AVTCORE WG M. Westerlund
Internet-Draft Ericsson
Updates: 3550, 3551 (if approved) C. Perkins
Intended status: Standards Track University of Glasgow
Expires: April 25, 2013 J. Lennox
Vidyo
October 22, 2012
Multiple Media Types in an RTP Session
draft-ietf-avtcore-multi-media-rtp-session-01
Abstract
This document specifies how an RTP session can contain media streams
with media from multiple media types such as audio, video, and text.
This has been restricted by the RTP Specification, and thus this
document updates RFC 3550 and RFC 3551 to enable this behavior for
applications that satisfy the applicability for using multiple media
types in a single RTP session.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 25, 2013.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. NAT and Firewalls . . . . . . . . . . . . . . . . . . . . 4
3.2. No Transport Level QoS . . . . . . . . . . . . . . . . . . 5
3.3. Architectural Equality . . . . . . . . . . . . . . . . . . 5
4. Overview of Solution . . . . . . . . . . . . . . . . . . . . . 5
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Usage of the RTP session . . . . . . . . . . . . . . . . . 6
5.2. Signalled Support . . . . . . . . . . . . . . . . . . . . 7
5.3. Homogeneous Multi-party . . . . . . . . . . . . . . . . . 7
5.4. Reduced number of Payload Types . . . . . . . . . . . . . 8
5.5. Stream Differentiation . . . . . . . . . . . . . . . . . . 8
5.6. Non-compatible Extensions . . . . . . . . . . . . . . . . 9
6. RTP Session Specification . . . . . . . . . . . . . . . . . . 9
6.1. RTP Session . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Sender Source Restrictions . . . . . . . . . . . . . . . . 11
6.3. Payload Type Applicability . . . . . . . . . . . . . . . . 12
6.4. RTCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Extension Considerations . . . . . . . . . . . . . . . . . . . 14
7.1. RTP Retransmission . . . . . . . . . . . . . . . . . . . . 14
7.2. Generic FEC . . . . . . . . . . . . . . . . . . . . . . . 14
8. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. SDP-Based Signalling . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
When the Real-time Transport Protocol (RTP) [RFC3550] was designed,
close to 20 years ago, IP networks were very different compared to
the ones in 2012 when this is written. The almost ubiquitous
deployment of Network Address Translators (NAT) and Firewalls has
increased the cost and likely-hood of communication failure when
using many different transport flows. Thus there exists a pressure
to reduce the number of concurrent transport flows.
RTP [RFC3550] recommends against sending several different types of
media, for example audio and video, in a single RTP session. The RTP
profile for Audio and Video Conferences with Minimal Control (RTP/
AVP) [RFC3551] mandates a similar restriction. The motivation for
these limitations is partly to allow lower layer Quality of Service
(QoS) mechanisms to be used, and partly due to limitations of the
RTCP timing rules that require all media in a session to have similar
bandwidth. The Session Description Protocol (SDP) [RFC4566], as one
of the dominant signalling method for establishing RTP session, has
enforced this rule, simply by not allowing multiple media types for a
given receiver destination or set of ICE candidates, which is the
most common method to determine which RTP session the packets are
intended for.
The fact that these limitations have been in place for so long a
time, in addition to RFC 3550 being written without fully considering
multiple media types in an RTP session, does result in a number of
considerations being needed when allowing this behavior. This
document provides such considerations regarding applicability as well
as functionality, including normative specification of behavior.
First, some basic definitions are provided. This is followed by a
background that discusses the motivation in more detail. A overview
of the solution of how to provide multiple media types in one RTP
session is then presented. Next is the formal applicability this
specification have followed by the normative specification. This is
followed by a discussion how some RTP/RTCP Extensions should function
in the case of multiple media types in one RTP session. A
specification of the requirements on signalling from this
specification and a look how this is realized in SDP using Bundle
[I-D.ietf-mmusic-sdp-bundle-negotiation]. The document ends with the
security considerations.
2. Definitions
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2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2.2. Terminology
The following terms are used with supplied definitions:
Endpoint: A single entity sending or receiving RTP packets. It may
be decomposed into several functional blocks, but as long as it
behaves as a single RTP stack entity it is classified as a single
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.
RTP Session: As defined by [RFC3550], the endpoints belonging to the
same RTP Session are those that share a single SSRC space. That
is, those endpoints can see an SSRC identifier transmitted by any
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.
3. Motivation
This section discusses in more detail the main motivations why
allowing multiple media types in the same RTP session is suitable.
3.1. NAT and Firewalls
The existence of NATs and Firewalls at almost all Internet access has
had implications on protocols like RTP that were designed to use
multiple transport flows. First of all, the NAT/FW traversal
solution one uses needs to ensure that all these transport flows are
established. This has three different impacts:
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1. Increased delay to perform the transport flow establishment
2. The more transport flows, the more state and the more resource
consumption in the NAT and Firewalls. When the resource
consumption in NAT/FWs reaches their limits, unexpected behaviors
usually occur.
3. More transport flows means a higher risk that some transport flow
fails to be established, thus preventing the application to
communicate.
Using fewer transport flows reduces the risk of communication
failure, improved establishment behavior and less load on NAT and
Firewalls.
3.2. No Transport Level QoS
Many RTP-using applications don't utilize any network level Quality
of Service functions. Nor do they expect or desire any separation in
network treatment of its media packets, independent of whether they
are audio, video or text. When an application has no such desire, it
doesn't need to provide a transport flow structure that simplifies
flow based QoS.
3.3. Architectural Equality
For applications that don't require different lower-layer QoS for
different media types, and that have no special requirements for RTP
extensions or RTCP reporting, the requirement to separate different
media into different RTP sessions may seem unnecessary. Provided the
media flows have similar bandwidth requirements, so that the RTCP
timing rules work, using the same RTP session for several types of
media at once appears a reasonable choice. The architecture should
be agnostic about the type of media being carried in an RTP session
to the extent possible given the constraints of the protocol.
4. Overview of Solution
The goal of the solution is to enable having one or more RTP
sessions, where each RTP session may contain two or more media types.
This includes having multiple RTP sessions containing a given media
type, for example having three sessions containing video and audio.
The solution is quite straightforward. The first step is to override
the SHOULD and SHOULD NOT language of the RTP specification
[RFC3550]. Similar change is needed to a sentence in Section 6 of
[RFC3551] that states that "different media types SHALL NOT be
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interleaved or multiplexed within a single RTP Session". This is
resolved by appropriate exception clauses given that this
specification and its applicability is followed.
Within an RTP session where multiple media types have been configured
for use, an SSRC may send only one type of media during its lifetime
(i.e., it can switch between different audio codecs, since those are
both the same type of media, but cannot switch between audio and
video). Different SSRCs must be used for the different media
sources, the same way multiple media sources of the same media type
already have to do. The payload type will inform a receiver which
media type the SSRC is being used for. Thus the payload type must be
unique across all of the payload configurations independent of media
type that may be used in the RTP session.
Some few extra considerations within the RTP sessions also needs to
be considered. RTCP bandwidth and regular reporting suppression
(AVPF and SAVPF) should be considered to be configured. Certain
payload types like FEC also need additional rules.
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
session exists, and how that then is configured. Thus document
documents some existing requirements, while an external reference
defines how this is accomplished in SDP.
5. Applicability
This specification has limited applicability and any one intending to
use it must ensure that their application and usage meets the below
criteria.
5.1. Usage of the RTP session
Before choosing to use this specification, an application implementer
needs to ensure that they don't have a need for different RTP
sessions between the media types for some reason. The main rule is
that if one expects to have equal treatment of all media packets,
then this specification might be suitable. The equal treatment
include anything from network level up to RTCP reporting and
feedback. The document Guidelines for using the Multiplexing
Features of RTP [I-D.westerlund-avtcore-multiplex-architecture] gives
more detailed guidance on aspects to consider when choosing how to
use RTP and specifically sessions. RTP-using applications that need
or would prefer multiple RTP sessions, but do not require the
functionalities or behaviors that multiple transport flows give, can
consider using Multiple RTP Sessions on a Single Lower-Layer
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Transport [I-D.westerlund-avtcore-transport-multiplexing].
The second important consideration is that all media flows to be sent
within a single RTP session need to have similar bandwidth. This is
due to limitations of the RTCP timing rules, and the need for a
common RTCP reporting interval across all participants in a session
to avoid problems with premature SSRC timeouts. If an RTP session
contains flows with very different bandwidths, for example low-rate
audio coupled with high-quality video, this will result in either
excessive or insufficient RTCP for some flows, depending how the RTCP
session bandwidth, and hence reporting interval, is configured. This
is discussed further in Section 6.4.
5.2. Signalled Support
Usage of this specification is not compatible with anyone following
RFC 3550 and intending to have different RTP sessions for each media
type. Therefore there must be mutual agreement to use multiple media
types in one RTP session by all participants within an RTP session.
This agreement must in most cases be determined using signalling.
This requirement can be a problem for signalling solutions that can't
negotiate with all participants. For declarative signalling
solutions, mandating that the session is using multiple media types
in one RTP session can be a way of attempting to ensure that all
participants in the RTP session follow the requirement. However, for
signalling solutions that lack methods for enforcing that a receiver
supports a specific feature, this can still cause issues.
5.3. Homogeneous Multi-party
In multiparty communication scenarios it is important to separate two
different cases. One case is where the RTP session contains multiple
participants in a common RTP session. This occurs for example in Any
Source Multicast (ASM) and Transport Translator topologies as defined
in RTP Topologies [RFC5117]. It may also occur in some
implementations of RTP mixers that share the same SSRC/CSRC 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
SSRC/CSRC values, all participants in multiparty communication are
required to support multiple media types in an RTP session. An
participant using two or more RTP sessions towards a multiparty
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
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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
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
participant and a central node it is possible to have a mix of single
RTP session users and multiple RTP session users as long as one is
willing to remap the SSRCs used by a participant with multiple RTP
sessions into non-used values in the single RTP session SSRC space
for each of the participants using a single RTP session with multiple
media types. It can be noted that this type of implementation is
required to understand any type of RTP/RTCP extension being used in
the RTP sessions to correctly be able to translate them between the
RTP sessions.
5.4. Reduced number of Payload Types
An RTP session with multiple media types in it have only a single
7-bit Payload Type range for all its payload types. Within the 128
available values, only 96 or less if "Multiplexing RTP Data and
Control Packets on a Single Port" [RFC5761] is used, all the
different RTP payload configurations for all the media types must
fit. For most applications this will not be a real problem, but the
limitation exists and could be encountered.
5.5. Stream Differentiation
If network level differentiation of the media streams of different
media types are desired using this specification can cause severe
limitations. All media streams in an RTP session, independent of the
media type, will be sent over the same underlying transport flow.
Any flow-based Quality of Service (QoS) mechanism will be unable to
provide differentiated treatment between different media types, e.g.
to prioritize audio over video. If that is desired, separate RTP
sessions over different underlying transport flows needs to be used.
Any marking-based QoS scheme like DiffServ is not affected unless a
network ingress marks based on flows.
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5.6. Non-compatible Extensions
There exist some RTP and RTCP extensions that rely on the existence
of multiple RTP sessions. If the goal of using an RTP session with
multiple media types is to have only a single RTP session, then these
extensions can't be used. If one has no need to have different RTP
sessions for the media types but is willing to have multiple RTP
sessions, one for the main media transmission and one for the
extension, they can be used. It should be noted that this assumes
that it is possible to get the extension working when the related RTP
session contains multiple media types.
Identified RTP/RTCP extensions that require multiple RTP Sessions
are:
RTP Retransmission: RTP Retransmission [RFC4588] has a session
multiplexed mode. It also has a SSRC multiplexed mode that can be
used instead. So use the mode that is suitable for the RTP
application.
XOR-Based FEC: The RTP Payload Format for Generic Forward Error
Correction [RFC5109] and its predecessor [RFC2733] requires a
separate RTP session unless the FEC data is carried in RTP Payload
for Redundant Audio Data [RFC2198] which has another set of
restrictions.
Note that the Source-Specific Media Attributes [RFC5576]
specification defines an SDP syntax (the "FEC" semantic of the
"ssrc-group" attribute) to signal FEC relationships between
multiple media streams within a single RTP session. However, this
can't be used as the FEC repair packets are required to have the
same SSRC value as the source packets being protected. [RFC5576]
does not normatively update and resolve that restriction.
6. RTP Session Specification
This section defines what needs to be done or avoided to make an RTP
session with multiple media types function without issues.
6.1. RTP Session
Section 5.2 of "RTP: A Transport Protocol for Real-Time Applications"
[RFC3550] states:
For example, in a teleconference composed of audio and video media
encoded separately, each medium SHOULD be carried in a separate
RTP session with its own destination transport address.
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Separate audio and video streams SHOULD NOT be carried in a single
RTP session and demultiplexed based on the payload type or SSRC
fields.
This specification changes both of these sentences. The first
sentence is changed to:
For example, in a teleconference composed of audio and video media
encoded separately, each medium SHOULD be carried in a separate
RTP session with its own destination transport address, unless
specification [RFCXXXX] is followed and the application meets the
applicability constraints.
The second sentence is changed to:
Separate audio and video streams SHOULD NOT be carried in a single
RTP session and demultiplexed based on the payload type or SSRC
fields, unless multiplexed based on both SSRC and payload type and
usage meets what Multiple Media Types in an RTP Session [RFCXXXX]
specifies.
Second paragraph of Section 6 in RTP Profile for Audio and Video
Conferences with Minimal Control [RFC3551] says:
The payload types currently defined in this profile are assigned
to exactly one of three categories or media types: audio only,
video only and those combining audio and video. The media types
are marked in Tables 4 and 5 as "A", "V" and "AV", respectively.
Payload types of different media types SHALL NOT be interleaved or
multiplexed within a single RTP session, but multiple RTP sessions
MAY be used in parallel to send multiple media types. An RTP
source MAY change payload types within the same media type during
a session. See the section "Multiplexing RTP Sessions" of RFC
3550 for additional explanation.
This specifications purpose is to violate that existing SHALL NOT
under certain conditions. Thus also this sentence must be changed to
allow for multiple media type's payload types in the same session.
The above sentence is changed to:
Payload types of different media types SHALL NOT be interleaved or
multiplexed within a single RTP session unless as specifified and
under the restriction in Multiple Media Types in an RTP Session
[RFCXXXX]. Multiple RTP sessions MAY be used in parallel to send
multiple media types.
RFC-Editor Note: Please replace RFCXXXX with the RFC number of this
specification when assigned.
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We can now go on and discuss the five bullets that are motivating the
previous in Section 5.2 of the RTP Specification [RFC3550]. They are
repeated here for the reader's convenience:
1. If, say, two audio streams shared the same RTP session and the
same SSRC value, and one were to change encodings and thus
acquire a different RTP payload type, there would be no general
way of identifying which stream had changed encodings.
2. An SSRC is defined to identify a single timing and sequence
number space. Interleaving multiple payload types would require
different timing spaces if the media clock rates differ and would
require different sequence number spaces to tell which payload
type suffered packet loss.
3. The RTCP sender and receiver reports (see Section 6.4 of RFC
3550) can only describe one timing and sequence number space per
SSRC and do not carry a payload type field.
4. An RTP mixer would not be able to combine interleaved streams of
incompatible media into one stream.
5. Carrying multiple media in one RTP session precludes: the use of
different network paths or network resource allocations if
appropriate; reception of a subset of the media if desired, for
example just audio if video would exceed the available bandwidth;
and receiver implementations that use separate processes for the
different media, whereas using separate RTP sessions permits
either single- or multiple-process implementations.
Bullets 1 to 3 are all related to that each media source must use one
or more unique SSRCs to avoid these issues as mandated below
(Section 6.2). Bullet 4 can be served by two arguments, first of all
each SSRC will commonly a native media type, communicated through the
RTP payload type, allowing a middlebox to do media type specific
operations. The second argument is that in many contexts blind
combining without additional contexts are anyway not suitable.
Regarding bullet 5 this is a understood and explicitly stated
applicability limitations for the method described in this document.
6.2. Sender Source Restrictions
A SSRC in the RTP session MUST only send one media type (audio,
video, text etc.) during the SSRC's lifetime. 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 streams of encoded
audio on the same SSRC, you can't send one audio and one video
encoding on the same SSRC. Each media encoding when made into an RTP
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stream needs to have the sole control over the sequence number and
timestamp space. If not, one would not be able to detect packet loss
for that particular stream. Nor can one easily determine which clock
rate a particular SSRCs timestamp shall increase with.
6.3. Payload Type Applicability
Most Payload Types have a native media type, like an audio codec is
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
example, transport robustification mechanisms like RTP Retransmission
[RFC4588] and Generic FEC [RFC5109] inherit their media type from
what they protect. RTP Retransmission is explicitly bound to the
payload type it is protecting, and thus will inherit it. However
Generic FEC is a excellent example of an RTP payload type that has no
natural media type. The media type for what it protects is not
relevant as it is the recovered RTP packets that have a particular
media type, and thus Generic FEC is best categorized as an
application media type.
The above discussion is relevant to what limitations exist for RTP
payload type usage within an RTP session that has multiple media
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
single media type when used with independent RTP sessions for source
and repair data.
Note a particular SSRC carrying Generic FEC will clearly only
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
one be applicable to both. However, in cases when the signalling
is setup to enable fallback to using separate RTP sessions, then
using a different media type, e.g. application, than the media
being protected can create issues.
6.4. RTCP
An RTP session has a single set of parameters that configure the
session bandwidth, the RTCP sender and receiver fractions (e.g., via
the SDP "b=RR:" and "b=RS: lines), and the parameters of the RTP/AVPF
profile [RFC4585] (e.g., trr-int) if that profile (or its secure
extension, RTP/SAVPF [RFC5124]) is used. As a consequence, the RTCP
reporting interval will be the same for every SSRC in an RTP session.
This uniform RTCP reporting interval can result in RTCP reports being
sent more often than is considered desirable for a particular media
type. For example, if an audio flow is multiplexed with a high
quality video flow where the session bandwidth is configured to match
the video bandwidth, this can result in the RTCP packets having a
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greater bandwidth allocation than the audio data rate. If the
reduced minimum RTCP interval described in Section 6.2 of [RFC3550]
is used in the session, which might be appropriate for video where
rapid feedback is wanted, the audio sources could be required to send
RTCP packets more often than they send audio data packets. This is
clearly undesirable, and while the mismatch can be reduced through
careful tuning of the RTCP parameters, particularly trr_int in RTP/
AVPF sessions, it is inherent in the design of the RTCP timing rules,
and affects all RTP sessions containing flows with mismatched
bandwidth.
(tbd: A future version of this draft needs to provide details of
the extent of this problem, recommendations for how to tune the
RTCP bandwidth fraction and trr_int, and when the mismatch is so
great that it's better to use separate RTP sessions. The
recommendations will likely be different for RTP/AVP and RTP/AVPF
sessions, since trr_int offers a potential solution that is not
suitable in legacy session.)
Having multiple media types in one RTP session also results in more
SSRCs being present in this RTP session. This increasing the amount
of cross reporting between the SSRCs. From an RTCP perspective, two
RTP sessions with half the number of SSRCs in each will be slightly
more efficient. If someone needs either the higher efficiency due to
the lesser number of SSRCs or the fact that one can't tailor RTCP
usage per media type, they need to use independent RTP sessions.
When it comes to handling multiple SSRCs in an RTP session there is a
clarification under discussion in Real-Time Transport Protocol (RTP)
Considerations for Multi-Stream Endpoints
[I-D.lennox-avtcore-rtp-multi-stream]. When it comes to configuring
RTCP the need for regular periodic reporting needs to be weighted
against any feedback or control messages being sent. The
applications using AVPF or SAVPF are RECOMMENDED to consider setting
trr-int parameter to a value suitable for the applications needs,
thus potentially reducing the need for regular reporting and thus
releasing more bandwidth for use for feedback or control.
Another aspect of an RTP session with multiple media types is that
the used RTCP packets, RTCP Feedback Messages, or RTCP XR metrics
used may not be applicable to all media types. Instead all RTP/RTCP
endpoints need to correlate the media type of the SSRC being
referenced in an messages/packet and only use those that apply to
that particular SSRC and its media type. Signalling solutions may
have shortcomings when it comes to indicate that a particular set of
RTCP reports or feedback messages only apply to a particular media
type within an RTP session.
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7. Extension Considerations
This section discusses the impact on some RTP/RTCP extensions due to
usage of multiple media types in on RTP session. Only extensions
where something worth noting has been included.
7.1. RTP Retransmission
SSRC-multiplexed RTP retransmission [RFC4588] is actually very
straightforward. Each retransmission RTP payload type is explicitly
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 that have retransmission enabled for them.
Session-multiplexed RTP retransmission is also possible to use where
an retransmission session contains the retransmissions of the
associated payload types in the source RTP session. The only
difference to previously is that the source RTP session is one which
contains multiple media types. Thus it is even more likely that only
a subset of the source RTP session's payload types and SSRCs are
actually retransmitted.
Open Issue: When using SDP to signal retransmission for one RTP
session with multiple media types and one RTP session for the
retransmission data will cause a situation where one will have
multiple m= lines grouped using FID and the ones belonging to
respective RTP session being grouped using BUNDLE. This usage may
contradict both the FID semantics [RFC5888] and an assumption in the
RTP retransmission specification [RFC4588].
7.2. Generic FEC
The RTP Payload Format for Generic Forward Error Correction
[RFC5109], and also its predecessor [RFC2733], requires some
considerations, and they are different depending on what type of
configuration of usage one has.
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 must 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".
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In stream, using RTP Payload for Redundant Audio Data [RFC2198]
combining repair and source data in the same packets. This is
possible to use within a single RTP session. However, the usage and
configuration of the payload types can create an issue. First of all
it might be required to have one payload type per media type for the
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.
Secondly each combination of source payload and its FEC repair data
must be an explicit configured payload type. This has potential for
making the limitation of RTP payload types available into a real
issue.
8. Signalling
The Signalling requirements
Establishing an RTP session with multiple media types requires
signalling. This signalling needs to fulfill the following
requirements:
1. Ensure that any participant in the RTP session is aware that this
is an RTP session with multiple media types.
2. Ensure that the payload types in use in the RTP session are using
unique values, with no overlap between the media types.
3. Configure the RTP session level parameters, such as RTCP RR and
RS bandwidth, AVPF trr-int, underlying transport, the RTCP
extensions in use, and security parameters, commonly for the RTP
session.
4. RTP and RTCP functions that can be bound to a particular media
type should be reused when possible also for other media types,
instead of having to be configured for multiple code-points.
Note: In some cases one will not have a choice but to use
multiple configurations.
8.1. SDP-Based Signalling
The signalling of multiple media types in one RTP session in SDP is
specified in "Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers"
[I-D.ietf-mmusic-sdp-bundle-negotiation].
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9. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
10. Security Considerations
Having an RTP session with multiple media types doesn't change the
methods for securing a particular RTP session. One possible
difference is that the different media have often had different
security requirements. When combining multiple media types in one
session, their security requirements must also be combined by
selecting the most demanding for each property. Thus having multiple
media types may 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
do not add any additional requirements compared to normal RTP, except
for the need to be able to ensure that the participants are aware
that it is a multiple media type session. If not that is ensured it
can cause issues in the RTP session for both the unaware and the
aware one. Similar issues can also be produced in an normal RTP
session by creating configurations for different end-points that
doesn't match each other.
11. Acknowledgements
The authors would like to thank Christer Holmberg for the feedback on
the document.
12. References
12.1. Normative References
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C. and H. Alvestrand, "Multiplexing Negotiation
Using Session Description Protocol (SDP) Port Numbers",
draft-ietf-mmusic-sdp-bundle-negotiation-01 (work in
progress), August 2012.
[I-D.lennox-avtcore-rtp-multi-stream]
Lennox, J. and M. Westerlund, "Real-Time Transport
Protocol (RTP) Considerations for Endpoints Sending
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Multiple Media Streams",
draft-lennox-avtcore-rtp-multi-stream-00 (work in
progress), July 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
12.2. Informative References
[I-D.westerlund-avtcore-multiplex-architecture]
Westerlund, M., Burman, B., Perkins, C., and H.
Alvestrand, "Guidelines for using the Multiplexing
Features of RTP",
draft-westerlund-avtcore-multiplex-architecture-02 (work
in progress), July 2012.
[I-D.westerlund-avtcore-transport-multiplexing]
Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a
Single Lower-Layer Transport",
draft-westerlund-avtcore-transport-multiplexing-03 (work
in progress), July 2012.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997.
[RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
for Generic Forward Error Correction", RFC 2733,
December 1999.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
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Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
Authors' Addresses
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
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Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US
Email: jonathan@vidyo.com
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