One document matched: draft-ietf-avtext-rtp-grouping-taxonomy-01.xml
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<front>
<title abbrev="RTP Grouping Taxonomy">A Taxonomy of Grouping Semantics and
Mechanisms for Real-Time Transport Protocol (RTP) Sources</title>
<author fullname="Jonathan Lennox" initials="J." surname="Lennox">
<organization abbrev="Vidyo">Vidyo, Inc.</organization>
<address>
<postal>
<street>433 Hackensack Avenue</street>
<street>Seventh Floor</street>
<city>Hackensack</city>
<region>NJ</region>
<code>07601</code>
<country>US</country>
</postal>
<email>jonathan@vidyo.com</email>
</address>
</author>
<author fullname="Kevin Gross" initials="K." surname="Gross">
<organization abbrev="AVA">AVA Networks, LLC</organization>
<address>
<postal>
<street/>
<city>Boulder</city>
<region>CO</region>
<country>US</country>
</postal>
<email>kevin.gross@avanw.com</email>
</address>
</author>
<author fullname="Suhas Nandakumar" initials="S" surname="Nandakumar">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>US</country>
</postal>
<email>snandaku@cisco.com</email>
</address>
</author>
<author fullname="Gonzalo Salgueiro" initials="G" surname="Salgueiro">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>7200-12 Kit Creek Road</street>
<city>Research Triangle Park</city>
<region>NC</region>
<code>27709</code>
<country>US</country>
</postal>
<email>gsalguei@cisco.com</email>
</address>
</author>
<author fullname="Bo Burman" initials="B." surname="Burman">
<organization>Ericsson</organization>
<address>
<postal>
<street>Farogatan 6</street>
<city>SE-164 80 Kista</city>
<country>Sweden</country>
</postal>
<phone>+46 10 714 13 11</phone>
<email>bo.burman@ericsson.com</email>
</address>
</author>
<date day="14" month="February" year="2014"/>
<area>Real Time Applications and Infrastructure (RAI)</area>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<!-- TODO: more keywords -->
<abstract>
<t>The terminology about, and associations among, Real-Time Transport
Protocol (RTP) sources can be complex and somewhat opaque. This document
describes a number of existing and proposed relationships among RTP
sources, and attempts to define common terminology for discussing
protocol entities and their relationships.</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>The existing taxonomy of sources in RTP is often regarded as
confusing and inconsistent. Consequently, a deep understanding of how
the different terms relate to each other becomes a real challenge.
Frequently cited examples of this confusion are (1) how different
protocols that make use of RTP use the same terms to signify different
things and (2) how the complexities addressed at one layer are often
glossed over or ignored at another.</t>
<t>This document attempts to provide some clarity by reviewing the
semantics of various aspects of sources in RTP. As an organizing
mechanism, it approaches this by describing various ways that RTP
sources can be grouped and associated together.</t>
<t>All non-specific references to ControLling mUltiple streams for
tElepresence (CLUE) in this document map to <xref
target="I-D.ietf-clue-framework"/> and all references to Web Real-Time
Communications (WebRTC) map to <xref
target="I-D.ietf-rtcweb-overview"/>.</t>
</section>
<!--TODO: Remove heading level 4 entirely and replace with prose-->
<section title="Concepts">
<t>This section defines concepts that serve to identify and name various
transformations and streams in a given RTP usage. For each concept an
attempt is made to list any alternate definitions and usages that
co-exist today along with various characteristics that further describes
the concept. These concepts are divided into two categories, one related
to the chain of streams and transformations that media can be subject
to, the other for entities involved in the communication.</t>
<section title="Media Chain">
<t>In the context of this memo, Media is a sequence of synthetic or
<xref target="physical-stimulus">Physical Stimulus</xref> (sound
waves, photons, key-strokes), represented in digital form. Synthesized
Media is typically generated directly in the digital domain.</t>
<t>This section contains the concepts that can be involved in taking
Media at a sender side and transporting it to a receiver, which may
recover a sequence of physical stimulus. This chain of concepts is of
two main types, streams and transformations. Streams are time-based
sequences of samples of the physical stimulus in various
representations, while transformations changes the representation of
the streams in some way.</t>
<t>The below examples are basic ones and it is important to keep in
mind that this conceptual model enables more complex usages. Some will
be further discussed in later sections of this document. In general
the following applies to this model:<list style="symbols">
<t>A transformation may have zero or more inputs and one or more
outputs.</t>
<t>A Stream is of some type.</t>
<t>A Stream has one source transformation and one or more sink
transformation (with the exception of <xref
target="physical-stimulus">Physical Stimulus</xref> that can have
no source or sink transformation).</t>
<t>Streams can be forwarded from a transformation output to any
number of inputs on other transformations that support that
type.</t>
<t>If the output of a transformation is sent to multiple
transformations, those streams will be identical; it takes a
transformation to make them different.</t>
<t>There are no formal limitations on how streams are connected to
transformations, this may include loops if required by a
particular transformation.</t>
</list>It is also important to remember that this is a conceptual
model. Thus real-world implementations may look different and have
different structure.</t>
<t>To provide a basic understanding of the relationships in the chain
we below first introduce the concepts for the <xref
target="fig-sender-chain">sender side</xref>. This covers physical
stimulus until media packets are emitted onto the network.</t>
<figure align="center" anchor="fig-sender-chain"
title="Sender Side Concepts in the Media Chain">
<artwork><![CDATA[ Physical Stimulus
|
V
+--------------------+
| Media Capture |
+--------------------+
|
Raw Stream
V
+--------------------+
| Media Source |<- Synchronization Timing
+--------------------+
|
Source Stream
V
+--------------------+
| Media Encoder |
+--------------------+
|
Encoded Stream +-----------+
V | V
+--------------------+ | +--------------------+
| Media Packetizer | | | Media Redundancy |
+--------------------+ | +--------------------+
| | |
+------------+ Redundancy Packet Stream
Source Packet Stream |
V V
+--------------------+ +--------------------+
| Media Transport | | Media Transport |
+--------------------+ +--------------------+
]]></artwork>
</figure>
<t>In <xref target="fig-sender-chain"/> we have included a branched
chain to cover the concepts for using redundancy to improve the
reliability of the transport. The Media Transport concept is an
aggregate that is decomposed below in <xref
target="media-transport"/>.</t>
<t>Below we review a <xref target="fig-receiver-chain">receiver media
chain</xref> matching the sender side to look at the inverse
transformations and their attempts to recover possibly identical
streams as in the sender chain. Note that the streams out of a reverse
transformation, like the Source Stream out the Media Decoder are in
many cases not the same as the corresponding ones on the sender side,
thus they are prefixed with a "Received" to denote a potentially
modified version. The reason for not being the same lies in the
transformations that can be of irreversible type. For example, lossy
source coding in the Media Encoder prevents the Source Stream out of
the Media Decoder to be the same as the one fed into the Media
Encoder. Other reasons include packet loss or late loss in the Media
Transport transformation that even Media Repair, if used, fails to
repair. It should be noted that some transformations are not always
present, like Media Repair that cannot operate without Redundancy
Packet Streams.</t>
<figure align="center" anchor="fig-receiver-chain"
title="Receiver Side Concepts of the Media Chain">
<artwork><![CDATA[+--------------------+ +--------------------+
| Media Transport | | Media Transport |
+--------------------+ +--------------------+
| |
Received Packet Stream Received Redundancy PS
| |
| +-------------------+
V V
+--------------------+
| Media Repair |
+--------------------+
|
Repaired Packet Stream
V
+--------------------+
| Media Depacketizer |
+--------------------+
|
Received Encoded Stream
V
+--------------------+
| Media Decoder |
+--------------------+
|
Received Source Stream
V
+--------------------+
| Media Sink |--> Synchronization Information
+--------------------+
|
Received Raw Stream
V
+--------------------+
| Media Renderer |
+--------------------+
|
V
Physical Stimulus
]]></artwork>
</figure>
<section anchor="physical-stimulus" title="Physical Stimulus">
<t>The physical stimulus is a physical event that can be measured
and converted to digital form by an appropriate sensor or
transducer. This include sound waves making up audio, photons in a
light field that is visible, or other excitations or interactions
with sensors, like keystrokes on a keyboard.</t>
</section>
<section anchor="media-capture" title="Media Capture">
<t>Media Capture is the process of transforming the <xref
target="physical-stimulus">Physical Stimulus</xref> into digital
Media using an appropriate sensor or transducer. The Media Capture
performs a digital sampling of the physical stimulus, usually
periodically, and outputs this in some representation as a <xref
target="raw-stream">Raw Stream</xref>. This data is due to its
periodical sampling, or at least being timed asynchronous events,
some form of a stream of media data. The Media Capture is normally
instantiated in some type of device, i.e. media capture device.
Examples of different types of media capturing devices are digital
cameras, microphones connected to A/D converters, or keyboards.</t>
<t>Alternate usages:<list style="symbols">
<t>The CLUE WG uses the term "Capture Device" to identify a
physical capture device.</t>
<t>WebRTC WG uses the term "Recording Device" to refer to the
locally available capture devices in an end-system.</t>
</list>Characteristics:<list style="symbols">
<t>A Media Capture is identified either by hardware/manufacturer
ID or via a session-scoped device identifier as mandated by the
application usage.</t>
<t>A Media Capture can generate an <xref
target="encoded-stream">Encoded Stream </xref> if the capture
device support such a configuration.</t>
</list></t>
</section>
<section anchor="raw-stream" title="Raw Stream">
<t>The time progressing stream of digitally sampled information,
usually periodically sampled and provided by a <xref
target="media-capture">Media Capture</xref>. A Raw Stream can also
contain synthesized Media that may not require any explicit Media
Capture, since it is already in an appropriate digital form.</t>
</section>
<section anchor="media-source" title="Media Source">
<t>A Media Source is the logical source of a reference clock
synchronized, time progressing, digital media stream, called a <xref
target="source-stream">Source Stream</xref>. This transformation
takes one or more <xref target="raw-stream">Raw Streams</xref> and
provides a Source Stream as output. This output has been
synchronized with some reference clock, even if just a system local
wall clock.</t>
<t>The output can be of different types. One type is directly
associated with a particular Media Capture's Raw Stream. Others are
more conceptual sources, like an <xref
target="fig-media-source-mixer">audio mix of multiple Raw
Streams</xref>, a mixed selection of the three loudest inputs
regarding speech activity, a selection of a particular video based
on the current speaker, i.e. typically based on other Media
Sources.</t>
<figure align="center" anchor="fig-media-source-mixer"
title="Conceptual Media Source in form of Audio Mixer">
<artwork><![CDATA[ Raw Raw Raw
Stream Stream Stream
| | |
V V V
+--------------------------+
| Media Source |<-- Reference Clock
| Mixer |
+--------------------------+
|
V
Source Stream
]]></artwork>
</figure>
<t>The CLUE WG uses the term "Media Capture" for this purpose. A
CLUE Media Capture is identified via indexed notation. The terms
Audio Capture and Video Capture are used to identify Audio Sources
and Video Sources respectively. Concepts such as "Capture Scene",
"Capture Scene Entry" and "Capture" provide a flexible framework to
represent media captured spanning spatial regions.</t>
<t>The WebRTC WG defines the term "RtcMediaStreamTrack" to refer to
a Media Source. An "RtcMediaStreamTrack" is identified by the ID
attribute.</t>
<!--MW: I think the below SDP is a bit misplaced. Do we need a special section to discuss
relation to SDP terminology. Or should this be focused and other interpretations
be added?-->
<t>Typically a Media Source is mapped to a single m=line via the
Session Description Protocol (SDP) <xref target="RFC4566"/> unless
mechanisms such as Source-Specific attributes are in place <xref
target="RFC5576"/>. In the latter cases, an m=line can represent
either multiple Media Sources, multiple <xref
target="packet-stream">Packet Streams</xref>, or both.</t>
<t>Characteristics:<list style="symbols">
<t>At any point, it can represent a physical captured source or
conceptual source.</t>
<!--MW: Put back a discussion of relation between Media Capture and Media sources?-->
</list></t>
</section>
<section anchor="source-stream" title="Source Stream">
<t>A time progressing stream of digital samples that has been
synchronized with a reference clock and comes from particular <xref
target="media-source">Media Source</xref>.</t>
</section>
<section anchor="media-encoder" title="Media Encoder">
<t>A Media Encoder is a transform that is responsible for encoding
the media data from a <xref target="source-stream">Source
Stream</xref> into another representation, usually more compact,
that is output as an <xref target="encoded-stream">Encoded
Stream</xref>.</t>
<t>The Media Encoder step commonly includes pre-encoding
transformations, such as scaling, resampling etc. The Media Encoder
can have a significant number of configuration options that affects
the properties of the encoded stream. This include properties such
as bit-rate, start points for decoding, resolution, bandwidth or
other fidelity affecting properties. The actually used codec is also
an important factor in many communication systems, not only its
parameters.</t>
<t>Scalable Media Encoders need special mentioning as they produce
multiple outputs that are potentially of different types. A scalable
Media Encoder takes one input Source Stream and encodes it into
multiple output streams of two different types; at least one Encoded
Stream that is independently decodable and one or more <xref
target="dependent-stream">Dependent Streams</xref> that requires at
least one Encoded Stream and zero or more Dependent Streams to be
possible to decode. A Dependent Stream's dependency is one of the
grouping relations this document discusses further in <xref
target="lms"/>.</t>
<figure align="center" anchor="fig-scalable-media-encoder"
title="Scalable Media Encoder Input and Outputs">
<artwork><![CDATA[ Source Stream
|
V
+--------------------------+
| Scalable Media Encoder |
+--------------------------+
| | ... |
V V V
Encoded Dependent Dependent
Stream Stream Stream
]]></artwork>
</figure>
<t>There are also other variants of encoders, like so-called
Multiple Description Coding (MDC). Such Media Encoder produce
multiple independent and thus individually decodable Encoded Streams
that are possible to combine into a Received Source Stream that is
somehow a better representation of the original Source Stream than
using only a single Encoded Stream.</t>
<t>Alternate usages:<list style="symbols">
<t>Within the SDP usage, an SDP media description (m=line)
describes part of the necessary configuration required for
encoding purposes.</t>
<t>CLUE's "Capture Encoding" provides specific encoding
configuration for this purpose.</t>
</list>Characteristics:<list style="symbols">
<t>A Media Source can be multiply encoded by different Media
Encoders to provide various encoded representations.</t>
</list></t>
</section>
<section anchor="encoded-stream" title="Encoded Stream">
<t>A stream of time synchronized encoded media that can be
independently decoded.</t>
<t>Characteristics:<list style="symbols">
<t>Due to temporal dependencies, an Encoded Stream may have
limitations in where decoding can be started. These entry
points, for example Intra frames from a video encoder, may
require identification and their generation may be event based
or configured to occur periodically.</t>
</list></t>
</section>
<section anchor="dependent-stream" title="Dependent Stream">
<t>A stream of time synchronized encoded media fragments that are
dependent on one or more <xref target="encoded-stream">Encoded
Streams</xref> and zero or more Dependent Streams to be possible to
decode.</t>
<t>Characteristics:<list style="symbols">
<t>Each Dependent Stream has a set of dependencies. These
dependencies must be understood by the parties in a multi-media
session that intend to use a Dependent Stream.</t>
</list></t>
</section>
<section anchor="media_packetizer" title="Media Packetizer">
<t>The transformation of taking one or more <xref
target="encoded-stream">Encoded</xref> or <xref
target="dependent-stream">Dependent Stream</xref> and put their
content into one or more sequences of packets, normally RTP packets,
and output <xref target="packet-stream">Source Packet
Streams</xref>. This step includes both generating RTP payloads as
well as RTP packets.</t>
<t>The Media Packetizer can use multiple inputs when producing a
single Packet Stream. One such example is <xref target="sstmst">SST
packetization when using SVC</xref>.</t>
<t>The Media Packetizer can also produce multiple Packet Streams,
for example when Encoded and/or Dependent Streams are distributed
over multiple Packet Streams. One example of this is <xref
target="sstmst">MST packetization when using SVC</xref>.</t>
<t>Alternate usages:<list style="symbols">
<t>An RTP sender is part of the Media Packetizer.</t>
</list>Characteristics:<list style="symbols">
<t>The Media Packetizer will select which Synchronization
source(s) (SSRC) <xref target="RFC3550"/> in which RTP sessions
that are used.</t>
<t>Media Packetizer can combine multiple Encoded or Dependent
Streams into one or more Packet Streams.</t>
</list></t>
</section>
<section anchor="packet-stream" title="Packet Stream">
<t>A stream of RTP packets containing media data, source or
redundant. The Packet Stream is identified by an SSRC belonging to a
particular RTP session. The RTP session is identified as discussed
in <xref target="rtp-session"/>.</t>
<t>A Source Packet Stream is a packet stream containing at least
some content from an Encoded Stream. Source material is any media
material that is produced for transport over RTP without any
additional redundancy applied to cope with network transport losses.
Compare this with the <xref
target="redundancy-packet-stream">Redundancy Packet
Stream</xref>.</t>
<t>Alternate usages:<list style="symbols">
<t>The term "Stream" is used by the CLUE WG to define an encoded
Media Source sent via RTP. "Capture Encoding", "Encoding Groups"
are defined to capture specific details of the encoding
scheme.</t>
<t>RFC3550 <xref target="RFC3550"/> uses the terms media stream,
audio stream, video stream and streams of (RTP) packets
interchangeably. It defines the SSRC as the "The source of a
stream of RTP packets, ...".</t>
<t>The equivalent mapping of a Packet Stream in SDP <xref
target="RFC4566"/> is defined per usage. For example, each Media
Description (m=line) and associated attributes can describe one
Packet Stream OR properties for multiple Packet Streams OR for
an RTP session (via <xref target="RFC5576"/> mechanisms for
example).</t>
</list></t>
<t>Characteristics:<list style="symbols">
<t>Each Packet Stream is identified by a unique Synchronization
source (SSRC) <xref target="RFC3550"/> that is carried in every
RTP and RTP Control Protocol (RTCP) packet header in a specific
RTP session context.</t>
<t>At any given point in time, a Packet Stream can have one and
only one SSRC. SSRC collision is a valid reason to change SSRC
for a Packet Stream, since the Packet Stream itself is not
changed in any way, only the identifying SSRC number.</t>
<t>Each Packet Stream defines a unique RTP sequence numbering
and timing space.</t>
<t>Several Packet Streams may map to a single Media Source via
the source transformations.</t>
<t>Several Packet Streams can be carried over a single RTP
Session.</t>
</list></t>
</section>
<section anchor="media-redundancy" title="Media Redundancy">
<t>Media redundancy is a transformation that generates redundant or
repair packets sent out as a Redundancy Packet Stream to mitigate
network transport impairments, like packet loss and delay.</t>
<t>The Media Redundancy exists in many flavors; they may be
generating independent Repair Streams that are used in addition to
the Source Stream (<xref target="RFC4588">RTP Retransmission</xref>
and some <xref target="RFC5109">FEC</xref>), they may generate a new
Source Stream by combining redundancy information with source
information (Using <xref target="RFC5109">XOR FEC</xref> as a <xref
target="RFC2198">redundancy payload</xref>), or completely replace
the source information with only redundancy packets.</t>
</section>
<section anchor="redundancy-packet-stream"
title="Redundancy Packet Stream">
<t>A <xref target="packet-stream">Packet Stream</xref> that contains
no original source data, only redundant data that may be combined
with one or more <xref target="received-packet-stream">Received
Packet Stream</xref> to produce <xref
target="repaired-packet-stream">Repaired Packet Streams</xref>.</t>
</section>
<section anchor="media-transport" title="Media Transport">
<t>A Media Transport defines the transformation that the <xref
target="packet-stream">Packet Streams</xref> are subjected to by the
end-to-end transport from one RTP sender to one specific RTP
receiver (an RTP session may contain multiple RTP receivers per
sender). Each Media Transport is defined by a transport association
that is identified by a 5-tuple (source address, source port,
destination address, destination port, transport protocol). Each
transport association normally contains only a single RTP session,
although a proposal exists for sending <xref
target="I-D.westerlund-avtcore-transport-multiplexing">multiple RTP
sessions over one transport association</xref>.</t>
<t>Characteristics:<list style="symbols">
<t>Media Transport transmits Packet Streams of RTP Packets from
a source transport address to a destination transport
address.</t>
</list></t>
<t>The Media Transport concept sometimes needs to be decomposed into
more steps to enable discussion of what a sender emits that gets
transformed by the network before it is received by the receiver.
Thus we provide also this <xref target="fig-media-transport">Media
Transport decomposition</xref>.</t>
<figure align="center" anchor="fig-media-transport"
title="Decomposition of Media Transport">
<artwork><![CDATA[ Packet Stream
|
V
+--------------------------+
| Media Transport Sender |
+--------------------------+
|
Sent Packet Stream
V
+--------------------------+
| Network Transport |
+--------------------------+
|
Transported Packet Stream
V
+--------------------------+
| Media Transport Receiver |
+--------------------------+
|
V
Received Packet Stream
]]></artwork>
</figure>
<section anchor="media-transport-sender"
title="Media Transport Sender">
<t>The first transformation within the <xref
target="media-transport">Media Transport</xref> is the Media
Transport Sender, where the sending <xref
target="end-point">End-Point</xref> takes a Packet Stream and
emits the packets onto the network using the transport association
established for this Media Transport thus creating a <xref
target="sent-packet-stream">Sent Packet Stream</xref>. In this
process it transforms the Packet Stream in several ways. First, it
gains the necessary protocol headers for the transport
association, for example IP and UDP headers, thus forming
IP/UDP/RTP packets. In addition, the Media Transport Sender may
queue, pace or otherwise affect how the packets are emitted onto
the network. Thus adding delay, jitter and inter packet spacings
that characterize the Sent Packet Stream.</t>
</section>
<section anchor="sent-packet-stream" title="Sent Packet Stream">
<t>The Sent Packet Stream is the Packet Stream as entering the
first hop of the network path to its destination. The Sent Packet
Stream is identified using network transport addresses, like for
IP/UDP the 5-tuple (source IP address, source port, destination IP
address, destination port, and protocol (UDP)).</t>
</section>
<section anchor="network-transport" title="Network Transport">
<t>Network Transport is the transformation that the <xref
target="sent-packet-stream">Sent Packet Stream</xref> is subjected
to by traveling from the source to the destination through the
network. These transformations include, loss of some packets,
varying delay on a per packet basis, packet duplication, and
packet header or data corruption. These transformations produces a
<xref target="transported-packet-stream">Transported Packet
Stream</xref> at the exit of the network path.</t>
</section>
<section anchor="transported-packet-stream"
title="Transported Packet Stream">
<t>The Packet Stream that is emitted out of the network path at
the destination, subjected to the <xref
target="network-transport">Network Transport's
transformation</xref>.</t>
</section>
<section title="Media Transport Receiver">
<t>The receiver <xref target="end-point">End-Point's</xref>
transformation of the <xref
target="transported-packet-stream">Transported Packet
Stream</xref> by its reception process that result in the <xref
target="received-packet-stream">Received Packet Stream</xref>.
This transformation includes transport checksums being verified
and if non-matching, causing discarding of the corrupted packet.
Other transformations can include delay variations in receiving a
packet on the network interface and providing it to the
application.</t>
</section>
</section>
<section anchor="received-packet-stream"
title="Received Packet Stream">
<t>The <xref target="packet-stream">Packet Stream</xref> resulting
from the Media Transport's transformation, i.e. subjected to packet
loss, packet corruption, packet duplication and varying transmission
delay from sender to receiver.</t>
</section>
<section anchor="received-redundancy-ps"
title="Received Redundandy Packet Stream">
<t>The <xref target="redundancy-packet-stream">Redundancy Packet
Stream</xref> resulting from the Media Transport's transformation,
i.e. subjected to packet loss, packet corruption, and varying
transmission delay from sender to receiver.</t>
</section>
<section title="Media Repair">
<t>A Transformation that takes as input one or more <xref
target="packet-stream">Source Packet Streams</xref> as well as <xref
target="redundancy-packet-stream">Redundancy Packet Streams</xref>
and attempts to combine them to counter the transformations
introduced by the <xref target="media-transport">Media
Transport</xref> to minimize the difference between the <xref
target="source-stream">Source Stream</xref> and the <xref
target="received-source-stream">Received Source Stream</xref> after
<xref target="media-decoder">Media Decoder</xref>. The output is a
<xref target="repaired-packet-stream">Repaired Packet
Stream</xref>.</t>
</section>
<section anchor="repaired-packet-stream"
title="Repaired Packet Stream">
<t>A <xref target="received-packet-stream">Received Packet
Stream</xref> for which <xref
target="received-redundancy-ps">Received Redundancy Packet
Stream</xref> information has been used to try to re-create the
<xref target="packet-stream">Packet Stream</xref> as it was before
<xref target="media-transport">Media Transport</xref>.</t>
</section>
<section title="Media Depacketizer">
<t>A Media Depacketizer takes one or more <xref
target="packet-stream">Packet Streams</xref> and depacketizes them
and attempts to reconstitute the <xref
target="encoded-stream">Encoded Streams</xref> or <xref
target="dependent-stream">Dependent Streams</xref> present in those
Packet Streams.</t>
<t>It should be noted that in practical implementations, the Media
Depacketizer and the Media Decoder may be tightly coupled and share
information to improve or optimize the overall decoding process in
various ways. It is however not expected that there would be any
benefit in defining a taxonomy for those detailed (and likely very
implementation-dependent) steps.</t>
</section>
<section anchor="received-encoded-stream"
title="Received Encoded Stream">
<t>The received version of an <xref target="encoded-stream">Encoded
Stream</xref>.</t>
</section>
<section anchor="media-decoder" title="Media Decoder">
<t>A Media Decoder is a transformation that is responsible for
decoding <xref target="encoded-stream">Encoded Streams</xref> and
any <xref target="dependent-stream">Dependent Streams</xref> into a
<xref target="source-stream">Source Stream</xref>.</t>
<t>It should be noted that in practical implementations, the Media
Decoder and the Media Depacketizer may be tightly coupled and share
information to improve or optimize the overall decoding process in
various ways. It is however not expected that there would be any
benefit in defining a taxonomy for those detailed (and likely very
implementation-dependent) steps.</t>
<t>Alternate usages:<list style="symbols">
<t>Within the context of SDP, an m=line describes the necessary
configuration and identification (RTP Payload Types) required to
decode either one or more incoming Media Streams.</t>
</list></t>
<t>Characteristics:<list style="symbols">
<t>A Media Decoder is the entity that will have to deal with any
errors in the encoded streams that resulted from corruptions or
failures to repair packet losses. This as a media decoder
generally is forced to produce some output periodically. It thus
commonly includes concealment methods.</t>
</list></t>
</section>
<section anchor="received-source-stream"
title="Received Source Stream">
<t>The received version of a <xref target="source-stream">Source
Stream</xref>.</t>
</section>
<section anchor="media-sink" title="Media Sink">
<t>The Media Sink receives a <xref target="source-stream">Source
Stream</xref> that contains, usually periodically, sampled media
data together with associated synchronization information. Depending
on application, this Source Stream then needs to be transformed into
a <xref target="raw-stream">Raw Stream</xref> that is sent in
synchronization with the output from other Media Sinks to a <xref
target="media-render">Media Render</xref>. The media sink may also
be connected with a <xref target="media-source">Media Source</xref>
and be used as part of a conceptual Media Source.</t>
<t>Characteristics:<list style="symbols">
<t>The Media Sink can further transform the Source Stream into a
representation that is suitable for rendering on the Media
Render as defined by the application or system-wide
configuration. This include sample scaling, level adjustments
etc.</t>
</list></t>
</section>
<section title="Received Raw Stream">
<t>The received version of a <xref target="raw-stream">Raw
Stream</xref>.</t>
</section>
<section anchor="media-render" title="Media Render">
<t>A Media Render takes a <xref target="raw-stream">Raw
Stream</xref> and converts it into <xref
target="physical-stimulus">Physical Stimulus</xref> that a human
user can perceive. Examples of such devices are screens, D/A
converters connected to amplifiers and loudspeakers.</t>
<t>Characteristics:<list style="symbols">
<t>An End Point can potentially have multiple Media Renders for
each media type.</t>
</list></t>
</section>
</section>
<section anchor="communication-entities" title="Communication Entities">
<t>This section contains concept for entities involved in the
communication.</t>
<section anchor="end-point" title="End Point">
<t>A single addressable 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
"End Point".</t>
<t>Alternate usages:<list style="symbols">
<t>The CLUE Working Group (WG) uses the terms "Media Provider"
and "Media Consumer" to describes aspects of End Point
pertaining to sending and receiving functionalities.</t>
</list></t>
<t>Characteristics:<list style="symbols">
<t>End Points can be identified in several different ways. While
RTCP Canonical Names (CNAMEs) <xref target="RFC3550"/> provide a
globally unique and stable identification mechanism for the
duration of the Communication Session (see <xref
target="comm-session"/>), their validity applies exclusively
within a <xref target="syncontext">Synchronization
Context</xref>. Thus one End Point can have multiple CNAMEs.
Therefore, mechanisms outside the scope of RTP, such as
application defined mechanisms, must be used to ensure End Point
identification when outside this Synchronization Context.</t>
</list></t>
</section>
<section anchor="rtp-session" title="RTP Session">
<t>An RTP session is an association among a group of participants
communicating with RTP. It is a group communications channel which
can potentially carry a number of Packet Streams. Within an RTP
session, every participant can find meta-data and control
information (over RTCP) about all the Packet Streams in the RTP
session. The bandwidth of the RTCP control channel is shared between
all participants within an RTP Session.</t>
<t>Alternate usages:<list style="symbols">
<t>Within the context of SDP, a singe m=line can map to a single
RTP Session or multiple m=lines can map to a single RTP Session.
The latter is enabled via multiplexing schemes such as BUNDLE
<xref target="I-D.ietf-mmusic-sdp-bundle-negotiation"/>, for
example, which allows mapping of multiple m=lines to a single
RTP Session.</t>
</list></t>
<t>Characteristics:<list style="symbols">
<t>Typically, an RTP Session can carry one ore more Packet
Streams.</t>
<t>An RTP Session shares a single SSRC space as defined in
RFC3550 <xref target="RFC3550"/>. That is, the End Points
participating in an RTP Session can see an SSRC identifier
transmitted by any of the other End Points. An End Point can
receive an SSRC either as SSRC or as a Contributing source
(CSRC) in RTP and RTCP packets, as defined by the endpoints'
network interconnection topology.</t>
<t>An RTP Session uses at least two <xref
target="media-transport">Media Transports</xref>, one for
sending and one for receiving. Commonly, the receiving one is
the reverse direction of the same one as used for sending. An
RTP Session may use many Media Transports and these define the
session's network interconnection topology. A single Media
Transport can normally not transport more than one RTP Session,
unless a solution for multiplexing multiple RTP sessions over a
single Media Transport is used. One example of such a scheme is
<xref
target="I-D.westerlund-avtcore-transport-multiplexing">Multiple
RTP Sessions on a Single Lower-Layer Transport</xref>.</t>
<t>Multiple RTP Sessions can be related.</t>
</list></t>
</section>
<section anchor="participant" title="Participant">
<t>A participant is an entity reachable by a single signaling
address, and is thus related more to the signaling context than to
the media context.</t>
<t>Characteristics:<list style="symbols">
<t>A single signaling-addressable entity, using an
application-specific signaling address space, for example a SIP
URI.</t>
<t>A participant can have several <xref
target="multimedia-session">Multimedia Sessions</xref>.</t>
<t>A participant can have several associated transport flows,
including several separate local transport addresses for those
transport flows.</t>
<!--MW: I can't understand what the purpose is of the last bullet regarding many
transport flows. It needs to be aligned with the rest of the concept language.
But I am unable to change it because I don't understand what one attempts
to say.
BoB: Speculatively, it is just trying to prohibit definig a Participant as
being one end of a single Media Transport. This bullet is then not needed,
as a single Multimedia Session can already have multiple Media Transports.
-->
</list></t>
</section>
<section anchor="multimedia-session" title="Multimedia Session">
<t>A multimedia session is an association among a group of
participants engaged in the communication via one or more <xref
target="rtp-session">RTP Sessions</xref>. It defines logical
relationships among <xref target="media-source">Media Sources</xref>
that appear in multiple RTP Sessions.</t>
<t>Alternate usages:<list style="symbols">
<t>RFC4566 <xref target="RFC4566"/> defines a multimedia session
as a set of multimedia senders and receivers and the data
streams flowing from senders to receivers.</t>
<t>RFC3550 <xref target="RFC3550"/> defines it as set of
concurrent RTP sessions among a common group of participants.
For example, a video conference (which is a multimedia session)
may contain an audio RTP session and a video RTP session.</t>
</list></t>
<t>Characteristics:<list style="symbols">
<t>A Multimedia Session can be composed of several parallel RTP
Sessions with potentially multiple Packet Streams per RTP
Session.</t>
<t>Each participant in a Multimedia Session can have a multitude
of Media Captures and Media Rendering devices.</t>
</list></t>
</section>
<section anchor="comm-session" title="Communication Session">
<t>A Communication Session is an association among group of
participants communicating with each other via a set of Multimedia
Sessions.</t>
<t>Alternate usages:<list style="symbols">
<t>The <xref target="RFC4566">Session Description Protocol
(SDP)</xref> defines a multimedia session as a set of multimedia
senders and receivers and the data streams flowing from senders
to receivers. In that definition it is however not clear if a
multimedia session includes both the sender's and the receiver's
view of the same RTP Packet Stream.</t>
</list></t>
<t>Characteristics:<list style="symbols">
<t>Each participant in a Communication Session is identified via
an application-specific signaling address.</t>
<t>A Communication Session is composed of at least one
Multimedia Session per participant, involving one or more
parallel RTP Sessions with potentially multiple Packet Streams
per RTP Session.</t>
</list></t>
<t>For example, in a full mesh communication, the Communication
Session consists of a set of separate Multimedia Sessions between
each pair of Participants. Another example is a centralized
conference, where the Communication Session consists of a set of
Multimedia Sessions between each Participant and the conference
handler.</t>
</section>
</section>
</section>
<section title="Relations at Different Levels">
<t>This section uses the concepts from previous section and look at
different types of relationships among them. These relationships occur
at different levels and for different purposes. The section is organized
such as to look at the level where a relation is required. The reason
for the relationship may exist at another step in the media handling
chain. For example, using Simulcast (discussed in <xref
target="simulcast"/>) needs to determine relations at Packet Stream
level, however the reason to relate Packet Streams is that multiple
Media Encoders use the same Media Source, i.e. to be able to identify a
common Media Source.</t>
<section title="Media Source Relations">
<t><xref target="media-source">Media Sources</xref> are commonly
grouped and related to an <xref target="end-point">End Point</xref> or
a <xref target="participant">Participant</xref>. This occurs for
several reasons; both application logic as well as media handling
purposes. These cases are further discussed below.</t>
<section anchor="syncontext" title="Synchronization Context">
<t>A Synchronization Context defines a requirement on a strong
timing relationship between the Media Sources, typically requiring
alignment of clock sources. Such relationship can be identified in
multiple ways as listed below. A single Media Source can only belong
to a single Synchronization Context, since it is assumed that a
single Media Source can only have a single media clock and requiring
alignment to several Synchronization Contexts (and thus reference
clocks) will effectively merge those into a single Synchronization
Context.</t>
<!--MW: The following paragraph may be quite misplaced. Should be reconsidered when improving
text for the relations between RTP Sessions, Multimedia Sessions and Communication
Sessions.-->
<t>A single Multimedia Session can contain media from one or more
Synchronization Contexts. An example of that is a Multimedia Session
containing one set of audio and video for communication purposes
belonging to one Synchronization Context, and another set of audio
and video for presentation purposes (like playing a video file) with
a separate Synchronization Context that has no strong timing
relationship and need not be strictly synchronized with the audio
and video used for communication.</t>
<section title="RTCP CNAME">
<t>RFC3550 <xref target="RFC3550"/> describes Inter-media
synchronization between RTP Sessions based on RTCP CNAME, RTP and
Network Time Protocol (NTP) <xref target="RFC5905"/> formatted
timestamps of a reference clock. As indicated in <xref
target="I-D.ietf-avtcore-clksrc"/>, despite using NTP format
timestamps, it is not required that the clock be synchronized to
an NTP source.</t>
</section>
<section title="Clock Source Signaling">
<t><xref target="I-D.ietf-avtcore-clksrc"/> provides a mechanism
to signal the clock source in SDP both for the reference clock as
well as the media clock, thus allowing a Synchronization Context
to be defined beyond the one defined by the usage of CNAME source
descriptions.</t>
</section>
<section title="CLUE Scenes">
<t>In CLUE "Capture Scene", "Capture Scene Entry" and "Captures"
define an implied Synchronization Context.</t>
</section>
<section title="Implicitly via RtcMediaStream">
<t>The WebRTC WG defines "RtcMediaStream" with one or more
"RtcMediaStreamTracks". All tracks in a "RtcMediaStream" are
intended to be possible to synchronize when rendered.</t>
</section>
<section title="Explicitly via SDP Mechanisms">
<t>RFC5888 <xref target="RFC5888"/> defines m=line grouping
mechanism called "Lip Synchronization (LS)" for establishing the
synchronization requirement across m=lines when they map to
individual sources.</t>
<t>RFC5576 <xref target="RFC5576"/> extends the above mechanism
when multiple media sources are described by a single m=line.</t>
</section>
</section>
<section title="End Point">
<t>Some applications requires knowledge of what Media Sources
originate from a particular <xref target="end-point">End
Point</xref>. This can include such decisions as packet routing
between parts of the topology, knowing the End Point origin of the
Packet Streams.</t>
<t>In RTP, this identification has been overloaded with the
Synchronization Context through the usage of the source description
CNAME item. This works for some usages, but sometimes it breaks
down. For example, if an End Point has two sets of Media Sources
that have different Synchronization Contexts, like the audio and
video of the human participant as well as a set of Media Sources of
audio and video for a shared movie. Thus, an End Point may have
multiple CNAMEs. The CNAMEs or the Media Sources themselves can be
related to the End Point.</t>
</section>
<section title="Participant">
<t>In communication scenarios, it is commonly needed to know which
Media Sources that originate from which <xref
target="participant">Participant</xref>. Thus enabling the
application to for example display Participant Identity information
correctly associated with the Media Sources. This association is
currently handled through the signaling solution to point at a
specific Multimedia Session where the Media Sources may be
explicitly or implicitly tied to a particular End Point.</t>
<t>Participant information becomes more problematic due to Media
Sources that are generated through mixing or other conceptual
processing of Raw Streams or Source Streams that originate from
different Participants. This type of Media Sources can thus have a
dynamically varying set of origins and Participants. RTP contains
the concept of Contributing Sources (CSRC) that carries such
information about the previous step origin of the included media
content on RTP level.</t>
</section>
<section title="WebRTC MediaStream">
<t>An RtcMediaStream, in addition to requiring a single
Synchronization Context as discussed above, is also an explicit
grouping of a set of Media Sources, as identified by
RtcMediaStreamTracks, within the RtcMediaStream.</t>
</section>
</section>
<section title="Packetization Time Relations">
<t>At RTP Packetization time, there exists a possibility for a number
of different types of relationships between <xref
target="encoded-stream">Encoded Streams</xref>, <xref
target="dependent-stream">Dependent Streams</xref> and <xref
target="packet-stream">Packet Streams</xref>. These are caused by
grouping together or distributing these different types of streams
into Packet Streams. This section will look at such relationships.</t>
<section anchor="sstmst"
title="Single and Multi-Session Transmission of SVC">
<t><xref target="RFC6190">Scalable Video Coding</xref> has a mode of
operation called Single Session Transmission (SST), where Encoded
Streams and Dependent Streams from the SVC Media Encoder are sent in
a single <xref target="rtp-session">RTP Session</xref> using the SVC
RTP Payload format. There is another mode of operation where Encoded
Streams and Dependent Streams are distributed across multiple RTP
Sessions, called Multi-Session Transmission (MST). Regardless if
used with SST or MST, as they are defined, each of those RTP
Sessions may contain one or more Packet Streams (SSRC) per Media
Source.</t>
<t>To elaborate, what could be called SST-SingleStream (SST-SS) uses
a single Packet Stream in a single RTP Session to send all Encoded
and Dependent Streams. Similarly, SST-MultiStream (SST-MS) uses
multiple Packet Streams in a single RTP Session to send the Encoded
and Dependent Streams. MST-SS uses a single Packet Stream in each of
multiple RTP Sessions and MST-MS uses multiple Packet Streams in
each of the multiple RTP Sessions:</t>
<texttable align="center">
<ttcol> </ttcol>
<ttcol>Single RTP Session</ttcol>
<ttcol>Multiple RTP Sessions</ttcol>
<c>Single Packet Stream</c>
<c>SST-SS</c>
<c>MST-SS</c>
<c>Multiple Packet Streams</c>
<c>SST-MS</c>
<c>MST-MS</c>
</texttable>
</section>
<section title="Multi-Channel Audio">
<t>There exist a number of RTP payload formats that can carry
multi-channel audio, despite the codec being a mono encoder.
Multi-channel audio can be viewed as multiple Media Sources sharing
a common Synchronization Context. These are independently encoded by
a Media Encoder and the different Encoded Streams are then
packetized together in a time synchronized way into a single Source
Packet Stream using the used codec's RTP Payload format. Example of
such codecs are, <xref target="RFC3551">PCMA and PCMU</xref>, <xref
target="RFC4867">AMR</xref>, and <xref
target="RFC5404">G.719</xref>.</t>
</section>
<section title="Redundancy Format">
<t>The <xref target="RFC2198">RTP Payload for Redundant Audio
Data</xref> defines how one can transport redundant audio data
together with primary data in the same RTP payload. The redundant
data can be a time delayed version of the primary or another time
delayed Encoded Stream using a different Media Encoder to encode the
same Media Source as the primary, as depicted below in <xref
target="fig-red-rfc2198"/>.</t>
<figure align="center" anchor="fig-red-rfc2198"
title="Concept for usage of Audio Redundancy with different Media Encoders">
<artwork><![CDATA[+--------------------+
| Media Source |
+--------------------+
|
Source Stream
|
+------------------------+
| |
V V
+--------------------+ +--------------------+
| Media Encoder | | Media Encoder |
+--------------------+ +--------------------+
| |
| +------------+
Encoded Stream | Time Delay |
| +------------+
| |
| +------------------+
V V
+--------------------+
| Media Packetizer |
+--------------------+
|
V
Packet Stream ]]></artwork>
</figure>
<t>The Redundancy format is thus providing the necessary meta
information to correctly relate different parts of the same Encoded
Stream, or in the case <xref target="fig-red-rfc2198">depicted
above</xref> relate the Received Source Stream fragments coming out
of different Media Decoders to be able to combine them together into
a less erroneous Source Stream.</t>
</section>
</section>
<section title="Packet Stream Relations">
<t>This section discusses various cases of relationships among Packet
Streams. This is a common relation to handle in RTP due to that Packet
Streams are separate and have their own SSRC, implying independent
sequence numbers and timestamp spaces. The underlying reasons for the
Packet Stream relationships are different, as can be seen in the cases
below. The different Packet Streams can be handled within the same RTP
Session or different RTP Sessions to accomplish different transport
goals. This separation of Packet Streams is further discussed in <xref
target="packet-stream-separation"/>.</t>
<section anchor="simulcast" title="Simulcast">
<t>A Media Source represented as multiple independent Encoded
Streams constitutes a simulcast of that Media Source. <xref
target="fig-simulcast"/> below represents an example of a Media
Source that is encoded into three separate and different Simulcast
streams, that are in turn sent on the same Media Transport flow.
When using Simulcast, the Packet Streams may be sharing RTP Session
and Media Transport, or be separated on different RTP Sessions and
Media Transports, or be any combination of these two. It is other
considerations that affect which usage is desirable, as discussed in
<xref target="packet-stream-separation"/>.</t>
<figure anchor="fig-simulcast"
title="Example of Media Source Simulcast">
<artwork align="center"><![CDATA[ +----------------+
| Media Source |
+----------------+
Source Stream |
+----------------------+----------------------+
| | |
v v v
+------------------+ +------------------+ +------------------+
| Media Encoder | | Media Encoder | | Media Encoder |
+------------------+ +------------------+ +------------------+
| Encoded | Encoded | Encoded
| Stream | Stream | Stream
v v v
+------------------+ +------------------+ +------------------+
| Media Packetizer | | Media Packetizer | | Media Packetizer |
+------------------+ +------------------+ +------------------+
| Source | Source | Source
| Packet | Packet | Packet
| Stream | Stream | Stream
+-----------------+ | +-----------------+
| | |
V V V
+-------------------+
| Media Transport |
+-------------------+
]]></artwork>
</figure>
<t>The simulcast relation between the Packet Streams is the common
Media Source. In addition, to be able to identify the common Media
Source, a receiver of the Packet Stream may need to know which
configuration or encoding goals that lay behind the produced Encoded
Stream and its properties. This to enable selection of the stream
that is most useful in the application at that moment.</t>
</section>
<section anchor="lms" title="Layered Multi-Stream">
<t>Layered Multi-Stream (LMS) is a mechanism by which different
portions of a layered encoding of a Source Stream are sent using
separate Packet Streams (sometimes in separate RTP Sessions). LMSs
are useful for receiver control of layered media.</t>
<t>A Media Source represented as an Encoded Stream and multiple
Dependent Streams constitutes a Media Source that has layered
dependencies. The figure below represents an example of a Media
Source that is encoded into three dependent layers, where two layers
are sent on the same Media Transport using different Packet Streams,
i.e. SSRCs, and the third layer is sent on a separate Media
Transport, i.e. a different RTP Session.</t>
<figure align="center" anchor="fig-ddp"
title="Example of Media Source Layered Dependency">
<artwork align="center"><![CDATA[ +----------------+
| Media Source |
+----------------+
|
|
V
+---------------------------------------------------------+
| Media Encoder |
+---------------------------------------------------------+
| | |
Encoded Stream Dependent Stream Dependent Stream
| | |
V V V
+----------------+ +----------------+ +----------------+
|Media Packetizer| |Media Packetizer| |Media Packetizer|
+----------------+ +----------------+ +----------------+
| | |
Packet Stream Packet Stream Packet Stream
| | |
+------+ +------+ |
| | |
V V V
+-----------------+ +-----------------+
| Media Transport | | Media Transport |
+-----------------+ +-----------------+
]]></artwork>
</figure>
<t>As an example, the <xref target="sstmst">SVC MST</xref> relation
needs to identify the common Media Encoder origin for the Encoded
and Dependent Streams. The SVC RTP Payload RFC is not particularly
explicit about how this relation is to be implemented. When using
different RTP Sessions, thus different Media Transports, and as long
as there is only one Packet Stream per Media Encoder and a single
Media Source in each RTP Session (<xref
target="sstmst">MST-SS</xref>), common SSRC and CNAMEs can be used
to identify the common Media Source. When multiple Packet Streams
are sent from one Media Encoder in the same RTP Session (SST-MS),
then CNAME is the only currently specified RTP identifier that can
be used. In cases where multiple Media Encoders use multiple Media
Sources sharing Synchronization Context, and thus having a common
CNAME, additional heuristics need to be applied to create the MST
relationship between the Packet Streams.</t>
</section>
<section anchor="repair" title="Robustness and Repair">
<t>Packet Streams may be protected by Redundancy Packet Streams
during transport. Several approaches listed below can achieve the
same result; <list style="symbols">
<t>Duplication of the original Packet Stream</t>
<t>Duplication of the original Packet Stream with a time
offset,</t>
<t>Forward Error Correction (FEC) techniques, and</t>
<t>Retransmission of lost packets (either globally or
selectively).</t>
</list></t>
<t/>
<section title="RTP Retransmission">
<t>The <xref target="fig-rtx">figure below</xref> represents an
example where a Media Source's Source Packet Stream is protected
by a <xref target="RFC4588">retransmission (RTX) flow</xref>. In
this example the Source Packet Stream and the Redundancy Packet
Stream share the same Media Transport.</t>
<figure align="center" anchor="fig-rtx"
title="Example of Media Source Retransmission Flows">
<artwork align="center"><![CDATA[+--------------------+
| Media Source |
+--------------------+
|
V
+--------------------+
| Media Encoder |
+--------------------+
| Retransmission
Encoded Stream +--------+ +---- Request
V | V V
+--------------------+ | +--------------------+
| Media Packetizer | | | RTP Retransmission |
+--------------------+ | +--------------------+
| | |
+------------+ Redundancy Packet Stream
Source Packet Stream |
| |
+---------+ +---------+
| |
V V
+-----------------+
| Media Transport |
+-----------------+
]]></artwork>
</figure>
<t>The <xref target="fig-rtx">RTP Retransmission example</xref>
helps illustrate that this mechanism works purely on the Source
Packet Stream. The RTP Retransmission transform buffers the sent
Source Packet Stream and upon requests emits a retransmitted
packet with some extra payload header as a Redundancy Packet
Stream. The <xref target="RFC4588">RTP Retransmission
mechanism</xref> is specified so that there is a one to one
relation between the Source Packet Stream and the Redundancy
Packet Stream. Thus a Redundancy Packet Stream needs to be
associated with its Source Packet Stream upon being received. This
is done based on CNAME selectors and heuristics to match requested
packets for a given Source Packet Stream with the original
sequence number in the payload of any new Redundancy Packet Stream
using the RTX payload format. In cases where the Redundancy Packet
Stream is sent in a separate RTP Session from the Source Packet
Stream, these sessions are related, e.g. using the <xref
target="RFC5888">SDP Media Grouping's</xref> FID semantics.</t>
</section>
<section title="Forward Error Correction">
<t>The <xref target="fig-fec">figure below</xref> represents an
example where two Media Sources' Source Packet Streams are
protected by FEC. Source Packet Stream A has a Media Redundancy
transformation in FEC Encoder 1. This produces a Redundancy Packet
Stream 1, that is only related to Source Packet Stream A. The FEC
Encoder 2, however takes two Source Packet Streams (A and B) and
produces a Redundancy Packet Stream 2 that protects them together,
i.e. Redundancy Packet Stream 2 relate to two Source Packet
Streams (a FEC group). FEC decoding, when needed due to packet
loss or packet corruption at the receiver, requires knowledge
about which Source Packet Streams that the FEC encoding was based
on.</t>
<t>In <xref target="fig-fec"/> all Packet Streams are sent on the
same Media Transport. This is however not the only possible
choice. Numerous combinations exist for spreading these Packet
Streams over different Media Transports to achieve the
communication application's goal.</t>
<figure align="center" anchor="fig-fec"
title="Example of FEC Flows">
<artwork align="center"><![CDATA[+--------------------+ +--------------------+
| Media Source A | | Media Source B |
+--------------------+ +--------------------+
| |
V V
+--------------------+ +--------------------+
| Media Encoder A | | Media Encoder B |
+--------------------+ +--------------------+
| |
Encoded Stream Encoded Stream
V V
+--------------------+ +--------------------+
| Media Packetizer A | | Media Packetizer B |
+--------------------+ +--------------------+
| |
Source Packet Stream A Source Packet Stream B
| |
+-----+-------+-------------+ +-------+------+
| V V V |
| +---------------+ +---------------+ |
| | FEC Encoder 1 | | FEC Encoder 2 | |
| +---------------+ +---------------+ |
| | | |
| Redundancy PS 1 Redundancy PS 2 |
V V V V
+----------------------------------------------------------+
| Media Transport |
+----------------------------------------------------------+
]]></artwork>
</figure>
<t>As FEC Encoding exists in various forms, the methods for
relating FEC Redundancy Packet Streams with its source information
in Source Packet Streams are many. The <xref target="RFC5109">XOR
based RTP FEC Payload format</xref> is defined in such a way that
a Redundancy Packet Stream has a one to one relation with a Source
Packet Stream. In fact, the RFC requires the Redundancy Packet
Stream to use the same SSRC as the Source Packet Stream. This
requires to either use a separate RTP session or to use the <xref
target="RFC2198">Redundancy RTP Payload format</xref>. The
underlying relation requirement for this FEC format and a
particular Redundancy Packet Stream is to know the related Source
Packet Stream, including its SSRC.</t>
<t><!--MW: Here we could add something about FECFRAME and generalized block FEC that can
protect multiple Packet Streams with one Redundancy Packet Stream. However, that do require
use of explicit Source Packet Information.--></t>
</section>
</section>
<section anchor="packet-stream-separation"
title="Packet Stream Separation">
<t>Packet Streams can be separated exclusively based on their SSRCs
or at the RTP Session level or at the Multi-Media Session level as
explained below.</t>
<t>When the Packet Streams that have a relationship are all sent in
the same RTP Session and are uniquely identified based on their SSRC
only, it is termed an SSRC-Only Based Separation. Such streams can
be related via RTCP CNAME to identify that the streams belong to the
same End Point. <xref target="RFC5576"/>-based approaches, when
used, can explicitly relate various such Packet Streams.</t>
<t>On the other hand, when Packet Streams that are related but are
sent in the context of different RTP Sessions to achieve separation,
it is known as RTP Session-based separation. This is commonly used
when the different Packet Streams are intended for different Media
Transports.</t>
<t>Several mechanisms that use RTP Session-based separation rely on
it to enable an implicit grouping mechanism expressing the
relationship. The solutions have been based on using the same SSRC
value in the different RTP Sessions to implicitly indicate their
relation. That way, no explicit RTP level mechanism has been needed,
only signaling level relations have been established using semantics
from <xref target="RFC5888">Grouping of Media lines
framework</xref>. Examples of this are <xref target="RFC4588">RTP
Retransmission</xref>, <xref target="RFC6190">SVC Multi-Session
Transmission</xref> and <xref target="RFC5109">XOR Based FEC</xref>.
RTCP CNAME explicitly relates Packet Streams across different RTP
Sessions, as explained in the previous section. Such a relationship
can be used to perform inter-media synchronization.</t>
<t>Packet Streams that are related and need to be associated can be
part of different Multimedia Sessions, rather than just different
RTP sessions within the same Multimedia Session context. This puts
further demand on the scope of the mechanism(s) and its handling of
identifiers used for expressing the relationships.</t>
</section>
</section>
<section title="Multiple RTP Sessions over one Media Transport">
<t><xref target="I-D.westerlund-avtcore-transport-multiplexing"/>
describes a mechanism that allow several RTP Sessions to be carried
over a single underlying Media Transport. The main reasons for doing
this are related to the impact of using one or more Media Transports.
Thus using a common network path or potentially have different ones.
There is reduced need for NAT/FW traversal resources and no need for
flow based QoS.</t>
<t>However, Multiple RTP Sessions over one Media Transport makes it
clear that a single Media Transport 5-tuple is not sufficient to
express which RTP Session context a particular Packet Stream exists
in. Complexities in the relationship between Media Transports and RTP
Session already exist as one RTP Session contains multiple Media
Transports, e.g. even a Peer-to-Peer RTP Session with RTP/RTCP
Multiplexing requires two Media Transports, one in each direction. The
relationship between Media Transports and RTP Sessions as well as
additional levels of identifiers need to be considered in both
signaling design and when defining terminology.</t>
</section>
</section>
<section anchor="topologies" title="Topologies and Communication Entities">
<t>This section reviews some communication topologies and looks at the
relationship among the communication entities that are defined in <xref
target="communication-entities"/>. It does not deal with discussions
about the streams and their relation to the transport. Instead, it
covers the aspects that enable the transport of those streams. For
example, the <xref target="media-transport">Media Transports</xref> that
exists between the <xref target="end-point">End Points</xref> that are
part of an <xref target="rtp-session">RTP session</xref> and their
relationship to the <xref target="multimedia-session">Multi-Media
Session</xref> between <xref target="participant">Participants</xref>
and the established <xref target="comm-session">Communication
session</xref> are explained.</t>
<t>The text provided below is neither any exhaustive listing of possible
topologies, nor does it cover all topologies described in <xref
target="I-D.ietf-avtcore-rtp-topologies-update"/>.</t>
<section title="Point-to-Point Communication">
<t><xref target="fig-p2p-basic"/> shows a very basic point-to-point
communication session between A and B. It uses two different audio and
video RTP sessions between A's and B's end points. Assume that the
Multi-media session shared by the participants is established using
SIP (i.e., there is a SIP Dialog between A and B). The high level
representation of this communication scenario can be demonstrated
using <xref target="fig-p2p-basic"/>.</t>
<figure align="center" anchor="fig-p2p-basic"
title="Point to Point Communication">
<artwork><![CDATA[
+---+ +---+
| A |<------->| B |
+---+ +---+
]]></artwork>
</figure>
<t>However, this picture gets slightly more complex when redrawn using
the communication entities concepts defined earlier in this
document.</t>
<figure align="center" anchor="fig-p2p"
title="Point to Point Communication Session with two RTP Sessions">
<artwork><![CDATA[
+-----------------------------------------------------------+
| Communication Session |
| |
| +----------------+ +----------------+ |
| | Participant A | +-------------+ | Participant B | |
| | | | Multi-Media | | | |
| | +-------------+|<=>| Session |<=>|+-------------+ | |
| | | End Point A || |(SIP Dialog) | || End Point B | | |
| | | || +-------------+ || | | |
| | | +-----------++---------------------++-----------+ | | |
| | | | RTP Session| | | | | |
| | | | Audio |---Media Transport-->| | | | |
| | | | |<--Media Transport---| | | | |
| | | +-----------++---------------------++-----------+ | | |
| | | || || | | |
| | | +-----------++---------------------++-----------+ | | |
| | | | RTP Session| | | | | |
| | | | Video |---Media Transport-->| | | | |
| | | | |<--Media Transport---| | | | |
| | | +-----------++---------------------++-----------+ | | |
| | +-------------+| |+-------------+ | |
| +----------------+ +----------------+ |
+-----------------------------------------------------------+
]]></artwork>
</figure>
<t><xref target="fig-p2p"/> shows the two RTP Sessions only exist
between the two End Points A and B and over their respective Media
Transports. The Multi-Media Session establishes the association
between the two Participants and configures these RTP sessions and the
Media Transports that are used.</t>
</section>
<section anchor="central-conferencing" title="Centralized Conferencing">
<t>This section looks at the centralized conferencing communication
topology, where a number of participants, like A, B, C, and D in <xref
target="fig-central-conf-basic"/>, communicate using an RTP mixer.</t>
<figure anchor="fig-central-conf-basic"
title="Centralized Conferincing using an RTP Mixer">
<artwork><![CDATA[+---+ +------------+ +---+
| A |<---->| |<---->| B |
+---+ | | +---+
| Mixer |
+---+ | | +---+
| C |<---->| |<---->| D |
+---+ +------------+ +---+
]]></artwork>
</figure>
<t>In this case each of the Participants establish their Multi-media
session with the Conference Bridge. Thus, negotiation for the
establishment of the used RTP sessions and their configuration happens
between these entities. The participants have their End Points (A, B,
C, D) and the Conference Bridge has the host running the RTP mixer,
referred to as End Point M in <xref target="fig-central-conf"/>.
However, despite the individual establishment of four Multi-Media
Sessions and the corresponding Media Transports for each of the RTP
sessions between the respective End Points and the Conference Bridge,
there is actually only two RTP sessions. One for audio and one for
Video, as these RTP sessions are, in this topology, shared between all
the Participants.</t>
<figure anchor="fig-central-conf"
title="Centralized Conferencing with Two Participants A and B communicating over a Conference Bridge">
<artwork><![CDATA[+-------------------------------------------------------------------+
| Communication Session |
| |
| +----------------+ +----------------+ |
| | Participant A | +-------------+ | Conference | |
| | | | Multi-Media | | Bridge | |
| | +-------------+|<=====>| Session A |<=====>|+-------------+ | |
| | | End Point A || |(SIP Dialog) | || End Point M | | |
| | | || +-------------+ || | | |
| | | +-----------++-----------------------------++-----------+ | | |
| | | | RTP Session| | | | | |
| | | | Audio |-------Media Transport------>| | | | |
| | | | |<------Media Transport-------| | | | |
| | | +-----------++-----------------------------++------+ | | | |
| | | || || | | | | |
| | | +-----------++-----------------------------++----+ | | | | |
| | | | RTP Session| | | | | | | |
| | | | Video |-------Media Transport------>| | | | | | |
| | | | |<------Media Transport-------| | | | | | |
| | | +-----------++-----------------------------++ | | | | | |
| | +-------------+| || | | | | | |
| +----------------+ || | | | | | |
| || | | | | | |
| +----------------+ || | | | | | |
| | Participant B | +-------------+ || | | | | | |
| | | | Multi-Media | || | | | | | |
| | +-------------+|<=====>| Session B |<=====>|| | | | | | |
| | | End Point B || |(SIP Dialog) | || | | | | | |
| | | || +-------------+ || | | | | | |
| | | +-----------++-----------------------------++ | | | | | |
| | | | RTP Session| | | | | | | |
| | | | Video |-------Media Transport------>| | | | | | |
| | | | |<------Media Transport-------| | | | | | |
| | | +-----------++-----------------------------++----+ | | | | |
| | | || || | | | | |
| | | +-----------++-----------------------------++------+ | | | |
| | | | RTP Session| | | | | |
| | | | Audio |-------Media Transport------>| | | | |
| | | | |<------Media Transport-------| | | | |
| | | +-----------++-----------------------------++-----------+ | | |
| | +-------------+| |+-------------+ | |
| +----------------+ +----------------+ |
+-------------------------------------------------------------------+
]]></artwork>
</figure>
<t>It is important to stress that in the case of <xref
target="fig-central-conf"/>, it might appear that the Multi-Media
Sessions context is scoped between A and B over M. This might not be
always true and they can have contexts that extend further. In this
case the RTP session, its common SSRC space goes beyond what occurs
between A and M and B and M respectively.</t>
</section>
<section title="Full Mesh Conferencing">
<t>This section looks at the case where the three Participants (A, B
and C) wish to communicate. They establish individual Multi-Media
Sessions and RTP sessions between themselves and the other two peers.
Thus, each providing two copies of their media to every other
participant. <xref target="fig-full-mesh-basic"/> shows a high level
representation of such a topology.</t>
<figure align="center" anchor="fig-full-mesh-basic"
title="Full Mesh Conferencing with three Participants A, B and C">
<artwork><![CDATA[+---+ +---+
| A |<---->| B |
+---+ +---+
^ ^
\ /
\ /
v v
+---+
| C |
+---+
]]></artwork>
</figure>
<t>In this particular case there are two aspects worth noting. The
first is there will be multiple Multi-Media Sessions per Communication
Session between the participants. This, however, hasn't been true in
the earlier examples; the Centralized Conferencing in<xref
target="central-conferencing"/> being the exception. The second aspect
is consideration of whether one needs to maintain relationships
between entities and concepts, for example Media Sources, between
these different Multi-Media Sessions and between Packet Streams in the
independent RTP sessions configured by those Multi-Media Sessions.</t>
<figure align="center" anchor="fig-full-mesh"
title="Full Mesh Conferencing between three Participants A, B and C">
<artwork><![CDATA[ +-----------------------------------------+
| Participant A |
+----------+ | +--------------------------------------+|
| Multi- | | | End Point A ||
| Media |<======>| | ||
| Session | | |+-------+ +-------+ +-------+ ||
| 1 | | || RTP 1 |<----| MS A1 |---->| RTP 2 | ||
+----------+ | || | +-------+ | | ||
^^ | +|-------|-------------------|-------|-+|
|| +--|-------|-------------------|-------|--+
|| | | ^^ | |
VV | | || | |
+-------------------------|-------|----+ || | |
| Participant B | | | VV | |
| +-----------------------|-------|---+| +----------+ | |
| | End Point B +----->| | || | Multi- | | |
| | | +-------+ || | Media | | |
| | +-------+ | +-------+ || | Session | | |
| | | MS B1 |------+----->| RTP 3 | || | 2 | | |
| | +-------+ | | || +----------+ | |
| +-----------------------|-------|---+| ^^ | |
+-------------------------|-------|----+ || | |
^^ | | || | |
|| | | VV | |
|| +--|-------|-------------------|-------|--+
VV | | | Participant C | | |
+----------+ | +|-------|-------------------|-------|-+|
| Multi- | | || | End Point C | | ||
| Media |<======>| |+-------+ +-------+ ||
| Session | | | ^ +-------+ ^ ||
| 3 | | | +---------| MS C1 |---------+ ||
+----------+ | | +-------+ ||
| +--------------------------------------+|
+-----------------------------------------+
]]></artwork>
</figure>
<t>For the sake of clarity, <xref target="fig-full-mesh"/> above does
not include all these concepts. The Media Sources (MS) from a given
End Point is sent to the two peers. This requires encoding and Media
Packetization to enable the Packet Streams to be sent over Media
Transports in the context of the RTP sessions depicted. The RTP
sessions 1, 2, and 3 are independent, and established in the context
of each of the Multi-Media Sessions 1, 2 and 3. The joint
communication session the full figure represents (not shown here as it
was <xref target="fig-central-conf"/> in order to save space),
however, combines the received representations of the peers' Media
Sources and plays them back.</t>
<t>It is noteworthy that the full mesh conferencing topologies
described here have the potential for creating loops. For example, if
one compares the above full mesh with a mixing three party
communication session as <xref target="fig-three-relay">depicted in
</xref>. In this example A's Media Source A1 is sent to B over a
Multi-Media Session (A-B). In B the Media Source A1 is mixed with
Media Source B1 and the resulting Media Source (MS AB) is sent to C
over a Multi-Media Session (B-C). If C and A would establish a
Multi-Media Session (A-C) and C would act in the same role as B, then
A would receive a Media Source from C that contains a mix of A, B and
C's individual Media Sources. This would result in A playing out a
time delay version of its own signal (i.e., the system has created an
echo path).</t>
<figure anchor="fig-three-relay"
title="Mixing Three Party Communication Session">
<artwork><![CDATA[+--------------+ +--------------+ +--------------+
| A | | B +-------+ | | C |
| | | | MS B1 | | | |
| | | +-------+ | | |
| +-------+ | | | | | |
| | MS A1 |----|--->|-----+ MS AB -|--->| |
| +-------+ | | | | |
+--------------+ +--------------+ +--------------+
]]></artwork>
</figure>
<t>The looping issue can be avoided, detected or prevented using two
general methods. The first method is to use great care when setting up
and establishing the communication session if participants have any
mixing or forwarding capacity, so that one doesn't end up getting back
a partial or full representation of one's own media believing it is
someone else's. The other method is to maintain some unique
identifiers at the communication session level for all Media Sources
and ensure that any Packet Streams received identify those Media
Sources that contributed to the content of the Packet Stream.</t>
</section>
<section title="Source-Specific Multicast">
<t>In one-to-many media distribution cases (e.g., IPTV), where one
Media Sender or a set of Media Senders is allowed to send Packet
Streams on a particular Source-Specific Multicast (SSM) group to many
receivers (R), there are some different aspects to consider. <xref
target="fig-ssm-basic"/> presents a high level SSM system for RTP/RTCP
defined in <xref target="RFC5760"/>. In this case, several Media
Senders sends their Packet Streams to the Distribution Source, which
is the only one allowed to send to the SSM group. The Receivers
joining the SSM group can provide RTCP feedback on its reception by
sending unicast feedback to a Feedback Target (FT).</t>
<figure anchor="fig-ssm-basic"
title="Source-Specific Multicast Communication Topology">
<artwork><![CDATA[+--------+ +-----+
|Media | | | Source-Specific
|Sender 1|<----->| D S | Multicast (SSM)
+--------+ | I O | +--+----------------> R(1)
| S U | | | |
+--------+ | T R | | +-----------> R(2) |
|Media |<----->| R C |->+ | : | |
|Sender 2| | I E | | +------> R(n-1) | |
+--------+ | B | | | | | |
: | U | +--+--> R(n) | | |
: | T +-| | | | |
: | I | |<---------+ | | |
+--------+ | O |F|<---------------+ | |
|Media | | N |T|<--------------------+ |
|Sender M|<----->| | |<-------------------------+
+--------+ +-----+ RTCP Unicast
FT = Feedback Target
]]></artwork>
</figure>
<t>Here the Media Transport from the Distribution Source to all the
SSM receivers (R) have the same 5-tuple, but in reality have different
paths. Also, the Multi-Media Sessions between the Distribution Source
and the individual receivers are normally identical. This is due to
one-way communication from the Distribution Source to the receiver of
configuration information. This is information typically embedded in
Electronic Program Guides (EPGs), distributed by the Session
Announcement Protocol (SAP) <xref target="RFC2974"/> or other one-way
protocols. In some cases load balancing occurs, for example, by
providing the receiver with a set of Feedback Targets and then it
randomly selects one out of the set.</t>
<t>This scenario varies significantly from previously described
communication topologies due to the asymmetric nature of the RTP
Session context across the Distribution Source. The Distribution
Source forms a focal point in collecting the unicasted RTCP feedback
from the receivers and then re-distributing it to the Media Senders.
Each Media Sender and the Distribution Source establish their own
Multi-Media Session Context for the underlying RTP Sessions but with
shared RTCP context across all the receivers.</t>
<t>To improve the readability,<xref target="fig-ssm-basic"> </xref>
intentionally hides the details of the various entities . Expanding on
this, one can think of Media Senders being part of one or more
Multi-Media Sessions grouped under a Communication Session. The Media
Sender in this scenario refers to the Media Packetizer transformation
<xref target="media_packetizer"/>. The Packet Stream generated by such
a Media Sender can be part of its own RTP Session or can be
multiplexed with other Packet Streams within an End Point. The latter
case requires careful consideration since the re-distributed RTCP
packets now correspond to a single RTP Session Context across all the
Media Senders.</t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<t>This document simply tries to clarify the confusion prevalent in RTP
taxonomy because of inconsistent usage by multiple technologies and
protocols making use of the RTP protocol. It does not introduce any new
security considerations beyond those already well documented in the RTP
protocol <xref target="RFC3550"/> and each of the many respective
specifications of the various protocols making use of it.</t>
<t>Hopefully having a well-defined common terminology and understanding
of the complexities of the RTP architecture will help lead us to better
standards, avoiding security problems.</t>
</section>
<section title="Acknowledgement">
<t>This document has many concepts borrowed from several documents such
as WebRTC <xref target="I-D.ietf-rtcweb-overview"/>, CLUE <xref
target="I-D.ietf-clue-framework"/>, Multiplexing Architecture <xref
target="I-D.westerlund-avtcore-transport-multiplexing"/>. The authors
would like to thank all the authors of each of those documents.</t>
<t>The authors would also like to acknowledge the insights, guidance and
contributions of Magnus Westerlund, Roni Even, Paul Kyzivat, Colin
Perkins, Keith Drage, Harald Alvestrand, and Alex Eleftheriadis.</t>
</section>
<section title="Contributors">
<t>Magnus Westerlund has contributed the concept model for the media
chain using transformations and streams model, including rewriting
pre-existing concepts into this model and adding missing concepts. The
first proposal for updating the relationships and the topologies based
on this concept was also performed by Magnus.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>This document makes no request of IANA.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.3550"?>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.2198'?>
<?rfc include='reference.RFC.2974'?>
<?rfc include="reference.RFC.3264"?>
<?rfc include='reference.RFC.3551'?>
<?rfc include="reference.RFC.4566"?>
<?rfc include='reference.RFC.4588'?>
<?rfc include='reference.RFC.4867'?>
<?rfc include='reference.RFC.5109'?>
<?rfc include='reference.RFC.5404'?>
<?rfc include="reference.RFC.5576"?>
<?rfc include='reference.RFC.5760'?>
<?rfc include="reference.RFC.5888"?>
<?rfc include="reference.RFC.5905"?>
<?rfc include='reference.RFC.6190'?>
<?rfc include="reference.RFC.6222"?>
<?rfc include="reference.I-D.ietf-clue-framework"?>
<?rfc include="reference.I-D.ietf-rtcweb-overview"?>
<?rfc include="reference.I-D.ietf-mmusic-sdp-bundle-negotiation"?>
<?rfc include="reference.I-D.ietf-avtcore-clksrc"?>
<?rfc include="reference.I-D.westerlund-avtcore-transport-multiplexing"?>
<?rfc include='reference.I-D.ietf-avtcore-rtp-topologies-update'?>
</references>
<section title="Changes From Earlier Versions">
<t>NOTE TO RFC EDITOR: Please remove this section prior to
publication.</t>
<section title="Modifications Between WG Version -00 and -03">
<t><list style="symbols">
<t>WG version -00 text is identical to individual draft -03</t>
<t>Amended description of SVC SST and MST encodings with respect
to concepts defined in this text</t>
<t>Removed UML as normative reference, since the text no longer
uses any UML notation</t>
<t>Removed a number of level 4 sections and moved out text to the
level above</t>
</list></t>
</section>
<section title="Modifications Between Version -02 and -03">
<t><list style="symbols">
<t>Section 4 rewritten (and new communication topologies added) to
reflect the major updates to Sections 1-3</t>
<t>Section 8 removed (carryover from initial -00 draft)</t>
<t>General clean up of text, grammar and nits</t>
</list></t>
</section>
<section title="Modifications Between Version -01 and -02">
<t><list style="symbols">
<t>Section 2 rewritten to add both streams and transformations in
the media chain.</t>
<t>Section 3 rewritten to focus on exposing relationships.</t>
</list></t>
</section>
<section title="Modifications Between Version -00 and -01">
<t><list style="symbols">
<t>Too many to list</t>
<t>Added new authors</t>
<t>Updated content organization and presentation</t>
</list></t>
</section>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 10:14:54 |