Network Working Group Stephan Wenger INTERNET-DRAFT Umesh Chandra Expires: April 2006 Nokia Magnus Westerlund Bo Burman Ericsson October 24, 2005 Codec Control Messages in the Audio-Visual Profile with Feedback (AVPF) draft-wenger-avt-avpf-ccm-01.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 (2005). 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 Tradeoff. Wenger, Chandra, Westerlund [Page 1] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 relaying MCU................8 2.3.4. Point to Multipoint using content modifying MCU.......9 2.3.5. Combining Topologies..................................9 3. Motivation (Informative).......................................10 3.1. Use Cases.................................................10 3.2. Using the Media Path......................................12 3.3. Using AVPF................................................12 3.3.1. Reliability..........................................13 3.4. Multicast.................................................13 3.5. Feedback Messages.........................................13 3.5.1. Full Intra Request Command...........................13 3.5.1.1. Reliability.....................................14 3.5.2. Freeze Request Indication............................14 3.5.3. Temporal Spatial Tradeoff Request and Acknowledgement 15 3.5.3.1. Point-to-point..................................16 3.5.3.2. Point-to-Multipoint using multicast or relaying MCU16 3.5.3.3. Point-to-Multipoint using content modifying MCU.17 3.5.3.4. Reliability.....................................17 3.5.4. Temporary Maximum Media Bit-rate Request and Acknowledgement ............................................................17 3.5.4.1. MCU based Multi-point operation.................18 3.5.4.2. Point-to-Multipoint using Multicast or relaying MCU20 3.5.4.3. Point-to-point operation........................20 3.5.4.4. Reliability.....................................20 4. RTCP Receiver Report Extensions................................20 4.1. Design Principles of the Extension Mechanism..............21 4.2. Transport Layer Feedback Messages.........................21 4.2.1. Temporary Maximum Media Bit-rate Request (TMMBR).....22 4.2.1.1. Semantics.......................................22 4.2.1.2. Message Format..................................23 4.2.1.3. Timing Rules....................................24 4.2.2. Temporary Maximum Media Bit-rate Acknowledgement (TMMBA) 24 4.2.2.1. Semantics.......................................24 4.2.2.2. Message Format..................................25 4.2.2.3. Timing Rules....................................25 4.3. Payload Specific Feedback Messages........................25 4.3.1. Full Intra Request (FIR).............................26 4.3.1.1. Semantics.......................................26 4.3.1.2. Message Format..................................28 4.3.1.3. Timing Rules....................................28 4.3.1.4. Remarks.........................................29 Wenger, Chandra, Westerlund Standards Track [Page 2] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 4.3.2. Temporal-Spatial Tradeoff Request (TSTR).............29 4.3.2.1. Semantics.......................................29 4.3.2.2. Message Format..................................29 4.3.2.3. Timing Rules....................................30 4.3.2.4. Remarks.........................................30 4.3.3. Temporal-Spatial Tradeoff Acknowledgement (TSTA).....31 4.3.3.1. Semantics.......................................31 4.3.3.2. Message Format..................................31 4.3.3.3. Timing Rules....................................32 4.3.3.4. Remarks.........................................32 4.3.4. Freeze Indication..........Error! Bookmark not defined. 4.3.4.1. Semantics.............Error! Bookmark not defined. 4.3.4.2. Message Format........Error! Bookmark not defined. 4.3.4.3. Timing Rules..........Error! Bookmark not defined. 4.3.4.4. Remarks...............Error! Bookmark not defined. 5. Congestion Control.............................................32 6. Security Considerations........................................32 7. SDP Definitions................................................33 7.1. Extension of rtcp-fb attribute............................33 7.2. Offer-Answer..............................................34 7.3. Examples..................................................35 8. IANA Considerations............................................36 9. Open Issues....................................................37 10. Acknowledgements..............................................37 11. References....................................................38 11.1. Normative references.....................................38 11.2. Informative references...................................38 12. Authors' Addresses............................................38 12. List of Changes relative to previous draft....................39 13................................................................39 Wenger, Chandra, Westerlund Standards Track [Page 3] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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, Chandra, Westerlund Standards Track [Page 4] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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. The message is acknowledged by its receiver. 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 Tradeoff 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 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 TMMBA - Temporary Maximum Media Bit-rate Acknowledgement TMMBR - Temporary Maximum Media Bit-rate Request PLI - Picture Loss Indication TSTA - Temporal Spatial Tradeoff Acknowledgement TSTR - Temporal Spatial Tradeoff Request 2.2. Terminology 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]. Wenger, Chandra, Westerlund Standards Track [Page 5] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 Note that 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 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. Wenger, Chandra, Westerlund Standards Track [Page 6] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 four basic topologies that are relevant for codec control. Further topologies can be constructed by combining them, see Section 2.3.5. 2.3.1. Point to Point The Point to Point (PtP) topology (Figure 1) is the simplest and consists of two end-points with unicast capabilities between them. +---+ +---+ | A |<------->| B | +---+ +---+ Figure 1 - Point to Point The main properties of this topology is that A send to B and only B, while B send to A and only A. This avoids all complexities of handling multiple participants and combining the requirements from them. Wenger, Chandra, Westerlund Standards Track [Page 7] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 2.3.2. Point to Multi-point using Multicast +-----+ +---+ / \ +---+ | A |--- / \---| B | +---+ / Multi- \ +---+ + Cast + +---+ \ Network / +---+ | C |----\ /---| D | +---+ \ / +---+ +-----+ Figure 2 - Point to Multipoint using Multicast The Point to Multipoint (PtM) using multicast topology is defined as the transmission from any participant to reach all the other participants (unless packet loss occurs). The number of participants can be one or many. However 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 relaying MCU +---+ +------------+ +---+ | A |------| Multipoint |------| B | +---+ | Control | +---+ | Unit | +---+ | (MCU) | +---+ | C |------| |------| D | +---+ +------------+ +---+ Figure 3 - Point to Multipoint using relaying MCU The PtM using relaying MCU is defined such that each participant uses unicast traffic between itself and the MCU. The MCU relays that traffic to all other participants. This relaying is performed for all media traffic and RTCP control traffic. However, the MCU may also originate RTCP control traffic to control the session or report on status as it sees it. In this usage the codec control messages are conveyed transparently to the media-transmitting participant for handling. The MCU does not, by itself, take action on the control messages it relayes. Wenger, Chandra, Westerlund Standards Track [Page 8] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 2.3.4. Point to Multipoint using content modifying MCU +---+ +------------+ +---+ | A |<---->| Multipoint |<---->| B | +---+ | Control | +---+ | Unit | +---+ | (MCU) | +---+ | C |<---->| |<---->| D | +---+ +------------+ +---+ Figure 4 - Point to Multipoint using content modifying MCU In a PtM scenario using a content modifying MCU, each participant runs a 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. 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; however it still indicates the original sender of the content. 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 indicated through the RTP CSRC field. In both scenarios, 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. Mixing forms of the two scenarios are possible. 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. 2.3.5. Combining Topologies An MCU can be used to combine the different topologies depending on what is most suitable or possible. Different combinations that are possible (non exhaustive): - Employing a relaying MCU to allow participants without multicast capabilities to join a PtM Multicast session. The MCU joins the Wenger, Chandra, Westerlund Standards Track [Page 9] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 multicast group as one participant of the multicast group. The MCU relays all traffic received on the multicast address to the participant using unicast. It also forwards the unicast participant's traffic to the multicast group. - Utilizing an MCU session that employs both transcoding and mixing, depending on participant and network capabilities. Due to limited bandwidth, processing capabilities, etc, the MCU can perform transcoding of content to what is suitable for some participants, while others receive unmodified, relayed traffic. 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. 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 tradeoff in temporal/spatial resolution. For example, one user may prefer a higher frame rate and a lower Wenger, Chandra, Westerlund Standards Track [Page 10] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 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: Wenger, Chandra, Westerlund Standards Track [Page 11] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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). Secondly, the signalling path quite commonly contains several signalling entities, e.g. SIP-proxies and application servers. 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. Wenger, Chandra, Westerlund Standards Track [Page 12] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 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 analogon applies. 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. Wenger, Chandra, Westerlund Standards Track [Page 13] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 Usage in multicast is possible; however aggregation of the commands is recommended. A receiver that receives a request closely (within 2*RTT) 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. 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. Therfore, 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 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. 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. A content modifying MCU that receives an FIR from a media receiver is responsible to ensure that a decoder refresh point is delivered to the requesting receiver. Due to a participants request it may be necessary for the MCU to generate FIR commands itself. These two legs are handled independently of each other from a reliability perspective. 3.5.2. Freeze Request Indication The Freeze Request Indication instructs the video decoder to complete the decoding of the current video picture and subsequently display it until either a timeout period has elapsed, or until the reception of a signal (in band in the video stream) that indicates the release of the frozen picture. Note that a freeze picture release signal is part of the at least the H.261, H.263 and H.264 video coding Wenger, Chandra, Westerlund Standards Track [Page 14] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 specifications. Coding schemes that support picture freeze release in their bitstreams are required to use freeze release to signal the remote end to resume decoding. Historically, the freeze indication has been used in MCUs according to use case 5. Nowadays, most MCUs operate media aware and simply stop sending media data of the old stream, at a defined picture boundary. The new stream is spliced in at a decoder refresh point. Hence, for modern MCUs, the Freeze indication is of much less relevance. However, a mechanism known as gradual decoder refresh may make the Freeze indication attractive again. Using a gradual decoder refresh, a new user can join a conference by listening in to a sequence of pictures (spanning a perhaps a second of video), which are guaranteed to gradually refresh for a complete reference picture. The associated problems in the video encoding are non-trivial, but solvable, and applications exist where they have been solved successfully. In order to shield the user from the slow and annoying gradual built-up of the picture, a stop of the rendering is desirable. The freeze picture indication can serve for this purpose (although other, more complex means (that may involve control protocols) may also be available. Usage of RTCP feedback messages for indication of Freeze Request Indication has one substantial issue. The late delivery of a Freeze request will usually result in annoying picture artifacts that will remain in a frozen picture until freeze release happens. Ideally, the freeze indication requires synchronous delivery with the media data. The only obvious solution we found (apart from pushing the problem to the media coding standardization) appears to somehow splice the freeze request into the forward media stream. Such a possibility exist using header extensions [Singer]. Due to isssue with performing Freeze Request in RTCP and the possibility to perform it in the media path it will not be specified in this document. 3.5.3. Temporal Spatial Tradeoff Request and Acknowledgement The Temporal Spatial Tradeoff 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 tradeoff that is used henceforth. Wenger, Chandra, Westerlund Standards Track [Page 15] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 tradeoff 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 tradeoff 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 tradeoff values for the same stream and/or endpoint encoder. This memo does not specify a Mixer's or endpoint's behaviour to the suggested tradeoff 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 tradeoff chosen by its own criteria (which may or may not be based on the tradeoff conveyed by TSTR). In other words, the tradeoff 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. 3.5.3.1. Point-to-point In this most trivial case, the media sender typically adjusts its temporal/spatial tradeoff based on the requested value in TSTR, and within its capabilities. The TSTA message conveys back the new tradeoff value (which may be identical to the old one if, for example, the sender is not capable to adjust its tradeoff). 3.5.3.2. Point-to-Multipoint using multicast or relaying MCU The problem here lies in the fact that TSTR messages from different receivers may be received unsynchronized, and possibly with different requested tradeoffs (because of different user preferences). It is not specified here, and open to the implementation, how the media sender is tuning its tradeoff. One possible strategy would be to select the mean, or median, of all tradeoff requests received. Another would be to prioritize certain participants, for example Wenger, Chandra, Westerlund Standards Track [Page 16] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 session moderators, and hence their input treated as of higher importance. Again, all TSTR messages need to be acknowledged by TSTA, and the value conveyed back has to reflect the decision made. 3.5.3.3. Point-to-Multipoint using content modifying MCU In this scenario the MCU receives the TSTR message from a participant. As the MCU can receive multiple requests from different participants, it needs to determine the future tradeoff for the whole session. This can be implemented in several ways, e.g. by averging the participants requests, prioritizing certain participants, or use session default values. If the MCU changes its tradeoff, 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 tradeoff it includes that value in the acknowledgement. Even if a MCU performs transcoding, it is very difficult to deliver media with the requested tradeoff, unless the content the MCU receives is already close to that tradeoff. Only in cases where the original source has substantially higher bit-rate, it is likely that transcoding can result in requested trade-off. 3.5.3.4. Reliability A request and reception acknowledgement mechanism is specified. The Temporal Spatial Tradeoff Acknowledge (TSTA) message informs the request-sender that its request has been received, and what tradeoff is used henceforth. This acknowledgment mechanism is desirable for at least the following reasons: o A change in the tradeoff cannot be directly identified from the media bit stream, o User feedback cannot be implemented without information of the chosen tradeoff value, according to the media sender's constraints, o Repetitive sending of messages requesting an unimplementable tradeoff can be avoided. 3.5.4. Temporary Maximum Media Bit-rate Request and Acknowledgement A receiver or MCU 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 (relaying MCU) and 2.3.4 (content modifying MCU), 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. Wenger, Chandra, Westerlund Standards Track [Page 17] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 relaying MCUs. It may also be local between the participant and the MCU, in case of content modifying MCUs. 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), 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 goverend by a congesiton control algorithm. TMMBR only indicates known limitations, usually in the local environement, and does not provide any guarantees. When 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. 3.5.4.1. MCU based Multi-point operation Assume a small multipart conference is ongoing, as depicted in Figure 3 of 2.3.3 or Figure 4 of 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 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 Wenger, Chandra, Westerlund Standards Track [Page 18] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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. 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 content modifying MCU, the RTCP RR is being sent between the RTP receiver in the endpoint and the RTP sender in the MCU 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 MCU. This endpoint doesn't see the aforementioned RTCP RRs, and hence needs explicitly informed about desired bandwidth adjustments. In this topology it is the MCU's responsibility to aggregate the different bit-rates, which the different links may support, into the bit rate requested. This aggregation may also take into account that the MCU may contain certain transcoding capabilities (as in 2.3.4), which can be employed for those few of the session participants that have the lowest available bit-rates. It is the MCU's responsibility to take into consideration the multiple max media bit rates, which it learns from the receivers, and select the lowest of those bit rate values. The MCU may also support certain transcoding capabilities, which can be employed for some of the receivers so as not to reduce Wenger, Chandra, Westerlund Standards Track [Page 19] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 the conference bit rate to a lowest common denominator, which would affect the user experience of all users. 3.5.4.2. Point-to-Multipoint using Multicast or relaying MCU 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 3.5.4.1 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). 3.5.4.3. 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 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. 3.5.4.4. Reliability A request and reception acknowledgement mechanism is required. Temporary Maximum Media Bit-rate Acknowledgement (TMMBA) is used to allow the TMMBR sender to know that the recipient has received the request. This is desirable behaviour as the result of TMMBR is not immediately identifiable through inspection of the media stream. Unless acknowledged, it can be expected that multiple TMMBR will be sent in an attempt to limit the probability of congestion and degraded media quality. 4. RTCP Receiver Report Extensions This memo specifies 5 new feedback messages. The Full Intra Request (FIR), Temporal-Spatial Tradeoff Request (TSTR), and Temporal-Spatial Tradeoff Acknowledgement (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 Acknowledgement (TMMBA) 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. Wenger, Chandra, Westerlund Standards Track [Page 20] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 two 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, that 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 an acknowledgement due to their nature, which is motivated separately for each message b) the acknowledgement cannot be easily derived from the media bit stream. 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 Wenger, Chandra, Westerlund Standards Track [Page 21] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 Acknowledgement 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 MUST 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 discussions. 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 relay MCU topologies. The above mentioned requirement implies that a receiver desiring to set a maximum bit-rate to all active media sender must address them all individually (which can be done in a single or in multiple TMMBR requests). A TMMBR message MAY be repeated if no TMMBA has been received at the time of transmission of the next RTCP packet. A repeated TMMBR request SHALL NOT change any of the SSRC or FCI fields of the request relative to the first transmission with the same sequence number. A TMMBR sender MAY change a value of the request prior to receving a TMMBA, however, in this case it SHALL increment the sequence number. Please note that the media sender's state is now undetermined in regards to the set maximum bit-rate until a TMMBA is received at the media receiver. Wenger, Chandra, Westerlund Standards Track [Page 22] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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. However the need for TMMBR in multicast and relaying MCU usage is limited and the operation is not optimized for these cases. If multiple maximum bit-rates are set by different media recievers in a given session, where the media is common to all the receivers (for example multicast), then the sender SHOULD set its sending bit rate to the lowest value received. For sessions with a larger number of participants using the lowest common denominator 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 partioning 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) consist 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seq. nr | Maximum bit-rate in units of 128 bits/s | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5 - Syntax for the TMMBR message SSRC: The SSRC value of the target of this specific maximum bit- rate 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 Wenger, Chandra, Westerlund Standards Track [Page 23] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 request. A repetition SHALL NOT increase the sequence number. Initial value is arbitary. Maximum bit-rate: The temporary maximum media bit-rate value in units of 128 bit/s. This provides range from 0 to 2147483647 bits/s (~2.15 Gbit/s) with a resolution 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. 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 Acknowledgement (TMMBA) The FCI field of the TMMBA Feedback message SHALL contain one or more TMMBA FCI entries. 4.2.2.1. Semantics This feedback message is used to acknowledge the reception of a TMMBR. It SHALL be sent for each TMMBR targeted to this receiver, i.e. for each TMMBR received in which the "SSRC" field in a TMMBR FCI entry is identical to the receiving entities SSRC. The acknowledgement SHALL be sent also for any recevied request, even if the request is repeated. If each recevied request didn't generate a acknowledgement then no reliability against losses of acknowledgement would exist. The TMMBA feedback message's "SSRC of packet sender" SHALL be set to the SSRC of the acknowledger. The "SSRC of media source" is not used and SHALL be set to 0. The receiver of TMMBR messages can acknowledge one or more TMMBR message in the same TMMBA feedback message. The FCI entry's SSRC field identifies the sender of the TMMBR requests, and the sequence number identifies which particular request, that is being acknowledged. The media sender SHALL acknowledge only the highest sequence number (modulo 256) if serveral TMMBR request with different sequence numbers has been received from the same SSRC. Wenger, Chandra, Westerlund Standards Track [Page 24] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 4.2.2.2. Message Format The TMMBA 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seq. nr | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6 - Syntax for the TMMBA message 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. Reserved: All bits SHALL be set to 0 and SHALL be ignored on reception. The length field value of the FB message SHALL be 2+2*N, where N is the number of TMMBA FCI entries. 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 MAY 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) Wenger, Chandra, Westerlund Standards Track [Page 25] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 4: Full Intra Request Command (FIR) 5: Temporal-Spatial Tradeoff Request (TSTR) 6: Temporal-Spatial Tradeoff Acknowledgement (TSTA) 7-14: unassigned 15: Application layer FB message 16-30: unassigned 31: reserved for future expansion of the sequence number space 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. Wenger, Chandra, Westerlund Standards Track [Page 26] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 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 any considerations such as the ones mentioned above into account. 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 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, 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 Wenger, Chandra, Westerlund Standards Track [Page 27] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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. 4.3.1.2. Message Format Full Intra Request uses one additional FCI field, the content of which is depicted in Figure 8. The length of the FB message MUST be set to 3. 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 7 - 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 arbitary. 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, Chandra, Westerlund Standards Track [Page 28] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 Tradeoff 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 tradeoff by sending a TSTR message to an encoder. If the encoder is capable of adjusting its temporal-spatial tradeoff, it SHOULD take the received TSTR message into account for future coded 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, but with identical SSRC and sequence numbers, is left open to the implementation. The selected tradeoff SHALL be communicated to the media receivers by the means of the TSTA message. 4.3.2.2. Message Format The Temporal-Spatial Tradeoff Request uses one additional FCI field, the content of which is depicted in Figure 8. The length of the FB message MUST be set to 3. 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 Wenger, Chandra, Westerlund Standards Track [Page 29] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seq nr. | | Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8 - 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 arbitary. 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. 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. 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 will likely stay constant during the lifetime of a session. However, all video compression standards have means to adjust the spatial quality at a 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. Wenger, Chandra, Westerlund Standards Track [Page 30] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 4.3.3. Temporal-Spatial Tradeoff Acknowledgement (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 receiver, 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 Tradeoff index that will be used as a result of the request. This is not necessary 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 limits the selection. 4.3.3.2. Message Format The Temporal-Spatial Tradeoff Acknowledgement uses one additional FCI field, the content of which is depicted in Figure 9. The length of the FB message MUST be set to 3. 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. | | Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9 - 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 tradeoff value the media sender is using henceforth. Wenger, Chandra, Westerlund Standards Track [Page 31] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 Informative note: The returned tradeoff value (Index) may differ from the requested one, for example in cases where a media encoder cannot tune its tradeoff, 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. A change of the Temporary Maximum Media Bit-rate value can only mitigate congestion, but not cause congestion. An increase of the value REQUIRES that the value is chosen such that any transmission up to that value is allowed by the used congestion control mechanism, at the time of sending. A reduction of the value may result 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 Wenger, Chandra, Westerlund Standards Track [Page 32] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 this type of attack, authentication and integrity protection of the setup signalling is required. Spoofing of feedback messages 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. Sending TSTR that result in a video quality different from the user's desire, rendering the session less useful. c. 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 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 Wenger, Chandra, Westerlund Standards Track [Page 33] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 "ccci" which indicates the support of codec control commands using RTCP feedback messages. The "ccci" 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 ccci 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 tradeoff request. In ABNF for rtcp-fb-val defined in [AVPF], there is a placeholder called rtcp-fb-id to define new feedback types. The ccci is defined as a new feedback type in this document and the ABNF for the parameters for ccci 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-ccci-param / ; empty rtcp-fb-ccci-param = "ccci" SP ccci-param ccci-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 Wenger, Chandra, Westerlund Standards Track [Page 34] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 ccci parameters, which it does not understand or does not wish to use in this particular media session. The answerer MUST NOT add new ccci 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 commands and indications - 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 ccci tstr a=rtcp-fb:98 ccci fir 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 Wenger, Chandra, Westerlund Standards Track [Page 35] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 a=rtcp-fb:98 ccci 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 ccci tstr a=rtcp-fb:98 ccci fir a=rtcp-fb:98 ccci tmmbr The answerer only wishes to support FIR and TSTO message as the codec control messages and the answerer SDP is <---------------- Answer 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 ccci tstr a=rtcp-fb:98 ccci fir 8. IANA Considerations The new value of ccci for the rtcp-fb attribute needs to be registered with IANA. Value name: ccci Long Name: Codec Control Commands and Indications Reference: RFC XXXX Wenger, Chandra, Westerlund Standards Track [Page 36] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 For use with ''ccci'' the following values also needs to be registered. Value name: fir Long name: Full Intra Request Command Usable with: ccci Reference: RFC XXXX Value name: tmmbr Long name: Temporary Maximum Media Bit-rate Usable with: ccci Reference: RFC XXXX Value name: tstr Long name: temporal Spatial Trade Off Usable with: ccci Reference: RFC XXXX 9. Open Issues As this draft is under development, certain open issues are to be resolved. Please provide feedback on the following open issues: 1. For the TSTA, should it be possible to indicate both semantic positive (will take it into account) and negative (request received but will ignore it) acknowledgement? OR should support from an end-point only be negotiated at session setup time? 2. How strict transmission rules should different messages have? For example should the acknowledgement have to be sent using early or immediate feedback if availalbe? Or is regular RTCP timing sufficient? 3. "Dave Singer expressed concern that repeating requests does not always work; might want a method to stop a receiver making repeated requests to a sender that cannot satisfy them. " Is this still an issue with the current definitions? 4. TMMA: relay back "chosen" maximum bit rate? Could be helpful for resource management in receiver. 10. Acknowledgements The authors would like to thank Andrea Basso, Orit Levin, Nermeen Ismail for their work on the requirement and discussion draft [Basso]. Wenger, Chandra, Westerlund Standards Track [Page 37] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 11. References 11.1. 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. 11.2. 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. Any 3GPP document can be downloaded from the 3GPP webserver, "http://www.3gpp.org/", see specifications. 12. Authors' Addresses Stephan Wenger Nokia Corporation Wenger, Chandra, Westerlund Standards Track [Page 38] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 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 13. List of Changes relative to previous draft The following changes since draft version 00 has been made: - The draft is restructured to remove redundancy in text. The motivation has been cleaned up and should be easier to read. - Freeze picture has been been removed from this draft for separate developement if interest exist. - Added a section on the usage scenarios (topologies) considered in the document. - All message formats has been restructured to allow several targets in a single message for better efficiency when multiple media senders needs to be sent requests or commands. - Added "semantic Ack" to the acknowledgement messages Full Copyright Statement Wenger, Chandra, Westerlund Standards Track [Page 39] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 Copyright (C) The Internet Society (2005). 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 Funding for the RFC Editor function is currently provided by the Internet Society. Wenger, Chandra, Westerlund Standards Track [Page 40] INTERNET-DRAFT AVPF RTCP-RR Extensions July 11, 2005 RFC Editor Considerations The RFC editor is requested to replace all occurrences of XXXX with the RFC number this document receives. Wenger, Chandra, Westerlund Standards Track [Page 41]