Network Working Group M. Westerlund
Internet-Draft B. Burman
Intended status: Standards Track L. Hamm
Expires: April 25, 2013 Ericsson
October 22, 2012
Codec Operation Point RTCP Extension
draft-westerlund-avtext-codec-operation-point-01
Abstract
The Audio-visual Profile with Feedback (AVPF) specification defines a
framework and messages for fast feedback and media control over RTCP.
The Codec Control Messages (CCM) specification defines an extension
to AVPF, by specifying additional messages for codec control and
feedback. This specification extends CCM, by specifying messages
that let participants dynamically communicate a set of codec
configuration parameters, which enables better optimization of
resource efficiency and quality of media transmission.
Status of this Memo
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This Internet-Draft will expire on April 25, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Requirements Language . . . . . . . . . . . . . . . . . . 7
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Problem Description . . . . . . . . . . . . . . . . . . . 7
3.2. Legacy Methods . . . . . . . . . . . . . . . . . . . . . . 10
3.2.1. Relation to SDP . . . . . . . . . . . . . . . . . . . 10
3.2.2. Relation to RTCP . . . . . . . . . . . . . . . . . . . 10
4. Use Cases for COP . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Point to Point . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Media Receiver to RTP Mixer . . . . . . . . . . . . . . . 12
4.3. RTP Mixer to Media Sender . . . . . . . . . . . . . . . . 13
4.4. Media Receiver in Multicast or with RTP Transport
Translator . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Message Structure . . . . . . . . . . . . . . . . . . . . 21
6.2. Codec Configuration Parameter Use . . . . . . . . . . . . 22
6.3. Operation Point . . . . . . . . . . . . . . . . . . . . . 23
6.4. Request . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.5. Notification . . . . . . . . . . . . . . . . . . . . . . . 25
6.6. Status Report . . . . . . . . . . . . . . . . . . . . . . 26
6.7. Adding and Removing Operation Points . . . . . . . . . . . 27
7. Codec Control Message Extension . . . . . . . . . . . . . . . 27
7.1. COP Message . . . . . . . . . . . . . . . . . . . . . . . 28
7.2. FCI Format . . . . . . . . . . . . . . . . . . . . . . . . 28
7.2.1. Message Item Format . . . . . . . . . . . . . . . . . 29
7.2.2. Message Item Types . . . . . . . . . . . . . . . . . . 30
7.2.3. Operation Point Identification . . . . . . . . . . . . 30
7.3. Codec Operation Point Notification . . . . . . . . . . . . 31
7.3.1. Message Format . . . . . . . . . . . . . . . . . . . . 31
7.3.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 32
7.3.3. Timing Rules . . . . . . . . . . . . . . . . . . . . . 35
7.4. Codec Operation Point Request . . . . . . . . . . . . . . 35
7.4.1. Message Format . . . . . . . . . . . . . . . . . . . . 35
7.4.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 36
7.4.3. Timing Rules . . . . . . . . . . . . . . . . . . . . . 38
7.5. Codec Operation Point Status . . . . . . . . . . . . . . . 38
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7.5.1. Message Format . . . . . . . . . . . . . . . . . . . . 38
7.5.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 40
7.5.3. Timing Rules . . . . . . . . . . . . . . . . . . . . . 41
7.6. Handling in Mixers and Translators . . . . . . . . . . . . 42
7.6.1. COPN . . . . . . . . . . . . . . . . . . . . . . . . . 42
7.6.2. COPR . . . . . . . . . . . . . . . . . . . . . . . . . 43
7.6.3. COPS . . . . . . . . . . . . . . . . . . . . . . . . . 43
8. Parameter Types . . . . . . . . . . . . . . . . . . . . . . . 43
8.1. Parameter Format . . . . . . . . . . . . . . . . . . . . . 43
8.2. ALT . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
8.3. ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
8.4. Payload Type . . . . . . . . . . . . . . . . . . . . . . . 48
8.5. Bitrate . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.6. Token Bucket Size . . . . . . . . . . . . . . . . . . . . 50
8.7. Framerate . . . . . . . . . . . . . . . . . . . . . . . . 51
8.8. Horizontal Pixels . . . . . . . . . . . . . . . . . . . . 52
8.9. Vertical Pixels . . . . . . . . . . . . . . . . . . . . . 52
8.10. Sample Aspect Ratio . . . . . . . . . . . . . . . . . . . 53
8.11. Picture Aspect Ratio . . . . . . . . . . . . . . . . . . . 54
8.12. Channels . . . . . . . . . . . . . . . . . . . . . . . . . 54
8.13. Sampling Rate . . . . . . . . . . . . . . . . . . . . . . 55
8.14. Maximum RTP Packet Size . . . . . . . . . . . . . . . . . 56
8.15. Maximum RTP Packet Rate . . . . . . . . . . . . . . . . . 57
8.16. Application Data Unit Aggregation . . . . . . . . . . . . 58
9. SDP Extensions . . . . . . . . . . . . . . . . . . . . . . . . 59
9.1. Extension of the rtcp-fb Attribute . . . . . . . . . . . . 59
9.2. Offer/Answer Usage . . . . . . . . . . . . . . . . . . . . 60
9.3. Declarative Usage . . . . . . . . . . . . . . . . . . . . 61
10. Codec Sub-Stream Identification . . . . . . . . . . . . . . . 61
10.1. H.264 AVC . . . . . . . . . . . . . . . . . . . . . . . . 62
10.2. H.264 SVC . . . . . . . . . . . . . . . . . . . . . . . . 62
11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
11.1. SDP Offer/Answer . . . . . . . . . . . . . . . . . . . . . 63
11.2. Dynamic Video Re-sizing . . . . . . . . . . . . . . . . . 65
11.3. Illegal Request . . . . . . . . . . . . . . . . . . . . . 67
11.4. Reference Response to Modification of Scalable Layer . . . 68
11.5. Successful Request to Add Codec Operation Point . . . . . 70
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 72
13. Security Considerations . . . . . . . . . . . . . . . . . . . 72
14. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 72
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 73
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 73
16.1. Normative References . . . . . . . . . . . . . . . . . . . 73
16.2. Informative References . . . . . . . . . . . . . . . . . . 74
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 75
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1. Introduction
Multimedia real-time communication services, such as video telephony
and videoconferencing, use the real-time transport (RTP/RTCP)
[RFC3550] protocol to transmit media streams, such as audio and
video. A session establishment protocol, such as SIP [RFC3261], in
combination with a capability negotiation protocol, such as SDP
offer/answer [RFC3264] is normally used to establish the session and
negotiate media capabilities. In some cases, a set of codec
parameters is negotiated that does not express any specific limit or
capability, but just describes a certain codec configuration.
During session establishment, the participating endpoints normally
have limited knowledge about the session environment, e.g. whether
the session will be point-to-point or contain some multiparty
scenario, how users will interact with the application, how network
conditions will vary during the session, etc. To take those
variations into account, the participants can renegotiate session
parameters to better suit the communication environment. At times,
when variations or changes are frequent in nature, it will require
the needed reaction time to be short, which may make repeated session
renegotiation inefficient and/or too slow. In addition, variations
may not even affect negotiated session parameters, if the variations
occur within the negotiated boundaries.
The above scenario can become critical especially in cases where a
given media stream is transmitted towards, and received by, multiple
receivers. In multiparty environments, scalable encoding or
simulcast can be used to make the system more efficient and provide
better quality to participants that are capable of receiving and
utilizing the higher quality. These use cases result in that a
sending party is requested to deliver multiple encoder operation
points.
The Audio-Visual Profile with Feedback (AVPF) specification [RFC4585]
defines a framework and messages for fast feedback and media control
over RTCP. The Codec Control Messages (CCM) specification [RFC5104]
defines an extension to AVPF, by specifying additional messages for
codec control and feedback. This specification extends CCM, by
specifying messages that let participants dynamically communicate a
set of codec configuration parameters, which enable better
optimization of resource usage and quality of media transmission.
The codec configuration parameters specified in this document focus
on some basic audio and video properties, such as video resolution,
video frame rate, media stream bit-rate, audio sampling rate, number
of audio channels, maximum RTP packet size and rate. Additional
parameters can be standardized in the future.
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The codec control messages are not meant to replace the configuration
performed using e.g. SDP. Instead, the messages can be used to
communicate dynamic and frequent changes that take place within
boundaries that have been negotiated as part of the session
establishment.
2. Definitions
2.1. Terminology
The following terms and abbreviations are used in this document:
Bandwidth: The network resource needed to transport a certain
bitrate and any transport overhead, measured in bits per second.
There will be spare network bandwidth when the (media) data
bitrate and overhead is less than the available bandwidth.
Similarly, data will have to be buffered when the available
bandwidth excluding transport overhead is less than the bitrate
used by the sender, or the excess data will be lost. The
available bandwidth typically varies dynamically over time.
Bitrate: The amount of (media) data transmitted per time unit,
measured in bits per second, utilizing some amount of the
available network bandwidth resource. In the context of this
specification and unless otherwise specified, it excludes IP/UDP/
RTP overhead. Depending on the (media) data source, the bitrate
can either be constant or vary dynamically over time.
Codec Configuration Parameter: The configurable value describing a
certain codec property, which may impact user-perceived media
fidelity, encoded media stream characteristics, or both. The
parameter has a type (codec parameter type, see below) and a
value, where the type describes what kind of codec property is
controlled, and the value describes the property setting as well
as how the value should be used in comparison operations. A
single parameter value can express one specific value or an open-
ended range. A pair of parameter values with different comparison
types can describe a value range. Such value range can also be
combined with a third, target value within that range.
Codec Operation Point: Also denoted just operation point. A set of
codec configuration parameter values, describing the
characteristics of one single encoding. For scalable encoding, it
describes the resulting characteristics from combining a set of
dependent sub-streams.
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Codec Parameter Type: The specific type of a codec configuration
parameter. Each parameter type defines what unit the value has.
This specification defines a number of generally useful parameter
types in Section 8 that can be used to control codec operation.
Encoding: A particular encoding is the resulting media stream from
applying a certain choice of codec configuration parameters to the
encoder. The media stream will have a certain fidelity (quality)
from that encoding through the choice of sampling, bit-rate and
other configuration parameters.
Endpoint: A host or node that has a presence in the RTP session with
one or more Synchronization Sources (SSRC)s.
Mixer: An RTP session centralized node that generates media streams
based on incoming media streams from other endpoints. See Topo-
Mixer in RTP Topologies [RFC5117].
RTP Session: An association among a set of participants
communicating with RTP. The distinguishing feature of an RTP
session (defined in [RFC3550]) is that each RTP session maintains
a full, separate space of SSRC identifiers. Each participant in
the RTP session can see SSRC or CSRC identifiers from the other
participants, either by RTP, RTCP, or both.
Sub-Stream: An individually decodeable part of a scalable media
stream, including all dependent sub-streams. The characteristics
of a certain sub-stream can be described by a codec operation
point.
Translator: An RTP session centralized node that forwards all media
streams from other endpoints, modified to some extent, e.g.
addressing, encoding, fidelity. See Topo-Translator in RTP
Topologies [RFC5117].
2.2. Abbreviations
AVC: Advanced Video Coding
AVPF: Extended RTP Profile for RTCP-Based Feedback
CCP: Codec Configuration Parameter
COP: Codec Operation Point
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COPN: Codec Operation Point Notification
COPR: Codec Operation Point Request
COPS: Codec Operation Point Status
CPT: Codec Parameter Type
FCI: Feedback Control Information
FMT: Feedback Message Type
GUI: Graphical User Interface
MST: Multi-Session Transmission
MVC: Multiview Video Coding
OP: Operation Point
OPID: Operation Point Identification number
PPS: Picture Parameter Set
SPS: Sequence Parameter Set
SST: Single-Session Transmission
SVC: Scalable Video Coding
TLV: Type-Length-Value
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 [RFC2119].
3. Motivation
3.1. Problem Description
Networks can contain endpoints with different capabilities, including
CPU power, capture and render device fidelity (e.g. image
resolution), and codecs. In addition, the characteristics and
properties of networks can vary, which endpoints have to cope with.
For example, in videoconferencing and telepresence services, a large
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number of endpoints may participate, and there may be a large number
of media streams associated with the session. Such multiparty
scenarios typically use entities for media mixing, switching and
transcoding. The aim is to provide the best possible quality to each
endpoint, taking endpoint and network capabilities into
consideration.
Many communication services today use codecs that can be configured
in a number of different ways. Often, the codecs have multiple
properties that can be configured and those properties may also be
inter-related, often in complex ways. One example is the H.264 (AVC)
[H264] video codec and its scalable (SVC) and multiview (MVC)
versions. Most other video codecs, and codecs for many other types
of media, also have multiple configurable properties. Such
configurable properties will be referred to as "codec configuration
parameters" in this specification.
There can be several reasons to change the media rate or other
encoding or packetization properties during an ongoing communication
session. Reasons can be that the available network bandwidth varies,
or that other network properties change, such as effective MTU or
packet rate limitations. Other reasons can be that the quality or
representation of the media rendered to the end user changes, maybe
as a direct result of the user manipulating the GUI (e.g. changing
window position or size), or the relative importance of the received
media stream changes (e.g. active or non-active speaker in a
conferencing scenario), or the user selects to show some other
content source that is available among the advertised media streams.
The codec changes above can be made directly between endpoints in a
point-to-point scenario, or they may involve, and be acted upon, by
media aware intermediaries (e.g. RTP mixers). An RTP mixer can do
transcoding to provide each receiver with media streams of adapted
quality, but transcoding has drawbacks as it always consumes
processing power, typically impacts media quality in a negative way,
and often introduces additional delays.
In order to avoid separate transcoding towards each endpoint, an RTP
mixer can, by taking the capabilities of the endpoints into account,
decide to request specific codec configurations from sending
endpoints, which will minimize the need for transcoding. Also, in
scenarios where no RTP mixers are used and transmitted media reaches
multiple endpoints, the sender will have to take into account that
each endpoint may have different capabilities. The use cases section
(Section 4) shows different use cases, with and without RTP mixers.
Resource optimization involving bandwidth is expected to be one of
the major reasons for changing encoding properties, since it is
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desirable to avoid using more bandwidth than absolutely necessary,
especially considering that
o the expectation for high media quality will continue to increase;
o the bitrate required to transmit the media, despite increasingly
efficient media coding, can due to the above also be expected to
increase;
o the available bandwidth is commonly a scarce and/or costly
resource and will continue to be in the future;
o the relation between media bitrate and media codec configuration,
the used set of media codec property values, is typically complex
and the mapping between each individual codec property and bitrate
is not linear;
o the used media bitrate does not uniquely identify the media codec
configuration, but there are multiple codec configurations that
can generate the same media bitrate;
o the media receiver preferences how the codec property values
should be set for a certain media bitrate will vary with the
specific end-user service requirements (for example, but not
limited to, users with special needs) and the current media stream
role in the application;
o the communication scenarios will not be limited to point-to-point,
potentially involving multiple and at least partly conflicting
constraints from different receivers.
Other resources that may be desirable to optimize include, but not
limited to, endpoint and middle node processing (CPU) utilization,
and transport quality (QoS).
A media receiver cannot be assumed to know exactly what codec
configuration will be best for the media sender to use, given that
the sender needs to take multiple aspects into account, including
implementation limitations in the actual encoder. It should be more
likely to find a value acceptable to both sender and receiver if the
receiver can indicate an acceptable range instead of just a single
value.
When an RTP mixer distributes streams to multiple receivers with
different media quality requirements, it is sometimes possible to
avoid targeted transcoding for every single receiver. That can be
accomplished if the media sender has the ability to produce multiple
media versions, such as for example scalable encoding or simulcast.
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Thus, there is a need to both address specific media versions and
describe the fact that multiple media versions with different
configurations should be used.
3.2. Legacy Methods
3.2.1. Relation to SDP
The session description protocol (SDP) [RFC4566] is commonly used to
negotiate and configure codecs, as well as to establish RTP/RTCP
session parameters during session establishment and ongoing sessions,
e.g. by using it in conjunction with SIP [RFC3261] and SDP Offer/
Answer [RFC3264].
As described in Section 3.1, many of the underlying reasons which
make media receivers desire certain codec encoding properties are
highly dynamic in nature and using SIP/SDP to renegotiate the session
will in many cases be too slow to be useful. SIP messages containing
an SDP may become quite large for sessions containing many media
types, and since there is no defined way to send a partial SDP, even
very small changes require sending the entire SDP. Most of the
current defined properties in SDP are oriented to be common for all
media streams in the same RTP session, at least the ones sharing the
same RTP Payload Type, rather than being specific to one media stream
(e.g. "a=fmtp:98 profile-level-id=42C00C").
The mechanism in this specification does not replace SDP, or the SDP
Offer/Answer mechanism. It is expected that SDP is used in order to
negotiate and configure boundary values for codec properties, and COP
can then be used to communicate specific values within those
boundaries, as long as there is no impact on the values negotiated
using SDP. It is possible to establish communication sessions even
if one or more endpoints do not support COP.
3.2.2. Relation to RTCP
As discussed in CCM, regular RTCP reporting or extended reports
[RFC3611] can to some extent be used to reconfigure an encoder, but
the reported measures seldom map directly back to encoding properties
and they typically cannot express an unwanted situation in terms of
encoding properties and what the receiver would like to receive
instead. Communicating codec properties indirectly as a set of
network properties will require interpretation by both sender and
receiver and will thus risk misinterpretations and ambiguity. Since
it is likely that a decoder is able to identify unwanted
characteristics of the media stream in terms of encoding properties,
the most straight forward approach is to convey those properties
directly to the encoder.
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Responsive techniques to control encoding are already available, e.g.
Codec Control Messages (CCM) [RFC5104]. Although highly applicable,
the possibilities to control encoding is however not explicit enough,
both in terms of the amount of available parameters to control, and
the fact that they may be inter-related, alternative, or both.
Some codecs define codec-specific methods to enable receiver control
of some encoding aspects, but it should be beneficial for
interoperability to use codec agnostic signaling instead.
4. Use Cases for COP
This section discusses a number of use cases for codec operation
points.
4.1. Point to Point
This set of use cases focuses on communication, which is directly
point to point between a media sender and a receiver. There is no
need for further forwarding of the media streams. Thus, the goal
should be to produce a media stream, transport it to the media
receiver, where it is consumed as optimal as possible for the
application. Thanks to this one-to-one mapping between encoder and
decoder, great flexibility exists to produce a media stream tailored
to the receiver's needs, given the constraints that exist from media
sender, transport network and the receiver.
Some constraints are static (and thus suitable for session
configuration signalling), but others are highly dynamical and
desirable to adapt to during the session:
Video Resolution in GUI: In a video communication application,
including WebRTC based ones, the window where the media senders
media stream is presented may change, for example due to the user
modifying the size of the window. It might also be due to other
application related actions, like selecting to show a
collaborative work space and thus reducing the area used to show
the remote video. In both of these cases it is the receiver side
that knows how big the actual screen area is and what the most
suitable resolution would be. It appears suitable to let the
receiver request the media sender to send a media stream
conforming to the displayed video size.
Network Bit-rate Limitations: If the receiver discovers a network
bandwidth limitation, it can choose to meet it by requesting media
stream bit-rate limitations. Especially in cases where a media
sender provides multiple media streams, the relative distribution
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of available bit-rate can help the application to provide the most
suitable experience in a constrained situation.
CPU Constraint: A media receiver may become constrained in the
amount of available processing resources. This may occur in the
middle of a session for example due to the user selecting a power
saving mode, or starting additional applications requiring
resources. When this occurs, the receiving application can select
which and how much to constrain codec parameters to best suit the
needs of the application. For example, if lower framerate is a
better constraint than lower resolution.
4.2. Media Receiver to RTP Mixer
This section considers a multiparty session with a centralized media
intermediary, like an RTP mixer, where the media receiver uses COP to
affect the delivered media.
+------------+ +---+
| |--RTP-->| B |
| |<--COP--| |
| | +---+
| |
+---+ | | +---+
| A |-RTP->| Mixer |--RTP-->| C |
+---+ | | +---+
| |
| | +---+
| |--RTP-->| D |
+------------+ +---+
Figure 1: Receiver (B) using COP to adapt a media stream
In the above Figure 1 we focus on the possible usages of COP by a
media receiver, like B. Here the functional role of the intermediary
becomes important (Topo-Mixer) [RFC5117]. An RTP mixer uses its own
SSRC(s) to channel selected media streams to B from other
participants like A. If the intermediary is instead a translator, the
Receiver B can see A's SSRC(s) directly instead of possibly showing
up as CSRC. We will in this section focus on the mixer case. The
RTP translator case is further discussed in Section 4.4.
The RTP mixer's usage of its own SSRC allows mixer to receiver media
flows to be associated with a role or purpose in the application
rather than a given media source. Based on the assumption that the
set of available stream roles are connected to the specific use case
or application, it is likely that the set of stream roles (for
example most active speaker) provided from a mixer will change less
often than the original media source representing that role is
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changed. It is further assumed that the desirable media
characteristics related to a specific role will be fairly constant.
To minimize the amount of signaling needed to modify stream
characteristics, it could thus be appropriate to let a stream
represent a role rather than limiting it to represent the original
source. When there exist multiple RTP streams from the mixer to a
receiver, the receiver can use COP to request an operations point
that better suits the receiver's needs on each particular stream
(role) of the media stream. COP also allows the receiver to select
its desired trade-off in properties and quality between multiple
delivered media streams.
There exist different reasons why B would need to indicate changes in
its capabilities to receive a particular media stream:
Network Path: The receiver detects changes in the network that on a
mid to long term will result in a new capability regarding the
maximum bit-rate that can be supported.
Bandwidth Trade-off: In an application receiving multiple media
streams, if the receiving application likes to change the relative
bit-rate trade-off between the streams.
Presentation or GUI Changes: If the presentation or graphical user
interface (GUI) changes on the receiving side this results in
other requirements or needs on the media streams. For example if
the application window is resized by the user, the amount of
screen estate to present the different video elements changes. To
optimize the video quality in relation to bit-rate the receiver
indicates the new preferred video resolution.
In all the above cases the receiver sends a COP request to the mixer
for new codec operation points on mixer controlled media stream(s).
It then becomes the mixer's responsibility to determine if and how
the requested COPs can be supported. For example by requesting new
operations points from the media source as discussed in Section 4.3.
The selection of another media source to deliver in a media stream
can result in that the mixer may have to update the receiver on the
properties of the operations point.
4.3. RTP Mixer to Media Sender
This section looks at the usage of COP in cases of multiparty with
centralized media intermediary, like an RTP mixer, selecting and
requesting tailored media stream or streams a media sender delivers
to the intermediary for further forwarding or manipulation. This
usage can be simplified to the media streams from one media sender
(A), which is currently being delivered to multiple receivers (B-D)
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as depicted in Figure 2.
+------------+ +---+
| |--RTP-->| B |
| | +---+
+---+ | |
| A |<-COP-| | +---+
| |-RTP->| Mixer |--RTP-->| C |
+---+ | | +---+
| |
| | +---+
| |--RTP-->| D |
+------------+ +---+
Figure 2: Mixer using COP to adapt media streams to multiple
receivers
The media path from the mixer to B, C and D are different and thus
the available resources may vary between them. In addition B, C and
D may have different capabilities when it comes to handling media
streams. These limitations can be learned by the mixer through
session configuration signalling, media transmission feedback (e.g.
RTCP), or usage of COP by the receivers (See Section 4.2).
Limitations are also expected to be updated during the session
lifetime.
The media sender (A) has certain capabilities and what is possible to
do will depend on A's capabilities and what has been configured
between A and the mixer. Let's consider different capabilities of A
and how they influence the usage of COP to affect the media stream(s)
delivered to the mixer.
Single Media Encoding: If A can only provide a single media encoding
of a particular media source, the mixer has to make a choice on
what property it would like to request for that media stream. The
most basic choice is to request the lowest common denominator
across the receiver population. If the mixer has certain
capabilities for media transcoding it could select to request
another operation point for the media encoding with higher quality
and then transcode to some few receivers. That enables a higher
quality to several receivers while still being able to serve
endpoints with the least capabilities. In these cases the mixer
has to send COP requests that indicate only a single operation
point with parameters matching the restrictions in the best
possible way.
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Scalable Media Encoding: If A is capable of producing a scalable
media stream encoding, the mixer can request multiple operation
points for the same media stream. For example, if A is capable of
producing three different operation points, the mixer in the above
Figure 2 would be able to request scalability layers that match
the capabilities of all three receivers B, C and D. If several
receivers have similar capabilities, the mixer may choose to
request fewer operation points. In this case, other than in the
single media encoding, the mixer must determine which packets or
parts of packets to send to each receiver based on their
capabilities. This requires that the mixer is capable of
identifying in the media stream which scalability layer matches a
requested operation point. Thus, it is desirable that the media
sender can indicate to the mixer which layer matches a given
operation point.
Simulcast Media: If A and the mixer have negotiated the usage of
simulcasted media encoding of the media source, then the mixer can
adopt several operation points to best suit the receivers, just
like for scalable encoding. When simulcasting, the mixer will
however have to send one COP request per media stream it actually
wants to affect. It is necessary to ensure that configuration
changes over multiple media streams from the same media source
take place. Compared to scalable media, the mixer does not need
not strip away layers to match a particular operation point but
can forward entirely self-contained media streams.
The use of COP as described above can be triggered by a multitude of
reasons. We will here discuss some of them. We already mentioned
that bit-rate adaptation (congestion control) on the mixer to
receiver path can indicate a need to change an operation point.
Another reason is when a new session participant joins that has
certain receiver capabilities (both decoding or other hardware, as
well as network path related), thus potentially changing the optimal
set of operation points. There also exist a number of different
cases where the desired application behavior results in changes in
desired operation points, like change of active speakers,
reconfiguration of the display layout, etc.
It is important to remember that Figure 2 only presents the view of a
single media sender. In most communication sessions there are
multiple media senders, and the mixer will need to take the
combination of media streams from multiple media senders into account
when choosing what is to be sent to a given receiver. Thus changes
at one media sender can result in related changes of the operation
points at the other media senders.
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4.4. Media Receiver in Multicast or with RTP Transport Translator
This section covers the usage of COP in multicast transported RTP
sessions, as well as when transport translators (Topo-Translator)
[RFC5117] are used. Transport translators can be used to emulate any
source multicast (ASM) over unicast. Multicast usages also include
Source Specific Multicast (SSM) [RFC4607], which according to "RTP
Control Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback" [RFC5760] has two main modes: simple
mode, and summary feedback mode. SSM modes affect the usage of COP
functionalities.
+---+ +------------+ +---+
| A |<---->| |<---->| B |
+---+ | | +---+
| Translator |
+---+ | | +---+
| C |<---->| |<---->| D |
+---+ +------------+ +---+
Figure 3: RTP translator topology
A transport translator [RFC5117], which main purpose is to forward
any incoming packets to all the other session participants, emulates
an ASM session (see Figure 3). As anyone can send to all other in
both cases, there are some properties in these large scale sessions
with many participants which require extra consideration.
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+-----+ +-----+ +-----+
| MS1 | | MS2 | .... | MSm |
+-----+ +-----+ +-----+
^ ^ ^
| | |
V V V
+---------------------------------+
| Distribution Source |
+--------+ |
| FT Agg | |
+--------+------------------------+
^ ^ |
: . |
: +...................+
: | .
: / \ .
+------+ / \ +-----+
| FT1 |<----+ +----->| FT2 |
+------+ / \ +-----+
^ ^ / \ ^ ^
: : / \ : :
: : / \ : :
: : / \ : :
: ./\ /\. :
: /. \ / .\ :
: V . V V . V :
+----+ +----+ +----+ +----+
| R1 | | R2 | ... |Rn-1| | Rn |
+----+ +----+ +----+ +----+
Figure 4: SSM based RTP session
In the above Figure 4, the media senders (MS1 ... MSm) send their
media streams and RTCP traffic to the distribution source (DS). The
DS forwards the RTP and RTCP traffic from the media senders to the
SSM group. Using the RTCP extension for unicast RTCP feedback
[RFC5760], the receivers (R1...Rn) send their RTCP traffic to their
configured feedback target. This sample session has two feedback
targets to scale with the amount of receivers. RTCP messages that
need to go to a media sender are forwarded to the FT aggregator part
of the distribution source for further forwarding over the unicast
paths between the distribution source and the media senders. The
feedback target and the feedback aggregator also forward all RTCP
messages from receivers in simple mode, and aggregate it in summary
mode. Some RTCP messages from a receiver may still have to be
forwarded over the SSM group.
COP needs to support some reasonable functionality over the different
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multiparty topologies described above and it is important that COP
does not cause significant issues in any of the environments.
In the basic case, where only a single multicast group exists, there
is a well known problem associated with adapting content and bit-rate
to the receiver population. The more receivers, the larger the
potential for non-matching requirements in requests from the
different receivers. One strategy for meeting this is to use the
lowest common denominator among the requests from the receiver
population. This normally results in sub-optimal quality for a
significant part of the session participants, the main benefit being
that all participants will be able to receive some content.
Because of the above limitations of operation within a single group,
the usage of COP in larger groups becomes difficult unless the
parameters that can be adopted and affected by COP requests are such
that a limited set of participants is expected to request them, and
the impact for the others are limited or acceptable. The authors
therefore expects the usage of COP in large groups to be limited and
this specification focuses on operation in smaller groups. However,
as it is not possible to define the threshold when a group changes
from being small to be too large to work well with COP in the generic
case, it is important that COP can operate safely in a large group,
although the possibilities to satisfy the request may be severely
limited.
There also exist use cases for COP where the media application uses
multiple multicast groups to enable multiple operation points and
allows each receiver to join the multicast groups that suits the
participant's capabilities. An example of such usage would be
Scalable Video Coding (SVC) using the Multi-Session Transport (MST)
mode of the SVC RTP payload format [RFC6190]. The SVC MST RTP
streams that are sent in each group can still contain multiple
scalability layers. One could combine coarse-grained control on the
operation points by having the receiver join a particular session
with a more fine-grained control using COP to adjust the included
scalability layers to the receiver's needs, such as lower CPU load.
5. Requirements
The solution outlined in this specification should fulfill the
following requirements:
REQ-1: Enable dynamic control of possibly inter-related codec
properties during an ongoing media session.
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REQ-2: Be media type agnostic, to the furthest extent possible, and
at least cover audio and video media.
REQ-3: Be codec agnostic (within the same media type), to the
furthest extent possible.
REQ-4: Work with different media transmission types, i.e. single-
stream, simulcast, single-stream scalable, and multi-stream
scalable transmission.
REQ-5: Work with un-encrypted as well as encrypted media.
REQ-6: Be extensible, making it simple to add control and
description of new codec properties.
REQ-7: Complement rather than conflict with other codec
configuration methods such as other RTCP based techniques and SDP.
REQ-8: Support configurable parameters that are directly visible in
the media stream as well as those that are not visible in the
media stream.
In addition, Guidelines for Extending RTCP [RFC5968] should be
followed.
6. Solution Overview
The mechanism described in this specification especially targets
heterogeneous multiparty scenarios where different endpoints require
differently encoded media from the same source, but its use in other
situations is not precluded. In fact, point-to-point scenarios are
considered to be of equal importance but not more demanding that the
multiparty case. In the targeted scenario, the media stream from one
encoder is sent to multiple decoders. Hence, the encoder must
possibly provide an encoding with multiple operation points, suitable
for the receivers. This is only possible with so-called scalable
codecs, but some codecs may have inherent scalability features
without being generally considered as scalable (e.g. H.264/AVC
temporal scalability through non-reference frames). Multiparty
services often involve a media mixer (Topo-Mixer) [RFC5117] as a
central network node.
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+---+
| S |
+---+
|
v
+-------+
| Mixer |
+-------+
/ | \
v v v
+---+ +---+ +---+
| A | | B | | C |
+---+ +---+ +---+
Figure 5: RTP mixer topology
The solution defined in this specification is targeted for automatic
control of codec parameters, not as a direct result of user
interaction, although the automatic control can in turn be triggered
by user interaction. It can be used during an active session to
quickly adapt to changes in media receiver available bandwidth and/or
preferences for one or more codec properties, while still conforming
to the session configuration, like SDP offer/answer negotiated
minimum or maximum limits (depending on individual SDP property
semantics). Some codec property changes will also motivate to
renegotiate the SDP, but the scope of this specification intends to
cover only changes that lie within the SDP negotiated set and thus do
not impact the SDP.
Three message types are defined to support the solution: a request, a
notification, and a status report:
Request (COPR): A media receiver requesting a media sender to adjust
one or more of it's media encoding parameters for a media stream.
The request is normally based on a specific set of media encoding
parameters that the media sender has explicitly notified the media
receiver about in a notification.
Notification (COPN): A media sender notifying a media receiver of
the currently used media encoding parameters for a media stream.
The notification is initiated by the media sender, typically
whenever the media encoding parameters changed significantly from
what was previously used. The reason for the change can either be
local to the media sender (user, endpoint or network), or it can
be the result of one or more requests from remote endpoints.
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Status Report (COPA): A media sender reporting to a request sender
(media receiver) on request reception status, which specific
request from the media receiver that was received and considered
in setting current media encoding parameters, and the
identification of the media stream that is considered to fulfill
the request. The status report can also indicate various error
conditions, such as reception of invalid or failing requests.
More details about the individual messages are found in the following
sub-sections.
6.1. Message Structure
A COP message is sent from an RTP session participant in its role
either as media receiver or media sender. Each message can contain
one or more message items of one or more message types, all
originating from a single media source.
The individual message items each relate only to a single operation
point, describing part of an atomic notification or request.
The general structure is outlined below:
+--------------------------------------+
| AVPF PSFB FMT="COP" |
| SSRC of Packet Sender |
| SSRC of Media Source |
| +----------------------------------+ |
| | COP Message Item 0 | |
| +----------------------------------+ |
| | (Codec Configuration Parameters) | |
| +----------------------------------+ |
| +----------------------------------+ |
| | COP Message Item 1 | |
| +----------------------------------+ |
| | (Codec Configuration Parameters) | |
| +----------------------------------+ |
| ... |
+--------------------------------------+
Figure 6: COP message structure
Note that the request is the only COP message item defined in this
specification that is sent in the media receiver role and makes use
of "SSRC of media source" as the targeted media stream for the
request. Both the notification and the status report message items
are sent in the media sender role, reporting on the message sender's
own configuration and thus relate only to the "SSRC of packet
sender", being agnostic to the "SSRC of media source" field.
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It is for example possible to collocate COPS and COPN messages for
the same media source in the same COP FCI. It is also possible to
co-locate one or more COPR referring to a single "SSRC of media
source" with one or more COPN and/or COPS relating to a single "SSRC
of packet sender" within a single COP message.
Multiple message items of the same type in the same COP message are
used to describe a notification, status or request for a media stream
containing multiple operation points (see Section 6.3).
Multiple COP messages are needed to be able to refer to multiple
different "SSRC of packet sender" and/or "SSRC of media source".
6.2. Codec Configuration Parameter Use
The codec configuration parameters that are applicable to a certain
codec may be specific to the media type (audio, video, ...), and may
also be codec specific. Some codec properties (described by codec
configuration parameters) have to be explicitly enabled by (non-RTCP
based) capability signaling to be possible or permitted to use.
An endpoint implementing this specification does not need to support
all available codec configuration parameters defined herein or in
extensions to this specification. A certain parameter could be
unnecessary for a certain codec or media stream, even if it is
generally supported by the endpoint. This specification therefore
defines capability signaling that allows a COP receiver to declare
explicit support per parameter type on a per codec level. The set of
codec configuration parameters that can be used for a certain media
stream by a COP sender is thus restricted by the combination of
applicability, capability signaling and explicit receiver parameter
support signaling.
Any codec configuration parameter that is applicable and feasible to
use, but is not included as part of an operation point, has a default
value. This default value is defined for each parameter type, but
should preferably whenever possible be taken from capability
signaling. It is not necessary to use all defined parameter types in
a media stream description. Some parameter types can, depending on
media type or codec, either be unnecessary, or not possible to
describe or control in detail, in which case they can be left out.
This means that the effective value is "undefined" within the limits
set by capability signaling (outside the scope of this
specification).
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6.3. Operation Point
The codec configuration parameters contained in a single message item
jointly constitute a description of an operation point for a specific
media stream from a media sender.
For the purpose of COP signaling, each operation point is identified
with an identity number (OPID), which is scoped by the media sender's
RTP SSRC identification, and can be chosen freely by the media
sender. The need for this media sub-stream identification only
appears with scalable coding or other media encoding methods that
introduce separable and configurable sub-streams within the same
SSRC. An OPID thus refers to such configurable sub-stream, described
by a set of related codec configuration parameters.
+--RTP Session 1 ---------------------+
Media Source 1----+-+-> SSRC1 --> Sub-Stream 1 -> OPID1 |
(MIC, Camera) | \-> Sub-Stream 2 -> OPID2 |
| |
Media Source 2-+--+---> SSRC2 --> Sub-Stream 1 -> OPID3 |
| | \-> Sub-Stream 2 -> OPID4 |
| | \-> Sub-Stream 3 -> OPID5 |
| +-------------------------------------+
|
| +--RTP Session 2 ---------------------+
+--+---> SSRC3 --> Sub-Stream 1 -> OPID6 |
| \-> Sub-Stream 2 -> OPID7 |
+-------------------------------------+
Figure 7: Relation of OPID to media source, RTP session and SSRC
Figure 7 depicts the possible relations between media sources, RTP
sessions, RTP streams (SSRCs), RTP sub-streams, and the OPID.
For example, a single video camera may be encoded using SVC for a
combined SST and MST transmission configuration. In that case a
subset of scalability layers is sent as SST in the first RTP session
using SSRC2. Another set of scalability layers is transported in the
second RTP session as another SST using SSRC3. The RTP packet stream
from each SSRC can thus contain several sub-streams, each identified
with its own OPID. As a result, a single media source is present in
two RTP sessions, using two different SSRCs (2 and 3) containing a
total of five sub-streams (OPID 3 to 7).
Since an operation point is expected to change over time, as a result
of media receiver requests (Section 6.4), resulting from local media
sender considerations (Section 6.5), or both, the operation point
(OPID) is version handled. The version is scoped by SSRC and OPID.
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It is expected that all encoders dividing a media stream into sub-
streams will include some means to identify those sub-streams in the
media stream. However, it is also expected that such identification
is in general codec specific. There is thus a need to map the codec
agnostic COP OPID identification to codec specific identification,
and this specification therefore includes a method for such mapping
(Section 10).
6.4. Request
The request is sent by a media receiver, which can be either an
endpoint or a middle node such as an RTP mixer. The receiver of the
request may similarly be either the original media sender or a RTP
mixer. Included in the request is a description of the desired codec
configuration for a specific media (sub-)stream. The parameter
values communicated in a notification (Section 6.5) of that
(sub-)stream are taken as a starting point when deciding what
parameters and parameter values to choose for the request, and only
parameters with changed values need to be included the request. The
media receiver can of course use other sources of information when
choosing parameters and values, for example observation of the
received media stream and capability signaling.
It is not required to receive a notification beforehand to be able to
create a meaningful request. The request can include a set of
changed properties for existing streams, but it can also request the
addition or removal of one or more media sub-streams having certain
properties, in which case there will be no notification to base the
request on. A media receiver may also want to send a request prior
to having received any notifications for existing streams, and can
then base the request on other information such as for example
observing the media stream or use information from the capability
signaling. In case there is no existing stream and OPID to refer to
in the request, a "provisional" OPID MUST be chosen in the request,
which will have to be mapped back to an existing (sub-)stream and
"real" OPID through methods defined in this specification
(Section 10).
The media sender receiving a specific request is not required to
reconfigure the encoder accordingly, even if it should try to do so.
The media sender is allowed to take other (previous or concurrent)
requests and any local considerations into account, possibly
modifying some of the parameter values, or even to reject the request
completely if it is not seen as feasible. It is thus not possible
for a media receiver to uniquely see from the media stream or even
from a notification if the media sender received the request or if
the request was lost and needs to be resent.
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A request should be based on a notification, but there may be
situations where a request is sent approximately simultaneously with
a new notification for the same stream. In that case, there is a
risk that the request is based on the wrong set of codec properties
compared to the new notification. It is therefore necessary to have
the set of codec properties version controlled, identified by an
OPID. If a notification announces a specific version of the
operation point, where the version is updated every time it is
changed, the request can refer to that specific version and any mis-
reference can be clearly identified and resolved. In addition, it
allows for easy identification of repeated notifications and requests
by checking the operation point identification and the version,
without the need to parse through all codec properties for changes.
6.5. Notification
The notification is sent by a media sender and describes a media
stream or sub-stream in terms of a defined, finite set of codec
properties. That same set of codec properties can also be used in a
request (Section 6.4). The notification and the set of defined
properties is important to be known at the media receiver since it is
rarely possible to see from the media stream itself what controllable
properties were used to generate the stream. The set of codec
properties and their values used to describe a certain media stream
at a certain point in time are henceforth called a codec
configuration. Each operation point in this codec configuration is
implemented using an RTP payload type, defined by capability
signaling outside the scope of this specification.
It must be possible for a media sender to change the codec
configuration not only based on requests from media receivers, but
also based on local limitations, considerations, or user actions.
This implies that the notification can be sent standalone and not
only as a response to a request (compare TMMBR and TMMBN [RFC5104]).
To avoid that media receivers have to guess what codec configuration
is used, a media sender should always send a notification when the
codec configuration for a stream changes. Loss of a notification
messages should not be critical since a media receiver could either
fall back to infer the approximate codec configuration from the media
stream itself, or simply wait with a request until the next
notification is sent.
A notification can potentially contain a large amount of codec
properties. However, parameters that are not enabled by codec and
COP capability signaling, or inherently are not part of the used
codec will not be included. The notification only describes the
currently used codec configuration, and each parameter of an
operation point will be described by a single value. To further
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limit the amount of properties to be sent, it is possible to rely on
parameter defaults (listed by individual parameter type definitions)
whenever those values are acceptable.
The media receiver could want to take local action at the time when
the codec configuration in the media stream changes. Using the same
reasoning as above, this may not be possible to see from the media
stream itself. This functionality is explicitly enabled by including
the RTP time stamp in the notification, where the time stamp
describes a time (possibly in the future) when the codec
configuration is (estimated to be) effective.
It is not required that a media sender sends notifications for all
media streams or sub-streams. However, the non-announced streams or
sub-streams will then not be accessible to media receiver control
(Section 6.4). Any media or transport resources occupied by those
non-announced streams (in COP terms) must be excluded from the total
amount of available resources when deciding feasible parameter value
ranges for the announced streams.
6.6. Status Report
The status report is sent by a media sender and is needed to confirm
reception of a request OPID to avoid unnecessary retransmission of
requests. Loss of a status report will likely trigger a request
retransmission, except when the request sender can infer from the
media stream or a notification that the stream is now acceptable.
The status report is not a required acknowledgement of every request,
but instead reports on the last received request, identified by a
request sequence number in addition to the OPID. This decoupling of
requests and status reports reduces the needed amount of status
reports in case of frequently updated requests and/or lack of
resources to send status reports.
If a request is somehow not acceptable to a media sender, the status
report can also indicate failure and a reason for failure.
In case the OPID in the request is a "provisional" OPID
(Section 6.4), the status report responds with that exact OPID, but
also includes a reference to a "real" media (sub-)stream
identification or OPID that the media sender considers appropriate
for the request.
No description of any codec configuration is included in a status
report, even if the corresponding request was successful. The codec
configuration is only carried in the notification (Section 6.5)
message. Multiple status reports targeted for multiple request
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senders can through media (sub-)stream identification and OPID point
to the same notification message, reducing the need to repeat
applicable codec configuration parameters with every accepted
request.
6.7. Adding and Removing Operation Points
A media sender can unilaterally create a new operation point by
simply selecting a free OPID identifier and use COPN to announce it.
To remove an operation point, the media sender simply stops
announcing it in COPN. This procedure can be used both for entire
media streams containing a single operation point and to add/remove
sub-streams in media streams containing multiple operation points.
The media receiver can request a new operation point to be created by
using a COPR with an unused identifier and by setting a flag to
indicate that this requests a new OPID. The media sender then
decides if it honors the request or not, and announces the new OPID
as described above.
The media receiver can indicate that it is no longer interested in
receiving an operation point corresponding to a media sub-stream by
not including any COPR message item for it in a single COP message.
The media receiver can indicate a wish to continue to receive an
unmodified operation point using a COPR without any codec properties
(no change).
7. Codec Control Message Extension
This specification specifies a new feedback message, COP, for codec
control of real-time media, as an extension to the AVPF [RFC4585] and
CCM [RFC5104] specifications. The AVPF specification outlines a
mechanism for fast feedback messages over RTCP, which is applicable
for IP based real-time media transport and communication services.
It defines both transport layer and payload-specific feedback
messages. This specification targets the payload-specific type,
since a certain codec is typically described by a payload type.
AVPF defines three and CCM defines four payload-specific feedback
messages (PSFB). All AVPF and CCM messages are identified by means
of the feedback message type (FMT) parameter. This specification
specifies one additional payload-specific feedback message.
One new PSFB FMT value is assigned in this specification:
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TBA1: Codec Operation Point (COP)
This section defines the feedback message structure, message items
and their semantics with the exception of the actual codec
configuration parameters which are defined in the next section
(Section 8).
7.1. COP Message
The COP message is a payload-specific AVPF CCM message identified by
the PSFB FMT value listed above. It carries one or more COP message
items, each with either a request for, a description of a certain
"operation point"; a set of codec parameters, or a request status
indication.
Not all message items makes use of the "SSRC of media source" in the
common packet header. "SSRC of media source" SHALL be set to 0 if no
message item that makes use of it is included in the FCI.
7.2. FCI Format
The COP FCI MUST contain one or more codec operation point message
items. The maximum number of COP message items in a COP message is
limited by the [RFC4585] Common Packet Format 'length' field.
The definition of the AVPF feedback message format mandates that the
FCI part is a multiple of 32-bit words. The below defined message
items will not be 32-bit word aligned. Therefore it is sometimes
necessary to insert one to three padding bytes at the end of the FCI.
The number of padding bytes are determined by a receiver by comparing
the sum of the message items and the feedback message length fields.
The padding byte MUST be set to zero (0) and ignored on reception.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|FMT=TBA1 | PT=206 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COP message item header #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COP message item payload #1 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | COP message item header #2 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | COP message item payload #2 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: COP message item payload #N | Padding (0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: COP RTCP Message Structure
7.2.1. Message Item Format
All codec operation point message items share a common header format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type | Payload Length | OPID |N| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (Message Item Payload) :
Figure 9: COP message item header format
The message header fields are:
Type (3 bits): Message item type. Three item types are defined in
this specification, COPR, COPN and COPS, with values as listed in
Table 1 below. More item types MAY be defined in extensions to
this specification. Message items with a type field that has an
unknown value SHALL be ignored by the receiver.
Payload Length (13 bits): The total length in bytes of all data
belonging to this message, following the message item header, i.e.
anything following the Version field.
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OPID (8 bits): Operation point ID. Some (typically scalable) codecs
are capable of encoding into multiple simultaneous operation
points using the same SSRC, and each operation point can then be
referenced by OPID. MUST be unique within the scope of an SSRC
when N flag is not set. MUST be set to 0 for message items not
using the field. See also Section 7.2.3.
N (1 bit): A "New OPID" flag, indicating that the OPID value is
chosen arbitrarily and is not meant to refer to any existing
operation point. The message sender SHOULD NOT use an already
known OPID in combination with the N flag. See also individual
message item definitions.
Version (7 bits): Referencing a specific version of the codec
configuration identified by the OPID.
7.2.2. Message Item Types
The message types defined in this specification are:
+-------+-------------------------------------------+
| Value | Message Item Type |
+-------+-------------------------------------------+
| 0 | Codec Operation Point Notification (COPN) |
| 1 | Codec Operation Point Request (COPR) |
| 2 | Codec Operation Point Status (COPS) |
| 3-6 | Unassigned |
| 7 | Reserved for future extensions |
+-------+-------------------------------------------+
Table 1: Message Item Type Values
Each message type defined in this specification is described in
detail in subsequent sections.
7.2.3. Operation Point Identification
All RTP media streams belonging to the same session can per
definition be identified by the SSRC. However, identification of any
sub-streams contained in the same RTP media stream (SSRC) needs to
use some other identification method, scoped by the SSRC. This is
the case for a media stream containing more than one operation point,
like for example SVC [RFC6190] streams being sent using Single Stream
Transport (SST) RTP packetization.
The encoding of and restrictions for such sub-stream (operation
point) identification will in general be codec specific. Therefore,
the OPID used in this specification is merely an SSRC-unique
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identification number. It is however necessary to create a mapping
between this generic number and the codec specific sub-stream
identification that can be found in the media stream. This mapping
is achieved by including the ID parameter (Section 8.3) in a message
item carrying a certain OPID.
In Section 10, codec specific ID parameter formats are defined for a
few of the most common codecs that supports scalability.
7.3. Codec Operation Point Notification
7.3.1. Message Format
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type | Payload Length | OPID |N| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transition Time Stamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|Payload Type | Codec Configuration Parameters :
+-+-+-+-+-+-+-+-+ :
: :
Figure 10: COPN format
The COPN-specific message fields are (see also message item format
(Section 7.2.1)):
Type (3 bits): Set to 0, as listed in Table 1.
OPID (8 bits): The OPID which is described by the codec
configuration parameters.
N (1 bit): Not used by COPN and SHALL be set to 0 by senders.
Version (7 bits): Referencing a specific version of the codec
configuration identified by the OPID. SHALL be increased by 1
modulo 2^8 whenever the used codec configuration referenced by the
OPID is changed. A repeated message SHALL NOT increase the
Version. The initial value SHOULD be chosen randomly.
Transition Time Stamp (32 bits): The RTP Time Stamp value when the
listed codec configuration parameters will be effective in the
media stream, using the same time line as RTP packets for the
referenced SSRC (media sender SSRC). The Time Stamp value MAY
express either a time in the past or in the future, and need not
map exactly to an actual RTP Time Stamp present in an RTP packet
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for that SSRC. The same timestamp value SHOULD be used for
subsequent transmissions of the identical set of codec
configuration parameters for the same OPID and version.
R (1 bit): Reserved. MUST be set to 0 by senders and MUST be
ignored by receivers implementing this specification. MAY be
defined differently by extensions to this specification.
Payload Type (7 bits): SHALL be identical to the RTP header Payload
Type valid for the (sub-)stream described by this OPID.
Codec Configuration Parameters (variable length): Contains zero or
more TLV carrying codec configuration parameters as defined in
parameter types (Section 8).
7.3.2. Semantics
This message is used to inform the media receiver(s) about used codec
configuration parameters at the media sender. The available codec
parameter types that can be used to describe the codec configuration
are defined in Section 8.
Some codecs may have clear inband indications in the encoded media
stream of how one or more of the codec configuration parameters are
configured. For those codecs and codec configuration parameters,
COPN is not strictly necessary. Still, for some codecs and / or for
some codec configuration parameters, it is not unambiguously possible
to see individual codec configuration parameter values from the
encoded media stream, or even possible to see some codec
configuration parameters at all, motivating use of COPN.
COPN SHOULD be scheduled for transmission when it becomes known that
there are media receivers in the RTP session that did not yet receive
any codec configuration parameters for an active operation point, or
whenever the effective codec configuration parameters has changed
significantly, but MAY be scheduled for transmission at any time.
The media sender decides what amount of change is required to be
considered significant.
The reason for a codec configuration parameter change can either be
local to the sending terminal, for example as a result of user
interaction or some algorithmic decision, or resulting from reception
of one or more COPR messages (Section 7.4).
If a media sender can no longer fulfill the established codec
configuration parameter restrictions of a operation point that was
previously described by a COPN, it MAY change any codec configuration
parameter or even remove the entire operation point, and SHOULD then
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signal this at the earliest opportunity by sending an updated COPN to
the media receiver(s).
An OPID can implicitly be indicated as no longer being used by
omitting that OPID from the set of COPN message items in the COP PSFB
message. All OPIDs that the media sender intends to use at the
latest time indicated by any transition timestamp value in the set of
COPN present in the COP PSFB message, MUST be included in that COP
message.
All operation points referred by a COPS (Section 7.5) SHOULD also be
detailed by a COPN message contained in the same or in a subsequent
COP feedback message, even if the operation point did not change
significantly from previous COPN.
Note that the OPID Version of that COPN, subsequent to COPS, will be
equal or larger than the Version indicated in the COPS. The Version
difference may be larger than one (taking field wraparound into
account) depending on the number of updated COPN sent since the COPR
that triggered the COPS. See also description of those messages
below.
Note: COPN may be seen as a more explicit and elaborate version of
the TSTN message of [RFC5104] and most of the considerations detailed
there for TSTN also apply to COPN.
7.3.2.1. Parameters
The media sender decides what codec configuration parameters to use
in the COPN to describe an operation point. It is RECOMMENDED that
all codec configuration parameters that were accepted as restrictions
based on received COPR messages are included. All codec
configuration parameters significantly more restrictive than implicit
or explicit restrictions set by capability signaling (outside the
scope of this specification) SHOULD also be included. Any codec
configuration parameter that are either not applicable to the Payload
Type or not enabled by capability signaling MUST NOT be included.
All codec configuration parameters not covered by the above
restrictions MAY be included.
When the operation point has dependency to other operation points
(such as in scalable coding), the values to use for codec
configuration parameters MUST describe the result when all
dependencies are utilized. For example, assume an operation point
describing a base layer with 15 Hz framerate, and a dependent
operation point describing an enhancement layer adding another 15 Hz
to the base layer, resulting in 30 Hz framerate when both layers are
combined. The correct parameter value to use for that latter,
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dependent "enhancement" operation point is 30 Hz, not the 15 Hz
difference.
The value of a codec configuration parameter that was not included in
a COPN message SHOULD either be inferred from other signaling, e.g.
session setup or capability negotiation, outside the scope of this
specification, or if such signaling is not available or not
applicable, use the default value as defined per parameter type
(Section 8).
An operation point describes one specific setting of codec
parameters, and a COPN message therefore MUST NOT include the ALT
parameter type (Section 8.2) in the codec parameters describing the
operation point.
7.3.2.2. Relation to COPR
To limit RTCP bandwidth and avoid bandwidth expansion, COPN is not
mandated as response to every received COPR (Section 7.4).
A media sender implementing this specification SHOULD take requested
operation points from COPR messages into account for future encoding,
but MAY decide to use other codec configuration parameter values than
those requested, e.g. as a result of multiple (possibly
contradicting) COPR messages from different media receivers, or any
media sender policies, rules or limitations. Thus, a COPN message
operation point MAY use other codec configuration parameters and
other values than those requested in a COPR.
The media sender SHOULD try to maintain OPIDs between COPR and COPN
when COPR sender suggests a new OPID value (N flag is set) in the
COPR, but MAY use another OPID in COPN. Examples where other OPID
values have to be chosen are for example when the suggested OPID
conflicts with an already existing OPID, or when the media sender
decides that a the suggested new OPID can be fulfilled by an already
existing OPID.
Even if a COPR references an existing OPID (N flag cleared), the
media sender may have to take other aspects than a specific COPR into
account when choosing how many operation points to use, and the exact
contents of those operation points. See the description on COPS
(Section 7.5) on how to achieve mapping between a suggested new OPID
and what OPID will actually be used.
When OPID cannot be kept the same between COPN and COPR, the mapping
SHALL be done using identical ID parameters (Section 8.3) in the COPS
and COPN resulting from the COPR. Further details are described in
the section on COPS (Section 7.5).
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Since COPR references a certain COPN OPID, Version, and COPN is send
unreliably and may be lost, COPN senders MUST keep at least the two
last COPN Versions for each SSRC, OPID tuple and SHOULD keep at least
four.
7.3.3. Timing Rules
The timing follows the rules outlined in section 3 of AVPF [RFC4585].
This notification message may be time critical and SHOULD be sent
using early or immediate feedback RTCP timing, but MAY be sent using
regular RTCP timing.
A typical example when regular RTCP timing can be appropriate is when
the sent media stream is further restricted from what was described
by the most recent COPN, which should not cause any problems in the
media receivers. Similarly, it is likely appropriate to use early or
immediate timing when effective media stream restrictions urgently
needs to be removed, which may require media receivers to increase
their resource usage.
7.4. Codec Operation Point Request
7.4.1. Message Format
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type | Payload Length | OPID |N| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence No | Codec Configuration Parameters :
+-+-+-+-+-+-+-+-+ :
: :
Figure 11: COPR format
The COPR-specific message fields are:
Type (3 bits): Set to 1, as listed in Table 1.
OPID (8 bits): The OPID this request refers to for an existing OPID,
and an arbitrarily chosen but unique value in requests for new
operations points, i.e. with the N flag set.
N (1 bit): MUST be set to 0 when OPID references an existing OPID
announced in a COPN received from the targeted media sender, and
MUST be set to 1 otherwise.
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Version (7 bits): When N flag is not set (0), referencing a specific
version of the codec configuration identified by the OPID in a
COPN received from the targeted media sender. Not used and MUST
be set to 0 when N flag is set (1).
Sequence No (8 bits): Sequence Number. SHALL be incremented by 1
modulo 2^8 for every COPR that includes an updated set of
requested codec configuration parameters described by the same
OPID and Version as was used with the previous Sequence Number.
Sequence Number SHALL be kept unchanged in repetitions of this
message. Initial value SHOULD be chosen randomly.
Codec Configuration Parameters (variable length): Contains zero or
more TLV carrying codec configuration parameters as defined in
parameter types (Section 8).
7.4.2. Semantics
This message item is sent by a media receiver wanting to control one
or more codec configuration parameters of the targeted media sender.
The requested values MUST stay within the media capability negotiated
by other means than this specification. The available codec
configuration parameters that can be controlled are listed in
Section 8.
Note: COPR may be seen as a more explicit and elaborate version of
the TSTR message of [RFC5104] and most of the considerations detailed
there for TSTR also apply to COPR.
7.4.2.1. Sender Behavior
If at least one COPN (Section 7.3) is received for the targeted
stream, the codec configuration parameters for that stream (SSRC)
with defined OPID and Version are known to the COPR sender. The COPR
MUST refer to the OPID and Version of the most recently received COPN
(if any) for the targeted stream. Since it references a defined set
of codec configuration parameters from a COPN, the COPR SHOULD only
include the codec configuration parameters it wishes to change in the
message, but it MAY include also unchanged codec configuration
parameters.
If no COPN is received for the targeted stream, the COPR sender MUST
choose an arbitrary OPID and set the N flag to indicate that the OPID
does not refer to any existing operation point. In this case the
Version field is not used and MUST be set to 0. The OPID value SHALL
NOT be identical to any OPID from the same media source that the
media receiver is aware of and has received COPN for. Since in this
case no COPN reference exist, the COPR sender SHOULD include all
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codec configuration parameters that it wishes to include a specific
restriction for (other than the default). Note that for some codecs,
some codec configuration parameters may be possible to infer from the
media stream, but if the wanted restriction includes also those and
lacking a describing COPN, they SHOULD anyway be included explicitly
in the COPR.
Any codec configuration parameter that are not enabled by capability
signaling MUST NOT be included.
A COPR sender MUST increment the SN field modulo 2^8 with every new
COPR that includes any update to the codec configuration parameters
(referring to a specific version of an OPID compared to the
previously sent SN, as long as it does not receive any COPS
(Section 7.5) with the same OPID, Version, and SN as was used in the
most recently sent COPR. COPR having a later SN MUST be interpreted
as replacing any COPR with identical OPID and Version but with lower
SN, taking field wrap into account.
A COPR sender that did not receive any corresponding COPS, but did
receive a COPN with the same OPID and with a higher Version than was
used in the last COPR SHALL reconsider the COPR and MAY send an
updated COPR referencing the new Version.
If the capability negotiation has established that a codec supporting
scalable operation is used, and if the media receiver wishes to
request that scalability is used, it MAY do so by sending multiple
COPR with different OPID to the same media sender. The OPID and
Version used in such request MAY be based on an existing operation
point, but it MAY also indicate a desire to introduce scalability
into a previously non-scalable stream by choosing a new OPID
(indicated by setting the N flag). In any case, the resulting OPIDs
and sub-streams are identified through use of the ID parameter
(Section 8.3) in subsequent COPS and COPN. See also the description
of COPS (Section 7.5).
An operation point without any codec configuration parameters MAY be
used and MUST be interpreted as a request to keep the operation point
unchanged. This is especially useful when modifying some but not all
in a set of sub-streams.
When a COPR sender is receiving multiple operation points and wants
to continue to do so, it MUST include all operation points it still
wishes to receive in the COPR, also those that can be left unchanged.
An COPR MAY also describe alternative operation points that the media
sender can choose from, through use of one or more ALT parameters
(Section 8.2).
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Since COPR references a specific COPN using SSRC, OPID and Version, a
COPR sender typically needs to keep the latest Version of received
COPN for each SSRC and OPID, also including the codec configuration
parameters.
7.4.2.2. Media Sender Behavior
A media sender receiving a COPR SHOULD take the request into account
for future encoding, but MAY also take COPR from other media
receivers and other information available to the media sender into
account when deciding how to change encoding properties.
A media receiver sending COPR thus cannot always expect that all
parameter values of the request are fully honored, or even honored at
all. It can only know that the COPR was taken into account when
receiving a COPS (Section 7.5) from the media sender with a matching
OPID, Version and SN.
To what extent a COPR is honored is described by the chosen codec
configuration parameter values contained in a subsequent COPN message
(Section 7.3) with a later (taking wraparound into account) Version
than the one referred by the COPR.
7.4.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585].
This request message MAY be sent using Immediate, Early or Regular
timing depending on the application's needs.
A COPR sender that did not receive a corresponding COPS MAY choose to
retransmit the COPR, without increasing the SN.
When an RTP media receiver (SSRC) is timing out or leaves (BYE
received) from the RTP session, it SHALL implicitly imply that all
COPR restrictions put by that media receiver are removed.
7.5. Codec Operation Point Status
7.5.1. Message Format
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type | Payload Length | OPID |N| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of COPR sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence No | RC | Reason |Codec Configuration Parameters :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
: :
Figure 12: COPS format
The COPS-specific message fields are:
Type (3 bits): Set to 2, as listed in Table 1.
OPID (8 bits): MUST be set identical to the same field in the COPR
being reported on.
N (1 bit): MUST be set identical to the same field in the COPR being
reported on.
Version (7 bits): MUST be set identical to the same field in the
COPR being reported on.
SSRC of COPR sender (32 bits): MUST be set identical to the SSRC of
packet sender field in the common AVPF header part of the COPR
being reported on.
Sequence No (8 bits): MUST be set identical to the same field in the
COPR being reported on.
RC (3 bits): Return Code. Indicates degree of success or failure of
the COPR being reported on, as described in Table 2.
Reason (5 bits): Contains more detailed information on the reason
for success or failure, as described in Table 3 or extensions to
this specification.
Codec Configuration Parameters (variable): MAY contain an ID codec
configuration parameter providing codec specific media
identification of the OPID, subject to conditions outlined in the
text below, or MAY be empty.
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7.5.2. Semantics
The COPS message item indicates the request status related to a
certain SSRC OPID tuple by listing the latest received COPR
(Section 7.4) SN. It effectively informs the COPR sender that it no
longer needs to resend that COPR SN (or any previous SN).
COPS indicates that the specified COPR was successfully received by
the media sender targeted in the request. If the COPR suggested
codec configuration parameters could be understood (Table 2), they
may be taken into account, possibly together with COPR messages from
other receivers and other aspects applicable to the specific media
sender. The Return Code carries an indication to which extent the
COPR could be honored.
+-------+-------------------------------+
| Value | Meaning |
+-------+-------------------------------+
| 0 | Success |
| 1 | Partial success |
| 2 | Failure |
| 3-6 | Unassigned |
| 7 | Reserved for future extension |
+-------+-------------------------------+
Table 2: Return Code Values
A Success Return Code indicates that the resulting media
configuration is fully in line with the COPR.
A Partial Success Return Code indicates that the resulting media
configuration is not fully in line with the COPR, but that the media
sender regards the COPR to be sufficiently well represented by one or
more of the existing operation points.
A Failure Return code indicates that the media sender failed to take
the COPR into account, either due to some error condition or because
no media stream could be created or changed to comply.
The Reason Values defined below are independent of Return Code, but
all reasons may not be meaningful with all return codes. More
reasons MAY be defined in extensions to this specification.
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+-------+----------------------------------------------------------+
| Value | Meaning |
+-------+----------------------------------------------------------+
| 0 | Success |
| 1 | Unknown OPID |
| 2 | Too many operation points |
| 3 | Request violates capability limits |
| 4 | Too old operation point version |
| 5 | Unknown parameter type |
| 6 | Parameter value too long |
| 7 | Invalid comparison type |
| 8 | One or more parameter values in the request were changed |
| 9-31 | Unassigned |
+-------+----------------------------------------------------------+
Table 3: Reason Values
COPS is typically sent without any codec configuration parameters.
When the N flag was set in the related COPR, a non-failing COPS MUST
include an ID parameter (Section 8.3) identifying the actual sub-
stream that the media sender considers applicable to the COPR. The
OPID used by that sub-stream can be found through examining ID
parameters of subsequent COPN from the same media source for ID
values matching the one in COPS.
Senders implementing this specification MUST NOT use any other codec
configuration parameter types than ID in a COPS message. The
contained ID parameter points to the specific media (sub-)stream that
the media sender regards as applicable to the COPR.
When a COPR receiver has received multiple COPR messages from a
single COPR source with the same OPID but with several different
values of Version and/or SN, and for which it has not yet sent a
COPS, it SHALL only send COPS for the COPR with the Highest SN,
taking field wrap of those two fields into account.
7.5.3. Timing Rules
COPS SHALL be sent at the earliest opportunity after having received
a COPR, with the following exception:
A media sender that receives a COPR with a previously received
OPID, Version, and SN closely after sending a COPS for that same
OPID, Version, and SN (within 2 times the longest observed round
trip time, plus any AVPF-induced packet sending delays), SHOULD
await a repeated COPR before scheduling another COPS transmission
for that OPID, Version, and SN.
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The exception is introduced to avoid unnecessary COPS transmission
when there is a chance that already sent COPS or COPN may satisfy or
invalidate the COPR.
7.6. Handling in Mixers and Translators
7.6.1. COPN
Any media sender, including mixers and translators, that sends RTP
media marked with it's own SSRC and that implements this
specification SHALL also be prepared to send COPN, even if it is not
the originating media source. As a result of that, such media sender
may have to send updated COPN whenever the included media sources
(CSRC) changes, subject to rules laid out above (Section 7.3.2).
Note that this can be achieved in different ways, for example by
forwarding (possibly cached) COPN from the included CSRC when the
mixer is not performing transcoding.
In cases where a mixer or translator needs to forward a COPR from one
side (A) to the other (B) (as described in Section 7.6.2), the COPN
sent to the A side MAY need to be delayed until the mixer or
translator has received a corresponding COPN from the B side, as
indicated in Figure 13 below.
+-------+ 1. COPR +-------+ 2. COPR +-------+
| |-------->| |-------->| |
| A | 4. COPN | Mixer | 3. COPN | B |
| |<--------| |<--------| |
+-------+ +-------+ +-------+
Figure 13: Mixer delay of COPN
If a mixer or translator has decided to act partially (modify the
media stream with respect to some parameter types, but not all) on a
received COPR from the A side, and a COPN is received from the B side
indicating that the current media modifications are no longer
necessary, the mixer or translator SHOULD cease it's own actions that
are no longer needed. It SHOULD then also issue a COPN describing
the new situation to the A side, as indicated in Figure 14 below.
+-------+ 1. COPR +-------+ +-------+
| |-------->| | 2. COPR | |
| | 3. COPN | |-------->| |
| A |<--------| Mixer | 4. COPN | B |
| | 5. COPN | |<--------| |
| |<--------| | | |
+-------+ +-------+ +-------+
Figure 14: Mixer update of COPN
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7.6.2. COPR
A mixer or media translator that implements this specification and
encodes content sent to the media receiver issuing the COPR SHALL
consider the request to determine if it can fulfill it by changing
its own encoding parameters. A mixer encoding for multiple session
participants will need to consider the joint needs of all
participants when generating a COPR on its own behalf towards the
media sender.
A mixer or translator able to fulfill the COPR partially MAY act on
the parts it can fulfill (and SHALL then send COPS and COPN
accordingly), but SHOULD anyway forward the unaltered COPR towards
the media sender, since it is likely most efficient to make the
necessary codec configuration parameter changes directly at the
original media source.
A media translator that does not act on COP messages will forward
them unaltered, according to normal translator rules.
7.6.3. COPS
A mixer or media translator that implements this specification,
encoding content sent to media receivers and that acts on COPR SHALL
also report using COPS, just like any other media sender. An RTP
translator not knowing or acting on COPR will forward all COP
messages unaltered, according to normal RTP translator rules.
8. Parameter Types
This section defines the general codec configuration parameter (CCP)
TLV format. Then a number of different parameter formats are
defined. It is expected that a number of additional CCPs will be
defined in the future as the needs of different codecs are explored
or developed.
8.1. Parameter Format
COP message items MAY contain one or more codec configuration
parameters, encoded in TLV (Type-Length-Value) format, which SHOULD
then be interpreted as simultaneously applicable to the defined
operation point. Parameter values MUST be byte-aligned.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ParamType | C | Length | |
+---------------+---+-----------+ |
| |
/ Parameter Value /
/ +--------------+
| |
+------------------------------------------------+
Figure 15: Codec parameter format
ParamType (8 bits): The codec configuration parameter type, encoded
as defined in Table 4 and possible extensions to this
specification. A parameter with an unknown ParamType SHALL be
ignored on reception in a COPN and SHALL either be reported as
unknown in COPS or be ignored when received in COPR.
C (2 bits): Comparison Type, encoded as defined in Table 5, unless
specified otherwise by individual ParamType definitions. The
Comparison Type specifies what type of restriction the codec
configuration parameter value expresses and how it should be
compared to other codec configuration parameter values of the same
ParamType.
Exact: The parameter value is an exact value, and no other values
are acceptable. MUST NOT be used together with any other
Comparison Types for the same ParamType.
Minimum: The parameter value is an inclusive minimum restriction.
MAY be used together with Maximum and/or Target Comparison
Types for the same ParamType. If no minimum restriction is
specified, no specific minimum restriction exist.
Maximum: The parameter value is an inclusive maximum restriction.
MAY be used together with Minimum and/or Target Comparison
Types for the same ParamType. If no maximum restriction is
specified, no specific maximum restriction exist.
Target: The parameter value is a preferred target value, but
other values within a specified range are acceptable. This
type MUST be used together with at least one of Minimum and
Maximum Comparison Types for the same ParamType. If no target
is specified, no specific preference exist.
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Length (6 bits): The parameter value Length in bytes, excluding the
ParamType and the Length field itself. A Length of 0 indicates
that the parameter has no value, effectively constituting a wild-
carded parameter that can take on any value (expresses no specific
restriction). This is also the RECOMMENDED way to explicitly
remove a previously effective restriction.
Parameter Value (variable length): The actual parameter value,
encoded in a format defined by the specific ParamType definition.
The meaning of Multiple codec configuration parameters with the same
ParamType and the same Comparison Type included as part of the same
operation point is undefined and SHALL NOT be used.
A codec configuration parameter that is encoded in a way (including
incorrectly) that cannot be interpreted by the receiver SHALL be
ignored.
The below parameters encoded as signed or unsigned integers uses a
variable size representation in the value field. It is RECOMMENDED
to only include the minimal number of bytes necessary to represent
the value that is to be included in the parameter TLV. The length
field in the parameter TLV will explicitly indicate how many bytes
are present in the value field. All parameters using a variable size
representation of their value MUST define the maximum number of bytes
possible to include in the value field.
The ParamType values and the SDP tags (see Section 9) for the codec
configuration parameter types defined in this specification are
listed below.
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+--------+-------------------------------+--------------+
| Value | Meaning | Tag |
+--------+-------------------------------+--------------+
| 0 | ALT | alt |
| 1 | ID | id |
| 2 | Payload Type | pt |
| 3 | Bitrate | bitrate |
| 4 | Token Bucket Size | token-bucket |
| 5 | Framerate | framerate |
| 6 | Horizontal Pixels | hor-size |
| 7 | Vertical Pixels | ver-size |
| 8 | Sample Aspect Ratio | sar |
| 9 | Picture Aspect Ratio | par |
| 10 | Channels | channels |
| 11 | Sampling Rate | sampling |
| 12 | Maximum RTP Packet Size | max-rtp-size |
| 13 | Maximum RTP Packet Rate | max-rtp-rate |
| 14 | Frame Aggregation | aggregate |
| 15-254 | Undefined | |
| 255 | Reserved for future extension | |
+--------+-------------------------------+--------------+
Table 4: Parameter Type Values
The values of the defined parameter value comparison type are listed
below.
+-------+---------+
| Value | Meaning |
+-------+---------+
| 0 | Exact |
| 1 | Minimum |
| 2 | Maximum |
| 3 | Target |
+-------+---------+
Table 5: Comparison Type Values
The following sub-sections describe the syntax and semantics of the
different codec configuration parameter types defined in this
specification.
Unless explicitly specified in the sub-sections below, or in
extensions to this specification, all parameter type values are
binary encoded unsigned integers, most significant byte first (for
multi-byte values).
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8.2. ALT
This codec parameter type is a special parameter, separating the
codec configuration parameters preceding it from the ones that follow
into two separate, alternative operation points.
Type Value: 0
Tag: alt
Unit: Not applicable.
Semantics: A special parameter expressing an "alternative" relation
between the parameters preceding it and the parameters following
it. This SHOULD be interpreted as describing two alternate
operation points where one and only one SHALL be chosen, with the
operation point preceding ALT in the parameter list being
preferred. Multiple ALT parameters MAY be used in the same
parameter list, in which case each set of parameters to evaluate
can be either before the first ALT parameter, between two ALT
parameters, or after the last ALT parameter. Evaluating from the
top of the list and obeying the above preference rule, the first
acceptable set of parameters (not containing any ALT parameter) is
the one to choose.
Encoding: Not applicable.
Media Types: All.
Value Restrictions: MUST be used with the Length field set to 0.
Two ALT parameters MUST be separated by at least one parameter
other than ALT.
Default Value: Not applicable.
Comparison Types: MUST be set to 0.
Note:
8.3. ID
This codec parameter type is a special parameter that enables codec
specific identification of sub-streams, for example when there are
multiple sub-streams in a single SSRC. It can also be used to
reference OPID, when the used codec does not support or use sub-
streams. When used, it SHALL be listed first among the codec
parameters used to describe the sub-stream.
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Type Value: 1
Tag: id
Unit: Not applicable.
Semantics: A special parameter describing the, possibly codec
specific, media identification for the OPID.
Encoding: If used with non-scalable encoding, it MUST contain an
OPID (Section 7.2.1). If used with scalable encoding, this codec
specific encoding MUST be defined by Section 10. It MUST be
defined to occupy an integer number of bytes, where all bits in
the bytes are defined as part of the format.
Media Types: All.
Value Restrictions: If used with non-scalable encoding, any OPID
restrictions apply. If used with scalable encoding, any
restrictions MUST be defined by the definition of the codec
specific sub-stream identification definition (Section 10).
Default Value: Not set.
Comparison Types: MUST be set to 0.
Note: MAY be used whenever there is a need to identify an operation
point in codec native format, or when there is a need to map that
against an OPID.
8.4. Payload Type
Type Value: 2
Tag: pt
Unit: Not applicable.
Semantics: Referencing the RTP Payload Type to use for the OPID.
Encoding: The least significant 7 bits MUST use the same encoding as
the RTP Payload Type field in the RTP header. The most
significant bit MUST be set to 0.
Media Types: All.
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Value Restrictions: The same restrictions valid for RTP Payload Type
apply, i.e. 7-bit values 0-127. MUST be represented by a single
byte in the value field.
Default Value: Not set.
Comparison Types: MUST be set to 0.
Note: MAY be used whenever there is a need to specify codec
configuration parameters valid only for a certain RTP Payload
Type. What media type, codec and possible parameters that are
described by the RTP Payload Type is outside the scope of this
specification, but is typically defined in capability or call
setup signaling, for example SDP.
8.5. Bitrate
Type Value: 3
Tag: bitrate
Unit: Bits per second.
Semantics: Media level per second average media bitrate, excluding
IP/UDP/RTP overhead, but including RTP payload headers (similar to
b=TIAS from SDP signaling [RFC3890]), rounded up to the closest
integer.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: All.
Value Restrictions: A value of 0 MAY be used. The largest value
allowed is what is possible to represent in a 64-bit unsigned
integer value, i.e. a value between 0 and
18,446,744,073,709,551,615.
Default Value: Maximum value computed from capability or call setup
signaling, e.g. b= parameter from SDP. Note that it is often not
possible to achieve more than a rough estimation from such
computation.
Comparison Types: All. The Exact comparison type is meaningful only
for streams that are able to produce a set of predictable (e.g.
constant) packet sizes, sent at predictable (e.g. constant) inter-
packet intervals.
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Note: This parameter used with a maximum comparison type parameter
is significantly similar to CCM Temporary Maximum Media Bit Rate
(TMMBR). When being used with a maximum or exact comparison type
value of 0, it is also significantly similar to PAUSE
[I-D.westerlund-avtext-rtp-stream-pause]. Compared to those, this
parameter conveys significant extra information through the
relation to other parameters applied to the same operation point,
as well as the possibility to express other restrictions than a
maximum limit. When CCM TMMBR is supported in addition to this
specification, the Bitrate parameters from all operation points
within each SSRC should be considered and CCM TMMBR messages
SHOULD be sent for those SSRC that are found to be in the bounding
set (see CCM [RFC5104], section 3.5.4.2). When PAUSE is supported
in addition to this specification, the Bitrate parameters from all
operation points within each SSRC should be considered and CCM
PAUSE messages SHOULD be sent for those SSRC that contain only
operation points that are limited by a Bitrate maximum value of 0.
There only difference between setting the bitrate to 0 and
removing the OPID entirely is that increasing the bitrate from 0
just requires the bitrate parameter to be sent again, while re-
activating a removed OPID requires it to be fully re-defined
including all other parameters that are included in the OPID.
8.6. Token Bucket Size
Type Value: 4
Tag: token-bucket
Unit: Bytes.
Semantics: Media level token bucket [RFC2212] size excluding IP/UDP/
RTP overhead, but including RTP payload headers, describing the
bitrate variability over time as described in
[I-D.westerlund-mmusic-sdp-bw-attribute]. This parameter can be
combined with the parameter bitrate (Section 8.5) (above) to
provide token bucket fill rate plus bucket size for a complete
token bucket model.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: All.
Value Restrictions: A value of 0 is generally not meaningful and
SHOULD NOT be used. Values that can be represented using a 32-bit
unsigned integer, i.e. 0 to 4,294,967,295.
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Default Value: 4096 bytes.
Comparison Types: Maximum, Target.
Note: Changing the token bucket size does not imply changing the
average bitrate, it just changes the acceptable average bitrate
variation over time.
8.7. Framerate
Type Value: 5
Tag: framerate
Unit: 100th of a Hz. This definition allows e.g. distinguishing
between video encoded at 30 Hz (two-byte value 3000) and 29.97 Hz
(two-byte value 2997). It also allows for high speed video
cameras, like 1000 Hz (three-byte value 100000), and slow-scan
down to one frame every 100 seconds (one-byte value 1).
Semantics: The number of media frames to render per second.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: Mainly intended for video and timed image media types,
but MAY be used also for other media types.
Value Restrictions: A value of 0 MAY be used, meaning single-frame,
request based encoding (request procedure is out of scope for this
specification). Values that can be represented using a 32-bit
unsigned integer, i.e. 0 to 42,949,672.95 Hz.
Default Value: Maximum allowed by call setup and/or capability
signaling, e.g. a=framerate parameter from SDP [RFC4566], or
codec-specific configuration.
Comparison Types: All.
Note: A media frame is typically a set of semantically grouped
samples, e.g. the relation that a video image has to its
individual pixels, or the relation that an audio frame has to
individual audio samples. The value applies to encoded media
framerate, not the packet rate (Section 8.15) that may also be
changed as a result of different Frame Aggregation (Section 8.16).
When the COP end-point also makes use of CCM [RFC5104] TSTR/TSTN,
COPN with this parameter MAY be used in combination with TSTN to
explicitly indicate what framerate setting the TSTR resulted in,
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making it possible for the TSTR sender to adjust the used,
relative TSTR scale to more closely match what framerate was
actually received.
8.8. Horizontal Pixels
Type Value: 6
Tag: hor-size
Unit: Pixels.
Semantics: Horizontal image size.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: Video and image.
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used.
Default Value: Maximum allowed by call setup and/or capability
signaling. Values that can be represented using a 32-bit unsigned
integer, i.e. 1 to 4,294,967,295.
Comparison Types: All.
Note: The pixel and picture aspect ratios cannot be changed with
this parameter. Video encoders can typically describe both pixel
and picture aspect ratios as part of the encoded media stream. If
the COP end-point supports imageattr signaling [RFC6236], values
for this parameter SHOULD be chosen only among the negotiated set
in the SDP, and should be done so both for the media receiving
COPR sender and the media sending COPN sender, according to
imageattr values for the affected media stream direction.
8.9. Vertical Pixels
Type Value: 7
Tag: ver-size
Unit: Pixels.
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Semantics: Vertical image size.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: Video and image.
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used. Values that can be represented using a 32-bit
unsigned integer, i.e. 1 to 4,294,967,295.
Default Value: Maximum allowed by call setup and/or capability
signaling.
Comparison Types: All.
Note: See Note in Section 8.8.
8.10. Sample Aspect Ratio
Type Value: 8
Tag: sar
Unit: Unit-less value pair.
Semantics: The ratio between the intended horizontal distance
between the columns and the intended vertical distance between the
rows of the luma sample array in a frame, similar to what is
defined in [H241].
Encoding: Two binary encoded, unsigned 8-bit integers in order
horizontal, vertical.
Media Types: Video and image.
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used as value in either the horizontal or vertical
component. Component values that can be represented using an
8-bit unsigned integer, i.e. 1 to 255.
Default Value: The same as defined in [H241] when there is no
explicit indication, based on image size.
Comparison Types: All.
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Note: If the COP end-point supports imageattr signaling [RFC6236],
values for this parameter SHOULD be chosen only among the
negotiated set in the SDP, and should be done so both for the
media receiving COPR sender and the media sending COPN sender,
according to imageattr values for the affected media stream
direction.
8.11. Picture Aspect Ratio
Type Value: 9
Tag: par
Unit: Unit-less value pair.
Semantics: The ratio between the intended horizontal width and the
intended vertical height of a displayed picture, similar to what
is defined in [H241].
Encoding: Two binary encoded, unsigned 8-bit integers in order
horizontal, vertical.
Media Types: Video and image.
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used as value in either the horizontal or vertical
component. Component values that can be represented using an
8-bit unsigned integer, i.e. 1 to 255.
Default Value: The same as defined in [H241] when there is no
explicit indication, based on image size.
Comparison Types: All.
Note: If the COP end-point supports imageattr signaling [RFC6236],
values for this parameter SHOULD be chosen only among the
negotiated set in the SDP, and should be done so both for the
media receiving COPR sender and the media sending COPN sender,
according to imageattr values for the affected media stream
direction.
8.12. Channels
Type Value: 10
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Tag: channels
Unit: Unit-less.
Semantics: The number of media channels.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: All.
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used. Values that can be represented using a 16-bit
unsigned integer, i.e. 1 to 65,535.
Default Value: Taken from call setup or capability signaling, or 1
if no other value is available.
Comparison Types: All.
Note: This codec configuration parameter SHOULD NOT be used if the
capability negotiation did not establish that suitable multi-
channel coding is supported by both ends. For audio, the
interpretation and spatial mapping SHALL follow the one for the
indicated payload format. If no such channel mapping is defined
in the payload format, and if not specifically signalled by other
means, e.g. SDP, the channel configurations defined in [RFC3551]
SHALL be used. For video, it SHALL be interpreted as the number
of views in multiview coding, where the number 2 SHOULD represent
stereo (3D) coding, unless negotiated otherwise by means outside
of this specification, e.g. SDP. If multiple payload formats are
defined and if those do not share channel configurations, the
Payload Type parameter (Section 8.4) MUST be included as one of
the parameters for the OPID.
8.13. Sampling Rate
Type Value: 11
Tag: sampling
Unit: Hz.
Semantics: Frequency of the media sampling clock in Hz, as input to
the codec, per channel (Section 8.12).
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Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: Mainly intended for audio media, but MAY be used for
other media types.
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used. Values that can be represented using a 32-bit
unsigned integer, i.e. 1 to 4,294,967,295.
Default Value: Taken from call setup or capability signaling, e.g.
RTP TS rate from SDP m-line.
Comparison Types: All.
Note: The value refers to the media sample clock, not the media
Framerate (Section 8.7). It does not specify any codec-internal
up- or down-sampling that may take place as part of the encoding
process. If multiple channels (Section 8.12) are used and
different channels use different sampling rates, then this
parameter MUST NOT be used unless there is a known sampling rate
relationship and an ordering between the channels, in which case
the specified sampling rate value SHALL be taken as applicable to
the first channel of the ordered set. The relationship may e.g.
be known implicitly by each party through some specification, or
be negotiated using other means than this specification.
Typically only a limited subset of sampling frequencies makes
sense to the media encoder, and sometimes it is not possible to
change at all. For video, the sampling rate is very closely
connected to the image horizontal (Section 8.8), vertical
(Section 8.9) resolution, and framerate (Section 8.7), which are
more explicit and meaningful and SHOULD therefore be used instead.
For audio, changing sampling rate may require changing codec and
thus changing RTP payload type. The actual media sampling rate
may not be identical to the sampling rate specified for RTP Time
Stamps for that RTP Payload Type. E.g. almost all video codecs
use only 90 000 Hz sampling clock for RTP Time Stamps, while the
actual pixel sampling clock is typically in the range from a few
to several hundred MHz. Also some recent audio codecs use an RTP
Time Stamp rate that differ from the actual media sampling rate.
Aspects related to mid-stream changes of RTP Time Stamp rate is
described in [I-D.ietf-avtext-multiple-clock-rates].
8.14. Maximum RTP Packet Size
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Type Value: 12
Tag: max-rtp-size
Unit: Bytes.
Semantics: The maximum size of an RTP packet, including the RTP
header but excluding lower layers.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: All.
Value Restrictions: The meaning of a value less than the size of the
RTP header (12 bytes for current RTP specification [RFC3550]) is
not defined and SHOULD NOT be used. Values that can be
represented using a 32-bit unsigned integer, i.e. 0 to
4,294,967,295.
Default Value: 1400 bytes for IPv4, 1280 bytes for IPv6 or if IP
version cannot be determined.
Comparison Types: Maximum.
Note: The parameter should typically be used to adapt encoding to a
known or assumed MTU limitation, and MAY be used to assist MTU
path discovery in point-to-point as well as in RTP mixer or
translator topologies.
8.15. Maximum RTP Packet Rate
Type Value: 13
Tag: max-rtp-rate
Unit: RTP packets per second.
Semantics: Maximum number of RTP packets per second, calculated or
estimated as the largest value appearing during a one-second
sliding window, similar to the definition of "maxprate" [RFC3890].
Encoding: Binary encoded unsigned integer, most significant byte
first.
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Media Types: All.
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used. Values that can be represented using a 32-bit
unsigned integer, i.e. 1 to 4,294,967,295.
Default Value: Not set.
Comparison Types: Maximum.
Note: The parameter should typically be used to adapt encoding on a
network that is packet rate rather than bitrate limited, if such
property is known. This codec configuration parameter MUST NOT
exceed any negotiated "maxprate" [RFC3890] value, if present.
8.16. Application Data Unit Aggregation
Type Value: 14
Tag: aggregate
Unit: Milliseconds.
Semantics: The amount of non-redundant application data unit (ADU)
representing different RTP Time Stamps that should be included in
the RTP payload, henceforth in this specification called an "ADU
aggregate". An ADU aggregation value of 1 is equivalent to no
aggregation.
Encoding: Binary encoded unsigned integer, most significant byte
first.
Media Types: Mainly intended for audio, but MAY be used also for
other media, e.g. Real-Time Text [RFC4103].
Value Restrictions: The meaning of the value 0 is not defined and
SHALL NOT be used. Values that can be represented using a 16-bit
unsigned integer, i.e. 1 to 65,535.
Value Default Value: 1.
Comparison Types: All.
Note: To use this parameter, there MUST exist a defined way of
including multiple ADUs into the same RTP payload for the used RTP
Payload Type. There MUST also exist a known internal timing
relationship between individual ADUs within the RTP payload for
the used RTP Payload Type. Some payload formats (typically video)
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do not allow multiple ADUs (representing different sampling times)
in the RTP payload. This codec configuration parameter SHOULD NOT
be used unless the "maxprate" [RFC3890] and/or "ptime" parameters
are included in the SDP. The requested ADU aggregation level MUST
NOT cause exceeding the negotiated "maxprate" value, if present,
and SHOULD NOT exceed the negotiated "ptime" value, if present.
The requested frame aggregation level MUST NOT be in conflict with
any Maximum RTP Packet Size (Section 8.14) or Maximum RTP Packet
Rate (Section 8.15) parameters. The packet rate that may result
from different frame aggregation values is related to, but
semantically not the same as, media Framerate (Section 8.7).
9. SDP Extensions
As described in [RFC4585] and [RFC5104], the rtcp-fb attribute may be
used to negotiate capability to handle specific AVPF commands and
indications, and specifically the "ccm" feedback value is used for
codec control. All rules defined there related to use of "rtcp-fb"
and "ccm" also apply to the new feedback message defined in this
specification.
9.1. Extension of the rtcp-fb Attribute
In this document, a new "ccm" rtcp-fb-ccm-param is defined, according
to the method of extension described in [RFC5104]:
o "cop" indicates support for all COP message items defined in this
specification, and one or more of the codec configuration
parameters defined in this specification
The ABNF [RFC5234] for the new rtcp-fb-ccm-param is:
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rtcp-fb-ccm-param =/ SP "cop" 1*rtcp-fb-ccm-cop-param
; rtcp-fb-ccm-param defined in [RFC5104]
rtcp-fb-ccm-cop-param = SP "alt"
/ SP "id"
/ SP "pt"
/ SP "bitrate"
/ SP "token-bucket"
/ SP "framerate"
/ SP "hor-size"
/ SP "ver-size"
/ SP "sar"
/ SP "par"
/ SP "channels"
/ SP "sampling"
/ SP "max-rtp-size"
/ SP "max-rtp-rate"
/ SP "aggregate"
/ SP token ; for future extensions
; token defined in [RFC4566]
Figure 16: ABNF for cop
Token values for rtcp-fb-ccm-cop-param are defined in Table 4. Their
semantics are described in Section 8.
Supported parameter types are indicated by including one or more
rtcp-fb-ccm-cop-param.
9.2. Offer/Answer Usage
The usage of Offer/Answer [RFC3264] in this specification inherits
all applicable usage defined in [RFC5104].
In order to announce support, and willingness to use, the CCM "cop"
feedback message, an offerer or answerer SHALL indicate that
capability through the extended SDP rtcp-fb attribute, defined in
Section 9.1. The offerer or answerer MUST include a list of the
parameter types that it is willing to receive.
If an SDP offer does not indicate support of the CCM "cop" feedback
message, the answerer MUST NOT indicate support in the associated SDP
answer.
The answerer MAY add and/or remove parameter types that were not
present in the associated SDP offer. If the answerer adds parameter
types to the SDP answer, it MUST be able to receive such messages,
but the answerer MUST NOT send such messages towards the offerer.
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If an SDP answer does not indicate support of the CCM "cop" feedback
message, the offerer MUST NOT send such messages towards the
answerer.
The offerer and the answerer SHOULD NOT send any parameter types that
the remote party did not indicate receive support for. As described
in Section 8, a parameter with an unknown ParamType SHALL be ignored
on reception in a COPN and SHALL either be reported as unknown in
COPS or be ignored when received in COPR.
Entities MUST list all supported parameter types in every subsequent
SDP offer or answer associated with the session. If a parameter type
is not listed, it is an indication that the offerer or answerer is no
longer willing to receive such messages within the session.
9.3. Declarative Usage
Declarative use of the CCM "cop" does not differ from the Offer/
Answer usage.
10. Codec Sub-Stream Identification
The defined mechanism is not bound to a specific codec. It uses the
main characteristics of a chosen set of media types, including audio
and video. To what extent this mechanism can be applied depends on
which specific codec is used.
When using a codec that can produce separate sub-streams within a
single SSRC, those sub-streams can only be referred with a COP OPID
if there is a defined relation to the codec-specific sub-stream
identification. This is accomplished in this specification by
defining an ID parameter format using codec-specific sub-stream
identification for each such codec.
If such sub-streams have dependencies, the OPID describes the
characteristics of the sub-stream including all it's dependencies,
but excluding any sub-streams that are dependent on this sub-stream.
The sub-stream identification describes a single, payload specific
node in a dependency tree, and does in general not include any
identification of the sub-streams it depends on, or the dependency
structure between sub-streams. Any dependency structure must thus be
described by the media stream payload format and is out of scope for
this specification.
This section contains ID parameter format definitions for a few
selected codecs. The format definitions MUST use an integer number
of bytes and MUST define all bits in those bytes. Note, the ID
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parameter is interpreted in the context of a given SSRC and a
specific RTP payload type.
Extensions to this specification MAY add more codec-specific
definitions than the ones described in the sub-sections below. Such
definitions made in extensions to this specification SHOULD be
considered as an integrated part of this section, with respect to
usage with other mechanisms defined in this specification.
10.1. H.264 AVC
Some non-scalable video codecs such as H.264 AVC [H264] and
corresponding RTP payload format [RFC6184] can accomplish
simultaneous encoding of multiple operation points. H.264 AVC can
encode a video stream using limited-reference and non-reference
frames such that it enables limited temporal scalability, by use of
the nal_ref_id syntax element.
The ID parameter type is defined below:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved | N |
+-+-+-+-+-+-+-+-+
Figure 17: ID definition for AVC
Reserved (6 bits): Reserved. SHALL be set to 0 by senders and SHALL
be ignored by receivers implementing this specification. MAY be
defined differently by extensions to this specification.
N (2 bits): SHALL be identical to the highest value of the
nal_ref_idc H.264 NAL header syntax element valid for the sub-
bitstream described by this OPID, with the exception of
nal_ref_idc value 3 that is valid for and is part of all sub-
bitstreams.
10.2. H.264 SVC
This document specifies the usage of multiple, simultaneous codec
operation points and therefore maps well to scalable video coding.
Scalable video coding such as H.264 SVC (Annex G) [H264] uses three
scalability dimensions: temporal, spatial, and quality. It also
includes the possibility to use redundant encodings and priority
among sub-streams.
The ID SHALL be considered describing an SVC sub-bitstream, which is
defined in G.3.59 of H.264 [H264] and corresponding RTP payload
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format [RFC6190]. For use with H.264 SVC, ID SHALL be constructed as
defined below:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| PID | RPC | DID | QID | TID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: ID definition for SVC
R (1 bit): Reserved. SHALL be set to 0 by senders and SHALL be
ignored by receivers implementing this specification. MAY be
defined differently by extensions to this specification.
PID (6 bits): SHALL be identical to an unsigned binary integer
representation of the priority_id H.264 syntax element valid for
the sub-bitstream described by this OPID. SHALL be set to 0 if no
priority_id is available.
RPC (7 bits): SHALL be identical to an unsigned binary integer
representation of the redundant_pic_cnt H.264 syntax element valid
for the sub-bitstream described by this OPID. SHALL be set to 0
if no redundant_pic_cnt is available.
DID (3 bits): SHALL be identical to the dependency_id H.264 syntax
element valid for the sub-bitstream described by this OPID.
QID (4 bits): SHALL be identical to the quality_id H.264 syntax
element valid for the sub-bitstream described by this OPID.
TID (3 bits): SHALL be identical to the temporal_id H.264 syntax
element valid for the sub-bitstream described by this OPID
11. Examples
COP messages are binary encoded. However, in the following examples,
all COP messages are for clarity listed in symbolic, pseudo-code
form, where only COP message fields of interest to the example are
included, along with the COP parameters.
11.1. SDP Offer/Answer
The SDP capabilities for COP are defined as receiver capabilities,
meaning that there is no explicit indication what COP messages an
endpoint will use in the send direction. It is however reasonable to
expect that an endpoint can also send the same messages that it can
understand and act on when received. This is assumed in all the SDP
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examples below, but note that symmetric COP capabilities is not a
requirement.
The example below shows an SDP Offer, where support of CCM "cop"
message is announced for the video codecs.
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example
s=-
c=IN IP4 host.atlanta.example
t=0 0
m=audio 50000 RTP/AVP 0 8 97
b=AS:80
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 50010 RTP/AVPF 31 32
b=AS:600
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=rtcp-fb:31 ccm cop framerate bitrate token-rate
a=rtcp-fb:32 ccm cop hor-size ver-size framerate bitrate \
token-rate
Figure 19: SDP offer (COP support indicated)
Note that the offer contains two different video payload types, and
that the COP parameters differ between them, meaning that the
possibility for codec configuration also differ. In this case, the
MPEG-1 codec can control both framerate and image size, but for H.261
only the framerate can be controlled.
In the SDP Answer below, responding to the above offer, the answerer
supports CCM "cop" messages.
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example
s=-
c=IN IP4 host.biloxi.example
t=0 0
m=audio 52000 RTP/AVP 0
b=AS:80
a=rtpmap:0 PCMU/8000
m=video 52100 RTP/AVPF 32
b=AS:600
a=rtpmap:32 MPV/90000
a=rtcp-fb:32 ccm cop hor-size ver-size framerate bitrate \
token-rate packet-size
Figure 20: SDP answer (COP support indicated)
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Note that the answerer indicates support for more parameter types
than the offerer.
Below is another SDP Answer, also responding to the same offer above,
where the answerer does not support "cop".
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example
s=-
c=IN IP4 host.biloxi.example
t=0 0
m=audio 52000 RTP/AVP 0
b=AS:80
a=rtpmap:0 PCMU/8000
m=video 52100 RTP/AVPF 32
b=AS:600
a=rtpmap:32 MPV/90000
Figure 21: SDP answer (COP support not indicated)
11.2. Dynamic Video Re-sizing
In this example, two COP-enabled endpoints communicate in an audio/
video session. The receiving endpoint has a graphical user interface
that can be dynamically changed by the user. This user interaction
includes the ability to change the size of the receiving video
window, which is also indicated in the previous SDP example
(Section 11.1).
At some point during the established communication, a notification
about current video stream codec operation point is sent to the
resizable window endpoint that receives the video stream.
COPN {SSRC:123456, OPID:123, Version:5,
bitrate(max):325000,
token-bucket(exact):1000,
framerate(exact):15,
hor-size(exact):320,
ver-size(exact):240}
Figure 22: COPN for QVGA 15 Hz
Some time later the user of the resizable window endpoint reduces the
size of the video window. As a result of the resize operation, the
video window can no longer make full use of the received video
resolution, wasting bandwidth and decoder processing resources. The
resizable window endpoint thus decides to notify the video stream
sender about the changed conditions by sending a request for a video
stream of smaller size:
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COPR {SSRC:123456, OPID:123, Version:5,
hor-size(target):243,
ver-size(target):185}
Figure 23: COPR for 243x185
The COPR refers to the previously received COPN with the same OPID
and Version, and thus need only list parameters that need be changed.
The request could arguably contain also other parameters that are
potentially affected by the spatial resolution, such as the bitrate,
but that can be omitted since the media sender is not slaved to the
request but is allowed to make it's own decisions based on the
request.
The request sender has chosen to use target type values instead of an
exact value for the horizontal and vertical sizes, which can be
interpreted as "anything sufficiently similar is acceptable". The
target values is in this example chosen to correspond exactly to the
resized video display area. Many video coding algorithms operate
most efficiently when the image size is some even multiple, and this
way of expressing the request explicitly leaves room for the media
sender to take such aspect into account.
The media sender (COPR receiver) responds with the following:
COPS {SSRC:123456, OPID:123, Version:5,
Partial Success,
One or more parameter values in the request were changed}
COPN {SSRC:123456, OPID:123, Version:6,
bitrate(max):240000,
token-bucket(exact):1000,
framerate(exact):15,
hor-size(exact):240,
ver-size(exact):176}
Figure 24: COPS and COPN for partial success
It can be noted that the updated COPN (version 6) indicates that the
media sender has, in addition to reducing the video horizontal and
vertical size, chosen to also reduce the bitrate. This bitrate
reduction was not in the request, but is a reasonable decision taken
by the media sender. It can also be seen that the horizontal and
vertical sizes are not chosen identical to the request, but is in
fact adjusted to be even multiples of 16, which is a local
restriction of the fictitious video encoder in this example. To
handle the mismatch of the request and the resulting video stream,
the video receiver can perform some local action such as for example
automatic readjustment of the resized window, image scaling (possibly
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combined with cropping), or padding.
11.3. Illegal Request
In this example, the sent request is asking the media sender to go
beyond what is negotiated in the SDP. The SDP Offer below indicates
to use video with H.264 Constrained Baseline Profile at level 1.1.
v=0
o=alice 2893746526 2893746526 IN IP4 host.atlanta.example
s=-
c=IN IP4 host.atlanta.example
t=0 0
m=audio 49160 RTP/AVP 96
b=AS:80
a=rtpmap:96 G722/16000
m=video 51920 RTP/AVPF 97
b=AS:200
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=42e00b
a=rtcp-fb:97 ccm cop framerate bitrate token-rate
Figure 25: SDP offer with H.264 level 1.1
Assuming this offer is accepted and that the answerer also supports
COP, further assume that this COP message exchange occurs at some
time during the established communication:
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Media Sender Media Receiver
------------ --------------
COPN {SSRC:9876, OPID:67, ->
Version:2,
bitrate(exact):190000,
token-bucket(exact):500,
framerate(exact):10,
hor-size(exact):320,
ver-size(exact):240}
<- COPR {SSRC:9876, OPID:67,
Version:2,
framerate(exact):10,
hor-size(exact):352,
ver-size(exact):288}
COPS {SSRC:9876, OPID:67, ->
Version:2,
Failure,
Request violates capability limits}
Figure 26: COP message exchange indicating failure
The failure above is due to a combination of frame size and frame
rate that exceeds H.264 level 1.1, which would thus exceed the limits
established by SDP Offer/Answer. The maximum permitted framerate for
352x288 pixels (CIF) is 7.6 Hz for H.264 level 1.1, as defined in
Annex A of [H264].
11.4. Reference Response to Modification of Scalable Layer
When scalable coding is used, each layer correspond to a codec
operation point. A media receiver can thus target a request towards
a single layer. Assume a video encoding with three framerate layers,
announced in a (multiple operation point) notification as:
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COPN {SSRC:9876, OPID:67, Version:2, ID:2
bitrate(exact):190000,
token-bucket(exact):500,
framerate(exact):10,
hor-size(exact):320,
ver-size(exact):240}
COPN {SSRC:9876, OPID:73, Version:1,
bitrate(exact):350000, ID:1
token-bucket(exact):600,
framerate(exact):30,
hor-size(exact):320,
ver-size(exact):240}
COPN {SSRC:9876, OPID:95, Version:5, ID:0
bitrate(exact):400000,
token-bucket(exact):800,
framerate(exact):60,
hor-size(exact):320,
ver-size(exact):240}
Figure 27: COPN indicating three framerate layers
Assume further that the media receiver is not pleased with the low
framerate of OPID 67, wanting to increase it from 10 Hz to 25-30 Hz.
Note that the media receiver still wants to receive the other layers
unchanged, not remove them, and thus has to explicitly indicate this
by including them without parameters.
COPR {SSRC:9876, OPID:67, Version:2,
framerate(greater):25,
framerate(less):30}
COPR {SSRC:9876, OPID:73, Version:1}
COPR {SSRC:9876, OPID:95, Version:5}
Figure 28: COPR requesting to change one layer
The media sender decides it cannot meet the request for OPID 67, but
instead considers (an unmodified) OPID 73 (with ID 1) to be a
sufficiently good match:
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COPS {SSRC:9876, OPID:67, Version:2,
Partial Success,
One or more parameter values in the request were changed,
ID:1}
(COPN for the other two OPIDs omitted here for brevity)
COPN {OSSRC:9876, OPID:73, Version:1, ID:1
bitrate(exact):350000,
token-bucket(exact):600,
framerate(exact):30,
hor-size(exact):320,
ver-size(exact):240}
Figure 29: COPS and COPN with layer modification partial success
The COPS indicates partial success and uses the ID number to refer
another OPID, describing the best compromise that can currently be
used to meet the request. COPS does not contain the referred OPID,
but ID should be defined in a codec-specific way that makes it
possible to identify the layer directly in the media stream. If the
corresponding OPID is needed, for example to attempt another request
targeting that, it can be found by searching the active set of COPN
for matching ID values.
11.5. Successful Request to Add Codec Operation Point
In this example, the media receiver is receiving a non-scalable
stream from a codec that can support scalability, and wishes to add a
scalability layer. Assume the existing OPID from the media sender is
announced as:
COPN {SSRC:3492, OPID:4, Version:2,
bitrate(exact):350000,
token-bucket(exact):600,
framerate(exact):30,
hor-size(exact):320,
ver-size(exact):240}
Figure 30: COPN with single operation point
The media receiver constructs a request for multiple streams by
including multiple requests for different OPID. Since the new stream
does not exist, it has no OPID from the media sender and the receiver
chooses a random value as reference and indicates that it is a new,
temporary OPID. The request for the new stream includes all
parameters that the media receiver has an opinion on, and leaves the
other parameters to be chosen by the media sender. In this case it
is a request for identical frame size and doubled framerate.
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COPR {SSRC:3492, OPID:4, Version:2}
COPR {SSRC:3492, OPID:237, New, Version:0,
framerate(exact):60,
hor-size(exact):320,
ver-size(exact):240}
Figure 31: COPR requesting to add operation point
The media sender decides it can start layered encoding with the
requested parameters. The status response to the new OPID contains a
reference to an ID that is included as part of the matching,
subsequent COPN. Note that since both the original and the new
streams are now part of a scalable set, they must both be identified
with ID parameters to be able to distinguish between them. The media
sender has chosen an OPID for the new stream in the COPN, which need
not be identical to the temporary one in the request, but the new
stream can anyway be uniquely identified through the ID that is
announced in both the COPS and COPN.
Note that since the ID has a defined relation to the media sub-stream
identification, decoding of that new sub-stream can start immediately
after receiving the COPS. It may however not be possible to describe
the new stream in COP parameter terms until the COPN is received
(depending on COP parameter visibility directly in the media stream).
COPS {SSRC:3492, OPID:4, Version:2,
Success, Success,
ID:1}
COPS {SSRC:3492, OPID:237, New, Version:0,
Success, Success,
ID:0}
COPN {SSRC:3492, OPID:4, Version:2, ID:1,
bitrate(exact):350000,
token-bucket(exact):600,
framerate(exact):30,
hor-size(exact):320,
ver-size(exact):240}
COPN {SSRC:3492, OPID:9, Version:0, ID:0,
bitrate(exact):390000,
token-bucket(exact):600,
framerate(exact):60,
hor-size(exact):320,
ver-size(exact):240}
Figure 32: COPS and COPN indicating operation point added
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12. IANA Considerations
Following the guidelines in [RFC4566], in [RFC4585], and in
[RFC3550], the IANA is requested to register:
1. The 'cop' tag to be used with ccm under rtcp-fb AVPF attribute in
SDP.
2. The FMT number TBA1 to be allocated to the COP feedback message
from this specification.
3. A registry listing registered values for 'cop' message item type,
with initial values from Table 1.
4. A registry listing registered values and tag names for 'cop'
parameter type, with initial values from Table 4.
13. Security Considerations
This document extends the CCM [RFC5104] and defines new messages,
i.e. COPR, COPN and COPS. 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 control or indicate the codec
parameters of any source transmission. In order to prevent this
type of attack, a strong authentication and integrity protection
mechanism is needed.
2. Denial of Service (DoS) - An attacker can falsely set codec
parameters for all the source streams which MAY result in Denial
of Service (DoS). An Authentication protocol MAY save from this
attack.
3. Man-in-Middle Attack (MiMT) - The codec configuration and
notification of changes of the RTP source is prone to a Man-in-
Middle attack. The public key authentication May be used to
prevent MiMT.
14. Open Issues
There is currently no defined way for a media receiver to indicate
that it wants to release the restrictions it previously had on an
operation point, if the media stream contains only a single operation
point.
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15. Acknowledgements
The authors would like to thank Prof. Dr.-Ing. Markus Kampmann at
Fachhochschule Koblenz University of Applied Sciences and Prof. Dr.-
Ing. Frank Hartung at Multimediatechnik, Audio- und Videotechnik at
Fachhochschule Aachen for fruitful contributions and discussions
during the initial stages of writing this specification. The authors
would also like to thank Christer Holmberg for feedback on the
specification.
16. References
16.1. Normative References
[H241] ITU-T Recommendation H.241, "Extended video procedures and
control signals for H.300 series terminals", May 2006.
[H264] ITU-T Recommendation H.264, "Advanced video coding for
generic audiovisual services", March 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3890] Westerlund, M., "A Transport Independent Bandwidth
Modifier for the Session Description Protocol (SDP)",
RFC 3890, September 2004.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[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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[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184, May 2011.
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
May 2011.
[RFC6236] Johansson, I. and K. Jung, "Negotiation of Generic Image
Attributes in the Session Description Protocol (SDP)",
RFC 6236, May 2011.
16.2. Informative References
[I-D.ietf-avtext-multiple-clock-rates]
Petit-Huguenin, M., "Support for multiple clock rates in
an RTP session", draft-ietf-avtext-multiple-clock-rates-02
(work in progress), January 2012.
[I-D.westerlund-avtext-rtp-stream-pause]
Akram, A., Burman, B., Grondal, D., and M. Westerlund,
"RTP Media Stream Pause and Resume",
draft-westerlund-avtext-rtp-stream-pause-02 (work in
progress), July 2012.
[I-D.westerlund-mmusic-sdp-bw-attribute]
Frankkila, T., Westerlund, M., and B. Burman, "Extensible
Bandwidth Attribute for SDP",
draft-westerlund-mmusic-sdp-bw-attribute-00 (work in
progress), October 2011.
[RFC2212] Shenker, S., Partridge, C., and R. Guerin, "Specification
of Guaranteed Quality of Service", RFC 2212,
September 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611,
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November 2003.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[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.
[RFC5968] Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP)", RFC 5968, September 2010.
Authors' Addresses
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
Bo Burman
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 13 11
Email: bo.burman@ericsson.com
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Laurits Hamm
Ericsson
Ericsson Allee 1
DE-52134 Herzogenrath
Germany
Phone: +49 2407 575 6779
Email: laurits.hamm@ericsson.com
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