Internet Engineering Task Force M. Montagud, Ed.
Internet-Draft F. Boronat
Intended status: Standards Track UPV
Expires: August 13, 2015 H. Stokking
TNO
February 9, 2015
Early Event-Driven (EED) RTCP Feedback for Rapid IDMS
draft-montagud-avtcore-eed-rtcp-idms-00
Abstract
RFC 7272 defines two RTCP messages to enable Inter-Destination Media
Synchronization (IDMS). However, if such RTCP messages are exchanged
using the Regular RTCP reporting rules specified in RFC 3550,
unacceptable delays can be originated when exchanging the
synchronization information conveyed into RTCP packets. Accordingly,
this document proposes Early Event-Driven (EED) RTCP reporting rules
and messages to enable higher interactivity, dynamism, flexibility
and accuracy when using RTP/RTCP for IDMS. Such IDMS extensions are
targeted at providing faster reaction on dynamic situations, such as
out-of-sync situations and channel change delays, as well as a finer
granularity for synchronizing media-related events, while still
adhering to the RTCP bandwidth bounds specified in RFC 3550.
Besides, a new RTP/AVPF transport layer feedback message is proposed
to allow the request of re-IDMS setting instructions (e.g., in case
of RTCP packet loss) as well as rapid accommodation of latecomers in
on-going sessions. Finally, this document also discusses various
situations in which the reporting of Reduced-Size RTCP packets (RFC
3556) can be applicable and beneficial for IDMS.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 13, 2015.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. RTP/RTCP for IDMS (RFC 7272) . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Background: RTCP Reporting Rules . . . . . . . . . . . . . . 7
3.1. Regular RTCP Feedback (RFC 3550) . . . . . . . . . . . . 7
3.2. Early RTCP Feedback (RFC 4585) . . . . . . . . . . . . . 9
3.3. Rapid Synchronization of RTP Flows (RFC 6051) . . . . . . 10
3.4. SDP Modifiers for RTCP Bandwidth (RFC 3556) . . . . . . . 11
4. Early Event-Driven (EED) RTCP Feedback for IDMS . . . . . . . 12
4.1. Immediate Initial RTCP IDMS Settings . . . . . . . . . . 12
4.2. Dynamic EED Reporting of IDMS Settings . . . . . . . . . 13
4.3. Rapid (Re-)Synchronization Request . . . . . . . . . . . 16
4.4. Reduction of Channel Change Delays . . . . . . . . . . . 18
5. Reduced-Size RTCP Reporting for IDMS . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 23
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
RTP (Real-Time Transport Protocol) and RTCP (RTP Control Protocol)
protocols provide support for both intra-media and inter-media
synchronization [RFC3550]. Moreover, [RFC7272] extends the RTP/RTCP
capabilities to also enable Inter-Destination Media Synchronization
(IDMS).
However, if the proposed RTCP messages for IDMS in [RFC7272] are
exchanged using the Regular RTCP reporting rules specified in
[RFC3550], this can lead to unnaceptable performance in some delay-
sensitive applications requiring stringent synchronization
requirements [Montagud2012]. This is because the RTCP packets are
exchanged in a pre-scheduled and inflexible manner, uniquely based on
preserving the allowed traffic bounds specified in [RFC3550]. There
is no support for timely feedback that would allow to repair or to
manage dynamic events of interest close to their occurrence.
Accordingly, there may be a variable time lag (from few seconds to
several minutes in large-scale environments) between detecting an
event (e.g., an out-of-sync situation) and being able to send an
appropriate RTCP packet to handle it. Moreover, the RTCP reports may
even not be received at the target side, since RTCP is sent over UDP,
which does not provide a reliable control channel.
This document proposes further extensions to RTP/RTCP to allow for a
more strategic and efficient usage of the RTCP channel for IDMS. In
particular, the RTCP extensions from [RFC4585] and [RFC6051]
encourage the specification of novel RTCP reporting rules and
messages, which in conjunction we call Early Event-Driven (EED) RTCP
Feedback for IDMS, to enable higher interactivity, dynamism,
flexibility and accuracy when using RTP/RTCP for IDMS. Such IDMS
extensions are backward compatible with the solution specified in
[RFC7272], while still adhering to the RTCP traffic bounds specified
in [RFC3550].
More specifically, the proposed EED RTCP Feedback for IDMS provides
the following key benefits: i) earlier correction of out-of-sync
situations; ii) higher granularity for synchronizing the presentation
of dynamically triggered media-related events (e.g., to ensure that
important pieces of media content are simultaneously watched by all
the users); iii) ability of dynamically requesting IDMS setting
instructions (e.g., in case of RTCP packet loss); iv) dynamic and
rapid accommodation of latecomers in on-going sessions; and v)
reduction of channel-change (i.e., zapping) delays. Additionally,
this document also discusses various situations in which the
reporting of Reduced-Size RTCP packets [RFC3556] can be applicable
and beneficial for IDMS.
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The proposed RTCP extensions for IDMS in this document are applicable
to and can have a potentially high impact on a wide spectrum of
scenarios with demanding IDMS characteristics [Montagud2012], such as
Social TV, multi-party conferencing, and synchronous e-learning.
A preliminary evaluation of such proposals can be found in
[Montagud2013].
1.1. RTP/RTCP for IDMS (RFC 7272)
IDMS refers to the playout of media streams at two or more
geographically distributed locations in a time-synchronized manner.
A large number of applications requiring IDMS can be found in
[Montagud2012]. Some relevant examples are: Social TV, multi-party
conferencing, networked loudspeakers and networked multi-player
games.
[RFC7272] extends the RTP/RTCP mechanisms to exchange useful feedback
information for IDMS between Synchronization Clients (SC) and a
centralized Media Synchronization Application Server (MSAS). First,
an RTCP XR block for IDMS, called "IDMS report", is defined to enable
the SCs to report on packet reception and/or presentation times for a
specific RTP stream. Second, a new RTCP IDMS packet type, called
"IDMS Settings packet", is defined to allow the MSAS the transmission
of IDMS setting instructions to the distributed SCs, based on the
collected IDMS timings from them. This IDMS Settings packet will
provide a common target playout point, referred to a specific RTP
packet (concretely, to its generation timestamp) to which all the SCs
belonging to a specific synchronization group (identified by the
SyncGroupId field, specified in [RFC7272]) must synchronize.
Besides, a new Session Description Protocol (SDP) parameter, called
"rtcp-idms", is defined to notify about the usage of the above IDMS
messages and to manage the group membership, thus allowing the co-
existence of various independent groups of users in IDMS-enabled
sessions.
Figure 1 shows the architectural solution for IDMS adopted in
[RFC7272], wich is based on a sync-maestro scheme [Montagud2012]. In
this particular case, the SCs are co-located with RTP Receivers and
the MSAS is co-located with the RTP Sender (although it could be co-
located with an SC or with a third-party entity). The MSAS is
responsible for collecting the IDMS reports from all the SCs
belonging to a specific group, computing the delay differences among
them and, if needed, sending IDMS Settings packets to make them to
enforce the required adjustments to achieve IDMS. Although other
architectural solutions are feasible, such as a distributed or a
master/slave scheme [Montagud2012], this document also focuses on the
sync-maestro scheme.
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+-----------------------+ +-----------------------+
| | | |
| RTP Sender | | RTP Receiver |
| | RTCP RR + | |
| +-----------------+ | XR for IDMS | +-----------------+ |
| | | | <-------------- | | | |
| | Media | | | | | |
| | Synchronization | | | | Synchronization | |
| | Application | | | | Client | |
| | Server | | RTCP SR + | | (SC) | |
| | (MSAS) | | IDMS Settings | | | |
| | | | --------------> | | | |
| +-----------------+ | | +-----------------+ |
| | | |
+-----------------------+ +-----------------------+
Figure 1: IDMS Architecture
2. Terminology
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].
The terminology defined in "RTP: A Transport Protocol for Real-Time
Applications" [RFC3550], "Extended RTP Profile for Real-time
Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)"
[RFC4585], "Rapid Synchronisation of RTP Flows" [RFC6051], and in
"RTCP for Inter-Destination Media Synchronization" [RFC7272], apply.
The naming convention for RTCP is sometimes confusing. Below is a
list of RTCP related terms and their meaning. Those concepts were
defined in [RFC3550], [RFC4585], and [RFC5506], but refreshed here
for a better understanding of this document:
o RTCP packet: There are different types of RTCP packets, each one
reporting on a specific metric or event of interest. It consists
of a fixed header part followed by structured fields or elements
depending on the information conveyed in that RTCP packet type.
o Regular RTCP mode: Mode of operation in which no preferred
transmission of feedback messages is allowed. Instead, RTCP
packets are sent following the rules specified in [RFC3550].
o Regular RTCP packet: RTCP packet that is sent using Regular RTCP
mode.
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o Event: An observation that is (potentially) of interest to the
participants of a media session and thus useful to be reported by
means of an RTCP feedback message in a timely fashion.
o Regular RTCP packet: RTCP packet that is sent using Regular RTCP
mode.
o Early RTCP mode: Mode of operation in which a participant is
capable of reporting events of interest in a timely fashion (i.e.,
close to their occurrence).
o Early RTCP packet: RTCP packet that is transmitted earlier than
the scheduled transmission time when following the reporting rules
specified in [RFC3550]. In [RFC4585], Early RTCP packets may be
sent either as "Immediate Feedback" or as "Early RTCP" modes,
depending on the group size. In the context of this document,
Early RTCP packets are sent by the MSAS, which is a single
centralized entity. Therefore, no dithering interval is needed to
avoid feedback implosion, and RTCP packets can be sent in an
"Immediate Feedback" mode. Sending an Early RTCP packet is also
referred to as sending Early Feedback in this document.
o Compound RTCP packet: A collection of two or more RTCP packets.
In [RFC3550], it is mandatory to send RTCP packets as compound
packets including at least an RTCP RR or SR packet, followed by an
SDES packet with the CNAME item for the transmitting source
identifier. Often the term "compound" is left out, so the
interpretation of RTCP packet is therefore dependent on the
context.
o Minimal compound RTCP packet: A compound RTCP packet that contains
only mandatory information, such as encryption prefix (if
necessary), exactly one RTCP RR or SR packet, exactly one SDES
packet with the CNAME item, and possible additional Feedback
Messages [RFC4585] that must be sent as an Early RTCP packet.
o (Full) Compound RTCP packet: A compound RTCP packet that conforms
to the requirements on minimal compound RTCP packets and contains
any additional number of RTCP packets (additional RRs, further
SDES items, etc.). RTCP compound packets are typically sent when
using Regular RTCP Feedback [RFC3550], although they may be also
sent when using Early RTCP Feedback [RFC4585].
o Reduced-Size RTCP packet: It only contains one or more RTCP
packets, being much smaller than (minimal and full) compound RTCP
packets [RFC3550]. Such RTCP packets are sent when using Early
RTCP Feedback, but must not be sent when using Regular RTCP
Feedback. The full definition is in Section 4.1 of [RFC5506].
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Besides, the definition of three key concepts will help understanding
(the proposed IDMS extensions in) this document:.
o Inter-Stream Synchronization Delay (or Latency): It refers to the
time difference between the instant at which a user joins an on-
going multimedia session, probably involving different media
(e.g., audio and video, or when using layered and/or multi-
description media) carried in separate streams, and the instant at
which these correlated streams can be presented to that user in a
synchronized manner. It is defined in [RFC6051].
o IDMS Delay (or Latency): It refers to the time difference between
the instant at which an IDMS-related event occurs (e.g., a user
joins an on-going session or an out-of-sync situation is detected)
and the instant at which the IDMS Setting instructions to handle/
repair such event have been received and processed at the target
side/s or SCs.
o Latecomer: In multi-party multimedia services, users may join and
leave the session quite frequently. A user who joins a session in
progress is usually called a latecomer (or late joiner) [RFC6051].
3. Background: RTCP Reporting Rules
In this section, an overview of the RTCP reporting rules is provided.
This is important to help understanding the benefits of the RTCP
reporting rules and reports proposed in this document.
3.1. Regular RTCP Feedback (RFC 3550)
During the media session's lifetime, the participants of an RTP
Session (i.e., senders and receivers) regularly exchange RTCP reports
(typically conveyed into compound RTCP packets) to inform mainly
about QoS (Quality of Service) statistics, either in a unicast or
multicast way (depending on the specific networked environment). On
the one hand, a low frequency of RTCP feedback reporting can lead to
faulty behavior due to outdated statistics. On the other hand,
excessive reports can be redundant and cause unnecessary control
traffic, probably leading to potential congestion situations.
Also, if the RTCP packets were exchanged at a constant rate, the
control traffic would grow linearly with the number of participants.
Accordingly, a trade-off between up-to-date information and the
amount of control traffic must be met. This would allow an
application to (automatically) scale over session sizes ranging from
few participants to tens of thousands.
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The total amount of control traffic added by RTCP should be limited
to a small (so that the primary function of media data transport is
not impaired) and known (so that each participant can independently
calculate its share) fraction of the allocated RTP session bandwidth.
A fraction of 5 % is recommended in [RFC3550]. In such process,
media senders are given special consideration to allow a more
frequent report exchange of their RTCP statistics, some of which are
indeed very relevant for multimedia synchronization. In particular,
if the proportion of senders constitute less than one quarter of the
session membership, this percentage is further divided into two
parts, where 25 % must be dedicated to active senders and the
remaining can be consumed by receivers. Otherwise, the RTCP
bandwidth is equally shared between senders and receivers.
Based on the above aspects, the RTCP report interval is dynamically
and locally computed in each RTP entity, every time an RTCP packet is
sent, according to the available session bandwidth, the average size
of all received and sent RTCP packets, the number of participants in
the session, their role (senders or receivers), as well as the
unicast or multicast nature of the session (see Section 6.2 of
[RFC3550] for further details).
This (deterministically) calculated RTCP report interval should also
have a lower bound to avoid having bursts (cumpling) of RTCP packets
when the number of participants is small and the law of large numbers
is not helping to smooth out the traffic overhead. This would also
help to avoid excessive frequent reports during transient outages
like a network partition. The recommended value in [RFC3550] for
that fixed minimum interval is 5 s.
Besides, a delay should be imposed to each participant before sending
the first RTCP packet upon joining the session. This allows a
quicker convergence of the RTCP report interval to the correct value.
This initial delay may be set to half the minimum RTCP report
interval (i.e., 2.5 s) in multicast sessions, whilst it may be set to
zero in unicast sessions.
In some cases (e.g. if the data rate is high and the application
demands more frequent RTCP reports), an implementation may scale the
minimum RTCP interval to a smaller value given by 360 divided by the
session bandwidth (in kbps). This yields to an interval smaller than
5 s when the session bandwidth becomes greater than 72 kbps. In
multicast sessions, only active senders may use that reduced minimum
interval, whilst in unicast sessions it also may be used by
receivers. In the above cases, however, the minimum interval of 5 s
must be still taken into account during the membership accounting
procedure to not prematurely time out participants (who can indeed be
using it) because of inactivity.
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After that, the interval between RTCP packets is varied randomly over
the range [0.5, 1.5] times that RTCP report interval to prevent
floods of RTCP reports (i.e., to avoid that all RTCP packets are sent
and received almost at the same time, in every report interval).
Additionally, "timer reconsideration" algorithms are introduced to
allow for a more rapid adaptation of the RTCP report interval in
large-scale sessions, where the membership can largely vary (e.g.,
many receivers join and leave the session quite frequently). To
compensate for the fact that the "timer reconsideration" algorithms
converge to a lower value than the intended average RTCP bandwidth,
the (randomized) report interval is finally divided by e-3/2=1.21828.
3.2. Early RTCP Feedback (RFC 4585)
In [RFC4585], further RTCP reporting mechanisms are specified to
enable receivers to provide, statistically, more immediate RTCP
feedback to the senders. This Early RTCP Feedback profile, which is
known as RTP Audio-Visual Profile with Feedback (RTP/AVPF), allows
for short-term adaptation and efficient feedback-based repairing
mechanisms to be implemented, while maintaining the RTCP bandwidth
constraints and preserving scalability to large groups.
The RTCP report interval specified in [RFC3550] is denoted as Regular
RTCP interval in [RFC4585]. In addition, [RFC4585] enables to send
RTCP reports earlier that the next scheduled Regular RTCP
transmission time if a receiver detects the need to inform about
media stream related events (e.g., picture or slice loss) close to
their occurrence.
[NOTE] A feedback suppression mechanism is adopted, in which
receivers wait for a random dithering interval to avoid feedback
implosion (i.e., lots of receivers reporting on the same event).
The reporting rules for Regular RTCP packets in [RFC4585] are similar
than the ones in [RFC3550]. However, the recommended minimum RTCP
report interval of 5 s in [RFC3550] is dropped in [RFC4585].
Instead, an optional attribute, called "trr-int", is specified as an
offset parameter (in ms) to the computed Regular RTCP Report
interval.
Note that providing "trr-int" as an independent variable is intended
to restrain from sending too frequent Regular RTCP packets (i.e.,
saving RTCP bandwidth) while enabling higher flexibility to transmit
Early RTCP packets (i.e., using the saved RTCP bandwidth) in response
to dynamic events. This could not be achieved by reducing the
overall RTCP bandwidth, because the frequency of Early RTCP packets
would be affected as well. Values between 4 and 5 s for "trr-int"
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are recommended to assure interworking with RTP entities only using
Regular RTCP Feedback. However, as "trr-int" is an optional
attribute, it may be set to zero (default value) if a specific
application would benefit from a higher frequency of Regular RTCP
packets. In such a case, the only difference between the RTCP timing
rules from [RFC3550] and [RFC4585] for transmitting Regular RTCP
packets resides in the minimum value for the report interval, which
is dropped in [RFC4585].
In order to preserve the RTCP traffic bounds, only one Early RTCP
packet can be transmitted between two consecutive Regular RTCP
packets (i.e. receivers cannot send two consecutive Early RTCP
packets). After sending an Early RTCP packet, the RTCP reporting
engine must schedule the transmission time for the next RTCP packet
by skipping the next Regular RTCP interval.
Furthermore, [RFC4585] defines a small number of general-purpose
feedback messages as well as a format for codec- and application-
specific feedback information for transmission in the RTCP payloads.
3.3. Rapid Synchronization of RTP Flows (RFC 6051)
When using RTP streaming, the inter-stream synchronization delay can
greatly increase in certain scenarios, especially in large multicast
groups or when Multipoint Conference Units (MCU) are involved in the
media delivery process. This increase of delay can be inacceptable
and annoying to users, resulting in an overall poor Qualilty of
Experience (QoE).
The aim of [RFC6051] is to minimize the inter-stream synchronization
delay when using RTP/RTCP-based streaming. The motivation is that a
receiver cannot synchronize playout of the incoming media streams
until compound RTCP packets, including a Source Description or SDES
packet (including the media source identification) and a Sender
Report or SR (including timing correlation parameters), are received
for all the involved RTP senders in a multimedia session. In most
implementations, media data will not be played out (watched or
listened) until inter-stream synchronization is (initially) achieved.
If there is no packet loss, this gives an expected delay equal to the
average time for receiving the first RTCP packet from the RTP Session
with the longest RTCP report interval. This delay is even more
problematic if an RTCP SR packet from one of the involved RTP
sessions is lost.
[NOTE] Note that the inter-stream synchronization delay depends on
the specific instant at which a user joins the multimedia session or
each RTP session (e.g., the user may first receive the RTCP packets
from the RTP session with the longest RTCP interval), as well as on
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the impact of the randomization processes in all the involved RTP
sessions.
In [RFC6051], three backward compatible extensions to the RTP/RTCP
protocols are proposed to reduce the inter-stream synchronization
delay. First, the RTCP timing rules are updated to allow Single
Source Multicast (SSM) senders [RFC5760] the immediate transmission
of an initial compound RTCP packet upon joining each RTP session in a
multimedia session (in parallel with the initial RTP packets). The
rationale for not allowing the transmission of immediate RTCP packets
to SSM receivers is to avoid feedback implosion in case that many
receivers join the session almost simultaneously ("flash crowd"
effect). This is clearly not an issue for SSM senders, since there
can be at most one sender. Likewise, feedback implosion is a concern
for Any Source Multicast (ASM) sessions, so [RFC6051] does not
propose changes to the RTCP timing rules in these kinds of multicast
environments. Second, a new RTP/AVPF transport layer feedback
message (this type of RTCP messages is defined in [RFC4585]), called
RTCP-SR-REQ, is defined to allow requesting the generation of an
Early RTCP SR packet from the media sender. This enables rapid
(re-)synchronization in case that an RTCP SR has not been received
for a long period (e.g. due to packet loss or in sessions with large
RTCP reporting intervals). Likewise, this enables latecomers to
achieve inter-stream synchronization as soon as possible upon joining
the session. Finally, new RTP header extensions are defined to
enable the inclusion of metadata (in particular, NTP-based
timestamps) in RTP data packets for in-band synchronization, thus
avoiding the need for receiving RTCP SR packets before streams can be
synchronized. These RTP header extensions do not eliminate the need
for RTCP SR messages, but both mechanisms must be used for the
synchronization control process. The use of RTCP SR packets for
inter-stream synchronization allows backwards compatibility, but also
provides higher robustness in the presence of middle boxes (e.g., RTP
translators) that might strip RTP header extensions.
An accurate and rapid inter-stream synchronization is especially
relevant when using layered, multi-description and multi-view media
encodings. This is because all the individual RTP streams need to be
synchronized before starting the decoding processes.
3.4. SDP Modifiers for RTCP Bandwidth (RFC 3556)
In some applications, it may be appropriate to specify the RTCP
bandwidth independently of the allocated RTP session bandwidth.
Accordingly, [RFC3556] defines two additional Session Description
Protocol (SDP) attributes to specify modifiers for the RTCP bandwidth
for senders and receivers.
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On the one hand, using a separate parameter allows rate-adaptive
applications to set an RTCP bandwidth consistent with a "typical"
data bandwidth that is lower than the maximum bandwidth specified by
the session bandwidth parameter. This allows the RTCP bandwidth to
be kept under 5% of the session bandwidth when the rate has been
adapted downward, e.g. based on the stability of the network
conditions. On the other hand, there may be applications that send
data at very low rates, but need to communicate quite frequent RTCP
information. These applications may need to specify an RTCP
bandwidth that is higher than 5% of the data bandwidth.
If any of the SDP attributes for the RTCP bandwidth modifiers are
omitted, the default value for that parameter is the one specified in
the RTP profile in use for the session. [RFC3556] does not impose
limits on the values that may be specified for both RTCP bandwidth
modifiers, other than that they must be non-negative. However, the
RTP specification and the appropriate RTP profile may specify limits.
4. Early Event-Driven (EED) RTCP Feedback for IDMS
This section introduces the EED RTCP reporting mechanisms proposed in
this document to enable higher flexibility, dynamism, interactivity
and accuracy when using RTP/RTCP for IDMS, with a sync-maestro
architecture.
4.1. Immediate Initial RTCP IDMS Settings
[RFC6051] relaxes the timing rules for unicast and layered sessions
so that SSM senders are allowed to transmit an initial compound RTCP
packet (containing an RTCP SR packet and an RTCP SDES packet with a
CNAME item) immediately on joining each RTP session in a multimedia
session, in parallel with the initial RTP data packets. Hence, the
inter-stream synchronization delay is significantly reduced, provided
that the initial RTCP packet is not lost.
The same rationale for reducing the inter-stream synchronization
delay in [RFC6051] can also be extrapolated to IDMS. In such a case,
it would also be desirable the transmission of a nearly-immediate
RTCP IDMS Settings packet by the MSAS upon establishing a multimedia
session.
[NOTE]: Note that in this document, the terms (nearly-)immediate,
close-to-instant and Early are used as synonymous. This is because
the MSAS is a single centralized entity in the media session, and the
Early RTCP packets can be sent immediately by this entity without
requiring a contention algorithm, as required for receivers in
[RFC4585].
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If the MSAS is integrated within the RTP Server, it MUST send the
IDMS Settings packet in parallel with the initial RTP data packets.
If the Sync Manager is co-located within a SC or a third party entity
(that also needs to be an RTP receiver for that session), it MUST
send the IDMS Settings packet as soon as it receives the initial RTP
data packets from the RTP Sender. In either case, as the MSAS is a
single centralized RTP entity, it is also allowed to transmit Early
RTCP packets [RFC6051]. This way, the SCs can start consuming the
media in a synchronized manner earlier, thus ensuring a reduction of
the IDMS Delay.
This mechanism is purely a local change to the MSAS that can be
implemented as a configurable option, as stated in [RFC6051].
4.2. Dynamic EED Reporting of IDMS Settings
During the media session's lifetime, if Regular RTCP Feedback is
used, the MSAS may have to wait a nearly-complete RTCP reporting
interval to be able to send a new compound RTCP packet (including an
IDMS Settings packet) after detecting an out-of-sync situation, which
might potentially take several seconds (up to 5 s or even more)
[RFC3550].
This is illustrated in Figure 2. In such a case, if an event (e.g.
out-of-sync situation) is detected just after the transmission of an
RTCP packet (at instant t_r1), the next RTCP packet cannot be sent
until the next randomized (over the scheduled transmission instant,
t_d2) RTCP transmission time (at instant t_r2). The figure shows the
worst case, in which the randomized RTCP report interval at that
moment is near the upper limit, i.e.:.
(t_r2 - t_r1) = 1.5 * (t_d2 - t_r1)
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R A N D O M I N T E R V A L S --> [0.5;1.5]*[t_d(n)- t_d(n-1)]
____/\____ ____/\____ ____/\____
/ \ / \ / \
t_r1 t_r2 t_r3
| | |
| | |
|-------(---&-|-+-o-)----(-----|----+)----#-(---+-|-----)------>
| | | | | | | Time
| Event | Event is | Event is |
| Occurs | Detected | Handled or |
t_d0 t_d1 t_d2 Repaired t_d3
\_____ _____/ \_____ _____/ \_____ _____/
\/ \/ \/
S C H E D U L E D R T C P R E P O R T I N T E R V A L S
\______ ______/\________ _________/\____ ____/
\/ \/ \/
R A N D O M I Z E D R T C P R E P O R T I N T E R V A L S
\_ _/\______ _______/\_ _/
\/ \/ \/
DD M S A S AD
D E L A Y
\_____________ ____________/
\/
I D M S D E L A Y (R E G U L A R R T C P I N T E R V A L)
Figure 2: Regular RTCP Timing Rules
Legend
-------
& Event Occurs
O Event is detected
# Event is handled/repaired
( ) Random Interval
| t_d(n): Scheduled (Deterministic) RTCP Transmission Times
+ t_r(n): Real (Randomized) RTCP Transmission Times
X RTCP Transmission Restriction (Cancellation)
DD Detection Delay
AD Adjustment Delay
Figure 3: Legend
Therefore, the contribution of the MSAS delay (i.e., the time
interval since an event is detected and an IDMS Settings packet to
handle or to repair it is transmitted) to the total IDMS Delay
becomes a serious barrier for those use cases requiring stringent
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synchronization levels (e.g., networked loudspeakers, or networked
games) [Montagud2012].
Accordingly, the MSAS is also allowed to dynamically send Early RTCP
IDMS Settings packets once detecting events throughout the duration
of the media session. This is illustrated in Figure 4. In such a
case, an RTCP IDMS Settings packet is sent just after the detection
of an event, despite that this moment moment is earlier than the next
regular RTCP transmission time. Consequently, the IDMS Delay is
significantly reduced, mainly due to the fact that the MSAS delay has
been minimized (due to the inmediate transmission of the IDMS
Settings packet).
R A N D O M I N T E R V A L S --> [0.5;1.5]*[t_d(n)- t_d(n-1)]
____/\____ ____/\____ ____/\____
/ \ / \ / \
t_r1 t_r2 Skipped t_r3
| | Handled | |
| | | | |
|-------(---&-|-+-o+)----(#----|----x)------(---+-|-----)------>
| | | | | | Time
| Event | Event is | |
| Occurs | Detected | |
t_d0 t_d1 t_d2 t_d3
\_____ _____/ \_____ _____/ \_____ _____/
\/ \/ \/
S C H E D U L E D R T C P R E P O R T I N T E R V A L S
\______ ______/|_ _|\____________ ____________/
\/ | \/
E V E N T ? D R I V E N R T C P R E P O R T I N T E R V A L S
|_|
|
M S A S D E L A Y
\_____ _____/
\/
I D M S D E L A Y (E E D R T C P I N T E R V A L)
Figure 4: Early Event-Driven (EED) RTCP Timing Rules
Note that if "trr-int" is set to zero, only one Early RTCP packet can
be transmitted between two consecutive Regular RTCP packets to
preserve the RTCP traffic bounds [RFC3550]. It means that an Early
RTCP packet can only be sent if the previous transmitted RTCP packet
was a Regular RTCP packet. Hence, after sending an Early RTCP
packet, the RTCP reporting engine MUST schedule the sending time for
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the next RTCP packet by delaying (i.e., skipping) one more Regular
RTCP report interval, as in [RFC4585].
In case of high frequency of events, setting an offset value for the
Regular RTCP report interval, by means of using the "trr-int"
attribute, can help to save RTCP bandwidth (by restraining the
transmission of too frequent Regular RTCP packets) while being able
to use the (saved) bandwidth when events occur. This is out of scope
of this document.
Therefore, the proposed EED RTCP reporting rules are based on the
fact that not all the feedback reports from the MSAS are of equal
importance. This means that some feedback reports from the MSAS need
to be reported in a timely fashion.
The dynamic EED reporting of IDMS Settings packets is also be very
useful to provide playout hints for specific events that must be
presented to all the involved users in a fine-grained synchronized
way with the piece of content they refer to. Those events can be
media-related events whose timing can be known in advance (e.g.,
commercials, start of the match in a sports event...), but the
events' timing could be even unknown (e.g., a goal in a football
match...) or dynamically triggered by either operators (e.g., a TV
quiz show, in-game actions, interesting scenes...) or users (e.g.,
shared service control, interactive instant messaging...).
Therefore, the use of EED RTCP Feedback for IDMS implies an
interaction between the application-layer (through which operator or
user generated events are triggered) and the transport/control layer
(i.e., RTP/RTCP protocols) in order to translate the high-level
(i.e., content-based or action-based) events into lower level calls
(i.e., transmission of Early RTCP packets), as well as their
alignment in terms of timelines. These are not severe issues, since
the MSAS will be co-located with the Media Sender in most
implementations. However, this differs from the use of Regular RTCP
Feedback for IDMS, in which the IDMS adjustments are purely based on
packet-level timestamps.
4.3. Rapid (Re-)Synchronization Request
If the initial compound RTCP packet (including an SR, SDES and IDMS
Settings packets) is lost, the affected SCs will not be able to
synchronize the media playout until the report interval has passed,
and the next RTCP packet can be sent. This is undesirable.
[RFC6051] defines a new RTP/AVPF transport layer feedback message to
request the generation of an Early RTCP SR, allowing rapid inter-
stream (re-)synchronization.
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A similar mechanism is proposed in this document to be applied for
IDMS purposes. A new RTP/AVPF transport layer feedback message
[RFC4585], called RTCP-IDMS-REQ, is introduced to request the rapid
generation (and transmission) of an RTCP IDMS Settings packet by the
MSAS (see Figure 5).
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=TBD | PT=RTPFB=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Media Stream Correlation Identifier (SyncGroupId) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(TBD: To Be Determined)
Figure 5: RTCP-IDMS-REQ Feedback Message
The Payload Type (PT) of this type of RTCP message should be 205, as
specified in [RFC4585], the Frame Message Type (FMT) MUST be assigned
by IANA (Internet Assigned Numbers Authority), and its length must be
equal to 3. The SSRC of the packet sender MUST indicate the SC
sending the packet, while the SSRC of the media source field MUST
indicate the source of the media stream the SC (who sends this RTCP-
IDMS-REQ) is unable to synchronize. In contrast to the RTCP-SR-REQ
feedback message defined in [RFC6051], in which the Feedback Control
Information (FCI) part is kept empty, in the RTCP-IDMS-REQ it must
carry the SyncGroupId (specified in [RFC7272]) to which the sender of
this message belongs.
Once a new RTCP-IDMS-REQ is received by the MSAS, it MUST generate an
Early RTCP IDMS Settings packet as soon as possible, while complying
with the Early RTCP feedback rules.
This mechanism can also be employed if a SC has not received RTCP
IDMS Settings in a (configurable) long time interval. The RTCP-IDMS-
REQ packet MAY be repeated once per RTCP reporting interval if no
RTCP IDMS Settings packet is forthcoming. Likewise, the MSAS MAY
ignore RTCP-IDMS-REQ packets if its regular schedule for RTCP
transmission will allow the SC to synchronize within a reasonable
time interval.
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Although this mechanism is similar to the one in [RFC6051] to request
rapid RTCP SRs, it is especially necessary since, in most
implementations,the IDMS Settings packets will not be regularly sent
in each RTCP report interval, as RTCP SRs, but only when the detected
asynchrony exceeds an allowable threshold..
When using SSM sessions with unicast feedback, it is possible that
the feedback target and the MSAS are not co-located. If a feedback
target receives an RTCP-IDMS-REQ feedback message in such a case, the
request SHOULD be forwarded to the MSAS. However, the mechanism to
be used for forwarding such requests is not defined in this document.
If the feedback target provides a network management interface, it
might be useful to provide a log of which SCs send RTCP-IDMS-REQ
feedback packets and which do not, since the later will experience
slower stream synchronization.
4.4. Reduction of Channel Change Delays
The participation and support of latecomers are key issues to enable
dynamic IDMS-enabled sessions. One widely applicability of the RTCP-
IDMS-REQ messages is the dynamic and rapid accommodation of
latecomers.
Once a latecomer joins an IDMS-enabled session, it MUST send an RTCP-
IDMS-REQ to the MSAS of that session. Then, the MSAS MUST generate
an Early RTCP IDMS Settings packet to rapidly bring the latecomer up-
to-date, thus minimizing the IDMS Delay experienced by that latecomer
(i.e. the time interval between joining and acquiring IDMS).
Immediately after receiving the RTCP IDMS Settings packet, the
latecomer will begin to play out the media stream in a time
synchronized way with the other SCs, thus becoming an additional
member in the IDMS-enabled session, as shown in Figure 5. This will
prevent from both long annoying startup delays and initial playout
inconsistencies.
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MSAS i-th SC j-th SC k-th SC
(Media Sender) (Latecomer)
| | | |
| :::::: Media Session Set-up :::::: |
| (see Section 3 in [RFC7272]) |
| ... ... ... |
|RTCP IDMS Packet | | |
|---------------->|---------------->| |
|===> | | |
|===> RTP Packets | | |
|===> | | |
| ... ... ... |
| RR + IDMS XR | | |
|<++++++++++++++++| | |
| | RR + IDMS XR | |
|<++++++++++++++++|+++++++++++++++++| |
|RTCP IDMS Packet |RTCP IDMS Packet | |
|---------------->|---------------->| |
| ... ... x t0 k-th SC
| | | RTCP AVPF | joins
| | | IDMS-REQ Packet |
t2 x<****************|*****************|*****************x t1
| | | |
|RTCP IDMS Packet | | |
t3 x---------------->|---------------->|---------------->x t4
| | | | IDMS Settings
| | | | reception
| ... |
| | | x t5: In Sync!
| | | RR + IDMS XR |
|<++++++++++++++++|+++++++++++++++++|+++++++++++++++++x
| ... |
Figure 6: IDMS Operation and Rapid Accommodation of Latecomers
The timing diagram for the RTCP exchange processes is illustrated in
Figure 6. It can be seen that, if enabling EED RTCP Feedback for
IDMS, the IDMS delay (which is the time interval between joining and
acquiring IDMS, represented by (t5 - t0) in Figure 6) for the
latecomer (k-th SC) can be significantly reduced, mainly due to the
fact that the MSAS delay ((t3 - t2) in Figure 6) can be minimized.
Although the above discussion has been focused for latecomers, it is
also applicable for reducing channel change (i.e., zapping) delays in
IDMS-enable sessions, which is currently a hot research topic in the
IPTV area. Concretely, the relevance of channel change delays and
their variability in IDMS-sensitive services is threefold. First, as
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for inter-stream synchronization, the required time to receive the
IDMS setting instructions must not contribute to further increase the
channel change delays. Second, apart from the magnitudes of channel
change delays, their variability (i.e., the delay variation for each
user) will also impact the IDMS performance. Third, when a group of
users are watching IPTV together and they (simultaneously) change (or
must change) to another channel, any playout time differences among
them will also influence the resulting delay. Therefore, in this
context, the use of EED RTCP Feedback for IDMS is very beneficial
because: i) it significantly reduces the time needed to receive the
initial RTCP IDMS Settings packet; and ii) it enables the
compensation of the delay differences when changing channels.
Two additional mechanisms could contribute to further reduce the IDMS
Delay. The first one consists of employing priority mechanisms for
the transport of RTCP messages, e.g., by adopting a Differentiated
Services (DiffServ) policy. This would help to decrease the Round
Trip Time (RTT) delays and the loss probability for RTCP packets (out
of the scope of this document). The second one is based on the
transmission of Early RTCP-IDMS-REQ messages by latecomers upon
joining the session. According to [RFC6051], the delay since joining
and sending an RTCP-IDMS-REQ message ((t1-t0) in Figure 6) should not
be reduced to avoid flooding of requests at specific time instants
(e.g., at the time a broadcasted sport event begins). Although we
initially adhere to this standard compliant rule in this document, we
believe that it should be discussed within the AVTCORE WG if this
flash crowd effect is a real limiting issue in real SSM scenarios
(e.g., networked quiz shows, gaming, IPTV, etc.). Our initial
assumption is that the upstream bandwidth availability by the SCs
(which is not used for other purposes) and the aggregation and re-
distribution mechanisms by Feedback Targets [RFC5760] do not entail a
real constraint for allowing the transmission of Early RTCP-IDMS-REQ
by the SCs. Moreover, it is assumed in [RFC6051] that all SCs switch
channels simultaneously, but even though using automated procedures
(e.g., through notifications via the Electronic Program Guides in
IPTV), this would not be a matter of a few seconds, but most probably
of minutes.
5. Reduced-Size RTCP Reporting for IDMS
RTCP SR and RR contain relevant QoS statistics usable for monitoring
of the media stream transmission and thus enable media adaptation.
The regular transmission of these RTCP packets becomes very useful to
infer trends between successive reports due to the dynamic nature of
the conveyed information. Likewise, the tracking of the session size
thanks to that regular reports warrants bounded RTCP traffic
bandwidth. Moreover, compound RTCP packets are useful to establish
and maintain multimedia synchronization. Due to the above issues,
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the regular transmission of compound RTCP packets must be maintained
throughout the RTP session's lifetime.
However, [RFC5506] defines certain changes to the RTCP reporting
rules to allow feedback messages to be sent as Reduced-Size RTCP
packets under certain conditions when using the RTP/AVPF profile
[RFC4585]. The motivation is that, in some cases, it is more useful
to report certain events of interest the more frequently or the
sooner as possible to enable short-term adaptation, rather than
sending full compound RTCP packets including periodic statistics.
In low bitrate links (e.g. radio access technologies), there are some
benefits of reporting Reduced-Size RTCP packets. First, if channels
conditions are poor, smaller RTCP packets are much more likely to be
successfully transmitted than larger compound RTCP packets, and they
will also introduce lower traffic overhead. The last issue is
critical, as those messages are likely to occur when channel
conditions are poor (e.g., for reporting picture or slice loss).
Second, the serialization time when transmitting small size RTCP
packets is shorter than when transmitting full (larger) RTCP compound
packets. Third, when the bandwidth availability is scarce, smaller
RTCP packets will enable more frequent feedback messages.
In high bitrate environments, the above issues are not real
limitations, but using Reduced-Size RTCP packets may also be
beneficial to reduce the processing delay and complexity of RTCP
packets.
Independently of the link type, additional benefits with sending
feedback in small Reduced-Size RTCP packets can be cited. For
instance, when using Early RTCP Feedback [RFC4585], receivers
typically need to send frequent event-driven feedback messages. In
such cases, if using Reduced-Sized RTCP packets, the risk that the
RTCP bandwidth becomes too high during periods of heavy feedback
signaling is reduced.
More details about use cases for, benefits of and issues with
Reduced-Size RTP reporting can be found in [RFC5506].
Accordingly, the use of Reduced-Size RTCP packets MAY also be
beneficial when enabling the EED RTCP reporting rules for IDMS
proposed in this document. [RFC5506] specifies that in Immediate
Feedback mode, as it is the case of RTCP IDMS Settings packets, all
feedback messages may be sent as Reduced-Size RTCP packets. However,
it is also stated that Reduced-Size RTCP packets shall not be sent
until at least one compound RTCP packet has been transmitted.
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This implies that if the use of non-compound RTCP packets [RFC5506]
has been previously negotiated between the participants in an IDMS-
enabled session, both the RTCP-IDMS-REQ and the RTCP IDMS Settings
packets MAY be sent as non-compound RTCP packets, but only if a
compound RTCP packet has been already sent by the senders of those
feedback messages.
The Reduced-Size RTCP reporting features do not apply to the first
IDMS Settings packet, which SHOULD be sent in parallel with the
initial RTP data packets (Section 4.1). In such a case, SCs would
also be benefited by the reception of a compound RTCP packet
including SR and SDES packets. This is also the case when a
latecomer sends an Early RTCP-IDMS-REQ. Here, the media sender and
the MSAS would also need an SDES packet from that latecomer for
membership accounting.
However, the use of Reduced-Size RTCP reporting MAY be beneficial
when an RTCP-IDMS-REQ is sent by an active SC that has not received
an RTCP IDMS Settings packet for a long period, requesting re-
synchronization setting instructions. Moreover, Reduced-Size RTCP
reporting MAY be especially useful for Dynamic EED transmission of
IDMS Settings packets (Section 4.2), e.g. immediately after detecting
an out-of-sync situation, or when a specific media event is targeted
to be simultaneously presented in all the SCs.
Reduced-Size RTCP packets can become substantially smaller than
compound packets. A compound packet is forced to contain both an RR
or an SR and an SDES with at least the CNAME item. The RR containing
a report block for a single source is 32 bytes, and an SR is 52
bytes. Both may be larger if they contain report blocks for multiple
sources. The SDES packet containing a CNAME item will be 10 bytes
plus the CNAME string length. Here, it is reasonable that the CNAME
string is at least 10 bytes to get a decent collision resistance. If
the recommended form of user@host is used, then most strings will be
longer than 20 characters. Additionally, for IDMS purposes: i) SCs
will regularly send RTCP XR blocks for IDMS, which consist of 10
32-bit words (40 bytes); ii) the MSAS will send RTCP IDMS Settings
packets, which consists of 9 32-bit words (36 bytes).
Therefore, Reduced-Size RTCP packets can become at least 70-80 bytes
smaller than RTCP compound packets, thus reducing the mean RTCP
packet size, decreasing sporadic traffic peaks, reducing the
transmission delay, and increasing the overall RTCP feedback
frequency.
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6. Security Considerations
This document does not impose security considerations, beyond the
ones described in [RFC3550], [RFC4585], [RFC6051], and [RFC7272].
7. IANA Considerations
This document defines a new RTP/AVPF transport layer feedback message
[RFC4585], called RTCP-IDMS-REQ, whose FMT Value needs to be assigned
by IANA..
8. Contributors
Authors would like to thank the co-authors of [RFC7272] for their
collaboration on specifying the RTCP extensions for IDMS, which are
starting point of this work.
9. Conclusions
This document proposes Early Event-Driven RTCP reporting rules to
enable higher interactivity, dynamism and accuracy when using RTP/
RTCP for IDMS.
10. References
10.1. 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.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC
3556, July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November
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.
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[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback", RFC 5760, February 2010.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.
[RFC7272] van Brandenburg, R., Stokking, H., van Deventer, O.,
Boronat, F., Montagud, M., and K. Gross, "Inter-
Destination Media Synchronization (IDMS) Using the RTP
Control Protocol (RTCP)", RFC 7272, June 2014.
10.2. Informative References
[Montagud2012]
Montagud, M., Boronat, F., Stokking, H., and R. van
Brandenburg, ""Inter-Destination Multimedia
Synchronization; Schemes, Use Cases and Standardization",
Multimedia Systems Journal (Springer), 18(6), pp.
459-482", November 2012, <http://link.springer.com/
article/10.1007%2Fs00530-012-0278-9#page-1>.
[Montagud2013]
Montagud, M., Boronat, F., and H. Stokking, ""Early Event-
Driven (EED) RTCP Feedback for Rapid IDMS", The 21st ACM
International Conference on Multimedia (ACM MM 2013),
Barcelona (Spain)", October 2013,
<http://dl.acm.org/citation.cfm?id=2502114>.
[RFC5968] Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP)", RFC 5968, September 2010.
Authors' Addresses
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Mario Montagud (editor)
Universitat Politecnica de Valencia
C/ Paraninf, 1
Grau de Gandia, Valencia 46730
Spain
Phone: +34 962 849 341
Email: mamontor@upv.es
Fernando Boronat
Universitat Politecnica de Valencia
C/ Paraninf, 1
Grau de Gandia, Valencia 46730
Spain
Phone: +34 962 849 341
Email: fboronat@dcom.upv.es
Hans Stokking
TNO
Brassersplein 2
Delft 2612CT
The Netherlands
Phone: +31-88-866-7000
Email: hans.stokking@tno.nl
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