draft-ietf-avtcore-multi-media-rtp-session-06.txt   draft-ietf-avtcore-multi-media-rtp-session-07.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: April 11, 2015 J. Lennox Expires: September 10, 2015 J. Lennox
Vidyo Vidyo
October 08, 2014 March 9, 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-06 draft-ietf-avtcore-multi-media-rtp-session-07
Abstract Abstract
This document specifies how an RTP session can contain media 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.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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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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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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 April 11, 2015. This Internet-Draft will expire on September 10, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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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
4. Overview of Solution . . . . . . . . . . . . . . . . . . . . 5 4. Overview of Solution . . . . . . . . . . . . . . . . . . . . 5
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . 12 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 . . . . . . . . . . . . . . . . . . 13 7. Extension Considerations . . . . . . . . . . . . . . . . . . 12
7.1. RTP Retransmission . . . . . . . . . . . . . . . . . . . 13 7.1. RTP Retransmission . . . . . . . . . . . . . . . . . . . 13
7.2. Generic FEC . . . . . . . . . . . . . . . . . . . . . . . 13 7.2. Generic FEC . . . . . . . . . . . . . . . . . . . . . . . 14
8. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 14 8. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. SDP-Based Signalling . . . . . . . . . . . . . . . . . . 15 8.1. SDP-Based Signalling . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 15 12.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . 16 12.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
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 different to those deployed close to 20 years ago, IP networks were different to those deployed
at the time of this writing. The virtually ubiquitous deployment of at the time of this writing. The virtually ubiquitous deployment of
Network Address Translators (NAT) and Firewalls has since increased Network Address Translators (NAT) and Firewalls has since increased
the cost and likely-hood of communication failure when using many the cost and likely-hood of communication failure when using many
different transport flows. Hence, there is pressure to reduce the different transport flows. Hence, there is pressure to reduce the
number of concurrent transport flows used by RTP applications. number of concurrent transport flows used by RTP applications.
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sessions, and has enforced this rule by not allowing multiple media sessions, and has enforced this rule by not allowing multiple media
types for a given destination or set of ICE candidates. types for a given destination or set of ICE candidates.
The fact that these limitations have been in place for so long, in The fact that these limitations have been in place for so long, in
addition to RFC 3550 being written without fully considering the use addition to RFC 3550 being written without fully considering the use
of multiple media types in an RTP session, results in a number of of multiple media types in an RTP session, results in a number of
issues when allowing this behaviour. This memo updates [RFC3550] and issues when allowing this behaviour. This memo updates [RFC3550] and
[RFC3551] with important considerations regarding applicability and [RFC3551] with important considerations regarding applicability and
functionality when using multiple types of media in an RTP session, functionality when using multiple types of media in an RTP session,
including normative specification of behaviour. This memo makes no including normative specification of behaviour. This memo makes no
changes to RTP behaviour when using multiple streams of media of the changes to RTP behaviour when using multiple RTP streams with media
same type (e.g., multiple audio streams or multiple video streams) in of the same type (e.g., multiple audio streams or multiple video
a single RTP session. 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 This memo is structured as follows. First, some basic definitions
are provided. This is followed by a background that discusses the are provided. This is followed by a background that discusses the
motivation in more detail. A overview of the solution of how to motivation in more detail. A overview of the solution of how to
provide multiple media types in one RTP session is then presented. provide multiple media types in one RTP session is then presented.
Next is the formal applicability this specification have followed by Next is the formal applicability this specification have followed by
the normative specification. This is followed by a discussion how the normative specification. This is followed by a discussion how
some RTP/RTCP Extensions are expected to function in the case of some RTP/RTCP Extensions are expected to function in the case of
multiple media types in one RTP session. A specification of the multiple media types in one RTP session. A specification of the
requirements on signalling from this specification and a look how requirements on signalling from this specification and a look how
this is realized in SDP using Bundle this is realized in SDP using Bundle
[I-D.ietf-mmusic-sdp-bundle-negotiation]. The memo ends with the [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: Media Type: The general type of media data used by a real-time
application. The media type corresponds to the value used in the
Endpoint: A single entity sending or receiving RTP packets. It can <media> field of an SDP m= line. The media types defined at the
be decomposed into several functional blocks, but as long as it time of this writing are "audio", "video", "text", "application",
behaves as a single RTP stack entity it is classified as a single and "message".
endpoint.
Media Stream: A sequence of RTP packets using a single SSRC that
together carries part or all of the content of a specific Media
Type from a specific sender source within a given RTP session.
Media Type: Audio, video, text or application whose form and meaning
are defined by a specific real-time application.
QoS: Quality of Service, i.e. network mechanisms that 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.
RTP Session: As defined by [RFC3550], the endpoints belonging to the The terms Encoded Stream, Endpoint, Media Source, RTP Session, and
same RTP Session are those that share a single SSRC space. That RTP Stream are used as defined in
is, those endpoints can see an SSRC identifier transmitted by any [I-D.ietf-avtext-rtp-grouping-taxonomy].
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
Session scope is decided by the endpoints' network interconnection
topology, in combination with RTP and RTCP forwarding strategies
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
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
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(i.e., it can switch between different audio codecs, since those are (i.e., it can switch between different audio codecs, since those are
both the same type of media, but cannot switch between audio and both the same type of media, but cannot switch between audio and
video). Different SSRCs MUST be used for the different media video). Different SSRCs MUST be used for the different media
sources, the same way multiple media sources of the same media type sources, the same way multiple media sources of the same media type
already have to do. The payload type will inform a receiver which 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 media type the SSRC is being used for. Thus the payload type MUST be
unique across all of the payload configurations independent of media unique across all of the payload configurations independent of media
type that is used in the RTP session. type that is used in the RTP session.
Some few extra considerations within the RTP sessions also needs to Some few extra considerations within the RTP sessions also needs to
be considered. RTCP bandwidth and regular reporting suppression (RTP be considered. RTCP bandwidth and regular reporting suppression
/AVPF and RTP/SAVPF) SHOULD be configured to reduce the impact for (RTP/AVPF and RTP/SAVPF) SHOULD be configured to reduce the impact
bit-rate variations between streams and media types. It is also for bit-rate variations between RTP streams and media types. It is
clarified how timeout calculations are to be done to avoid any also clarified how timeout calculations are to be done to avoid any
issues. Certain payload types like FEC also need additional rules. issues. Certain payload types like FEC also need additional rules.
The final important part of the solution to this is to use signalling The final important part of the solution to this is to use signalling
and ensure that agreement on using multiple media types in an RTP and ensure that agreement on using multiple media types in an RTP
session exists, and how that then is configured. This memo describes session exists, and how that then is configured. This memo describes
some existing requirements, while an external reference defines how some existing requirements, while an external reference defines how
this is accomplished in SDP. this is accomplished in SDP.
5. Applicability 5. Applicability
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Before choosing to use this specification, an application implementer Before choosing to use this specification, an application implementer
needs to ensure that they don't have a need for different RTP 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 sessions between the media types for some reason. The main rule is
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.
prefer multiple RTP sessions, but do not require the functionalities
or behaviours that multiple transport flows give, can consider using There is some work in progress
Multiple RTP Sessions on a Single Lower-Layer Transport [I-D.westerlund-avtcore-transport-multiplexing] that attempt to
[I-D.westerlund-avtcore-transport-multiplexing]. 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 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 RTCP requirements. There are limitations in the RTCP timing similar RTCP requirements. There are limitations in the RTCP timing
rules, 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 RTCP requirements, for example due to media streams very different RTCP requirements, for example due to RTP Streams
bandwidth consumption and packet rate, for example low-rate audio bandwidth consumption and packet rate, for example low-rate audio
coupled with high-quality video, this can result in either excessive coupled with high-quality video, this can result in either excessive
or insufficient RTCP for some flows, depending how the RTCP session 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
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in one RTP session can be a way of attempting to ensure that all in one RTP session can be a way of attempting to ensure that all
participants in the RTP session follow the requirement. However, for participants in the RTP session follow the requirement. However, for
signalling solutions that lack methods for enforcing that a receiver signalling solutions that lack methods for enforcing that a receiver
supports a specific feature, this can still cause issues. supports a specific feature, this can still cause issues.
5.3. Homogeneous Multi-party 5.3. Homogeneous Multi-party
In multiparty communication scenarios it is important to separate two In multiparty communication scenarios it is important to separate two
different cases. One case is where the RTP session contains multiple different cases. One case is where the RTP session contains multiple
participants in a common RTP session. This occurs for example in Any participants in a common RTP session. This occurs for example in Any
Source Multicast (ASM) and Transport Translator topologies as defined Source Multicast (ASM) and Relay (Transport Translator) topologies as
in RTP Topologies [RFC5117]. It can also occur in some defined in RTP Topologies [I-D.ietf-avtcore-rtp-topologies-update].
implementations of RTP mixers that share the same SSRC/CSRC space It can also occur in some implementations of RTP mixers that share
across all participants. The second case is when the RTP session is the same SSRC/CSRC space across all participants. The second case is
terminated in a middlebox and the other participants sources are when the RTP session is terminated in a middlebox and the other
projected or switched into each RTP session and rewritten on RTP participants sources are projected or switched into each RTP session
header level including SSRC mappings. and rewritten on RTP header level including SSRC mappings.
For the first case, with a common RTP session or at least shared SSRC For the first case, with a common RTP session or at least shared
/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 media
types. The reason is that in case of multiple RTP sessions, the same types. The reason is that in case of multiple RTP sessions, the same
SSRC value can be use in both RTP sessions without any issues, but SSRC value can be use in both RTP sessions without any issues, but
when collapsed to a single session there is an SSRC collision. In when collapsed to a single session there is an SSRC collision. In
addition some collisions can't be represented in the multiple addition some collisions can't be represented in the multiple
separate RTP sessions. For example, in a session with audio and separate RTP sessions. For example, in a session with audio and
video, an SSRC value used for video will not show up in the Audio RTP video, an SSRC value used for video will not show up in the Audio RTP
session at the participant using multiple RTP sessions, and thus not session at the participant using multiple RTP sessions, and thus not
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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 sessions. It might also suffer issues due to differencies in
configured RTCP bandwidth and other parameters between the RTP 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 RTP stream or media source identity name space that is
interconnect parts are needed. common across all 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
Control Packets on a Single Port" [RFC5761] is used, all the Control Packets on a Single Port" [RFC5761] is used, all the
different RTP payload configurations for all the media types need to different RTP payload configurations for all the media types need to
fit in the available space. For most applications this will not be a fit in the available space. For most applications this will not be a
real problem, but the limitation exists and could be encountered. real problem, but the limitation exists and could be encountered.
5.5. Stream Differentiation 5.5. Stream Differentiation
If network level differentiation of the media streams of different If network level differentiation of the RTP streams with different
media types are desired using this specification can cause severe media types is desired, using this specification can cause severe
limitations. All media streams in an RTP session, independent of the limitations. All RTP 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.
Marking-based QoS scheme like DiffServ can be affected if network Marking-based QoS schemes like DiffServ can be affected if a network
ingress is the one that performs markings based on flows. Endpoint ingress is the one that performs, markings based on flows. Endpoint
marking where the network API supports marking on individual packet marking where the network API supports marking on individual packet
level will be unaffected by this specification. However, there exist level will be unaffected by this specification. However, there exist
limitations as discussed in [I-D.ietf-avtcore-multiplex-guidelines] limitations, as discussed in [I-D.ietf-dart-dscp-rtp], on how
exist for how different traffic classes can be applied on a single different traffic classes can be applied on different packets or RTP
RTP media stream. streams within a single transport flow.
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
that it is possible to get the extension working when the related RTP that it is possible to get the extension working when the related RTP
session contains multiple media types. session contains multiple media types.
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XOR-Based FEC: The RTP Payload Format for Generic Forward Error XOR-Based FEC: The RTP Payload Format for Generic Forward Error
Correction [RFC5109] and its predecessor [RFC2733] requires a Correction [RFC5109] and its predecessor [RFC2733] requires a
separate RTP session unless the FEC data is carried in RTP Payload separate RTP session unless the FEC data is carried in RTP Payload
for Redundant Audio Data [RFC2198]. However, using the Generic for Redundant Audio Data [RFC2198]. However, using the Generic
FEC with the Redundancy payload has another set of restrictions, FEC with the Redundancy payload has another set of restrictions,
see Section 7.2. see Section 7.2.
Note that the Source-Specific Media Attributes [RFC5576] Note that the Source-Specific Media Attributes [RFC5576]
specification defines an SDP syntax (the "FEC" semantic of the specification defines an SDP syntax (the "FEC" semantic of the
"ssrc-group" attribute) to signal FEC relationships between "ssrc-group" attribute) to signal FEC relationships between
multiple media streams within a single RTP session. However, this 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 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 value as the source packets being protected. [RFC5576] does not
normatively update and resolve that restriction. There is ongoing normatively update and resolve that restriction. There is ongoing
work on an ULP extension to allow it be use FEC streams within the work on an ULP extension to allow it be use FEC RTP streams within
same RTP Session as the source stream the same RTP Session as the source stream
[I-D.lennox-payload-ulp-ssrc-mux]. [I-D.lennox-payload-ulp-ssrc-mux].
6. RTP Session Specification 6. RTP Session Specification
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 6.1. 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"
skipping to change at page 10, line 24 skipping to change at page 10, line 7
sentence is changed to: sentence is changed to:
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 streams SHOULD NOT be carried in a single Separate audio and video media sources SHOULD NOT be carried in a
RTP session and demultiplexed based on the payload type or SSRC single RTP session and demultiplexed based on the payload type or
fields, unless multiplexed based on both SSRC and payload type and SSRC fields, unless multiplexed based on both SSRC and payload
usage meets what Multiple Media Types in an RTP Session [RFCXXXX] type and usage meets what Multiple Media Types in an RTP Session
specifies. [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
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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 encoded streams of
audio on the same SSRC, you can't send one audio and one video audio on the same SSRC, you can't send one encoded audio and one
encoding on the same SSRC. Each media encoding when made into an RTP encoded video stream on the same SSRC. Each encoded stream when made
stream needs to have the sole control over the sequence number and into an RTP stream needs to have the sole control over the sequence
timestamp space. If not, one would not be able to detect packet loss number and timestamp space. If not, one would not be able to detect
for that particular stream. Nor can one easily determine which clock packet loss for that particular encoded stream. Nor can one easily
rate a particular SSRCs timestamp will increase with. For additional determine which clock rate a particular SSRCs timestamp will increase
arguments why RTP payload type based multiplexing of multiple media with. For additional arguments why RTP payload type based
streams doesn't work see Appendix A in multiplexing of multiple media sources doesn't work see
[I-D.ietf-avtcore-multiplex-guidelines]. [I-D.ietf-avtcore-multiplex-guidelines].
6.3. Payload Type Applicability 6.3. Payload Type Applicability
Most Payload Types have a native media type, like an audio codec is Most Payload Types have a native media type, like an audio codec is
natural belonging to the audio media type. However, there exist a natural belonging to the audio media type. However, there exist a
number of RTP payload types that don't have a native media type. For number of RTP payload types that don't have a native media type. For
example, transport robustness mechanisms like RTP Retransmission example, transport robustness mechanisms like RTP Retransmission
[RFC4588] and Generic FEC [RFC5109] inherit their media type from [RFC4588] and Generic FEC [RFC5109] inherit their media type from
what they protect. RTP Retransmission is explicitly bound to the what they protect. RTP Retransmission is explicitly bound to the
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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 RTP 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.
7. Extension Considerations 7. Extension Considerations
This section discusses the impact on some RTP/RTCP extensions due to This section discusses the impact on some RTP/RTCP extensions due to
usage of multiple media types in on RTP session. Only extensions usage of multiple media types in on RTP session. Only extensions
where something worth noting has been included. where something worth noting has been included.
7.1. RTP Retransmission 7.1. RTP Retransmission
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 that 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
associated payload types in the source RTP session. The only associated payload types in the source RTP session. The only
difference to previously is that the source RTP session is one which difference to the previous case is if the source RTP session is one
contains multiple media types. Thus it is even more likely that only which contains multiple media types. This results in the
a subset of the source RTP session's payload types and SSRCs are retransmission streams in the RTP session for the retransmission
actually retransmitted. having multiple associated media types.
Open Issue: When using SDP to signal retransmission for one RTP When using SDP signalling for a multiple media type RTP session, i.e.
session with multiple media types and one RTP session for the BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], the session
retransmission data will cause a situation where one will have multiplexed case do require some recommendations on how to signal
multiple m= lines grouped using FID and the ones belonging to this. To avoid breaking the semantics of the FID grouping as defined
respective RTP session being grouped using BUNDLE. This usage might by [RFC5888] each media line can only be included in one FID group.
contradict both the FID semantics [RFC5888] and an assumption in the FID is used by RTP retransmission to indicate the SDP media lines
RTP retransmission specification [RFC4588]. that is a source and retransmission pair. Thus, for SDP using
BUNDLE, each original media source (m= line) that is retransmitted
needs a corresponding media line in the retransmission RTP session.
In case there are multiple media lines for retransmission, these
media lines will form a independent BUNDLE group from the BUNDLE
group with the source streams.
Below is an SDP example (Figure 1) which shows the grouping
structures. This example is not legal SDP and only the most
important attributes has been left in place. Note that this SDP is
not an initial BUNDLE offer. As can be seen there are two bundle
groups, one for the source RTP session and one for the
retransmissions. Then each of the media sources are grouped with its
retransmission flow using FID, resulting in three more groupings.
a=group:BUNDLE foo bar fiz
a=group:BUNDLE zoo kelp glo
a=group:FID foo zoo
a=group:FID bar kelp
a=group:FID fiz glo
m=audio 10000 RTP/AVP 0
a=mid:foo
a=rtpmap:0 PCMU/8000
m=video 10000 RTP/AVP 31
a=mid:bar
a=rtpmap:31 H261/90000
m=video 10000 RTP/AVP 31
a=mid:fiz
a=rtpmap:31 H261/90000
m=audio 40000 RTP/AVPF 99
a=rtpmap:99 rtx/90000
a=fmtp:99 apt=0;rtx-time=3000
a=mid:zoo
m=video 40000 RTP/AVPF 100
a=rtpmap:100 rtx/90000
a=fmtp:199 apt=31;rtx-time=3000
a=mid:kelp
m=video 40000 RTP/AVPF 100
a=rtpmap:100 rtx/90000
a=fmtp:199 apt=31;rtx-time=3000
a=mid:glo
Figure 1: SDP example of Session Multiplexed RTP Retransmission
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".
If one uses SDP signalling with BUNDLE
[I-D.ietf-mmusic-sdp-bundle-negotiation], then the RTP session
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 will be grouped using media line grouping using the FEC or FEC-
FR [RFC5956] grouping. This is very similar to the situation that
arise for RTP retransmission with session multiplexing discussed
above inSection 7.1.
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 51 skipping to change at page 16, line 51
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-05 (work in progress), draft-ietf-avtcore-rtp-multi-stream-06 (work in progress),
July 2014. October 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,
"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-11 (work in progress), September 2014. negotiation-17 (work in progress), March 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 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-02 (work in progress), January 2014. multiplex-guidelines-03 (work in progress), October 2014.
[I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-06 (work in progress),
March 2015.
[I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
"A Taxonomy of Grouping Semantics and Mechanisms for Real-
Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
rtp-grouping-taxonomy-06 (work in progress), March 2015.
[I-D.ietf-dart-dscp-rtp]
Black, D. and P. Jones, "Differentiated Services
(DiffServ) and Real-time Communication", draft-ietf-dart-
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
(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 17, line 12 skipping to change at page 18, line 30
[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.
[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,
January 2008.
[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.
Authors' Addresses [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956, September
2010.
Authors' Addresses
Magnus Westerlund Magnus Westerlund
Ericsson Ericsson
Farogatan 6 Farogatan 6
SE-164 80 Kista SE-164 80 Kista
Sweden Sweden
Phone: +46 10 714 82 87 Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com Email: magnus.westerlund@ericsson.com
Colin Perkins Colin Perkins
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