RFC 7728
RTP Stream Pause and Resume
Internet Engineering Task Force (IETF) B. Burman Request for Comments: 7728 A. Akram Updates: 5104 Ericsson Category: Standards Track R. Even ISSN: 2070-1721 Huawei Technologies M. Westerlund Ericsson February 2016 RTP Stream Pause and ResumeAbstract
With the increased popularity of real-time multimedia applications, it is desirable to provide good control of resource usage, and users also demand more control over communication sessions. This document describes how a receiver in a multimedia conversation can pause and resume incoming data from a sender by sending real-time feedback messages when using the Real-time Transport Protocol (RTP) for real- time data transport. This document extends the Codec Control Message (CCM) RTP Control Protocol (RTCP) feedback package by explicitly allowing and describing specific use of existing CCMs and adding a group of new real-time feedback messages used to pause and resume RTP data streams. This document updates RFC 5104. Status of This Memo This is an Internet Standards Track document. 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). Further information on Internet Standards is available in 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/rfc7728.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 2.3. Requirements Language . . . . . . . . . . . . . . . . . . 7 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Point to Point . . . . . . . . . . . . . . . . . . . . . 8 3.2. RTP Mixer to Media Sender . . . . . . . . . . . . . . . . 9 3.3. RTP Mixer to Media Sender in Point to Multipoint . . . . 10 3.4. Media Receiver to RTP Mixer . . . . . . . . . . . . . . . 11 3.5. Media Receiver to Media Sender across RTP Mixer . . . . . 11 4. Design Considerations . . . . . . . . . . . . . . . . . . . . 12 4.1. Real-Time Nature . . . . . . . . . . . . . . . . . . . . 12 4.2. Message Direction . . . . . . . . . . . . . . . . . . . . 12 4.3. Apply to Individual Sources . . . . . . . . . . . . . . . 12 4.4. Consensus . . . . . . . . . . . . . . . . . . . . . . . . 13 4.5. Message Acknowledgments . . . . . . . . . . . . . . . . . 13 4.6. Request Retransmission . . . . . . . . . . . . . . . . . 14 4.7. Sequence Numbering . . . . . . . . . . . . . . . . . . . 14 4.8. Relation to Other Solutions . . . . . . . . . . . . . . . 14 5. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 15 5.1. Expressing Capability . . . . . . . . . . . . . . . . . . 16 5.2. PauseID . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.3. Requesting to Pause . . . . . . . . . . . . . . . . . . . 17 5.4. Media Sender Pausing . . . . . . . . . . . . . . . . . . 18 5.5. Requesting to Resume . . . . . . . . . . . . . . . . . . 19 5.6. TMMBR/TMMBN Considerations . . . . . . . . . . . . . . . 20 6. Participant States . . . . . . . . . . . . . . . . . . . . . 22 6.1. Playing State . . . . . . . . . . . . . . . . . . . . . . 22 6.2. Pausing State . . . . . . . . . . . . . . . . . . . . . . 22 6.3. Paused State . . . . . . . . . . . . . . . . . . . . . . 23 6.3.1. RTCP BYE Message . . . . . . . . . . . . . . . . . . 24 6.3.2. SSRC Time-Out . . . . . . . . . . . . . . . . . . . . 24 6.4. Local Paused State . . . . . . . . . . . . . . . . . . . 24 7. Message Format . . . . . . . . . . . . . . . . . . . . . . . 26 8. Message Details . . . . . . . . . . . . . . . . . . . . . . . 28 8.1. PAUSE . . . . . . . . . . . . . . . . . . . . . . . . . . 29 8.2. PAUSED . . . . . . . . . . . . . . . . . . . . . . . . . 30 8.3. RESUME . . . . . . . . . . . . . . . . . . . . . . . . . 31 8.4. REFUSED . . . . . . . . . . . . . . . . . . . . . . . . . 32 8.5. Transmission Rules . . . . . . . . . . . . . . . . . . . 32 9. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 33 9.1. Offer/Answer Use . . . . . . . . . . . . . . . . . . . . 37 9.2. Declarative Use . . . . . . . . . . . . . . . . . . . . . 39
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 39 10.1. Offer/Answer . . . . . . . . . . . . . . . . . . . . . . 40 10.2. Point-to-Point Session . . . . . . . . . . . . . . . . . 41 10.3. Point to Multipoint Using Mixer . . . . . . . . . . . . 45 10.4. Point to Multipoint Using Translator . . . . . . . . . . 47 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50 12. Security Considerations . . . . . . . . . . . . . . . . . . . 50 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 52 13.1. Normative References . . . . . . . . . . . . . . . . . . 52 13.2. Informative References . . . . . . . . . . . . . . . . . 53 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 54 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 551. Introduction
As real-time communication attracts more people, more applications are created; multimedia conversation applications is one example. Multimedia conversation further exists in many forms, for example, peer-to-peer chat application and multiparty video conferencing controlled by central media nodes, such as RTP Mixers. Multimedia conferencing may involve many participants; each has its own preferences for the communication session, not only at the start but also during the session. This document describes several scenarios in multimedia communication where a conferencing node or participant chooses to temporarily pause an incoming RTP [RFC3550] stream and later resume it when needed. The receiver does not need to terminate or inactivate the RTP session and start all over again by negotiating the session parameters, for example, using SIP [RFC3261] with the Session Description Protocol (SDP) [RFC4566] offer/answer [RFC3264]. Centralized nodes, like RTP Mixers or Multipoint Control Units (MCUs) that use either logic based on voice activity, other measurements, or user input could reduce the resources consumed in both the sender and the network by temporarily pausing the RTP streams that aren't required by the RTP Mixer. If the number of conference participants are greater than what the conference logic has chosen to present simultaneously to receiving participants, some participant RTP streams sent to the RTP Mixer may not need to be forwarded to any other participant. Those RTP streams could then be temporarily paused. This becomes especially useful when the media sources are provided in multiple encoding versions (Simulcast) [SDP-SIMULCAST] or with Multi-Session Transmission (MST) of scalable encoding such as Scalable Video Coding (SVC) [RFC6190]. There may be some of the
defined encodings or a combination of scalable layers that are not used or cannot be used all of the time. As an example, a centralized node may choose to pause such unused RTP streams without being explicitly requested to do so, maybe due to temporarily limited network or processing resources. It may then also send an explicit indication that the streams are paused. As the set of RTP streams required at any given point in time is highly dynamic in such scenarios, using the out-of-band signaling channel for pausing, and even more importantly resuming, an RTP stream is difficult due to the performance requirements. Instead, the pause and resume signaling should be in the media plane and go directly between the affected nodes. When using RTP [RFC3550] for media transport, using "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)" [RFC4585] appears appropriate. No currently existing RTCP feedback message explicitly supports pausing and resuming an incoming RTP stream. As this affects the generation of packets and may even allow the encoding process to be paused, the functionality appears to match Codec Control Messages (CCMs) in the RTP Audio-Visual Profile with Feedback (AVPF) [RFC5104]. This document defines the solution as a CCM extension. The Temporary Maximum Media Bitrate Request (TMMBR) message of CCM is used by video conferencing systems for flow control. It is desirable to be able to use that method with a bitrate value of zero for pause, whenever possible. This specification updates RFC 5104 by adding the new pause and resume semantics to the TMMBR and Temporary Maximum Media Bitrate Notification (TMMBN) messages.2. Definitions
2.1. Abbreviations
AVPF: Audio-Visual Profile with Feedback (RFC 4585) CCM: Codec Control Message (RFC 5104) CNAME: Canonical Name (RTCP Source Description) CSRC: Contributing Source (RTP) FCI: Feedback Control Information (AVPF) FIR: Full Intra Refresh (CCM) FMT: Feedback Message Type (AVPF)
MCU: Multipoint Control Unit MTU: Maximum Transfer Unit PT: Payload Type (RTP) RTP: Real-time Transport Protocol (RFC 3550) RTCP: RTP Control Protocol (RFC 3550) RTCP RR: RTCP Receiver Report RTCP SR: RTCP Sender Report SDP: Session Description Protocol (RFC 4566) SIP: Session Initiation Protocol (RFC 3261) SSRC: Synchronization Source (RTP) SVC: Scalable Video Coding TMMBR: Temporary Maximum Media Bitrate Request (CCM) TMMBN: Temporary Maximum Media Bitrate Notification (CCM) UA: User Agent (SIP) UDP: User Datagram Protocol (RFC 768)2.2. Terminology
In addition to the following, the definitions from RTP [RFC3550], AVPF [RFC4585], CCM [RFC5104], and RTP Taxonomy [RFC7656] also apply in this document. Feedback Messages: CCM [RFC5104] categorized different RTCP feedback messages into four types: Request, Command, Indication, and Notification. This document places the PAUSE and RESUME messages into the Request category, PAUSED as an Indication, and REFUSED as a Notification. PAUSE: Request from an RTP stream receiver to pause a stream RESUME: Request from an RTP stream receiver to resume a paused stream
PAUSED: Indication from an RTP stream sender that a stream is
paused
REFUSED: Notification from an RTP stream sender that a PAUSE or
RESUME request will not be honored
Mixer: The intermediate RTP node that receives an RTP stream from
different endpoints, combines them to make one RTP stream, and
forwards them to destinations, in the sense described for Topo-
Mixer in "RTP Topologies" [RFC7667].
Participant: A member that is part of an RTP session, acting as the
receiver, sender, or both.
Paused sender: An RTP stream sender that has stopped its
transmission, i.e., no other participant receives its RTP
transmission, based on having received either a PAUSE request,
defined in this specification, or a local decision.
Pausing receiver: An RTP stream receiver that sends a PAUSE request,
defined in this specification, to another participant(s).
Stream: Used as a short term for RTP stream, unless otherwise noted.
Stream receiver: Short for RTP stream receiver; the RTP entity
responsible for receiving an RTP stream, usually a Media
Depacketizer.
Stream sender: Short for RTP stream sender; the RTP entity
responsible for creating an RTP stream, usually a Media
Packetizer.
2.3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
3. Use Cases
This section discusses the main use cases for RTP stream pause and resume. The RTCWEB WG's use case and requirements document [RFC7478] defines the following API requirements in Appendix A, which is also used by the W3C WebRTC WG: A8 The web API must provide means for the web application to mute/ unmute a stream or stream component(s). When a stream is sent to a peer, mute status must be preserved in the stream received by the peer. A9 The web API must provide means for the web application to cease the sending of a stream to a peer. This document provides means to optimize transport usage by stopping the sending of muted streams and starting the sending of streams again when unmuted. Here, it is assumed that "mute" above can be taken to apply also to media other than audio. At the time of publication for this specification, the RTCWEB WG did not specify any pause/resume functionality.3.1. Point to Point
This is the most basic use case with an RTP session containing two endpoints. Each endpoint sends one or more streams. +---+ +---+ | A |<------->| B | +---+ +---+ Figure 1: Point to Point The usage of RTP stream pause in this use case is to temporarily halt delivery of streams that the sender provides but the receiver does not currently use. This can, for example, be due to minimized applications where the video stream is not actually shown on any display, or it is not used in any other way, such as being recorded. In this case, since there is only a single receiver of the stream, pausing or resuming a stream does not impact anyone else other than the sender and the single receiver of that stream.
3.2. RTP Mixer to Media Sender
One of the most commonly used topologies in centralized conferencing is based on the RTP Mixer [RFC7667]. The main reason for this is that it provides a very consistent view of the RTP session towards each participant. That is accomplished through the Mixer originating its own streams, identified by distinct SSRC values, and any RTP streams sent to the participants will be sent using those SSRC values. If the Mixer wants to identify the underlying media sources for its conceptual streams, it can identify them using CSRC. The stream the Mixer provides can be an actual mix of multiple media sources, but it might also be switching received streams as described in Sections 3.6 - 3.8 of "RTP Topologies" [RFC7667]. +---+ +-----------+ +---+ | A |<---->| |<---->| B | +---+ | | +---+ | Mixer | +---+ | | +---+ | C |<---->| |<---->| D | +---+ +-----------+ +---+ Figure 2: RTP Mixer in Unicast Only Which streams from clients B, C, and D that are delivered to a given receiver, A, can depend on several things: o The RTP Mixer's own logic and measurements, such as voice activity on the incoming audio streams. o The number of sent media sources exceed what is reasonable to present simultaneously at any given receiver. o A human controlling the conference that determines how the media should be mixed. This would be more common in lecture or similar applications where regular listeners may be prevented from breaking into the session unless approved by the moderator. o The streams may also be part of a Simulcast [SDP-SIMULCAST] or scalable encoded (for Multi-Session Transmission) [RFC6190], thus providing multiple versions that can be delivered by the RTP stream sender. These examples indicate that there are numerous reasons why a particular stream would not currently be in use but must be available for use at very short notice if any dynamic event occurs that causes a different stream selection to be done in the Mixer.
Because of this, it would be highly beneficial if the Mixer could request the RTP stream sender to pause a particular stream. The Mixer also needs to be able to request the RTP stream sender to resume delivery with minimal delay. In some cases, especially when the Mixer sends multiple RTP streams per receiving client, there may be situations that make it desirable for the Mixer to pause some of its sent RTP streams, even without being explicitly asked to do so by the receiving client. Such situations can, for example, be caused by a temporary lack of available Mixer network or processing resources. An RTP stream receiver that no longer receives an RTP stream could interpret this as an error condition and try to take action to re-establish the RTP stream. Such action would likely be undesirable if the RTP stream was in fact deliberately paused by the Mixer. Undesirable RTP stream receiver actions could be avoided if the Mixer is able to explicitly indicate that an RTP stream is deliberately paused. Just as for point to point (Section 3.1), there is only a single receiver of the stream, the RTP Mixer, and pausing or resuming a stream does not affect anyone else other than the sender and single receiver of that stream.3.3. RTP Mixer to Media Sender in Point to Multipoint
This use case is similar to the previous section; however, the RTP Mixer is involved in three domains that need to be separated: the Multicast Network (including participants A and C), participant B, and participant D. The difference from above is that A and C share a multicast domain, which is depicted below. +-----+ +---+ / \ +-----------+ +---+ | A |<---/ \ | |<---->| B | +---+ / Multi- \ | | +---+ + cast +->| Mixer | +---+ \ Network / | | +---+ | C |<---\ / | |<---->| D | +---+ \ / +-----------+ +---+ +-----+ Figure 3: RTP Mixer in Point to Multipoint If the RTP Mixer pauses a stream from A, it will not only pause the stream towards itself but will also stop the stream from arriving to C, which C is heavily impacted by, might not approve of, and should thus have a say on.
If the Mixer resumes a paused stream from A, it will be resumed also towards C. In this case, if C is not interested, it can simply ignore the stream and is not impacted as much as above. In this use case, there are several receivers of a stream, and the Mixer must take special care so as not to pause a stream that is still wanted by some receivers.3.4. Media Receiver to RTP Mixer
In this use case, the direction of the request to pause is the opposite compared to the two previous use cases. An endpoint in Figure 2 could potentially request to pause the delivery of a given stream. Possible reasons include those in the point-to-point case (Section 3.1) above. When the RTP Mixer is only connected to individual unicast paths, the use case and any considerations are identical to the point-to-point use case. However, when the endpoint requesting stream pause is connected to the RTP Mixer through a multicast network, such as A or C in Figure 3, the use case instead becomes identical to the one in Section 3.3, only with reverse direction of the streams and pause/ resume requests.3.5. Media Receiver to Media Sender across RTP Mixer
An endpoint, like A in Figure 2, could potentially request to pause the delivery of a given stream, like one of B's, over any of the SSRCs used by the Mixer by sending a pause request for the CSRC identifying the stream. However, the authors are of the opinion that this is not a suitable solution for several reasons: 1. The Mixer might not include CSRC in its stream indications. 2. An endpoint cannot rely on the CSRC to correctly identify the stream to be paused when the delivered media is some type of mix. A more elaborate stream identification solution is needed to support this in the general case. 3. The endpoint cannot determine if a given stream is still needed by the RTP Mixer to deliver to another session participant. Due to the above reasons, we exclude this use case from further consideration.
4. Design Considerations
This section describes the requirements that this specification needs to meet.4.1. Real-Time Nature
The first section (Section 1) of this specification describes some possible reasons why a receiver may pause an RTP sender. Pausing and resuming is time dependent, i.e., a receiver may choose to pause an RTP stream for a certain duration, after which the receiver may want the sender to resume. This time dependency means that the messages related to pause and resume must be transmitted to the sender in a timely fashion in order for them to be purposeful. The pause operation is arguably not as time critical as the resume operation, since it mainly provides a reduction of resource usage. Timely handling of the resume operation is, however, likely to directly impact the end-user's perceived quality experience, since it affects the availability of media that the user expects to receive more or less instantly. It may also be highly desirable for a receiver to quickly learn that an RTP stream is intentionally paused on the RTP sender's own behalf.4.2. Message Direction
It is the responsibility of an RTP stream receiver that wants to pause or resume a stream from the sender(s) to transmit PAUSE and RESUME messages. An RTP stream sender that wants to pause itself can often simply do it, but sometimes this will adversely affect the receiver and an explicit indication that the RTP stream is paused may then help. Any indication that an RTP stream is paused is the responsibility of the RTP stream sender and may in some cases not even be needed by the stream receiver.4.3. Apply to Individual Sources
The PAUSE and RESUME messages apply to single RTP streams identified by their SSRC, which means the receiver targets the sender's SSRC in the PAUSE and RESUME requests. If a paused sender starts sending with a new SSRC, the receivers will need to send a new PAUSE request in order to pause it. PAUSED indications refer to a single one of the sender's own paused SSRC.
4.4. Consensus
An RTP stream sender should not pause an SSRC that some receiver still wishes to receive. The reason is that in RTP topologies where the stream is shared between multiple receivers, a single receiver on that shared network must not single-handedly cause the stream to be paused without letting all other receivers voice their opinions on whether or not the stream should be paused. Such shared networks can, for example, be multicast, a mesh with a joint RTP session, or a transport Translator-based network. A consequence of this is that a newly joining receiver first needs to learn the existence of paused streams and secondly should be able to resume any paused stream. A newly joining receiver can, for example, be detected through an RTCP Receiver Report containing both a new SSRC and a CNAME that does not already occur in the session. Any single receiver wanting to resume a stream should also cause it to be resumed. An important exception to this is when the RTP stream sender is aware of conditions that make it desirable or even necessary to pause the RTP stream on its own behalf, without being explicitly asked to do so. Such local consideration in the RTP sender takes precedence over RTP receiver wishes to receive the stream.4.5. Message Acknowledgments
RTP and RTCP does not guarantee reliable data transmission. It uses whatever assurance the lower-layer transport protocol can provide. However, this is commonly UDP that provides no reliability guarantees. Thus, it is possible that a PAUSE and/or RESUME message transmitted from an RTP endpoint does not reach its destination, i.e., the targeted RTP stream sender. When PAUSE or RESUME reaches the RTP stream sender and is effective, i.e., an active RTP stream sender pauses or a resuming RTP stream sender has media data to transmit, it is immediately seen from the arrival or non-arrival of RTP packets for that RTP stream. Thus, no explicit acknowledgments are required in this case. In some cases, when a PAUSE or RESUME message reaches the RTP stream sender, it will not be able to pause or resume the stream due to some local consideration, for example, lack of data to transmit. In this error condition, a negative acknowledgment may be needed to avoid unnecessary retransmission of requests (Section 4.6).
4.6. Request Retransmission
When the stream is not affected as expected by a PAUSE or RESUME request, the request may have been lost and the sender of the request will need to retransmit it. The retransmission should take the round-trip time into account, and will also need to take the normal RTCP bandwidth and timing rules applicable to the RTP session into account, when scheduling retransmission of feedback. When it comes to resume requests or unsolicited paused indications that are more time critical, the best performance may be achieved by repeating the message as often as possible until a sufficient number have been sent to reach a high probability of message delivery or at an explicit indication that the message was delivered. For resume requests, such explicit indication can be delivery of the RTP stream being requested to be resumed.4.7. Sequence Numbering
A PAUSE request message will need to have a sequence number to separate retransmissions from new requests. A retransmission keeps the sequence number unchanged, while it is incremented every time a new PAUSE request is transmitted that is not a retransmission of a previous request. Since RESUME always takes precedence over PAUSE and is even allowed to avoid pausing a stream, there is a need to keep strict ordering of PAUSE and RESUME. Thus, RESUME needs to share sequence number space with PAUSE and implicitly reference which PAUSE it refers to. For the same reasons, the explicit PAUSED indication also needs to share sequence number space with PAUSE and RESUME.4.8. Relation to Other Solutions
A performance comparison between SIP/SDP and RTCP signaling technologies was made and included in draft versions of this specification. Using SIP and SDP to carry pause and resume information means that they will need to traverse the entire signaling path to reach the signaling destination (either the remote endpoint or the entity controlling the RTP Mixer) across any signaling proxies that potentially also have to process the SDP content to determine if they are expected to act on it. The amount of bandwidth required for a signaling solution based on SIP/SDP is in the order of at least 10 times more than an RTCP-based solution. Especially for a UA sitting on mobile wireless access, this will risk introducing delays that are too long (Section 4.1) to provide a good user experience, and the bandwidth cost may also be considered infeasible compared to an RTCP-based solution. RTCP data sent
through the media path, which is likely shorter (contains fewer intermediate nodes) than the signaling path, may have to traverse a few intermediate nodes anyway. The amount of processing and buffering required in intermediate nodes to forward those RTCP messages is, however, believed to be significantly less than for intermediate nodes in the signaling path. Based on those considerations, RTCP is chosen as the signaling protocol for the pause and resume functionality.5. Solution Overview
The proposed solution implements pause and resume functionality based on sending AVPF RTCP feedback messages from any RTP session participant that wants to pause or resume a stream targeted at the stream sender, as identified by the sender SSRC. This solution reuses CCM TMMBR and TMMBN [RFC5104] to the extent possible and defines a small set of new RTCP feedback messages where new semantics is needed. A single feedback message specification is used to implement the new messages. The message consists of a number of Feedback Control Information (FCI) blocks, where each block can be a PAUSE request, a RESUME request, a PAUSED indication, a REFUSED notification, or an extension to this specification. This structure allows a single feedback message to handle pause functionality on a number of streams. The PAUSED functionality is also defined in such a way that it can be used as a standalone by the RTP stream sender to indicate a local decision to pause, and it can inform any receiver of the fact that halting media delivery is deliberate and which RTP packet was the last transmitted. Special considerations that apply when using TMMBR/TMMBN for pause and resume purposes are described in Section 5.6. This specification applies to both the new messages defined herein as well as their TMMBR/TMMBN counterparts, except when explicitly stated otherwise. An obvious exception is any reference to the message parameters that are only available in the messages defined here. For example, any reference to PAUSE in the text below is equally applicable to TMMBR 0, and any reference to PAUSED is equally applicable to TMMBN 0. Therefore, and for brevity, TMMBR/TMMBN will not be mentioned in the text, unless there is specific reason to do so. This section is intended to be explanatory and therefore intentionally contains no mandatory statements. Such statements can instead be found in other parts of this specification.
5.1. Expressing Capability
An endpoint can use an extension to CCM SDP signaling to declare capability to understand the messages defined in this specification. Capability to understand only a subset of messages is possible, to support partial implementation, which is specifically believed to be feasible for the 'RTP Mixer to Media Sender' use case (Section 3.2). In that use case, only the RTP Mixer has capability to request the media sender to pause or resume. Consequently, in that same use case, only the media sender has capability to pause and resume its sent streams based on requests from the RTP Mixer. Allowing for partial implementation of this specification is not believed to hamper interoperability, as long as the subsets are well defined and describe a consistent functionality, including a description of how a more capable implementation must perform fallback. For the case when TMMBR/TMMBN are used for pause and resume purposes, it is possible to explicitly express joint support for TMMBR and TMMBN, but not for TMMBN only.5.2. PauseID
All messages defined in this specification (Section 8) contain a PauseID, satisfying the design consideration on sequence numbering (Section 4.7). This PauseID is scoped by and thus a property of the targeted RTP stream (SSRC) and is not only a sequence number for individual messages. Instead, it numbers an entire "pause and resume operation" for the RTP stream, typically keeping PauseID constant for multiple, related messages. The PauseID value used during such operation is called the current PauseID. A new "pause and resume operation" is defined to start when the RTP stream sender resumes the RTP stream after it was being paused. The current PauseID is then incremented by one in modulo arithmetic. In the subsequent descriptions below, it is sometimes necessary to refer to PauseID values that were already used as the current PauseID, which is denoted as the past PauseID. It should be noted that since PauseID uses modulo arithmetic, a past PauseID may have a larger value than the current PauseID. Since PauseID uses modulo arithmetic, it is also useful to define what PauseID values are considered "past" to clearly separate it from what could be considered "future" PauseID values. Half of the entire PauseID value range is chosen to represent a past PauseID, while a quarter of the PauseID value range is chosen to represent future values. The remaining quarter of the PauseID value range is intentionally left undefined in that respect.
5.3. Requesting to Pause
An RTP stream receiver can choose to send a PAUSE request at any time, subject to AVPF timing rules. The PAUSE request contains the current PauseID (Section 5.2). When a non-paused RTP stream sender receives the PAUSE request, it continues to send the RTP stream while waiting for some time to allow other RTP stream receivers in the same RTP session that saw this PAUSE request to disapprove by sending a RESUME (Section 5.5) for the same stream and with the same current PauseID as in the PAUSE being disapproved. If such a disapproving RESUME arrives at the RTP stream sender during the hold-off period before the stream is paused, the pause is not performed. In point-to-point configurations, the hold- off period may be set to zero. Using a hold-off period of zero is also appropriate when using TMMBR 0 and is in line with the semantics for that message. If the RTP stream sender receives further PAUSE requests with the current PauseID while waiting as described above, those additional requests are ignored. If the PAUSE request is lost before it reaches the RTP stream sender, it will be discovered by the RTP stream receiver because it continues to receive the RTP stream. It will also not see any PAUSED indication (Section 5.4) for the stream. The same condition can be caused by the RTP stream sender having received a disapproving RESUME from stream receiver A for a PAUSE request sent by stream sender B, except that the PAUSE sender (B) did not receive the RESUME (from A) and may instead think that the PAUSE was lost. In both cases, a PAUSE request can be retransmitted using the same current PauseID. If using TMMBR 0, the request MAY be retransmitted when the requester fails to receive a TMMBN 0 confirmation. If the pending stream pause is aborted due to a disapproving RESUME, the pause and resume operation for that PauseID is concluded, the current PauseID is updated, and any new PAUSE must therefore use the new current PauseID to be effective. An RTP stream sender receiving a PAUSE not using the current PauseID informs the RTP stream receiver sending the ineffective PAUSE of this condition by sending a REFUSED notification that contains the current PauseID value. A situation where an ineffective PauseID is chosen can appear when a new RTP stream receiver joins a session and wants to PAUSE a stream but does not yet know the current PauseID to use. The REFUSED
notification will then provide sufficient information to create a valid PAUSE. The required extra signaling round trip is not considered harmful, since it is assumed that pausing a stream is not time critical (Section 4.1). There may be local considerations making it impossible or infeasible to pause the stream, and the RTP stream sender can then respond with a REFUSED. In this case, if the used current PauseID would otherwise have been effective, REFUSED contains the same current PauseID as in the PAUSE request. Note that when using TMMBR 0 as PAUSE, that request cannot be refused (TMMBN > 0) due to the existing restriction in Section 4.2.2.2 of [RFC5104] that TMMBN shall contain the current bounding set, and the fact that a TMMBR 0 will always be the most restrictive point in any bounding set, regardless of the bounding set overhead value. If the RTP stream sender receives several identical PAUSE requests for an RTP stream that was already responded to at least once with REFUSED and the condition causing REFUSED remains, those additional REFUSED notifications should be sent with regular RTCP timing. A single REFUSED can respond to several identical PAUSE requests.5.4. Media Sender Pausing
An RTP stream sender can choose to pause the stream at any time. This can be either a result of receiving a PAUSE or based on some local sender consideration. When it does, it sends a PAUSED indication, containing the current PauseID. Note that the current PauseID in an unsolicited PAUSED (without having received a PAUSE) is incremented compared to a previously sent PAUSED. It also sends the PAUSED indication in the next two regular RTCP reports, given that the pause condition is then still effective. There is no reply to a PAUSED indication; it is simply an explicit indication of the fact that an RTP stream is paused. This can be helpful for the RTP stream receiver, for example, to quickly understand that transmission is deliberately and temporarily suspended and no specific corrective action is needed. The RTP stream sender may want to apply some local consideration to exactly when the RTP stream is paused, for example, completing some media unit or a forward error correction block, before pausing the stream. The PAUSED indication also contains information about the RTP extended highest sequence number when the pause became effective. This provides RTP stream receivers with firsthand information that allows them to know whether they lost any packets just before the
stream paused or when the stream is resumed again. This allows RTP stream receivers to quickly and safely take into account that the stream is paused in, for example, retransmission or congestion control algorithms. If the RTP stream sender receives PAUSE requests with the current PauseID while the stream is already paused, those requests are ignored. As long as the stream is being paused, the PAUSED indication MAY be sent together with any regular RTCP Sender Report (SR) or Receiver Report (RR). Including PAUSED in this way allows RTP stream receivers to join while the stream is paused and to quickly know that there is a paused stream, what the last sent extended RTP sequence number is, and what the current PauseID is, which enables them to construct valid PAUSE and RESUME requests at a later stage. When the RTP stream sender learns that a new endpoint has joined the RTP session, for example, by a new SSRC and a CNAME that was not previously seen in the RTP session, it should send PAUSED indications for all its paused streams at its earliest opportunity. In addition, it should continue to include PAUSED indications in at least two regular RTCP reports.5.5. Requesting to Resume
An RTP stream receiver can request the RTP stream sender to resume a stream with a RESUME request at any time, subject to AVPF timing rules. The RTP stream receiver must include the current PauseID in the RESUME request for it to be effective. A pausing RTP stream sender that receives a RESUME including the current PauseID resumes the stream at the earliest opportunity. Receiving RESUME requests for a stream that is not paused does not require any action and can be ignored. There may be local considerations at the RTP stream sender, for example, that the media device is not ready, making it temporarily impossible to resume the stream at that point in time, and the RTP stream sender can then respond with a REFUSED containing the current PauseID. When receiving such REFUSED with a current PauseID identical to the one in the sent RESUME, RTP stream receivers should avoid sending further RESUME requests for some reasonable amount of time to allow the condition to clear. An RTP stream sender having sent a REFUSED SHOULD resume the stream through local considerations (see below) when the condition that caused the REFUSED is no longer true.
If the RTP stream sender receives several identical RESUME requests for an RTP stream that was already at least once responded to with REFUSED and the condition causing REFUSED remains, those additional REFUSED notifications should be sent with regular RTCP timing. A single REFUSED can respond to several identical RESUME requests. A pausing RTP stream sender can apply local considerations and can resume a paused RTP stream at any time. If TMMBR 0 was used to pause the RTP stream, resumption is prevented by protocol, even if the RTP sender would like to resume due to local considerations. If TMMBR/ TMMBN signaling is used, the RTP stream is paused due to local considerations (Section 5.4), and the RTP stream sender thus owns the TMMBN bounding set, the RTP stream can be resumed due to local considerations. When resuming a paused stream, especially for media that makes use of temporal redundancy between samples such as video, it may not be appropriate to use such temporal dependency in the encoding between samples taken before the pause and at the time instant the stream is resumed. Should such temporal dependency between media samples before and after the media was paused be used by the RTP stream sender, it requires the RTP stream receiver to have saved the samples from before the pause for successful continued decoding when resuming. The use of this temporal dependency of media samples from before the pause is left up to the RTP stream sender. If temporal dependency on samples from before the pause is not used when the RTP stream is resumed, the first encoded sample after the pause will not contain any temporal dependency on samples before the pause (for video it may be a so-called intra picture). If temporal dependency on samples from before the pause is used by the RTP stream sender when resuming, and if the RTP stream receiver did not save any sample from before the pause, the RTP stream receiver can use a FIR request [RFC5104] to explicitly ask for a sample without temporal dependency (for video a so-called intra picture), even at the same time as sending the RESUME.5.6. TMMBR/TMMBN Considerations
As stated above, TMMBR/TMMBN may be used to provide pause and resume functionality for the point-to-point case. If the topology is not point to point, TMMBR/TMMBN cannot safely be used for pause or resume. This use is expected to be mainly for interworking with implementations that don't support the messages defined in this specification (Section 8) but make use of TMMBR/TMMBN to achieve a similar effect.
This is a brief summary of what functionality is provided when using
TMMBR/TMMBN:
TMMBR 0: Corresponds to PAUSE, without the requirement for any hold-
off period to wait for RESUME before pausing the RTP stream.
TMMBR > 0: Corresponds to RESUME when the RTP stream was previously
paused with TMMBR 0. Since there is only a single RTP stream
receiver, there is no need for the RTP stream sender to delay
resuming the stream until after sending TMMBN > 0 or to apply the
hold-off period specified in [RFC5104] before increasing the
bitrate from zero. The bitrate value used when resuming after
pausing with TMMBR 0 is either according to known limitations or
based on starting a stream with the configured maximum for the
stream or session, for example, given by "b=" line in SDP.
TMMBN 0: Corresponds to PAUSED when the RTP stream was paused with
TMMBR 0 but may, just as PAUSED, also be used unsolicited. An
unsolicited RTP stream pause based on local sender considerations
uses the RTP stream's own SSRC as the TMMBR restriction owner in
the TMMBN message bounding set. It also corresponds to a REFUSED
notification when a stream is requested to be resumed with
TMMBR > 0, thus resulting in the stream sender becoming the owner
of the bounding set in the TMMBN message.
TMMBN > 0: Cannot be used as a REFUSED notification when a stream is
requested to be paused with TMMBR 0, for reasons stated in
Section 5.3.
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