draft-ietf-avtcore-multi-media-rtp-session-07.txt   draft-ietf-avtcore-multi-media-rtp-session-08.txt 
AVTCORE WG M. Westerlund AVTCORE WG M. Westerlund
Internet-Draft Ericsson Internet-Draft Ericsson
Updates: 3550, 3551 (if approved) C. Perkins Updates: 3550, 3551 (if approved) C. Perkins
Intended status: Standards Track University of Glasgow Intended status: Standards Track University of Glasgow
Expires: September 10, 2015 J. Lennox Expires: January 08, 2016 J. Lennox
Vidyo Vidyo
March 9, 2015 July 07, 2015
Sending Multiple Types of Media in a Single RTP Session Sending Multiple Types of Media in a Single RTP Session
draft-ietf-avtcore-multi-media-rtp-session-07 draft-ietf-avtcore-multi-media-rtp-session-08
Abstract Abstract
This document specifies how an RTP session can contain RTP Streams This document specifies how an RTP session can contain RTP Streams
with media from multiple media types such as audio, video, and text. with media from multiple media types such as audio, video, and text.
This has been restricted by the RTP Specification, and thus this This has been restricted by the RTP Specification, and thus this
document updates RFC 3550 and RFC 3551 to enable this behaviour for document updates RFC 3550 and RFC 3551 to enable this behaviour for
applications that satisfy the applicability for using multiple media applications that satisfy the applicability for using multiple media
types in a single RTP session. types in a single RTP session.
skipping to change at page 1, line 38 skipping to change at page 1, line 38
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Background and Motivation . . . . . . . . . . . . . . . . . . 3
4. Overview of Solution . . . . . . . . . . . . . . . . . . . . 5 4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Using Multiple Media Types in a Single RTP Session . . . . . 7
5.1. Usage of the RTP session . . . . . . . . . . . . . . . . 6 5.1. Allowing Multiple Media Types in an RTP Session . . . . . 7
5.2. Signalled Support . . . . . . . . . . . . . . . . . . . . 6 5.2. Demultiplexing Media Streams . . . . . . . . . . . . . . 8
5.3. Homogeneous Multi-party . . . . . . . . . . . . . . . . . 7 5.3. Per-SSRC Media Type Restrictions . . . . . . . . . . . . 8
5.4. Reduced number of Payload Types . . . . . . . . . . . . . 8 5.4. RTCP Considerations . . . . . . . . . . . . . . . . . . . 9
5.5. Stream Differentiation . . . . . . . . . . . . . . . . . 8 6. Extension Considerations . . . . . . . . . . . . . . . . . . 9
5.6. Non-compatible Extensions . . . . . . . . . . . . . . . . 8 6.1. RTP Retransmission Payload Format . . . . . . . . . . . . 9
6. RTP Session Specification . . . . . . . . . . . . . . . . . . 9 6.2. RTP Payload Format for Generic FEC . . . . . . . . . . . 11
6.1. RTP Session . . . . . . . . . . . . . . . . . . . . . . . 9 6.3. RTP Payload Format for Redundant Audio . . . . . . . . . 11
6.2. Sender Source Restrictions . . . . . . . . . . . . . . . 11 7. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Payload Type Applicability . . . . . . . . . . . . . . . 12 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6.4. RTCP Considerations . . . . . . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Extension Considerations . . . . . . . . . . . . . . . . . . 12 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
7.1. RTP Retransmission . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2. Generic FEC . . . . . . . . . . . . . . . . . . . . . . . 14 11.1. Normative References . . . . . . . . . . . . . . . . . . 13
8. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 15 11.2. Informative References . . . . . . . . . . . . . . . . . 14
8.1. SDP-Based Signalling . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
When the Real-time Transport Protocol (RTP) [RFC3550] was designed, The Real-time Transport Protocol [RFC3550] was designed to use
close to 20 years ago, IP networks were different to those deployed separate RTP sessions to transport different types of media. This
at the time of this writing. The virtually ubiquitous deployment of implies that different transport layer flows are used for different
Network Address Translators (NAT) and Firewalls has since increased media streams. For example, a video conferencing application might
the cost and likely-hood of communication failure when using many send audio and video traffic RTP flows on separate UDP ports. With
different transport flows. Hence, there is pressure to reduce the increased use of network address/port translation, firewalls, and
number of concurrent transport flows used by RTP applications. other middleboxes it is, however, becoming difficult to establish
multiple transport layer flows between endpoints. Hence, there is
The RTP specification recommends against sending several different pressure to reduce the number of concurrent transport flows used by
types of media, for example audio and video, in a single RTP session. RTP applications.
The RTP profile for Audio and Video Conferences with Minimal Control This memo updates [RFC3550] and [RFC3551] to allow multiple media
(RTP/AVP) [RFC3551] mandates a similar restriction. The motivation types to be sent in a single RTP session in certain cases, thereby
for these limitations is partly to allow lower layer Quality of reducing the number of transport layer flows that are needed. It
Service (QoS) mechanisms to be used, and partly due to limitations of makes no changes to RTP behaviour when using multiple RTP streams
the RTCP timing rules that assumes all media in a session to have containing media of the same type (e.g., multiple audio streams or
similar bandwidth. The Session Description Protocol (SDP) [RFC4566] multiple video streams) in a single RTP session, however
is one of the dominant signalling methods for establishing RTP [I-D.ietf-avtcore-rtp-multi-stream] provides important clarifications
sessions, and has enforced this rule by not allowing multiple media to RTP behaviour in that case.
types for a given destination or set of ICE candidates.
The fact that these limitations have been in place for so long, in This memo is structured as follows. Section 2 defines terminology.
addition to RFC 3550 being written without fully considering the use Section 3 further describes the background to, and motivation for,
of multiple media types in an RTP session, results in a number of this memo and Section 4 describes the scenarios where this memo is
issues when allowing this behaviour. This memo updates [RFC3550] and applicable. (tbd: fixme)
[RFC3551] with important considerations regarding applicability and
functionality when using multiple types of media in an RTP session,
including normative specification of behaviour. This memo makes no
changes to RTP behaviour when using multiple RTP streams with media
of the same type (e.g., multiple audio streams or multiple video
streams) in a single RTP session. Instead it relies on the
clarifications in [I-D.ietf-avtcore-rtp-multi-stream].
This memo is structured as follows. First, some basic definitions 2. Terminology
are provided. This is followed by a background that discusses the
motivation in more detail. A overview of the solution of how to
provide multiple media types in one RTP session is then presented.
Next is the formal applicability this specification have followed by
the normative specification. This is followed by a discussion how
some RTP/RTCP Extensions are expected to function in the case of
multiple media types in one RTP session. A specification of the
requirements on signalling from this specification and a look how
this is realized in SDP using Bundle
[I-D.ietf-mmusic-sdp-bundle-negotiation]. The memo ends with the
security considerations.
2. Definitions The terms Encoded Stream, Endpoint, Media Source, RTP Session, and
RTP Stream are used as defined in
[I-D.ietf-avtext-rtp-grouping-taxonomy]. We also define the
following terms:
Media Type: The general type of media data used by a real-time Media Type: The general type of media data used by a real-time
application. The media type corresponds to the value used in the application. The media type corresponds to the value used in the
<media> field of an SDP m= line. The media types defined at the <media> field of an SDP m= line. The media types defined at the
time of this writing are "audio", "video", "text", "application", time of this writing are "audio", "video", "text", "application",
and "message". and "message".
Quality of Service (QoS): Network mechanisms that are intended to Quality of Service (QoS): Network mechanisms that are intended to
ensure that the packets within a flow or with a specific marking ensure that the packets within a flow or with a specific marking
are transported with certain properties. are transported with certain properties.
The terms Encoded Stream, Endpoint, Media Source, RTP Session, and
RTP Stream are used as defined in
[I-D.ietf-avtext-rtp-grouping-taxonomy].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Motivation 3. Background and Motivation
The existence of NATs and Firewalls at almost all Internet access has
had implications on protocols like RTP that were designed to use
multiple transport flows. First of all, the NAT/FW traversal
solution needs to ensure that all these transport flows are
established. This has three consequences:
1. Increased delay to perform the transport flow establishment
2. The more transport flows, the more state and the more resource
consumption in the NAT and Firewalls. When the resource
consumption in NAT/FWs reaches their limits, unexpected
behaviours usually occur.
3. More transport flows means a higher risk that some transport flow
fails to be established, thus preventing the application to
communicate.
Using fewer transport flows reduces the risk of communication
failure, improved establishment behaviour and less load on NAT and
Firewalls.
Furthermore, we note that many RTP-using applications don't utilize RTP was designed to support multimedia sessions, containing multiple
any network level Quality of Service (QoS) functions. Nor do they types of media sent simultaneously, by using multiple transport layer
expect or desire any separation in network treatment of its media flows. The existence of network address translators, firewalls, and
packets, independent of whether they are audio, video or text. When other middleboxes complicates this, however, since a mechanism is
an application has no such desire, it doesn't need to provide a needed to ensure that all the transport layer flows needed by the
transport flow structure that simplifies flow based QoS. application can be established. This has three consequences:
For applications that don't require different lower-layer QoS for 1. increased delay to establish a complete session, since each of
different media types, and that have no special requirements for RTP the transport layer flows needs to be negotiated and established;
extensions or RTCP reporting, the requirement to separate different
media into different RTP sessions might seem unnecessary. Provided
the application accepts that all media flows will get similar RTCP
reporting, using the same RTP session for several types of media at
once appears a reasonable choice. The architecture ought to be
agnostic about the type of media being carried in an RTP session to
the extent possible given the constraints of the protocol.
4. Overview of Solution 2. increased state and resource consumption in the middleboxes, that
can lead to unexpected behaviour when middlebox resource limits
are reached; and
The goal of the solution is to enable each RTP session to contain 3. increased risk that a subset of the transport layer flows will
more than just one media type. This includes having multiple RTP fail to be established, thus preventing the application from
sessions containing a given media type, for example having three communicating.
sessions containing both video and audio.
The solution is quite straightforward. The first step is to override Using fewer transport layer flows can hence be seen to reduce the
the SHOULD and SHOULD NOT language of the RTP specification risk of communication failure, and can lead to improved reliability
[RFC3550]. Similar change is needed to a sentence in Section 6 of and performance.
[RFC3551] that states that "different media types SHALL NOT be
interleaved or multiplexed within a single RTP Session". This is
resolved by appropriate exception clauses given that this
specification and its applicability is followed.
Within an RTP session where multiple media types have been configured One of the benefits of using multiple transport layer flows is that
for use, an SSRC can only send one type of media during its lifetime it makes it easy to use network layer quality of service (QoS)
(i.e., it can switch between different audio codecs, since those are mechanisms to give differentiated performance for different flows.
both the same type of media, but cannot switch between audio and However, we note that many RTP-using application don't use network
video). Different SSRCs MUST be used for the different media QoS features, and don't expect or desire any separation in network
sources, the same way multiple media sources of the same media type treatment of their media packets, independent of whether they are
already have to do. The payload type will inform a receiver which audio, video or text. When an application has no such desire, it
media type the SSRC is being used for. Thus the payload type MUST be doesn't need to provide a transport flow structure that simplifies
unique across all of the payload configurations independent of media flow based QoS.
type that is used in the RTP session.
Some few extra considerations within the RTP sessions also needs to Given this, it might seem desirable for RTP-based applications to
be considered. RTCP bandwidth and regular reporting suppression send all their media streams bundled into one RTP session, that runs
(RTP/AVPF and RTP/SAVPF) SHOULD be configured to reduce the impact on a single transport layer flow. Unfortunately, this is prohibited
for bit-rate variations between RTP streams and media types. It is by the RTP specification, since RTP makes certain assumptions that
also clarified how timeout calculations are to be done to avoid any can be incompatible with sending multiple media types in a single RTP
issues. Certain payload types like FEC also need additional rules. session. Specifically, the RTP control protocol (RTCP) timing rules
assume that all RTP media flows in a single RTP session have broadly
similar RTCP reporting and feedback requirements, which can be
problematic when different types of media are multiplexed together.
Certain RTP extensions also make assumptions that are incompatible
with sending different media types in a single RTP session.
The final important part of the solution to this is to use signalling This memo updates [RFC3550] and [RFC3551] to allow RTP sessions to
and ensure that agreement on using multiple media types in an RTP contain more than just one media type, and gives guidance on when it
session exists, and how that then is configured. This memo describes is safe to perform such multiplexing.
some existing requirements, while an external reference defines how
this is accomplished in SDP.
5. Applicability 4. Applicability
This specification has limited applicability, and anyone intending to This specification has limited applicability, and anyone intending to
use it needs to ensure that their application and usage meets the use it MUST ensure that their application and use meets the following
below criteria. criteria:
5.1. Usage of the RTP session
Before choosing to use this specification, an application implementer
needs to ensure that they don't have a need for different RTP
sessions between the media types for some reason. The main rule is
that if one expects to have equal treatment of all media packets,
then this specification might be suitable. The equal treatment
include anything from network level up to RTCP reporting and
feedback. The document Guidelines for using the Multiplexing
Features of RTP [I-D.ietf-avtcore-multiplex-guidelines] gives more
detailed guidance on aspects to consider when choosing how to use RTP
and specifically sessions.
There is some work in progress
[I-D.westerlund-avtcore-transport-multiplexing] that attempt to
address a solution for RTP-using applications that need or would
prefer multiple RTP sessions, but do not require the
functionalities or behaviours that multiple transport flows give.
The second important consideration is the resulting behaviour when
media flows to be sent within a single RTP session does not have
similar RTCP requirements. There are limitations in the RTCP timing
rules, and this implies a common RTCP reporting interval across all
participants in a session. If an RTP session contains flows with
very different RTCP requirements, for example due to RTP Streams
bandwidth consumption and packet rate, for example low-rate audio
coupled with high-quality video, this can result in either excessive
or insufficient RTCP for some flows, depending how the RTCP session
bandwidth, and hence reporting interval, is configured. This is
discussed further in Section 6.4.
5.2. Signalled Support
Usage of this specification is not compatible with anyone following Equal treatment of media: The use of a single RTP session enforces
RFC 3550 and intending to have different RTP sessions for each media similar treatment on all types of media used within the session.
type. Therefore there needs to be mutual agreement to use multiple Applications that require significantly different network QoS or
media types in one RTP session by all participants within that RTP RTCP configuration for different media streams are better suited
session. This agreement has to be determined using signalling in by sending those media streams on separate RTP session, using
most cases. separate transport layer flows for each, since that gives greater
flexibility. Further guidance is given in
[I-D.ietf-avtcore-multiplex-guidelines] and
[I-D.ietf-dart-dscp-rtp].
This requirement can be a problem for signalling solutions that can't Compatible Media Requirements: The RTCP timing rules enforce a
negotiate with all participants. For declarative signalling single RTCP reporting interval for all participants in an RTP
solutions, mandating that the session is using multiple media types session. Flows with very different media requirements, for
in one RTP session can be a way of attempting to ensure that all example a low-rate audio flow with no feedback needs and a high-
participants in the RTP session follow the requirement. However, for quality video flow with different repair mechanisms, cannot be
signalling solutions that lack methods for enforcing that a receiver multiplexed together since this results in either excessive or
supports a specific feature, this can still cause issues. insufficient RTCP for some flows, depending how the RTCP session
bandwidth, and hence reporting interval, is configured.
5.3. Homogeneous Multi-party Signalled Support: The extensions defined in this memo are not
compatible with unmodified [RFC3550]-compatible endpoints. Their
use requires signalling and mutual agreement by all participants
within an RTP session. This requirement can be a problem for
signalling solutions that can't negotiate with all participants.
For declarative signalling solutions, mandating that the session
is using multiple media types in one RTP session can be a way of
attempting to ensure that all participants in the RTP session
follow the requirement. However, for signalling solutions that
lack methods for enforcing that a receiver supports a specific
feature, this can still cause issues.
In multiparty communication scenarios it is important to separate two Consistent support for multiple media types in a single RTP session:
different cases. One case is where the RTP session contains multiple In multiparty communication scenarios it is important to separate
participants in a common RTP session. This occurs for example in Any two different cases. One case is where the RTP session contains
Source Multicast (ASM) and Relay (Transport Translator) topologies as multiple participants in a common RTP session. This occurs for
defined in RTP Topologies [I-D.ietf-avtcore-rtp-topologies-update]. example in Any Source Multicast (ASM) and Relay (Transport
It can also occur in some implementations of RTP mixers that share Translator) topologies as defined in RTP Topologies
the same SSRC/CSRC space across all participants. The second case is [I-D.ietf-avtcore-rtp-topologies-update]. It can also occur in
when the RTP session is terminated in a middlebox and the other some implementations of RTP mixers that share the same SSRC/CSRC
participants sources are projected or switched into each RTP session space across all participants. The second case is when the RTP
and rewritten on RTP header level including SSRC mappings. session is terminated in a middlebox and the other participants
sources are projected or switched into each RTP session and
rewritten on RTP header level including SSRC mappings.
For the first case, with a common RTP session or at least shared For the first case, with a common RTP session or at least shared
SSRC/CSRC values, all participants in multiparty communication are SSRC/CSRC values, all participants in multiparty communication are
REQUIRED to support multiple media types in an RTP session. An REQUIRED to support multiple media types in an RTP session. An
participant using two or more RTP sessions towards a multiparty participant using two or more RTP sessions towards a multiparty
session can't be collapsed into a single session with multiple media session can't be collapsed into a single session with multiple
types. The reason is that in case of multiple RTP sessions, the same media types. The reason is that in case of multiple RTP sessions,
SSRC value can be use in both RTP sessions without any issues, but the same SSRC value can be use in both RTP sessions without any
when collapsed to a single session there is an SSRC collision. In issues, but when collapsed to a single session there is an SSRC
addition some collisions can't be represented in the multiple collision. In addition some collisions can't be represented in
separate RTP sessions. For example, in a session with audio and the multiple separate RTP sessions. For example, in a session
video, an SSRC value used for video will not show up in the Audio RTP with audio and video, an SSRC value used for video will not show
session at the participant using multiple RTP sessions, and thus not up in the Audio RTP session at the participant using multiple RTP
trigger any collision handling. Thus any application using this type sessions, and thus not trigger any collision handling. Thus any
of RTP session structure MUST have a homogeneous support for multiple application using this type of RTP session structure MUST have a
media types in one RTP session, or be forced to insert a translator homogeneous support for multiple media types in one RTP session,
node between that participant and the rest of the RTP session. or be forced to insert a translator node between that participant
and the rest of the RTP session.
For the second case of separate RTP sessions for each multiparty For the second case of separate RTP sessions for each multiparty
participant and a central node it is possible to have a mix of single participant and a central node it is possible to have a mix of
RTP session users and multiple RTP session users as long as one is single RTP session users and multiple RTP session users as long as
willing to remap the SSRCs used by a participant with multiple RTP one is willing to remap the SSRCs used by a participant with
sessions into non-used values in the single RTP session SSRC space multiple RTP sessions into non-used values in the single RTP
for each of the participants using a single RTP session with multiple session SSRC space for each of the participants using a single RTP
media types. It can be noted that this type of implementation has to session with multiple media types. It can be noted that this type
understand all types of RTP/RTCP extension being used in the RTP of implementation has to understand all types of RTP/RTCP
sessions to correctly be able to translate them between the RTP extension being used in the RTP sessions to correctly be able to
sessions. It might also suffer issues due to differencies in translate them between the RTP sessions. It might also suffer
configured RTCP bandwidth and other parameters between the RTP issues due to differencies in configured RTCP bandwidth and other
sessions. It can also negatively impact the possibility for loop parameters between the RTP sessions. It can also negatively
detection, as SSRC/CSRC can't be used to detect the loops, instead impact the possibility for loop detection, as SSRC/CSRC can't be
some other RTP stream or media source identity name space that is used to detect the loops, instead some other RTP stream or media
common across all interconnect parts are needed. source identity name space that is common across all interconnect
parts are needed.
5.4. Reduced number of Payload Types
An RTP session with multiple media types in it have only a single
7-bit Payload Type range for all its payload types. Within the 128
available values, only 96 or less if "Multiplexing RTP Data and
Control Packets on a Single Port" [RFC5761] is used, all the
different RTP payload configurations for all the media types need to
fit in the available space. For most applications this will not be a
real problem, but the limitation exists and could be encountered.
5.5. Stream Differentiation
If network level differentiation of the RTP streams with different
media types is desired, using this specification can cause severe
limitations. All RTP streams in an RTP session, independent of the
media type, will be sent over the same underlying transport flow.
Any flow-based Quality of Service (QoS) mechanism will be unable to
provide differentiated treatment between different media types, e.g.
to prioritize audio over video. If differentiated treatment is
desired using flow-based QoS, separate RTP sessions over different
underlying transport flows needs to be used.
Marking-based QoS schemes like DiffServ can be affected if a network
ingress is the one that performs, markings based on flows. Endpoint
marking where the network API supports marking on individual packet
level will be unaffected by this specification. However, there exist
limitations, as discussed in [I-D.ietf-dart-dscp-rtp], on how
different traffic classes can be applied on different packets or RTP
streams within a single transport flow.
5.6. Non-compatible Extensions
There exist some RTP and RTCP extensions that rely on the existence
of multiple RTP sessions. If the goal of using an RTP session with
multiple media types is to have only a single RTP session, then these
extensions can't be used. If one has no need to have different RTP
sessions for the media types but is willing to have multiple RTP
sessions, one for the main media transmission and one for the
extension, they can be used. It is to be noted that this assumes
that it is possible to get the extension working when the related RTP
session contains multiple media types.
Identified RTP/RTCP extensions that require multiple RTP Sessions
are:
RTP Retransmission: RTP Retransmission [RFC4588] has a session
multiplexed mode. It also has a SSRC multiplexed mode that can be
used instead. So use the mode that is suitable for the RTP
application.
XOR-Based FEC: The RTP Payload Format for Generic Forward Error Ability to operate with limited payload type space: An RTP session
Correction [RFC5109] and its predecessor [RFC2733] requires a has only a single 7-bit payload type space for all its payload
separate RTP session unless the FEC data is carried in RTP Payload type numbers. Some applications might find this space limiting
for Redundant Audio Data [RFC2198]. However, using the Generic when media different media types and RTP payload formats are using
FEC with the Redundancy payload has another set of restrictions, within a single RTP session.
see Section 7.2.
Note that the Source-Specific Media Attributes [RFC5576] Avoids incompatible Extensions: Some RTP and RTCP extensions rely on
specification defines an SDP syntax (the "FEC" semantic of the the existence of multiple RTP sessions and relate media streams
"ssrc-group" attribute) to signal FEC relationships between between sessions. Others report on particular media types, and
multiple RTP streams within a single RTP session. However, this cannot be used with other media types. Applications that send
can't be used as the FEC repair packets need to have the same SSRC multiple types of media into a single RTP session need to avoid
value as the source packets being protected. [RFC5576] does not such extensions.
normatively update and resolve that restriction. There is ongoing
work on an ULP extension to allow it be use FEC RTP streams within
the same RTP Session as the source stream
[I-D.lennox-payload-ulp-ssrc-mux].
6. RTP Session Specification 5. Using Multiple Media Types in a Single RTP Session
This section defines what needs to be done or avoided to make an RTP This section defines what needs to be done or avoided to make an RTP
session with multiple media types function without issues. session with multiple media types function without issues.
6.1. RTP Session 5.1. Allowing Multiple Media Types in an RTP Session
Section 5.2 of "RTP: A Transport Protocol for Real-Time Applications" Section 5.2 of "RTP: A Transport Protocol for Real-Time Applications"
[RFC3550] states: [RFC3550] states:
For example, in a teleconference composed of audio and video media For example, in a teleconference composed of audio and video media
encoded separately, each medium SHOULD be carried in a separate encoded separately, each medium SHOULD be carried in a separate
RTP session with its own destination transport address. RTP session with its own destination transport address.
Separate audio and video streams SHOULD NOT be carried in a single Separate audio and video streams SHOULD NOT be carried in a single
RTP session and demultiplexed based on the payload type or SSRC RTP session and demultiplexed based on the payload type or SSRC
skipping to change at page 10, line 8 skipping to change at page 7, line 35
For example, in a teleconference composed of audio and video media For example, in a teleconference composed of audio and video media
encoded separately, each medium SHOULD be carried in a separate encoded separately, each medium SHOULD be carried in a separate
RTP session with its own destination transport address, unless RTP session with its own destination transport address, unless
specification [RFCXXXX] is followed and the application meets the specification [RFCXXXX] is followed and the application meets the
applicability constraints. applicability constraints.
The second sentence is changed to: The second sentence is changed to:
Separate audio and video media sources SHOULD NOT be carried in a Separate audio and video media sources SHOULD NOT be carried in a
single RTP session and demultiplexed based on the payload type or single RTP session, unless the guidelines specified in [RFCXXXX]
SSRC fields, unless multiplexed based on both SSRC and payload are followed.
type and usage meets what Multiple Media Types in an RTP Session
[RFCXXXX] specifies.
Second paragraph of Section 6 in RTP Profile for Audio and Video Second paragraph of Section 6 in RTP Profile for Audio and Video
Conferences with Minimal Control [RFC3551] says: Conferences with Minimal Control [RFC3551] says:
The payload types currently defined in this profile are assigned The payload types currently defined in this profile are assigned
to exactly one of three categories or media types: audio only, to exactly one of three categories or media types: audio only,
video only and those combining audio and video. The media types video only and those combining audio and video. The media types
are marked in Tables 4 and 5 as "A", "V" and "AV", respectively. are marked in Tables 4 and 5 as "A", "V" and "AV", respectively.
Payload types of different media types SHALL NOT be interleaved or Payload types of different media types SHALL NOT be interleaved or
multiplexed within a single RTP session, but multiple RTP sessions multiplexed within a single RTP session, but multiple RTP sessions
MAY be used in parallel to send multiple media types. An RTP MAY be used in parallel to send multiple media types. An RTP
source MAY change payload types within the same media type during source MAY change payload types within the same media type during
a session. See the section "Multiplexing RTP Sessions" of RFC a session. See the section "Multiplexing RTP Sessions" of RFC
3550 for additional explanation. 3550 for additional explanation.
This specifications purpose is to violate that existing SHALL NOT This specifications purpose is to violate that existing SHALL NOT
under certain conditions. Thus also this sentence has to be changed under certain conditions. Thus this sentence also has to be changed
to allow for multiple media type's payload types in the same session. to allow for multiple media type's payload types in the same session.
The above sentence is changed to: The above sentence is changed to:
Payload types of different media types SHALL NOT be interleaved or Payload types of different media types SHALL NOT be interleaved or
multiplexed within a single RTP session unless as specified and multiplexed within a single RTP session unless as specified and
under the restriction in Multiple Media Types in an RTP Session under the restriction in Multiple Media Types in an RTP Session
[RFCXXXX]. Multiple RTP sessions MAY be used in parallel to send [RFCXXXX]. Multiple RTP sessions MAY be used in parallel to send
multiple media types. multiple media types.
RFC-Editor Note: Please replace RFCXXXX with the RFC number of this RFC-Editor Note: Please replace RFCXXXX with the RFC number of this
specification when assigned. specification when assigned.
We can now go on and discuss the five bullets that are motivating the 5.2. Demultiplexing Media Streams
previous in Section 5.2 of the RTP Specification [RFC3550]. They are
repeated here for the reader's convenience:
1. If, say, two audio streams shared the same RTP session and the
same SSRC value, and one were to change encodings and thus
acquire a different RTP payload type, there would be no general
way of identifying which stream had changed encodings.
2. An SSRC is defined to identify a single timing and sequence
number space. Interleaving multiple payload types would require
different timing spaces if the media clock rates differ and would
require different sequence number spaces to tell which payload
type suffered packet loss.
3. The RTCP sender and receiver reports (see Section 6.4 of RFC
3550) can only describe one timing and sequence number space per
SSRC and do not carry a payload type field.
4. An RTP mixer would not be able to combine interleaved streams of
incompatible media into one stream.
5. Carrying multiple media in one RTP session precludes: the use of
different network paths or network resource allocations if
appropriate; reception of a subset of the media if desired, for
example just audio if video would exceed the available bandwidth;
and receiver implementations that use separate processes for the
different media, whereas using separate RTP sessions permits
either single- or multiple-process implementations.
Bullets 1 to 3 are all related to that each media source has to use
one or more unique SSRCs to avoid these issues as mandated below
(Section 6.2). Bullet 4 can be served by two arguments, first of all
each SSRC will be associated with a specific media type, communicated
through the RTP payload type, allowing a middlebox to do media type
specific operations. The second argument is that in many contexts
blind combining without additional contexts are anyway not suitable.
Regarding bullet 5 this is a understood and explicitly stated
applicability limitations for the method described in this document.
6.2. Sender Source Restrictions When receiving packets from a transport layer flow, an endpoint will
first separate the RTP and RTCP packets from the non-RTP packets, and
pass them to the RTP/RTCP protocol handler. The RTP and RTCP packets
are then demultiplexed based on their SSRC into the different media
streams. For each media stream, incoming RTCP packets are processed,
and the RTP payload type is used to select the appropriate media
decoder.
A SSRC in the RTP session MUST only send one media type (audio, This process remains the same irrespective of whether multiple media
video, text etc.) during the SSRC's lifetime. The main motivation is types are sent in a single RTP session or not. It is important to
that a given SSRC has its own RTP timestamp and sequence number note that the RTP payload type is never used to demultiplex media
spaces. The same way that you can't send two encoded streams of streams. Media streams are distinguished by SSRC, and the payload
audio on the same SSRC, you can't send one encoded audio and one type is then used to route data for a particular SSRC to the right
encoded video stream on the same SSRC. Each encoded stream when made media decoder.
into an RTP stream needs to have the sole control over the sequence
number and timestamp space. If not, one would not be able to detect
packet loss for that particular encoded stream. Nor can one easily
determine which clock rate a particular SSRCs timestamp will increase
with. For additional arguments why RTP payload type based
multiplexing of multiple media sources doesn't work see
[I-D.ietf-avtcore-multiplex-guidelines].
6.3. Payload Type Applicability 5.3. Per-SSRC Media Type Restrictions
Most Payload Types have a native media type, like an audio codec is An SSRC in an RTP session MUST NOT change media type during its
natural belonging to the audio media type. However, there exist a lifetime. For example, an SSRC cannot start sending audio, then
number of RTP payload types that don't have a native media type. For change to sending video. The lifetime of an SSRC ends when an RTCP
example, transport robustness mechanisms like RTP Retransmission BYE packet for that SSRC is sent, or when it ceases transmission for
[RFC4588] and Generic FEC [RFC5109] inherit their media type from long enough that it times out for the other participants in the
what they protect. RTP Retransmission is explicitly bound to the session.
payload type it is protecting, and thus will inherit it. However
Generic FEC is a excellent example of an RTP payload type that has no
natural media type. The media type for what it protects is not
relevant as it is the recovered RTP packets that have a particular
media type, and thus Generic FEC is best categorized as an
application media type.
The above discussion is relevant to what limitations exist for RTP The main motivation is that a given SSRC has its own RTP timestamp
payload type usage within an RTP session that has multiple media and sequence number spaces. The same way that you can't send two
types. In fact this document (Section 7.2) suggest that for usage of encoded streams of audio on the same SSRC, you can't send one encoded
Generic FEC (XOR-based) as defined in RFC 5109 can actually use a audio and one encoded video stream on the same SSRC. Each encoded
single media type when used with independent RTP sessions for source stream when made into an RTP stream needs to have the sole control
and repair data. over the sequence number and timestamp space. If not, one would not
be able to detect packet loss for that particular encoded stream.
Nor can one easily determine which clock rate a particular SSRCs
timestamp will increase with. For additional arguments why RTP
payload type based multiplexing of multiple media sources doesn't
work see [I-D.ietf-avtcore-multiplex-guidelines].
Note a particular SSRC carrying Generic FEC will clearly only Within an RTP session where multiple media types have been configured
protect a specific SSRC and thus that instance is bound to the for use, an SSRC can only send one type of media during its lifetime
SSRC's media type. For this specific case, it is possible to have (i.e., it can switch between different audio codecs, since those are
one be applicable to both. However, in cases when the signalling both the same type of media, but cannot switch between audio and
is setup to enable fall back to using separate RTP sessions, then video). Different SSRCs MUST be used for the different media
using a different media type, e.g. application, than the media sources, the same way multiple media sources of the same media type
being protected can create issues. already have to do. The payload type will inform a receiver which
media type the SSRC is being used for. Thus the payload type MUST be
unique across all of the payload configurations independent of media
type that is used in the RTP session.
6.4. RTCP Considerations 5.4. RTCP Considerations
Guidelines for handling RTCP when sending multiple RTP streams with When sending multiple types of media that have different rates in a
disparate rates in a single RTP session are outlined in single RTP session, endpoints MUST follow the guidelines for handling
[I-D.ietf-avtcore-rtp-multi-stream]. These guidelines apply when RTCP described in Section 7 of [I-D.ietf-avtcore-rtp-multi-stream].
sending multiple types of media in a single RTP session if the
different types of media have different rates.
7. Extension Considerations 6. Extension Considerations
This section discusses the impact on some RTP/RTCP extensions due to This section outlines known issues and incompatibilities with RTP and
usage of multiple media types in on RTP session. Only extensions RTCP extensions when multiple media types are used in a single RTP
where something worth noting has been included. sessions. Future extensions to RTP and RTCP need to consider, and
document, any potential incompatibility.
7.1. RTP Retransmission 6.1. RTP Retransmission Payload Format
SSRC-multiplexed RTP retransmission [RFC4588] is actually very SSRC-multiplexed RTP retransmission [RFC4588] is actually very
straightforward. Each retransmission RTP payload type is explicitly straightforward. Each retransmission RTP payload type is explicitly
connected to an associated payload type. If retransmission is only connected to an associated payload type. If retransmission is only
to be used with a subset of all payload types, this is not a problem, to be used with a subset of all payload types, this is not a problem,
as it will be evident from the retransmission payload types which as it will be evident from the retransmission payload types which
payload types have retransmission enabled for them. payload types have retransmission enabled for them.
Session-multiplexed RTP retransmission is also possible to use where Session-multiplexed RTP retransmission is also possible to use where
an retransmission session contains the retransmissions of the an retransmission session contains the retransmissions of the
skipping to change at page 14, line 34 skipping to change at page 11, line 5
a=rtpmap:100 rtx/90000 a=rtpmap:100 rtx/90000
a=fmtp:199 apt=31;rtx-time=3000 a=fmtp:199 apt=31;rtx-time=3000
a=mid:kelp a=mid:kelp
m=video 40000 RTP/AVPF 100 m=video 40000 RTP/AVPF 100
a=rtpmap:100 rtx/90000 a=rtpmap:100 rtx/90000
a=fmtp:199 apt=31;rtx-time=3000 a=fmtp:199 apt=31;rtx-time=3000
a=mid:glo a=mid:glo
Figure 1: SDP example of Session Multiplexed RTP Retransmission Figure 1: SDP example of Session Multiplexed RTP Retransmission
7.2. Generic FEC 6.2. RTP Payload Format for Generic FEC
The RTP Payload Format for Generic Forward Error Correction The RTP Payload Format for Generic Forward Error Correction
[RFC5109], and also its predecessor [RFC2733], requires some [RFC5109], and also its predecessor [RFC2733], requires some
considerations, and they are different depending on what type of considerations, and they are different depending on what type of
configuration of usage one has. configuration of usage one has.
Independent RTP Sessions, i.e. where source and repair data are sent Independent RTP Sessions, i.e. where source and repair data are sent
in different RTP sessions. As this mode of configuration requires in different RTP sessions. As this mode of configuration requires
different RTP session, there has to be at least one RTP session for different RTP session, there has to be at least one RTP session for
source data, this session can be one using multiple media types. The source data, this session can be one using multiple media types. The
skipping to change at page 15, line 9 skipping to change at page 11, line 29
theory be any of the defined ones. It is RECOMMENDED that one uses theory be any of the defined ones. It is RECOMMENDED that one uses
"Application". "Application".
If one uses SDP signalling with BUNDLE If one uses SDP signalling with BUNDLE
[I-D.ietf-mmusic-sdp-bundle-negotiation], then the RTP session [I-D.ietf-mmusic-sdp-bundle-negotiation], then the RTP session
carrying the FEC streams will be its own BUNDLE group. The media carrying the FEC streams will be its own BUNDLE group. The media
line with the source stream for the FEC and the FEC stream's media line with the source stream for the FEC and the FEC stream's media
line will be grouped using media line grouping using the FEC or FEC- line will be grouped using media line grouping using the FEC or FEC-
FR [RFC5956] grouping. This is very similar to the situation that FR [RFC5956] grouping. This is very similar to the situation that
arise for RTP retransmission with session multiplexing discussed arise for RTP retransmission with session multiplexing discussed
above inSection 7.1. above inSection 6.1.
The RTP Payload Format for Generic Forward Error Correction [RFC5109]
and its predecessor [RFC2733] requires a separate RTP session unless
the FEC data is carried in RTP Payload for Redundant Audio Data
[RFC2198].
Note that the Source-Specific Media Attributes [RFC5576]
specification defines an SDP syntax (the "FEC" semantic of the "ssrc-
group" attribute) to signal FEC relationships between multiple RTP
streams within a single RTP session. However, this can't be used as
the FEC repair packets need to have the same SSRC value as the source
packets being protected. [RFC5576] does not normatively update and
resolve that restriction. There is ongoing work on an ULP extension
to allow it be use FEC RTP streams within the same RTP Session as the
source stream [I-D.lennox-payload-ulp-ssrc-mux].
6.3. RTP Payload Format for Redundant Audio
In stream, using RTP Payload for Redundant Audio Data [RFC2198] In stream, using RTP Payload for Redundant Audio Data [RFC2198]
combining repair and source data in the same packets. This is combining repair and source data in the same packets. This is
possible to use within a single RTP session. However, the usage and possible to use within a single RTP session. However, the usage and
configuration of the payload types can create an issue. First of all configuration of the payload types can create an issue. First of all
it might be necessary to have one payload type per media type for the it might be necessary to have one payload type per media type for the
FEC repair data payload format, i.e. one for audio/ulpfec and one for FEC repair data payload format, i.e. one for audio/ulpfec and one
text/ulpfec if audio and text are combined in an RTP session. for text/ulpfec if audio and text are combined in an RTP session.
Secondly each combination of source payload and its FEC repair data Secondly each combination of source payload and its FEC repair data
has to be an explicit configured payload type. This has potential has to be an explicit configured payload type. This has potential
for making the limitation of RTP payload types available into a real for making the limitation of RTP payload types available into a real
issue. issue.
8. Signalling 7. Signalling
The Signalling requirements
Establishing an RTP session with multiple media types requires Establishing an RTP session with multiple media types requires
signalling. This signalling needs to fulfil the following signalling. This signalling needs to fulfil the following
requirements: requirements:
1. Ensure that any participant in the RTP session is aware that this 1. Ensure that any participant in the RTP session is aware that this
is an RTP session with multiple media types. is an RTP session with multiple media types.
2. Ensure that the payload types in use in the RTP session are using 2. Ensure that the payload types in use in the RTP session are using
unique values, with no overlap between the media types. unique values, with no overlap between the media types.
skipping to change at page 16, line 5 skipping to change at page 12, line 34
RS bandwidth, AVPF trr-int, underlying transport, the RTCP RS bandwidth, AVPF trr-int, underlying transport, the RTCP
extensions in use, and security parameters, commonly for the RTP extensions in use, and security parameters, commonly for the RTP
session. session.
4. RTP and RTCP functions that can be bound to a particular media 4. RTP and RTCP functions that can be bound to a particular media
type SHOULD be reused when possible also for other media types, type SHOULD be reused when possible also for other media types,
instead of having to be configured for multiple code-points. instead of having to be configured for multiple code-points.
Note: In some cases one will not have a choice but to use Note: In some cases one will not have a choice but to use
multiple configurations. multiple configurations.
8.1. SDP-Based Signalling
The signalling of multiple media types in one RTP session in SDP is The signalling of multiple media types in one RTP session in SDP is
specified in "Multiplexing Negotiation Using Session Description specified in "Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers" Protocol (SDP) Port Numbers"
[I-D.ietf-mmusic-sdp-bundle-negotiation]. [I-D.ietf-mmusic-sdp-bundle-negotiation].
9. IANA Considerations 8. Security Considerations
This document makes no request of IANA.
Note to RFC Editor: this section is to be removed on publication as
an RFC.
10. Security Considerations
Having an RTP session with multiple media types doesn't change the Having an RTP session with multiple media types doesn't change the
methods for securing a particular RTP session. One possible methods for securing a particular RTP session. One possible
difference is that the different media have often had different difference is that the different media have often had different
security requirements. When combining multiple media types in one security requirements. When combining multiple media types in one
session, their security requirements also have to be combined by session, their security requirements also have to be combined by
selecting the most demanding for each property. Thus having multiple selecting the most demanding for each property. Thus having multiple
media types can result in increased overhead for security for some media types can result in increased overhead for security for some
media types to ensure that all requirements are meet. media types to ensure that all requirements are meet.
Otherwise, the recommendations for how to configure and RTP session Otherwise, the recommendations for how to configure and RTP session
do not add any additional requirements compared to normal RTP, except do not add any additional requirements compared to normal RTP, except
for the need to be able to ensure that the participants are aware for the need to be able to ensure that the participants are aware
that it is a multiple media type session. If not that is ensured it that it is a multiple media type session. If not that is ensured it
can cause issues in the RTP session for both the unaware and the can cause issues in the RTP session for both the unaware and the
aware one. Similar issues can also be produced in an normal RTP aware one. Similar issues can also be produced in an normal RTP
session by creating configurations for different end-points that session by creating configurations for different end-points that
doesn't match each other. doesn't match each other.
11. Acknowledgements 9. IANA Considerations
This memo makes no request of IANA.
10. Acknowledgements
The authors would like to thank Christer Holmberg, Gunnar Hellstroem, The authors would like to thank Christer Holmberg, Gunnar Hellstroem,
and Charles Eckel for the feedback on the document. and Charles Eckel for the feedback on the document.
12. References 11. References
12.1. Normative References 11.1. Normative References
[I-D.ietf-avtcore-rtp-multi-stream] [I-D.ietf-avtcore-rtp-multi-stream]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins, Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session", "Sending Multiple Media Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-06 (work in progress), draft-ietf-avtcore-rtp-multi-stream-07 (work in progress),
October 2014. March 2015.
[I-D.ietf-mmusic-sdp-bundle-negotiation] [I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings, Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session "Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-17 (work in progress), March 2015. negotiation-22 (work in progress), June 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003. Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551, Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003. July 2003.
12.2. Informative References 11.2. Informative References
[I-D.ietf-avtcore-multiplex-guidelines] [I-D.ietf-avtcore-multiplex-guidelines]
Westerlund, M., Perkins, C., and H. Alvestrand, Westerlund, M., Perkins, C., and H. Alvestrand,
"Guidelines for using the Multiplexing Features of RTP to "Guidelines for using the Multiplexing Features of RTP to
Support Multiple Media Streams", draft-ietf-avtcore- Support Multiple Media Streams", draft-ietf-avtcore-
multiplex-guidelines-03 (work in progress), October 2014. multiplex-guidelines-03 (work in progress), October 2014.
[I-D.ietf-avtcore-rtp-topologies-update] [I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft- Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-06 (work in progress), ietf-avtcore-rtp-topologies-update-10 (work in progress),
March 2015. July 2015.
[I-D.ietf-avtext-rtp-grouping-taxonomy] [I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro, Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
"A Taxonomy of Grouping Semantics and Mechanisms for Real- B. Burman, "A Taxonomy of Semantics and Mechanisms for
Time Transport Protocol (RTP) Sources", draft-ietf-avtext- Real-Time Transport Protocol (RTP) Sources", draft-ietf-
rtp-grouping-taxonomy-06 (work in progress), March 2015. avtext-rtp-grouping-taxonomy-07 (work in progress), June
2015.
[I-D.ietf-dart-dscp-rtp] [I-D.ietf-dart-dscp-rtp]
Black, D. and P. Jones, "Differentiated Services Black, D. and P. Jones, "Differentiated Services
(DiffServ) and Real-time Communication", draft-ietf-dart- (DiffServ) and Real-time Communication", draft-ietf-dart-
dscp-rtp-10 (work in progress), November 2014. dscp-rtp-10 (work in progress), November 2014.
[I-D.lennox-payload-ulp-ssrc-mux] [I-D.lennox-payload-ulp-ssrc-mux]
Lennox, J., "Supporting Source-Multiplexing of the Real- Lennox, J., "Supporting Source-Multiplexing of the Real-
Time Transport Protocol (RTP) Payload for Generic Forward Time Transport Protocol (RTP) Payload for Generic Forward
Error Correction", draft-lennox-payload-ulp-ssrc-mux-00 Error Correction", draft-lennox-payload-ulp-ssrc-mux-00
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