMMBR/TMMBN for pause and resume purposes are also described.
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 REFUSE 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.
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 PAUSED indication is defined separately from
the others to support partial implementation, which is specifically
believed to be feasible for the RTP Mixer to Media Sender use case
(Section 3.2).
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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. This also applies when using TMMBR 0
in the point-to-point case.
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 wait period before the stream is paused, the pause is not
performed. In point-to-point configurations, the wait period may be
set to zero. Using a wait 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 or TMMBR 0 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) or TMMBN 0 for the stream. The
same condition can be caused by the RTP stream sender having received
a disapproving RESUME from a stream receiver A for a PAUSE request
sent by a stream sender B, but that the PAUSE sender (B) did not
receive the RESUME (from A) and may instead think that the PAUSE was
lost. In both cases, a 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.
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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 REFUSE response that contains the next
available PauseID value. This REFUSE 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
REFUSE 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 REFUSE. In this case, if the used PauseID would otherwise have
been effective, the REFUSE 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 REFUSE and the
condition causing REFUSE remains, those additional REFUSE should be
sent with regular RTCP timing. A single REFUSE 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. If the stream was
paused by a TMMBR 0, TMMBN 0 is used as PAUSED indication. What is
said on PAUSED in the rest of this paragraph apply also to the use of
TMMBN 0, except for PAUSED message parameters. Note that PauseID is
incremented when pausing locally (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.
The RTP stream sender may want to apply some local consideration to
exactly when the stream is paused, for example completing some media
unit or a forward error correction block, before pausing the stream.
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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.
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. If the stream was
paused with TMMBR 0, resuming the stream is made with TMMBR
containing a bitrate value larger than 0. The bitrate value used
when resuming after a PAUSE with TMMBR 0 is either according to known
limitations, or the configured maximum for the stream or session.
What is said on RESUME in the rest of this paragraph apply also to
the use of TMMBR with a bitrate value larger than 0, except for
RESUME message parameters.
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.
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There may be local considerations, 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 REFUSE containing the same PauseID as in the RESUME. When
receiving such REFUSE 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 REFUSE and the
condition causing REFUSE remains, those additional REFUSE should be
sent with regular RTCP timing. A single REFUSE can respond to
several identical RESUME requests.
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
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, 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 stream.
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TMMBR >0: Corresponds to RESUME when the 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.
TMMBN 0: Corresponds to PAUSED. Also corresponds to a REFUSE
indication when a stream is requested to be resumed with TMMBR >0.
TMMBN >0: Cannot be used as REFUSE 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 REFUSE in this section SHALL
be taken to apply to the extent possible also when TMMBR/TMMBN are
used (Section 5.5) for this functionality.
+------------------------------------------------------+
| Received RESUME |
v |
+---------+ Received PAUSE +---------+ Hold-off period +--------+
| Playing |---------------->| Pausing |---------------->| Paused |
| |<----------------| | | |
+---------+ Received RESUME +---------+ +--------+
^ | | PAUSE decision |
| | v |
| | PAUSE decision +---------+ PAUSE decision |
| +------------------>| Local |<--------------------+
+-------------------------| Paused |
RESUME decision +---------+
Figure 4: RTP Pause States
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.
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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 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 delay period (denoted by 'Hold-off period' in the
figure) 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.
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.
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.
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6.3.1. RTCP BYE Message
When a participant leaves the RTP session, it sends an RTCP BYE
message. In addition to the semantics described in 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.
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 in the next two
regular RTCP reports.
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.
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.
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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.
The Feedback Control Information (FCI) field consist of one or more
PAUSE, RESUME, PAUSED, REFUSE, or any future extension. These
messages have the following FCI format:
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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 REFUSE, 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: REFUSE 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.
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,
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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 REFUSE 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 revert to use those instead. 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 REFUSE (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
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
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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 REFUSE 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 MAY be sent either as a result of a valid PAUSE
(Section 8.1) request, when entering Paused State (Section 6.3), or
based on a RTP stream sender local decision, when entering Local
Paused State (Section 6.4).
PauseID MUST contain the available, valid value to be included in a
subsequent RESUME (Section 8.3).
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.
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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 REFUSE (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 REFUSE 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. REFUSE
REFUSE 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
REFUSE 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 REFUSE 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 REFUSE
responses are received before the scheduled REFUSE is sent, duplicate
REFUSE MUST NOT be scheduled for transmission. This effectively lets
a single REFUSE respond to several invalid PAUSE or RESUME requests.
If REFUSE containing a certain PauseID was already sent and yet more
PAUSE or RESUME messages are received that require additional REFUSE
with that specific PauseID to be scheduled, and unless the PauseID
number space has wrapped since REFUSE was last sent with that
PauseID, further REFUSE 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 REFUSE 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 REFUSE to clear.
An RTP stream receiver that sent a PAUSE or RESUME request and
receives a REFUSE containing a PauseID different from the request MAY
schedule another request using the PauseID from the REFUSE
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.
All messages defined in this specification MAY use either Regular,
Early or Immediate timings, taking the following into consideration:
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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.
o RESUME SHOULD always use Immediate or Early timing.
o The first transmission of REFUSE for each (non-wrapped) PauseID
SHOULD be sent with Immediate or Early timing, while subsequent
REFUSE 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. Like AVPF [RFC4585] and
CCM [RFC5104], it is RECOMMENDED to use the rtcp-fb attribute at
media level and it MUST NOT be used at session level. This
specification follows all the rules defined in AVPF for rtcp-fb
attribute relating to payload type in a session description.
Note: When TMMBR 0 / TMMBN 0 are used to implement pause and
resume functionality (with the restrictions described in this
memo), signaling rtcp-fb attribute with ccm tmmbr parameter is
sufficient and no further signaling is necessary.
This specification defines two new parameters to the "ccm" feedback
value defined in CCM [RFC5104], "pause" and "paused".
o "pause" represents the capability to understand the RTCP feedback
message and all of the defined FCIs of PAUSE, RESUME, PAUSED and
REFUSE. A direction sub-parameter is used to determine if a given
node desires to issue PAUSE or RESUME requests, can respond to
PAUSE or RESUME requests, or both.
o "paused" represents the functionality of supporting the playing
and local paused states and generate PAUSED FCI when a stream
delivery is paused. A direction sub-parameter is used to
determine if a given node desires to receive these indications,
intends to send them, or both.
The reason for this separation is to make it possible for partial
implementation of this specification, according to the different
roles in the use cases section (Section 3).
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A sub-parameter named "nowait", indicating that the hold-off time
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.
A sub-parameter named "dir" is used to indicate in which directions a
given node will use the pause or paused functionality. The node
being configured or issuing an offer or an answer uses the
directionality in the following way. Note that pause and paused have
separate and different definitions.
Direction ("dir") values for "pause" is defined as follows:
sendonly: The node intends to send PAUSE and RESUME requests for
other nodes' streams and is thus also capable of receiving PAUSED
and REFUSE. It will not support receiving PAUSE and RESUME
requests.
recvonly: The node supports receiving PAUSE and RESUME requests
targeted for streams sent by the node. It will send PAUSED and
REFUSE as needed. The node will not send any PAUSE and RESUME
requests.
sendrecv: The node supports receiving PAUSE and RESUME requests
targeted for streams sent by the node. The node intends to send
PAUSE and RESUME requests for other nodes' streams. Thus the node
is capable of sending and receiving all types of pause messages.
This is the default value. If the "dir" parameter is omitted, it
MUST be interpreted to represent this value.
Direction values for "paused" is defined as follows:
sendonly: The node intends to send PAUSED indications whenever it
pauses RTP stream delivery in any of its streams. It has no need
to receive PAUSED indications itself.
recvonly: The node desires to receive PAUSED indications whenever
any stream sent by another node is paused. It does not intend to
send any PAUSED indications.
sendrecv: The nodes desires to receive PAUSED indications and
intends to send PAUSED indications whenever any stream is paused.
This is the default value. If the "dir" parameter is omitted, it
MUST be interpreted to represent this value.
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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)
/ SP "paused" *(SP paused-attr)
pause-attr = direction
/ "nowait"
/ token ; for future extensions
paused-attr = direction
/ token ; for future extensions
direction = "dir=" direction-alts
direction-alts = "sendonly" / "recvonly" / "sendrecv"
Figure 6: ABNF
An endpoint implementing this specification and using SDP to signal
capability SHOULD indicate both of the new "pause" and "paused"
parameters with ccm signaling. When negotiating usage, it is
possible select either of them, noting that "pause" contain the full
"paused" functionality. A sender or receiver SHOULD NOT use the
messages from this specification towards receivers that did not
declare capability for it.
There MUST NOT be more than one "a=rtcp-fb" line with "pause" and one
with "paused" 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" or "paused" specification.
There MUST NOT be more than a single direction sub-parameter per
"pause" and "paused" parameter. There MUST NOT be more than a single
"nowait" sub-parameter per "pause" parameter.
9.1. Offer-Answer Use
An offerer implementing this specification needs to include "pause"
and/or "paused" CCM parameters with suitable directionality parameter
("dir") in the SDP, according to what messages it intends to send and
desires or is capable to receive in the session. It is RECOMMENDED
to include both "pause" and "paused" if "pause" is supported, to
enable at least the "paused" functionality if the answer only
supports "paused" or different directionality for the two
functionalities. The "pause" and "paused" functionalities are
negotiated independently, although the "paused" functionality is part
of the "pause" functionality. As a result, an answerer MAY remove
"pause" or "paused" lines from the SDP depending on the agreed mode
of functionality.
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In offer/answer, the "dir" parameter is interpreted based on the
agent providing the SDP. The node described in the offer is the
offerer, and the answerer is described in an answer. In other words,
an offer for "paused dir=sendonly" means that the offerer intends to
send PAUSED indications whenever it pauses RTP stream delivery in any
of its streams.
An answerer receiving an offer with a "pause" parameter with
dir=sendrecv MAY remove the pause line in its answer, respond with
pause keeping sendrecv for full bi-directionality, or it may change
dir value to either sendonly or recvonly based on its capabilities
and desired functionality. An offer with a "pause" parameter with
dir=sendonly or dir=recvonly is either completely removed or accepted
with reverse directionality, i.e. sendonly becomes recvonly or
recvonly becomes sendonly.
An answer receiving an offer with "paused" has the same choices as
for "pause" above. It should be noted that the directionality of
pause is the inverse of RTP stream direction, while the
directionality of paused is the same as the RTP stream direction.
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 "paused" line in the offer. If both
offer and answer contained a "nowait" parameter, then the hold-off
time 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 draft. First, it is normally
only necessary to include either "pause" or "paused" parameter to
indicate the level of functionality the node should use in this RTP
session. Including both is only necessary if some implementations
only understands "paused" and some other can understand both. Thus
indicating both means use pause if you understand it, and if you only
understand paused, use that.
The "dir" directionality parameter indicates how the configured node
should behave. For example "pause" with sendonly:
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sendonly: The node intends to send PAUSE and RESUME requests for
other nodes' streams and is thus also capable of receiving PAUSED
and REFUSE. It will not support receiving PAUSE and RESUME
requests.
In this example, the configured node should send PAUSE and RESUME
requests if has reason for it. It does not need to respond to any
PAUSE or RESUME requests as that is not supported.
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
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.
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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
a=rtcp-fb:* ccm paused
Figure 7: SDP Offer With Pause and Resume Capability
The offerer supports all of the messages defined in this
specification and offers a sendrecv stream. 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
both "pause" and "paused" parameters.
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 dir=sendonly
a=rtcp-fb:98 ccm paused
Figure 8: SDP Answer With Pause and Resume Capability
The answerer will not allow its sent streams to be paused or resumed
and thus support pause only in sendonly mode. It does support paused
and intends to send it, and also desires to receive PAUSED
indications. Thus paused in sendrecv mode is included in the answer.
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 time when being
requested to pause the stream.
When using TMMBR 0 / TMMBN 0 to achieve pause and resume
functionality, there are no differences in SDP compared to CCM
[RFC5104] and therefore no such examples are included here.
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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 time 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 9: Pause and Resume Operation in Point-to-Point
Figure 9 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 10: TMMBR Pause and Resume in Point-to-Point
Figure 10 describes the same point-to-point scenario as above, but
using TMMBR/TMMBN signaling.
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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 11: Pause and Resume Operation With Messages Lost
Figure 11 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 REFUSE 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: REFUSE(11) |
| ------------------------------> |
| |
| t4: RTP data |
| ------------------------------> |
: :
Figure 12: Pause Request is Refused in Point-to-Point
In Figure 12, the receiver requests to pause the sender, which
refuses to pause due to some consideration local to the sender and
responds with a REFUSE 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 13: 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 14: Pause and Resume Operation Between Two Participants Using
a Translator
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Figure 14 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 time 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 15: Pause and Resume Operation Between One Sender and Two
Receivers Through Translator
Figure 15 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
time, 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
As outlined in Section 7, this specification requests IANA to
allocate
1. The FMT number TBA1 to be allocated to the PAUSE and RESUME
functionality from this specification.
2. The 'pause' and 'paused' tags to be used with ccm under rtcp-fb
AVPF attribute in SDP.
3. The 'nowait' parameter to be used with the 'pause' and 'paused'
tags in SDP.
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4. A registry listing registered values for 'pause' Types.
5. PAUSE, RESUME, PAUSED, and REFUSE with the listed numbers in the
pause Type registry.
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.
14. Acknowledgements
Daniel Grondal made valuable contributions during the initial
versions of this draft.
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.
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[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.
15.2. Informative References
[I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-02 (work in progress),
May 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-01 (work in progress), February
2014.
[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-03 (work
in progress), October 2013.
[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.
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[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 -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.2. 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
Authors' Addresses
Bo Burman
Ericsson
Kistavagen 25
SE - 164 80 Kista
Sweden
Phone: +46107141311
Email: bo.burman@ericsson.com
URI: www.ericsson.com
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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- Kista 164 80
Sweden
Phone: +46107148287
Email: magnus.westerlund@ericsson.com
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