draft-ietf-avtcore-multiplex-guidelines-10.txt   draft-ietf-avtcore-multiplex-guidelines-11.txt 
Network Working Group M. Westerlund Network Working Group M. Westerlund
Internet-Draft B. Burman Internet-Draft B. Burman
Intended status: Informational Ericsson Intended status: Informational Ericsson
Expires: August 17, 2020 C. Perkins Expires: August 21, 2020 C. Perkins
University of Glasgow University of Glasgow
H. Alvestrand H. Alvestrand
Google Google
R. Even R. Even
Huawei Huawei
February 14, 2020 February 18, 2020
Guidelines for using the Multiplexing Features of RTP to Support Guidelines for using the Multiplexing Features of RTP to Support
Multiple Media Streams Multiple Media Streams
draft-ietf-avtcore-multiplex-guidelines-10 draft-ietf-avtcore-multiplex-guidelines-11
Abstract Abstract
The Real-time Transport Protocol (RTP) is a flexible protocol that The Real-time Transport Protocol (RTP) is a flexible protocol that
can be used in a wide range of applications, networks, and system can be used in a wide range of applications, networks, and system
topologies. That flexibility makes for wide applicability, but can topologies. That flexibility makes for wide applicability, but can
complicate the application design process. One particular design complicate the application design process. One particular design
question that has received much attention is how to support multiple question that has received much attention is how to support multiple
media streams in RTP. This memo discusses the available options and media streams in RTP. This memo discusses the available options and
design trade-offs, and provides guidelines on how to use the design trade-offs, and provides guidelines on how to use the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 17, 2020. This Internet-Draft will expire on August 21, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Subjects Out of Scope . . . . . . . . . . . . . . . . . . 5 2.2. Subjects Out of Scope . . . . . . . . . . . . . . . . . . 5
3. RTP Multiplexing Overview . . . . . . . . . . . . . . . . . . 5 3. RTP Multiplexing Overview . . . . . . . . . . . . . . . . . . 5
3.1. Reasons for Multiplexing and Grouping RTP Streams . . . . 5 3.1. Reasons for Multiplexing and Grouping RTP Streams . . . . 5
3.2. RTP Multiplexing Points . . . . . . . . . . . . . . . . . 6 3.2. RTP Multiplexing Points . . . . . . . . . . . . . . . . . 6
3.2.1. RTP Session . . . . . . . . . . . . . . . . . . . . . 7 3.2.1. RTP Session . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Synchronisation Source (SSRC) . . . . . . . . . . . . 8 3.2.2. Synchronisation Source (SSRC) . . . . . . . . . . . . 8
3.2.3. Contributing Source (CSRC) . . . . . . . . . . . . . 10 3.2.3. Contributing Source (CSRC) . . . . . . . . . . . . . 10
3.2.4. RTP Payload Type . . . . . . . . . . . . . . . . . . 10 3.2.4. RTP Payload Type . . . . . . . . . . . . . . . . . . 10
3.3. Issues Related to RTP Topologies . . . . . . . . . . . . 11 3.3. Issues Related to RTP Topologies . . . . . . . . . . . . 11
3.4. Issues Related to RTP and RTCP Protocol . . . . . . . . . 12 3.4. Issues Related to RTP and RTCP Protocol . . . . . . . . . 13
3.4.1. The RTP Specification . . . . . . . . . . . . . . . . 13 3.4.1. The RTP Specification . . . . . . . . . . . . . . . . 13
3.4.2. Multiple SSRCs in a Session . . . . . . . . . . . . . 14 3.4.2. Multiple SSRCs in a Session . . . . . . . . . . . . . 14
3.4.3. Binding Related Sources . . . . . . . . . . . . . . . 15 3.4.3. Binding Related Sources . . . . . . . . . . . . . . . 15
3.4.4. Forward Error Correction . . . . . . . . . . . . . . 16 3.4.4. Forward Error Correction . . . . . . . . . . . . . . 17
4. Considerations for RTP Multiplexing . . . . . . . . . . . . . 17 4. Considerations for RTP Multiplexing . . . . . . . . . . . . . 17
4.1. Interworking Considerations . . . . . . . . . . . . . . . 17 4.1. Interworking Considerations . . . . . . . . . . . . . . . 17
4.1.1. Application Interworking . . . . . . . . . . . . . . 17 4.1.1. Application Interworking . . . . . . . . . . . . . . 17
4.1.2. RTP Translator Interworking . . . . . . . . . . . . . 18 4.1.2. RTP Translator Interworking . . . . . . . . . . . . . 18
4.1.3. Gateway Interworking . . . . . . . . . . . . . . . . 18 4.1.3. Gateway Interworking . . . . . . . . . . . . . . . . 18
4.1.4. Multiple SSRC Legacy Considerations . . . . . . . . . 19 4.1.4. Multiple SSRC Legacy Considerations . . . . . . . . . 19
4.2. Network Considerations . . . . . . . . . . . . . . . . . 20 4.2. Network Considerations . . . . . . . . . . . . . . . . . 20
4.2.1. Quality of Service . . . . . . . . . . . . . . . . . 20 4.2.1. Quality of Service . . . . . . . . . . . . . . . . . 20
4.2.2. NAT and Firewall Traversal . . . . . . . . . . . . . 21 4.2.2. NAT and Firewall Traversal . . . . . . . . . . . . . 21
4.2.3. Multicast . . . . . . . . . . . . . . . . . . . . . . 22 4.2.3. Multicast . . . . . . . . . . . . . . . . . . . . . . 22
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| packets | packets
+-- v +-- v
| +------------+ | +------------+
| | Socket | Transport Protocol Demultiplexing | | Socket | Transport Protocol Demultiplexing
| +------------+ | +------------+
| || || | || ||
RTP | RTP/ || |+-----> DTLS (SRTP Keying, SCTP, etc) RTP | RTP/ || |+-----> DTLS (SRTP Keying, SCTP, etc)
Session | RTCP || +------> STUN (multiplexed using same port) Session | RTCP || +------> STUN (multiplexed using same port)
+-- || +-- ||
+-- || +-- ||
| (split by MID/RID and/or SSRC) | (split by SSRC) +---> Identify SSRC collision
| || || || || | || || || ||
| (associate with signalling by MID/RID)
| || || || || | || || || ||
RTP | +--+ +--+ +--+ +--+ Jitter buffer, RTP | +--+ +--+ +--+ +--+ Jitter buffer,
Streams | |PB| |PB| |PB| |PB| process RTCP, etc. Streams | |PB| |PB| |PB| |PB| process RTCP, etc.
| +--+ +--+ +--+ +--+ | +--+ +--+ +--+ +--+
+-- | | | | +-- | | | |
(select decoder based on PT) (select decoder based on PT)
+-- | / | / +-- | / | /
| +----+ | / | +----+ | /
| / | | | / | |
Payload | +---+ +---+ +---+ Payload | +---+ +---+ +---+
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correlating SSRCs within and between RTP Sessions as coming from the correlating SSRCs within and between RTP Sessions as coming from the
same endpoint. The main property attributed to SSRCs associated with same endpoint. The main property attributed to SSRCs associated with
the same CNAME is that they are from a particular synchronisation the same CNAME is that they are from a particular synchronisation
context and can be synchronised at playback. context and can be synchronised at playback.
An RTP receiver receiving a previously unseen SSRC value will An RTP receiver receiving a previously unseen SSRC value will
interpret it as a new source. It might in fact be a previously interpret it as a new source. It might in fact be a previously
existing source that had to change SSRC number due to an SSRC existing source that had to change SSRC number due to an SSRC
conflict. Use of the MID extension conflict. Use of the MID extension
[I-D.ietf-mmusic-sdp-bundle-negotiation] helps to identify which [I-D.ietf-mmusic-sdp-bundle-negotiation] helps to identify which
media source the apparently new source belongs to and use of the RID media source the new SSRC represents and use of the RID extension
extension [I-D.ietf-mmusic-rid] helps to identify what encoding or [I-D.ietf-mmusic-rid] helps to identify what encoding or redundancy
redundancy stream it represents, even though the SSRC changed. stream it represents, even though the SSRC changed. However, the
However, the originator of the previous SSRC ought to have ended the originator of the previous SSRC ought to have ended the conflicting
conflicting source by sending an RTCP BYE for it prior to starting to source by sending an RTCP BYE for it prior to starting to send with
send with the new SSRC, making the new SSRC a new source. the new SSRC, making the new SSRC a new source.
3.2.3. Contributing Source (CSRC) 3.2.3. Contributing Source (CSRC)
The Contributing Source (CSRC) is not a separate identifier. Rather The Contributing Source (CSRC) is not a separate identifier. Rather
an SSRC identifier is listed as a CSRC in the RTP header of a packet an SSRC identifier is listed as a CSRC in the RTP header of a packet
generated by an RTP mixer or video MCU/switch, if the corresponding generated by an RTP mixer or video MCU/switch, if the corresponding
SSRC was in the header of one of the packets that contributed to the SSRC was in the header of one of the packets that contributed to the
output. output.
It is not possible, in general, to extract media represented by an It is not possible, in general, to extract media represented by an
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with how the RTP session participants are interconnected, described with how the RTP session participants are interconnected, described
by RTP Topology [RFC7667]. by RTP Topology [RFC7667].
Even the most basic use case, denoted Topo-Point-to-Point in Even the most basic use case, denoted Topo-Point-to-Point in
[RFC7667], raises a number of considerations that are discussed in [RFC7667], raises a number of considerations that are discussed in
detail in following sections. They range over such aspects as: detail in following sections. They range over such aspects as:
o Does my communication peer support RTP as defined with multiple o Does my communication peer support RTP as defined with multiple
SSRCs per RTP session? SSRCs per RTP session?
o Do I need network differentiation in form of QoS ( Section 4.2.1)? o Do I need network differentiation in form of QoS (Section 4.2.1)?
o Can the application more easily process and handle the media o Can the application more easily process and handle the media
streams if they are in different RTP sessions? streams if they are in different RTP sessions?
o Do I need to use additional RTP streams for RTP retransmission or o Do I need to use additional RTP streams for RTP retransmission or
FEC? FEC?
For some point to multi-point topologies (e.g. Topo-ASM and Topo-SSM For some point to multi-point topologies (e.g. Topo-ASM and Topo-SSM
in [RFC7667]), multicast is used to interconnect the session in [RFC7667]), multicast is used to interconnect the session
participants. Special considerations (documented in Section 4.2.3) participants. Special considerations (documented in Section 4.2.3)
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