draft-ietf-avtcore-multi-media-rtp-session-08.txt   draft-ietf-avtcore-multi-media-rtp-session-09.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: January 08, 2016 J. Lennox Expires: January 21, 2016 J. Lennox
Vidyo Vidyo
July 07, 2015 July 20, 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-08 draft-ietf-avtcore-multi-media-rtp-session-09
Abstract Abstract
This document specifies how an RTP session can contain RTP Streams This document specifies how an RTP session can contain RTP Streams
with media from multiple media types such as audio, video, and text. with media from multiple media types such as audio, video, and text.
This has been restricted by the RTP Specification, and thus this This has been restricted by the RTP Specification, and thus this
document updates RFC 3550 and RFC 3551 to enable this behaviour for document updates RFC 3550 and RFC 3551 to enable this behaviour for
applications that satisfy the applicability for using multiple media applications that satisfy the applicability for using multiple media
types in a single RTP session. types in a single RTP session.
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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 January 08, 2016. This Internet-Draft will expire on January 21, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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publication of this document. Please review these documents publication of this document. Please review these documents
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background and Motivation . . . . . . . . . . . . . . . . . . 3 3. Background and Motivation . . . . . . . . . . . . . . . . . . 3
4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Using Multiple Media Types in a Single RTP Session . . . . . 7 5. Using Multiple Media Types in a Single RTP Session . . . . . 6
5.1. Allowing Multiple Media Types in an RTP Session . . . . . 7 5.1. Allowing Multiple Media Types in an RTP Session . . . . . 6
5.2. Demultiplexing Media Streams . . . . . . . . . . . . . . 8 5.2. Demultiplexing media types within an RTP session . . . . 7
5.3. Per-SSRC Media Type Restrictions . . . . . . . . . . . . 8 5.3. Per-SSRC Media Type Restrictions . . . . . . . . . . . . 8
5.4. RTCP Considerations . . . . . . . . . . . . . . . . . . . 9 5.4. RTCP Considerations . . . . . . . . . . . . . . . . . . . 8
6. Extension Considerations . . . . . . . . . . . . . . . . . . 9 6. Extension Considerations . . . . . . . . . . . . . . . . . . 8
6.1. RTP Retransmission Payload Format . . . . . . . . . . . . 9 6.1. RTP Retransmission Payload Format . . . . . . . . . . . . 9
6.2. RTP Payload Format for Generic FEC . . . . . . . . . . . 11 6.2. RTP Payload Format for Generic FEC . . . . . . . . . . . 10
6.3. RTP Payload Format for Redundant Audio . . . . . . . . . 11 6.3. RTP Payload Format for Redundant Audio . . . . . . . . . 11
7. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13 11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 14 11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction 1. Introduction
The Real-time Transport Protocol [RFC3550] was designed to use The Real-time Transport Protocol [RFC3550] was designed to use
separate RTP sessions to transport different types of media. This separate RTP sessions to transport different types of media. This
implies that different transport layer flows are used for different implies that different transport layer flows are used for different
media streams. For example, a video conferencing application might media streams. For example, a video conferencing application might
send audio and video traffic RTP flows on separate UDP ports. With send audio and video traffic RTP flows on separate UDP ports. With
increased use of network address/port translation, firewalls, and increased use of network address/port translation, firewalls, and
other middleboxes it is, however, becoming difficult to establish other middleboxes it is, however, becoming difficult to establish
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reducing the number of transport layer flows that are needed. It reducing the number of transport layer flows that are needed. It
makes no changes to RTP behaviour when using multiple RTP streams makes no changes to RTP behaviour when using multiple RTP streams
containing media of the same type (e.g., multiple audio streams or containing media of the same type (e.g., multiple audio streams or
multiple video streams) in a single RTP session, however multiple video streams) in a single RTP session, however
[I-D.ietf-avtcore-rtp-multi-stream] provides important clarifications [I-D.ietf-avtcore-rtp-multi-stream] provides important clarifications
to RTP behaviour in that case. to RTP behaviour in that case.
This memo is structured as follows. Section 2 defines terminology. This memo is structured as follows. Section 2 defines terminology.
Section 3 further describes the background to, and motivation for, Section 3 further describes the background to, and motivation for,
this memo and Section 4 describes the scenarios where this memo is this memo and Section 4 describes the scenarios where this memo is
applicable. (tbd: fixme) applicable. Section 5 discusses issues arising from the base RTP and
RTCP specification when using multiple types of media in a single RTP
session, while Section 6 considers the impact of RTP extensions. We
discuss signalling in Section 7. Finally, security considerations
are discussed in Section 8.
2. Terminology 2. Terminology
The terms Encoded Stream, Endpoint, Media Source, RTP Session, and The terms Encoded Stream, Endpoint, Media Source, RTP Session, and
RTP Stream are used as defined in RTP Stream are used as defined in
[I-D.ietf-avtext-rtp-grouping-taxonomy]. We also define the [I-D.ietf-avtext-rtp-grouping-taxonomy]. We also define the
following terms: following terms:
Media Type: The general type of media data used by a real-time Media Type: The general type of media data used by a real-time
application. The media type corresponds to the value used in the application. The media type corresponds to the value used in the
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RTP was designed to support multimedia sessions, containing multiple RTP was designed to support multimedia sessions, containing multiple
types of media sent simultaneously, by using multiple transport layer types of media sent simultaneously, by using multiple transport layer
flows. The existence of network address translators, firewalls, and flows. The existence of network address translators, firewalls, and
other middleboxes complicates this, however, since a mechanism is other middleboxes complicates this, however, since a mechanism is
needed to ensure that all the transport layer flows needed by the needed to ensure that all the transport layer flows needed by the
application can be established. This has three consequences: application can be established. This has three consequences:
1. increased delay to establish a complete session, since each of 1. increased delay to establish a complete session, since each of
the transport layer flows needs to be negotiated and established; the transport layer flows needs to be negotiated and established;
2. increased state and resource consumption in the middleboxes, that 2. increased state and resource consumption in the middleboxes that
can lead to unexpected behaviour when middlebox resource limits can lead to unexpected behaviour when middlebox resource limits
are reached; and are reached; and
3. increased risk that a subset of the transport layer flows will 3. increased risk that a subset of the transport layer flows will
fail to be established, thus preventing the application from fail to be established, thus preventing the application from
communicating. communicating.
Using fewer transport layer flows can hence be seen to reduce the Using fewer transport layer flows can hence be seen to reduce the
risk of communication failure, and can lead to improved reliability risk of communication failure, and can lead to improved reliability
and performance. and performance.
One of the benefits of using multiple transport layer flows is that One of the benefits of using multiple transport layer flows is that
it makes it easy to use network layer quality of service (QoS) it makes it easy to use network layer quality of service (QoS)
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One of the benefits of using multiple transport layer flows is that One of the benefits of using multiple transport layer flows is that
it makes it easy to use network layer quality of service (QoS) it makes it easy to use network layer quality of service (QoS)
mechanisms to give differentiated performance for different flows. mechanisms to give differentiated performance for different flows.
However, we note that many RTP-using application don't use network However, we note that many RTP-using application don't use network
QoS features, and don't expect or desire any separation in network QoS features, and don't expect or desire any separation in network
treatment of their media packets, independent of whether they are treatment of their media packets, independent of whether they are
audio, video or text. When an application has no such desire, it audio, video or text. When an application has no such desire, it
doesn't need to provide a transport flow structure that simplifies doesn't need to provide a transport flow structure that simplifies
flow based QoS. flow based QoS.
Given this, it might seem desirable for RTP-based applications to Given the above issues, it might seem appropriate for RTP-based
send all their media streams bundled into one RTP session, that runs applications to send all their media streams bundled into one RTP
on a single transport layer flow. Unfortunately, this is prohibited session, running over a single transport layer flow. However, this
by the RTP specification, since RTP makes certain assumptions that is prohibited by the RTP specification, because the design of RTP
can be incompatible with sending multiple media types in a single RTP makes certain assumptions that can be incompatible with sending
session. Specifically, the RTP control protocol (RTCP) timing rules multiple media types in a single RTP session. Specifically, the RTP
assume that all RTP media flows in a single RTP session have broadly control protocol (RTCP) timing rules assume that all RTP media flows
similar RTCP reporting and feedback requirements, which can be in a single RTP session have broadly similar RTCP reporting and
problematic when different types of media are multiplexed together. feedback requirements, which can be problematic when different types
Certain RTP extensions also make assumptions that are incompatible of media are multiplexed together. Various RTP extensions also make
assumptions about SSRC use and RTCP reporting that are incompatible
with sending different media types in a single RTP session. with sending different media types in a single RTP session.
This memo updates [RFC3550] and [RFC3551] to allow RTP sessions to This memo updates [RFC3550] and [RFC3551] to allow RTP sessions to
contain more than just one media type, and gives guidance on when it contain more than one media type in certain circumstances, and gives
is safe to perform such multiplexing. guidance on when it is safe to send multiple media types in a single
RTP session.
4. Applicability 4. Applicability
This specification has limited applicability, and anyone intending to This specification has limited applicability, and anyone intending to
use it MUST ensure that their application and use meets the following use it MUST ensure that their application and use meets the following
criteria: criteria:
Equal treatment of media: The use of a single RTP session enforces Equal treatment of media: The use of a single RTP session enforces
similar treatment on all types of media used within the session. similar treatment on all types of media used within the session.
Applications that require significantly different network QoS or Applications that require significantly different network QoS or
RTCP configuration for different media streams are better suited RTCP configuration for different media streams are better suited
by sending those media streams on separate RTP session, using by sending those media streams on separate RTP session, using
separate transport layer flows for each, since that gives greater separate transport layer flows for each, since that gives greater
flexibility. Further guidance is given in flexibility. Further guidance is given in
[I-D.ietf-avtcore-multiplex-guidelines] and [I-D.ietf-avtcore-multiplex-guidelines] and
[I-D.ietf-dart-dscp-rtp]. [I-D.ietf-dart-dscp-rtp].
Compatible Media Requirements: The RTCP timing rules enforce a Compatible RTCP Behaviour: The RTCP timing rules enforce a single
single RTCP reporting interval for all participants in an RTP RTCP reporting interval for all participants in an RTP session.
session. Flows with very different media requirements, for Flows with very different media sending rate or RTCP feedback
example a low-rate audio flow with no feedback needs and a high- requirements cannot be multiplexed together, since this leads to
quality video flow with different repair mechanisms, cannot be either excessive or insufficient RTCP for some flows, depending
multiplexed together since this results in either excessive or how the RTCP session bandwidth, and hence reporting interval, is
insufficient RTCP for some flows, depending how the RTCP session configured. For example, it is likely not feasible to find a
bandwidth, and hence reporting interval, is configured. single RTCP configuration that simultaneously suits both a low-
rate audio flow with no feedback and a high-quality video flow
with sophisticated RTCP-based feedback needs, making it difficult
to combine these into a single RTP session.
Signalled Support: The extensions defined in this memo are not Signalled Support: The extensions defined in this memo are not
compatible with unmodified [RFC3550]-compatible endpoints. Their compatible with unmodified [RFC3550]-compatible endpoints. Their
use requires signalling and mutual agreement by all participants use requires signalling and mutual agreement by all participants
within an RTP session. This requirement can be a problem for within an RTP session. This requirement can be a problem for
signalling solutions that can't negotiate with all participants. signalling solutions that can't negotiate with all participants.
For declarative signalling solutions, mandating that the session For declarative signalling solutions, mandating that the session
is using multiple media types in one RTP session can be a way of is using multiple media types in one RTP session can be a way of
attempting to ensure that all participants in the RTP session attempting to ensure that all participants in the RTP session
follow the requirement. However, for signalling solutions that follow the requirement. However, for signalling solutions that
lack methods for enforcing that a receiver supports a specific lack methods for enforcing that a receiver supports a specific
feature, this can still cause issues. feature, this can still cause issues.
Consistent support for multiple media types in a single RTP session: Consistent support for multiparty RTP sessions: If it is desired to
In multiparty communication scenarios it is important to separate send multiple types of media in a multiparty RTP session, then all
two different cases. One case is where the RTP session contains participants in that session need to support sending multiple type
multiple participants in a common RTP session. This occurs for of media in a single RTP session. It is not possible, in the
example in Any Source Multicast (ASM) and Relay (Transport general case, to implement a gateway that can interconnect an
Translator) topologies as defined in RTP Topologies endpoint using multiple types of media sent using separate RTP
[I-D.ietf-avtcore-rtp-topologies-update]. It can also occur in sessions, with one or more endpoints that send multiple types of
some implementations of RTP mixers that share the same SSRC/CSRC media in a single RTP session.
space across all participants. The second case is when the RTP
session is terminated in a middlebox and the other participants
sources are projected or switched into each RTP session and
rewritten on RTP header level including SSRC mappings.
For the first case, with a common RTP session or at least shared One reason for this is that the same SSRC value can safely be used
SSRC/CSRC values, all participants in multiparty communication are for different streams in multiple RTP sessions, but when collapsed
REQUIRED to support multiple media types in an RTP session. An to a single RTP session there is an SSRC collision. This would
participant using two or more RTP sessions towards a multiparty not be an issue, since SSRC collision detection will resolve the
session can't be collapsed into a single session with multiple conflict, except that some RTP payload formats and extensions use
media types. The reason is that in case of multiple RTP sessions, matching SSRCs to identify related flows, and break when a single
the same SSRC value can be use in both RTP sessions without any RTP session is used.
issues, but when collapsed to a single session there is an SSRC
collision. In addition some collisions can't be represented in
the multiple 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 session at the participant using multiple RTP
sessions, and thus not trigger any collision handling. Thus any
application using this type of RTP session structure MUST have a
homogeneous support for multiple media types in one RTP session,
or be forced to insert a translator node between that participant
and the rest of the RTP session.
For the second case of separate RTP sessions for each multiparty A middlebox that remaps SSRC values when combining multiple RTP
participant and a central node it is possible to have a mix of sessions into one also needs to be aware of all possible RTCP
single RTP session users and multiple RTP session users as long as packet types that might be used, so that it can remap the SSRC
one is willing to remap the SSRCs used by a participant with values in those packets. This is impossible to do without
multiple RTP sessions into non-used values in the single RTP restricting the set of RTCP packet types that can be used to those
session SSRC space for each of the participants using a single RTP that are known by the middlebox. Such a middlebox might also have
session with multiple media types. It can be noted that this type difficulty due to differences in configured RTCP bandwidth and
of implementation has to understand all types of RTP/RTCP other parameters between the RTP sessions.
extension being used in the RTP sessions to correctly be able to
translate them between the RTP sessions. It might also suffer Finally, the use of a middlebox that translates SSRC values can
issues due to differencies in configured RTCP bandwidth and other negatively impact the possibility for loop detection, as SSRC/CSRC
parameters between the RTP sessions. It can also negatively can't be used to detect the loops, instead some other RTP stream
impact the possibility for loop detection, as SSRC/CSRC can't be or media source identity name space that is common across all
used to detect the loops, instead some other RTP stream or media interconnect parts are needed.
source identity name space that is common across all interconnect
parts are needed.
Ability to operate with limited payload type space: An RTP session Ability to operate with limited payload type space: An RTP session
has only a single 7-bit payload type space for all its payload has only a single 7-bit payload type space for all its payload
type numbers. Some applications might find this space limiting type numbers. Some applications might find this space limiting
when media different media types and RTP payload formats are using when media different media types and RTP payload formats are using
within a single RTP session. within a single RTP session.
Avoids incompatible Extensions: Some RTP and RTCP extensions rely on Avoids incompatible Extensions: Some RTP and RTCP extensions rely on
the existence of multiple RTP sessions and relate media streams the existence of multiple RTP sessions and relate media streams
between sessions. Others report on particular media types, and between sessions. Others report on particular media types, and
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source MAY change payload types within the same media type during source MAY change payload types within the same media type during
a session. See the section "Multiplexing RTP Sessions" of RFC a session. See the section "Multiplexing RTP Sessions" of RFC
3550 for additional explanation. 3550 for additional explanation.
This specifications purpose is to violate that existing SHALL NOT This specifications purpose is to violate that existing SHALL NOT
under certain conditions. Thus this sentence also has to be changed under certain conditions. Thus this sentence also has to be changed
to allow for multiple media type's payload types in the same session. to allow for multiple media type's payload types in the same session.
The above sentence is changed to: The above sentence is changed to:
Payload types of different media types SHALL NOT be interleaved or Payload types of different media types SHALL NOT be interleaved or
multiplexed within a single RTP session unless as specified and multiplexed within a single RTP session unless [RFCXXXX] is used,
under the restriction in Multiple Media Types in an RTP Session and the application conforms to the applicability constraints.
[RFCXXXX]. Multiple RTP sessions MAY be used in parallel to send Multiple RTP sessions MAY be used in parallel to send multiple
multiple media types. media types.
RFC-Editor Note: Please replace RFCXXXX with the RFC number of this RFC-Editor Note: Please replace RFCXXXX with the RFC number of this
specification when assigned. specification when assigned.
5.2. Demultiplexing Media Streams 5.2. Demultiplexing media types within an RTP session
When receiving packets from a transport layer flow, an endpoint will When receiving packets from a transport layer flow, an endpoint will
first separate the RTP and RTCP packets from the non-RTP packets, and first separate the RTP and RTCP packets from the non-RTP packets, and
pass them to the RTP/RTCP protocol handler. The RTP and RTCP packets pass them to the RTP/RTCP protocol handler. The RTP and RTCP packets
are then demultiplexed based on their SSRC into the different media are then demultiplexed based on their SSRC into the different media
streams. For each media stream, incoming RTCP packets are processed, streams. For each media stream, incoming RTCP packets are processed,
and the RTP payload type is used to select the appropriate media and the RTP payload type is used to select the appropriate media
decoder. decoder. This process remains the same irrespective of whether
multiple media types are sent in a single RTP session or not.
This process remains the same irrespective of whether multiple media It is important to note that the RTP payload type is never used to
types are sent in a single RTP session or not. It is important to distinguish media streams. The RTP packets are demultiplexed into
note that the RTP payload type is never used to demultiplex media media streams based on their SSRC, then the RTP payload type is used
streams. Media streams are distinguished by SSRC, and the payload to select the correct media decoding pathway for each media stream.
type is then used to route data for a particular SSRC to the right
media decoder.
5.3. Per-SSRC Media Type Restrictions 5.3. Per-SSRC Media Type Restrictions
An SSRC in an RTP session MUST NOT change media type during its An SSRC in an RTP session can change between media formats of the
lifetime. For example, an SSRC cannot start sending audio, then same type, subject to certain restrictions [RFC7160], but MUST NOT
change to sending video. The lifetime of an SSRC ends when an RTCP change media type during its lifetime. For example, an SSRC can
BYE packet for that SSRC is sent, or when it ceases transmission for change between different audio formats, but cannot start sending
long enough that it times out for the other participants in the audio then change to sending video. The lifetime of an SSRC ends
session. when an RTCP BYE packet for that SSRC is sent, or when it ceases
transmission for long enough that it times out for the other
participants in the session.
The main motivation is that a given SSRC has its own RTP timestamp The main motivation is that a given SSRC has its own RTP timestamp
and sequence number spaces. The same way that you can't send two and sequence number spaces. The same way that you can't send two
encoded streams of audio on the same SSRC, you can't send one encoded encoded streams of audio on the same SSRC, you can't send one encoded
audio and one encoded video stream on the same SSRC. Each encoded audio and one encoded video stream on the same SSRC. Each encoded
stream when made into an RTP stream needs to have the sole control stream when made into an RTP stream needs to have the sole control
over the sequence number and timestamp space. If not, one would not over the sequence number and timestamp space. If not, one would not
be able to detect packet loss for that particular encoded stream. be able to detect packet loss for that particular encoded stream.
Nor can one easily determine which clock rate a particular SSRCs Nor can one easily determine which clock rate a particular SSRCs
timestamp will increase with. For additional arguments why RTP timestamp will increase with. For additional arguments why RTP
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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.
5.4. RTCP Considerations 5.4. RTCP Considerations
When sending multiple types of media that have different rates in a When sending multiple types of media that have different rates in a
single RTP session, endpoints MUST follow the guidelines for handling single RTP session, endpoints MUST follow the guidelines for handling
RTCP described in Section 7 of [I-D.ietf-avtcore-rtp-multi-stream]. RTCP described in Section 7 of [I-D.ietf-avtcore-rtp-multi-stream].
6. Extension Considerations 6. Extension Considerations
This section outlines known issues and incompatibilities with RTP and This section outlines known issues and incompatibilities with RTP and
RTCP extensions when multiple media types are used in a single RTP RTCP extensions when multiple media types are used in a single RTP
sessions. Future extensions to RTP and RTCP need to consider, and sessions. Future extensions to RTP and RTCP need to consider, and
document, any potential incompatibility. document, any potential incompatibility.
6.1. RTP Retransmission Payload Format 6.1. RTP Retransmission Payload Format
SSRC-multiplexed RTP retransmission [RFC4588] is actually very The RTP Retransmission Payload Format [RFC4588] can operate in either
straightforward. Each retransmission RTP payload type is explicitly SSRC-multiplexed mode or session-multiplex mode.
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,
as it will be evident from the retransmission payload types which
payload types have retransmission enabled for them.
Session-multiplexed RTP retransmission is also possible to use where In SSRC-multiplexed mode, retransmitted RTP packets are sent in the
an retransmission session contains the retransmissions of the same RTP session as the original packets, but use a different SSRC
associated payload types in the source RTP session. The only with the same RTCP SDES CNAME. If each endpoint sends only a single
difference to the previous case is if the source RTP session is one original RTP stream and a single retransmission RTP stream in the
which contains multiple media types. This results in the session, this is sufficient. If an endpoint sends multiple original
retransmission streams in the RTP session for the retransmission and retransmission RTP streams, as would occur when sending multiple
having multiple associated media types. media types in a single RTP session, then each original RTP stream
and the retransmission RTP stream have to be associated using
heuristics. By having retransmission requests outstanding for only
one SSRC not yet mapped, a receiver can determine the binding between
original and retransmission RTP stream. Another alternative is the
use of different RTP payload types, allowing the signalled "apt"
(associated payload type) parameter of the RTP retransmission payload
format to be used to associate retransmitted and original packets.
When using SDP signalling for a multiple media type RTP session, i.e. Session-multiplexed mode sends the retransmission RTP stream in a
BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], the session separate RTP session to the original RTP stream, but using the same
multiplexed case do require some recommendations on how to signal SSRC for each, with association being done by matching SSRCs between
this. To avoid breaking the semantics of the FID grouping as defined the two sessions. This is unaffected by the use of multiple media
by [RFC5888] each media line can only be included in one FID group. types in a single RTP session, since each media type will be sent
FID is used by RTP retransmission to indicate the SDP media lines using a different SSRC in the original RTP session, and the same
that is a source and retransmission pair. Thus, for SDP using SSRCs can be used in the retransmission session, allowing the streams
BUNDLE, each original media source (m= line) that is retransmitted to be associated. This can be signalled using SDP with the BUNDLE
needs a corresponding media line in the retransmission RTP session. [I-D.ietf-mmusic-sdp-bundle-negotiation] and FID grouping [RFC5888]
In case there are multiple media lines for retransmission, these extensions. These SDP extensions require each "m=" line to only be
media lines will form a independent BUNDLE group from the BUNDLE included in a single FID group, but the RTP retransmission payload
group with the source streams. format uses FID groups to indicate the m= lines that form an original
and retransmission pair. Accordingly, when using the BUNDLE
extension to allow multiple media types to be sent in a single RTP
session, each original media source (m= line) that is retransmitted
needs a corresponding m= 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 An example SDP fragment showing the grouping structures is provided
structures. This example is not legal SDP and only the most in Figure 1. This example is not legal SDP and only the most
important attributes has been left in place. Note that this SDP is important attributes have been left in place. Note that this SDP is
not an initial BUNDLE offer. As can be seen there are two bundle not an initial BUNDLE offer. As can be seen there are two bundle
groups, one for the source RTP session and one for the groups, one for the source RTP session and one for the
retransmissions. Then each of the media sources are grouped with its retransmissions. Then each of the media sources are grouped with its
retransmission flow using FID, resulting in three more groupings. retransmission flow using FID, resulting in three more groupings.
a=group:BUNDLE foo bar fiz a=group:BUNDLE foo bar fiz
a=group:BUNDLE zoo kelp glo a=group:BUNDLE zoo kelp glo
a=group:FID foo zoo a=group:FID foo zoo
a=group:FID bar kelp a=group:FID bar kelp
a=group:FID fiz glo a=group:FID fiz glo
skipping to change at page 11, line 7 skipping to change at page 10, line 40
a=mid:kelp a=mid:kelp
m=video 40000 RTP/AVPF 100 m=video 40000 RTP/AVPF 100
a=rtpmap:100 rtx/90000 a=rtpmap:100 rtx/90000
a=fmtp:199 apt=31;rtx-time=3000 a=fmtp:199 apt=31;rtx-time=3000
a=mid:glo a=mid:glo
Figure 1: SDP example of Session Multiplexed RTP Retransmission Figure 1: SDP example of Session Multiplexed RTP Retransmission
6.2. RTP Payload Format for Generic FEC 6.2. RTP Payload Format for Generic FEC
The RTP Payload Format for Generic Forward Error Correction The RTP Payload Format for Generic Forward Error Correction (FEC)
[RFC5109], and also its predecessor [RFC2733], requires some [RFC5109] (and its predecessor [RFC2733]) can either send the FEC
considerations, and they are different depending on what type of stream as a separate RTP stream, or it can send the FEC combined with
configuration of usage one has. the original RTP stream as a redundant encoding [RFC2198].
Independent RTP Sessions, i.e. where source and repair data are sent
in different RTP sessions. As this mode of configuration requires
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
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
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
"Application".
If one uses SDP signalling with BUNDLE When sending FEC as a separate stream, the RTP Payload Format for
[I-D.ietf-mmusic-sdp-bundle-negotiation], then the RTP session generic FEC requires that FEC stream to be sent in a separate RTP
carrying the FEC streams will be its own BUNDLE group. The media session to the original stream, using the same SSRC, with the FEC
line with the source stream for the FEC and the FEC stream's media stream being associated by matching the SSRC between sessions. The
line will be grouped using media line grouping using the FEC or FEC- RTP session used for the original streams can include multiple RTP
FR [RFC5956] grouping. This is very similar to the situation that streams, and those RTP stream can use multiple media types. The
arise for RTP retransmission with session multiplexing discussed repair session only needs one RTP Payload type to indicate FEC data,
above inSection 6.1. irrespective of the number of FEC streams sent, since the SSRC is
used to associate the FEC streams with the original streams. Hence,
it is RECOMMENDED that FEC stream use the "application/ulpfec" media
type for [RFC5109], and the "application/parityfec" media type for
[RFC2733]. It is legal, but NOT RECOMMENDED, to send FEC streams
using media specific payload format names (e.g., if an original RTP
session contains audio and video flows, for the associated FEC RTP
session where to use the "audio/ulpfec" and "video/ulpfec" payload
formats), since this unnecessarily uses up RTP payload type values,
and adds no value for demultiplexing since there might be multiple
streams of the same media type).
The RTP Payload Format for Generic Forward Error Correction [RFC5109] The combination of an original RTP session using multiple media types
and its predecessor [RFC2733] requires a separate RTP session unless with a associated generic FEC session can be signalled using SDP with
the FEC data is carried in RTP Payload for Redundant Audio Data the BUNDLE extension [I-D.ietf-mmusic-sdp-bundle-negotiation]. In
[RFC2198]. this case, the RTP session carrying the FEC streams will be its own
BUNDLE group. The m= line for each original stream and the m= line
for the corresponding FEC stream are grouped using the SDP grouping
framework with either the FEC [RFC4756] or the FEC-FR [RFC5956]
grouping. This is similar to the situation that arises for RTP
retransmission with session multiplexing discussed in Section 6.1.
Note that the Source-Specific Media Attributes [RFC5576] The Source-Specific Media Attributes [RFC5576] specification defines
specification defines an SDP syntax (the "FEC" semantic of the "ssrc- an SDP extension (the "FEC" semantic of the "ssrc-group" attribute)
group" attribute) to signal FEC relationships between multiple RTP to signal FEC relationships between multiple RTP streams within a
streams within a single RTP session. However, this can't be used as single RTP session. This cannot be used with generic FEC, since the
the FEC repair packets need to have the same SSRC value as the source FEC repair packets need to have the same SSRC value as the source
packets being protected. [RFC5576] does not normatively update and packets being protected. There is ongoing work on an ULP extension
resolve that restriction. There is ongoing work on an ULP extension
to allow it be use FEC RTP streams within the same RTP Session as the to allow it be use FEC RTP streams within the same RTP Session as the
source stream [I-D.lennox-payload-ulp-ssrc-mux]. source stream [I-D.lennox-payload-ulp-ssrc-mux].
When the FEC is sent as a redundant encoding, the considerations in
Section 6.3 apply.
6.3. RTP Payload Format for Redundant Audio 6.3. RTP Payload Format for Redundant Audio
In stream, using RTP Payload for Redundant Audio Data [RFC2198] The RTP Payload Format for Redundant Audio [RFC2198] can be used to
combining repair and source data in the same packets. This is protect audio streams. It can also be used along with the generic
possible to use within a single RTP session. However, the usage and FEC payload format to send original and repair data in the same RTP
configuration of the payload types can create an issue. First of all packets. Both are compatible with RTP sessions containing multiple
it might be necessary to have one payload type per media type for the media types.
FEC repair data payload format, i.e. one for audio/ulpfec and one
for text/ulpfec if audio and text are combined in an RTP session. This payload format requires each different redundant encoding use a
Secondly each combination of source payload and its FEC repair data different RTP payload type number. When used with generic FEC in
has to be an explicit configured payload type. This has potential sessions that contain multiple media types, this requires each media
for making the limitation of RTP payload types available into a real type use a different payload type for the FEC stream. For example,
issue. if audio and text are sent in a single RTP session with generic ULP
FEC sent as a redundant encoding for each, then payload types need to
be assigned for FEC using the audio/ulpfec and text/ulpfec payload
formats. If multiple original payload types of used in the session,
different redundant payload types need to be allocated for each one.
This has potential to rapidly exhaust the available RTP payload type
numbers.
7. Signalling 7. Signalling
Establishing an RTP session with multiple media types requires Establishing a single RTP session using multiple media types requires
signalling. This signalling needs to fulfil the following signalling. This signalling has to:
requirements:
1. Ensure that any participant in the RTP session is aware that this 1. ensure that any participant in the RTP session is aware that this
is an RTP session with multiple media types. is an RTP session with multiple media types;
2. Ensure that the payload types in use in the RTP session are using 2. ensure that the payload types in use in the RTP session are using
unique values, with no overlap between the media types. unique values, with no overlap between the media types;
3. Configure the RTP session level parameters, such as RTCP RR and 3. ensure RTP session level parameters, for example the RTCP RR and
RS bandwidth, AVPF trr-int, underlying transport, the RTCP RS bandwidth modifiers, the RTP/AVPF trr-int parameter, transport
extensions in use, and security parameters, commonly for the RTP protocol, RTCP extensions in use, and any security parameters,
session. are consistent across the session; and
4. RTP and RTCP functions that can be bound to a particular media 4. ensure that RTP and RTCP functions that can be bound to a
type SHOULD be reused when possible also for other media types, particular media type are reused where possible, rather than
instead of having to be configured for multiple code-points. configuring multiple code-points for the same thing.
Note: In some cases one will not have a choice but to use
multiple configurations.
The signalling of multiple media types in one RTP session in SDP is When using SDP signalling, the BUNDLE extension
specified in "Multiplexing Negotiation Using Session Description [I-D.ietf-mmusic-sdp-bundle-negotiation] is used to signal RTP
Protocol (SDP) Port Numbers" sessions containing multiple media types.
[I-D.ietf-mmusic-sdp-bundle-negotiation].
8. Security Considerations 8. Security Considerations
Having an RTP session with multiple media types doesn't change the RTP provides a range of strong security mechanisms that can be used
methods for securing a particular RTP session. One possible to secure sessions [RFC7201], [RFC7202]. The majority of these are
difference is that the different media have often had different independent of the type of media sent in the RTP session, however it
security requirements. When combining multiple media types in one is important to check that the security mechanism chosen is
session, their security requirements also have to be combined by compatible with all types of media sent within the session.
selecting the most demanding for each property. Thus having multiple
media types can result in increased overhead for security for some
media types to ensure that all requirements are meet.
Otherwise, the recommendations for how to configure and RTP session Sending multiple media types in a single RTP session will generally
do not add any additional requirements compared to normal RTP, except require that all use the same security mechanism, whereas media sent
for the need to be able to ensure that the participants are aware using different RTP sessions can be secured in different ways. When
that it is a multiple media type session. If not that is ensured it different media types have different security requirements, it might
can cause issues in the RTP session for both the unaware and the be necessary to send them using separate RTP sessions to meet those
aware one. Similar issues can also be produced in an normal RTP different requirements. This can have significant costs in terms of
session by creating configurations for different end-points that resource usage, session set-up time, etc.
doesn't match each other.
9. IANA Considerations 9. IANA Considerations
This memo makes no request of IANA. This memo makes no request of IANA.
10. Acknowledgements 10. Acknowledgements
The authors would like to thank Christer Holmberg, Gunnar Hellstroem, The authors would like to thank Christer Holmberg, Gunnar Hellstroem,
and Charles Eckel for the feedback on the document. and Charles Eckel for the feedback on the document.
11. References 11. References
11.1. Normative References 11.1. Normative References
skipping to change at page 13, line 27 skipping to change at page 13, line 18
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.
11. References 11. References
11.1. Normative References 11.1. Normative References
[I-D.ietf-avtcore-rtp-multi-stream] [I-D.ietf-avtcore-rtp-multi-stream]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins, Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session", "Sending Multiple Media Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-07 (work in progress), draft-ietf-avtcore-rtp-multi-stream-08 (work in progress),
March 2015. July 2015.
[I-D.ietf-mmusic-sdp-bundle-negotiation] [I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings, Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session "Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-22 (work in progress), June 2015. negotiation-22 (work in progress), June 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[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. DOI 10.17487/RFC3551, July 2003,
<http://www.rfc-editor.org/info/rfc3551>.
11.2. Informative References 11.2. Informative References
[I-D.ietf-avtcore-multiplex-guidelines] [I-D.ietf-avtcore-multiplex-guidelines]
Westerlund, M., Perkins, C., and H. Alvestrand, Westerlund, M., Perkins, C., and H. Alvestrand,
"Guidelines for using the Multiplexing Features of RTP to "Guidelines for using the Multiplexing Features of RTP to
Support Multiple Media Streams", draft-ietf-avtcore- Support Multiple Media Streams", draft-ietf-avtcore-
multiplex-guidelines-03 (work in progress), October 2014. multiplex-guidelines-03 (work in progress), October 2014.
[I-D.ietf-avtcore-rtp-topologies-update] [I-D.ietf-avtcore-rtp-topologies-update]
skipping to change at page 14, line 43 skipping to change at page 14, line 33
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-
westerlund-avtcore-transport-multiplexing-07 (work in westerlund-avtcore-transport-multiplexing-07 (work in
progress), October 2013. progress), October 2013.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- Handley, M., Bolot, J.C., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997. DOI 10.17487/RFC2198, September 1997,
<http://www.rfc-editor.org/info/rfc2198>.
[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, DOI
1999. 10.17487/RFC2733, December 1999,
<http://www.rfc-editor.org/info/rfc2733>.
[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, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[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. DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error [RFC4756] Li, A., "Forward Error Correction Grouping Semantics in
Correction", RFC 5109, December 2007. Session Description Protocol", RFC 4756, DOI 10.17487/
RFC4756, November 2006,
<http://www.rfc-editor.org/info/rfc4756>.
[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <http://www.rfc-editor.org/info/rfc5109>.
[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, DOI 10.17487/RFC5576, June 2009,
<http://www.rfc-editor.org/info/rfc5576>.
[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, DOI 10.17487/
RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.
[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, DOI 10.17487
/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>.
[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956, September the Session Description Protocol", RFC 5956, DOI 10.17487/
2010. RFC5956, September 2010,
<http://www.rfc-editor.org/info/rfc5956>.
[RFC7160] Petit-Huguenin, M. and G. Zorn, Ed., "Support for Multiple
Clock Rates in an RTP Session", RFC 7160, DOI 10.17487/
RFC7160, April 2014,
<http://www.rfc-editor.org/info/rfc7160>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://www.rfc-editor.org/info/rfc7201>.
[RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
2014, <http://www.rfc-editor.org/info/rfc7202>.
Authors' Addresses 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
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