AVTCORE WG                                                 M. Westerlund
Internet-Draft                                                  Ericsson
Updates: 3550 (if approved)                                   C. Perkins
Intended status: Standards Track                   University of Glasgow
Expires: January 10, 2013                                      J. Lennox
                                                            July 9, 2012

                 Multiple Media Types in an RTP Session


   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 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 January 10, 2013.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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  . . . . . . . . . . . . . . . . . . . .  6
     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 . . . . . . . . . . . . . . . . 10
     6.3.  Payload Type Applicability . . . . . . . . . . . . . . . . 10
     6.4.  RTCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Extension Considerations . . . . . . . . . . . . . . . . . . . 11
     7.1.  RTP Retransmission . . . . . . . . . . . . . . . . . . . . 12
     7.2.  Generic FEC  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Signalling . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  SDP-Based Signalling . . . . . . . . . . . . . . . . . . . 13
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     12.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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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] as defined recommends against having multiple media
   types, like audio and video, in the same RTP session.  The motivation
   for this is dependent on particular usage or dependencies on lower
   layer Quality of Service (QoS).  When these aren't present, there are
   no strong RTP reasons for not allowing multiple media types in one
   RTP session.  However, the Session Description Protocol (SDP)
   [RFC4566], as one of the dominant signalling method for establishing
   RTP session, has enforced this rule, 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.  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",
   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

   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

   Using fewer transport flows reduces the risk of communication
   failure, improved establishment behavior and less load on NAT and

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 desire any type of different treatment,
   neither on the transport level nor in RTP or RTCP reporting, using
   the same RTP session for both media types appears a reasonable
   choice.  The architecture should be neutral to media type, rather
   look at what it provides based on the application users choice.
   Therefore this bias should be removed and let the application
   designer make the choice if they need multiple RTP sessions or not
   based on other aspects.

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].  This is done by appropriate exception clauses given that
   this specification is followed.

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   Within an RTP session where multiple media types have been configured
   for use, a SSRC may only send one media type during its lifetime.
   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 must ensure that their application and usage meets the below
   criteria for usage.

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 Guidance on RTP Multiplexing Architecture
   [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 Transport

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

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   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
   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

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   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.

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

   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

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   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

      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.

      Separate audio and video streams SHOULD NOT be carried in a single
      RTP session and demultiplexed based on the payload type or SSRC

   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]

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   RFC-Editor Note: Please replace RFCXXXX with the RFC number of this
   specification when assigned.

   TBD: Discussion of the motivations in Section 5.2 of the RTP
   Specification [RFC3550].

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 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.  When it comes to Generic FEC, is an configured payload type
   allowed to be used to protect both audio SSRCs and Video SSRCs?  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 appears 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.

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   TBD: What recommendations are needed here?

6.4.  RTCP

   All SSRCs in an RTP session fall under the same set of RTCP
   configuration parameters, such as the RR and RS bandwidth and the
   trr-int parameter if AVPF or SAVPF is used.  This means that at least
   the regular reporting period by, and on, a source will be equal,
   independent of the media type for that source.  This should in most
   cases not be an issue, but it may result in more frequent reporting
   than is considered necessary for a particular media type or set of
   media sources.  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.

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 may
   contradict both the FID semantics [RFC5888] and an assumption in the
   RTP retransmission specification [RFC4588].

7.2.  Generic FEC


8.  Signalling

   The Signalling requirements

   Establishing an RTP session with multiple media types requires
   signalling.  This signalling needs to fulfill the following

   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

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   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"

9.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

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.

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12.  References

12.1.  Normative References

              Holmberg, C. and H. Alvestrand, "Multiplexing Negotiation
              Using Session Description Protocol (SDP) Port Numbers",
              draft-ietf-mmusic-sdp-bundle-negotiation-00 (work in
              progress), February 2012.

              Lennox, J. and M. Westerlund, "Real-Time Transport
              Protocol (RTP) Considerations for Multi-Stream Endpoints",
              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.

12.2.  Informative References

              Westerlund, M., Burman, B., and C. Perkins, "RTP
              Multiplexing Architecture",
              draft-westerlund-avtcore-multiplex-architecture-01 (work
              in progress), March 2012.

              Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a
              Single Lower-Layer Transport",
              draft-westerlund-avtcore-transport-multiplexing-02 (work
              in progress), March 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.

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   [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.

   [RFC5117]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
              January 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
   Farogatan 6
   SE-164 80 Kista

   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

   Email: jonathan@vidyo.com

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