RTP Media Stream Pause and Resume
draft-westerlund-avtext-rtp-stream-pause-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Authors | Azam Akram , Bo Burman , Daniel Grondal , Magnus Westerlund | ||
| Last updated | 2012-05-15 | ||
| Replaced by | draft-ietf-avtext-rtp-stream-pause, draft-ietf-avtext-rtp-stream-pause, RFC 7728 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-westerlund-avtext-rtp-stream-pause-01
Network Working Group A. Akram
Internet-Draft B. Burman
Intended status: Standards Track D. Grondal
Expires: November 16, 2012 M. Westerlund
Ericsson AB
May 15, 2012
RTP Media Stream Pause and Resume
draft-westerlund-avtext-rtp-stream-pause-01
Abstract
With the increased popularity of real-time multimedia applications,
users demand more control over communication sessions. This document
describes how a receiver in a multimedia conversation can pause and
resume incoming data from a sender by sending real-time feedback
messages when using Real-time Transport Protocol (RTP) for real time
data transport. This document extends the Codec Control Messages
(CCM) RTCP feedback package by adding a group of new real-time
feedback messages used to pause and resume RTP data streams.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 16, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Requirements Language . . . . . . . . . . . . . . . . . . 6
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Point to Point . . . . . . . . . . . . . . . . . . . . . . 6
3.2. RTP Mixer to Media Sender . . . . . . . . . . . . . . . . 6
3.3. Media Receiver to RTP mixer . . . . . . . . . . . . . . . 7
4. Design Considerations . . . . . . . . . . . . . . . . . . . . 8
4.1. Real-time Nature . . . . . . . . . . . . . . . . . . . . . 8
4.2. Message Direction . . . . . . . . . . . . . . . . . . . . 8
4.3. Apply to Individual Sources . . . . . . . . . . . . . . . 8
4.4. Consensus . . . . . . . . . . . . . . . . . . . . . . . . 8
4.5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 9
4.6. Retransmitting Requests . . . . . . . . . . . . . . . . . 9
4.7. Sequence Numbering . . . . . . . . . . . . . . . . . . . . 9
5. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 10
6. Participants States . . . . . . . . . . . . . . . . . . . . . 10
6.1. Paused State . . . . . . . . . . . . . . . . . . . . . . . 10
6.1.1. RTCP BYE Message . . . . . . . . . . . . . . . . . . . 11
6.1.2. SSRC Time-out . . . . . . . . . . . . . . . . . . . . 11
7. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Message Acknowledgements . . . . . . . . . . . . . . . . . 13
7.1.1. Negative-Acknowledgement (NACK) . . . . . . . . . . . 14
7.1.2. Pause-Acknowledgement (PACK) . . . . . . . . . . . . . 14
7.1.3. Resume-Acknowledgement (RACK) . . . . . . . . . . . . 15
7.1.4. REFUSE . . . . . . . . . . . . . . . . . . . . . . . . 15
7.2. Transmission Rules . . . . . . . . . . . . . . . . . . . . 15
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Point-to-Point Session . . . . . . . . . . . . . . . . . . 15
8.2. Point-to-multipoint using Mixer . . . . . . . . . . . . . 18
8.3. Point-to-multipoint using Translator . . . . . . . . . . . 20
9. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
13.1. Normative References . . . . . . . . . . . . . . . . . . . 24
13.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
As real-time communication attracts more people, more applications
are created; multimedia conversation applications being one example.
Multimedia conversation further exists in many forms, for example,
peer-to-peer chat application and multiparty video conferencing
controlled by central media nodes, such as RTP Mixers.
Video conferencing MAY involve many participants; each has its own
preferences and demands control over the communication session not
only from the start but also during the session. This document
describes several scenarios in multimedia communication where a
participant chooses to temporarily pause incoming data from specific
sources(s) and resuming it when needed. The receiver does not need
to terminate the session from the source(s) and start all over again
by negotiating the session parameters, for example using SIP
[RFC3261] with SDP Offer/Answer [RFC3264].
Centralized nodes, like RTP Mixers, which either uses logic based on
voice activity, other measurements, user input over properietary
interfaces, or Media Stream Selection
[I-D.westerlund-dispatch-stream-selection] could reduce the resources
consumed in both the media sender and the network by temporarily
pausing the media streams that aren't required by the RTP Mixer.
This becomes especially useful when the media sources are provided in
multiple encoding versions (Simulcast)
[I-D.westerlund-avtcore-rtp-simulcast] or with scalable encoding such
as SVC [RFC6190]. There may be some of the defined encodings or
combination of scalable layers that are not used all of the time.
As the the media streams required at any given point is highly
dynamic, using the out-of-band signalling channel for pausing and
even more importantly resuming a media stream is difficult due to the
performance requirements. Instead, the pause and resume signalling
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 approriate. No currently
existing RTCP feedback message supports pausing and resuming an
incoming data stream. As this is affects the generation of packets
and may even allow the encoding process to be paused, the
functionality appears to match Codec Control Messages in the RTP
Audio-Visual Profile with Feedback (AVPF) [RFC5104] and should thus
be defined as a Codec Control Message (CCM) extension.
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2. Definition
2.1. Abbreviations
RTP Real-time Transport Protocol
RTCP Real-time Transport Control Protocol
SSRC Synchronization Source
CSRC Contributing Source
FB Feedback
AVPF Audio-Visual Profile with Feedback
FMT Feedback Message Type
PT Payload Type
CCM Codec Control Messages
MCU Multipoint Control Unit
2.2. Terminology
In addition to following, the definitions from RTP [RFC3550], AVPF
[RFC4585] and CCM [RFC5104] also apply in this document.
Feedback Messages: CCM [RFC5104] categorised different RTCP feedback
messages into four types, Request, Command, Indication and
Notification. This document places the PAUSE and RESUME messages
into Request category as they need acknowledgement.
Acknowledgement: The confirmation from receiver to sender that the
message has been received.
Sender: The RTP entity that sends an RTP data stream.
Receiver: The RTP entity that receives an RTP data stream.
Mixer: The intermediate RTP node which receives a data stream from
different nodes, combines them to make one stream and forwards to
destinations, in the sense described in Topo-Mixer of RTP
Topologies [RFC5117].
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Participant: A member which is part of an RTP session, acting as
receiver, sender or both.
Paused Sender: An RTP sender which receives a PAUSE request, defined
in this memo, from all other members in a communication session
and stops its transmission, i.e. no other participant receives its
RTP transmission at any given time.
Pausing Receiver: An RTP receiver which sends a PAUSE request,
defined in this memo, to other participant(s).
2.3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Use Cases
This section discusses the main use cases for media stream pause and
resume.
3.1. Point to Point
This is the most basic use case with an RTP session containing two
end-points. Each end-point has one or more SSRCs.
+---+ +---+
| A |<------->| B |
+---+ +---+
Point to Point
The usage of media stream pause in this use case is to temporarily
halt media delivery of media streams that the sender provides but the
receiver doesn't currently use. This can for example de due to
minimized applications where the video stream isn't actually shown on
any display, and neither is it used in any other way, such as being
recorded.
3.2. RTP Mixer to Media Sender
One of the most commonly used topologies in centralized conferencing
is based on the RTP Mixer. 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 having its' own
SSRCs and any media sent to the participants will be sent using those
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SSRCs. If the Mixer wants to identify the underlying media sources
for its' conceptual streams, it can identify them using CSRC. The
media stream the Mixer provides can be an actual media mixing of
multiple media sources, but it might also be as simple as selecting
one of the underlying sources based on some Mixer policy or control
signalling.
+---+ +------------+ +---+
| A |<---->| |<---->| B |
+---+ | | +---+
| Mixer |
+---+ | | +---+
| C |<---->| |<---->| D |
+---+ +------------+ +---+
Figure 1: RTP Mixer
The media streams being delivered to a given receiver, A, can depend
on several things. It can either be the RTP Mixer's own logic and
measurements such as voice activity on the incoming audio streams.
It can also be a human controlling the conference that determines how
the media should be mixed; this would be more common in lecture or
similar applications where regular listners may be prevented from
breaking into the session unless approved by the moderator. The
media selection could also be under the user's control using a
protocol like Media Stream Selection
[I-D.westerlund-dispatch-stream-selection].The media streams may also
be simulcasted or scalably encoded, thus providing multiple versions
that can be delivered by the media sender. These examples indicate
that there are numerous reasons why a particular media 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 media
stream selection to be done in the Mixer.
Because of this, it would be highly beneficial if the Mixer could
request to pause a particular media stream from being delivered to
it. It also needs to be able to resume delivery with minimal delay.
3.3. Media Receiver to RTP mixer
An end-point like A in Figure 1 could potentially request to pause
the delivery of a given media 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 media stream. However, the authors are of the
opinion that this is not a suitable solution.
First of all, the Mixer might not include CSRC in it's stream
indications. Secondly, an end-point cannot rely on the CSRC to
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correctly identify the media stream be paused when the delivered
media is some type of mix. A media stream identification solution is
needed to support this.
In addition, pause is only part of the semantics when it comes to
selecting media streams. As can be seen in MESS
[I-D.westerlund-dispatch-stream-selection], it can be beneficial to
have both include and exclude semantics. In addition, substitution
and possibility to control in what local media stream the selected
media stream is to be provided gives richer functionality.
Due to the above reasons, we exclude this use case from
consideration.
4. Design Considerations
This section describes the requirements that this memo needs to meet.
4.1. Real-time Nature
The first section (Section 1) of this memo describes some possible
reasons why a receiver may pause an RTP sender. Pausing and resuming
is time-dependent, i.e. a receiver may choose to pause an RTP stream
for a certain duration after which the receiver may want the sender
to resume. This time dependency means that the messages related to
pause and resume must be transmitted to the sender in real-time in
order for them to be purposeful.
4.2. Message Direction
It is the responsibility of a receiver, who wants to pause or resume
a stream from the sender(s), to transmit PAUSE and RESUME messages.
A sender who likes to pause itself, can simply do it.
4.3. Apply to Individual Sources
The PAUSE and RESUME messages apply to single media 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.
4.4. Consensus
A sender must not pause an SSRC until all receivers that the sender
knows of have requested it to be paused. The reason is that in RTP
topologies where the media stream is shared between multiple
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receivers, a single receiver on that shared network, independent of
it being multicast or a transport Translator based, must not cause
the media stream to be paused without the consent of all other
receivers. A consequence of this is that a newly joining receiver
needs to cause the sender to resume a paused stream. Any receiver
wanting to resume a stream must also cause it to be resumed.
4.5. Acknowledgements
RTP does not guarantee reliable data transmission. It uses whatever
assurance the lower layer transport protocol can provide. However,
this is commonly UDP that provides no reliability guarantees. Thus
it is possible that a PAUSE and/or RESUME message transmitted from an
RTP end-point does not reach its destination, i.e. the targeted media
sender. In some cases when a PAUSE or RESUME message reaches the
media sender, it will not be able to pause the stream, instead the
sender awaits requests from other receivers as well to fulfill the
consensus requirement. In that case an RTP receiver MAY assume that
previous PAUSE or RESUME message was lost and falsely retransmit it.
In order to avoid this condition, the media sender target of a PAUSE
or RESUME request needs to send an acknowledgement in response to
each PAUSE and RESUME message.
4.6. Retransmitting Requests
As PAUSE or RESUME requests as well as Acknowledgments can be lost,
the sender of a request will need to retransmit it in case no
acknowlegement is received. The retransmission should take the round
trip time into account, and will also need to take the normal RTCP
bandwidth and timing rules applicable to the RTP session into
account, when scheduling retransmission of feedback.
When it comes to resume requests that are more time critical, the
best resume performance may be achieved by repeating the request as
often as possible until a sufficient number have been sent to reach a
high probability of request delivery, an acknowledgement has been
received, or the media stream gets delivered.
4.7. Sequence Numbering
Every PAUSE and RESUME request message will need to have a sequence
number to separate retransmissions from new requests. The sequence
number is incremented by one every time a new request is transmitted.
The PAUSE and RESUME message should share the same sequence number
space. The advantage of using same sequence number space is to avoid
the ambiguity which message to the request receiver should follow in
case of retransmissions. For example, if an RTP sender receives both
PAUSE and RESUME messages before deciding which message to respond to
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(may be due to late packet arrival or any other reason), it can
follow the message with higher sequence number.
Each acknowledgement will have the same sequence number as in the
message (PAUSE or RESUME) it is responding to.
5. Solution Overview
The PAUSE and RESUME functionality is based on sending RTCP feedback
messages from any RTP session participant that wants to pause or
resume a media stream targeted at the media stream sender, as
identified by the sender SSRC. A single Feedback message
specification is used. The message consists of a number of Feedback
Control Information (FCI) blocks, where each block can be a PAUSE
request, a RESUME request or one of four different kinds of
acknwoledgements. This structure allows a single feedback message to
request pause or resume on a number of media streams.
To ensure reliability of the established state at the targeted media
senders, acknowlegments are used. However, due to the requirement to
not pause until all RTP session receivers, i.e. the ones that send
RTCP Receiver Reports on the media sender's stream, are ok with it,
most acknowlegements will NACK. This NACK says the session
participant has established state for the media receiver that it
desires a paused state, but it couldn't comply due to other session
participants not having requested to pause the stream.
The transmission of any RTCP feedback messages follows the regular
AVPF defined timing rules and depends on the session's mode of
operation.
6. Participants States
This document introduces a new state the media stream in an RTP
sender can have, a paused state.
6.1. Paused State
A media stream is in paused state when the sender pauses its
transmission after receiving PAUSE requests from all other receiving
participants in the session, which means no participant is willing to
receive it's transmission. This requires the media stream sender to
track all RTP session participants to determine that all have
requested a pause state with the sender.
Following sub-sections discusses some potential issues when an RTP
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sender goes into paused state.
6.1.1. RTCP BYE Message
When a participant leaves the communication session, it sends an RTCP
BYE message. In addition to the semantics described in section 6.3.4
and 6.3.7 of RTP [RFC3550], following two conditions MUST also be
considered when an RTP participant sends an RTCP BYE message,
o If a paused sender sends an RTCP BYE message, receivers observing
this SHALL NOT send further PAUSE or RESUME requests to it.
o Since a sender pauses its transmission on receiving the PAUSE
requests from all receivers in a session, the sender keeps record
of all the receivers which do and which do not want to receive its
transmission. If a pausing receiver sends an RTCP BYE message
observed by the sender, the sender SHALL NOT consider that
receiver when it decides to pause its transmission.
These conditions are also valid if an RTP Translator is used in the
communication. When an RTP Mixer implementing this memo 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 above conditions also apply to the sender and receiver parts of
the RTP Mixer, respectively.
6.1.2. 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 other participant(s) for last five RTCP reporting
intervals it removes that participant from the receiver list.
7. Message Format
Section 6 of AVPF [RFC4585] defines three types of low-delay RTCP
feedback messages, i.e. Transport layer, Payload-specific, and
Application layer feedback messages. This document defines a new
Transport layer feedback message, this message is either a PAUSE
request, a RESUME request, or one of four different types of
acknowledgements in response to either PAUSE or RESUME requests.
The Transport layer feedback messages are identified by having the
RTCP payload type be RTPFB (205) as defined by AVPF [RFC4585]. The
PAUSE and RESUME messages are identified by Feedback Message Type
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(FMT) value in common packet header for feedback message defined in
section 6.1 of AVPF [RFC4585]. The PAUSE and RESUME transport
feedback message is identified by the FMT value = TBA1.
The Common Packet Format for Feedback Messages is defined by AVPF
[RFC4585] is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT | PT | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
For the PAUSE and RESUME messages, the following interpretation of
the packet fields will be:
FMT: The FMT value identifying the PAUSE and RESUME message: TBA1
PT: Payload Type = 205 (RTPFB)
Length: As defined by AVPF, i.e. he 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 request(s) or acknowledgments in the FCI.
SSRC of media source: Not used, SHALL be set to 0. The FCI
identifies the SSRC the request is for or whose request the
acknowlegement are on.
The Feedback Control Information (FCI) field consist of one or more
PAUSE, RESUME, or their acknowledgement 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Syntax of FCI Entry in the PAUSE and RESUME message
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The FCI fields have the following definitions:
Target SSRC (32 bits): For a Request message, the "Target SSRC"
value is the SSRC that this request is intended for. For any type
of Acknowlegement type defined in this document, the SSRC is the
SSRC who sent the request being acknowledged. A CSRC MUST NOT be
used as a target as the interpretation of such a request is
unclear.
Sequence Number (16 bits): Sequence number of the request that SHALL
be incremented by one for each new request. Both PAUSE and RESUME
messages SHALL share the same sequence number space. Each
requesting SSRC has its own sequence number space with each target
SSRC. In other words, A requesting B to PAUSE or RESUME has a
different sequence number space than A and C. Also, B requesting
of A to PAUSE or RESUME will have a different sequence number
space.
Type (4 bits): The pause feedback type, i.e. either PAUSE or RESUME
or their acknowledgements. The values are as follows,
0: PAUSE message
1: RESUME message
2: Pause-Acknowledgement (PACK)
3: Resume-Acknowledgement (RACK)
4: Negative-Acknowledgement (NACK)
5: REFUSE
6-15: Reserved for future use
Reserved: (12 bits): SHALL be ignored by receivers implementing this
memo and MUST be set to 0 by senders implementing this memo.
7.1. Message Acknowledgements
To let the sender of PAUSE and RESUME requests verify the reception
and the target's reaction to the request, the target of a PAUSE or
RESUME request SHALL send an acknowledgment for each request
received. All transmissions of request and acknowlegement are
governed by the transmission rules as defined by Section 7.2. A
request sender that hasn't received any acknowledgement after one
Round-Trip Time (RTT) MAY retransmit the request again.
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After having received an acknowledgement on a request, a receiver
SHOULD avoid sending further requests of the same type to the same
sender to avoid unnecessary bandwidth consumption. However, a
receiver MAY repeat a request of the same type, e.g. if it is for
some reason necessary to re-confirm the sender's opinion of the
receiver's request status. Consequently, a sender SHALL respond with
corresponding acknowledgement to all requests, even if the request
seems unnecessary and does not cause the sender to change state.
Every acknowledgement SHALL have the same sequence number as the
request message (PAUSE or RESUME) it acknowledges. The sender can
respond to PAUSE or RESUME requests in four different ways.
7.1.1. Negative-Acknowledgement (NACK)
In order for the sender to pause its transmission, it MUST receive
PAUSE request from all the receivers in a session. Consider there
are N receiving participants in a session. When a sender receives a
PAUSE request, it MUST check if it has received requests from N-1
participants. If the number of requesting participants are less than
N-1 it replies with NACK, which is the indication to the requester
that though the request has been received, the transmission can not
be paused at this stage because there are still some receiver(s) in
the session that want to receive it. If a pausing receiver is no
longer interested in pausing the SSRC, it MAY send an RESUME request
to the sender from which it has previously received a NACK. The
sender shall then reply with RACK to that receiver Section 7.1.3.
The NACK MUST only be sent in response to a PAUSE request. The NACK
MUST have the same sequence number as in the PAUSE request.
7.1.2. Pause-Acknowledgement (PACK)
When an RTP sender receives a PAUSE request from all the receivers in
a session, it sends a Pause-Acknowledgement (PACK) to the receivers
and enters into Paused state as discussed in Section 6.1. It means
that if there are N participants in a session and the sender receives
PAUSE request(s) from N-1th participant, it pauses its transmission
and sends a PACK to all the PAUSE requesters. The other participants
can detect that the media sender is paused based on it sending a
PACK.
The PACK MUST only be sent in response to a PAUSE request. The PACK
MUST contain the same sequence number as in the PAUSE request.
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7.1.3. Resume-Acknowledgement (RACK)
When an RTP sender receives a RESUME request from any of the
receivers in a session, it replies with Resume-Acknowledgement (RACK)
and resumes its transmission, if it is in Paused state (discussed in
Section 6.1).
The RACK MUST only be sent in response to a RESUME request. The RACK
MUST match the sequence number in RESUME request.
7.1.4. REFUSE
If any PAUSE and/or RESUME request can not be fulfilled by the sender
due to some reason, it replies with REFUSE acknowledgement.
The REFUSE MAY be sent in response to PAUSE or RESUME requests. The
REFUSE MUST contain the same sequence number as in the PAUSE/RESUME
request.
7.2. Transmission Rules
To be Written
8. Examples
Following are the use cases when there MAY be a need to use PAUSE and
RESUME messages,
1. Point-to-Point session
2. Point-to-multipoint using Mixer
3. Point-to-multipoint using Translator
8.1. 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 message to the sender, which pauses or resumes
transmission accordingly.
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+---------------+ +---------------+
| RTP Sender | | RTP Receiver |
+---------------+ +---------------+
t1: RTP data
| -------------------------------> |
| t2: PAUSE |
| <------------------------------- |
| |
| < RTP data paused > |
| t3: PACK |
| -------------------------------> |
| |
| t4: RESUME |
| <------------------------------- |
| t5: RACK |
| -------------------------------> |
| t6: RTP data |
| -------------------------------> |
Figure 3: The pause and resume operation in Point-to-Point scenario
Figure 3 shows the basic pause and resume operation in Point-to-Point
scenario. At time t1, an RTP sender sends data to a receiver. At
time t2, the RTP receiver requests the sender to pause the stream.
The sender pauses the data and replies with a Pause-Acknowledgement
(PACK). Some time later (at time t4) the receiver requests the
sender to resume, which resumes its transmission and replies with
Resume-Acknowledgement (RACK).
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+---------------+ +---------------+
| RTP Sender | | RTP Receiver |
+---------------+ +---------------+
t1: RTP data
| ------------------------------------> |
| t2: PAUSE, lost |
| <---X-------------- |
| |
| t3: RTP data |
| ------------------------------------> |
| |
| <Timeout, still receiving data> |
| t4: PAUSE |
| <------------------------------------ |
| < RTP data paused > |
| t5: PACK |
| ------------------------------------> |
| |
| t6: RESUME |
| <------------------------------------ |
| t7: RACK |
| ------------------------------------> |
| t8: RTP data |
| ------------------------------------> |
Figure 4: The pause and resume operation with PAUSE lost
Figure 4 describes what happens if a PAUSE message from an RTP
receiver does not reach the RTP sender. After sending a PAUSE
message, the receiver waits for a time-out to detect if the sender
has paused the data transmission or has sent any acknowledgement
according to the rules discussed in Section 7.1. As the PAUSE
message is lost on the way (at time t2), RTP data continues to reach
to the receiver. When the timer expires, receiver schedules
retransmit of the PAUSE message. If PAUSE message reaches to the RTP
sender, it stops streaming and replies with PACK. The same rules
apply to the RESUME message, i.e., the RTP receiver waits for a time-
out value after sending the RESUME message until it gets the
transmission or receives any acknowledgement.
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+---------------+ +---------------+
| RTP Sender | | RTP Receiver |
+---------------+ +---------------+
| t1: RTP data |
| ------------------------------> |
| t2: PAUSE |
| <------------------------------ |
| |
| < Can not pause RTP data > |
| t3: REFUSE |
| ------------------------------> |
| |
| t4: RTP data |
| ------------------------------> |
Figure 5: The pause request is refused in Point-to-Point scenario
In Figure 5, the receiver requests to pause the sender, which refuses
to pause due to session policy and responds with REFUSE message.
8.2. Point-to-multipoint using Mixer
An RTP Mixer is an intermediate node connecting different transport-
level clouds. The Mixer receives the streams from different RTP
sources, selects or combines them based on the application's need and
forwards the generated stream(s) to the destination. The Mixer puts
its' own SSRC(s) in RTP data packets instead of the original
source(s).
The Mixer keeps track of all the media streams delivered to the Mixer
and how they currently are used. It selects the video stream to
deliver to the receiver R based on the voice activity of the media
senders. The video stream will be delivered to R using M's SSRC and
with an CSRC indicating the orignal source.
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+-----+ +-----+ +-----+ +-----+
| R | | M | | S1 | | S2 |
+-----+ +-----| +-----+ +-----+
| | t1:RTP(S1) | |
| t2:RTP(M:S1) |<-----------------| |
|<-----------------| | |
| | t3:RTP(S2) | |
| |<------------------------------------|
| | t4: PAUSE(S2) | |
| |------------------------------------>|
| | | t5: PACK(S2) |
| |<------------------------------------|
| | | <S2:No RTP to M> |
| | t6: RESUME(S2) | |
| |------------------------------------->
| | | t7: RTP to M |
| |<------------------------------------|
| | | t8: RACK(S2) |
| |<------------------------------------|
| t9:RTP(M:S2) | | |
|<-----------------| | |
| | t10:PAUSE(S1) | |
| |----------------->| |
| | t11:PACK(S1) | |
| |<-----------------| |
| | <S1:No RTP to M> | |
Figure 6: The pause and resume operations for an Voice Activated
Mixer
The session starts at t1 with S1 being the most active speaker and
thus being selected as the single video stream to be delivered to R
(t2) using the Mixer SSRC but with the CSRC indicated after the colon
in the figure. Then S2 joins the session at t3 and starts delivering
media to the Mixer. As S2 has less voice activity then S1, the Mixer
decides to pause S2 at t4 by sending S2 a PAUSE request. At t5, S2
acknowledges with a PACK and at the same instant stops delivering RTP
to the Mixer. At t6, the user at S2 starts speaking and becomes the
most active speaker and the Mixer decides to switch the video stream
to S2, and therefore sends a RESUME request to S2. At t7, S2 has
received the RESUME request and acts on it by resuming RTP media
delivery to M. It also schedules the transmission of a RACK, which is
sent at t8. When the media from t7 arrives at the Mixer, it switches
this media into its SSRC (M) at t9 and changes the CSRC to S2. As S1
now becomes unused, the Mixer issues a PAUSE request to S1 at t10,
which is acknowledged at t11 with a PACK and the RTP media stream
from S1 stops being delivered.
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8.3. Point-to-multipoint using Translator
A transport Translator in an RTP session forwards the message from
one peer to all the others. Unlike Mixer, the Translator does not
mix the streams and change the SSRC of the message. These examples
are to show that the message can be safely used also in a transport
Translator case.
+-------------+ +-------------+ +--------------+
| Sender(S) | | Translator | | Receiver(R) |
+-------------+ +-------------| +--------------+
| t1: RTP(S) | |
|------------------>| |
| | t2: RTP (S) |
| |------------------>|
| | t3: PAUSE(S) |
| |<------------------|
| t4:PAUSE (S) | |
|<------------------| |
| < RTP data paused > |
| t5: PACK (S) | |
|------------------>| |
| | t6: PACK (S) |
| |------------------>|
| | |
| | t7: RESUME(S) |
| |<------------------|
|t8: RESUME(S) | |
|<------------------| |
| t9: RACK (S) | |
|------------------>| |
| | t10: RACK (S) |
| |------------------>|
| t11: RTP(S) | |
|------------------>| |
| | t12: RTP (S) |
| |------------------>|
Figure 7: The pause and resume operation between two participants
using the Translator
Figure 7 describes how a Translator can help the receiver in pausing
and resuming the sender. The sender S sends RTP data to the receiver
R through Translator, which just forwards the data without modifying
the SSRCs. The receiver sends PAUSE requests to the sender, which
checks that there is no other receiver which wants to receive the
data, hence pauses itself and replies with PACK. Similarly the
receiver resumes the sender by sending RESUME request through
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Translator.
+-----+ +-----+ +-----+ +-----+
| S | | T | | R1 | | R2 |
+-----+ +-----| +-----+ +-----+
| t1:RTP(S) | | |
|----------------->| | |
| | t2:RTP(S) | |
| |----------------->------------------>|
| | t3:PAUSE(S) | |
| |<-----------------| |
| t4:PAUSE(S) | | |
|<-----------------| | |
| t5:NACK(S) | | |
|----------------->| | |
| | t6:NACK(S) | |
| |----------------->| |
| |<RTP stream continues to R1 and R2> |
| | | t7: PAUSE(S) |
| |<------------------------------------|
| t8:PAUSE(S) | | |
|<-----------------| | |
| < Pauses RTP data stream > | |
| t9:PACK(S) | | |
|----------------->| | |
| | t10:PACK(S) | |
| |----------------->------------------>|
| | t11:RESUME(S) | |
| |<-----------------| |
| t12:RESUME(S) | | |
|<-----------------| | |
| t13:RACK(S) | | |
|----------------->| | |
| | t14:RACK(S) | |
| |----------------->| |
| t15:RTP(S) | | |
|----------------->| | |
| | t16:RTP(S) | |
| |----------------->------------------>|
Figure 8: The pause and resume operation between one sender and two
receivers through translator
Figure 8 explains the pause and resume operations when a transport
Translator is involved between a sender and two receivers in an RTP
session. Each message exchange is represented by the time it
happens. At time t1, Sender (S) starts sending media to the
Translator, which is forwarded to R1 and R2 through the Translator,
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T. R1 and R2 receives RTP data from Translator at t2. At this point
both R1 and R2 will send RTCP Receiver Reports to S informing that
they receive S's media stream.
After some time (at t3), R1 chooses to pause the stream. On
receiving the PAUSE request from R1, S checks if there are any other
receiver which still wants to receive the data. At this time, S
knows that R2 exists and has not indicated that it wants to pause the
stream. The sender S replies with NACK to R1 and continues to send
data to T which forwards to both R1 and R2. At t7, the receiver R2
also selects to pause the data by sending a PAUSE request. Now the
sender S knows that no receiver (neither R1 nor R2) want the stream,
it concludes that the stream must be paused. S now stops sending the
stream and replies with PACK to R1 and R2. When any of the receivers
(R1 or R2) choses to resume the stream from S, it sends a RESUME
request to the sender. In reply, the RTP sender sends a RACK to the
requesting RTP receiver and resumes streaming.
Consider an RTP session which includes one or more receivers, paused
sender(s), and a Translator. 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) resumes the transmission since there is now a
receiver in the session that did not pause the sender. It SHALL
depend on the new receiver to pause or continue that stream(s).
9. Signalling
The capability of handling PAUSE and RESUME messages MAY be exchanged
at a higher layer such as SDP. This document extends the rtcp-fb
attribute defined in section 4 of AVPF [RFC4585] to include the
request for pause and resume. Like AVPF [RFC4585] and CCM [RFC5104],
this document recommends to use the rtcp-fb attribute at media level
and it must not be used at session level. This memo follows all the
rules defined in AVPF for rtcp-fb attribute relating to payload type
in a session description.
Section 7.1 of CCM [RFC5104] defines a new feedback value "ccm",
which indicates the support of codec control using RTCP feedback.
The CCM [RFC5104] defines four different parameters which SHOULD be
used with the feedback value "ccm" to indicate the specific codec
control command.
This memo defines a new parameter, "pause", which aggregatively
represent the PAUSE, RESUME messages and their acknowledgements
(i.e., PACK, NACK, RACK and REFUSE). An endpoint implementing this
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memo and using SDP to signal capability MUST use the new "pause"
extension to ccm signaling. Similarly, a sender or receiver SHOULD
NOT use the messages from this memo towards receivers that did not
declare capability for it.
The below figure is an example how to show support for pausing and
resuming the stream according to this memo:
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Pausing Media
t=0 0
c=IN IP4 host.example.com
m=audio 49170 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm pause
Figure 9: An SDP example with pause and resume capability
10. IANA Considerations
As outlined in Section 7, this memo requests IANA to allocate
1. The 'pause' tag to be used with ccm under rtcp-fb AVPF attribute
in SDP.
2. The FMT number TBA1 to be allocated to the PAUSE and RESUME
functionality from this memo.
3. A registry listing registered values for 'pause' Types.
4. PAUSE, RESUME, PACK, RACK, NACK, and REFUSE with the listed
numbers in the pause Type registry.
11. Security Considerations
This document extends the CCM [RFC5104] and defines new messages,
i.e., PAUSE and RESUME. The exchange of these new messages MAY have
some security implications, which need to be addressed by the user.
Following are some important implications,
1. Identity spoofing - An attacker can spoof him/herself as an
authenticated user and can falsely pause or resume any source
transmission. In order to prevent this type of attack, a strong
authentication and integrity protection mechanism is needed.
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2. Denial of Service (DoS) - An attacker can falsely paused all the
source stream which MAY result in Denial of Service (DoS). An
Authentication protocol MAY save from this attack.
3. Man-in-Middle Attack (MiMT) - The pausing and resuming of the RTP
source is prone to a Man-in-Middle attack. The public key
authentication May be used to prevent MiMT.
12. Acknowledgements
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
13.2. Informative References
[I-D.westerlund-avtcore-rtp-simulcast]
Westerlund, M., Burman, B., Lindqvist, M., and F. Jansson,
"Using Simulcast in RTP sessions",
draft-westerlund-avtcore-rtp-simulcast-00 (work in
progress), October 2011.
[I-D.westerlund-dispatch-stream-selection]
Grondal, D., Burman, B., and M. Westerlund, "Media Stream
Selection (MESS)",
draft-westerlund-dispatch-stream-selection-00 (work in
progress), October 2011.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
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Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
May 2011.
Authors' Addresses
Azam Akram
Ericsson AB
Farogatan 6
SE - 164 80 Kista,
Sweden
Phone: +46107142658
Fax: +46107175550
Email: muhammad.azam.akram@ericsson.com
URI: www.ericsson.com
Bo Burman
Ericsson AB
Farogatan 6
SE - 164 80 Kista,
Sweden
Phone: +46107141311
Fax: +46107175550
Email: bo.burman@ericsson.com
URI: www.ericsson.com
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Daniel Grondal
Ericsson AB
Farogatan 6
SE - 164 80 Kista,
Sweden
Phone: +46107147505
Fax: +46107175550
Email: daniel.grondal@ericsson.com
URI: www.ericsson.com
Magnus Westerlund
Ericsson AB
Farogatan 6
SE- Kista 164 80,
Sweden
Phone: +46107148287
Fax:
Email: magnus.westerlund@ericsson.com
URI: www.ericsson.com
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