AVTCORE WG M. Westerlund
Internet-Draft Ericsson
Updates: 3550, 3551 (if approved) C. Perkins
Intended status: Standards Track University of Glasgow
Expires: July 17, 2014 J. Lennox
Vidyo
January 13, 2014
Sending Multiple Types of Media in a Single RTP Session
draft-ietf-avtcore-multi-media-rtp-session-04
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 behaviour for
applications that satisfy the applicability for using multiple media
types in a single RTP session.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 17, 2014.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. NAT and Firewalls . . . . . . . . . . . . . . . . . . . . 4
3.2. No Transport Level QoS . . . . . . . . . . . . . . . . . 4
3.3. Architectural Equality . . . . . . . . . . . . . . . . . 5
4. Overview of Solution . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . 8
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 Considerations . . . . . . . . . . . . . . . . . . . 12
7. Extension Considerations . . . . . . . . . . . . . . . . . . 12
7.1. RTP Retransmission . . . . . . . . . . . . . . . . . . . 13
7.2. Generic FEC . . . . . . . . . . . . . . . . . . . . . . . 13
8. Signalling . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. SDP-Based Signalling . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
10. Security Considerations . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
12.1. Normative References . . . . . . . . . . . . . . . . . . 15
12.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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 2013 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.
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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 assumes all media in a session to have similar
bandwidth. The Session Description Protocol (SDP) [RFC4566], as one
of the dominant signalling method for establishing RTP session, has
enforced this rule, simply by not allowing multiple media types for a
given receiver destination or set of ICE candidates, which is the
most common method to determine which RTP session the packets are
intended for.
The fact that these limitations have been in place for so long a
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 behaviour. This
document provides such considerations regarding applicability as well
as functionality, including normative specification of behaviour.
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 is expected to
function in the case of multiple media types in one RTP session. A
specification of the requirements on signalling from this
specification and a look how this is realized in SDP using Bundle
[I-D.ietf-mmusic-sdp-bundle-negotiation]. The document ends with the
security considerations.
2. Definitions
The following terms are used with supplied definitions:
Endpoint: A single entity sending or receiving RTP packets. It can
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.
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QoS: Quality of Service, i.e. network mechanisms that intended to
ensure that the packets within a flow or with a specific marking
are transported with certain properties.
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.
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].
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 needs to ensure that all these transport flows are
established. This has three consequences:
1. Increased delay to perform the transport flow establishment
2. The more transport flows, the more state and the more resource
consumption in the NAT and Firewalls. When the resource
consumption in NAT/FWs reaches their limits, unexpected
behaviours usually occur.
3. More transport flows means a higher risk that some transport flow
fails to be established, thus preventing the application to
communicate.
Using fewer transport flows reduces the risk of communication
failure, improved establishment behaviour and less load on NAT and
Firewalls.
3.2. No Transport Level QoS
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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 might seem unnecessary. Provided
the application accepts that all media flows will get similar RTCP
reporting, using the same RTP session for several types of media at
once appears a reasonable choice. The architecture ought to be
agnostic about the type of media being carried in an RTP session to
the extent possible given the constraints of the protocol.
4. Overview of Solution
The goal of the solution is to enable each RTP session to contain
more than just one media type. This includes having multiple RTP
sessions containing a given media type, for example having three
sessions containing both 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
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 can only send 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 is used in the RTP session.
Some few extra considerations within the RTP sessions also needs to
be considered. RTCP bandwidth and regular reporting suppression (RTP
/AVPF and RTP/SAVPF) SHOULD be configured to reduce the impact for
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bit-rate variations between streams and media types. It is also
clarified how timeout calculations are to be done to avoid any
issues. 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. This memo describes
some existing requirements, while an external reference defines how
this is accomplished in SDP.
5. Applicability
This specification has limited applicability, and anyone intending to
use it needs to 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.ietf-avtcore-multiplex-guidelines] 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 behaviours that multiple transport flows give, can consider using
Multiple RTP Sessions on a Single Lower-Layer Transport
[I-D.westerlund-avtcore-transport-multiplexing]. It needs to be
noted that some difference in treatment is still possible to achieve,
for example marking based QoS, or RTCP feedback traffic for only some
media streams.
The second important consideration is the resulting behaviour when
media flows to be sent within a single RTP session does not have
similar bandwidth. There are limitations in the RTCP timing rules,
and this implies a common RTCP reporting interval across all
participants in a session. If an RTP session contains flows with
very different bandwidths, for example low-rate audio coupled with
high-quality video, this can result in either excessive or
insufficient RTCP for some flows, depending how the RTCP session
bandwidth, and hence reporting interval, is configured. This is
discussed further in Section 6.4.
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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 needs to be mutual agreement to use multiple
media types in one RTP session by all participants within that RTP
session. This agreement has to be determined using signalling in
most cases.
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 can 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
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.
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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 has to
understand all types of RTP/RTCP extension being used in the RTP
sessions to correctly be able to translate them between the RTP
sessions. It can also negatively impact the possibility for loop
detection, as SSRC/CSRC can't be used to detect the loops, instead
some other media stream identity name space that is common across all
interconnect parts are needed.
5.4. Reduced number of Payload Types
An RTP session with multiple media types in it have only a single
7-bit Payload Type range for all its payload types. Within the 128
available values, only 96 or less if "Multiplexing RTP Data and
Control Packets on a Single Port" [RFC5761] is used, all the
different RTP payload configurations for all the media types need to
fit in the available space. For most applications this will not be a
real problem, but the limitation exists and could be encountered.
5.5. Stream Differentiation
If network level differentiation of the 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 differentiated treatment is
desired using flow-based QoS, 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, in which case the same
considerations as for flow based QoS applies.
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
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extension, they can be used. It is to be noted that this assumes
that it is possible to get the extension working when the related RTP
session contains multiple media types.
Identified RTP/RTCP extensions that require multiple RTP Sessions
are:
RTP Retransmission: RTP Retransmission [RFC4588] has a session
multiplexed mode. It also has a SSRC multiplexed mode that can be
used instead. So use the mode that is suitable for the RTP
application.
XOR-Based FEC: The RTP Payload Format for Generic Forward Error
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]. However, using the Generic
FEC with the Redundancy payload has another set of restrictions,
see Section 7.2.
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 need to have the same SSRC
value as the source packets being protected. [RFC5576] does not
normatively update and resolve that restriction. There is ongoing
work on an ULP extension to allow it be use FEC streams within the
same RTP Session as the source stream
[I-D.lennox-payload-ulp-ssrc-mux].
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.
Separate audio and video streams SHOULD NOT be carried in a single
RTP session and demultiplexed based on the payload type or SSRC
fields.
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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 has to 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 specified 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.
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:
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1. If, say, two audio streams shared the same RTP session and the
same SSRC value, and one were to change encodings and thus
acquire a different RTP payload type, there would be no general
way of identifying which stream had changed encodings.
2. An SSRC is defined to identify a single timing and sequence
number space. Interleaving multiple payload types would require
different timing spaces if the media clock rates differ and would
require different sequence number spaces to tell which payload
type suffered packet loss.
3. The RTCP sender and receiver reports (see Section 6.4 of RFC
3550) can only describe one timing and sequence number space per
SSRC and do not carry a payload type field.
4. An RTP mixer would not be able to combine interleaved streams of
incompatible media into one stream.
5. Carrying multiple media in one RTP session precludes: the use of
different network paths or network resource allocations if
appropriate; reception of a subset of the media if desired, for
example just audio if video would exceed the available bandwidth;
and receiver implementations that use separate processes for the
different media, whereas using separate RTP sessions permits
either single- or multiple-process implementations.
Bullets 1 to 3 are all related to that each media source has to use
one or more unique SSRCs to avoid these issues as mandated below
(Section 6.2). Bullet 4 can be served by two arguments, first of all
each SSRC will be associated with a specific media type, communicated
through the RTP payload type, allowing a middlebox to do media type
specific operations. The second argument is that in many contexts
blind combining without additional contexts are anyway not suitable.
Regarding bullet 5 this is a understood and explicitly stated
applicability limitations for the method described in this document.
6.2. Sender Source Restrictions
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
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 will increase with. For additional
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arguments why RTP payload type based multiplexing of multiple media
streams doesn't work see Appendix A in
[I-D.ietf-avtcore-multiplex-guidelines].
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 robustness 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 fall back 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 Considerations
Guidelines for handling RTCP when sending multiple media streams with
disparate rates in a single RTP session are outlined in
[I-D.ietf-avtcore-rtp-multi-stream]. These guidelines apply when
sending multiple types of media in a single RTP session if the
different types of media have different rates.
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.
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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 might
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 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".
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 necessary to have one payload type per media type for the
FEC repair data payload format, i.e. one for audio/ulpfec and one
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for text/ulpfec if audio and text are combined in an RTP session.
Secondly each combination of source payload and its FEC repair data
has to 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 fulfil 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].
9. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section is to be removed on publication as
an RFC.
10. Security Considerations
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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 also have to be combined by
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
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, Gunnar Hellstroem,
and Charles Eckel for the feedback on the document.
12. References
12.1. Normative References
[I-D.ietf-avtcore-rtp-multi-stream]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-01 (work in progress),
July 2013.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp-
bundle-negotiation-05 (work in progress), October 2013.
[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.
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[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.ietf-avtcore-multiplex-guidelines]
Westerlund, M., Perkins, C., and H. Alvestrand,
"Guidelines for using the Multiplexing Features of RTP to
Support Multiple Media Streams", draft-ietf-avtcore-
multiplex-guidelines-01 (work in progress), July 2013.
[I-D.lennox-payload-ulp-ssrc-mux]
Lennox, J., "Supporting Source-Multiplexing of the Real-
Time Transport Protocol (RTP) Payload for Generic Forward
Error Correction", draft-lennox-payload-ulp-ssrc-mux-00
(work in progress), February 2013.
[I-D.westerlund-avtcore-transport-multiplexing]
Westerlund, M. and C. Perkins, "Multiplexing Multiple RTP
Sessions onto a Single Lower-Layer Transport", draft-
westerlund-avtcore-transport-multiplexing-07 (work in
progress), October 2013.
[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.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
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[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.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
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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