Network Working Group Stephan Wenger INTERNET-DRAFT Umesh Chandra Expires: August 2006 Nokia Magnus Westerlund Bo Burman Ericsson March 6, 2006 Codec Control Messages in the Audio-Visual Profile with Feedback (AVPF) draft-wenger-avt-avpf-ccm-03.txt> Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document specifies a few extensions to the messages defined in the Audio-Visual Profile with Feedback (AVPF). They are helpful primarily in conversational multimedia scenarios where centralized multipoint functionalities are in use. However some are also usable in smaller multicast environments and point-to-point calls. The extensions discussed are Full Intra Request, Temporary Maximum Media Bit-rate and Temporal Spatial Trade-off. Wenger, et al. [Page 1] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 TABLE OF CONTENTS 1. Introduction....................................................4 2. Definitions.....................................................5 2.1. Glossary...................................................5 2.2. Terminology................................................5 2.3. Topologies.................................................7 2.3.1. Point to Point........................................7 2.3.2. Point to Multi-point using Multicast..................8 2.3.3. Point to Multipoint using the RFC 3550 translator.....8 2.3.4. Point to Multipoint using the RFC 3550 mixer model...11 2.3.5. Point to Multipoint using video switching MCU........13 2.3.6. Point to Multipoint using RTCP-terminating MCU.......14 2.3.7. Combining Topologies.................................15 3. Motivation (Informative).......................................15 3.1. Use Cases.................................................15 3.2. Using the Media Path......................................17 3.3. Using AVPF................................................18 3.3.1. Reliability..........................................18 3.4. Multicast.................................................18 3.5. Feedback Messages.........................................18 3.5.1. Full Intra Request Command...........................18 3.5.1.1. Reliability.....................................19 3.5.2. Temporal Spatial Trade-off Request and Acknowledgement20 3.5.2.1. Point-to-point..................................21 3.5.2.2. Point-to-Multipoint using Multicast or Translators21 3.5.2.3. Point-to-Multipoint using RTP Mixer.............21 3.5.2.4. Reliability.....................................22 3.5.3. Temporary Maximum Media Bit-rate Request.............22 3.5.3.1. MCU based Multi-point operation.................23 3.5.3.2. Point-to-Multipoint using Multicast or Translators24 3.5.3.3. Point-to-point operation........................24 3.5.3.4. Reliability.....................................25 4. RTCP Receiver Report Extensions................................26 4.1. Design Principles of the Extension Mechanism..............26 4.2. Transport Layer Feedback Messages.........................27 4.2.1. Temporary Maximum Media Bit-rate Request (TMMBR).....27 4.2.1.1. Semantics.......................................27 4.2.1.2. Message Format..................................29 4.2.1.3. Timing Rules....................................30 4.2.2. Temporary Maximum Media Bit-rate Notificiation (TMMBN)30 4.2.2.1. Semantics.......................................30 4.2.2.2. Message Format..................................31 4.2.2.3. Timing Rules....................................31 4.3. Payload Specific Feedback Messages........................31 4.3.1. Full Intra Request (FIR) command.....................32 4.3.1.1. Semantics.......................................32 4.3.1.2. Message Format..................................34 4.3.1.3. Timing Rules....................................34 4.3.1.4. Remarks.........................................35 Wenger, et al. Standards Track [Page 2] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 4.3.2. Temporal-Spatial Trade-off Request (TSTR)............35 4.3.2.1. Semantics.......................................35 4.3.2.2. Message Format..................................35 4.3.2.3. Timing Rules....................................36 4.3.2.4. Remarks.........................................36 4.3.3. Temporal-Spatial Trade-off Acknowledgement (TSTA)....37 4.3.3.1. Semantics.......................................37 4.3.3.2. Message Format..................................37 4.3.3.3. Timing Rules....................................38 4.3.3.4. Remarks.........................................38 5. Congestion Control.............................................38 6. Security Considerations........................................39 7. SDP Definitions................................................39 7.1. Extension of rtcp-fb attribute............................40 7.2. Offer-Answer..............................................41 7.3. Examples..................................................41 8. IANA Considerations............................................43 9. Acknowledgements...............................................43 10. References....................................................44 10.1. Normative references.....................................44 10.2. Informative references...................................44 11. Authors' Addresses............................................45 12. List of Changes relative to previous draft....................45 Wenger, et al. Standards Track [Page 3] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 1. Introduction When the Audio-Visual Profile with Feedback (AVPF) [AVPF] was developed, the main emphasis lied in the efficient support of point- to-point and small multipoint scenarios without centralized multipoint control. However, in practice, many small multipoint conferences operate utilizing devices known as Multipoint Control Units (MCUs). MCUs comprise mixers and translators (in RTP [RFC3550] terminology), but also signalling support. Long standing experience of the conversational video conferencing industry suggests that there is a need for a few additional feedback messages, to efficiently support MCU-based multipoint conferencing. Some of the messages have applications beyond centralized multipoint, and this is indicated in the description of the message. Some of the messages defined here are forward only, in that they do not require an explicit acknowledgement. Other messages require acknowledgement, leading to a two way communication model that could suggest to some to be useful for control purposes. It is not the intention of this memo to open up the use of RTCP to generalized control protocol functionality. All mentioned messages have relatively strict real-time constraints and are of transient nature, which make the use of more traditional control protocol means, such as SIP re-invites, undesirable. Furthermore, all messages are of a very simple format that can be easily processed by an RTP/RTCP sender/receiver. Finally, all messages infer only to the RTP stream they are related to, and not to any other property of a communication system. The Full Intra Request (FIR) Command requires the receiver of the message (and sender of the stream) to immediately insert a decoder refresh point (e.g. an IDR/Intra picture). In order to fulfil congestion control constraints, this may imply a significant drop in frame rate, as decoder refresh points are commonly much larger than regular predicted pictures. The use of this message is restricted to cases where no other means of decoder refresh can be employed, e.g. during the join-phase of a new participant in a multipoint conference. It is explicitly disallowed to use the FIR command for error resilience purposes, and instead it is referred to AVPF's PLI message, which reports lost pictures and has been included in AVPF for that purpose. The message does not require an acknowledgement, as the presence of a decoder refresh point can be easily derived from the media bit stream. Today, the FIR message appears to be useful primarily with video streams, but in the future it may become helpful also in conjunction with other media codecs that support temporal prediction across RTP packets. Wenger, et al. Standards Track [Page 4] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 The Temporary Maximum Media Bandwidth Request (TMMBR) Message allows to signal, from media receiver to media sender, the current maximum supported media bit-rate for a given media stream. Once a bandwidth limitation is established by the media sender, that sender notifies the initiator of the request, and all other session participants, by sending a TMMBN notification message. One usage scenarios comprises limiting media senders in multiparty conferencing to the slowest receiver's maximum media bandwidth reception/handling capability (the receiver's situation may have changed due to computational load, or it may be that the receiver has just joined the conference). Another application involves graceful bandwidth adaptation in scenarios where the upper limit connection bandwidth to a receiver changes but is known in the interval between these dynamic changes. The TMMBR message is useful for all media types that are not inherently of constant bit rate. Finally, the Temporal-Spatial Trade-off Request (TSTR) Message enables a video receiver to signal to the video sender its preference for spatial quality or high temporal resolution (frame rate). The receiver of the video stream generates this signal typically based on input from its user interface, so to react to explicit requests of the user. However, some implicit use forms are also known. For example, the trade-offs commonly used for live video and document camera content are different. Obviously, this indication is relevant only with respect to video transmission. The message is acknowledged by an announcement message indicating the newly chosen tradeoff, so to allow immediate user feedback. 2. Definitions 2.1. Glossary ASM - Asynchronous Multicast AVPF - The Extended RTP Profile for RTCP-based Feedback FEC - Forward Error Correction FIR - Full Intra Request MCU - Multipoint Control Unit MPEG - Moving Picture Experts Group PtM - Point to Multipoint PtP - Point to Point TMMBN - Temporary Maximum Media Bit-rate Notification TMMBR - Temporary Maximum Media Bit-rate Request PLI - Picture Loss Indication TSTA - Temporal Spatial Trade-off Announcement TSTR - Temporal Spatial Trade-off Request 2.2. Terminology Wenger, et al. Standards Track [Page 5] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Message: Codepoint defined by this specification, of one of the following types: Request: Message that requires Acknowledgement Acknowledgment: Message that answers a Request Command: Message that forces the receiver to an action Indication: Message that reports a situation Notification: See Indication. Note that, with the exception of "Notification", this terminology is in alignment with ITU-T Rec. H.245. Decoder Refresh Point: A bit string, packetised in one or more RTP packets, which completely resets the decoder to a known state. Typical examples of Decoder Refresh Points are H.261 Intra pictures and H.264 IDR pictures. However, there are also much more complex decoder refresh points. Typical examples for "hard" decoder refresh points are Intra pictures in H.261, H.263, MPEG 1, MPEG 2, and MPEG-4 part 2, and IDR pictures in H.264. "Gradual" decoder refresh points may also be used; see for example [11]. While both "hard" and "gradual" decoder refresh points are acceptable in the scope of this specification, in most cases the user experience will benefit from using a "hard" decoder refresh point. A decoder refresh point also contains all header information above the picture layer (or equivalent, depending on the video compression standard) that is conveyed in-band. In H.264, for example, a decoder refresh point contains parameter set NAL units that generate parameter sets necessary for the decoding of the following slice/data partition NAL units (and that are not conveyed out of band). To the best of the author's knowledge, the term "Decoder Wenger, et al. Standards Track [Page 6] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 Refresh Point" has been formally defined only in H.264; hence we are referring here to this video compression standard. Decoding: The operation of reconstructing the media stream. Rendering: The operation of presenting (parts of) the reconstructed media stream to the user. Stream thinning: The operation of removing some of the packets from a media stream. Stream thinning, preferably, is performed in a media aware fashion implying that the media packets are removed in the order of their relevance to the reproductive quality. However even when employing media-aware stream thinning, most media streams quickly lose quality when subject to increasing levels of thinning. Media-unaware stream thinning leads to even worse quality degradation. 2.3. Topologies This subsection defines several basic topologies that are relevant for codec control. The first four relate to the RTP system model utilizing multicast and/or unicast, as envisioned in RFC 3550. The last two topologies, in contrast, describe the widely deployed system model as used in most H.323 video conferences, where both the media streams and the RTCP control traffic terminate at the MCU. More topologies can be constructed by combining any of the models, see Section 2.3.7. 2.3.1. Point to Point The Point to Point (PtP) topology (Figure 1) consists of two end- points with unicast capabilities between them. Both RTP and RTCP traffic are conveyed endpoint to endpoint using unicast traffic only (even if this unicast traffic happens to be conveyed over an IP- multicast address). +---+ +---+ | A |<------->| B | +---+ +---+ Figure 1 - Point to Point The main property of this topology is that A sends to B and only B, while B sends to A and only A. This avoids all complexities of handling multiple endpoints and combining the requirements from them. Wenger, et al. Standards Track [Page 7] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 Do note that an endpoint may still use multiple RTP Synchronization Sources (SSRCs) in an RTP session. 2.3.2. Point to Multi-point using Multicast +-----+ +---+ / \ +---+ | A |----/ \---| B | +---+ / Multi- \ +---+ + Cast + +---+ \ Network / +---+ | C |----\ /---| D | +---+ \ / +---+ +-----+ Figure 2 - Point to Multipoint using Multicast We define Point to Multipoint (PtM) using multicast topology as a transmission model in which traffic from any participant reaches all the other participants, except for cases such as o packet loss occurs, o a participant participant does not wish to receive the traffic from a certain other participant, and therefore has not subscribed to the IP multicast group in question. In this sense, "traffic" encompasses both RTP and RTCP traffic. The number of participants can be between one and many -- as RTP and RTCP scales to very large multicast groups (the theoretical limit of RTP is approximately two billion participants). This draft is primarily interested in the subset of multicast session where the number of participants in the multicast group allows the participants to use early or immediate feedback as defined in AVPF. This document refers to those groups as as "small multicast groups". 2.3.3. Point to Multipoint using the RFC 3550 translator Two main categories of Translators can be distinguished. Transport Translators do not modify the media stream itself, but are concerned with transport parameters. Transport parameters, in the sense of this section, comprise the transport addresses to bridge different domains, and the media packetization to allow other transport protocols to be interconnected to a session (gateways). Media Translators, in contrast, modify the media stream itself. This process is commonly known as transcoding. The modification of the media stream can be as small as removing parts of the stream, and can Wenger, et al. Standards Track [Page 8] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 go all the way to a full transcoding utilizing a different media codec. Media translators are commonly used to connect entities without a common interoperability point. Stand-alone Media Translators are rare. Most commonly, a combination of Transport and Media Translators are used to translate both the media stream and the transport aspects of a stream between two transport domains (or clouds). Both Translator types share common attributes that separates them from mixers. For each media stream that the translator receives, it generates an individual stream in the other domain. However, a translator maintains a complete view of all existing participants between both domains. Therefore, the SSRC space is shared across the two domains. The RTCP translation process can be trivial, for example when Transport translators just need to adjust IP addresses, and can be quite complex in the case of media translators. See section 7.2 of [RFC 3550]. +-----+ +---+ / \ +------------+ +---+ | A |<---/ \ | |<---->| B | +---+ / Multi- \ | | +---+ + Cast +->| Translator | +---+ \ Network / | | +---+ | C |<---\ / | |<---->| D | +---+ \ / +------------+ +---+ +-----+ Figure 3 - Point to Multipoint using a Translator Figure 3 depicts an example of a Transport Translator performing at least IP address translation. It allows the (non multicast capable) participants B and D to take part in a multicasted session by having the translator forward their unicast traffic to the multicast addresses in use, and vice versa. It must also forward B's traffic to D and vice versa, to provide each of B and D with a complete view of the session. If B were behind a limited link, the translator may perform media transcoding to allow the traffic received from the other participants to reach B without overloading the link. When in the example depicted in Figure 3 the translator acts only as a Transport Translator, then the RTCP traffic can simply be forwarded, similar to the media traffic. However, when media translation occurs, the translator's task becomes substantially more Wenger, et al. Standards Track [Page 9] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 complex even with respect to the RTCP traffic. In this case, the translator needs to rewrite B's RTCP receiver report, before forwarding them to D and the multicast network. The rewriting is needed as the stream received by B is not the same stream as the other participants receive. For example, the number of packets transmitted to B may be lower than what D receives, due to the different media format. Therefore, if the receiver reports were forwarded without changes, the extended highest sequence number would indicate that B were substantially behind in reception -- while it most likely it would not be. Therefore, the translator must translate that number to a corresponding sequence number for the stream the translator received. Similar arguments can be made for most other fields in the RTCP receiver reports. +---+ +------------+ +---+ | A |<---->| Multipoint |<---->| B | +---+ | Control | +---+ | Unit | +---+ | (MCU) | +---+ | C |<---->| |<---->| D | +---+ +------------+ +---+ Figure 4 - MCU with RTP Translator (relay) with only unicast links A common MCU scenario is the one depicted in Figure 4 - MCU with RTP Translator (relay) with only unicast links. Herein, the MCU connects multiple users of a conference through unicast. This can be implemented using a very simple transport translator, which could be called a relay. The relay forwards all traffic it receives, both RTP and RTCP, to all other participants. In doing so, a multicast network is emulated without relying on a multicast capable network structure. Wenger, et al. Standards Track [Page 10] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 2.3.4. Point to Multipoint using the RFC 3550 mixer model A mixer is a middlebox that aggregates multiple RTP streams that are part of a session, by mixing the media data and generating a new RTP stream. One common application for a mixer is to allow a participant to receive a session with a reduced amount of resources. +-----+ +---+ / \ +-----------+ +---+ | A |<---/ \ | |<---->| B | +---+ / Multi- \ | | +---+ + Cast +->| Mixer | +---+ \ Network / | | +---+ | C |<---\ / | |<---->| D | +---+ \ / +-----------+ +---+ +-----+ Figure 5 - Point to Multipoint using RFC 3550 mixer model A mixer can be viewed as a device terminating the media streams received from other session participants. Using the media data from the received media streams, a mixer generates a media stream that is sent to the session participant. The content that the mixer provides is the mixed aggregate of what the mixer receives from the PtP or PtM links, which are part of the same conference session. The mixer is the content source, as it mixes the content (often in the uncompressed domain) and then encodes it for transmission to a participant. The CC and CSRC fields in the RTP header are used to indicate the contributors of to the newly generated stream. The SSRCs of the to-be-mixed streams on the mixer input appear as the CSRCs at the mixer output. That output stream uses a new SSRC that identifies the Mixer. The CSRC are forwarded between the two domains to allow for loop detection and identification of sources that are part of the global session. The mixer is responsible for generating RTCP packets in accordance with its role. It is a receiver and should therefore send reception reports for the media streams it receives. As a media sender itself it should also generate sender report for those media streams sent. The content of the SRs created by the mixer may or may not take into account the situation on its receiving side. Similarly, the content of RRs created by the mixer may or may not be based on the situation on the mixer's sending side. This is left open to the implementation. As specified in Section 7.3 of RFC 3550, a mixer must not forward RTCP unaltered between the two domains. Wenger, et al. Standards Track [Page 11] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 The mixer depicted in Figure 5 has three domains that needs to be separated; the multicast network, participant B and participant D. The Mixer produces different mixed streams to B and D, as the one to B may contain D and vice versa. However the mixer does only need one SSRC in each domain that is the receiving entity and transmitter of mixed content. In the multicast domain the mixer does not provide a mixed view of the other domains and only forwards media from B and D into the multicast network using B's and D's SSRC. The mixer is responsible for receiving the codec control messages and handles them appropriately. The definition of "appropriate" depends on the message itself and the context. In some cases, the reception of a codec control message may result in the generation and transmission of codec control messages by the mixer to the participants in the other domain. In other cases, a message is handled by the mixer itself and therefore not forwarded to any other domains. It should be noted that this form of mixing technology is not widely deployed. Most multipoint video conferences used today employ one of the models discussed in the next sections. When replacing the multicast network in Figure 5 (to the left of the mixer) with individual unicast links as depicted in Figure 6, the mixer model is very similar to the one discussed in section 2.3.6 below. +---+ +------------+ +---+ | A |<---->| Multipoint |<---->| B | +---+ | Control | +---+ | Unit | +---+ | (MCU) | +---+ | C |<---->| |<---->| D | +---+ +------------+ +---+ Figure 6 - RTP Mixer with only unicast links Wenger, et al. Standards Track [Page 12] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 2.3.5. Point to Multipoint using video switching MCU +---+ +------------+ +---+ | A |------| Multipoint |------| B | +---+ | Control | +---+ | Unit | +---+ | (MCU) | +---+ | C |------| |------| D | +---+ +------------+ +---+ Figure 7 - Point to Multipoint using relaying MCU This PtM topology is, today, perhaps the most widely deployed one. It reflects today's lack of wide deployment of IP multicast technologies on IP networks and the Internet, as well as the simplicity of content switching when compared to content mixing. The technology is commonly implemented in what is known as "Video Switching MCUs". A video switch MCU forwards to a participant a single media stream, selected from the available streams. The criteria for selection are often based on voice activity in the audio-visual conference, but other conference management mechanisms (like explicit floor control) are known to exist as well. The video switching MCU may also perform media translation to modify the content in bit-rate, encoding, resolution; however it still indicates the original sender of the content through the SSRC. The values of the CC and CSRC fields are retained. RTCP Sender Reports are forwarded for the currently selected sender. All RTCP receiver reports are freely forward between the participants. In addition, the MCU may also originate RTCP control traffic in order to control the session and/or report on status from its viewpoint. The video switching MCU has mostly the attributes of a translator. However its stream selection is a mixing behaviour. This behaviour has some RTP and RTCP issues associated with it. The suppression of all but one media stream results in that most participants see only a subset of the sent media streams at any given time; often a single stream per conference. Therefore, RTCP receiver reports only report on these streams. In consequence, the media senders that are not currently forwarded receive a view of the session that indicates their media streams disappearing somewhere en route. This makes the use of RTCP for congestion control very problematic. To avoid these issues the MCU needs to modify the RTCP RRs. Wenger, et al. Standards Track [Page 13] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 2.3.6. Point to Multipoint using RTCP-terminating MCU +---+ +------------+ +---+ | A |<---->| Multipoint |<---->| B | +---+ | Control | +---+ | Unit | +---+ | (MCU) | +---+ | C |<---->| |<---->| D | +---+ +------------+ +---+ Figure 8 - Point to Multipoint using content modifying MCU In this PtM scenario, each participant runs an RTP point-to-point session between itself and the MCU. The content that the MCU provides to each participant is either: a) A selection of the content received from the other participants, or b) The mixed aggregate of what the MCU receives from the other PtP links, which are part of the same conference session. In case a) the MCU may modify the content in bit-rate, encoding, resolution. No explicit RTP mechanism is used to establish the relationship between the original media sender and the version the MCU sends. In other words, the outgoing session typically uses a different SSRC, and may well use a different PT, even if this different PT happens to be mapped to the same media type. (This is the definition of this topology and distinguishes it from the topologies previously discussed). In case b) the MCU is the content source as it mixes the content and then encodes it for transmission to a participant. The participant's content that is included in the aggregated content is not indicated through any explicit RTP mechanism. For example, regardless of the number of streams that are aggregated, in the MCU generated streams CC is zero and therefore no CSRC fields are present. The MCU is responsible for receiving the codec control messages and handle them appropriately. In some cases, the reception of a codec control message may result in the generation and transmission of codec control messages by the MCU to some or all of the other participants. An MCU may transparently relay some codec control messages and intercept, modify, and (when appropriate) generate codec control messages of its own and transmit them to the media senders. The main feature that sets this topology apart from what RFC 3550 describes, is the lack of an explicit RTP level indication of all Wenger, et al. Standards Track [Page 14] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 participants. If one were using the mechanisms available in RTP and RTCP to signal this explicitly, the topology would follow the approach of an RTP mixer. The lack of explicit indication has at least the following potential problems: 1) Loop detection cannot be performed on the RTP level. When carelessly connecting two misconfigured MCUs, a loop could be generated. 2) There is no information about active media senders available in the RTP packet. As this information is missing, receivers cannot use it. It also deprive the participant's clients information about who are actively sending in a machine usable way. Thus preventing clients from doing indication of currently active speakers in user interfaces, etc. 2.3.7. Combining Topologies Topologies can be combined and linked to each other using mixers or translators. Care must however be taken to how the SSRC space is handled, mixers separate the SSRC space into two parts, while translators maintain the space across themselves. Any hybrid, like the video switching MCU, 2.3.5, requires considerable afterthought on how RTCP is dealt with. 3. Motivation (Informative) This section discusses the motivation and usage of the different video and media control messages. The video control messages have been under discussion for a long time, and a requirement draft was drawn up [Basso]. This draft has expired; however we do quote relevant parts out of that draft to provide motivation and requirements. 3.1. Use Cases There are a number of possible usages for the proposed feedback messages. Let's begin with looking through the use cases Basso et al. [Basso] proposed. Some of the use cases have been reformulated and commented: 1. An RTP video mixer composes multiple encoded video sources into a single encoded video stream. Each time a video source is added, the RTP mixer needs to request a decoder refresh point from the video source, so as to start an uncorrupted prediction chain on the spatial area of the mixed picture occupied by the data from the new video source. Wenger, et al. Standards Track [Page 15] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 2. An RTP video mixer that receives multiple encoded RTP video streams from conference participants, and dynamically selects one of the streams to be included in its output RTP stream. At the time of a bit stream change (determined through means such as voice activation or the user interface), the mixer requests a decoder refresh point from the remote source, in order to avoid using unrelated content as reference data for inter picture prediction. After requesting the decoder refresh point, the video mixer stops the delivery of the current RTP stream and monitors the RTP stream from the new source until it detects data belonging to the decoder refresh point. At that time, the RTP mixer starts forwarding the newly selected stream to the receiver(s). 3. An application needs to signal to the remote encoder a request of change of the desired trade-off in temporal/spatial resolution. For example, one user may prefer a higher frame rate and a lower spatial quality, and another use may prefer the opposite. This choice is also highly content dependent. Many current video conferencing systems offer in the user interface a mechanism to make this selection, usually in the form of a slider. The mechanism is helpful in point-to-point, centralized multipoint and non-centralized multipoint uses. 4. Use case 4 of the Basso draft applies only to AVPF's PLI and is not reproduced here. 5. A video mixer switches its output stream to a new video source, similar to use case 2. The video mixer instructs the receiving endpoints by means of a freeze message to complete the decoding of the current picture and then freezing the picture (stop rendering but continue decoding), until the freeze picture request is released. The freeze picture release codepoint is a mechanism that can be selected on a per picture basis and can be conveyed in-band in most video coding standards. Concurrently, the video mixer request a decoder refresh point from the new video source and immediately switches to the new source. Once the new source receives the request for the reference picture and acts on it, it produces a decoder refresh point with an embedded Freeze-Release. Once having received the decoder refresh point with the freeze release information, the receiving endpoints restart rendering and displays an uncorrupted new picture. The main benefit of this method as opposed to the one of use case 2 is that the video mixer does not have to discover the beginning of a decoder refresh point. 6. A video mixer dynamically selects one of the received video streams to be sent out to participants and tries to provide the highest bit rate possible to all participants, while minimizing stream transrating. One way of achieving this is to setup sessions with endpoints using the maximum bit rate accepted by that endpoint, and by the call admission method used by the mixer. By Wenger, et al. Standards Track [Page 16] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 means of commands that allow reducing the maximum media bitrate beyond what has been negotiated during session setup, the mixer can then reduce the maximum bit rate sent by endpoints to the lowest common denominator of all received streams. As the lowest common denominator changes due to endpoints joining, leaving, or network congestion, the mixer can adjust the limits to which endpoints can send their streams to match the new limit. The mixer then would request a new maximum bit rate, which is equal or less than the maximum bit-rate negotiated at session setup, for a specific media stream, and the remote endpoint can respond with the actual bit-rate that it can support. The picture Basso, et al draws up covers most applications we foresee. However we would like to extend the list with one additional use case: 7. The used congestion control algorithms (AMID and TFRC) probe for more bandwidth as long as there is something to send. With congestion control using packet-loss as the indication for congestion, this probing does generally result in reduced media quality (often to a point where the distortion is large enough to make the media unusable), due to packet loss and increased delay. In a number of deployment scenarios, especially cellular ones, the bottleneck link is often the last hop link. That cellular link also commonly has some type of QoS negotiation enabling the cellular device to learn the maximal bit-rate available over this last hop. Thus indicating the maximum available bit-rate to the transmitting part can be beneficial to prevent it from even trying to exceed the known hard limit that exists. For cellular or other mobile devices the available known bit-rate can also quickly change due to handover to another transmission technology, QoS renegotiation due to congestion, etc. To enable minimal disruption of service a possibility for quick convergence, especially in cases of reduced bandwidth, a media path signalling method is desired. 3.2. Using the Media Path There are multiple reasons why we propose to use the media path for the messages. First, systems employing MCUs are usually separating the control and media processing parts. As these messages are intended or generated by the media processing rather than the signalling part of the MCU, having them on the media path avoids interfaces and unnecessary control traffic between signalling and processing. If the MCU is physically decomposite, the use of the media path avoids the need for media control protocol extensions (e.g. in MEGACO [RFC3525]). Secondly, the signalling path quite commonly contains several signalling entities, e.g. SIP-proxies and application servers. Wenger, et al. Standards Track [Page 17] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 Avoiding signalling entities avoids delay for several reasons. Proxies have less stringent delay requirements than media processing and due to their complex and more generic nature may result in significant processing delay. The topological locations of the signalling entities are also commonly not optimized for minimal delay, rather other architectural goals. Thus the signalling path can be significantly longer in both geographical and delay sense. 3.3. Using AVPF The AVPF feedback message framework provides a simple way of implementing the new messages. Furthermore, AVPF implements rules controlling the timing of feedback messages so to avoid congestion through network flooding, which are re-used by reference. The signalling setup for AVPF allows each individual type of function to be configured or negotiated on a RTP session basis. 3.3.1. Reliability The use of RTCP messages implies that each message transfer is unreliable, unless the lower layer transport provides reliability. The different messages proposed in this specification have different requirements in terms of reliability. However, in all cases, the reaction to an (occasional) loss of a feedback message is specified. 3.4. Multicast The media related requests might be used with multicast. The RTCP timing rules specified in [RTP] and [AVPF] ensure that the messages do not cause overload of the RTCP connection. Inconsistent messages arriving at the RTP sender from different receivers are more problematic when multicast is employed. The reaction to inconsistencies depends on the message type, and is discussed for each message type separately. 3.5. Feedback Messages This section describes the semantics of the different feedback messages and how that applies to the different use cases. 3.5.1. Full Intra Request Command A Full Intra Request (FIR) command, when received by the designated media sender, requires that the media sender sends a "decoder refresh point" (see 2.2) at the earliest opportunity. The evaluation of such Wenger, et al. Standards Track [Page 18] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 opportunity includes the current encoder coding strategy and the current available network resources. FIR is also known as an "instantaneous decoder refresh request" or "video fast update request". Using a decoder refresh point implies refraining from using any picture sent prior to that point as a reference for the encoding process of any subsequent picture sent in the stream. For predictive media types that are not video, the analogue applies. For example, if in MPEG-4 systems scene updates are used, the decoder refresh point consists of the full representation of the scene and is not delta-coded relative to previous updates. Decoder Refresh points, especially Intra or IDR pictures are in general several times larger in size than predicted pictures. Thus, in scenarios in which the available bandwidth is small, the use of a decoder refresh point implies a delay that is significantly longer than the typical picture duration. Usage in multicast is possible; however aggregation of the commands is recommended. A receiver that receives a request closely (within 2 times the longest Round Trip Time (RTT) known) after sending a decoder refresh point should await a second request message to ensure that the media receiver has not been served by the previously delivered decoder refresh point. The reason for delaying 2 times the longest known RTT is to avoid sending unnecessary decoder refresh points. A session participant may have sent its own request while another participants request was in-flight to them. Thus suppressing those requests that may have been sent without knowledge about the other request avoids this issue. Full Intra Request is applicable in use-case 1, 2, and 5. 3.5.1.1. Reliability The FIR message results in the delivery of a decoder refresh point, unless the message is lost. Decoder refresh points are easily identifiable from the bit stream. Therefore, there is no need for protocol-level acknowledgement, and a simple command repetition mechanism is sufficient for ensuring the level of reliability required. However, the potential use of repetition does require a mechanism to prevent the recipient from responding to messages already received and responded to. To ensure the best possible reliability, a sender of FIR may repeat the FIR request until a response has been received. The repetition interval is determined by the RTCP timing rules the session operates under. Upon reception of a complete decoder refresh point or the detection of an attempt to send a decoder refresh point (which got Wenger, et al. Standards Track [Page 19] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 damaged due to a packet loss) the repetition of the FIR must stop. If another FIR is necessary, the request sequence number must be increased. To combat loss of the decoder refresh points sent, the sender that receives repetitions of the FIR 2*RTT after the transmission of the decoder refresh point shall send a new decoder refresh point. Two round trip times allow time for the request to arrive at the media sender and the decoder refresh point to arrive back to the requestor. A FIR sender shall not have more than one FIR request (different request sequence number) outstanding at any time per media sender in the session. An RTP Mixer that receives an FIR from a media receiver is responsible to ensure that a decoder refresh point is delivered to the requesting receiver. It may be necessary to generate FIR commands by the MCU. The two legs (FIR-requesting endpoint to MCU, and MCU to decoder refresh point generating MCU) are handled independently from each other from a reliability perspective. 3.5.2. Temporal Spatial Trade-off Request and Announcement The Temporal Spatial Trade-off Request (TSTR) instructs the video encoder to change its trade-off between temporal and spatial resolution. Index values from 0 to 31 indicate monotonically a desire for higher frame rate. In general the encoder reaction time may be significantly longer than the typical picture duration. See use case 3 for an example. The encoder decides if the request results in a change of the trade off. An acknowledgement process has been defined to provide feedback of the trade-off that is used henceforth. Informative note: TSTR and TSTA have been introduced primarily because it is believed that control protocol mechanisms, e.g. a SIP re-invite, are too heavyweight, and too slow to allow for a reasonable user experience. Consider, for example, a user interface where the remote user selects the temporal/spatial trade- off with a slider (as it is common in state-of-the-art video conferencing systems). An immediate feedback to any slider movement is required for a reasonable user experience. A SIP re- invite would require at least 2 round-trips more (compared to the TSTR/TSTA mechanism) and may involve proxies and other complex mechanisms. Even in a well-designed system, it may take a second or so until finally the new trade-off is selected. Furthermore the use of RTCP solves very efficiently the multicast use case. The use of TSTR and TSTA in multipoint scenarios is a non-trivial subject, and can be solved in many implementation specific ways. Problems are stemming from the fact that TSTRs will typically arrive unsynchronized, and may request different trade-off values for the same stream and/or endpoint encoder. This memo does not specify a Wenger, et al. Standards Track [Page 20] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 MCU's or endpoint's reaction to the reception of a suggested trade- off as conveyed in the TSTR -- we only require the receiver of a TSTR message to reply to it by sending a TSTA, carrying the new trade-off chosen by its own criteria (which may or may not be based on the trade-off conveyed by TSTR). In other words, the trade-off sent in TSTR is a non-binding recommendation; nothing more. With respect to TSTR/TSTA, four scenarios based on the topologies in section 2.3 needs to be distinguished. The scenarios are described in the following sub-clauses. 3.5.2.1. Point-to-point In this most trivial case, the media sender typically adjusts its temporal/spatial trade-off based on the requested value in TSTR, and within its capabilities. The TSTA message conveys back the new trade-off value (which may be identical to the old one if, for example, the sender is not capable to adjust its trade-off). 3.5.2.2. Point-to-Multipoint using Multicast or Translators RTCP Multicast is used either with media multicast according to Section 2.3.2, or following RFC 3550's translator model according to Section 2.3.3. In these cases, TSTR messages from different receivers may be received unsynchronized, and possibly with different requested trade-offs (because of different user preferences). This memo does not specify how the media sender tunes its trade-off. Possible strategies include selecting the mean, or median, of all trade-off requests received, prioritize certain participants, or continue using the previously selected trade-off (e.g. when the sender is not capable of adjusting it). Again, all TSTR messages need to be acknowledged by TSTA, and the value conveyed back has to reflect the decision made. 3.5.2.3. Point-to-Multipoint using RTP Mixer In this scenario the RTP Mixer receives all TSTR messages, and has the opportunity to act on them based on its own criteria. In most cases, the MCU should form a "consensus" of potentially conflicting TSTR messages arriving from different participants, and initiate its own TSTR message(s) to the media sender(s). The strategy of forming this "consensus" is open for the implementation, and can, for example, encompass averaging the participants' requests, prioritizing certain participants, or use session default values. If the Mixer changes its trade-off, it needs to request from the media sender(s) the use the new value, by creating a TSTR of its own. Upon reaching a decision on the used trade-off it includes that value in the acknowledgement. Wenger, et al. Standards Track [Page 21] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 Even if a Mixer or Translator performs transcoding, it is very difficult to deliver media with the requested trade-off, unless the content the MCU receives is already close to that trade-off. Only in cases where the original source has substantially higher quality (and bit-rate), it is likely that transcoding can result in the requested trade-off. 3.5.2.4. Reliability A request and reception acknowledgement mechanism is specified. The Temporal Spatial Trade-off Announcement (TSTA) message informs the request-sender that its request has been received, and what trade-off is used henceforth. This acknowledgment mechanism is desirable for at least the following reasons: o A change in the trade-off cannot be directly identified from the media bit stream, o User feedback cannot be implemented without information of the chosen trade-off value, according to the media sender's constraints, o Repetitive sending of messages requesting an unimplementable trade- off can be avoided. 3.5.3. Temporary Maximum Media Bit-rate Request A receiver, translator or mixer uses the Temporary Maximum Media Bit- rate Request (TMMBR, "timber") to request a sender to limit the maximum bit-rate for a media stream to, or below, the provided value. The primary usage for this is a scenario with MCU (use case 6), corresponding to topologies in 2.3.3 (translator) and 2.3.4(mixer), but also 2.3.1 (point-to-point). The temporary maximum media bit-rate messages are generic messages that can be applied to any media. The reasoning below assumes that the participants have negotiated a session maximum bit-rate, using the signalling protocol. This value can be global, for example in case of point-to-point, multicast, or translators. It may also be local between the participant and the peer or mixer. In both cases, the bit-rate negotiated in signalling is the one that the participant guarantees to be able to handle (encode and decode). In practice, the connectivity of the participant also bears an influence to the negotiated value -- it does not necessarily make much sense to negotiate a media bit rate that one's network interface does not support. An already established temporary bit-rate value may be changed at any time (subject to the timing rules of the feedback message sending), Wenger, et al. Standards Track [Page 22] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 and to any value between zero and the session maximum, as negotiated during signalling. Even if a sender has received a TMMBR message increasing the bit-rate, all increases must be governed by a congestion control algorithm. TMMBR only indicates known limitations, usually in the local environment, and does not provide any guarantees. If it is likely that the new bit-rate indicated by TMMBR will be valid for the remainder of the session, the TMMBR sender can perform a renegotiation of the session upper limit using the session signalling protocol. 3.5.3.1. MCU based Multi-point operation Assume a small multipart conference is ongoing, as depicted in Figure 6 of Section 2.3.4. All participants (A-D) have negotiated a common maximum bit-rate that this session can use. The conference operates over a number of unicast links between the participants and the MCU. The congestion situation on each of these links can easily be monitored by the participant in question and by the MCU, utilizing, for example, RTCP Receiver Reports. However, any given participant has no knowledge of the congestion situation of the connections to the other participants. Worse, without mechanisms similar to the ones discussed in this draft, the MCU (who is aware of the congestion situation on all connections it manages) has no standardized means to inform participants to slow down, short of forging its own receiver reports (which is undesirable). In principle, an MCU confronted with such a situation is obliged to thin or transcode streams intended for connections that detected congestion. In practice, stream thinning - if done media aware - is unfortunately a very difficult and cumbersome operation and adds undesirable delay. If done media unaware, it leads very quickly to unacceptable reproduced media quality. Hence, means to slow down senders even in the absence of congestion on their connections to the MCU are desirable. To allow the MCU to perform congestion control on the individual links, without performing transcoding, there is a need for a mechanism that enables the MCU to request the participant's media encoders to limit their maximum media bit-rate currently used. The MCU handles the detection of a congestion state between itself and a participant as follows: 1. Start thinning the media traffic to the supported bit-rate. 2. Use the TMMBR to request the media sender(s) to reduce the media bit-rate sent by them to the MCU, to a value that is in compliance with congestion control principles for the slowest link. Slow refers here to the available bandwidth and packet rate after congestion control. Wenger, et al. Standards Track [Page 23] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 3. As soon as the bit-rate has been reduced by the sending part, the MCU stops stream thinning implicitly, because there is no need for it any more as the stream is in compliance with congestion control. Above algorithms may suggest to some that there is no need for the TMMBR - it should be sufficient to solely rely on stream thinning. As much as this is desirable from a network protocol designer's viewpoint, it has the disadvantage that it doesn't work very well - the reproduced media quality quickly becomes unusable. It appears to be a reasonable compromise to rely on stream thinning as an immediate reaction tool to combat congestions, and have a quick control mechanism that instructs the original sender to reduce its bitrate. Note also that the standard RTCP receiver report cannot serve for the purpose mentioned. In an environment with RTP Mixers, the RTCP RR is being sent between the RTP receiver in the endpoint and the RTP sender in the Mixer only - as there is no multicast transmission. The stream that needs to be bandwidth-reduced, however, is the one between the original sending endpoint and the Mixer. This endpoint doesn't see the aforementioned RTCP RRs, and hence needs explicitly informed about desired bandwidth adjustments. In this topology it is the Mixer's responsibility to collect, and consider jointly, the different bit-rates which the different links may support, into the bit rate requested. This aggregation may also take into account that the Mixer may contain certain transcoding capabilities (as discussed in 2.3.4), which can be employed for those few of the session participants that have the lowest available bit- rates. 3.5.3.2. Point-to-Multipoint using Multicast or Translators In this topology, RTCP RRs are transmitted globally which allows for the detection of transmission problems such as congestion, on a medium timescale. As all media senders are aware of the congestion situation of all media receivers, the rationale of the use of TMMBR of section 3.5.3.1 does not apply. However, even in this case the congestion control response can be improved when the unicast links are employing congestion controlled transport protocols (such as TCP or DCCP). A peer may also report local limitation to the media sender. 3.5.3.3. Point-to-point operation In use case 7 it is possible to use TMMBR to improve the performance at times of changes in the known upper limit of the bit-rate. In Wenger, et al. Standards Track [Page 24] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 this use case the signalling protocol has established an upper limit for the session and media bit-rates. However at the time of transport link bit-rate reduction, a receiver could avoid serious congestion by sending a TMMBR to the sending side. 3.5.3.4. Reliability The result of TMMBR is not immediately identifiable through inspection of the media stream, and therefore a more explicit mechanism is needed. Using a statistically based retransmission scheme would only provide statistical guarantees of the request being received. It would also not avoid the retransmission of already received messages. In addition it does not allow for easy suppression of other participants requests. For the reasons mentioned, a mechanism based on notification is used. Upon the reception of a request a media sender sends a notification containing the current applicable limitation of the bit-rate, and which session participants that own that limit. That allows all other participants to suppress any request they may have, with limitation value equal or higher to the current one. The identity of the owner allows for small message sizes and media sender states. A media sender only keeps state for the SSRC of the current owner of the limitation; all other requests and their sources are not saved. Only the participant with the lowest value is allowed to remove or change its limitation. Otherwise anyone that ever set a limitation would need to remove it to allow the maximum bit-rate to be raised beyond that value. Wenger, et al. Standards Track [Page 25] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 4. RTCP Receiver Report Extensions This memo specifies 5 new feedback messages. The Full Intra Request (FIR), Temporal-Spatial Trade-off Request (TSTR), and Temporal- Spatial Trade-off Announcement (TSTA) are "Payload Specific Feedback Messages" in the sense of section 6.3 of AVPF [AVPF]. The Temporary Maximum Media Bit-rate Request (TMMBR) and Temporary Maximum Media Bit-rate Notification (TMMBN) are "Transport Layer Feedback Messages" in the sense of section 6.2 of AVPF. In the following subsections, the new feedback messages are defined, following a similar structure as in the AVPF specification's sections 6.2 and 6.3, respectively. 4.1. Design Principles of the Extension Mechanism RTCP was originally introduced as a channel to convey presence, reception quality statistics and hints on the desired media coding. A limited set of media control mechanisms have been introduced in early RTP payload formats for video formats, for example in RFC 2032 [RFC2032]. However, this specification, for the first time, suggests a two-way handshake for one of its messages. There is danger that this introduction could be misunderstood as the precedence for the use of RTCP as an RTP session control protocol. In order to prevent these misunderstandings, this subsection attempts to clarify the scope of the extensions specified in this memo, and strongly suggests that future extensions follow the rationale spelled out here, or compellingly explain why they divert from the rationale. In this memo, and in AVPF [AVPF], only such messages have been included which a) have comparatively strict real-time constraints, which prevent the use of mechanisms such as a SIP re-invite in most application scenarios. The real-time constraints are explained separately for each message where necessary b) are multicast-safe in that the reaction to potentially contradicting feedback messages is specified, as necessary for each message c) are directly related to activities of a certain media codec, class of media codecs (e.g. video codecs), or the given media stream. In this memo, a two-way handshake is only introduced for such messages that a) require a notification or acknowledgement due to their nature, which is motivated separately for each message b) the notification or acknowledgement cannot be easily derived from the media bit stream. Wenger, et al. Standards Track [Page 26] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 All messages in AVPF [AVPF] and in this memo follow a number of common design principles. In particular: a) Media receivers are not always implementing higher control protocol functionalities (SDP, XML parsers and such) in their media path. Therefore, simple binary representations are used in the feedback messages and not an (otherwise desirable) flexible format such as, for example, XML. 4.2. Transport Layer Feedback Messages Transport Layer FB messages are identified by the value RTPFB (205) as RTCP packet type. In AVPF, one message of this category had been defined. This memo specifies two more messages for a total of three messages of this type. They are identified by means of the FMT parameter as follows: 0: unassigned 1: Generic NACK (as per AVPF) 2: Maximum Media Bit-rate Request 3: Maximum Media Bit-rate Notification 4-30: unassigned 31: reserved for future expansion of the identifier number space The following subsection defines the formats of the FCI field for this type of FB message. 4.2.1. Temporary Maximum Media Bit-rate Request (TMMBR) The FCI field of a TMMBR Feedback message SHALL contain one or more FCI entries. 4.2.1.1. Semantics The TMMBR is used to indicate the highest bit-rate per sender of a media, which the receiver currently supports in this RTP session. The media sender MAY use any lower bit-rate, as it may need to address a congestion situation or other limiting factors. See section 5 (congestion control) for more discussion. The "SSRC of the packet sender" field indicates the source of the request, and the "SSRC of media source" is not used and SHALL be set to 0. The SSRC of media sender in the FCI field denotes the media sender the message applies to. This is useful in the multicast or translator topologies where each media sender may be addressed in a single TMMBR message using multiple FCIs. Wenger, et al. Standards Track [Page 27] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 A TMMBR FCI MAY be repeated in subsequent TMMBR messages if no applicable TMMBN FCI has been received at the time of transmission of the next RTCP packet. The bit-rate value of a TMMBR FCI MAY be changed from a previous TMMBR message and the next, regardless of the eventual reception of an applicable TMMBN FCI. Please note that a TMMBN message is sent by the media sender at the earliest possible point in time, as a result of any TMMBR messages received since the last sending of TMMBN. The TMMBN message indicates the limit and the owner of that limit at the time of the transmission of the message. The limit is the lowest of all values received since the last TMMBN was transmitted. A media receiver who is not the owner of the bandwidth limit when sending a TMMBR, MUST request a bandwidth lower than their knowledge of currently established bandwidth limit for this media sender. Therefore, all received requests for bandwidth limits greater or equal to the one currently established are ignored. A media receiver who is the owner of the current bandwidth limit, MAY lower the value further, raise the value or remove the restriction completely by setting the bandwidth limit equal to the session limit. Once a session participant receives the TMMBN in response to its TMMBR, with its own SSRC, it knows that it "owns" the bandwidth limitation. Only the "owner" of a bandwidth limitation can raise it or reset it to the session limit. Note that, due to the unreliable nature of transport of TMMBR and TMMBN, the above rules may lead to the sending of TMMBR messages disobeying the rules above. Furthermore, in multicast scenarios it can happen that more than one session participants believes it "owns" the current bandwidth limitation. This is not critical for a number of reasons: a) If a TMMBR message is lost in transmission, the media sender does not learn about the restrictions imposed on it. However, it also does not send a TMMBN message notifying reception of a request it has never received. Therefore, no new limit is established, the media receiver sending the more restrictive TMMBR is not the owner. Since this media receiver has not seen a notification corresponding to its request, it is free to re-send it. b) Similarly, if a TMMBN message gets lost, the media receiver that has sent the corresponding TMMBR request does not receive acknowledgement. In that case, it is also not the "owner" of the restriction and is free to re-send the request. c) If multiple competing TMMBR messages are sent by different session participants, then the resulting TMMBN indicates the lowest bandwidth requested; the owner is set to the sender of the TMMBR with the lowest requested bandwidth value. Wenger, et al. Standards Track [Page 28] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 TMMBR feedback SHOULD NOT be used if the underlying transport protocol is capable of providing similar feedback information from the receiver to the sender. It also important to consider the security risks involved with faked TMMBRs. See security considerations in Section 6. The feedback messages may be used in both multicast and unicast sessions of any of the specified topologies. For sessions with a larger number of participants using the lowest common denominator, as required by this mechanism, may not be the most suitable course of action. Larger session may need to consider other ways to support adapted bit-rate to participants, such as partitioning the session in different quality tiers, or use some other method of achieving bit-rate scalability. If the value set by a TMMBR message is expected to be permanent the TMMBR setting party is RECOMMENDED to renegotiate the session parameters to reflect that using the setup signalling. 4.2.1.2. Message Format The Feedback control information (FCI) consists of one or more TMMBR FCI entries with the following syntax: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Maximum bit-rate in units of 128 bits/s | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9 - Syntax for the TMMBR message SSRC: The SSRC value of the target of this specific maximum bit- rate request. Maximum bit-rate: The temporary maximum media bit-rate value in units of 128 bit/s. This provides range from 0 to 549755813888 bits/s (~550 Tbit/s) with a granularity of 128 bits/s. The length of the FB message is be set to 2+2*N where N is the number of TMMBR FCI entries. Wenger, et al. Standards Track [Page 29] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 4.2.1.3. Timing Rules The first transmission of the request message MAY use early or immediate feedback in cases when timeliness is desirable. Any repetition of a request message SHOULD use regular RTCP mode for its transmission timing. 4.2.2. Temporary Maximum Media Bit-rate Notification (TMMBN) The FCI field of the TMMBN Feedback message SHALL contain one TMMBN FCI entry. 4.2.2.1. Semantics This feedback message is used to notify the senders of any TMMBR message that one or more TMMBR messages have been received. It indicates to all participants the currently employed maximum bit-rate value and the "owner" of the current limitation. The "owner" of a limitation is the sender of the last (most restrictive) TMMBR message received by the media sender. The "SSRC of the packet sender" field indicates the source of the notification. The "SSRC of media source" SHALL be set to the SSRC of the media receiver that currently owns the bit-rate limitation. A TMMBN message SHALL be scheduled for transmission after the reception of a TMMBR message with a FCI including the session participant's SSRC. Only a single TMMBN SHALL be sent, even if more than one TMMBR messages are received between the scheduling of the transmission and the actual transmission of the TMMBN message. The TMMBN message indicates the limit and the owner of that limit at the time of transmitting the message. The limit SHALL be the lowest of all values received since the last TMMBN was transmitted. The one sending that request SHALL become the owner of the limit. The reception of a TMMBR message with a transmission limit greater or equal than the current limit SHALL still result in the transmission of a TMMBN message. However the limit and owner is not changed, unless it was from the owner, and the current limit and owner is indicated in the TMMBN message. This procedure allows session participants that haven't seen the last TMMBN message to get a correct view of this media sender's state. When a media sender determines an "owner" of a limitation has left the session, then the current limitation is removed, and the media sender SHALL send an TMMBN message indicating the maximum session bandwidth. Wenger, et al. Standards Track [Page 30] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 4.2.2.2. Message Format The TMMBN Feedback control information (FCI) entry has the following syntax: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Maximum bit-rate in units of 128 bits/s | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10 - Syntax for the TMMBN message Maximum bit-rate: The current temporary maximum media bit-rate value in units of 128 bit/s. The length field value of the FB message SHALL be 3. 4.2.2.3. Timing Rules The acknowledgement SHOULD be sent as soon as allowed by the applied timing rules for the session. Immediate or early feedback mode SHOULD be used for these messages. 4.3. Payload Specific Feedback Messages Payload-Specific FB messages are identified by the value PT=PSFB (206) as RTCP packet type. AVPF defines three payload-specific FB messages and one application layer FB message. This memo specifies three additional payload specific feedback messages. All are identified by means of the FMT parameter as follows: 0: unassigned 1: Picture Loss Indication (PLI) 2: Slice Lost Indication (SLI) 3: Reference Picture Selection Indication (RPSI) 4: Full Intra Request Command (FIR) 5: Temporal-Spatial Trade-off Request (TSTR) 6: Temporal-Spatial Trade-off Announcement (TSTA) 7-14: unassigned 15: Application layer FB message 16-30: unassigned 31: reserved for future expansion of the sequence number space Wenger, et al. Standards Track [Page 31] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 The following subsections define the new FCI formats for the payload- specific FB messages. 4.3.1. Full Intra Request (FIR) command The FIR command FB message is identified by PT=PSFB and FMT=4. There MUST be one or more FIR entry contained in the FCI field. 4.3.1.1. Semantics Upon reception of a FIR message, an encoder MUST send a decoder refresh point (see Section 2.2) as soon as possible. Note: Currently, video appears to be the only useful application for FIR, as it appears to be the only RTP payloads widely deployed that relies heavily on media prediction across RTP packet boundaries. However, use of FIR could also reasonably be envisioned for other media types that share essential properties with compressed video, namely cross-frame prediction (whatever a frame may be for that media type). One possible example may be the dynamic updates of MPEG-4 scene descriptions. It is suggested that payload formats for such media types refer to FIR and other message types defined in this specification and in AVPF, instead of creating similar mechanisms in the payload specifications. The payload specifications may have to explain how the payload specific terminologies map to the video-centric terminology used here. Note: In environments where the sender has no control over the codec (e.g. when streaming pre-recorded and pre-coded content), the reaction to this command cannot be specified. One suitable reaction of a sender would be to skip forward in the video bit stream to the next decoder refresh point. In other scenarios, it may be preferable not to react to the command at all, e.g. when streaming to a large multicast group. Other reactions may also be possible. When deciding on a strategy, a sender could take into account factors such as the size of the receiving multicast group, the "importance" of the sender of the FIR message (however "importance" may be defined in this specific application), the frequency of decoder refresh points in the content, and others. However the usage of FIR in a session which predominately handles pre-coded content shouldn't use the FIR at all. The sender MUST consider congestion control as outlined in section 5, which MAY restrict its ability to send a decoder refresh point quickly. Note: The relationship between the Picture Loss Indication and FIR is as follows. As discussed in section 6.3.1 of AVPF, a Picture Wenger, et al. Standards Track [Page 32] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 Loss Indication informs the decoder about the loss of a picture and hence the likeliness of misalignment of the reference pictures in encoder and decoder. Such a scenario is normally related to losses in an ongoing connection. In point-to-point scenarios, and without the presence of advanced error resilience tools, one possible option an encoder has is to send a decoder refresh point. However, there are other options including ignoring the PLI, for example if only one receiver of many has sent a PLI or when the embedded stream redundancy is likely to clean up the reproduced picture within a reasonable amount of time. The FIR, in contrast, leaves a real-time encoder no choice but to send a decoder refresh point. It disallows the encoder to take into account any considerations such as the ones mentioned above. Note: Mandating a maximum delay for completing the sending of a decoder refresh point would be desirable from an application viewpoint, but may be problematic from a congestion control point of view. "As soon as possible" as mentioned above appears to be a reasonable compromise. FIR SHALL NOT be sent as a reaction to picture losses - it is RECOMMENDED to use PLI instead. FIR SHOULD be used only in such situations where not sending a decoder refresh point would render the video unusable for the users. Note: a typical example where sending FIR is adequate is when, in a multipoint conference, a new user joins the session and no regular decoder refresh point interval is established. Another example would be a video switching MCU that changes streams. Here, normally, the MCU issues a freeze picture request (through protocol means outside this specification) to the receiver(s), switches the streams, and issues a FIR to the new sender so to force it to emit a decoder refresh point. The decoder refresh point includes normally a Freeze Picture Release (defined outside this specification), which re-starts the rendering process of the receivers. Both techniques mentioned are commonly used in MCU- based multipoint conferences. Other RTP payload specifications such as RFC 2032 [4] already define a feedback mechanism for certain codecs. An application supporting both schemes MUST use the feedback mechanism defined in this specification when sending feedback. For backward compatibility reasons, such an application SHOULD also be capable to receive and react to the feedback scheme defined in the respective RTP payload format, if this is required by that payload format. The "SSRC of the packet sender" field indicates the source of the request, and the "SSRC of media source" is not used and SHALL be set Wenger, et al. Standards Track [Page 33] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 to 0. The SSRC of media sender to which the FIR command applies to is in the FCI. 4.3.1.2. Message Format Full Intra Request uses one additional FCI field, the content of which is depicted in Figure 12. The length of the FB message MUST be set to 2+2*N, where N is the number of FCI entries. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seq. nr | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11 - Syntax for the FIR message SSRC: The SSRC value of the target of this specific FIR command. Seq. nr: Command sequence number. The sequence number space is unique for each tuple consisting of the SSRC of command source and the SSRC of the command target. The sequence number SHALL be increased by 1 modulo 256 for each new command. A repetition SHALL NOT increase the sequence number. Initial value is arbitrary. Reserved: All bits SHALL be set to 0 and SHALL be ignored on reception. The semantics of this FB message is independent of the RTP payload type. 4.3.1.3. Timing Rules The timing follows the rules outlined in section 3 of [AVPF]. FIR commands MAY be used with early or immediate feedback. The FIR feedback message MAY be repeated. If using immediate feedback mode the repetition SHOULD wait at least on RTT before being sent. In early or regular RTCP mode the repetition is sent in the next regular RTCP packet. Wenger, et al. Standards Track [Page 34] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 4.3.1.4. Remarks FIR messages typically trigger the sending of full intra or IDR pictures. Both are several times larger then predicted (inter) pictures. Their size is independent of the time they are generated. In most environments, especially when employing bandwidth-limited links, the use of an intra picture implies an allowed delay that is a significant multitude of the typical frame duration. An example: If the sending frame rate is 10 fps, and an intra picture is assumed to be 10 times as big as an inter picture, then a full second of latency has to be accepted. In such an environment there is no need for a particular short delay in sending the FIR message. Hence waiting for the next possible time slot allowed by RTCP timing rules as per [AVPF] may not have an overly negative impact on the system performance. 4.3.2. Temporal-Spatial Trade-off Request (TSTR) The TSTR FB message is identified by PT=PSFB and FMT=5. There MUST be one or more TSTR entry contained in the FCI field. 4.3.2.1. Semantics A decoder can suggest the use of a temporal-spatial trade-off by sending a TSTR message to an encoder. If the encoder is capable of adjusting its temporal-spatial trade-off, it SHOULD take into account the received TSTR message for future coding of pictures. A value of 0 suggests a high spatial quality and a value of 31 suggests a high frame rate. The values from 0 to 31 indicate monotonically a desire for higher frame rate. Actual values do not correspond to precise values of spatial quality or frame rate. The reaction to the reception of more than one TSTR messages by a media sender from different media receivers, is left open to the implementation. The selected trade-off SHALL be communicated to the media receivers by the means of the TSTA message. The "SSRC of the packet sender" field indicates the source of the request, and the "SSRC of media source" is not used and SHALL be set to 0. The SSRC of media sender to which the TSTR applies to is in the FCI entries. A TSTR message may contain multiple requests to different media senders, using multiple FCI entries. 4.3.2.2. Message Format Wenger, et al. Standards Track [Page 35] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 The Temporal-Spatial Trade-off Request uses one FCI field, the content of which is depicted in Figure 12. The length of the FB message MUST be set to 2+2*N, where N is the number of FCI entries included. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seq nr. | Reserved | Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12 - Syntax of the TSTR SSRC: The SSRC value of the target of this specific TSTR request. Seq. nr: Request sequence number. The sequence number space is unique for each tuple consisting of the SSRC of request source and the SSRC of the request target. The sequence number SHALL be increased by 1 modulo 256 for each new command. A repetition SHALL NOT increase the sequence number. Initial value is arbitrary. Index: An integer value between 0 and 31 that indicates the relative trade off that is requested. An index value of 0 index highest possible spatial quality, while 31 indicates highest possible temporal resolution. Reserved: All bits SHALL be set to 0 and SHALL be ignored on reception. 4.3.2.3. Timing Rules The timing follows the rules outlined in section 3 of [AVPF]. This request message is not time critical and SHOULD be sent using regular RTCP timing. An exception being if the user interface requires fast feedback to present for the user. 4.3.2.4. Remarks The term "spatial quality" does not necessarily refer to the resolution, measured by the number of pixels the reconstructed video is using. In fact, in most scenarios the video resolution stays constant during the lifetime of a session. However, all video compression standards have means to adjust the spatial quality at a Wenger, et al. Standards Track [Page 36] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 given resolution, normally referred to as Quantizer Parameter or QP. A numerically low QP results in a good reconstructed picture quality, whereas a numerically high QP yields a coarse picture. The typical reaction of an encoder to this request is to change its rate control parameters to use a lower frame rate and a numerically lower (on average) QP, or vice versa. The precise mapping of Index, frame rate, and QP is intentionally left open here, as it depends on factors such as compression standard employed, spatial resolution, content, bit rate, and many more. 4.3.3. Temporal-Spatial Trade-off Announcement (TSTA) The TSTA FB message is identified by PT=PSFB and FMT=6. There SHALL be one or more TSTA contained in the FCI field. 4.3.3.1. Semantics This feedback message is used to acknowledge the reception of a TSTR. A TSTA entry in a TSTA feedback message SHALL be sent for each TSTR entry targeted to this session participant, i.e. each TSTR received that in the SSRC field in the entry has the receiving entities SSRC. The acknowledgement SHALL be sent also for repetitions received. If the request receiver has received TSTR with several different sequence numbers from a single requestor it SHALL only respond to the request with the highest (modulo 256) sequence number. The TSTA SHALL include the Temporal-Spatial Trade-off index that will be used as a result of the request. This is not necessarily the same index as requested, as media sender may need to aggregate requests from several requesting session participants. It may also have some other policies or rules that limit the selection. A single TSTA message MAY acknowledge multiple requests using multiple FCI entries. 4.3.3.2. Message Format The Temporal-Spatial Trade-off Announcement uses one additional FCI field, the content of which is depicted in Figure 13. The length of the FB message MUST be set to 2+2*N, where N is the number of FCI entries. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Wenger, et al. Standards Track [Page 37] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 | Seq nr. | Reserved | Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13 - Syntax of the TSTA SSRC: The SSRC of the source of the TMMBR request that is acknowledged. Seq. nr: The sequence number value from the TMMBR request that is being acknowledged. Index: The trade-off value the media sender is using henceforth. Reserved: All bits SHALL be set to 0 and SHALL be ignored on reception. Informative note: The returned trade-off value (Index) may differ from the requested one, for example in cases where a media encoder cannot tune its trade-off, or when pre-recorded content is used. 4.3.3.3. Timing Rules The timing follows the rules outlined in section 3 of [AVPF]. This acknowledgement message is not extremely time critical and SHOULD be sent using regular RTCP timing. 4.3.3.4. Remarks None 5. Congestion Control The correct application of the AVPF timing rules prevents the network flooding by feedback messages. Hence, assuming a correct implementation, the RTCP channel cannot break its bit-rate commitment and introduce congestion. The reception of some of the feedback messages modifies the behaviour of the media senders or, more specifically, the media encoders. All of these modifications MUST only be performed within the bandwidth limits the applied congestion control provides. For example, when reacting to a FIR, the unusually high number of packets that form the decoder refresh point have to be paced in compliance with the congestion control algorithm, even if the user experience suffers from a slowly transmitted decoder refresh point. Wenger, et al. Standards Track [Page 38] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 A change of the Temporary Maximum Media Bit-rate value can only mitigate congestion, but not cause congestion. An increase of the value by a request REQUIRES the media sender to use congestion control when increasing its transmission rate to that value. A reduction of the value results in a reduced transmission bit-rate thus reducing the risk for congestion. 6. Security Considerations The defined messages have certain properties that have security implications. These must be addressed and taken into account by users of this protocol. The defined setup signalling mechanism is sensitive to modification attacks that can result in session creation with sub-optimal configuration, and, in the worst case, session rejection. To prevent this type of attack, authentication and integrity protection of the setup signalling is required. Spoofed or maliciously created feedback messages of the type defined in this specification can have the following implications: a. Severely reduced media bit-rate due to false TMMBR messages that sets the maximum to a very low value. b. The assignment of the ownership of a bit-rate limit with a TMMBN message to the wrong participant. Thus potentially freezing the mechanism until a correct TMMBN message reached the participants. c. Sending TSTR that result in a video quality different from the user's desire, rendering the session less useful. d. Frequent FIR commands will potentially reduce the frame-rate making the video jerky due to the frequent usage of decoder refresh points. To prevent these attacks there is need to apply authentication and integrity protection of the feedback messages. This can be accomplished against group external threats using the RTP profile that combines SRTP [SRTP] and AVPF into SAVPF [SAVPF]. In the MCU cases separate security contexts and filtering can be applied between the MCU and the participants thus protecting other MCU users from a misbehaving participant. 7. SDP Definitions Section 4 of [AVPF] defines new SDP attributes that are used for the capability exchange of the AVPF commands and indications, like Reference Picture selection, Picture loss indication etc. The defined SDP attribute is known as rtcp-fb and its ABNF is described in section 4.2 of [AVPF]. In this section we extend the rtcp-fb attribute to include the commands and indications that are described Wenger, et al. Standards Track [Page 39] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 in this document for codec control protocol. We also discuss the Offer/Answer implications for the codec control commands and indications. 7.1. Extension of rtcp-fb attribute As described in [AVPF], the rtcp-fb attribute is defined to indicate the capability of using RTCP feedback. As defined in AVPF the rtcp-fb attribute must only be used as a media level attribute and must not be provided at session level. All the rules described in [AVPF] for rtcp-fb attribute relating to payload type, multiple rtcp-fb attributes in a session description hold for the new feedback messages for codec control defined in this document. The ABNF for rtcp-fb attributed as defined in [AVPF] is Rtcp-fb-syntax = "a=rtcp-fb: " rtcp-fb-pt SP rtcp-fb-val CRLF Where rtcp-fb-pt is the payload type and rtcp-fb-val defines the type of the feedback message such as ack, nack, trr-int and rtcp-fb-id. For example to indicate the support of feedback of picture loss indication, the sender declares the following in SDP v=0 o=alice 3203093520 3203093520 IN IP4 host.example.com s=Media with feedback t=0 0 c=IN IP4 host.example.com m=audio 49170 RTP/AVPF 98 a=rtpmap:98 H263-1998/90000 a=rtcp-fb:98 nack pli In this document we define a new feedback value type called "ccm" which indicates the support of codec control using RTCP feedback messages. The "ccm" feedback value should be used with parameters, which indicates the support of which codec commands the session would use. In this draft we define three parameters, which can be used with the ccm feedback value type. o "fir" indicates the support of Full Intra Request o "tmmbr" indicates the support of Temporal Maximum Media Bit-rate o "tstr" indicates the support of temporal spatial trade-off request. In ABNF for rtcp-fb-val defined in [AVPF], there is a placeholder called rtcp-fb-id to define new feedback types. The ccm is defined as a new feedback type in this document and the ABNF for the parameters Wenger, et al. Standards Track [Page 40] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 for ccm are defined here (please refer section 4.2 of [AVPF] for complete ABNF syntax). Rtcp-fb-param = SP "app" [SP byte-string] / SP rtcp-fb-ccm-param / ; empty rtcp-fb-ccm-param = "ccm" SP ccm-param ccm-param = "fir" ; Full Intra Request / "tmmbr" ; Temporary max media bit rate / "tstr" ; Temporal Spatial Trade Off / token [SP byte-string] ; for future commands/indications byte-string = 7.2. Offer-Answer The Offer/Answer [RFC3264] implications to codec control protocol feedback messages are similar to as described in [AVPF]. The offerer MAY indicate the capability to support selected codec commands and indications. The answerer MUST remove all ccm parameters, which it does not understand or does not wish to use in this particular media session. The answerer MUST NOT add new ccm parameters in addition to what has been offered. The answer is binding for the media session and both offerer and answerer MUST only use feedback messages negotiated in this way. 7.3. Examples Example 1: The following SDP describes a point-to-point video call with H.263 with the originator of the call declaring its capability to support codec control messages - fir, tstr. The SDP is carried in a high level signalling protocol like SIP v=0 o=alice 3203093520 3203093520 IN IP4 host.example.com s=Point-to-Point call c=IN IP4 172.11.1.124 m=audio 49170 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 51372 RTP/AVPF 98 a=rtpmap:98 H263-1998/90000 a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm fir Wenger, et al. Standards Track [Page 41] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 In the above example the sender when it receives a TSTR message from the remote party can adjust the trade off as indicated in the RTCP TSTA feedback message. Example 2: The following SDP describes a SIP end point joining a video MCU that is hosting a multiparty video conferencing session. The participant supports only the FIR (Full Intra Request) codec control command and it declares it in its session description. The video MCU can send an FIR RTCP feedback message to this end point when it needs to send this participants video to other participants of the conference. v=0 o=alice 3203093520 3203093520 IN IP4 host.example.com s=Multiparty Video Call c=IN IP4 172.11.1.124 m=audio 49170 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 51372 RTP/AVPF 98 a=rtpmap:98 H263-1998/90000 a=rtcp-fb:98 ccm fir When the video MCU decides to route the video of this participant it sends an RTCP FIR feedback message. Upon receiving this feedback message the end point is mandated to generate a full intra request. Example 3: The following example describes the Offer/Answer implications for the codec control messages. The Offerer wishes to support all the commands and indications of codec control messages. The offered SDP is -------------> Offer v=0 o=alice 3203093520 3203093520 IN IP4 host.example.com s=Offer/Answer c=IN IP4 172.11.1.124 m=audio 49170 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 51372 RTP/AVPF 98 a=rtpmap:98 H263-1998/90000 a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm fir a=rtcp-fb:98 ccm tmmbr The answerer only wishes to support FIR and TSTR message as the codec control messages and the answerer SDP is <---------------- Answer Wenger, et al. Standards Track [Page 42] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 v=0 o=alice 3203093520 3203093524 IN IP4 host.anywhere.com s=Offer/Answer c=IN IP4 189.13.1.37 m=audio 47190 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 53273 RTP/AVPF 98 a=rtpmap:98 H263-1998/90000 a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm fir 8. IANA Considerations The new value of ccm for the rtcp-fb attribute needs to be registered with IANA. Value name: ccm Long Name: Codec Control Commands and Indications Reference: RFC XXXX For use with "ccm" the following values also needs to be registered. Value name: fir Long name: Full Intra Request Command Usable with: ccm Reference: RFC XXXX Value name: tmmbr Long name: Temporary Maximum Media Bit-rate Usable with: ccm Reference: RFC XXXX Value name: tstr Long name: temporal Spatial Trade Off Usable with: ccm Reference: RFC XXXX 9. Acknowledgements The authors would like to thank Andrea Basso, Orit Levin, Nermeen Ismail for their work on the requirement and discussion draft [Basso]. Wenger, et al. Standards Track [Page 43] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 10. References 10.1. Normative references [AVPF] draft-ietf-avt-rtcp-feedback-11.txt [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. 10.2. Informative references [Basso] A. Basso, et. al., "Requirements for transport of video control commands", draft-basso-avt-videoconreq-02.txt, expired Internet Draft, October 2004. [AVC] Joint Video Team of ITU-T and ISO/IEC JTC 1, Draft ITU-T Recommendation and Final Draft International Standard of Joint Video Specification (ITU-T Rec. H.264 | ISO/IEC 14496-10 AVC), Joint Video Team (JVT) of ISO/IEC MPEG and ITU-T VCEG, JVT-G050, March 2003. [SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [Singer] D. Singer, "A general mechanism for RTP Header Extensions," draft-ietf-avt-rtp-hdrext-00, Aug 11, 2005. [RFC2032] Turletti, T. and C. Huitema, "RTP Payload Format for H.261 Video Streams", RFC 2032, October 1996. [SAVPF] J. Ott, E. Carrara, "Extended Secure RTP Profile for RTCP- based Feedback (RTP/SAVPF)," draft-ietf-avt-profile-savpf- 02.txt, July, 2005. [RFC3525] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, "Gateway Control Protocol Version 1", RFC 3525, June 2003. Any 3GPP document can be downloaded from the 3GPP web server, "http://www.3gpp.org/", see specifications. Wenger, et al. Standards Track [Page 44] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 11. Authors' Addresses Stephan Wenger Nokia Corporation P.O. Box 100 FIN-33721 Tampere FINLAND Phone: +358-50-486-0637 EMail: Stephan.Wenger@nokia.com Umesh Chandra Nokia Research Center 6000 Connection Drive Irving, Texas 75063 USA Phone: +1-972-894-6017 Email: Umesh.Chandra@nokia.com Magnus Westerlund Ericsson Research Ericsson AB SE-164 80 Stockholm, SWEDEN Phone: +46 8 7190000 EMail: magnus.westerlund@ericsson.com Bo Burman Ericsson Research Ericsson AB SE-164 80 Stockholm, SWEDEN Phone: +46 8 7190000 EMail: bo.burman@ericsson.com 12. List of Changes relative to previous drafts The following changes since draft-wenger-avt-avpf-ccm-01 have been made: - The topologies have been rewritten and clarified. - The TMMBR mechanism has been completely revised to use notification and suppress messages in deployments with large common SSRC spaces. The following changes since draft-wenger-avt-avpf-ccm-02 have been made: Wenger, et al. Standards Track [Page 45] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 - Update of section 4.2.2.1 (TMMBN) as per discussions between Harikishan Desineni and Magnus Westerlund on the AVT list around Feb 21, 2006 - Section 2.3.4 clarified as per email exchange between Colin Perkins and Magnus Westerlund around Feb 24 - Section 3.5.2 and other occurrences throughout the draft, Temporal/Spatial Acknowledgement renamed to Temporal/Spatial Annoucement Full Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Wenger, et al. Standards Track [Page 46] INTERNET-DRAFT AVPF RTCP-RR Extensions March 6, 2006 Funding for the RFC Editor function is currently provided by the Internet Society. RFC Editor Considerations The RFC editor is requested to replace all occurrences of XXXX with the RFC number this document receives. Wenger, et al. Standards Track [Page 47]