| Rfc | 7728 |
|---|
| Title | RTP Stream Pause and Resume |
|---|
| Author | B. Burman, A. Akram, R. Even, M.
Westerlund |
|---|
| Date | February 2016 |
|---|
| Format: | TXT, HTML |
|---|
| Updates | RFC5104 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) B. Burman
Request for Comments: 7728 A. Akram
Updates: 5104 Ericsson
Category: Standards Track R. Even
ISSN: 2070-1721 Huawei Technologies
M. Westerlund
Ericsson
February 2016
RTP Stream Pause and Resume
Abstract
With the increased popularity of real-time multimedia applications,
it is desirable to provide good control of resource usage, and users
also demand more control over communication sessions. This document
describes how a receiver in a multimedia conversation can pause and
resume incoming data from a sender by sending real-time feedback
messages when using the Real-time Transport Protocol (RTP) for real-
time data transport. This document extends the Codec Control Message
(CCM) RTP Control Protocol (RTCP) feedback package by explicitly
allowing and describing specific use of existing CCMs and adding a
group of new real-time feedback messages used to pause and resume RTP
data streams. This document updates RFC 5104.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7728.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Requirements Language . . . . . . . . . . . . . . . . . . 7
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Point to Point . . . . . . . . . . . . . . . . . . . . . 8
3.2. RTP Mixer to Media Sender . . . . . . . . . . . . . . . . 9
3.3. RTP Mixer to Media Sender in Point to Multipoint . . . . 10
3.4. Media Receiver to RTP Mixer . . . . . . . . . . . . . . . 11
3.5. Media Receiver to Media Sender across RTP Mixer . . . . . 11
4. Design Considerations . . . . . . . . . . . . . . . . . . . . 12
4.1. Real-Time Nature . . . . . . . . . . . . . . . . . . . . 12
4.2. Message Direction . . . . . . . . . . . . . . . . . . . . 12
4.3. Apply to Individual Sources . . . . . . . . . . . . . . . 12
4.4. Consensus . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5. Message Acknowledgments . . . . . . . . . . . . . . . . . 13
4.6. Request Retransmission . . . . . . . . . . . . . . . . . 14
4.7. Sequence Numbering . . . . . . . . . . . . . . . . . . . 14
4.8. Relation to Other Solutions . . . . . . . . . . . . . . . 14
5. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Expressing Capability . . . . . . . . . . . . . . . . . . 16
5.2. PauseID . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.3. Requesting to Pause . . . . . . . . . . . . . . . . . . . 17
5.4. Media Sender Pausing . . . . . . . . . . . . . . . . . . 18
5.5. Requesting to Resume . . . . . . . . . . . . . . . . . . 19
5.6. TMMBR/TMMBN Considerations . . . . . . . . . . . . . . . 20
6. Participant States . . . . . . . . . . . . . . . . . . . . . 22
6.1. Playing State . . . . . . . . . . . . . . . . . . . . . . 22
6.2. Pausing State . . . . . . . . . . . . . . . . . . . . . . 22
6.3. Paused State . . . . . . . . . . . . . . . . . . . . . . 23
6.3.1. RTCP BYE Message . . . . . . . . . . . . . . . . . . 24
6.3.2. SSRC Time-Out . . . . . . . . . . . . . . . . . . . . 24
6.4. Local Paused State . . . . . . . . . . . . . . . . . . . 24
7. Message Format . . . . . . . . . . . . . . . . . . . . . . . 26
8. Message Details . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. PAUSE . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.2. PAUSED . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.3. RESUME . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.4. REFUSED . . . . . . . . . . . . . . . . . . . . . . . . . 32
8.5. Transmission Rules . . . . . . . . . . . . . . . . . . . 32
9. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.1. Offer/Answer Use . . . . . . . . . . . . . . . . . . . . 37
9.2. Declarative Use . . . . . . . . . . . . . . . . . . . . . 39
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 39
10.1. Offer/Answer . . . . . . . . . . . . . . . . . . . . . . 40
10.2. Point-to-Point Session . . . . . . . . . . . . . . . . . 41
10.3. Point to Multipoint Using Mixer . . . . . . . . . . . . 45
10.4. Point to Multipoint Using Translator . . . . . . . . . . 47
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50
12. Security Considerations . . . . . . . . . . . . . . . . . . . 50
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 52
13.1. Normative References . . . . . . . . . . . . . . . . . . 52
13.2. Informative References . . . . . . . . . . . . . . . . . 53
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 54
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 55
1. Introduction
As real-time communication attracts more people, more applications
are created; multimedia conversation applications is one example.
Multimedia conversation further exists in many forms, for example,
peer-to-peer chat application and multiparty video conferencing
controlled by central media nodes, such as RTP Mixers.
Multimedia conferencing may involve many participants; each has its
own preferences for the communication session, not only at the start
but also during the session. This document describes several
scenarios in multimedia communication where a conferencing node or
participant chooses to temporarily pause an incoming RTP [RFC3550]
stream and later resume it when needed. The receiver does not need
to terminate or inactivate the RTP session and start all over again
by negotiating the session parameters, for example, using SIP
[RFC3261] with the Session Description Protocol (SDP) [RFC4566]
offer/answer [RFC3264].
Centralized nodes, like RTP Mixers or Multipoint Control Units (MCUs)
that use either logic based on voice activity, other measurements, or
user input could reduce the resources consumed in both the sender and
the network by temporarily pausing the RTP streams that aren't
required by the RTP Mixer. If the number of conference participants
are greater than what the conference logic has chosen to present
simultaneously to receiving participants, some participant RTP
streams sent to the RTP Mixer may not need to be forwarded to any
other participant. Those RTP streams could then be temporarily
paused. This becomes especially useful when the media sources are
provided in multiple encoding versions (Simulcast) [SDP-SIMULCAST] or
with Multi-Session Transmission (MST) of scalable encoding such as
Scalable Video Coding (SVC) [RFC6190]. There may be some of the
defined encodings or a combination of scalable layers that are not
used or cannot be used all of the time. As an example, a centralized
node may choose to pause such unused RTP streams without being
explicitly requested to do so, maybe due to temporarily limited
network or processing resources. It may then also send an explicit
indication that the streams are paused.
As the set of RTP streams required at any given point in time is
highly dynamic in such scenarios, using the out-of-band signaling
channel for pausing, and even more importantly resuming, an RTP
stream is difficult due to the performance requirements. Instead,
the pause and resume signaling should be in the media plane and go
directly between the affected nodes. When using RTP [RFC3550] for
media transport, using "Extended RTP Profile for Real-time Transport
Control Protocol (RTCP)-Based Feedback (RTP/AVPF)" [RFC4585] appears
appropriate. No currently existing RTCP feedback message explicitly
supports pausing and resuming an incoming RTP stream. As this
affects the generation of packets and may even allow the encoding
process to be paused, the functionality appears to match Codec
Control Messages (CCMs) in the RTP Audio-Visual Profile with Feedback
(AVPF) [RFC5104]. This document defines the solution as a CCM
extension.
The Temporary Maximum Media Bitrate Request (TMMBR) message of CCM is
used by video conferencing systems for flow control. It is desirable
to be able to use that method with a bitrate value of zero for pause,
whenever possible. This specification updates RFC 5104 by adding the
new pause and resume semantics to the TMMBR and Temporary Maximum
Media Bitrate Notification (TMMBN) messages.
2. Definitions
2.1. Abbreviations
AVPF: Audio-Visual Profile with Feedback (RFC 4585)
CCM: Codec Control Message (RFC 5104)
CNAME: Canonical Name (RTCP Source Description)
CSRC: Contributing Source (RTP)
FCI: Feedback Control Information (AVPF)
FIR: Full Intra Refresh (CCM)
FMT: Feedback Message Type (AVPF)
MCU: Multipoint Control Unit
MTU: Maximum Transfer Unit
PT: Payload Type (RTP)
RTP: Real-time Transport Protocol (RFC 3550)
RTCP: RTP Control Protocol (RFC 3550)
RTCP RR: RTCP Receiver Report
RTCP SR: RTCP Sender Report
SDP: Session Description Protocol (RFC 4566)
SIP: Session Initiation Protocol (RFC 3261)
SSRC: Synchronization Source (RTP)
SVC: Scalable Video Coding
TMMBR: Temporary Maximum Media Bitrate Request (CCM)
TMMBN: Temporary Maximum Media Bitrate Notification (CCM)
UA: User Agent (SIP)
UDP: User Datagram Protocol (RFC 768)
2.2. Terminology
In addition to the following, the definitions from RTP [RFC3550],
AVPF [RFC4585], CCM [RFC5104], and RTP Taxonomy [RFC7656] also apply
in this document.
Feedback Messages: CCM [RFC5104] categorized different RTCP feedback
messages into four types: Request, Command, Indication, and
Notification. This document places the PAUSE and RESUME messages
into the Request category, PAUSED as an Indication, and REFUSED as
a Notification.
PAUSE: Request from an RTP stream receiver to pause a stream
RESUME: Request from an RTP stream receiver to resume a paused
stream
PAUSED: Indication from an RTP stream sender that a stream is
paused
REFUSED: Notification from an RTP stream sender that a PAUSE or
RESUME request will not be honored
Mixer: The intermediate RTP node that receives an RTP stream from
different endpoints, combines them to make one RTP stream, and
forwards them to destinations, in the sense described for Topo-
Mixer in "RTP Topologies" [RFC7667].
Participant: A member that is part of an RTP session, acting as the
receiver, sender, or both.
Paused sender: An RTP stream sender that has stopped its
transmission, i.e., no other participant receives its RTP
transmission, based on having received either a PAUSE request,
defined in this specification, or a local decision.
Pausing receiver: An RTP stream receiver that sends a PAUSE request,
defined in this specification, to another participant(s).
Stream: Used as a short term for RTP stream, unless otherwise noted.
Stream receiver: Short for RTP stream receiver; the RTP entity
responsible for receiving an RTP stream, usually a Media
Depacketizer.
Stream sender: Short for RTP stream sender; the RTP entity
responsible for creating an RTP stream, usually a Media
Packetizer.
2.3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
3. Use Cases
This section discusses the main use cases for RTP stream pause and
resume.
The RTCWEB WG's use case and requirements document [RFC7478] defines
the following API requirements in Appendix A, which is also used by
the W3C WebRTC WG:
A8 The web API must provide means for the web application to mute/
unmute a stream or stream component(s). When a stream is sent to
a peer, mute status must be preserved in the stream received by
the peer.
A9 The web API must provide means for the web application to cease
the sending of a stream to a peer.
This document provides means to optimize transport usage by stopping
the sending of muted streams and starting the sending of streams
again when unmuted. Here, it is assumed that "mute" above can be
taken to apply also to media other than audio. At the time of
publication for this specification, the RTCWEB WG did not specify any
pause/resume functionality.
3.1. Point to Point
This is the most basic use case with an RTP session containing two
endpoints. Each endpoint sends one or more streams.
+---+ +---+
| A |<------->| B |
+---+ +---+
Figure 1: Point to Point
The usage of RTP stream pause in this use case is to temporarily halt
delivery of streams that the sender provides but the receiver does
not currently use. This can, for example, be due to minimized
applications where the video stream is not actually shown on any
display, or it is not used in any other way, such as being recorded.
In this case, since there is only a single receiver of the stream,
pausing or resuming a stream does not impact anyone else other than
the sender and the single receiver of that stream.
3.2. RTP Mixer to Media Sender
One of the most commonly used topologies in centralized conferencing
is based on the RTP Mixer [RFC7667]. The main reason for this is
that it provides a very consistent view of the RTP session towards
each participant. That is accomplished through the Mixer originating
its own streams, identified by distinct SSRC values, and any RTP
streams sent to the participants will be sent using those SSRC
values. If the Mixer wants to identify the underlying media sources
for its conceptual streams, it can identify them using CSRC. The
stream the Mixer provides can be an actual mix of multiple media
sources, but it might also be switching received streams as described
in Sections 3.6 - 3.8 of "RTP Topologies" [RFC7667].
+---+ +-----------+ +---+
| A |<---->| |<---->| B |
+---+ | | +---+
| Mixer |
+---+ | | +---+
| C |<---->| |<---->| D |
+---+ +-----------+ +---+
Figure 2: RTP Mixer in Unicast Only
Which streams from clients B, C, and D that are delivered to a given
receiver, A, can depend on several things:
o The RTP Mixer's own logic and measurements, such as voice activity
on the incoming audio streams.
o The number of sent media sources exceed what is reasonable to
present simultaneously at any given receiver.
o A human controlling the conference that determines how the media
should be mixed. This would be more common in lecture or similar
applications where regular listeners may be prevented from
breaking into the session unless approved by the moderator.
o The streams may also be part of a Simulcast [SDP-SIMULCAST] or
scalable encoded (for Multi-Session Transmission) [RFC6190], thus
providing multiple versions that can be delivered by the RTP
stream sender.
These examples indicate that there are numerous reasons why a
particular stream would not currently be in use but must be available
for use at very short notice if any dynamic event occurs that causes
a different stream selection to be done in the Mixer.
Because of this, it would be highly beneficial if the Mixer could
request the RTP stream sender to pause a particular stream. The
Mixer also needs to be able to request the RTP stream sender to
resume delivery with minimal delay.
In some cases, especially when the Mixer sends multiple RTP streams
per receiving client, there may be situations that make it desirable
for the Mixer to pause some of its sent RTP streams, even without
being explicitly asked to do so by the receiving client. Such
situations can, for example, be caused by a temporary lack of
available Mixer network or processing resources. An RTP stream
receiver that no longer receives an RTP stream could interpret this
as an error condition and try to take action to re-establish the RTP
stream. Such action would likely be undesirable if the RTP stream
was in fact deliberately paused by the Mixer. Undesirable RTP stream
receiver actions could be avoided if the Mixer is able to explicitly
indicate that an RTP stream is deliberately paused.
Just as for point to point (Section 3.1), there is only a single
receiver of the stream, the RTP Mixer, and pausing or resuming a
stream does not affect anyone else other than the sender and single
receiver of that stream.
3.3. RTP Mixer to Media Sender in Point to Multipoint
This use case is similar to the previous section; however, the RTP
Mixer is involved in three domains that need to be separated: the
Multicast Network (including participants A and C), participant B,
and participant D. The difference from above is that A and C share a
multicast domain, which is depicted below.
+-----+
+---+ / \ +-----------+ +---+
| A |<---/ \ | |<---->| B |
+---+ / Multi- \ | | +---+
+ cast +->| Mixer |
+---+ \ Network / | | +---+
| C |<---\ / | |<---->| D |
+---+ \ / +-----------+ +---+
+-----+
Figure 3: RTP Mixer in Point to Multipoint
If the RTP Mixer pauses a stream from A, it will not only pause the
stream towards itself but will also stop the stream from arriving to
C, which C is heavily impacted by, might not approve of, and should
thus have a say on.
If the Mixer resumes a paused stream from A, it will be resumed also
towards C. In this case, if C is not interested, it can simply
ignore the stream and is not impacted as much as above.
In this use case, there are several receivers of a stream, and the
Mixer must take special care so as not to pause a stream that is
still wanted by some receivers.
3.4. Media Receiver to RTP Mixer
In this use case, the direction of the request to pause is the
opposite compared to the two previous use cases. An endpoint in
Figure 2 could potentially request to pause the delivery of a given
stream. Possible reasons include those in the point-to-point case
(Section 3.1) above.
When the RTP Mixer is only connected to individual unicast paths, the
use case and any considerations are identical to the point-to-point
use case.
However, when the endpoint requesting stream pause is connected to
the RTP Mixer through a multicast network, such as A or C in
Figure 3, the use case instead becomes identical to the one in
Section 3.3, only with reverse direction of the streams and pause/
resume requests.
3.5. Media Receiver to Media Sender across RTP Mixer
An endpoint, like A in Figure 2, could potentially request to pause
the delivery of a given stream, like one of B's, over any of the
SSRCs used by the Mixer by sending a pause request for the CSRC
identifying the stream. However, the authors are of the opinion that
this is not a suitable solution for several reasons:
1. The Mixer might not include CSRC in its stream indications.
2. An endpoint cannot rely on the CSRC to correctly identify the
stream to be paused when the delivered media is some type of mix.
A more elaborate stream identification solution is needed to
support this in the general case.
3. The endpoint cannot determine if a given stream is still needed
by the RTP Mixer to deliver to another session participant.
Due to the above reasons, we exclude this use case from further
consideration.
4. Design Considerations
This section describes the requirements that this specification needs
to meet.
4.1. Real-Time Nature
The first section (Section 1) of this specification describes some
possible reasons why a receiver may pause an RTP sender. Pausing and
resuming is time dependent, i.e., a receiver may choose to pause an
RTP stream for a certain duration, after which the receiver may want
the sender to resume. This time dependency means that the messages
related to pause and resume must be transmitted to the sender in a
timely fashion in order for them to be purposeful. The pause
operation is arguably not as time critical as the resume operation,
since it mainly provides a reduction of resource usage. Timely
handling of the resume operation is, however, likely to directly
impact the end-user's perceived quality experience, since it affects
the availability of media that the user expects to receive more or
less instantly. It may also be highly desirable for a receiver to
quickly learn that an RTP stream is intentionally paused on the RTP
sender's own behalf.
4.2. Message Direction
It is the responsibility of an RTP stream receiver that wants to
pause or resume a stream from the sender(s) to transmit PAUSE and
RESUME messages. An RTP stream sender that wants to pause itself can
often simply do it, but sometimes this will adversely affect the
receiver and an explicit indication that the RTP stream is paused may
then help. Any indication that an RTP stream is paused is the
responsibility of the RTP stream sender and may in some cases not
even be needed by the stream receiver.
4.3. Apply to Individual Sources
The PAUSE and RESUME messages apply to single RTP streams identified
by their SSRC, which means the receiver targets the sender's SSRC in
the PAUSE and RESUME requests. If a paused sender starts sending
with a new SSRC, the receivers will need to send a new PAUSE request
in order to pause it. PAUSED indications refer to a single one of
the sender's own paused SSRC.
4.4. Consensus
An RTP stream sender should not pause an SSRC that some receiver
still wishes to receive.
The reason is that in RTP topologies where the stream is shared
between multiple receivers, a single receiver on that shared network
must not single-handedly cause the stream to be paused without
letting all other receivers voice their opinions on whether or not
the stream should be paused. Such shared networks can, for example,
be multicast, a mesh with a joint RTP session, or a transport
Translator-based network. A consequence of this is that a newly
joining receiver first needs to learn the existence of paused streams
and secondly should be able to resume any paused stream. A newly
joining receiver can, for example, be detected through an RTCP
Receiver Report containing both a new SSRC and a CNAME that does not
already occur in the session. Any single receiver wanting to resume
a stream should also cause it to be resumed. An important exception
to this is when the RTP stream sender is aware of conditions that
make it desirable or even necessary to pause the RTP stream on its
own behalf, without being explicitly asked to do so. Such local
consideration in the RTP sender takes precedence over RTP receiver
wishes to receive the stream.
4.5. Message Acknowledgments
RTP and RTCP does not guarantee reliable data transmission. It uses
whatever assurance the lower-layer transport protocol can provide.
However, this is commonly UDP that provides no reliability
guarantees. Thus, it is possible that a PAUSE and/or RESUME message
transmitted from an RTP endpoint does not reach its destination,
i.e., the targeted RTP stream sender. When PAUSE or RESUME reaches
the RTP stream sender and is effective, i.e., an active RTP stream
sender pauses or a resuming RTP stream sender has media data to
transmit, it is immediately seen from the arrival or non-arrival of
RTP packets for that RTP stream. Thus, no explicit acknowledgments
are required in this case.
In some cases, when a PAUSE or RESUME message reaches the RTP stream
sender, it will not be able to pause or resume the stream due to some
local consideration, for example, lack of data to transmit. In this
error condition, a negative acknowledgment may be needed to avoid
unnecessary retransmission of requests (Section 4.6).
4.6. Request Retransmission
When the stream is not affected as expected by a PAUSE or RESUME
request, the request may have been lost and the sender of the request
will need to retransmit it. The retransmission should take the
round-trip time into account, and will also need to take the normal
RTCP bandwidth and timing rules applicable to the RTP session into
account, when scheduling retransmission of feedback.
When it comes to resume requests or unsolicited paused indications
that are more time critical, the best performance may be achieved by
repeating the message as often as possible until a sufficient number
have been sent to reach a high probability of message delivery or at
an explicit indication that the message was delivered. For resume
requests, such explicit indication can be delivery of the RTP stream
being requested to be resumed.
4.7. Sequence Numbering
A PAUSE request message will need to have a sequence number to
separate retransmissions from new requests. A retransmission keeps
the sequence number unchanged, while it is incremented every time a
new PAUSE request is transmitted that is not a retransmission of a
previous request.
Since RESUME always takes precedence over PAUSE and is even allowed
to avoid pausing a stream, there is a need to keep strict ordering of
PAUSE and RESUME. Thus, RESUME needs to share sequence number space
with PAUSE and implicitly reference which PAUSE it refers to. For
the same reasons, the explicit PAUSED indication also needs to share
sequence number space with PAUSE and RESUME.
4.8. Relation to Other Solutions
A performance comparison between SIP/SDP and RTCP signaling
technologies was made and included in draft versions of this
specification. Using SIP and SDP to carry pause and resume
information means that they will need to traverse the entire
signaling path to reach the signaling destination (either the remote
endpoint or the entity controlling the RTP Mixer) across any
signaling proxies that potentially also have to process the SDP
content to determine if they are expected to act on it. The amount
of bandwidth required for a signaling solution based on SIP/SDP is in
the order of at least 10 times more than an RTCP-based solution.
Especially for a UA sitting on mobile wireless access, this will risk
introducing delays that are too long (Section 4.1) to provide a good
user experience, and the bandwidth cost may also be considered
infeasible compared to an RTCP-based solution. RTCP data sent
through the media path, which is likely shorter (contains fewer
intermediate nodes) than the signaling path, may have to traverse a
few intermediate nodes anyway. The amount of processing and
buffering required in intermediate nodes to forward those RTCP
messages is, however, believed to be significantly less than for
intermediate nodes in the signaling path. Based on those
considerations, RTCP is chosen as the signaling protocol for the
pause and resume functionality.
5. Solution Overview
The proposed solution implements pause and resume functionality based
on sending AVPF RTCP feedback messages from any RTP session
participant that wants to pause or resume a stream targeted at the
stream sender, as identified by the sender SSRC.
This solution reuses CCM TMMBR and TMMBN [RFC5104] to the extent
possible and defines a small set of new RTCP feedback messages where
new semantics is needed.
A single feedback message specification is used to implement the new
messages. The message consists of a number of Feedback Control
Information (FCI) blocks, where each block can be a PAUSE request, a
RESUME request, a PAUSED indication, a REFUSED notification, or an
extension to this specification. This structure allows a single
feedback message to handle pause functionality on a number of
streams.
The PAUSED functionality is also defined in such a way that it can be
used as a standalone by the RTP stream sender to indicate a local
decision to pause, and it can inform any receiver of the fact that
halting media delivery is deliberate and which RTP packet was the
last transmitted.
Special considerations that apply when using TMMBR/TMMBN for pause
and resume purposes are described in Section 5.6. This specification
applies to both the new messages defined herein as well as their
TMMBR/TMMBN counterparts, except when explicitly stated otherwise.
An obvious exception is any reference to the message parameters that
are only available in the messages defined here. For example, any
reference to PAUSE in the text below is equally applicable to
TMMBR 0, and any reference to PAUSED is equally applicable to TMMBN
0. Therefore, and for brevity, TMMBR/TMMBN will not be mentioned in
the text, unless there is specific reason to do so.
This section is intended to be explanatory and therefore
intentionally contains no mandatory statements. Such statements can
instead be found in other parts of this specification.
5.1. Expressing Capability
An endpoint can use an extension to CCM SDP signaling to declare
capability to understand the messages defined in this specification.
Capability to understand only a subset of messages is possible, to
support partial implementation, which is specifically believed to be
feasible for the 'RTP Mixer to Media Sender' use case (Section 3.2).
In that use case, only the RTP Mixer has capability to request the
media sender to pause or resume. Consequently, in that same use
case, only the media sender has capability to pause and resume its
sent streams based on requests from the RTP Mixer. Allowing for
partial implementation of this specification is not believed to
hamper interoperability, as long as the subsets are well defined and
describe a consistent functionality, including a description of how a
more capable implementation must perform fallback.
For the case when TMMBR/TMMBN are used for pause and resume purposes,
it is possible to explicitly express joint support for TMMBR and
TMMBN, but not for TMMBN only.
5.2. PauseID
All messages defined in this specification (Section 8) contain a
PauseID, satisfying the design consideration on sequence numbering
(Section 4.7). This PauseID is scoped by and thus a property of the
targeted RTP stream (SSRC) and is not only a sequence number for
individual messages. Instead, it numbers an entire "pause and resume
operation" for the RTP stream, typically keeping PauseID constant for
multiple, related messages. The PauseID value used during such
operation is called the current PauseID. A new "pause and resume
operation" is defined to start when the RTP stream sender resumes the
RTP stream after it was being paused. The current PauseID is then
incremented by one in modulo arithmetic. In the subsequent
descriptions below, it is sometimes necessary to refer to PauseID
values that were already used as the current PauseID, which is
denoted as the past PauseID. It should be noted that since PauseID
uses modulo arithmetic, a past PauseID may have a larger value than
the current PauseID. Since PauseID uses modulo arithmetic, it is
also useful to define what PauseID values are considered "past" to
clearly separate it from what could be considered "future" PauseID
values. Half of the entire PauseID value range is chosen to
represent a past PauseID, while a quarter of the PauseID value range
is chosen to represent future values. The remaining quarter of the
PauseID value range is intentionally left undefined in that respect.
5.3. Requesting to Pause
An RTP stream receiver can choose to send a PAUSE request at any
time, subject to AVPF timing rules.
The PAUSE request contains the current PauseID (Section 5.2).
When a non-paused RTP stream sender receives the PAUSE request, it
continues to send the RTP stream while waiting for some time to allow
other RTP stream receivers in the same RTP session that saw this
PAUSE request to disapprove by sending a RESUME (Section 5.5) for the
same stream and with the same current PauseID as in the PAUSE being
disapproved. If such a disapproving RESUME arrives at the RTP stream
sender during the hold-off period before the stream is paused, the
pause is not performed. In point-to-point configurations, the hold-
off period may be set to zero. Using a hold-off period of zero is
also appropriate when using TMMBR 0 and is in line with the semantics
for that message.
If the RTP stream sender receives further PAUSE requests with the
current PauseID while waiting as described above, those additional
requests are ignored.
If the PAUSE request is lost before it reaches the RTP stream sender,
it will be discovered by the RTP stream receiver because it continues
to receive the RTP stream. It will also not see any PAUSED
indication (Section 5.4) for the stream. The same condition can be
caused by the RTP stream sender having received a disapproving RESUME
from stream receiver A for a PAUSE request sent by stream sender B,
except that the PAUSE sender (B) did not receive the RESUME (from A)
and may instead think that the PAUSE was lost. In both cases, a
PAUSE request can be retransmitted using the same current PauseID.
If using TMMBR 0, the request MAY be retransmitted when the requester
fails to receive a TMMBN 0 confirmation.
If the pending stream pause is aborted due to a disapproving RESUME,
the pause and resume operation for that PauseID is concluded, the
current PauseID is updated, and any new PAUSE must therefore use the
new current PauseID to be effective.
An RTP stream sender receiving a PAUSE not using the current PauseID
informs the RTP stream receiver sending the ineffective PAUSE of this
condition by sending a REFUSED notification that contains the current
PauseID value.
A situation where an ineffective PauseID is chosen can appear when a
new RTP stream receiver joins a session and wants to PAUSE a stream
but does not yet know the current PauseID to use. The REFUSED
notification will then provide sufficient information to create a
valid PAUSE. The required extra signaling round trip is not
considered harmful, since it is assumed that pausing a stream is not
time critical (Section 4.1).
There may be local considerations making it impossible or infeasible
to pause the stream, and the RTP stream sender can then respond with
a REFUSED. In this case, if the used current PauseID would otherwise
have been effective, REFUSED contains the same current PauseID as in
the PAUSE request. Note that when using TMMBR 0 as PAUSE, that
request cannot be refused (TMMBN > 0) due to the existing restriction
in Section 4.2.2.2 of [RFC5104] that TMMBN shall contain the current
bounding set, and the fact that a TMMBR 0 will always be the most
restrictive point in any bounding set, regardless of the bounding set
overhead value.
If the RTP stream sender receives several identical PAUSE requests
for an RTP stream that was already responded to at least once with
REFUSED and the condition causing REFUSED remains, those additional
REFUSED notifications should be sent with regular RTCP timing. A
single REFUSED can respond to several identical PAUSE requests.
5.4. Media Sender Pausing
An RTP stream sender can choose to pause the stream at any time.
This can be either a result of receiving a PAUSE or based on some
local sender consideration. When it does, it sends a PAUSED
indication, containing the current PauseID. Note that the current
PauseID in an unsolicited PAUSED (without having received a PAUSE) is
incremented compared to a previously sent PAUSED. It also sends the
PAUSED indication in the next two regular RTCP reports, given that
the pause condition is then still effective.
There is no reply to a PAUSED indication; it is simply an explicit
indication of the fact that an RTP stream is paused. This can be
helpful for the RTP stream receiver, for example, to quickly
understand that transmission is deliberately and temporarily
suspended and no specific corrective action is needed.
The RTP stream sender may want to apply some local consideration to
exactly when the RTP stream is paused, for example, completing some
media unit or a forward error correction block, before pausing the
stream.
The PAUSED indication also contains information about the RTP
extended highest sequence number when the pause became effective.
This provides RTP stream receivers with firsthand information that
allows them to know whether they lost any packets just before the
stream paused or when the stream is resumed again. This allows RTP
stream receivers to quickly and safely take into account that the
stream is paused in, for example, retransmission or congestion
control algorithms.
If the RTP stream sender receives PAUSE requests with the current
PauseID while the stream is already paused, those requests are
ignored.
As long as the stream is being paused, the PAUSED indication MAY be
sent together with any regular RTCP Sender Report (SR) or Receiver
Report (RR). Including PAUSED in this way allows RTP stream
receivers to join while the stream is paused and to quickly know that
there is a paused stream, what the last sent extended RTP sequence
number is, and what the current PauseID is, which enables them to
construct valid PAUSE and RESUME requests at a later stage.
When the RTP stream sender learns that a new endpoint has joined the
RTP session, for example, by a new SSRC and a CNAME that was not
previously seen in the RTP session, it should send PAUSED indications
for all its paused streams at its earliest opportunity. In addition,
it should continue to include PAUSED indications in at least two
regular RTCP reports.
5.5. Requesting to Resume
An RTP stream receiver can request the RTP stream sender to resume a
stream with a RESUME request at any time, subject to AVPF timing
rules. The RTP stream receiver must include the current PauseID in
the RESUME request for it to be effective.
A pausing RTP stream sender that receives a RESUME including the
current PauseID resumes the stream at the earliest opportunity.
Receiving RESUME requests for a stream that is not paused does not
require any action and can be ignored.
There may be local considerations at the RTP stream sender, for
example, that the media device is not ready, making it temporarily
impossible to resume the stream at that point in time, and the RTP
stream sender can then respond with a REFUSED containing the current
PauseID. When receiving such REFUSED with a current PauseID
identical to the one in the sent RESUME, RTP stream receivers should
avoid sending further RESUME requests for some reasonable amount of
time to allow the condition to clear. An RTP stream sender having
sent a REFUSED SHOULD resume the stream through local considerations
(see below) when the condition that caused the REFUSED is no longer
true.
If the RTP stream sender receives several identical RESUME requests
for an RTP stream that was already at least once responded to with
REFUSED and the condition causing REFUSED remains, those additional
REFUSED notifications should be sent with regular RTCP timing. A
single REFUSED can respond to several identical RESUME requests.
A pausing RTP stream sender can apply local considerations and can
resume a paused RTP stream at any time. If TMMBR 0 was used to pause
the RTP stream, resumption is prevented by protocol, even if the RTP
sender would like to resume due to local considerations. If TMMBR/
TMMBN signaling is used, the RTP stream is paused due to local
considerations (Section 5.4), and the RTP stream sender thus owns the
TMMBN bounding set, the RTP stream can be resumed due to local
considerations.
When resuming a paused stream, especially for media that makes use of
temporal redundancy between samples such as video, it may not be
appropriate to use such temporal dependency in the encoding between
samples taken before the pause and at the time instant the stream is
resumed. Should such temporal dependency between media samples
before and after the media was paused be used by the RTP stream
sender, it requires the RTP stream receiver to have saved the samples
from before the pause for successful continued decoding when
resuming. The use of this temporal dependency of media samples from
before the pause is left up to the RTP stream sender. If temporal
dependency on samples from before the pause is not used when the RTP
stream is resumed, the first encoded sample after the pause will not
contain any temporal dependency on samples before the pause (for
video it may be a so-called intra picture). If temporal dependency
on samples from before the pause is used by the RTP stream sender
when resuming, and if the RTP stream receiver did not save any sample
from before the pause, the RTP stream receiver can use a FIR request
[RFC5104] to explicitly ask for a sample without temporal dependency
(for video a so-called intra picture), even at the same time as
sending the RESUME.
5.6. TMMBR/TMMBN Considerations
As stated above, TMMBR/TMMBN may be used to provide pause and resume
functionality for the point-to-point case. If the topology is not
point to point, TMMBR/TMMBN cannot safely be used for pause or
resume. This use is expected to be mainly for interworking with
implementations that don't support the messages defined in this
specification (Section 8) but make use of TMMBR/TMMBN to achieve a
similar effect.
This is a brief summary of what functionality is provided when using
TMMBR/TMMBN:
TMMBR 0: Corresponds to PAUSE, without the requirement for any hold-
off period to wait for RESUME before pausing the RTP stream.
TMMBR > 0: Corresponds to RESUME when the RTP stream was previously
paused with TMMBR 0. Since there is only a single RTP stream
receiver, there is no need for the RTP stream sender to delay
resuming the stream until after sending TMMBN > 0 or to apply the
hold-off period specified in [RFC5104] before increasing the
bitrate from zero. The bitrate value used when resuming after
pausing with TMMBR 0 is either according to known limitations or
based on starting a stream with the configured maximum for the
stream or session, for example, given by "b=" line in SDP.
TMMBN 0: Corresponds to PAUSED when the RTP stream was paused with
TMMBR 0 but may, just as PAUSED, also be used unsolicited. An
unsolicited RTP stream pause based on local sender considerations
uses the RTP stream's own SSRC as the TMMBR restriction owner in
the TMMBN message bounding set. It also corresponds to a REFUSED
notification when a stream is requested to be resumed with
TMMBR > 0, thus resulting in the stream sender becoming the owner
of the bounding set in the TMMBN message.
TMMBN > 0: Cannot be used as a REFUSED notification when a stream is
requested to be paused with TMMBR 0, for reasons stated in
Section 5.3.
6. Participant States
This document introduces three new states for a stream in an RTP
sender, according to the figure and subsections below. Any
references to PAUSE, PAUSED, RESUME, and REFUSED in this section
SHALL be taken to apply to the extent possible also when TMMBR/TMMBN
are used (Section 5.6) for this functionality.
+------------------------------------------------------+
| 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 in Sender
6.1. Playing State
This state is not new but is the normal media sending state from
[RFC3550]. When entering the state, the current PauseID MUST be
incremented by one in modulo arithmetic. The RTP sequence number for
the first packet sent after a pause SHALL be incremented by one
compared to the highest RTP sequence number sent before the pause.
The first RTP timestamp for the first packet sent after a pause
SHOULD be set according to capture times at the source, meaning the
RTP timestamp 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.
If the RTP stream sender has set the hold-off period to 0 and
receives information that it was an incorrect decision and that there
are in fact several receivers of the stream, it MUST change the hold-
off period to be based on the above formula instead.
An RTP stream sender SHOULD use the following criteria to determine
if there is only a single receiver, unless it has explicit and more
reliable information:
o Observing only a single CNAME across all received SSRCs (CNAMEs
for received CSRCs are insignificant), or
o If RTCP reporting groups [MULTI-STREAM-OPT] is used, observing
only a single, endpoint external RTCP reporting group.
6.3. Paused State
An RTP stream is in paused state when the sender pauses its
transmission after receiving at least one PAUSE message and the hold-
off period has passed without receiving any RESUME message for that
stream. Pausing transmission SHOULD only be done when reaching an
appropriate place to pause in the stream, like a media boundary that
avoids a media receiver to trigger repair or concealment actions.
When entering the state, the RTP stream sender SHALL send a PAUSED
indication to all known RTP stream receivers, and SHALL also repeat
PAUSED in the next two regular RTCP reports, as long as it is then
still in paused state.
Pausing an RTP stream MUST NOT affect the sending of RTP keepalive
[RFC6263][RFC5245] applicable to that RTP stream.
The following subsections discuss 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 Sections 6.3.4
and 6.3.7 of RTP [RFC3550], the 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 caused the RTP stream to pause. If that
receiver sends an RTCP BYE message observed by the sender, the
sender SHALL resume the RTP stream. No receivers that were in the
RTP session when the stream was paused objected that the stream
was paused, but if there were so far undetected receivers added to
the session during pause, those may not have learned about the
existence of the paused stream because either there was no PAUSED
sent for the paused RTP stream or those receivers did not support
PAUSED. Resuming the stream when the pausing party leaves the RTP
session allows those potentially undetected receivers to learn
that the stream exists.
6.3.2. SSRC Time-Out
Section 6.3.5 in RTP [RFC3550] describes the SSRC time-out of an RTP
participant. Every RTP participant maintains a sender and receiver
list in a session. If a participant does not get any RTP or RTCP
packets from some other participant for the last five RTCP reporting
intervals, it removes that participant from the receiver list. Any
streams that were paused by that removed participant SSRC SHALL be
resumed.
6.4. Local Paused State
This state can be entered at any time, based on local decision from
the RTP stream sender. Pausing transmission SHOULD only be done when
reaching an appropriate place to pause in the stream, like a media
boundary that avoids a media receiver to trigger repair or
concealment actions.
As with paused state (Section 6.3), the RTP stream sender SHALL send
a PAUSED indication to all known RTP stream receivers, when entering
the state, unless the stream was already in paused state
(Section 6.3). Such PAUSED indication SHALL be repeated a sufficient
number of times to reach a high probability that the message is
correctly delivered, stopping such repetition whenever leaving the
state.
When using TMMBN 0 as a PAUSED indication and when already in paused
state, the actions when entering local paused state depends on the
bounding set overhead value in the received TMMBR 0 that caused the
paused state and the bounding set overhead value used in (the RTP
stream sender's own) TMMBN 0:
TMMBN 0 overhead <= TMMBR 0 overhead: The RTP stream sender SHALL
NOT send any new TMMBN 0 replacing that active (more restrictive)
bounding set, even if entering local paused state.
TMMBN 0 overhead > TMMBR 0 overhead: The RTP stream sender SHALL
send TMMBN 0 with itself in the TMMBN bounding set when entering
local paused state.
The case above, when using TMMBN 0 as a PAUSED indication, being in
local paused state, and having received a TMMBR 0 with a bounding set
overhead value greater than the value the RTP stream sender would
itself use in a TMMBN 0, requires further consideration and is for
clarity henceforth referred to as "restricted local paused state".
As indicated in Figure 4, local paused state has higher precedence
than paused state (Section 6.3), and RESUME messages alone cannot
resume a paused RTP stream as long as the local decision still
applies. An RTP stream sender in local paused state is responsible
for leaving the state whenever the conditions that caused the
decision to enter the state no longer apply.
If the RTP stream sender is in restricted local paused state, it
cannot leave that state until the TMMBR 0 limit causing the state is
removed by a TMMBR > 0 (RESUME). If the RTP stream sender then needs
to stay in local paused state due to local considerations, it MAY
continue pausing the RTP stream by entering local paused state and
MUST then act accordingly, including sending a TMMBN 0 with itself in
the bounding set.
Pausing an RTP stream MUST NOT affect the sending of RTP keepalive
[RFC6263][RFC5245] applicable to that RTP stream.
When leaving the local paused state, the stream state SHALL become
Playing, regardless of whether or not there were any RTP stream
receivers that sent PAUSE for that stream during the local paused
state, effectively clearing the RTP stream sender's memory for that
stream.
7. Message Format
Section 6 of AVPF [RFC4585] defines three types of low-delay RTCP
feedback messages, i.e., transport-layer, payload-specific, and
application-layer feedback messages. This document defines a new
transport-layer feedback message, which is further subtyped into
either a PAUSE request, a RESUME request, a PAUSED indication, or a
REFUSED notification.
The transport-layer feedback messages are identified by having the
RTCP payload type be RTPFB (205) as defined by AVPF [RFC4585]. This
transport-layer feedback message, containing one or more of the
subtyped messages, is henceforth referred to as the PAUSE-RESUME
message. The specific FCI format is identified by a Feedback Message
Type (FMT) value in a common packet header for the feedback message
defined in Section 6.1 of AVPF [RFC4585]. The PAUSE-RESUME
transport-layer feedback message FCI is identified by FMT value = 9.
The Common Packet Format for feedback messages defined by AVPF
[RFC4585] is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT | PT | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
Figure 5: AVPF Common Feedback Message Packet Format
For the PAUSE-RESUME message defined in this memo, the following
interpretations of the packet fields apply:
FMT: The FMT value identifying the PAUSE-RESUME FCI: 9
PT: Payload Type = 205 (RTPFB)
Length: As defined by AVPF, i.e., the length of this packet in
32-bit words minus one, including the header and any padding.
SSRC of packet sender: The SSRC of the RTP session participant
sending the messages in the FCI. Note, for endpoints that have
multiple SSRCs in an RTP session, any of its SSRCs MAY be used to
send any of the pause message types.
SSRC of media source: Not used; SHALL be set to 0. The FCI
identifies the SSRC the message is targeted for.
The FCI field consists of one or more PAUSE, RESUME, PAUSED, or
REFUSED messages or any future extension. These messages have the
following FCI format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Res | Parameter Len | PauseID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Type Specific :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Syntax of FCI Entry in the PAUSE and RESUME Message
The FCI fields have the following definitions:
Target SSRC (32 bits): For a PAUSE-RESUME message, this value is the
SSRC that the request is intended for. For PAUSED, it MUST be the
SSRC being paused. If pausing is the result of a PAUSE request,
the value in PAUSED is effectively the same as Target SSRC in a
related PAUSE request. For REFUSED, it MUST be the Target SSRC of
the PAUSE or RESUME request that cannot change state. A CSRC MUST
NOT be used as a target as the interpretation of such a request is
unclear.
Type (4 bits): The pause feedback type. The values defined in this
specification are as follows:
0: PAUSE request message.
1: RESUME request message.
2: PAUSED indication message.
3: REFUSED notification message.
4-15: Reserved for future use. FCI fields with these Type values
SHALL be ignored on reception by receivers and MUST NOT be used
by senders implementing this specification.
Res: (4 bits): Type Specific reserved. It 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, as described in
Section 5.2. SHALL be incremented by one modulo 2^16 for each new
PAUSE message, unless the message is retransmitted. The initial
value SHOULD be 0. The PauseID is scoped by the Target SSRC,
meaning that PAUSE, RESUME, and PAUSED messages therefore share
the same PauseID space for a specific Target SSRC.
Type Specific (variable): Defined per pause feedback type. MAY be
empty. A receiver implementing this specification MUST be able to
skip and ignore any unknown Type Specific data, even for Type
values defined in this specification.
8. Message Details
This section contains detailed explanations of each message defined
in this specification. All transmissions of requests and indications
are governed by the transmission rules as defined by Section 8.5.
Any references to PAUSE, PAUSED, RESUME, and REFUSED in this section
SHALL be taken to apply to the extent possible and also when TMMBR/
TMMBN are used (Section 5.6) for this functionality. TMMBR/TMMBN MAY
be used instead of the messages defined in this specification when
the effective topology is point to point. This use is expected to be
mainly for interworking with implementations that don't support the
messages defined in this specification but make use of TMMBR/TMMBN to
achieve a similar effect. If either sender or receiver learns that
the topology is not point to point, TMMBR/TMMBN MUST NOT be used for
pause/resume functionality. If the messages defined in this
specification are supported in addition to TMMBR/TMMBN by all
involved parties, pause/resume signaling MUST use messages from this
specification. If the topology is not point to point and the
messages defined in this specification are not supported, pause/
resume functionality with TMMBR/TMMBN MUST NOT be used.
For the scope of this specification, a past PauseID (Section 5.2) is
defined as having a value between and including (PauseID - 2^15) MOD
2^16 and (PauseID - 1) MOD 2^16, where "MOD" is the modulo operator.
Similarly, a future PauseID is defined as having a value between and
including (PauseID + 1) MOD 2^16 and (PauseID + 2^14) MOD 2^16. It
is intentional that future PauseID is not defined as the entire range
outside that of past PauseID. The remaining range of PauseID is
simply "not current".
8.1. PAUSE
An RTP stream receiver MAY schedule PAUSE for transmission at any
time.
PAUSE has no defined Type Specific parameters.
PauseID SHOULD be the current PauseID, as indicated by PAUSED
(Section 8.2), REFUSED (Section 8.4), or implicitly determined by
previously received PAUSE or RESUME (Section 8.3) requests. A
randomly chosen PauseID MAY be used if it was not possible to
retrieve current PauseID information, in which case the PAUSE will
either succeed or the current PauseID can be found in the returned
REFUSED (Section 8.4).
It can be noted that as a result of what is described in Section 6.1,
PauseID is incremented by one, in modulo arithmetic, for each PAUSE
request that is not a retransmission, compared to what was used in
the last PAUSED indication sent by the media sender. PauseID in the
message is supposed to match current PauseID at the RTP stream
sender.
If an RTP stream receiver that sent a PAUSE with a certain PauseID
for a Target SSRC receives a RESUME or a REFUSED with the same
PauseID for the same Target SSRC, it is RECOMMENDED that it refrains
from scheduling further PAUSE requests for some appropriate time.
This is because the RESUME indicates that there are other receivers
that still wish to receive the stream, and the REFUSED indicates that
the RTP stream sender is currently not able to pause the stream.
What is an appropriate time can vary from application to application
and will also depend on the importance of achieving the bandwidth
saving, but 2-5 regular RTCP intervals is expected to be appropriate.
If the targeted RTP stream does not pause, if no PAUSED indication
with a future PauseID compared to the one used in PAUSE is received,
and if no REFUSED with the current or a future PauseID is received
within 2 * RTT + T_dither_max, the PAUSE MAY be scheduled for
retransmission, using the same current PauseID. RTT is the observed
round trip to the RTP stream sender, and T_dither_max is defined in
Section 3.4 of [RFC4585]. An RTP stream receiver in a bi-directional
RTP communication will generally have an RTT estimate to the RTP
stream sender, e.g., from RTCP SR/RR as described in Section 6.4 of
[RFC3550]. However, RTP stream receivers that don't send any RTP
streams will lack an RTT estimate unless they use additional
mechanisms, such as the "Receiver Reference Time Report Block" part
of RTCP XR [RFC3611]. RTP stream receivers that lack an RTT estimate
to the sender SHOULD use 500 ms as the default value.
When an RTP stream sender in playing state (Section 6.1) receives a
PAUSE with the current PauseID, and unless local considerations
currently make it impossible to pause the stream, it SHALL enter
pausing state (Section 6.2) and act accordingly.
If an RTP stream sender receives a PAUSE with the current PauseID
while in pausing, paused (Section 6.3), or local paused (Section 6.4)
states, the received PAUSE SHALL be ignored.
8.2. PAUSED
The PAUSED indication, if supported, MUST be sent whenever entering
paused state (Section 6.3) or local paused state (Section 6.4).
PauseID in the PAUSED message MUST contain the current PauseID that
can be included in a subsequent RESUME (Section 8.3). For local
paused state, this means that PauseID in the message is the current
PauseID, just as if the RTP stream sender had sent a PAUSE to itself.
PAUSED SHALL contain a fixed-length 32-bit parameter at the start of
the Type Specific field with the extended RTP sequence number of the
last RTP packet sent before the RTP stream was paused, in the same
format as the extended highest sequence number received
(Section 6.4.1 of [RFC3550]).
After having entered paused or local paused state and thus having
sent PAUSED once, PAUSED MUST also be included in (at least) the next
two regular RTCP reports, given that the pause condition is then
still effective.
PAUSED indications MAY be retransmitted, subject to transmission
rules (Section 8.5), to increase the probability that the message
reaches the receiver in a timely fashion. This can be especially
important when entering local paused state. The number of
repetitions to use could be tuned to observed loss rate and desired
loss probability, for example, based on RTCP reports received from
the intended message target.
While remaining in paused or local paused states, PAUSED MAY be
included in all compound RTCP reports, as long as the negotiated RTCP
bandwidth is not exceeded.
When in paused or local paused states, whenever the RTP stream sender
learns that there are endpoints that did not previously receive the
stream, for example, by RTCP reports with an SSRC and a CNAME that
were not previously seen in the RTP session, it is RECOMMENDED to
send PAUSED at the earliest opportunity and also to include it in (at
least) the next two regular RTCP reports, given that the pause
condition is then still effective.
8.3. RESUME
An RTP stream receiver MAY schedule RESUME for transmission whenever
it wishes to resume a paused stream or disapprove a stream from being
paused.
PauseID SHOULD be the current PauseID, as indicated by PAUSED
(Section 8.2) or implicitly determined by previously received PAUSE
(Section 8.1) or RESUME requests. A randomly chosen PauseID MAY be
used if it was not possible to retrieve current PauseID information,
in which case the RESUME will either succeed or the current PauseID
can be found in a returned REFUSED (Section 8.4).
If an RTP stream receiver that sent a RESUME with a certain PauseID
receives a REFUSED with the same PauseID, it is RECOMMENDED that it
refrains from scheduling further RESUME requests for some appropriate
time since the REFUSE indicates that it is currently not possible to
resume the stream. What is an appropriate time can vary from
application to application and will also depend on the importance of
resuming the stream, but 1-2 regular RTCP intervals is expected to be
appropriate.
RESUME requests MAY be retransmitted, subject to transmission rules
(Section 8.5), to increase the probability that the message reaches
the receiver in a timely fashion. The number of repetitions to use
could be tuned to observed loss rate and desired loss probability,
for example, based on RTCP reports received from the intended message
target. Such retransmission SHOULD stop as soon as RTP packets from
the targeted stream are received or when a REFUSED with the current
PauseID for the targeted RTP stream is received.
RESUME has no defined Type Specific parameters.
When an RTP stream sender in pausing (Section 6.2), paused
(Section 6.3), or local paused state (Section 6.4) receives a RESUME
with the current PauseID, and unless local considerations currently
make it impossible to resume the stream, it SHALL enter playing state
(Section 6.1) and act accordingly. If the RTP stream sender is
incapable of honoring a RESUME request with the current PauseID, or
if it receives a RESUME request with a PauseID that is not the
current PauseID while in paused or pausing state, the RTP stream
sender SHALL schedule a REFUSED message for transmission as specified
below.
If an RTP stream sender in playing state receives a RESUME containing
either the current PauseID or a past PauseID, the received RESUME
SHALL be ignored.
8.4. REFUSED
If an RTP stream sender receives a PAUSE (Section 8.1) or RESUME
(Section 8.3) request containing the current PauseID, where the
requested action cannot be fulfilled by the RTP stream sender due to
some local consideration, it SHALL schedule transmission of a REFUSED
notification containing the current PauseID from the rejected
request.
REFUSED has no defined Type Specific parameters.
If an RTP stream sender receives a PAUSE or RESUME request with a
PauseID that is not the current PauseID, it SHALL schedule a REFUSED
notification containing the current PauseID, except if the RTP stream
sender is in playing state and receives a RESUME with a past PauseID,
in which case the RESUME SHALL be ignored.
If several PAUSE or RESUME requests that would render identical
REFUSED notifications are received before the scheduled REFUSED is
sent, duplicate REFUSED notifications MUST NOT be scheduled for
transmission. This effectively lets a single REFUSED respond to
several ineffective PAUSE or RESUME requests.
An RTP stream receiver that sent a PAUSE or RESUME request and
receives a REFUSED containing the same PauseID as in the request
SHOULD refrain from sending an identical request for some appropriate
time to allow the condition that caused REFUSED to clear. For PAUSE,
an appropriate time is suggested in Section 8.1. For RESUME, an
appropriate time is suggested in Section 8.3.
An RTP stream receiver that sent a PAUSE or RESUME request and
receives a REFUSED containing a PauseID different from the request
MAY schedule another request using the PauseID from the REFUSED
notification.
8.5. Transmission Rules
The transmission of any RTCP feedback messages defined in this
specification MUST follow the normal AVPF-defined timing rules and
depend on the session's mode of operation.
All messages defined in this specification, as well as TMMBR/TMMBN
used for pause/resume purposes (Section 5.6), can use either Regular,
Early, or Immediate timings but should make a trade-off between
timely transmission (Section 4.1) and RTCP bandwidth consumption.
This can be achieved by taking the following into consideration:
o It is recommended that PAUSE use Early or Immediate timing, except
for retransmissions where RTCP bandwidth can motivate the use of
Regular timing.
o The first transmission of PAUSED for each (non-wrapped) PauseID is
recommended to be sent with Immediate or Early timing to stop
unnecessary repetitions of PAUSE. It is recommended that
subsequent transmissions of PAUSED for that PauseID use Regular
timing to avoid excessive PAUSED RTCP bandwidth caused by multiple
PAUSE requests.
o It is recommended that unsolicited PAUSED (sent when entering
local paused state (Section 6.4)) 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, to avoid RTP
stream receivers that take unnecessary corrective action when the
RTP stream is no longer received, after which it is recommended
that PAUSE uses Regular timing (as for PAUSED triggered by PAUSE
above).
o RESUME is often time critical, and it is recommended that it
always uses Immediate or Early timing.
o The first transmission of REFUSED for each (non-wrapped) PauseID
is recommended to be sent with Immediate or Early timing to stop
unnecessary repetitions of PAUSE or RESUME. It is recommended
that subsequent REFUSED notifications for that PauseID use Regular
timing to avoid excessive REFUSED RTCP bandwidth caused by
multiple unreasonable requests.
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 the 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 the "rtcp-fb" attribute with the "ccm"
and "tmmbr" parameters is sufficient and no further signaling is
necessary. There is, however, no guarantee that TMMBR/TMMBN
implementations predating this specification work exactly as
described here when used with a bitrate value of 0.
The "pause" parameter has two optional attributes, which are "nowait"
and "config":
o "nowait" indicates that the hold-off period defined in Section 6.2
can be set to 0, reducing the latency before the stream can be
paused after receiving a PAUSE request. This condition occurs
when there will only be a single receiver per direction in the RTP
session, for example, in point-to-point sessions. It is also
possible to use in scenarios using unidirectional media. The
conditions that allow "nowait" to be set (Section 6.2) also
indicate that it would be possible to use CCM TMMBR/TMMBN as
pause/resume signaling.
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 subset is
implemented:
1 Full implementation of this specification. This is the default
configuration. A missing "config" pause 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 it may pause sent RTP streams due to local
considerations and then intend 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 it 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 it also does
not support receiving PAUSED indications.
6 The implementation supports sent and received RTP streams being
paused due to local considerations and thus supports sending
and receiving PAUSED indications.
7 The implementation supports and desires to receive PAUSED
indications for received RTP streams but does not pause or send
PAUSED indications for sent RTP streams. It does not support
any other messages defined in this specification.
8 The implementation supports pausing sent RTP streams and
sending PAUSED indications for them but does not support
receiving PAUSED indications for received RTP streams. It does
not support any other messages defined in this specification.
All implementers of this specification are encouraged to include full
support for all messages ("config=1"), but it is recognized that this
is sometimes not meaningful for implementations operating in an
environment where only parts of the functionality provided by this
specification are needed. The above defined "config" functionality
subsets provide a trade-off between completeness and the need for
implementation interoperability, achieving at least a level of
functionality corresponding to what is desired by the least-capable
party when used as specified here. Implementing any functionality
subsets other than those defined above is NOT RECOMMENDED.
When signaling a "config" value other than 1, an implementation MUST
ignore non-supported messages on reception and SHOULD omit sending
messages not supported by the remote peer. One example where it can
be motivated to send messages that some receivers do not support is
when there are multiple message receivers with different message
support (different "config" values). That approach avoids letting
the least-capable receiver limit the functionality provided to
others. The below table summarizes per-message send and receive
support for the different "config" pause attribute values ("X"
indicating support and "-" indicating non-support):
+---+-----------------------------+-----------------------------+
| # | Send | Receive |
| | PAUSE RESUME PAUSED REFUSED | PAUSE RESUME PAUSED REFUSED |
+---+-----------------------------+-----------------------------+
| 1 | X X X X | X X X X |
| 2 | X X X - | - - X X |
| 3 | - - X X | X X X - |
| 4 | X X - - | - - X X |
| 5 | - - X X | X X - - |
| 6 | - - X - | - - X - |
| 7 | - - - - | - - X - |
| 8 | - - X - | - - - - |
+---+-----------------------------+-----------------------------+
Figure 7: Supported Messages for Different "config" Values
In the above description of partial implementations, "config" values
2 and 4 correspond to the RTP Mixer in the 'RTP Mixer to Media
Sender' use case (Section 3.2), and "config" values 3 and 5
correspond to the media sender in that same use case. For that use
case, it should be clear that an RTP Mixer implementing only "config"
values 3 or 5 will not provide a working solution. Similarly, for
that use case, a media sender implementing only "config" values 2 or
4 will not provide a working solution. Both the RTP Mixer and the
media sender will of course work when implementing the full set of
messages, corresponding to "config=1".
A partial implementation is not suitable for pause/resume support
between cascaded RTP Mixers, but it would require support
corresponding to "config=1" between such RTP Mixers. This is because
an RTP Mixer is then also a media sender towards the other RTP Mixer,
requiring support for the union of "config" values 2 and 3 or
"config" values 4 and 5, which effectively becomes "config=1".
As can be seen from Figure 7 above, "config" values 2 and 3 differ
from "config" values 4 and 5 only in that in the latter, the PAUSE/
RESUME message sender (e.g., the RTP Mixer side) does not support
local pause (Section 6.4) for any of its own streams and therefore
also does not support sending PAUSED.
Partial implementations that only support local pause functionality
can declare this capability through "config" values 6-8.
Viable fallback rules between different "config" values are described
in Section 9.1 and Figure 9.
This is the resulting ABNF [RFC5234], extending the existing ABNF in
Section 7.1 of CCM [RFC5104]:
rtcp-fb-ccm-param =/ SP "pause" *(SP pause-attr)
pause-attr = pause-config ; partial message support
/ "nowait" ; no hold-off period
/ byte-string ; for future extensions
pause-config = "config=" pause-config-value
pause-config-value = 1*2DIGIT
; byte-string as defined in RFC 4566
Figure 8: ABNF
An endpoint implementing this specification and using SDP to signal
capability SHOULD indicate the new "pause" parameter with "ccm"
signaling but MAY instead use existing "ccm tmmbr" signaling
[RFC5104] if the limitations in functionality when using TMMBR/TMMBN
as described in this specification (Section 5.6) are considered
acceptable. In that case, no partial message support is possible.
The messages from this specification (Section 8) SHOULD NOT be used
towards receivers that did not declare capability to receive those
messages.
The pause functionality can normally be expected to work
independently of the payload type. However, there might exist
situations where an endpoint needs to restrict or at least configure
the capabilities differently depending on the payload type carrying
the media stream. Reasons for this might relate to capabilities to
correctly handle media boundaries and avoid any pause or resume
operation to occur where it would leave a receiver or decoder with no
choice than to attempt to repair or discard the media received just
prior to or at the point of resuming.
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 the payload type, in which case "*" SHALL be
interpreted as applicable to all listed payload types that do not
have an explicit "pause" specification. The "config" pause attribute
MUST NOT appear more than once for each "pause" CCM parameter. The
"nowait" pause attribute MUST NOT appear more than once for each
"pause" CCM parameter.
9.1. Offer/Answer Use
An offerer implementing this specification needs to include the
"pause" CCM parameter with a 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" pause attribute and its message
directions are interpreted based on the agent providing the SDP. The
offerer is described in an offer, and the answerer is described in an
answer.
An answerer receiving an offer with a "pause" CCM line and a "config"
pause attribute with a certain value, describing a certain capability
to send and receive messages, MAY change the "config" pause attribute
value in the answer to another configuration. The permitted answers
are listed in the below table.
SDP Offer "config" value | Permitted SDP Answer "config" values
-------------------------+-------------------------------------
1 | 1, 2, 3, 4, 5, 6, 7, 8
2 | 3, 4, 5, 6, 7, 8
3 | 2, 4, 5, 6, 7, 8
4 | 5, 6, 7, 8
5 | 4, 6, 7, 8
6 | 6, 7, 8
7 | 8
8 | 7
Figure 9: "config" Values in Offer/Answer
An offer or answer omitting the "config" pause attribute MUST be
interpreted as equivalent to "config=1". Implementations of this
specification MUST NOT use any "config" values other than those
defined above in an offer or answer and MUST remove the "pause" CCM
line in the answer when receiving an offer with a "config" value it
does not understand. In all cases, the answerer MAY also completely
remove any "pause" CCM line to indicate that it does not understand
or desire to use any pause functionality for the affected payload
types.
If the offerer believes that itself and the intended answerer are
likely the only endpoints in the RTP session, it MAY include the
"nowait" pause attribute on the "pause" line in the offer. If an
answerer receives the "nowait" pause attribute on the "pause" line in
the SDP, and if it has information that the offerer and itself are
not the only endpoints in the RTP session, it MUST NOT include any
"nowait" pause attribute on its "pause" line in the SDP answer. The
answerer MUST NOT add "nowait" on the "pause" line in the answer
unless it is present on the "pause" line in the offer. If both offer
and answer contain a "nowait" pause attribute, then the hold-off
period is configured to 0 at both the offerer and answerer.
Unknown pause attributes MUST be ignored in the offer and MUST then
be omitted from the answer.
If both "pause" and "tmmbr" are present in the offer, both MAY be
included also in the answer, in which case TMMBR/TMMBN MUST NOT be
used for pause/resume purposes (with a bitrate value of 0), to avoid
signaling ambiguity.
9.2. Declarative Use
In declarative use, the SDP is used to configure the node receiving
the SDP. This has implications on the interpretation of the SDP
signaling extensions defined in this specification.
First, the "config" pause attribute and its message directions are
interpreted based on the node receiving the SDP, and it describes the
RECOMMENDED level of operation. If the joining client does not
support the indicated "config" value, some RTP session stream
optimizations may not be possible in that some RTP streams will not
be paused by the joining client, and/or the joining client may not be
able to resume and receive wanted streams because they are paused.
Second, the "nowait" pause attribute, 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 the Session Announcement Protocol (SAP) [RFC2974] will
not be in a point-to-point session, thus "nowait" cannot be included.
A Real-Time Streaming Protocol (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.
Unknown pause attributes MUST be ignored.
If both "pause" and "tmmbr" are present in the SDP, TMMBR/TMMBN MUST
NOT be used for pause/resume purposes (with a bitrate value of 0) to
avoid signaling ambiguity.
10. Examples
The following examples show use of PAUSE and RESUME messages,
including use of offer/answer:
1. Offer/Answer
2. Point-to-Point Session
3. Point to Multipoint using Mixer
4. Point to Multipoint using Relay
10.1. Offer/Answer
The below figures contain an example of how to show support for
pausing and resuming the streams, as well as indicating whether or
not the hold-off period can be set to 0.
v=0
o=alice 3203093520 3203093520 IN IP4 alice.example.com
s=Pausing Media
t=0 0
c=IN IP4 alice.example.com
m=audio 49170 RTP/AVPF 98 99
a=rtpmap:98 G719/48000
a=rtpmap:99 PCMA/8000
a=rtcp-fb:* ccm pause nowait
Figure 10: SDP Offer with Pause and Resume Capability
The offerer supports all of the messages defined in this
specification, leaving out the optional "config" pause 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" pause
attribute 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 11: SDP Answer with Pause and Resume Capability
The answerer will not allow its sent streams to be paused or resumed
and thus restricts the answer to indicate "config=2". It also
supports pausing its own RTP streams due to local considerations,
which is why "config=2" is chosen rather than "config=4". The
answerer somehow knows that it will not be a point-to-point RTP
session and has therefore removed "nowait" from the "pause" line,
meaning that the offerer must use a non-zero hold-off period when
being requested to pause the stream.
When using TMMBR 0 / TMMBN 0 to achieve pause and resume
functionality, there are no differences in SDP compared to CCM
[RFC5104]; 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 > |
| t7: PAUSED(4) |
| -------------------------------> |
: :
Figure 12: Pause and Resume Operation in Point to Point
Figure 12 shows the basic pause and resume operation in a
point-to-point scenario. At time t1, an RTP sender sends data to a
receiver. At time t2, the RTP receiver requests the sender to pause
the stream, using PauseID 3 (which it knew since before in this
example). The sender pauses the data and replies with a PAUSED
containing the same PauseID. Some time later (at time t4), the
receiver requests the sender to resume, which resumes its
transmission. The next PAUSE, sent at time t6, contains an updated
PauseID (4), with a corresponding PAUSED being sent at time t7.
+---------------+ +---------------+
| RTP Sender | | RTP Receiver |
+---------------+ +---------------+
: t1: RTP data :
| -------------------------------> |
| t2: TMMBR 0 |
| <------------------------------- |
| < RTP data paused > |
| t3: TMMBN 0 |
| -------------------------------> |
: < Some time passes > :
| t4: TMMBR 150000 |
| <------------------------------- |
| t5: RTP data |
| -------------------------------> |
: < Some time passes > :
| t6: TMMBR 0 |
| <------------------------------- |
| < RTP data paused > |
| t7: TMMBN 0 |
| -------------------------------> |
: :
Figure 13: TMMBR Pause and Resume in Point to Point
Figure 13 describes the same point-to-point scenario as above, but
using TMMBR/TMMBN signaling.
+---------------+ +----------------+
| RTP Sender A | | RTP Receiver B |
+---------------+ +----------------+
: t1: RTP data :
| -------------------------------> |
| < RTP data paused > |
| t2: TMMBN {A:0} |
| -------------------------------> |
: < Some time passes > :
| t3: TMMBR 0 |
| <------------------------------- |
| t4: TMMBN {A:0,B:0} |
| -------------------------------> |
: < Some time passes > :
| t5: TMMBN {B:0} |
| -------------------------------> |
: < Some time passes > :
| t6: TMMBR 80000 |
| <------------------------------- |
| t7: RTP data |
| -------------------------------> |
: :
Figure 14: Unsolicited PAUSED Using TMMBN
Figure 14 describes the case when an RTP stream sender (A) chooses to
pause an RTP stream due to local considerations. Both 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 in receiving 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 in
receiving 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.
+---------------+ +---------------+
| RTP Sender | | RTP Receiver |
+---------------+ +---------------+
: t1: RTP data :
| ------------------------------------> |
| t2: PAUSE(7), lost |
| <---X-------------- |
| |
| t3: RTP data |
| ------------------------------------> |
: :
| < Time-out, still receiving data > |
| t4: PAUSE(7) |
| <------------------------------------ |
| < RTP data paused > |
| t5: PAUSED(7) |
| ------------------------------------> |
: < Some time passes > :
| t6: RESUME(7), lost |
| <---X-------------- |
| t7: RESUME(7) |
| <------------------------------------ |
| t8: RTP data |
| ------------------------------------> |
| t9: RESUME(7) |
| <------------------------------------ |
: :
Figure 15: Pause and Resume Operation with Messages Lost
Figure 15 describes what happens if a PAUSE message from an RTP
stream receiver does not reach the RTP stream sender. After sending
a PAUSE message, the RTP stream receiver waits for a time-out to
detect if the RTP stream sender has paused the data transmission or
has sent a 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
requests are sent, and another RESUME is sent already at time t7,
which now reaches the RTP stream sender that consequently resumes the
stream at time t8. The time interval between t6 and t7 can vary but
may, for example, be one RTCP feedback transmission interval as
determined by the AVPF rules.
The RTP stream receiver did not realize that the RTP stream was
resumed in time to stop yet another scheduled RESUME from being sent
at time t9. This is, however, harmless since RESUME contains a past
PauseID and will be ignored by the RTP stream sender. It will also
not cause the RTP stream to be resumed even if the stream was paused
again based on a PAUSE from some other receiver before receiving the
RESUME, since the current PauseID was updated compared to the one in
the stray RESUME, which contains a past PauseID and will be ignored
by the RTP stream sender.
+---------------+ +---------------+
| RTP Sender | | RTP Receiver |
+---------------+ +---------------+
: t1: RTP data :
| ------------------------------> |
| t2: PAUSE(11) |
| <------------------------------ |
| |
| < Cannot pause RTP data > |
| t3: REFUSED(11) |
| ------------------------------> |
| |
| t4: RTP data |
| ------------------------------> |
: :
Figure 16: Pause Request is Refused in Point to Point
In Figure 16, the receiver requests to pause the sender, which
refuses to pause due to some consideration local to the sender and
responds with a REFUSED message.
10.3. Point to Multipoint Using Mixer
An RTP Mixer is an intermediate node connecting different transport-
level clouds. The Mixer receives streams from different RTP sources,
selects or combines them based on the application's needs, and
forwards the generated stream(s) to the destination. The Mixer
typically puts its own SSRC(s) in RTP data packets instead of the
original source(s).
The Mixer keeps track of all the streams delivered to the Mixer and
how they are currently used. In this example, Mixer (M) selects the
video stream to deliver to the RTP stream receiver (R) based on the
voice activity of the RTP stream senders (S1 and S2). The video
stream will be delivered to R using M's SSRC and with a CSRC
indicating the original source.
Note that PauseID is not of any significance for the example and is
therefore omitted in the description.
+-----+ +-----+ +-----+ +-----+
| R | | M | | S1 | | S2 |
+-----+ +-----+ +-----+ +-----+
: : t1:RTP(S1) : :
| t2:RTP(M:S1) |<-----------------| |
|<-----------------| | |
| | t3:RTP(S2) | |
| |<------------------------------------|
| | t4: PAUSE(S2) | |
| |------------------------------------>|
| | | t5: PAUSED(S2) |
| |<------------------------------------|
| | | <S2:No RTP to M> |
| | t6: RESUME(S2) | |
| |------------------------------------>|
| | | t7: RTP to M |
| |<------------------------------------|
| t8:RTP(M:S2) | | |
|<-----------------| | |
| | t9:PAUSE(S1) | |
| |----------------->| |
| | t10:PAUSED(S1) | |
| |<-----------------| |
| | <S1:No RTP to M> | |
: : : :
Figure 17: Pause and Resume Operation for a Voice-Activated Mixer
The session starts at t1 with S1 being the most active speaker and
thus being selected as the single video stream to be delivered to R
(t2) using M's SSRC but with S1 as the CSRC (indicated after the
colon in the figure). Then S2 joins the session at t3 and starts
delivering an RTP stream to M. As S2 has less voice activity then
S1, M decides to pause S2 at t4 by sending S2 a PAUSE request. At
t5, S2 acknowledges with PAUSED and at the same instant stops
delivering RTP to M. At t6, the user at S2 starts speaking and
becomes the most active speaker and M 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 stream from t7 arrives at M, it
switches this RTP stream into its SSRC (M) at t8 and changes the CSRC
to S2. As S1 now becomes unused, M issues a PAUSE request to S1 at
t9, which is acknowledged at t10 with PAUSED, and the RTP stream from
S1 stops being delivered.
10.4. Point to Multipoint Using Translator
A transport Relay in an RTP session forwards the message from one
peer to all the others. Unlike Mixer, the Relay does not mix the
streams or change the SSRC of the messages or RTP media. These
examples are to show that the messages defined in this specification
can be safely used also in a transport Relay case. The parentheses
in the figures contains (Target SSRC, PauseID) information for the
messages defined in this specification.
+-------------+ +-------------+ +-------------+
| Sender(S) | | Relay | | Receiver(R) |
+-------------+ +-------------+ +-------------+
: t1: RTP(S) : :
|------------------>| |
| | t2: RTP (S) |
| |------------------>|
| | t3: PAUSE(S,3) |
| |<------------------|
| t4:PAUSE(S,3) | |
|<------------------| |
: <Sender waiting for possible RESUME> :
| < RTP data paused > |
| t5: PAUSED(S,3) | |
|------------------>| |
| | t6: PAUSED(S,3) |
| |------------------>|
: : :
| | t7: RESUME(S,3) |
| |<------------------|
| t8: RESUME(S,3) | |
|<------------------| |
| t9: RTP (S) | |
|------------------>| |
| | t10: RTP (S) |
| |------------------>|
: : :
Figure 18: Pause and Resume Operation between Two Participants Using
a Relay
Figure 18 describes how a Relay can help the receiver (R) in pausing
and resuming the sender (S). S sends RTP data to R through the
Relay, which just forwards the data without modifying the SSRCs. R
sends a PAUSE request to S 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, and when no disapproving RESUME messages are received, it
pauses itself and replies with PAUSED. Similarly R resumes S by
sending a RESUME request through the Relay. Since this describes
only a single pause and resume operation for a single RTP stream
sender, all messages use a single PauseID; in this example, it's
three.
+-----+ +-----+ +-----+ +-----+
| S | | Rel | | R1 | | R2 |
+-----+ +-----+ +-----+ +-----+
: t1:RTP(S) : : :
|----------------->| | |
| | t2:RTP(S) | |
| |----------------->------------------>|
| | t3:PAUSE(S,7) | |
| |<-----------------| |
| t4:PAUSE(S,7) | | |
|<-----------------|------------------------------------>|
| | | t5:RESUME(S,7) |
| |<------------------------------------|
| t6:RESUME(S,7) | | |
|<-----------------|----------------->| |
| | <RTP stream continues to R1 and R2> |
| | | t7: PAUSE(S,8) |
| |<------------------------------------|
| t8:PAUSE(S,8) | | |
|<-----------------|----------------->| |
: : : :
| < Pauses RTP stream > | |
| t9:PAUSED(S,8) | | |
|----------------->| | |
| | t10:PAUSED(S,8) | |
| |----------------->------------------>|
: : : :
| | t11:RESUME(S,8) | |
| |<-----------------| |
| t12:RESUME(S,8) | | |
|<-----------------|------------------------------------>|
| t13:RTP(S) | | |
|----------------->| | |
| | t14:RTP(S) | |
| |----------------->------------------>|
: : : :
Figure 19: Pause and Resume Operation between One Sender and Two
Receivers through Relay
Figure 19 explains the pause and resume operations when a transport
Relay (Rel) is involved between a sender (S) and two receivers (R1
and R2) in an RTP session. Each message exchange is represented by
the time it happens. At time t1, S starts sending an RTP stream to
Rel, which forwards it to R1 and R2. R1 and R2 receives RTP data
from Rel at t2. At this point, both R1 and R2 will send RTCP
Receiver Reports to S informing that they received 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 is 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 sender S by Rel. S sees the RESUME at time t6
and continues to send data to Rel, which forwards it to both R1 and
R2. At t7, R2 chooses to pause the stream by sending a PAUSE request
with an updated PauseID. S still knows that there is 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 messages, 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) choose to resume the
stream from S, in this example R1, it sends a RESUME request to S
(also seen by R2). S immediately resumes the stream.
Consider also an RTP session that includes one or more receivers,
paused sender(s), and a Relay. Further assume that a new participant
joins the session, which is not aware of the paused sender(s). On
receiving knowledge about the newly joined participant, e.g., any RTP
traffic or RTCP report (i.e., either SR or RR) from the newly joined
participant, the paused sender(s) immediately sends PAUSED
indications for the paused streams since there is now a receiver in
the session that did not pause the sender(s) and may want to receive
the streams. Having this information, the newly joined participant
has the same possibility as any other participant to resume the
paused streams.
11. IANA Considerations
Per this specification, IANA has made the following registrations:
1. A new value for media stream pause/resume has been registered in
the "FMT Values for RTPFB Payload Types" registry located at the
time of publication at: <http://www.iana.org/assignments/rtp-
parameters>
Value: 9
Name: PAUSE-RESUME
Long Name: Media Pause/Resume
Reference: RFC 7728
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>
Value Name: pause
Long Name: Media Pause/Resume
Usable with: ccm
Reference: RFC 7728
12. Security Considerations
This document extends CCM [RFC5104] and defines new messages, i.e.,
PAUSE, RESUME, PAUSED, and REFUSED. The exchange of these new
messages has some security implications, which need to be addressed
by the user.
The messages defined in this specification can have a substantial
impact on the perceived media quality if used in a malicious way.
First of all, there is the risk for Denial of Service (DoS) on any
RTP session that uses the PAUSE-RESUME functionality. By injecting
one or more PAUSE requests into the RTP session, an attacker can
potentially prevent any media from flowing, especially when the hold-
off period is zero. The injection of PAUSE messages is quite simple,
requiring knowledge of the SSRC and the PauseID. This information is
visible to an on-path attacker unless RTCP messages are encrypted.
Even off-path attacks are possible as signaling messages often carry
the SSRC value, while the 16-bit PauseID has to be guessed or tried.
The way of protecting the RTP session from these injections is to
perform source authentication combined with message integrity to
prevent other than intended session participants from sending these
messages. The security solution should provide replay protection.
Otherwise, if a session is long lived enough for the PauseID value to
wrap, an attacker could replay old messages at the appropriate time
to influence the media sender state. There exist several different
choices for securing RTP sessions to prevent this type of attack.
The Secure Real-time Transport Protocol (SRTP) is the most common,
but also other methods exist as discussed in "Options for Securing
RTP Sessions" [RFC7201].
Most of the methods for securing RTP, however, do not provide source
authentication of each individual participant in a multiparty use
case. In case one of the session participants is malicious, it can
wreck significant havoc within the RTP session and similarly cause a
DoS on the RTP session from within. That damage can also be
attempted to be obfuscated by having the attacker impersonate other
endpoints within the session. These attacks can be mitigated by
using a solution that provides true source authentication of all
participants' RTCP packets. However, that has other implications.
For multiparty sessions including a middlebox, that middlebox is
RECOMMENDED to perform checks on all forwarded RTCP packets so that
each participant only uses its set of SSRCs to prevent the attacker
from utilizing another participant's SSRCs. An attacker that can
send a PAUSE request that does not reach any participants other than
the media sender can cause a stream to be paused without providing
opportunity for opposition. This is mitigated in multiparty
topologies that ensure that requests are seen by all or most of the
RTP session participants, enabling these participants to send a
RESUME. In topologies with middleboxes that consume and process
PAUSE requests, the middlebox can also mitigate such behavior as it
will commonly not generate or forward a PAUSE message if it knows of
another participant having use for the media stream.
The above text has been focused on using the PAUSE message as the
tool for malicious impact on the RTP session. That is because of the
greater impact from denying users access to RTP media streams. In
contrast, if an attacker attempts to use RESUME in a malicious
purpose, it will result in the media streams being delivered.
However, such an attack basically prevents the use of the pause and
resume functionality. Thus, it potentially forces a reduction of the
media quality due to limitation in available resources, like
bandwidth that must be shared.
The session establishment signaling is also a potential venue of
attack, as that can be used to prevent the enabling of pause and
resume functionality by modifying the signaling messages. The above
mitigation of attacks based on source authentication also requires
the signaling system to securely handle identities and assert that
only the intended identities are allowed into the RTP session and
provided with the relevant security contexts.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[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,
DOI 10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <http://www.rfc-editor.org/info/rfc5104>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263,
DOI 10.17487/RFC6263, June 2011,
<http://www.rfc-editor.org/info/rfc6263>.
13.2. Informative References
[MULTI-STREAM-OPT]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session:
Grouping RTCP Reception Statistics and Other Feedback",
Work in Progress, draft-ietf-avtcore-rtp-multi-stream-
optimisation-11, December 2015.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326,
DOI 10.17487/RFC2326, April 1998,
<http://www.rfc-editor.org/info/rfc2326>.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
October 2000, <http://www.rfc-editor.org/info/rfc2974>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<http://www.rfc-editor.org/info/rfc3611>.
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
DOI 10.17487/RFC6190, May 2011,
<http://www.rfc-editor.org/info/rfc6190>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://www.rfc-editor.org/info/rfc7201>.
[RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use Cases and Requirements", RFC 7478,
DOI 10.17487/RFC7478, March 2015,
<http://www.rfc-editor.org/info/rfc7478>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<http://www.rfc-editor.org/info/rfc7667>.
[SDP-SIMULCAST]
Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
"Using Simulcast in SDP and RTP Sessions", Work in
Progress, draft-ietf-mmusic-sdp-simulcast-04, February
2016.
Acknowledgments
Daniel Grondal made valuable contributions during the initial
versions of this document. The authors would also like to thank Emil
Ivov, Christian Groves, David Mandelberg, Meral Shirazipour, Spencer
Dawkins, Bernard Aboba, and Ben Campbell, who provided valuable
review comments.
Contributors
Daniel Grondal contributed in the creation and writing of early
versions of this specification. Christian Groves contributed
significantly to the SDP "config" pause attribute and its use in
offer/answer.
Authors' Addresses
Bo Burman
Ericsson
Kistavagen 25
SE - 164 80 Kista
Sweden
Email: bo.burman@ericsson.com
Azam Akram
Ericsson
Farogatan 6
SE - 164 80 Kista
Sweden
Phone: +46107142658
Email: akram.muhammadazam@gmail.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