draft-ietf-avtcore-multi-media-rtp-session-04.txt   draft-ietf-avtcore-multi-media-rtp-session-05.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: July 17, 2014 J. Lennox Expires: August 18, 2014 J. Lennox
Vidyo Vidyo
January 13, 2014 February 14, 2014
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-04 draft-ietf-avtcore-multi-media-rtp-session-05
Abstract Abstract
This document specifies how an RTP session can contain media streams This document specifies how an RTP session can contain media 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.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 17, 2014. This Internet-Draft will expire on August 18, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. NAT and Firewalls . . . . . . . . . . . . . . . . . . . . 4
3.2. No Transport Level QoS . . . . . . . . . . . . . . . . . 4
3.3. Architectural Equality . . . . . . . . . . . . . . . . . 5
4. Overview of Solution . . . . . . . . . . . . . . . . . . . . 5 4. Overview of Solution . . . . . . . . . . . . . . . . . . . . 5
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Usage of the RTP session . . . . . . . . . . . . . . . . 6 5.1. Usage of the RTP session . . . . . . . . . . . . . . . . 6
5.2. Signalled Support . . . . . . . . . . . . . . . . . . . . 7 5.2. Signalled Support . . . . . . . . . . . . . . . . . . . . 6
5.3. Homogeneous Multi-party . . . . . . . . . . . . . . . . . 7 5.3. Homogeneous Multi-party . . . . . . . . . . . . . . . . . 7
5.4. Reduced number of Payload Types . . . . . . . . . . . . . 8 5.4. Reduced number of Payload Types . . . . . . . . . . . . . 8
5.5. Stream Differentiation . . . . . . . . . . . . . . . . . 8 5.5. Stream Differentiation . . . . . . . . . . . . . . . . . 8
5.6. Non-compatible Extensions . . . . . . . . . . . . . . . . 8 5.6. Non-compatible Extensions . . . . . . . . . . . . . . . . 8
6. RTP Session Specification . . . . . . . . . . . . . . . . . . 9 6. RTP Session Specification . . . . . . . . . . . . . . . . . . 9
6.1. RTP Session . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. RTP Session . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Sender Source Restrictions . . . . . . . . . . . . . . . 11 6.2. Sender Source Restrictions . . . . . . . . . . . . . . . 11
6.3. Payload Type Applicability . . . . . . . . . . . . . . . 12 6.3. Payload Type Applicability . . . . . . . . . . . . . . . 12
6.4. RTCP Considerations . . . . . . . . . . . . . . . . . . . 12 6.4. RTCP Considerations . . . . . . . . . . . . . . . . . . . 12
7. Extension Considerations . . . . . . . . . . . . . . . . . . 12 7. Extension Considerations . . . . . . . . . . . . . . . . . . 12
7.1. RTP Retransmission . . . . . . . . . . . . . . . . . . . 13 7.1. RTP Retransmission . . . . . . . . . . . . . . . . . . . 13
7.2. Generic FEC . . . . . . . . . . . . . . . . . . . . . . . 13 7.2. Generic FEC . . . . . . . . . . . . . . . . . . . . . . . 13
8. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 14 8. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. SDP-Based Signalling . . . . . . . . . . . . . . . . . . 14 8.1. SDP-Based Signalling . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
10. Security Considerations . . . . . . . . . . . . . . . . . . . 14 10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
12.1. Normative References . . . . . . . . . . . . . . . . . . 15 12.1. Normative References . . . . . . . . . . . . . . . . . . 15
12.2. Informative References . . . . . . . . . . . . . . . . . 16 12.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction 1. Introduction
When the Real-time Transport Protocol (RTP) [RFC3550] was designed, When the Real-time Transport Protocol (RTP) [RFC3550] was designed,
close to 20 years ago, IP networks were very different compared to close to 20 years ago, IP networks were different to those deployed
the ones in 2013 when this is written. The almost ubiquitous at the time of this writing. The virtually ubiquitous deployment of
deployment of Network Address Translators (NAT) and Firewalls has Network Address Translators (NAT) and Firewalls has since increased
increased the cost and likely-hood of communication failure when the cost and likely-hood of communication failure when using many
using many different transport flows. Thus there exists a pressure different transport flows. Hence, there is pressure to reduce the
to reduce the number of concurrent transport flows. number of concurrent transport flows used by RTP applications.
RTP [RFC3550] recommends against sending several different types of The RTP specification recommends against sending several different
media, for example audio and video, in a single RTP session. The RTP types of media, for example audio and video, in a single RTP session.
profile for Audio and Video Conferences with Minimal Control (RTP/
AVP) [RFC3551] mandates a similar restriction. The motivation for
these limitations is partly to allow lower layer Quality of Service
(QoS) mechanisms to be used, and partly due to limitations of the
RTCP timing rules that assumes all media in a session to have similar
bandwidth. The Session Description Protocol (SDP) [RFC4566], as one
of the dominant signalling method for establishing RTP session, has
enforced this rule, simply by not allowing multiple media types for a
given receiver destination or set of ICE candidates, which is the
most common method to determine which RTP session the packets are
intended for.
The fact that these limitations have been in place for so long a The RTP profile for Audio and Video Conferences with Minimal Control
time, in addition to RFC 3550 being written without fully considering (RTP/AVP) [RFC3551] mandates a similar restriction. The motivation
multiple media types in an RTP session, does result in a number of for these limitations is partly to allow lower layer Quality of
considerations being needed when allowing this behaviour. This Service (QoS) mechanisms to be used, and partly due to limitations of
document provides such considerations regarding applicability as well the RTCP timing rules that assumes all media in a session to have
as functionality, including normative specification of behaviour. similar bandwidth. The Session Description Protocol (SDP) [RFC4566]
is one of the dominant signalling methods for establishing RTP
sessions, and has enforced this rule by not allowing multiple media
types for a given destination or set of ICE candidates.
First, some basic definitions are provided. This is followed by a The fact that these limitations have been in place for so long, in
background that discusses the motivation in more detail. A overview addition to RFC 3550 being written without fully considering the use
of the solution of how to provide multiple media types in one RTP of multiple media types in an RTP session, results in a number of
session is then presented. Next is the formal applicability this issues when allowing this behaviour. This memo updates [RFC3550] and
specification have followed by the normative specification. This is [RFC3551] with important considerations regarding applicability and
followed by a discussion how some RTP/RTCP Extensions is expected to functionality when using multiple types of media in an RTP session,
function in the case of multiple media types in one RTP session. A including normative specification of behaviour. This memo makes no
specification of the requirements on signalling from this changes to RTP behaviour when using multiple streams of media of the
specification and a look how this is realized in SDP using Bundle same type (e.g., multiple audio streams or multiple video streams) in
[I-D.ietf-mmusic-sdp-bundle-negotiation]. The document ends with the a single RTP session.
This memo is structured as follows. First, some basic definitions
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. security considerations.
2. Definitions 2. Definitions
The following terms are used with supplied definitions: The following terms are used with supplied definitions:
Endpoint: A single entity sending or receiving RTP packets. It can Endpoint: A single entity sending or receiving RTP packets. It can
be decomposed into several functional blocks, but as long as it be decomposed into several functional blocks, but as long as it
behaves as a single RTP stack entity it is classified as a single behaves as a single RTP stack entity it is classified as a single
endpoint. endpoint.
Media Stream: A sequence of RTP packets using a single SSRC that Media Stream: A sequence of RTP packets using a single SSRC that
together carries part or all of the content of a specific Media together carries part or all of the content of a specific Media
Type from a specific sender source within a given RTP session. Type from a specific sender source within a given RTP session.
Media Type: Audio, video, text or application whose form and meaning Media Type: Audio, video, text or application whose form and meaning
are defined by a specific real-time application. are defined by a specific real-time application.
QoS: Quality of Service, i.e. network mechanisms that intended to QoS: Quality of Service, i.e. network mechanisms that 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.
RTP Session: As defined by [RFC3550], the endpoints belonging to the RTP Session: As defined by [RFC3550], the endpoints belonging to the
same RTP Session are those that share a single SSRC space. That same RTP Session are those that share a single SSRC space. That
is, those endpoints can see an SSRC identifier transmitted by any is, those endpoints can see an SSRC identifier transmitted by any
one of the other endpoints. An endpoint can receive an SSRC one of the other endpoints. An endpoint can receive an SSRC
either as SSRC or as CSRC in RTP and RTCP packets. Thus, the RTP either as SSRC or as CSRC in RTP and RTCP packets. Thus, the RTP
Session scope is decided by the endpoints' network interconnection Session scope is decided by the endpoints' network interconnection
topology, in combination with RTP and RTCP forwarding strategies topology, in combination with RTP and RTCP forwarding strategies
deployed by endpoints and any interconnecting middle nodes. deployed by endpoints and any interconnecting middle nodes.
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. Motivation
This section discusses in more detail the main motivations why
allowing multiple media types in the same RTP session is suitable.
3.1. NAT and Firewalls
The existence of NATs and Firewalls at almost all Internet access has The existence of NATs and Firewalls at almost all Internet access has
had implications on protocols like RTP that were designed to use had implications on protocols like RTP that were designed to use
multiple transport flows. First of all, the NAT/FW traversal multiple transport flows. First of all, the NAT/FW traversal
solution needs to ensure that all these transport flows are solution needs to ensure that all these transport flows are
established. This has three consequences: established. This has three consequences:
1. Increased delay to perform the transport flow establishment 1. Increased delay to perform the transport flow establishment
2. The more transport flows, the more state and the more resource 2. The more transport flows, the more state and the more resource
consumption in the NAT and Firewalls. When the resource consumption in the NAT and Firewalls. When the resource
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behaviours usually occur. behaviours usually occur.
3. More transport flows means a higher risk that some transport flow 3. More transport flows means a higher risk that some transport flow
fails to be established, thus preventing the application to fails to be established, thus preventing the application to
communicate. communicate.
Using fewer transport flows reduces the risk of communication Using fewer transport flows reduces the risk of communication
failure, improved establishment behaviour and less load on NAT and failure, improved establishment behaviour and less load on NAT and
Firewalls. Firewalls.
3.2. No Transport Level QoS Furthermore, we note that many RTP-using applications don't utilize
Many RTP-using applications don't utilize any network level Quality any network level Quality of Service (QoS) functions. Nor do they
of Service functions. Nor do they expect or desire any separation in expect or desire any separation in network treatment of its media
network treatment of its media packets, independent of whether they packets, independent of whether they are audio, video or text. When
are audio, video or text. When an application has no such desire, it an application has no such desire, it doesn't need to provide a
doesn't need to provide a transport flow structure that simplifies transport flow structure that simplifies flow based QoS.
flow based QoS.
3.3. Architectural Equality
For applications that don't require different lower-layer QoS for For applications that don't require different lower-layer QoS for
different media types, and that have no special requirements for RTP different media types, and that have no special requirements for RTP
extensions or RTCP reporting, the requirement to separate different extensions or RTCP reporting, the requirement to separate different
media into different RTP sessions might seem unnecessary. Provided media into different RTP sessions might seem unnecessary. Provided
the application accepts that all media flows will get similar RTCP the application accepts that all media flows will get similar RTCP
reporting, using the same RTP session for several types of media at reporting, using the same RTP session for several types of media at
once appears a reasonable choice. The architecture ought to be once appears a reasonable choice. The architecture ought to be
agnostic about the type of media being carried in an RTP session to agnostic about the type of media being carried in an RTP session to
the extent possible given the constraints of the protocol. the extent possible given the constraints of the protocol.
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that if one expects to have equal treatment of all media packets, that if one expects to have equal treatment of all media packets,
then this specification might be suitable. The equal treatment then this specification might be suitable. The equal treatment
include anything from network level up to RTCP reporting and include anything from network level up to RTCP reporting and
feedback. The document Guidelines for using the Multiplexing feedback. The document Guidelines for using the Multiplexing
Features of RTP [I-D.ietf-avtcore-multiplex-guidelines] gives more Features of RTP [I-D.ietf-avtcore-multiplex-guidelines] gives more
detailed guidance on aspects to consider when choosing how to use RTP detailed guidance on aspects to consider when choosing how to use RTP
and specifically sessions. RTP-using applications that need or would and specifically sessions. RTP-using applications that need or would
prefer multiple RTP sessions, but do not require the functionalities prefer multiple RTP sessions, but do not require the functionalities
or behaviours that multiple transport flows give, can consider using or behaviours that multiple transport flows give, can consider using
Multiple RTP Sessions on a Single Lower-Layer Transport Multiple RTP Sessions on a Single Lower-Layer Transport
[I-D.westerlund-avtcore-transport-multiplexing]. It needs to be [I-D.westerlund-avtcore-transport-multiplexing].
noted that some difference in treatment is still possible to achieve,
for example marking based QoS, or RTCP feedback traffic for only some
media streams.
The second important consideration is the resulting behaviour when The second important consideration is the resulting behaviour when
media flows to be sent within a single RTP session does not have media flows to be sent within a single RTP session does not have
similar bandwidth. There are limitations in the RTCP timing rules, similar RTCP requirements. There are limitations in the RTCP timing
and this implies a common RTCP reporting interval across all rules, and this implies a common RTCP reporting interval across all
participants in a session. If an RTP session contains flows with participants in a session. If an RTP session contains flows with
very different bandwidths, for example low-rate audio coupled with very different RTCP requirements, for example due to media streams
high-quality video, this can result in either excessive or bandwidth consumption and packet rate, for example low-rate audio
insufficient RTCP for some flows, depending how the RTCP session 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 bandwidth, and hence reporting interval, is configured. This is
discussed further in Section 6.4. discussed further in Section 6.4.
5.2. Signalled Support 5.2. Signalled Support
Usage of this specification is not compatible with anyone following Usage of this specification is not compatible with anyone following
RFC 3550 and intending to have different RTP sessions for each media RFC 3550 and intending to have different RTP sessions for each media
type. Therefore there needs to be mutual agreement to use multiple type. Therefore there needs to be mutual agreement to use multiple
media types in one RTP session by all participants within that RTP media types in one RTP session by all participants within that RTP
session. This agreement has to be determined using signalling in session. This agreement has to be determined using signalling in
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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 single
RTP session users and multiple RTP session users as long as one is RTP session users and multiple RTP session users as long as one is
willing to remap the SSRCs used by a participant with multiple RTP willing to remap the SSRCs used by a participant with multiple RTP
sessions into non-used values in the single RTP session SSRC space sessions into non-used values in the single RTP session SSRC space
for each of the participants using a single RTP session with multiple for each of the participants using a single RTP session with multiple
media types. It can be noted that this type of implementation has to media types. It can be noted that this type of implementation has to
understand all types of RTP/RTCP extension being used in the RTP understand all types of RTP/RTCP extension being used in the RTP
sessions to correctly be able to translate them between the RTP sessions to correctly be able to translate them between the RTP
sessions. It might also suffer issues due to differencies in
configured RTCP bandwidth and other parameters between the RTP
sessions. It can also negatively impact the possibility for loop sessions. It can also negatively impact the possibility for loop
detection, as SSRC/CSRC can't be used to detect the loops, instead detection, as SSRC/CSRC can't be used to detect the loops, instead
some other media stream identity name space that is common across all some other media stream identity name space that is common across all
interconnect parts are needed. interconnect parts are needed.
5.4. Reduced number of Payload Types 5.4. Reduced number of Payload Types
An RTP session with multiple media types in it have only a single 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 7-bit Payload Type range for all its payload types. Within the 128
available values, only 96 or less if "Multiplexing RTP Data and available values, only 96 or less if "Multiplexing RTP Data and
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If network level differentiation of the media streams of different If network level differentiation of the media streams of different
media types are desired using this specification can cause severe media types are desired using this specification can cause severe
limitations. All media streams in an RTP session, independent of the limitations. All media streams in an RTP session, independent of the
media type, will be sent over the same underlying transport flow. media type, will be sent over the same underlying transport flow.
Any flow-based Quality of Service (QoS) mechanism will be unable to Any flow-based Quality of Service (QoS) mechanism will be unable to
provide differentiated treatment between different media types, e.g. provide differentiated treatment between different media types, e.g.
to prioritize audio over video. If differentiated treatment is to prioritize audio over video. If differentiated treatment is
desired using flow-based QoS, separate RTP sessions over different desired using flow-based QoS, separate RTP sessions over different
underlying transport flows needs to be used. underlying transport flows needs to be used.
Any marking-based QoS scheme like DiffServ is not affected unless a Marking-based QoS scheme like DiffServ can be affected if network
network ingress marks based on flows, in which case the same ingress is the one that performs markings based on flows. Endpoint
considerations as for flow based QoS applies. 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-avtcore-multiplex-guidelines]
exist for how different traffic classes can be applied on a single
RTP media stream.
5.6. Non-compatible Extensions 5.6. Non-compatible Extensions
There exist some RTP and RTCP extensions that rely on the existence 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 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 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 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 for the media types but is willing to have multiple RTP
sessions, one for the main media transmission and one for the sessions, one for the main media transmission and one for the
extension, they can be used. It is to be noted that this assumes extension, they can be used. It is to be noted that this assumes
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each SSRC will be associated with a specific media type, communicated each SSRC will be associated with a specific media type, communicated
through the RTP payload type, allowing a middlebox to do media type through the RTP payload type, allowing a middlebox to do media type
specific operations. The second argument is that in many contexts specific operations. The second argument is that in many contexts
blind combining without additional contexts are anyway not suitable. blind combining without additional contexts are anyway not suitable.
Regarding bullet 5 this is a understood and explicitly stated Regarding bullet 5 this is a understood and explicitly stated
applicability limitations for the method described in this document. applicability limitations for the method described in this document.
6.2. Sender Source Restrictions 6.2. Sender Source Restrictions
A SSRC in the RTP session MUST only send one media type (audio, A SSRC in the RTP session MUST only send one media type (audio,
video, text etc.) during the SSRC's lifetime. The main motivation video, text etc.) during the SSRC's lifetime. The main motivation is
is that a given SSRC has its own RTP timestamp and sequence number that a given SSRC has its own RTP timestamp and sequence number
spaces. The same way that you can't send two streams of encoded spaces. The same way that you can't send two streams of encoded
audio on the same SSRC, you can't send one audio and one video audio on the same SSRC, you can't send one audio and one video
encoding on the same SSRC. Each media encoding when made into an RTP encoding on the same SSRC. Each media encoding when made into an RTP
stream needs to have the sole control over the sequence number and 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 timestamp space. If not, one would not be able to detect packet loss
for that particular stream. Nor can one easily determine which clock for that particular stream. Nor can one easily determine which clock
rate a particular SSRCs timestamp will increase with. For additional rate a particular SSRCs timestamp will increase with. For additional
arguments why RTP payload type based multiplexing of multiple media arguments why RTP payload type based multiplexing of multiple media
streams doesn't work see Appendix A in streams doesn't work see Appendix A in
[I-D.ietf-avtcore-multiplex-guidelines]. [I-D.ietf-avtcore-multiplex-guidelines].
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types. In fact this document (Section 7.2) suggest that for usage of types. In fact this document (Section 7.2) suggest that for usage of
Generic FEC (XOR-based) as defined in RFC 5109 can actually use a Generic FEC (XOR-based) as defined in RFC 5109 can actually use a
single media type when used with independent RTP sessions for source single media type when used with independent RTP sessions for source
and repair data. and repair data.
Note a particular SSRC carrying Generic FEC will clearly only Note a particular SSRC carrying Generic FEC will clearly only
protect a specific SSRC and thus that instance is bound to the protect a specific SSRC and thus that instance is bound to the
SSRC's media type. For this specific case, it is possible to have SSRC's media type. For this specific case, it is possible to have
one be applicable to both. However, in cases when the signalling one be applicable to both. However, in cases when the signalling
is setup to enable fall back to using separate RTP sessions, then is setup to enable fall back to using separate RTP sessions, then
using a different media type, e.g. application, than the media using a different media type, e.g. application, than the media
being protected can create issues. being protected can create issues.
6.4. RTCP Considerations 6.4. RTCP Considerations
Guidelines for handling RTCP when sending multiple media streams with Guidelines for handling RTCP when sending multiple media streams with
disparate rates in a single RTP session are outlined in disparate rates in a single RTP session are outlined in
[I-D.ietf-avtcore-rtp-multi-stream]. These guidelines apply when [I-D.ietf-avtcore-rtp-multi-stream]. These guidelines apply when
sending multiple types of media in a single RTP session if the sending multiple types of media in a single RTP session if the
different types of media have different rates. different types of media have different rates.
skipping to change at page 13, line 37 skipping to change at page 13, line 37
contradict both the FID semantics [RFC5888] and an assumption in the contradict both the FID semantics [RFC5888] and an assumption in the
RTP retransmission specification [RFC4588]. RTP retransmission specification [RFC4588].
7.2. Generic FEC 7.2. 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
repair session only needs one RTP Payload type indicating repair repair session only needs one RTP Payload type indicating repair
data, i.e. x/ulpfec or x/parityfec depending if RFC 5109 or RFC 2733 data, i.e. x/ulpfec or x/parityfec depending if RFC 5109 or RFC 2733
is used. The media type in this session is not relevant and can in is used. The media type in this session is not relevant and can in
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".
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 FEC repair data payload format, i.e. one for audio/ulpfec and one for
for text/ulpfec if audio and text are combined in an RTP session. 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 8. Signalling
The Signalling requirements The Signalling requirements
Establishing an RTP session with multiple media types requires Establishing an RTP session with multiple media types requires
skipping to change at page 15, line 34 skipping to change at page 15, line 37
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 12. References
12.1. Normative References 12.1. 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-01 (work in progress), draft-ietf-avtcore-rtp-multi-stream-02 (work in progress),
July 2013. January 2014.
[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,
"Multiplexing Negotiation Using Session Description "Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp- Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp-
bundle-negotiation-05 (work in progress), October 2013. bundle-negotiation-05 (work in progress), October 2013.
[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.
skipping to change at page 16, line 15 skipping to change at page 16, line 15
[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 12.2. 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-01 (work in progress), July 2013. multiplex-guidelines-02 (work in progress), January 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
(work in progress), February 2013. (work in progress), February 2013.
[I-D.westerlund-avtcore-transport-multiplexing] [I-D.westerlund-avtcore-transport-multiplexing]
Westerlund, M. and C. Perkins, "Multiplexing Multiple RTP Westerlund, M. and C. Perkins, "Multiplexing Multiple RTP
Sessions onto a Single Lower-Layer Transport", draft- Sessions onto a Single Lower-Layer Transport", draft-
skipping to change at page 16, line 41 skipping to change at page 16, line 41
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997. September 1997.
[RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
for Generic Forward Error Correction", RFC 2733, December for Generic Forward Error Correction", RFC 2733, December
1999. 1999.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. 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.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588, Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006. July 2006.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error [RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007. Correction", RFC 5109, December 2007.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008. January 2008.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009. (SDP)", RFC 5576, June 2009.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010. Control Packets on a Single Port", RFC 5761, April 2010.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010. Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
 End of changes. 28 change blocks. 
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