RFC 7656
A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources
Internet Engineering Task Force (IETF) J. Lennox Request for Comments: 7656 Vidyo Category: Informational K. Gross ISSN: 2070-1721 AVA S. Nandakumar G. Salgueiro Cisco Systems B. Burman, Ed. Ericsson November 2015 A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) SourcesAbstract
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 properties and relationships among RTP sources and defines common terminology for discussing protocol entities and their relationships. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7656.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Media Chain . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.1. Physical Stimulus . . . . . . . . . . . . . . . . . . 10 2.1.2. Media Capture . . . . . . . . . . . . . . . . . . . . 10 2.1.3. Raw Stream . . . . . . . . . . . . . . . . . . . . . 10 2.1.4. Media Source . . . . . . . . . . . . . . . . . . . . 11 2.1.5. Source Stream . . . . . . . . . . . . . . . . . . . . 11 2.1.6. Media Encoder . . . . . . . . . . . . . . . . . . . . 12 2.1.7. Encoded Stream . . . . . . . . . . . . . . . . . . . 13 2.1.8. Dependent Stream . . . . . . . . . . . . . . . . . . 13 2.1.9. Media Packetizer . . . . . . . . . . . . . . . . . . 13 2.1.10. RTP Stream . . . . . . . . . . . . . . . . . . . . . 14 2.1.11. RTP-Based Redundancy . . . . . . . . . . . . . . . . 14 2.1.12. Redundancy RTP Stream . . . . . . . . . . . . . . . . 15 2.1.13. RTP-Based Security . . . . . . . . . . . . . . . . . 15 2.1.14. Secured RTP Stream . . . . . . . . . . . . . . . . . 16 2.1.15. Media Transport . . . . . . . . . . . . . . . . . . . 16 2.1.16. Media Transport Sender . . . . . . . . . . . . . . . 17 2.1.17. Sent RTP Stream . . . . . . . . . . . . . . . . . . . 18 2.1.18. Network Transport . . . . . . . . . . . . . . . . . . 18 2.1.19. Transported RTP Stream . . . . . . . . . . . . . . . 18 2.1.20. Media Transport Receiver . . . . . . . . . . . . . . 18 2.1.21. Received Secured RTP Stream . . . . . . . . . . . . . 19 2.1.22. RTP-Based Validation . . . . . . . . . . . . . . . . 19 2.1.23. Received RTP Stream . . . . . . . . . . . . . . . . . 19 2.1.24. Received Redundancy RTP Stream . . . . . . . . . . . 19 2.1.25. RTP-Based Repair . . . . . . . . . . . . . . . . . . 19 2.1.26. Repaired RTP Stream . . . . . . . . . . . . . . . . . 19 2.1.27. Media Depacketizer . . . . . . . . . . . . . . . . . 20 2.1.28. Received Encoded Stream . . . . . . . . . . . . . . . 20 2.1.29. Media Decoder . . . . . . . . . . . . . . . . . . . . 20 2.1.30. Received Source Stream . . . . . . . . . . . . . . . 20 2.1.31. Media Sink . . . . . . . . . . . . . . . . . . . . . 21 2.1.32. Received Raw Stream . . . . . . . . . . . . . . . . . 21 2.1.33. Media Render . . . . . . . . . . . . . . . . . . . . 21 2.2. Communication Entities . . . . . . . . . . . . . . . . . 22 2.2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . 23 2.2.2. RTP Session . . . . . . . . . . . . . . . . . . . . . 23 2.2.3. Participant . . . . . . . . . . . . . . . . . . . . . 24 2.2.4. Multimedia Session . . . . . . . . . . . . . . . . . 24 2.2.5. Communication Session . . . . . . . . . . . . . . . . 25 3. Concepts of Inter-Relations . . . . . . . . . . . . . . . . . 25 3.1. Synchronization Context . . . . . . . . . . . . . . . . . 26 3.1.1. RTCP CNAME . . . . . . . . . . . . . . . . . . . . . 26 3.1.2. Clock Source Signaling . . . . . . . . . . . . . . . 26
3.1.3. Implicitly via RtcMediaStream . . . . . . . . . . . . 26
3.1.4. Explicitly via SDP Mechanisms . . . . . . . . . . . . 26
3.2. Endpoint . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3. Participant . . . . . . . . . . . . . . . . . . . . . . . 27
3.4. RtcMediaStream . . . . . . . . . . . . . . . . . . . . . 27
3.5. Multi-Channel Audio . . . . . . . . . . . . . . . . . . . 28
3.6. Simulcast . . . . . . . . . . . . . . . . . . . . . . . . 28
3.7. Layered Multi-Stream . . . . . . . . . . . . . . . . . . 30
3.8. RTP Stream Duplication . . . . . . . . . . . . . . . . . 32
3.9. Redundancy Format . . . . . . . . . . . . . . . . . . . . 33
3.10. RTP Retransmission . . . . . . . . . . . . . . . . . . . 33
3.11. Forward Error Correction . . . . . . . . . . . . . . . . 35
3.12. RTP Stream Separation . . . . . . . . . . . . . . . . . . 36
3.13. Multiple RTP Sessions over one Media Transport . . . . . 37
4. Mapping from Existing Terms . . . . . . . . . . . . . . . . . 37
4.1. Telepresence Terms . . . . . . . . . . . . . . . . . . . 37
4.1.1. Audio Capture . . . . . . . . . . . . . . . . . . . . 37
4.1.2. Capture Device . . . . . . . . . . . . . . . . . . . 37
4.1.3. Capture Encoding . . . . . . . . . . . . . . . . . . 38
4.1.4. Capture Scene . . . . . . . . . . . . . . . . . . . . 38
4.1.5. Endpoint . . . . . . . . . . . . . . . . . . . . . . 38
4.1.6. Individual Encoding . . . . . . . . . . . . . . . . . 38
4.1.7. Media Capture . . . . . . . . . . . . . . . . . . . . 38
4.1.8. Media Consumer . . . . . . . . . . . . . . . . . . . 38
4.1.9. Media Provider . . . . . . . . . . . . . . . . . . . 39
4.1.10. Stream . . . . . . . . . . . . . . . . . . . . . . . 39
4.1.11. Video Capture . . . . . . . . . . . . . . . . . . . . 39
4.2. Media Description . . . . . . . . . . . . . . . . . . . . 39
4.3. Media Stream . . . . . . . . . . . . . . . . . . . . . . 39
4.4. Multimedia Conference . . . . . . . . . . . . . . . . . . 39
4.5. Multimedia Session . . . . . . . . . . . . . . . . . . . 40
4.6. Multipoint Control Unit (MCU) . . . . . . . . . . . . . . 40
4.7. Multi-Session Transmission (MST) . . . . . . . . . . . . 40
4.8. Recording Device . . . . . . . . . . . . . . . . . . . . 41
4.9. RtcMediaStream . . . . . . . . . . . . . . . . . . . . . 41
4.10. RtcMediaStreamTrack . . . . . . . . . . . . . . . . . . . 41
4.11. RTP Receiver . . . . . . . . . . . . . . . . . . . . . . 41
4.12. RTP Sender . . . . . . . . . . . . . . . . . . . . . . . 41
4.13. RTP Session . . . . . . . . . . . . . . . . . . . . . . . 41
4.14. Single-Session Transmission (SST) . . . . . . . . . . . . 41
4.15. SSRC . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5. Security Considerations . . . . . . . . . . . . . . . . . . . 42
6. Informative References . . . . . . . . . . . . . . . . . . . 42
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 45
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
1. Introduction
The existing taxonomy of sources in the Real-time Transport Protocol (RTP) [RFC3550] has previously been 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. This document improves 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 are transformed on their way between sender and receiver, and how they can be grouped and associated together. All non-specific references to ControLling mUltiple streams for tElepresence (CLUE) in this document map to [CLUE-FRAME], and all references to Web Real-time Communications (WebRTC) map to [WEBRTC-OVERVIEW].2. Concepts
This section defines concepts that serve to identify and name various transformations and streams in a given RTP usage. For each concept, alternate definitions and usages that coexist today are listed along with various characteristics that further describe the concept. These concepts are divided into two categories: one is related to the chain of streams and transformations that Media can be subject to, and the other is for entities involved in the communication.2.1. Media Chain
In the context of this document, media is a sequence of synthetic or Physical Stimuli (Section 2.1.1) -- for example, sound waves, photons, key strokes -- represented in digital form. Synthesized media is typically generated directly in the digital domain. 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 stimuli. 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 change the representation of the streams in some way.
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:
o A transformation may have zero or more inputs and one or more
outputs.
o A stream is of some type, such as audio, video, real-time text,
etc.
o A stream has one source transformation and one or more sink
transformations (with the exception of physical stimulus
(Section 2.1.1) that may lack source or sink transformation).
o Streams can be forwarded from a transformation output to any
number of inputs on other transformations that support that type.
o If the output of a transformation is sent to multiple
transformations, those streams will be identical; it takes a
transformation to make them different.
o There are no formal limitations on how streams are connected to
transformations.
It is also important to remember that this is a conceptual model.
Thus, real-world implementations may look different and have a
different structure.
To provide a basic understanding of the relationships in the chain,
we first introduce the concepts for the sender side (Figure 1). This
covers physical stimuli until media packets are emitted onto the
network.
Physical Stimulus | V +----------------------+ | Media Capture | +----------------------+ | Raw Stream V +----------------------+ | Media Source |<- Synchronization Timing +----------------------+ | Source Stream V +----------------------+ | Media Encoder | +----------------------+ | Encoded Stream +------------+ V | V +----------------------+ | +----------------------+ | Media Packetizer | | | RTP-Based Redundancy | +----------------------+ | +----------------------+ | | | +-------------+ Redundancy RTP Stream Source RTP Stream | V V +----------------------+ +----------------------+ | RTP-Based Security | | RTP-Based Security | +----------------------+ +----------------------+ | | Secured RTP Stream Secured Redundancy RTP Stream V V +----------------------+ +----------------------+ | Media Transport | | Media Transport | +----------------------+ +----------------------+ Figure 1: Sender Side Concepts in the Media Chain In Figure 1, 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 in Section 2.1.15.
In Figure 2, we review a receiver media chain matching the sender side, to look at the inverse transformations and their attempts to recover identical streams as in the sender chain, subject to what may be lossy compression and imperfect media transport. Note that the streams out of a reverse transformation, like the Source Stream out of 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 from being the same as the one fed into the media encoder. Other reasons include packet loss in the media transport transformation that even RTP-based Repair, if used, fails to repair.
+----------------------+ +----------------------+ | Media Transport | | Media Transport | +----------------------+ +----------------------+ Received | Received | Secured Secured RTP Stream Redundancy RTP Stream V V +----------------------+ +----------------------+ | RTP-Based Validation | | RTP-Based Validation | +----------------------+ +----------------------+ | | Received RTP Stream Received Redundancy RTP Stream | | | +--------------------+ V V +----------------------+ | RTP-Based Repair | +----------------------+ | Repaired RTP Stream V +----------------------+ | Media Depacketizer | +----------------------+ | Received Encoded Stream V +----------------------+ | Media Decoder | +----------------------+ | Received Source Stream V +----------------------+ | Media Sink |--> Synchronization Information +----------------------+ | Received Raw Stream V +----------------------+ | Media Render | +----------------------+ | V Physical Stimulus Figure 2: Receiver Side Concepts of the Media Chain
2.1.1. Physical Stimulus
The physical stimulus is a physical event in the analog domain that can be sampled and converted to digital form by an appropriate sensor or transducer. This includes sound waves making up audio, photons in a light field, or other excitations or interactions with sensors, like keystrokes on a keyboard.2.1.2. Media Capture
Media Capture is the process of transforming the analog physical stimulus (Section 2.1.1) 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 Raw Stream (Section 2.1.3). This data is considered "media", because it includes data that is periodically sampled or made up of a set of timed asynchronous events. 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. Characteristics: o A media capture is identified either by hardware/manufacturer ID or via a session-scoped device identifier as mandated by the application usage. o A media capture can generate an Encoded Stream (Section 2.1.7) if the capture device supports such a configuration. o The nature of the media capture may impose constraints on the clock handling in some of the subsequent steps. For example, many audio or video capture devices are not completely free in selecting the sample rate.2.1.3. Raw Stream
A raw stream is the time progressing stream of digitally sampled information, usually periodically sampled and provided by a media capture (Section 2.1.2). 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.
2.1.4. Media Source
A Media Source is the logical source of a time progressing digital media stream synchronized to a reference clock. This stream is called a source stream (Section 2.1.5). This transformation takes one or more raw streams (Section 2.1.3) and provides a source stream as output. The output is synchronized with a reference clock (Section 3.1), which can be as simple as a system local wall clock or as complex as an NTP synchronized clock. 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 audio mix of multiple source streams (Figure 3). Mixing multiple streams typically requires that the input streams are possible to relate in time, meaning that they have to be source streams (Section 2.1.5) rather than raw streams. In Figure 3, the generated source stream is a mix of the three input source streams. Source Source Source Stream Stream Stream | | | V V V +--------------------------+ | Media Source |<-- Reference Clock | Mixer | +--------------------------+ | V Source Stream Figure 3: Conceptual Media Source in the form of an Audio Mixer Another possible example of a conceptual media source is a video surveillance switch, where the input is multiple source streams from different cameras, and the output is one of those source streams based on some selection criteria, such as round robin or some video activity measure.2.1.5. Source Stream
A source stream is a stream of digital samples that has been synchronized with a reference clock and comes from a particular media source (Section 2.1.4).
2.1.6. Media Encoder
A media encoder is a transform that is responsible for encoding the media data from a source stream (Section 2.1.5) into another representation, usually more compact, that is output as an encoded stream (Section 2.1.7). 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 includes properties such as codec, bitrate, start points for decoding, resolution, bandwidth, or other fidelity affecting properties. Scalable media encoders need special attention as they produce multiple outputs that are potentially of different types. As shown in Figure 4, 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 Dependent Streams (Section 2.1.8). Decoding requires at least one encoded stream and zero or more dependent streams. A dependent stream's dependency is one of the grouping relations this document discusses further in Section 3.7. Source Stream | V +--------------------------+ | Scalable Media Encoder | +--------------------------+ | | ... | V V V Encoded Dependent Dependent Stream Stream Stream Figure 4: Scalable Media Encoder Input and Outputs There are also other variants of encoders, like so-called Multiple Description Coding (MDC). Such media encoders produce multiple independent and thus individually decodable encoded streams. However, (logically) combining multiple of these encoded streams into a single Received Source Stream during decoding leads to an improvement in perceptual reproduced quality when compared to decoding a single encoded stream. Creating multiple encoded streams from the same source stream, where the encoded streams are neither in a scalable nor in an MDC
relationship is commonly utilized in simulcast [SDP-SIMULCAST] environments.2.1.7. Encoded Stream
A stream of time synchronized encoded media that can be independently decoded. 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.2.1.8. Dependent Stream
A stream of time synchronized encoded media fragments that are dependent on one or more encoded streams (Section 2.1.7) and zero or more dependent streams to be possible to decode. Each dependent stream has a set of dependencies. These dependencies must be understood by the parties in a Multimedia Session (Section 2.2.4) that intend to use a dependent stream.2.1.9. Media Packetizer
The transformation of taking one or more encoded (Section 2.1.7) or dependent streams (Section 2.1.8) and putting their content into one or more sequences of packets, normally RTP Packets, and output Source RTP Streams (Section 2.1.10). This step includes both generating RTP Payloads as well as RTP packets. The Media Packetizer then selects which synchronization source(s) (SSRC) [RFC3550] and RTP Sessions (Section 2.2.2) to use. The media packetizer can combine multiple encoded or dependent streams into one or more RTP Streams: o The media packetizer can use multiple inputs when producing a single RTP stream. One such example is Single RTP stream on a Single media Transport (SRST) packetization when using Scalable Video Coding (SVC) (Section 3.7). o The media packetizer can also produce multiple RTP streams, for example, when encoded and/or dependent streams are distributed over multiple RTP streams. One example of this is Multiple RTP streams on Multiple media Transports (MRMT) packetization when using SVC (Section 3.7).
2.1.10. RTP Stream
An RTP stream is a stream of RTP packets containing media data, source or redundant. The RTP stream is identified by an SSRC belonging to a particular RTP Session. The RTP session is identified as discussed in Section 2.2.2. A source RTP stream is an RTP stream directly related to an encoded stream (Section 2.1.7), targeted for transport over RTP without any additional RTP-based Redundancy (Section 2.1.11) applied. Characteristics: o Each RTP stream is identified by an SSRC [RFC3550] that is carried in every RTP and RTP Control Protocol (RTCP) packet header. The SSRC is unique in a specific RTP session context. o At any given point in time, an RTP stream can have one and only one SSRC, but SSRCs for a given RTP stream can change over time. SSRC collision and clock rate change [RFC7160] are examples of valid reasons to change SSRC for an RTP stream. In those cases, the RTP stream itself is not changed in any significant way, only the identifying SSRC number. o Each SSRC defines a unique RTP sequence numbering and timing space. o Several RTP streams, each with their own SSRC, may represent a single media source. o Several RTP streams, each with their own SSRC, can be carried in a single RTP session.2.1.11. RTP-Based Redundancy
RTP-based redundancy is defined here as a transformation that generates redundant or repair packets sent out as a Redundancy RTP Stream (Section 2.1.12) to mitigate Network Transport (Section 2.1.18) impairments, like packet loss and delay. Note that this excludes the type of redundancy that most suitable media encoders (Section 2.1.6) may add to the media format of the encoded stream (Section 2.1.7) that makes it cope better with RTP packet losses. The RTP-based redundancy exists in many flavors: they may generate independent repair streams that are used in addition to the source stream (like RTP Retransmission (Section 3.10) and some special types of Forward Error Correction (FEC) (Section 3.11), like RTP stream
duplication (Section 3.8)); they may generate a new source stream by combining redundancy information with source information (using XOR FEC as a redundancy payload (Section 3.9)); or they may completely replace the source information with only redundancy packets.2.1.12. Redundancy RTP Stream
A redundancy RTP stream is an RTP stream (Section 2.1.10) that contains no original source data, only redundant data, which may either be used as standalone or be combined with one or more Received RTP Streams (Section 2.1.23) to produce Repaired RTP Streams (Section 2.1.26).2.1.13. RTP-Based Security
The optional RTP-based Security transformation applies security services such as authentication, integrity protection, and confidentiality to an input RTP stream, like what is specified in "The Secure Real-time Transport Protocol (SRTP)" [RFC3711], producing a Secured RTP Stream (Section 2.1.14). Either an RTP stream (Section 2.1.10) or a redundancy RTP stream (Section 2.1.12) can be used as input to this transformation. In SRTP and the related Secure RTCP (SRTCP), all of the above- mentioned security services are optional, except for integrity protection of SRTCP, which is mandatory. Also confidentiality (encryption) is effectively optional in SRTP, since it is possible to use a NULL encryption algorithm. As described in [RFC7201], the strength of SRTP data origin authentication depends on the cryptographic transform and key management used. For example, in group communication, where it is sometimes possible to authenticate group membership but not the actual RTP stream sender. RTP-based security and RTP-based redundancy can be combined in a few different ways. One way is depicted in Figure 1, where an RTP stream and its corresponding redundancy RTP stream are protected by separate RTP-based security transforms. In other cases, like when a Media Translator is adding FEC in Section 3.2.1.3 of [RTP-TOPOLOGIES], a middlebox can apply RTP-based redundancy to an already secured RTP stream instead of a source RTP stream. One example of that is depicted in Figure 5 below.
Source RTP Stream +------------+ V | V +----------------------+ | +----------------------+ | RTP-Based Security | | | RTP-Based Redundancy | +----------------------+ | +----------------------+ | | | | | Redundancy RTP Stream +-------------+ | | V | +----------------------+ Secured RTP Stream | RTP-Based Security | | +----------------------+ | | | Secured Redundancy RTP Stream V V +----------------------+ +----------------------+ | Media Transport | | Media Transport | +----------------------+ +----------------------+ Figure 5: Adding Redundancy to a Secured RTP Stream In this case, the redundancy RTP stream may already have been secured for confidentiality (encrypted) by the first RTP-based security, and it may therefore not be necessary to apply additional confidentiality protection in the second RTP-based security. To avoid attacks and negative impact on RTP-based Repair (Section 2.1.25) and the resulting repaired RTP stream (Section 2.1.26), it is, however, still necessary to have this second RTP-based security apply both authentication and integrity protection to the redundancy RTP stream.2.1.14. Secured RTP Stream
A secured RTP stream is a source or redundancy RTP stream that is protected through RTP-based security (Section 2.1.13) by one or more of the confidentiality, integrity, or authentication security services.2.1.15. Media Transport
A media transport defines the transformation that the RTP streams (Section 2.1.10) are subjected to by the end-to-end transport from one RTP Sender (Section 4.12) to one specific RTP Receiver (Section 4.11) (an RTP session (Section 2.2.2) may contain multiple RTP receivers per sender). Each media transport is defined by a transport association that is normally identified by a 5-tuple (source address, source port, destination address, destination port, transport protocol), but a proposal exists for sending multiple transport associations on a single 5-tuple [TRANSPORT-MULTIPLEX].
Characteristics:
o Media transport transmits RTP streams of RTP packets from a source
transport address to a destination transport address.
o Each media transport contains only a single RTP session.
o A single RTP session can span multiple media transports.
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 media transport decomposition (Figure 6).
RTP Stream
|
V
+--------------------------+
| Media Transport Sender |
+--------------------------+
|
Sent RTP Stream
V
+--------------------------+
| Network Transport |
+--------------------------+
|
Transported RTP Stream
V
+--------------------------+
| Media Transport Receiver |
+--------------------------+
|
V
Received RTP Stream
Figure 6: Decomposition of Media Transport
2.1.16. Media Transport Sender
The first transformation within the media transport (Section 2.1.15)
is the Media Transport Sender. The sending Endpoint (Section 2.2.1)
takes an RTP stream and emits the packets onto the network using the
transport association established for this media transport, thereby
creating a Sent RTP Stream (Section 2.1.17). In the process, it
transforms the RTP stream in several ways. First, it generates 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, intentionally pace, or otherwise affect how the packets are emitted onto the network, thereby potentially introducing delay and delay variations [RFC5481] that characterize the sent RTP stream.2.1.17. Sent RTP Stream
The sent RTP stream is the RTP stream as entering the first hop of the network path to its destination. The sent RTP stream is identified using network transport addresses, like the 5-tuple (source IP address, source port, destination IP address, destination port, and protocol (UDP)) for IP/UDP.2.1.18. Network Transport
Network transport is the transformation that subjects the sent RTP stream (Section 2.1.17) to traveling from the source to the destination through the network. This transformation can result in loss of some packets, delay, and delay variation on a per-packet basis, packet duplication, and packet header or data corruption. This transformation produces a Transported RTP Stream (Section 2.1.19) at the exit of the network path.2.1.19. Transported RTP Stream
The transported RTP stream is the RTP stream that is emitted out of the network path at the destination, subjected to the network transport's transformation (Section 2.1.18).2.1.20. Media Transport Receiver
The Media Transport Receiver is the receiver endpoint's (Section 2.2.1) transformation of the transported RTP stream (Section 2.1.19) by its reception process, which results in the received RTP stream (Section 2.1.23). This transformation includes transport checksums being verified. Sensible system designs typically either discard packets with mismatching checksums or pass them on while somehow marking them in the resulting received RTP stream so to alert subsequent transformations about the possible corrupt state. In this context, it is worth noting that there is typically some probability for corrupt packets to pass through undetected (with a seemingly correct checksum). Other transformations can compensate for delay variations in receiving a packet on the network interface and providing it to the application (de-jitter buffer).
2.1.21. Received Secured RTP Stream
This is the secured RTP stream (Section 2.1.14) resulting from the media transport (Section 2.1.15) aggregate transformation.2.1.22. RTP-Based Validation
RTP-based Validation is the reverse transformation of RTP-based security (Section 2.1.13). If this transformation fails, the result is either not usable and must be discarded or may be usable but cannot be trusted. If the transformation succeeds, the result can be a received RTP stream (Section 2.1.23) or a Received Redundancy RTP Stream (Section 2.1.24), depending on what was input to the corresponding RTP-based security transformation, but it can also be a Received Secured RTP Stream (Section 2.1.21) in case several RTP- based security transformations were applied.2.1.23. Received RTP Stream
The received RTP stream is the RTP stream (Section 2.1.10) resulting from the media transport's aggregate transformation (Section 2.1.15), i.e., subjected to packet loss, packet corruption, packet duplication, delay, and delay variation from sender to receiver.2.1.24. Received Redundancy RTP Stream
The received redundancy RTP stream is the redundancy RTP stream (Section 2.1.12) resulting from the media transport's aggregate transformation, i.e., subjected to packet loss, packet corruption, packet duplication, delay, and delay variation from sender to receiver.2.1.25. RTP-Based Repair
RTP-based repair is a transformation that takes as input zero or more received RTP streams (Section 2.1.23) and one or more received redundancy RTP streams (Section 2.1.24) and produces one or more repaired RTP streams (Section 2.1.26) that are as close to the corresponding sent source RTP streams (Section 2.1.10) as possible, using different RTP-based repair methods, for example, the ones referred to in RTP-based redundancy (Section 2.1.11).2.1.26. Repaired RTP Stream
A repaired RTP stream is a received RTP stream (Section 2.1.23) for which received redundancy RTP stream (Section 2.1.24) information has been used to try to recover the source RTP stream (Section 2.1.10) as it was before media transport (Section 2.1.15).
2.1.27. Media Depacketizer
A Media Depacketizer takes one or more RTP streams (Section 2.1.10), depacketizes them, and attempts to reconstitute the encoded streams (Section 2.1.7) or dependent streams (Section 2.1.8) present in those RTP streams. In practical implementations, the media depacketizer and the media decoder may be tightly coupled and share information to improve or optimize the overall decoding and error concealment process. It is, however, not expected that there would be any benefit in defining a taxonomy for those detailed (and likely very implementation- dependent) steps.2.1.28. Received Encoded Stream
The Received Encoded Stream is the received version of an encoded stream (Section 2.1.7).2.1.29. Media Decoder
A media decoder is a transformation that is responsible for decoding encoded streams (Section 2.1.7) and any dependent streams (Section 2.1.8) into a source stream (Section 2.1.5). 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. A media decoder has to deal with any errors in the encoded streams that resulted from corruption or failure to repair packet losses. Therefore, it commonly is robust to error and losses, and includes concealment methods.2.1.30. Received Source Stream
The received source stream is the received version of a source stream (Section 2.1.5).
2.1.31. Media Sink
The Media Sink receives a source stream (Section 2.1.5) 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 raw stream (Section 2.1.3) that is conveyed to the Media Render (Section 2.1.33) and synchronized with the output from other media sinks. The media sink may also be connected with a media source (Section 2.1.4) and be used as part of a conceptual media source. 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 includes sample scaling, level adjustments, etc.2.1.32. Received Raw Stream
The Received Raw Stream is the received version of a raw stream (Section 2.1.3).2.1.33. Media Render
A media render takes a raw stream (Section 2.1.3) and converts it into physical stimulus (Section 2.1.1) that a human user can perceive. Examples of such devices are screens and D/A converters connected to amplifiers and loudspeakers. An endpoint can potentially have multiple media renders for each media type.
2.2. Communication Entities
This section contains concepts for entities involved in the communication. +------------------------------------------------------------+ | Communication Session | | | | +----------------+ +----------------+ | | | Participant A | +------------+ | Participant B | | | | | | Multimedia | | | | | | +------------+ |<==>| Session |<==>| +------------+ | | | | | Endpoint A | | | | | | Endpoint B | | | | | | | | +------------+ | | | | | | | | +----------+-+----------------------+-+----------+ | | | | | | | RTP | | | | | | | | | | | | Session |-+---Media Transport----+>| | | | | | | | | Audio |<+---Media Transport----+-| | | | | | | | | | | ^ | | | | | | | | | +----------+-+----------|-----------+-+----------+ | | | | | | | | v | | | | | | | | | | +-----------------+ | | | | | | | | | | | Synchronization | | | | | | | | | | | | Context | | | | | | | | | | | +-----------------+ | | | | | | | | | | ^ | | | | | | | | +----------+-+----------|-----------+-+----------+ | | | | | | | RTP | | v | | | | | | | | | | Session |<+---Media Transport----+-| | | | | | | | | Video |-+---Media Transport----+>| | | | | | | | | | | | | | | | | | | | +----------+-+----------------------+-+----------+ | | | | | +------------+ | | +------------+ | | | +----------------+ +----------------+ | +------------------------------------------------------------+ Figure 7: Example Point-to-Point Communication Session with Two RTP Sessions Figure 7 shows a high-level example representation of a very basic point-to-point Communication Session between Participants A and B. It uses two different audio and video RTP sessions between A's and B's endpoints, where each RTP session is a group communications channel that can potentially carry a number of RTP streams. It is using separate media transports for those RTP sessions. The multimedia session shared by the participants can, for example, be established using SIP (i.e., there is a SIP dialog between A and B).
The terms used in Figure 7 are further elaborated in the subsections below.2.2.1. Endpoint
An endpoint is 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 "endpoint". Characteristics: o Endpoints can be identified in several different ways. While RTCP Canonical Names (CNAMEs) [RFC3550] provide a globally unique and stable identification mechanism for the duration of the communication session (see Section 2.2.5), their validity applies exclusively within a Synchronization Context (Section 3.1). Thus, one endpoint can handle multiple CNAMEs, each of which can be shared among a set of endpoints belonging to the same participant (Section 2.2.3). Therefore, mechanisms outside the scope of RTP, such as application-defined mechanisms, must be used to provide endpoint identification when outside this synchronization context. o An endpoint can be associated with at most one participant (Section 2.2.3) at any single point in time. o In some contexts, an endpoint would typically correspond to a single "host", for example, a computer using a single network interface and being used by a single human user. In other contexts, a single "host" can serve multiple participants, in which case each participant's endpoint may share properties, for example, the IP address part of a transport address.2.2.2. RTP Session
An RTP session is an association among a group of participants communicating with RTP. It is a group communications channel that can potentially carry a number of RTP streams. Within an RTP session, every participant can find metadata and control information (over RTCP) about all the RTP streams in the RTP session. The bandwidth of the RTCP control channel is shared between all participants within an RTP session. Characteristics: o An RTP session can carry one or more RTP streams.
o An RTP session shares a single SSRC space as defined in [RFC3550]. That is, the endpoints participating in an RTP session can see an SSRC identifier transmitted by any of the other endpoints. An endpoint 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. o An RTP session uses at least two media transports (Section 2.1.15): one for sending and one for receiving. Commonly, the receiving media transport is the reverse direction of the media transport used for sending. An RTP session may use many media transports and these define the session's network interconnection topology. o A single media transport always carries a single RTP session. o Multiple RTP sessions can be conceptually related, for example, originating from or targeted for the same participant (Section 2.2.3) or endpoint (Section 2.2.1), or by containing RTP streams that are somehow related (Section 3).2.2.3. Participant
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. Characteristics: o A single signaling-addressable entity, using an application- specific signaling address space, for example, a SIP URI. o A participant can participate in several multimedia sessions (Section 2.2.4). o A participant can be comprised of several associated endpoints (Section 2.2.1).2.2.4. Multimedia Session
A multimedia session is an association among a group of participants (Section 2.2.3) engaged in the communication via one or more RTP sessions (Section 2.2.2). It defines logical relationships among media sources (Section 2.1.4) that appear in multiple RTP sessions.
Characteristics:
o A multimedia session can be composed of several RTP sessions with
potentially multiple RTP streams per RTP session.
o Each participant in a multimedia session can have a multitude of
media captures and media rendering devices.
o A single multimedia session can contain media from one or more
synchronization contexts (Section 3.1). 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.
2.2.5. Communication Session
A communication session is an association among two or more
participants (Section 2.2.3) communicating with each other via one or
more multimedia sessions (Section 2.2.4).
Characteristics:
o Each participant in a communication session is identified via an
application-specific signaling address.
o A communication session is composed of participants that share at
least one multimedia session, involving one or more parallel RTP
sessions with potentially multiple RTP streams per RTP session.
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.
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