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diff --git a/doc/rfc/rfc7728.txt b/doc/rfc/rfc7728.txt new file mode 100644 index 0000000..4aedaf6 --- /dev/null +++ b/doc/rfc/rfc7728.txt @@ -0,0 +1,3083 @@ + + + + + + +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 Resume + +Abstract + + 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. + + + + + + + + + + + + +Burman, et al. Standards Track [Page 1] + +RFC 7728 RTP Stream Pause February 2016 + + +Copyright Notice + + Copyright (c) 2016 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (http://trustee.ietf.org/license-info) in effect on the date of + publication of this document. Please review these documents + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + the Trust Legal Provisions and are provided without warranty as + described in the Simplified BSD License. + + This document may contain material from IETF Documents or IETF + Contributions published or made publicly available before November + 10, 2008. The person(s) controlling the copyright in some of this + material may not have granted the IETF Trust the right to allow + modifications of such material outside the IETF Standards Process. + Without obtaining an adequate license from the person(s) controlling + the copyright in such materials, this document may not be modified + outside the IETF Standards Process, and derivative works of it may + not be created outside the IETF Standards Process, except to format + it for publication as an RFC or to translate it into languages other + than English. + + + + + + + + + + + + + + + + + + + + + + + + + +Burman, et al. Standards Track [Page 2] + +RFC 7728 RTP Stream Pause February 2016 + + +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 + + + + + +Burman, et al. Standards Track [Page 3] + +RFC 7728 RTP Stream Pause February 2016 + + + 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 . . . . . . . . . . . . . . . . . . . . . . . 55 + +1. 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 + + + + + +Burman, et al. Standards Track [Page 4] + +RFC 7728 RTP Stream Pause February 2016 + + + 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) + + + + +Burman, et al. Standards Track [Page 5] + +RFC 7728 RTP Stream Pause February 2016 + + + 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 + + + + + +Burman, et al. Standards Track [Page 6] + +RFC 7728 RTP Stream Pause February 2016 + + + 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]. + + + + + + + + + + + + + +Burman, et al. Standards Track [Page 7] + +RFC 7728 RTP Stream Pause February 2016 + + +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. + + + + + + + + +Burman, et al. Standards Track [Page 8] + +RFC 7728 RTP Stream Pause February 2016 + + +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. + + + + +Burman, et al. Standards Track [Page 9] + +RFC 7728 RTP Stream Pause February 2016 + + + 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. + + + + +Burman, et al. Standards Track [Page 10] + +RFC 7728 RTP Stream Pause February 2016 + + + 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. + + + + + +Burman, et al. Standards Track [Page 11] + +RFC 7728 RTP Stream Pause February 2016 + + +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. + + + + + + + + + +Burman, et al. Standards Track [Page 12] + +RFC 7728 RTP Stream Pause February 2016 + + +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). + + + + + + + + +Burman, et al. Standards Track [Page 13] + +RFC 7728 RTP Stream Pause February 2016 + + +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 + + + +Burman, et al. Standards Track [Page 14] + +RFC 7728 RTP Stream Pause February 2016 + + + 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. + + + +Burman, et al. Standards Track [Page 15] + +RFC 7728 RTP Stream Pause February 2016 + + +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. + + + + + + + +Burman, et al. Standards Track [Page 16] + +RFC 7728 RTP Stream Pause February 2016 + + +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 + + + +Burman, et al. Standards Track [Page 17] + +RFC 7728 RTP Stream Pause February 2016 + + + 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 + + + +Burman, et al. Standards Track [Page 18] + +RFC 7728 RTP Stream Pause February 2016 + + + 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. + + + + +Burman, et al. Standards Track [Page 19] + +RFC 7728 RTP Stream Pause February 2016 + + + 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. + + + + + + +Burman, et al. Standards Track [Page 20] + +RFC 7728 RTP Stream Pause February 2016 + + + 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. + + + + + + + + + + + + + + + + + + + + + + + +Burman, et al. Standards Track [Page 21] + +RFC 7728 RTP Stream Pause February 2016 + + +6. Participant States + + This document introduces three new states for a stream in an RTP + sender, according to the figure and subsections below. Any + references to PAUSE, PAUSED, RESUME, and REFUSED in this section + SHALL be taken to apply to the extent possible also when TMMBR/TMMBN + are used (Section 5.6) for this functionality. + + +------------------------------------------------------+ + | Received RESUME | + v | + +---------+ Received PAUSE +---------+ Hold-off period +--------+ + | Playing |---------------->| Pausing |---------------->| Paused | + | |<----------------| | | | + +---------+ Received RESUME +---------+ +--------+ + ^ | | PAUSE decision | + | | v | + | | PAUSE decision +---------+ PAUSE decision | + | +------------------>| Local |<--------------------+ + +-------------------------| Paused | + RESUME decision +---------+ + + Figure 4: RTP Pause States in Sender + +6.1. Playing State + + This state is not new but is the normal media sending state from + [RFC3550]. When entering the state, the current PauseID MUST be + incremented by one in modulo arithmetic. The RTP sequence number for + the first packet sent after a pause SHALL be incremented by one + compared to the highest RTP sequence number sent before the pause. + The first RTP timestamp for the first packet sent after a pause + SHOULD be set according to capture times at the source, meaning the + RTP timestamp difference compared to before the pause reflects the + time the RTP stream was paused. + +6.2. Pausing State + + In this state, the RTP stream sender has received at least one PAUSE + message for the stream in question. The RTP stream sender SHALL wait + during a hold-off period for the possible reception of RESUME + messages for the RTP stream being paused before actually pausing RTP + stream transmission. The hold-off period to wait SHALL be long + enough to allow another RTP stream receiver to respond to the PAUSE + with a RESUME, if it determines that it would not like to see the + stream paused. This hold-off period is determined by the formula: + + 2 * RTT + T_dither_max, + + + +Burman, et al. Standards Track [Page 22] + +RFC 7728 RTP Stream Pause February 2016 + + + where RTT is the longest round trip known to the RTP stream sender + and T_dither_max is defined in Section 3.4 of [RFC4585]. The hold- + off period MAY be set to 0 by some signaling (Section 9) means when + it can be determined that there is only a single receiver, for + example, in point to point or some unicast situations. + + If the RTP stream sender has set the hold-off period to 0 and + receives information that it was an incorrect decision and that there + are in fact several receivers of the stream, it MUST change the hold- + off period to be based on the above formula instead. + + An RTP stream sender SHOULD use the following criteria to determine + if there is only a single receiver, unless it has explicit and more + reliable information: + + o Observing only a single CNAME across all received SSRCs (CNAMEs + for received CSRCs are insignificant), or + + o If RTCP reporting groups [MULTI-STREAM-OPT] is used, observing + only a single, endpoint external RTCP reporting group. + +6.3. Paused State + + An RTP stream is in paused state when the sender pauses its + transmission after receiving at least one PAUSE message and the hold- + off period has passed without receiving any RESUME message for that + stream. Pausing transmission SHOULD only be done when reaching an + appropriate place to pause in the stream, like a media boundary that + avoids a media receiver to trigger repair or concealment actions. + + When entering the state, the RTP stream sender SHALL send a PAUSED + indication to all known RTP stream receivers, and SHALL also repeat + PAUSED in the next two regular RTCP reports, as long as it is then + still in paused state. + + Pausing an RTP stream MUST NOT affect the sending of RTP keepalive + [RFC6263][RFC5245] applicable to that RTP stream. + + The following subsections discuss some potential issues when an RTP + sender goes into paused state. These conditions are also valid if an + RTP Translator is used in the communication. When an RTP Mixer + implementing this specification is involved between the participants + (which forwards the stream by marking the RTP data with its own + SSRC), it SHALL be a responsibility of the Mixer to control sending + PAUSE and RESUME requests to the sender. The below conditions also + apply to the sender and receiver parts of the RTP Mixer, + respectively. + + + + +Burman, et al. Standards Track [Page 23] + +RFC 7728 RTP Stream Pause February 2016 + + +6.3.1. RTCP BYE Message + + When a participant leaves the RTP session, it sends an RTCP BYE + message. In addition to the semantics described in Sections 6.3.4 + and 6.3.7 of RTP [RFC3550], the following two conditions MUST also be + considered when an RTP participant sends an RTCP BYE message: + + o If a paused sender sends an RTCP BYE message, receivers observing + this SHALL NOT send further PAUSE or RESUME requests to it. + + o Since a sender pauses its transmission on receiving the PAUSE + requests from any receiver in a session, the sender MUST keep + record of which receiver caused the RTP stream to pause. If that + receiver sends an RTCP BYE message observed by the sender, the + sender SHALL resume the RTP stream. No receivers that were in the + RTP session when the stream was paused objected that the stream + was paused, but if there were so far undetected receivers added to + the session during pause, those may not have learned about the + existence of the paused stream because either there was no PAUSED + sent for the paused RTP stream or those receivers did not support + PAUSED. Resuming the stream when the pausing party leaves the RTP + session allows those potentially undetected receivers to learn + that the stream exists. + +6.3.2. SSRC Time-Out + + Section 6.3.5 in RTP [RFC3550] describes the SSRC time-out of an RTP + participant. Every RTP participant maintains a sender and receiver + list in a session. If a participant does not get any RTP or RTCP + packets from some other participant for the last five RTCP reporting + intervals, it removes that participant from the receiver list. Any + streams that were paused by that removed participant SSRC SHALL be + resumed. + +6.4. Local Paused State + + This state can be entered at any time, based on local decision from + the RTP stream sender. Pausing transmission SHOULD only be done when + reaching an appropriate place to pause in the stream, like a media + boundary that avoids a media receiver to trigger repair or + concealment actions. + + As with paused state (Section 6.3), the RTP stream sender SHALL send + a PAUSED indication to all known RTP stream receivers, when entering + the state, unless the stream was already in paused state + (Section 6.3). Such PAUSED indication SHALL be repeated a sufficient + + + + + +Burman, et al. Standards Track [Page 24] + +RFC 7728 RTP Stream Pause February 2016 + + + number of times to reach a high probability that the message is + correctly delivered, stopping such repetition whenever leaving the + state. + + When using TMMBN 0 as a PAUSED indication and when already in paused + state, the actions when entering local paused state depends on the + bounding set overhead value in the received TMMBR 0 that caused the + paused state and the bounding set overhead value used in (the RTP + stream sender's own) TMMBN 0: + + TMMBN 0 overhead <= TMMBR 0 overhead: The RTP stream sender SHALL + NOT send any new TMMBN 0 replacing that active (more restrictive) + bounding set, even if entering local paused state. + + TMMBN 0 overhead > TMMBR 0 overhead: The RTP stream sender SHALL + send TMMBN 0 with itself in the TMMBN bounding set when entering + local paused state. + + The case above, when using TMMBN 0 as a PAUSED indication, being in + local paused state, and having received a TMMBR 0 with a bounding set + overhead value greater than the value the RTP stream sender would + itself use in a TMMBN 0, requires further consideration and is for + clarity henceforth referred to as "restricted local paused state". + + As indicated in Figure 4, local paused state has higher precedence + than paused state (Section 6.3), and RESUME messages alone cannot + resume a paused RTP stream as long as the local decision still + applies. An RTP stream sender in local paused state is responsible + for leaving the state whenever the conditions that caused the + decision to enter the state no longer apply. + + If the RTP stream sender is in restricted local paused state, it + cannot leave that state until the TMMBR 0 limit causing the state is + removed by a TMMBR > 0 (RESUME). If the RTP stream sender then needs + to stay in local paused state due to local considerations, it MAY + continue pausing the RTP stream by entering local paused state and + MUST then act accordingly, including sending a TMMBN 0 with itself in + the bounding set. + + Pausing an RTP stream MUST NOT affect the sending of RTP keepalive + [RFC6263][RFC5245] applicable to that RTP stream. + + When leaving the local paused state, the stream state SHALL become + Playing, regardless of whether or not there were any RTP stream + receivers that sent PAUSE for that stream during the local paused + state, effectively clearing the RTP stream sender's memory for that + stream. + + + + +Burman, et al. Standards Track [Page 25] + +RFC 7728 RTP Stream Pause February 2016 + + +7. Message Format + + Section 6 of AVPF [RFC4585] defines three types of low-delay RTCP + feedback messages, i.e., transport-layer, payload-specific, and + application-layer feedback messages. This document defines a new + transport-layer feedback message, which is further subtyped into + either a PAUSE request, a RESUME request, a PAUSED indication, or a + REFUSED notification. + + The transport-layer feedback messages are identified by having the + RTCP payload type be RTPFB (205) as defined by AVPF [RFC4585]. This + transport-layer feedback message, containing one or more of the + subtyped messages, is henceforth referred to as the PAUSE-RESUME + message. The specific FCI format is identified by a Feedback Message + Type (FMT) value in a common packet header for the feedback message + defined in Section 6.1 of AVPF [RFC4585]. The PAUSE-RESUME + transport-layer feedback message FCI is identified by FMT value = 9. + + The Common Packet Format for feedback messages defined by AVPF + [RFC4585] is: + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |V=2|P| FMT | PT | Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SSRC of packet sender | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SSRC of media source | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : Feedback Control Information (FCI) : + : : + + Figure 5: AVPF Common Feedback Message Packet Format + + For the PAUSE-RESUME message defined in this memo, the following + interpretations of the packet fields apply: + + FMT: The FMT value identifying the PAUSE-RESUME FCI: 9 + + PT: Payload Type = 205 (RTPFB) + + Length: As defined by AVPF, i.e., the length of this packet in + 32-bit words minus one, including the header and any padding. + + + + + + + +Burman, et al. Standards Track [Page 26] + +RFC 7728 RTP Stream Pause February 2016 + + + SSRC of packet sender: The SSRC of the RTP session participant + sending the messages in the FCI. Note, for endpoints that have + multiple SSRCs in an RTP session, any of its SSRCs MAY be used to + send any of the pause message types. + + SSRC of media source: Not used; SHALL be set to 0. The FCI + identifies the SSRC the message is targeted for. + + The FCI field consists of one or more PAUSE, RESUME, PAUSED, or + REFUSED messages or any future extension. These messages have the + following FCI format: + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Target SSRC | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Type | Res | Parameter Len | PauseID | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : Type Specific : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Figure 6: Syntax of FCI Entry in the PAUSE and RESUME Message + + The FCI fields have the following definitions: + + Target SSRC (32 bits): For a PAUSE-RESUME message, this value is the + SSRC that the request is intended for. For PAUSED, it MUST be the + SSRC being paused. If pausing is the result of a PAUSE request, + the value in PAUSED is effectively the same as Target SSRC in a + related PAUSE request. For REFUSED, it MUST be the Target SSRC of + the PAUSE or RESUME request that cannot change state. A CSRC MUST + NOT be used as a target as the interpretation of such a request is + unclear. + + Type (4 bits): The pause feedback type. The values defined in this + specification are as follows: + + 0: PAUSE request message. + + 1: RESUME request message. + + 2: PAUSED indication message. + + 3: REFUSED notification message. + + + + + + +Burman, et al. Standards Track [Page 27] + +RFC 7728 RTP Stream Pause February 2016 + + + 4-15: Reserved for future use. FCI fields with these Type values + SHALL be ignored on reception by receivers and MUST NOT be used + by senders implementing this specification. + + Res: (4 bits): Type Specific reserved. It SHALL be ignored by + receivers implementing this specification and MUST be set to 0 by + senders implementing this specification. + + Parameter Len (8 bits): Length of the Type Specific field in 32-bit + words. MAY be 0. + + PauseID (16 bits): Message sequence identification, as described in + Section 5.2. SHALL be incremented by one modulo 2^16 for each new + PAUSE message, unless the message is retransmitted. The initial + value SHOULD be 0. The PauseID is scoped by the Target SSRC, + meaning that PAUSE, RESUME, and PAUSED messages therefore share + the same PauseID space for a specific Target SSRC. + + Type Specific (variable): Defined per pause feedback type. MAY be + empty. A receiver implementing this specification MUST be able to + skip and ignore any unknown Type Specific data, even for Type + values defined in this specification. + +8. Message Details + + This section contains detailed explanations of each message defined + in this specification. All transmissions of requests and indications + are governed by the transmission rules as defined by Section 8.5. + + Any references to PAUSE, PAUSED, RESUME, and REFUSED in this section + SHALL be taken to apply to the extent possible and also when TMMBR/ + TMMBN are used (Section 5.6) for this functionality. TMMBR/TMMBN MAY + be used instead of the messages defined in this specification when + the effective topology is point to point. This use is expected to be + mainly for interworking with implementations that don't support the + messages defined in this specification but make use of TMMBR/TMMBN to + achieve a similar effect. If either sender or receiver learns that + the topology is not point to point, TMMBR/TMMBN MUST NOT be used for + pause/resume functionality. If the messages defined in this + specification are supported in addition to TMMBR/TMMBN by all + involved parties, pause/resume signaling MUST use messages from this + specification. If the topology is not point to point and the + messages defined in this specification are not supported, pause/ + resume functionality with TMMBR/TMMBN MUST NOT be used. + + For the scope of this specification, a past PauseID (Section 5.2) is + defined as having a value between and including (PauseID - 2^15) MOD + 2^16 and (PauseID - 1) MOD 2^16, where "MOD" is the modulo operator. + + + +Burman, et al. Standards Track [Page 28] + +RFC 7728 RTP Stream Pause February 2016 + + + Similarly, a future PauseID is defined as having a value between and + including (PauseID + 1) MOD 2^16 and (PauseID + 2^14) MOD 2^16. It + is intentional that future PauseID is not defined as the entire range + outside that of past PauseID. The remaining range of PauseID is + simply "not current". + +8.1. PAUSE + + An RTP stream receiver MAY schedule PAUSE for transmission at any + time. + + PAUSE has no defined Type Specific parameters. + + PauseID SHOULD be the current PauseID, as indicated by PAUSED + (Section 8.2), REFUSED (Section 8.4), or implicitly determined by + previously received PAUSE or RESUME (Section 8.3) requests. A + randomly chosen PauseID MAY be used if it was not possible to + retrieve current PauseID information, in which case the PAUSE will + either succeed or the current PauseID can be found in the returned + REFUSED (Section 8.4). + + It can be noted that as a result of what is described in Section 6.1, + PauseID is incremented by one, in modulo arithmetic, for each PAUSE + request that is not a retransmission, compared to what was used in + the last PAUSED indication sent by the media sender. PauseID in the + message is supposed to match current PauseID at the RTP stream + sender. + + If an RTP stream receiver that sent a PAUSE with a certain PauseID + for a Target SSRC receives a RESUME or a REFUSED with the same + PauseID for the same Target SSRC, it is RECOMMENDED that it refrains + from scheduling further PAUSE requests for some appropriate time. + This is because the RESUME indicates that there are other receivers + that still wish to receive the stream, and the REFUSED indicates that + the RTP stream sender is currently not able to pause the stream. + What is an appropriate time can vary from application to application + and will also depend on the importance of achieving the bandwidth + saving, but 2-5 regular RTCP intervals is expected to be appropriate. + + If the targeted RTP stream does not pause, if no PAUSED indication + with a future PauseID compared to the one used in PAUSE is received, + and if no REFUSED with the current or a future PauseID is received + within 2 * RTT + T_dither_max, the PAUSE MAY be scheduled for + retransmission, using the same current PauseID. RTT is the observed + round trip to the RTP stream sender, and T_dither_max is defined in + Section 3.4 of [RFC4585]. An RTP stream receiver in a bi-directional + RTP communication will generally have an RTT estimate to the RTP + stream sender, e.g., from RTCP SR/RR as described in Section 6.4 of + + + +Burman, et al. Standards Track [Page 29] + +RFC 7728 RTP Stream Pause February 2016 + + + [RFC3550]. However, RTP stream receivers that don't send any RTP + streams will lack an RTT estimate unless they use additional + mechanisms, such as the "Receiver Reference Time Report Block" part + of RTCP XR [RFC3611]. RTP stream receivers that lack an RTT estimate + to the sender SHOULD use 500 ms as the default value. + + When an RTP stream sender in playing state (Section 6.1) receives a + PAUSE with the current PauseID, and unless local considerations + currently make it impossible to pause the stream, it SHALL enter + pausing state (Section 6.2) and act accordingly. + + If an RTP stream sender receives a PAUSE with the current PauseID + while in pausing, paused (Section 6.3), or local paused (Section 6.4) + states, the received PAUSE SHALL be ignored. + +8.2. PAUSED + + The PAUSED indication, if supported, MUST be sent whenever entering + paused state (Section 6.3) or local paused state (Section 6.4). + + PauseID in the PAUSED message MUST contain the current PauseID that + can be included in a subsequent RESUME (Section 8.3). For local + paused state, this means that PauseID in the message is the current + PauseID, just as if the RTP stream sender had sent a PAUSE to itself. + + PAUSED SHALL contain a fixed-length 32-bit parameter at the start of + the Type Specific field with the extended RTP sequence number of the + last RTP packet sent before the RTP stream was paused, in the same + format as the extended highest sequence number received + (Section 6.4.1 of [RFC3550]). + + After having entered paused or local paused state and thus having + sent PAUSED once, PAUSED MUST also be included in (at least) the next + two regular RTCP reports, given that the pause condition is then + still effective. + + PAUSED indications MAY be retransmitted, subject to transmission + rules (Section 8.5), to increase the probability that the message + reaches the receiver in a timely fashion. This can be especially + important when entering local paused state. The number of + repetitions to use could be tuned to observed loss rate and desired + loss probability, for example, based on RTCP reports received from + the intended message target. + + While remaining in paused or local paused states, PAUSED MAY be + included in all compound RTCP reports, as long as the negotiated RTCP + bandwidth is not exceeded. + + + + +Burman, et al. Standards Track [Page 30] + +RFC 7728 RTP Stream Pause February 2016 + + + When in paused or local paused states, whenever the RTP stream sender + learns that there are endpoints that did not previously receive the + stream, for example, by RTCP reports with an SSRC and a CNAME that + were not previously seen in the RTP session, it is RECOMMENDED to + send PAUSED at the earliest opportunity and also to include it in (at + least) the next two regular RTCP reports, given that the pause + condition is then still effective. + +8.3. RESUME + + An RTP stream receiver MAY schedule RESUME for transmission whenever + it wishes to resume a paused stream or disapprove a stream from being + paused. + + PauseID SHOULD be the current PauseID, as indicated by PAUSED + (Section 8.2) or implicitly determined by previously received PAUSE + (Section 8.1) or RESUME requests. A randomly chosen PauseID MAY be + used if it was not possible to retrieve current PauseID information, + in which case the RESUME will either succeed or the current PauseID + can be found in a returned REFUSED (Section 8.4). + + If an RTP stream receiver that sent a RESUME with a certain PauseID + receives a REFUSED with the same PauseID, it is RECOMMENDED that it + refrains from scheduling further RESUME requests for some appropriate + time since the REFUSE indicates that it is currently not possible to + resume the stream. What is an appropriate time can vary from + application to application and will also depend on the importance of + resuming the stream, but 1-2 regular RTCP intervals is expected to be + appropriate. + + RESUME requests MAY be retransmitted, subject to transmission rules + (Section 8.5), to increase the probability that the message reaches + the receiver in a timely fashion. The number of repetitions to use + could be tuned to observed loss rate and desired loss probability, + for example, based on RTCP reports received from the intended message + target. Such retransmission SHOULD stop as soon as RTP packets from + the targeted stream are received or when a REFUSED with the current + PauseID for the targeted RTP stream is received. + + RESUME has no defined Type Specific parameters. + + When an RTP stream sender in pausing (Section 6.2), paused + (Section 6.3), or local paused state (Section 6.4) receives a RESUME + with the current PauseID, and unless local considerations currently + make it impossible to resume the stream, it SHALL enter playing state + (Section 6.1) and act accordingly. If the RTP stream sender is + incapable of honoring a RESUME request with the current PauseID, or + if it receives a RESUME request with a PauseID that is not the + + + +Burman, et al. Standards Track [Page 31] + +RFC 7728 RTP Stream Pause February 2016 + + + current PauseID while in paused or pausing state, the RTP stream + sender SHALL schedule a REFUSED message for transmission as specified + below. + + If an RTP stream sender in playing state receives a RESUME containing + either the current PauseID or a past PauseID, the received RESUME + SHALL be ignored. + +8.4. REFUSED + + If an RTP stream sender receives a PAUSE (Section 8.1) or RESUME + (Section 8.3) request containing the current PauseID, where the + requested action cannot be fulfilled by the RTP stream sender due to + some local consideration, it SHALL schedule transmission of a REFUSED + notification containing the current PauseID from the rejected + request. + + REFUSED has no defined Type Specific parameters. + + If an RTP stream sender receives a PAUSE or RESUME request with a + PauseID that is not the current PauseID, it SHALL schedule a REFUSED + notification containing the current PauseID, except if the RTP stream + sender is in playing state and receives a RESUME with a past PauseID, + in which case the RESUME SHALL be ignored. + + If several PAUSE or RESUME requests that would render identical + REFUSED notifications are received before the scheduled REFUSED is + sent, duplicate REFUSED notifications MUST NOT be scheduled for + transmission. This effectively lets a single REFUSED respond to + several ineffective PAUSE or RESUME requests. + + An RTP stream receiver that sent a PAUSE or RESUME request and + receives a REFUSED containing the same PauseID as in the request + SHOULD refrain from sending an identical request for some appropriate + time to allow the condition that caused REFUSED to clear. For PAUSE, + an appropriate time is suggested in Section 8.1. For RESUME, an + appropriate time is suggested in Section 8.3. + + An RTP stream receiver that sent a PAUSE or RESUME request and + receives a REFUSED containing a PauseID different from the request + MAY schedule another request using the PauseID from the REFUSED + notification. + +8.5. Transmission Rules + + The transmission of any RTCP feedback messages defined in this + specification MUST follow the normal AVPF-defined timing rules and + depend on the session's mode of operation. + + + +Burman, et al. Standards Track [Page 32] + +RFC 7728 RTP Stream Pause February 2016 + + + All messages defined in this specification, as well as TMMBR/TMMBN + used for pause/resume purposes (Section 5.6), can use either Regular, + Early, or Immediate timings but should make a trade-off between + timely transmission (Section 4.1) and RTCP bandwidth consumption. + This can be achieved by taking the following into consideration: + + o It is recommended that PAUSE use Early or Immediate timing, except + for retransmissions where RTCP bandwidth can motivate the use of + Regular timing. + + o The first transmission of PAUSED for each (non-wrapped) PauseID is + recommended to be sent with Immediate or Early timing to stop + unnecessary repetitions of PAUSE. It is recommended that + subsequent transmissions of PAUSED for that PauseID use Regular + timing to avoid excessive PAUSED RTCP bandwidth caused by multiple + PAUSE requests. + + o It is recommended that unsolicited PAUSED (sent when entering + local paused state (Section 6.4)) always use Immediate or Early + timing, until PAUSED for that PauseID is considered delivered at + least once to all receivers of the paused RTP stream, to avoid RTP + stream receivers that take unnecessary corrective action when the + RTP stream is no longer received, after which it is recommended + that PAUSE uses Regular timing (as for PAUSED triggered by PAUSE + above). + + o RESUME is often time critical, and it is recommended that it + always uses Immediate or Early timing. + + o The first transmission of REFUSED for each (non-wrapped) PauseID + is recommended to be sent with Immediate or Early timing to stop + unnecessary repetitions of PAUSE or RESUME. It is recommended + that subsequent REFUSED notifications for that PauseID use Regular + timing to avoid excessive REFUSED RTCP bandwidth caused by + multiple unreasonable requests. + +9. Signaling + + The capability of handling messages defined in this specification MAY + be exchanged at a higher layer such as SDP. This document extends + the "rtcp-fb" attribute defined in Section 4 of AVPF [RFC4585] to + include the request for pause and resume. This specification follows + all the rules defined in AVPF [RFC4585] and CCM [RFC5104] for an + "rtcp-fb" attribute relating to the payload type in a session + description. + + + + + + +Burman, et al. Standards Track [Page 33] + +RFC 7728 RTP Stream Pause February 2016 + + + This specification defines a new parameter "pause" to the "ccm" + feedback value defined in CCM [RFC5104], representing the capability + to understand the RTCP feedback message and all of the defined FCIs + of PAUSE, RESUME, PAUSED, and REFUSED. + + Note: When TMMBR 0 / TMMBN 0 are used to implement pause and + resume functionality (with the restrictions described in this + specification), signaling the "rtcp-fb" attribute with the "ccm" + and "tmmbr" parameters is sufficient and no further signaling is + necessary. There is, however, no guarantee that TMMBR/TMMBN + implementations predating this specification work exactly as + described here when used with a bitrate value of 0. + + The "pause" parameter has two optional attributes, which are "nowait" + and "config": + + o "nowait" indicates that the hold-off period defined in Section 6.2 + can be set to 0, reducing the latency before the stream can be + paused after receiving a PAUSE request. This condition occurs + when there will only be a single receiver per direction in the RTP + session, for example, in point-to-point sessions. It is also + possible to use in scenarios using unidirectional media. The + conditions that allow "nowait" to be set (Section 6.2) also + indicate that it would be possible to use CCM TMMBR/TMMBN as + pause/resume signaling. + + o "config" allows for partial implementation of this specification + according to the different roles in the use-cases section + (Section 3) and takes a value that describes what subset is + implemented: + + 1 Full implementation of this specification. This is the default + configuration. A missing "config" pause attribute MUST be + treated equivalent to providing a "config" value of 1. + + 2 The implementation intends to send PAUSE and RESUME requests + for received RTP streams and is thus also capable of receiving + PAUSED and REFUSED. It does not support receiving PAUSE and + RESUME requests, but it may pause sent RTP streams due to local + considerations and then intend to send PAUSED for them. + + 3 The implementation supports receiving PAUSE and RESUME requests + targeted for RTP streams it sends. It will send PAUSED and + REFUSED as needed. The node will not send any PAUSE and RESUME + requests but supports and desires receiving PAUSED if received + RTP streams are paused. + + + + + +Burman, et al. Standards Track [Page 34] + +RFC 7728 RTP Stream Pause February 2016 + + + 4 The implementation intends to send PAUSE and RESUME requests + for received RTP streams and is thus also capable of receiving + PAUSED and REFUSED. It cannot pause any RTP streams it sends, + and thus does not support receiving PAUSE and RESUME requests, + and it also does not support sending PAUSED indications. + + 5 The implementation supports receiving PAUSE and RESUME requests + targeted for RTP streams it sends. It will send PAUSED and + REFUSED as needed. It does not support sending PAUSE and + RESUME requests to pause received RTP streams, and it also does + not support receiving PAUSED indications. + + 6 The implementation supports sent and received RTP streams being + paused due to local considerations and thus supports sending + and receiving PAUSED indications. + + 7 The implementation supports and desires to receive PAUSED + indications for received RTP streams but does not pause or send + PAUSED indications for sent RTP streams. It does not support + any other messages defined in this specification. + + 8 The implementation supports pausing sent RTP streams and + sending PAUSED indications for them but does not support + receiving PAUSED indications for received RTP streams. It does + not support any other messages defined in this specification. + + All implementers of this specification are encouraged to include full + support for all messages ("config=1"), but it is recognized that this + is sometimes not meaningful for implementations operating in an + environment where only parts of the functionality provided by this + specification are needed. The above defined "config" functionality + subsets provide a trade-off between completeness and the need for + implementation interoperability, achieving at least a level of + functionality corresponding to what is desired by the least-capable + party when used as specified here. Implementing any functionality + subsets other than those defined above is NOT RECOMMENDED. + + When signaling a "config" value other than 1, an implementation MUST + ignore non-supported messages on reception and SHOULD omit sending + messages not supported by the remote peer. One example where it can + be motivated to send messages that some receivers do not support is + when there are multiple message receivers with different message + support (different "config" values). That approach avoids letting + the least-capable receiver limit the functionality provided to + others. The below table summarizes per-message send and receive + support for the different "config" pause attribute values ("X" + indicating support and "-" indicating non-support): + + + + +Burman, et al. Standards Track [Page 35] + +RFC 7728 RTP Stream Pause February 2016 + + + +---+-----------------------------+-----------------------------+ + | # | Send | Receive | + | | PAUSE RESUME PAUSED REFUSED | PAUSE RESUME PAUSED REFUSED | + +---+-----------------------------+-----------------------------+ + | 1 | X X X X | X X X X | + | 2 | X X X - | - - X X | + | 3 | - - X X | X X X - | + | 4 | X X - - | - - X X | + | 5 | - - X X | X X - - | + | 6 | - - X - | - - X - | + | 7 | - - - - | - - X - | + | 8 | - - X - | - - - - | + +---+-----------------------------+-----------------------------+ + + Figure 7: Supported Messages for Different "config" Values + + In the above description of partial implementations, "config" values + 2 and 4 correspond to the RTP Mixer in the 'RTP Mixer to Media + Sender' use case (Section 3.2), and "config" values 3 and 5 + correspond to the media sender in that same use case. For that use + case, it should be clear that an RTP Mixer implementing only "config" + values 3 or 5 will not provide a working solution. Similarly, for + that use case, a media sender implementing only "config" values 2 or + 4 will not provide a working solution. Both the RTP Mixer and the + media sender will of course work when implementing the full set of + messages, corresponding to "config=1". + + A partial implementation is not suitable for pause/resume support + between cascaded RTP Mixers, but it would require support + corresponding to "config=1" between such RTP Mixers. This is because + an RTP Mixer is then also a media sender towards the other RTP Mixer, + requiring support for the union of "config" values 2 and 3 or + "config" values 4 and 5, which effectively becomes "config=1". + + As can be seen from Figure 7 above, "config" values 2 and 3 differ + from "config" values 4 and 5 only in that in the latter, the PAUSE/ + RESUME message sender (e.g., the RTP Mixer side) does not support + local pause (Section 6.4) for any of its own streams and therefore + also does not support sending PAUSED. + + Partial implementations that only support local pause functionality + can declare this capability through "config" values 6-8. + + Viable fallback rules between different "config" values are described + in Section 9.1 and Figure 9. + + + + + + +Burman, et al. Standards Track [Page 36] + +RFC 7728 RTP Stream Pause February 2016 + + + This is the resulting ABNF [RFC5234], extending the existing ABNF in + Section 7.1 of CCM [RFC5104]: + + rtcp-fb-ccm-param =/ SP "pause" *(SP pause-attr) + pause-attr = pause-config ; partial message support + / "nowait" ; no hold-off period + / byte-string ; for future extensions + pause-config = "config=" pause-config-value + pause-config-value = 1*2DIGIT + ; byte-string as defined in RFC 4566 + + Figure 8: ABNF + + An endpoint implementing this specification and using SDP to signal + capability SHOULD indicate the new "pause" parameter with "ccm" + signaling but MAY instead use existing "ccm tmmbr" signaling + [RFC5104] if the limitations in functionality when using TMMBR/TMMBN + as described in this specification (Section 5.6) are considered + acceptable. In that case, no partial message support is possible. + The messages from this specification (Section 8) SHOULD NOT be used + towards receivers that did not declare capability to receive those + messages. + + The pause functionality can normally be expected to work + independently of the payload type. However, there might exist + situations where an endpoint needs to restrict or at least configure + the capabilities differently depending on the payload type carrying + the media stream. Reasons for this might relate to capabilities to + correctly handle media boundaries and avoid any pause or resume + operation to occur where it would leave a receiver or decoder with no + choice than to attempt to repair or discard the media received just + prior to or at the point of resuming. + + There MUST NOT be more than one "a=rtcp-fb" line with "pause" + applicable to a single payload type in the SDP, unless the additional + line uses "*" as the payload type, in which case "*" SHALL be + interpreted as applicable to all listed payload types that do not + have an explicit "pause" specification. The "config" pause attribute + MUST NOT appear more than once for each "pause" CCM parameter. The + "nowait" pause attribute MUST NOT appear more than once for each + "pause" CCM parameter. + +9.1. Offer/Answer Use + + An offerer implementing this specification needs to include the + "pause" CCM parameter with a suitable configuration attribute + ("config") in the SDP, according to what messages it intends to send + and desires to receive in the session. + + + +Burman, et al. Standards Track [Page 37] + +RFC 7728 RTP Stream Pause February 2016 + + + In SDP offer/answer, the "config" pause attribute and its message + directions are interpreted based on the agent providing the SDP. The + offerer is described in an offer, and the answerer is described in an + answer. + + An answerer receiving an offer with a "pause" CCM line and a "config" + pause attribute with a certain value, describing a certain capability + to send and receive messages, MAY change the "config" pause attribute + value in the answer to another configuration. The permitted answers + are listed in the below table. + + SDP Offer "config" value | Permitted SDP Answer "config" values + -------------------------+------------------------------------- + 1 | 1, 2, 3, 4, 5, 6, 7, 8 + 2 | 3, 4, 5, 6, 7, 8 + 3 | 2, 4, 5, 6, 7, 8 + 4 | 5, 6, 7, 8 + 5 | 4, 6, 7, 8 + 6 | 6, 7, 8 + 7 | 8 + 8 | 7 + + Figure 9: "config" Values in Offer/Answer + + An offer or answer omitting the "config" pause attribute MUST be + interpreted as equivalent to "config=1". Implementations of this + specification MUST NOT use any "config" values other than those + defined above in an offer or answer and MUST remove the "pause" CCM + line in the answer when receiving an offer with a "config" value it + does not understand. In all cases, the answerer MAY also completely + remove any "pause" CCM line to indicate that it does not understand + or desire to use any pause functionality for the affected payload + types. + + If the offerer believes that itself and the intended answerer are + likely the only endpoints in the RTP session, it MAY include the + "nowait" pause attribute on the "pause" line in the offer. If an + answerer receives the "nowait" pause attribute on the "pause" line in + the SDP, and if it has information that the offerer and itself are + not the only endpoints in the RTP session, it MUST NOT include any + "nowait" pause attribute on its "pause" line in the SDP answer. The + answerer MUST NOT add "nowait" on the "pause" line in the answer + unless it is present on the "pause" line in the offer. If both offer + and answer contain a "nowait" pause attribute, then the hold-off + period is configured to 0 at both the offerer and answerer. + + Unknown pause attributes MUST be ignored in the offer and MUST then + be omitted from the answer. + + + +Burman, et al. Standards Track [Page 38] + +RFC 7728 RTP Stream Pause February 2016 + + + If both "pause" and "tmmbr" are present in the offer, both MAY be + included also in the answer, in which case TMMBR/TMMBN MUST NOT be + used for pause/resume purposes (with a bitrate value of 0), to avoid + signaling ambiguity. + +9.2. Declarative Use + + In declarative use, the SDP is used to configure the node receiving + the SDP. This has implications on the interpretation of the SDP + signaling extensions defined in this specification. + + First, the "config" pause attribute and its message directions are + interpreted based on the node receiving the SDP, and it describes the + RECOMMENDED level of operation. If the joining client does not + support the indicated "config" value, some RTP session stream + optimizations may not be possible in that some RTP streams will not + be paused by the joining client, and/or the joining client may not be + able to resume and receive wanted streams because they are paused. + + Second, the "nowait" pause attribute, if included, is followed as + specified. It is the responsibility of the declarative SDP sender to + determine if a configured node will participate in a session that + will be point to point, based on the usage. For example, a + conference client being configured for an any source multicast + session using the Session Announcement Protocol (SAP) [RFC2974] will + not be in a point-to-point session, thus "nowait" cannot be included. + A Real-Time Streaming Protocol (RTSP) [RFC2326] client receiving a + declarative SDP may very well be in a point-to-point session, + although it is highly doubtful that an RTSP client would need to + support this specification, considering the inherent PAUSE support in + RTSP. + + Unknown pause attributes MUST be ignored. + + If both "pause" and "tmmbr" are present in the SDP, TMMBR/TMMBN MUST + NOT be used for pause/resume purposes (with a bitrate value of 0) to + avoid signaling ambiguity. + +10. Examples + + The following examples show use of PAUSE and RESUME messages, + including use of offer/answer: + + 1. Offer/Answer + + 2. Point-to-Point Session + + 3. Point to Multipoint using Mixer + + + +Burman, et al. Standards Track [Page 39] + +RFC 7728 RTP Stream Pause February 2016 + + + 4. Point to Multipoint using Relay + +10.1. Offer/Answer + + The below figures contain an example of how to show support for + pausing and resuming the streams, as well as indicating whether or + not the hold-off period can be set to 0. + + v=0 + o=alice 3203093520 3203093520 IN IP4 alice.example.com + s=Pausing Media + t=0 0 + c=IN IP4 alice.example.com + m=audio 49170 RTP/AVPF 98 99 + a=rtpmap:98 G719/48000 + a=rtpmap:99 PCMA/8000 + a=rtcp-fb:* ccm pause nowait + + Figure 10: SDP Offer with Pause and Resume Capability + + The offerer supports all of the messages defined in this + specification, leaving out the optional "config" pause attribute. + The offerer also believes that it will be the sole receiver of the + answerer's stream as well as that the answerer will be the sole + receiver of the offerer's stream and thus includes the "nowait" pause + attribute for the "pause" parameter. + + This is the SDP answer: + + v=0 + o=bob 293847192 293847192 IN IP4 bob.example.com + s=- + t=0 0 + c=IN IP4 bob.example.com + m=audio 49202 RTP/AVPF 98 + a=rtpmap:98 G719/48000 + a=rtcp-fb:98 ccm pause config=2 + + Figure 11: SDP Answer with Pause and Resume Capability + + The answerer will not allow its sent streams to be paused or resumed + and thus restricts the answer to indicate "config=2". It also + supports pausing its own RTP streams due to local considerations, + which is why "config=2" is chosen rather than "config=4". The + answerer somehow knows that it will not be a point-to-point RTP + session and has therefore removed "nowait" from the "pause" line, + meaning that the offerer must use a non-zero hold-off period when + being requested to pause the stream. + + + +Burman, et al. Standards Track [Page 40] + +RFC 7728 RTP Stream Pause February 2016 + + + When using TMMBR 0 / TMMBN 0 to achieve pause and resume + functionality, there are no differences in SDP compared to CCM + [RFC5104]; therefore, no such examples are included here. + +10.2. Point-to-Point Session + + This is the most basic scenario, which involves two participants, + each acting as a sender and/or receiver. Any RTP data receiver sends + PAUSE or RESUME messages to the sender, which pauses or resumes + transmission accordingly. The hold-off period before pausing a + stream is 0. + + +---------------+ +---------------+ + | RTP Sender | | RTP Receiver | + +---------------+ +---------------+ + : t1: RTP data : + | -------------------------------> | + | t2: PAUSE(3) | + | <------------------------------- | + | < RTP data paused > | + | t3: PAUSED(3) | + | -------------------------------> | + : < Some time passes > : + | t4: RESUME(3) | + | <------------------------------- | + | t5: RTP data | + | -------------------------------> | + : < Some time passes > : + | t6: PAUSE(4) | + | <------------------------------- | + | < RTP data paused > | + | t7: PAUSED(4) | + | -------------------------------> | + : : + + Figure 12: Pause and Resume Operation in Point to Point + + Figure 12 shows the basic pause and resume operation in a + point-to-point scenario. At time t1, an RTP sender sends data to a + receiver. At time t2, the RTP receiver requests the sender to pause + the stream, using PauseID 3 (which it knew since before in this + example). The sender pauses the data and replies with a PAUSED + containing the same PauseID. Some time later (at time t4), the + receiver requests the sender to resume, which resumes its + transmission. The next PAUSE, sent at time t6, contains an updated + PauseID (4), with a corresponding PAUSED being sent at time t7. + + + + + +Burman, et al. Standards Track [Page 41] + +RFC 7728 RTP Stream Pause February 2016 + + + +---------------+ +---------------+ + | RTP Sender | | RTP Receiver | + +---------------+ +---------------+ + : t1: RTP data : + | -------------------------------> | + | t2: TMMBR 0 | + | <------------------------------- | + | < RTP data paused > | + | t3: TMMBN 0 | + | -------------------------------> | + : < Some time passes > : + | t4: TMMBR 150000 | + | <------------------------------- | + | t5: RTP data | + | -------------------------------> | + : < Some time passes > : + | t6: TMMBR 0 | + | <------------------------------- | + | < RTP data paused > | + | t7: TMMBN 0 | + | -------------------------------> | + : : + + Figure 13: TMMBR Pause and Resume in Point to Point + + Figure 13 describes the same point-to-point scenario as above, but + using TMMBR/TMMBN signaling. + + + + + + + + + + + + + + + + + + + + + + + + +Burman, et al. Standards Track [Page 42] + +RFC 7728 RTP Stream Pause February 2016 + + + +---------------+ +----------------+ + | RTP Sender A | | RTP Receiver B | + +---------------+ +----------------+ + : t1: RTP data : + | -------------------------------> | + | < RTP data paused > | + | t2: TMMBN {A:0} | + | -------------------------------> | + : < Some time passes > : + | t3: TMMBR 0 | + | <------------------------------- | + | t4: TMMBN {A:0,B:0} | + | -------------------------------> | + : < Some time passes > : + | t5: TMMBN {B:0} | + | -------------------------------> | + : < Some time passes > : + | t6: TMMBR 80000 | + | <------------------------------- | + | t7: RTP data | + | -------------------------------> | + : : + + Figure 14: Unsolicited PAUSED Using TMMBN + + Figure 14 describes the case when an RTP stream sender (A) chooses to + pause an RTP stream due to local considerations. Both A and the RTP + stream receiver (B) use TMMBR/TMMBN signaling for pause/resume + purposes. A decides to pause the RTP stream at time t2 and uses + TMMBN 0 to signal PAUSED, including itself in the TMMBN bounding set. + At time t3, despite the fact that the RTP stream is still paused, B + decides that it is no longer interested in receiving the RTP stream + and signals PAUSE by sending a TMMBR 0. As a result of that, the + bounding set now contains both A and B, and A sends out a new TMMBN + reflecting that. After a while, at time t5, the local considerations + that caused A to pause the RTP stream no longer apply, causing it to + remove itself from the bounding set and to send a new TMMBN + indicating this. At time t6, B decides that it is now interested in + receiving the RTP stream again and signals RESUME by sending a TMMBR + containing a bitrate value greater than 0, causing A to resume + sending RTP data. + + + + + + + + + + +Burman, et al. Standards Track [Page 43] + +RFC 7728 RTP Stream Pause February 2016 + + + +---------------+ +---------------+ + | RTP Sender | | RTP Receiver | + +---------------+ +---------------+ + : t1: RTP data : + | ------------------------------------> | + | t2: PAUSE(7), lost | + | <---X-------------- | + | | + | t3: RTP data | + | ------------------------------------> | + : : + | < Time-out, still receiving data > | + | t4: PAUSE(7) | + | <------------------------------------ | + | < RTP data paused > | + | t5: PAUSED(7) | + | ------------------------------------> | + : < Some time passes > : + | t6: RESUME(7), lost | + | <---X-------------- | + | t7: RESUME(7) | + | <------------------------------------ | + | t8: RTP data | + | ------------------------------------> | + | t9: RESUME(7) | + | <------------------------------------ | + : : + + Figure 15: Pause and Resume Operation with Messages Lost + + Figure 15 describes what happens if a PAUSE message from an RTP + stream receiver does not reach the RTP stream sender. After sending + a PAUSE message, the RTP stream receiver waits for a time-out to + detect if the RTP stream sender has paused the data transmission or + has sent a PAUSED indication according to the rules discussed in + Section 6.3. As the PAUSE message is lost on the way (at time t2), + RTP data continues to reach to the RTP stream receiver. When the + timer expires, the RTP stream receiver schedules a retransmission of + the PAUSE message, which is sent at time t4. If the PAUSE message + now reaches the RTP stream sender, it pauses the RTP stream and + replies with PAUSED. + + At time t6, the RTP stream receiver wishes to resume the stream again + and sends a RESUME, which is lost. This does not cause any severe + effect, since there is no requirement to wait until further RESUME + requests are sent, and another RESUME is sent already at time t7, + which now reaches the RTP stream sender that consequently resumes the + stream at time t8. The time interval between t6 and t7 can vary but + + + +Burman, et al. Standards Track [Page 44] + +RFC 7728 RTP Stream Pause February 2016 + + + may, for example, be one RTCP feedback transmission interval as + determined by the AVPF rules. + + The RTP stream receiver did not realize that the RTP stream was + resumed in time to stop yet another scheduled RESUME from being sent + at time t9. This is, however, harmless since RESUME contains a past + PauseID and will be ignored by the RTP stream sender. It will also + not cause the RTP stream to be resumed even if the stream was paused + again based on a PAUSE from some other receiver before receiving the + RESUME, since the current PauseID was updated compared to the one in + the stray RESUME, which contains a past PauseID and will be ignored + by the RTP stream sender. + + +---------------+ +---------------+ + | RTP Sender | | RTP Receiver | + +---------------+ +---------------+ + : t1: RTP data : + | ------------------------------> | + | t2: PAUSE(11) | + | <------------------------------ | + | | + | < Cannot pause RTP data > | + | t3: REFUSED(11) | + | ------------------------------> | + | | + | t4: RTP data | + | ------------------------------> | + : : + + Figure 16: Pause Request is Refused in Point to Point + + In Figure 16, the receiver requests to pause the sender, which + refuses to pause due to some consideration local to the sender and + responds with a REFUSED message. + +10.3. Point to Multipoint Using Mixer + + An RTP Mixer is an intermediate node connecting different transport- + level clouds. The Mixer receives streams from different RTP sources, + selects or combines them based on the application's needs, and + forwards the generated stream(s) to the destination. The Mixer + typically puts its own SSRC(s) in RTP data packets instead of the + original source(s). + + The Mixer keeps track of all the streams delivered to the Mixer and + how they are currently used. In this example, Mixer (M) selects the + video stream to deliver to the RTP stream receiver (R) based on the + voice activity of the RTP stream senders (S1 and S2). The video + + + +Burman, et al. Standards Track [Page 45] + +RFC 7728 RTP Stream Pause February 2016 + + + stream will be delivered to R using M's SSRC and with a CSRC + indicating the original source. + + Note that PauseID is not of any significance for the example and is + therefore omitted in the description. + + +-----+ +-----+ +-----+ +-----+ + | R | | M | | S1 | | S2 | + +-----+ +-----+ +-----+ +-----+ + : : t1:RTP(S1) : : + | t2:RTP(M:S1) |<-----------------| | + |<-----------------| | | + | | t3:RTP(S2) | | + | |<------------------------------------| + | | t4: PAUSE(S2) | | + | |------------------------------------>| + | | | t5: PAUSED(S2) | + | |<------------------------------------| + | | | <S2:No RTP to M> | + | | t6: RESUME(S2) | | + | |------------------------------------>| + | | | t7: RTP to M | + | |<------------------------------------| + | t8:RTP(M:S2) | | | + |<-----------------| | | + | | t9:PAUSE(S1) | | + | |----------------->| | + | | t10:PAUSED(S1) | | + | |<-----------------| | + | | <S1:No RTP to M> | | + : : : : + + Figure 17: Pause and Resume Operation for a Voice-Activated Mixer + + The session starts at t1 with S1 being the most active speaker and + thus being selected as the single video stream to be delivered to R + (t2) using M's SSRC but with S1 as the CSRC (indicated after the + colon in the figure). Then S2 joins the session at t3 and starts + delivering an RTP stream to M. As S2 has less voice activity then + S1, M decides to pause S2 at t4 by sending S2 a PAUSE request. At + t5, S2 acknowledges with PAUSED and at the same instant stops + delivering RTP to M. At t6, the user at S2 starts speaking and + becomes the most active speaker and M decides to switch the video + stream to S2 and therefore quickly sends a RESUME request to S2. At + t7, S2 has received the RESUME request and acts on it by resuming RTP + stream delivery to M. When the RTP stream from t7 arrives at M, it + switches this RTP stream into its SSRC (M) at t8 and changes the CSRC + to S2. As S1 now becomes unused, M issues a PAUSE request to S1 at + + + +Burman, et al. Standards Track [Page 46] + +RFC 7728 RTP Stream Pause February 2016 + + + t9, which is acknowledged at t10 with PAUSED, and the RTP stream from + S1 stops being delivered. + +10.4. Point to Multipoint Using Translator + + A transport Relay in an RTP session forwards the message from one + peer to all the others. Unlike Mixer, the Relay does not mix the + streams or change the SSRC of the messages or RTP media. These + examples are to show that the messages defined in this specification + can be safely used also in a transport Relay case. The parentheses + in the figures contains (Target SSRC, PauseID) information for the + messages defined in this specification. + + +-------------+ +-------------+ +-------------+ + | Sender(S) | | Relay | | Receiver(R) | + +-------------+ +-------------+ +-------------+ + : t1: RTP(S) : : + |------------------>| | + | | t2: RTP (S) | + | |------------------>| + | | t3: PAUSE(S,3) | + | |<------------------| + | t4:PAUSE(S,3) | | + |<------------------| | + : <Sender waiting for possible RESUME> : + | < RTP data paused > | + | t5: PAUSED(S,3) | | + |------------------>| | + | | t6: PAUSED(S,3) | + | |------------------>| + : : : + | | t7: RESUME(S,3) | + | |<------------------| + | t8: RESUME(S,3) | | + |<------------------| | + | t9: RTP (S) | | + |------------------>| | + | | t10: RTP (S) | + | |------------------>| + : : : + + Figure 18: Pause and Resume Operation between Two Participants Using + a Relay + + Figure 18 describes how a Relay can help the receiver (R) in pausing + and resuming the sender (S). S sends RTP data to R through the + Relay, which just forwards the data without modifying the SSRCs. R + sends a PAUSE request to S which, in this example, knows that there + + + +Burman, et al. Standards Track [Page 47] + +RFC 7728 RTP Stream Pause February 2016 + + + may be more receivers of the stream and waits a non-zero hold-off + period to see if there is any other receiver that wants to receive + the data, and when no disapproving RESUME messages are received, it + pauses itself and replies with PAUSED. Similarly R resumes S by + sending a RESUME request through the Relay. Since this describes + only a single pause and resume operation for a single RTP stream + sender, all messages use a single PauseID; in this example, it's + three. + + +-----+ +-----+ +-----+ +-----+ + | S | | Rel | | R1 | | R2 | + +-----+ +-----+ +-----+ +-----+ + : t1:RTP(S) : : : + |----------------->| | | + | | t2:RTP(S) | | + | |----------------->------------------>| + | | t3:PAUSE(S,7) | | + | |<-----------------| | + | t4:PAUSE(S,7) | | | + |<-----------------|------------------------------------>| + | | | t5:RESUME(S,7) | + | |<------------------------------------| + | t6:RESUME(S,7) | | | + |<-----------------|----------------->| | + | | <RTP stream continues to R1 and R2> | + | | | t7: PAUSE(S,8) | + | |<------------------------------------| + | t8:PAUSE(S,8) | | | + |<-----------------|----------------->| | + : : : : + | < Pauses RTP stream > | | + | t9:PAUSED(S,8) | | | + |----------------->| | | + | | t10:PAUSED(S,8) | | + | |----------------->------------------>| + : : : : + | | t11:RESUME(S,8) | | + | |<-----------------| | + | t12:RESUME(S,8) | | | + |<-----------------|------------------------------------>| + | t13:RTP(S) | | | + |----------------->| | | + | | t14:RTP(S) | | + | |----------------->------------------>| + : : : : + + Figure 19: Pause and Resume Operation between One Sender and Two + Receivers through Relay + + + +Burman, et al. Standards Track [Page 48] + +RFC 7728 RTP Stream Pause February 2016 + + + Figure 19 explains the pause and resume operations when a transport + Relay (Rel) is involved between a sender (S) and two receivers (R1 + and R2) in an RTP session. Each message exchange is represented by + the time it happens. At time t1, S starts sending an RTP stream to + Rel, which forwards it to R1 and R2. R1 and R2 receives RTP data + from Rel at t2. At this point, both R1 and R2 will send RTCP + Receiver Reports to S informing that they received S's stream. + + After some time (at t3), R1 chooses to pause the stream. On + receiving the PAUSE request from R1 at t4, S knows that there is at + least one receiver that may still want to receive the data and uses a + non-zero hold-off period to wait for possible RESUME messages. R2 + did also receive the PAUSE request at time t4 and since it still + wants to receive the stream, it sends a RESUME for it at time t5, + which is forwarded to sender S by Rel. S sees the RESUME at time t6 + and continues to send data to Rel, which forwards it to both R1 and + R2. At t7, R2 chooses to pause the stream by sending a PAUSE request + with an updated PauseID. S still knows that there is more than one + receiver (R1 and R2) that may want the stream and again waits a non- + zero hold-off period, after which, and not having received any + disapproving RESUME messages, it concludes that the stream must be + paused. S now stops sending the stream and replies with PAUSED to R1 + and R2. When any of the receivers (R1 or R2) choose to resume the + stream from S, in this example R1, it sends a RESUME request to S + (also seen by R2). S immediately resumes the stream. + + Consider also an RTP session that includes one or more receivers, + paused sender(s), and a Relay. Further assume that a new participant + joins the session, which is not aware of the paused sender(s). On + receiving knowledge about the newly joined participant, e.g., any RTP + traffic or RTCP report (i.e., either SR or RR) from the newly joined + participant, the paused sender(s) immediately sends PAUSED + indications for the paused streams since there is now a receiver in + the session that did not pause the sender(s) and may want to receive + the streams. Having this information, the newly joined participant + has the same possibility as any other participant to resume the + paused streams. + + + + + + + + + + + + + + +Burman, et al. Standards Track [Page 49] + +RFC 7728 RTP Stream Pause February 2016 + + +11. IANA Considerations + + Per this specification, IANA has made the following registrations: + + 1. A new value for media stream pause/resume has been registered in + the "FMT Values for RTPFB Payload Types" registry located at the + time of publication at: <http://www.iana.org/assignments/rtp- + parameters> + + Value: 9 + + Name: PAUSE-RESUME + + Long Name: Media Pause/Resume + + Reference: RFC 7728 + + 2. A new value "pause" to be registered with IANA in the "Codec + Control Messages" registry located at the time of publication at: + <http://www.iana.org/assignments/sdp-parameters> + + Value Name: pause + + Long Name: Media Pause/Resume + + Usable with: ccm + + Reference: RFC 7728 + +12. Security Considerations + + This document extends CCM [RFC5104] and defines new messages, i.e., + PAUSE, RESUME, PAUSED, and REFUSED. The exchange of these new + messages has some security implications, which need to be addressed + by the user. + + The messages defined in this specification can have a substantial + impact on the perceived media quality if used in a malicious way. + First of all, there is the risk for Denial of Service (DoS) on any + RTP session that uses the PAUSE-RESUME functionality. By injecting + one or more PAUSE requests into the RTP session, an attacker can + potentially prevent any media from flowing, especially when the hold- + off period is zero. The injection of PAUSE messages is quite simple, + requiring knowledge of the SSRC and the PauseID. This information is + visible to an on-path attacker unless RTCP messages are encrypted. + Even off-path attacks are possible as signaling messages often carry + the SSRC value, while the 16-bit PauseID has to be guessed or tried. + The way of protecting the RTP session from these injections is to + + + +Burman, et al. Standards Track [Page 50] + +RFC 7728 RTP Stream Pause February 2016 + + + perform source authentication combined with message integrity to + prevent other than intended session participants from sending these + messages. The security solution should provide replay protection. + Otherwise, if a session is long lived enough for the PauseID value to + wrap, an attacker could replay old messages at the appropriate time + to influence the media sender state. There exist several different + choices for securing RTP sessions to prevent this type of attack. + The Secure Real-time Transport Protocol (SRTP) is the most common, + but also other methods exist as discussed in "Options for Securing + RTP Sessions" [RFC7201]. + + Most of the methods for securing RTP, however, do not provide source + authentication of each individual participant in a multiparty use + case. In case one of the session participants is malicious, it can + wreck significant havoc within the RTP session and similarly cause a + DoS on the RTP session from within. That damage can also be + attempted to be obfuscated by having the attacker impersonate other + endpoints within the session. These attacks can be mitigated by + using a solution that provides true source authentication of all + participants' RTCP packets. However, that has other implications. + For multiparty sessions including a middlebox, that middlebox is + RECOMMENDED to perform checks on all forwarded RTCP packets so that + each participant only uses its set of SSRCs to prevent the attacker + from utilizing another participant's SSRCs. An attacker that can + send a PAUSE request that does not reach any participants other than + the media sender can cause a stream to be paused without providing + opportunity for opposition. This is mitigated in multiparty + topologies that ensure that requests are seen by all or most of the + RTP session participants, enabling these participants to send a + RESUME. In topologies with middleboxes that consume and process + PAUSE requests, the middlebox can also mitigate such behavior as it + will commonly not generate or forward a PAUSE message if it knows of + another participant having use for the media stream. + + The above text has been focused on using the PAUSE message as the + tool for malicious impact on the RTP session. That is because of the + greater impact from denying users access to RTP media streams. In + contrast, if an attacker attempts to use RESUME in a malicious + purpose, it will result in the media streams being delivered. + However, such an attack basically prevents the use of the pause and + resume functionality. Thus, it potentially forces a reduction of the + media quality due to limitation in available resources, like + bandwidth that must be shared. + + The session establishment signaling is also a potential venue of + attack, as that can be used to prevent the enabling of pause and + resume functionality by modifying the signaling messages. The above + mitigation of attacks based on source authentication also requires + + + +Burman, et al. Standards Track [Page 51] + +RFC 7728 RTP Stream Pause February 2016 + + + the signaling system to securely handle identities and assert that + only the intended identities are allowed into the RTP session and + provided with the relevant security contexts. + +13. References + +13.1. Normative References + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, + DOI 10.17487/RFC2119, March 1997, + <http://www.rfc-editor.org/info/rfc2119>. + + [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model + with Session Description Protocol (SDP)", RFC 3264, + DOI 10.17487/RFC3264, June 2002, + <http://www.rfc-editor.org/info/rfc3264>. + + [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. + Jacobson, "RTP: A Transport Protocol for Real-Time + Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, + July 2003, <http://www.rfc-editor.org/info/rfc3550>. + + [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session + Description Protocol", RFC 4566, DOI 10.17487/RFC4566, + July 2006, <http://www.rfc-editor.org/info/rfc4566>. + + [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, + "Extended RTP Profile for Real-time Transport Control + Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, + DOI 10.17487/RFC4585, July 2006, + <http://www.rfc-editor.org/info/rfc4585>. + + [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, + "Codec Control Messages in the RTP Audio-Visual Profile + with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104, + February 2008, <http://www.rfc-editor.org/info/rfc5104>. + + [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax + Specifications: ABNF", STD 68, RFC 5234, + DOI 10.17487/RFC5234, January 2008, + <http://www.rfc-editor.org/info/rfc5234>. + + [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment + (ICE): A Protocol for Network Address Translator (NAT) + Traversal for Offer/Answer Protocols", RFC 5245, + DOI 10.17487/RFC5245, April 2010, + <http://www.rfc-editor.org/info/rfc5245>. + + + +Burman, et al. Standards Track [Page 52] + +RFC 7728 RTP Stream Pause February 2016 + + + [RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for + Keeping Alive the NAT Mappings Associated with RTP / RTP + Control Protocol (RTCP) Flows", RFC 6263, + DOI 10.17487/RFC6263, June 2011, + <http://www.rfc-editor.org/info/rfc6263>. + +13.2. Informative References + + [MULTI-STREAM-OPT] + Lennox, J., Westerlund, M., Wu, W., and C. Perkins, + "Sending Multiple Media Streams in a Single RTP Session: + Grouping RTCP Reception Statistics and Other Feedback", + Work in Progress, draft-ietf-avtcore-rtp-multi-stream- + optimisation-11, December 2015. + + [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time + Streaming Protocol (RTSP)", RFC 2326, + DOI 10.17487/RFC2326, April 1998, + <http://www.rfc-editor.org/info/rfc2326>. + + [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session + Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, + October 2000, <http://www.rfc-editor.org/info/rfc2974>. + + [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, + A., Peterson, J., Sparks, R., Handley, M., and E. + Schooler, "SIP: Session Initiation Protocol", RFC 3261, + DOI 10.17487/RFC3261, June 2002, + <http://www.rfc-editor.org/info/rfc3261>. + + [RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., + "RTP Control Protocol Extended Reports (RTCP XR)", + RFC 3611, DOI 10.17487/RFC3611, November 2003, + <http://www.rfc-editor.org/info/rfc3611>. + + [RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis, + "RTP Payload Format for Scalable Video Coding", RFC 6190, + DOI 10.17487/RFC6190, May 2011, + <http://www.rfc-editor.org/info/rfc6190>. + + [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP + Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014, + <http://www.rfc-editor.org/info/rfc7201>. + + [RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real- + Time Communication Use Cases and Requirements", RFC 7478, + DOI 10.17487/RFC7478, March 2015, + <http://www.rfc-editor.org/info/rfc7478>. + + + +Burman, et al. Standards Track [Page 53] + +RFC 7728 RTP Stream Pause February 2016 + + + [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and + B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms + for Real-Time Transport Protocol (RTP) Sources", RFC 7656, + DOI 10.17487/RFC7656, November 2015, + <http://www.rfc-editor.org/info/rfc7656>. + + [RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667, + DOI 10.17487/RFC7667, November 2015, + <http://www.rfc-editor.org/info/rfc7667>. + + [SDP-SIMULCAST] + Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty, + "Using Simulcast in SDP and RTP Sessions", Work in + Progress, draft-ietf-mmusic-sdp-simulcast-04, February + 2016. + +Acknowledgments + + Daniel Grondal made valuable contributions during the initial + versions of this document. The authors would also like to thank Emil + Ivov, Christian Groves, David Mandelberg, Meral Shirazipour, Spencer + Dawkins, Bernard Aboba, and Ben Campbell, who provided valuable + review comments. + +Contributors + + Daniel Grondal contributed in the creation and writing of early + versions of this specification. Christian Groves contributed + significantly to the SDP "config" pause attribute and its use in + offer/answer. + + + + + + + + + + + + + + + + + + + + + +Burman, et al. Standards Track [Page 54] + +RFC 7728 RTP Stream Pause February 2016 + + +Authors' Addresses + + Bo Burman + Ericsson + Kistavagen 25 + SE - 164 80 Kista + Sweden + + Email: bo.burman@ericsson.com + + + Azam Akram + Ericsson + Farogatan 6 + SE - 164 80 Kista + Sweden + + Phone: +46107142658 + Email: akram.muhammadazam@gmail.com + URI: www.ericsson.com + + Roni Even + Huawei Technologies + Tel Aviv + Israel + + Email: roni.even@mail01.huawei.com + + + Magnus Westerlund + Ericsson + Farogatan 6 + SE - 164 80 Kista + Sweden + + Phone: +46107148287 + Email: magnus.westerlund@ericsson.com + + + + + + + + + + + + + + +Burman, et al. Standards Track [Page 55] + |