Network Working Group Q. Wu Internet-Draft F. Xia Intended status: Standards Track R. Even Expires: April 23, 2011 Huawei October 20, 2010 RTCP Report Extension for Feedback Suppression draft-wu-avt-retransmission-supression-rtp-05 Abstract When packet loss close to the media source or intermediary of the session is detected as a loss by a large number of receivers, large number of feedback requests used to ask for the lost RTP packets are all addressed to the same media source, or a designated feedback target. This may result in a phenomenon known variously as a "feedback storm " or "feedback implosion ". This document specifies an extension to the RTCP feedback messages defined in the Audio-Visual Profile with Feedback (AVPF). This extension allows an intermediate node in the network (such as an RTP translator) inform downstream receivers that packet loss was detected and sending a feedback message concerning a specified set of RTP packets may be unnecessary, or even harmful. Receivers respond to receipt of a feedback suppression message by not sending a feedback message (e.g. a NACK) that they otherwise would, This in turn reduces load on both the feedback target and on the network. The proposed extension is useful for single-source multicast sessions. In addition, it can be applied to any other types of RTP sessions and topologies that might benefit from feedback suppression mechanism. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on April 23, 2011. Wu, et al. Expires April 23, 2011 [Page 1] Internet-Draft Feedback Suppression October 2010 Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Wu, et al. Expires April 23, 2011 [Page 2] Internet-Draft Feedback Suppression October 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5 4. RTCP Feedback Report Extension . . . . . . . . . . . . . . . . 7 4.1. Transport Layer Feedback: NACK Suppression Report . . . . 7 4.2. Payload Specific Feedback: FIR suppression report . . . . 8 5. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Example Use Cases . . . . . . . . . . . . . . . . . . . . . . 9 6.1. Source Specific Multicast (SSM) use case . . . . . . . . . 10 6.1.1. Simple Feedback Model . . . . . . . . . . . . . . . . 11 6.1.2. Distribution Source Feedback Summary Model . . . . . . 12 6.2. Unicast based Rapid Acquisition of Multicast Stream (RAMS) use case . . . . . . . . . . . . . . . . . . . . . 13 6.3. RTP transport translator use case . . . . . . . . . . . . 13 6.4. Multipoint Control Unit (MCU) use case . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 8. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 15 9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 16 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 10.1. Normative References . . . . . . . . . . . . . . . . . . . 16 10.2. Informative References . . . . . . . . . . . . . . . . . . 17 Appendix A. Example scenarios for NACK Implosion . . . . . . . . 17 A.1. Scenario 1: One or more media source, One distribution source . . . . . . . . . . . . . . . . . . . . . . . . . . 17 A.2. Scenario 2:One media source, Two distribution sources in cascade . . . . . . . . . . . . . . . . . . . . . . . . 18 A.3. Scenario 3:One media source, Two distribution sources in parallel . . . . . . . . . . . . . . . . . . . . . . . 18 Appendix B. Applicability of Feedback Suppression . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 Wu, et al. Expires April 23, 2011 [Page 3] Internet-Draft Feedback Suppression October 2010 1. Introduction RTCP feedback messages [RFC4585]allow the receivers in an RTP session to report events and ask for action from the media source (or a delegated feedback target). There are cases where multiple receivers may initiate the same, or an equivalent message towards the same media source. When the receiver count is large, this behavior may overload the media source or the network or both. One common case is receivers utilizing RTP retransmission as a packet loss recovery technique in a real-time application such as streaming video or audio.[RFC4585]Feedback is accomplished using the RTCP NACK message which conveys the RTP sequence number(s) of the lost packet(s). This information can then be used by the media or distribution source to retransmit the missing RTP packets using the RTP retransmission payload format[RFC4588]. However, in topologies utilizing transport translators (Topo-Trn- Translator) or large-scale multicast transmission (Topo-Multicast) as defined in [RFC5117], packet loss can occur on either an upstream link or a downstream aggregate link of the intermediate network element (e.g., Distribution Source, MCU, Transport Translator). Where there are many receivers, this may result in a Feedback implosion [RFC5740] towards the media or distribution source, i.e., large number of feedback requests to the same multicast sender for retransmission of the same RTP packets. This phenomenon goes by a number of alternate names, such as the "feedback storm" or the "NACK storm" terminology of [DVB-IPTV]. In an attempt to increase its robustness against the loss of a feedback message or of retransmission packets, a receiver may send multiple feedbacks for the same detected packet loss, which may aggravate the feedback implosion. Another use case involves video Fast Update requests. A storm of these feedback messages can occur in conversational multimedia scenarios like Topo-Video-switch-MCU [RFC5117]. In this scenario, packet loss may happen on an upstream link of an intermediate network element such as a Multipoint Control Unit(MCU). Receivers missing the packets issue fast update requests (i.e., Full Intra Request(FIR) described in [RFC5104]), which results in an implosion of FIR requests from receivers to the same media source. As these feedback storms propagate (e.g., NACK implosion or Fast update implosion), the network may be permeated with more and more feedback traffic, resulting in a positive feedback loop as the network is also saturated with media traffic. RTCP feedback storms may cause short term overload and, and in extreme cases to pose a possible risk of increasing network congestion on the control channel (e.g. RTCP feedback), the data channel (i.e. RTP retransmission), Wu, et al. Expires April 23, 2011 [Page 4] Internet-Draft Feedback Suppression October 2010 or Both if the receivers are not correctly implemented In order to mitigate these behaviors, the current text in [RFC5760] allows the distribution source to filter out the NACK messages and [DVB-IPTV] suggests sending a NACK message from server to the client (or receiver). However NACK is defined as a receiver report sent from a client to the server and therefore exhibits a semantic mismatch when used as a suppression indication from the server (or intermediary) to the client. This document instead specifies a newly message for this function. It further is more precise in the intended uses and less likely to be confusing to receivers. It tells receivers explicitly that feedback for a particular packet loss is not needed and can provide an early indication before the receiver reacts to the loss and invokes its packet loss repair machinery. Receivers respond to receipt of a feedback suppression message by not sending a feedback message (e.g. a NACK) that they otherwise would, This in turn reduces load on both the feedback target and on the network. Also the intermediate node may initiate its own feedback toward the media source to provoke a retransmission. When the media source receives the request from the intermediate node, the media source resends the lost packets to the receivers by using the RTP retransmission payload format [RFC4588] or resends a new refresh point for FIR Initiator [RFC5104], depending on the type of feedback it received. 2. Terminology The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Protocol Overview This document extends the RTCP feedback messages defined in the Audio-Visual Profile with Feedback (AVPF) and define the Feedback Suppression message. The Feedback Suppression message asks a receiver to not send feedback messages for particular packets (indicated by their RTP sequence numbers) independent of whether the receiver detected the packet loss or detected a need for a decoder refresh point). In order to detect packet loss before the receivers perceive it, one or more intermediate nodes are placed between the media source and Wu, et al. Expires April 23, 2011 [Page 5] Internet-Draft Feedback Suppression October 2010 receiver (e.g., Distribution server, MCU, RTP translator). These intermediaries monitor for packet loss upstream of themselves by checking RTP sequence numbers, just as receivers do. Upon detecting (or suspecting) an upstream loss, the intermediary may send Feedback Suppression message towards the receivers as defined in this specification. These intermediate nodes need to take into account such factors as the tolerable application delay, the network dynamics, and the media type. When the packet loss is detected upstream of the intermediary and additional latency is tolerable, the intermediate node may itself send a feedback message asking for the suspected lost packet or ask for the correct decoder refresh point. Because it has already provided the necessary feedback toward the source, the intermediate node can be reasonably certain that it will help the situation by sending a Feedback Suppression message to all the relevant receivers, thereby indicating that the receivers should not themselves transmit feedback messages. When the receiver receives such Feedback Suppression message, if the receiver understands this message it will not send packet loss request (e.g., NACK) for the missing packets reported in the message. If it did not understand this new message, the host MAY send packet loss request(i.e., Feedback messages) to the specified media source. A receiver may still have sent a Feedback message before receiving a feedback suppression message, but further feedback messages for those sequence numbers will be suppressed by this technique. RTCP Feedback Suppression follows the same semantic model as RTCP NACK - it conveys the packet receipt/loss events at the sequence number level and considers missing packets as unrepaired. But unlike RTCP NACK, the Feedback Suppression messages can be generated at intermediate nodes who are not RTP receivers and sent to the corresponding receivers. Intermediaries downstream of an intermediary detecting loss obviously SHOULD NOT initiate their own additional feedback suppression messages for the same packet sequence numbers. They may either simply forward the Feedback Suppression message received from upstream, or augment (or replace) it with a feedback suppression message that reflects the loss pattern they have themselves seen. Since feedback suppression interacts strongly with repair timing, it has to work together with feedback to not adversely impact the repair of lost source packets. In some cases where the loss was detected and repair initiated much closer to the source, the delay for the receiver to recover from packet loss can be reduced through the combination of intermediary feedback to the source and feedback suppression downstream. In all (properly operating) cases, the risk Wu, et al. Expires April 23, 2011 [Page 6] Internet-Draft Feedback Suppression October 2010 of increasing network congestion is decreased. This document registers two new RTCP Feedback messages for Feedback Suppression. Applications that are employing one or more loss-repair methods MAY use Feedback Suppression together with their existing loss-repair methods either for every packet they expect to receive, or for an application-specific subset of the RTP packets in a session. In other words, receivers MAY ignore Feedback Suppression messages, but SHOULD react to them unless they have good reason to still send feedback messages despite having been requested to suppress them. 4. RTCP Feedback Report Extension 4.1. Transport Layer Feedback: NACK Suppression Report The NACK implosion Suppression message is an extension to the RTCP feedback report and identified by RTCP packet type value PT=RTPFB and FMT=TBD. The FCI field MUST contain one or more NACK Suppression Early Indication (NSEI) entries. Each entry applies to a different media source, identified by its SSRC. The Feedback Control Information (FCI) for NSEI uses the similar format of message Types defined in the section 4.3.1.1 of [RFC5104]. The format is shown in Figure 1. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PID | BLP | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Message Format for the NSEI report SSRC (32 bits): The SSRC value of the media source that is requested to send the lost packet. Wu, et al. Expires April 23, 2011 [Page 7] Internet-Draft Feedback Suppression October 2010 Packet ID (PID): 16 bits The PID field is used to specify a lost packet. The PID field refers to the RTP sequence number of the lost packet. bitmask of following lost packets (BLP): 16 bits The BLP allows for reporting losses of any of the 16 RTP packets immediately following the RTP packet indicated by the PID. The BLP's definition is identical to that given in [RFC4585]. 4.2. Payload Specific Feedback: FIR suppression report The FIR implosion Suppression message is an extension to the RTCP receiver feedback report and identified by RTCP packet type value PT=PSFB and FMT=TBD. The FCI field MUST contain one or more FIR suppression Early Indication (FSEI) entries. Each entry applies to a different media source, identified by its SSRC. The Feedback Control Information (FCI) for FSEI uses the similar format of message Types defined in the section 4.3.1.1 of [RFC5104]. The format is shown in Figure 2. 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 2: Message Format for the FSEI report SSRC (32 bits): The SSRC value of the media source that is requested to send a decoder refresh point. Seq nr:8bits Command sequence number. The sequence number space is unique for each pairing 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. Wu, et al. Expires April 23, 2011 [Page 8] Internet-Draft Feedback Suppression October 2010 Reserved: 24 bits All bits SHALL be set to 0 by the media source and SHALL be ignored on reception. 5. SDP Signaling A new feedback value "fss" needs to be defined for the Feedback Storm Suppression message to be used with Session Description Protocol (SDP)[RFC4566] using the Augmented Backus-Naur Form (ABNF)[RFC4585]. The "fss" feedback value SHOULD be used with parameters that indicate the feedback suppression supported. In this document, we define two such parameters, namely: o "fsei" denotes support of fir suppression early indication (fsei). o "nsei" denotes support of NACK suppression early indication. In the ABNF for rtcp-fb-val defined in [RFC4585], there is a placeholder called rtcp-fb-id to define new feedback types. "fss" is defined as a new feedback type in this document, and the ABNF for the parameters for fss is defined here (please refer to section 4.2 of [RFC4585] for complete ABNF syntax). rtcp-fb-val =/ "fss" rtcp-fb-fss-param rtcp-fb-fss-param = SP "nsei";nack suppression early indication / SP "fsei";fir suppression early indication / SP token [SP byte-string] ; for future commands/indications byte-string = Refer to Section 4.2 of [RFC4585] for a detailed description and the full syntax of the "rtcp-fb" attribute. 6. Example Use Cases The operation of feedback suppression is similar for all types of RTP sessions and topologies [RFC5117], however the exact messages used and the scenarios in which suppression is employed differ for various use cases. The following sections outline the intended use cases for feedback suppression and give an overview of the particular mechanisms. Wu, et al. Expires April 23, 2011 [Page 9] Internet-Draft Feedback Suppression October 2010 6.1. Source Specific Multicast (SSM) use case In SSM RTP sessions as described in [RFC5760], one or more Media Sources send RTP packets to a Distribution Source. The Distribution Source relays the RTP packets to the receivers using a source- specific multicast group. In order to avoid the forms of NACK implosion described in section 1,the distribution source SHOULD choose to include the support for loss detection. How the packet loss detection works is beyond of scope of this document. When upstream link or downstream aggregate link packet loss occurs, the distribution source creates a Feedback Suppression report and sent it to all the RTP receivers, over the multicast channel. Another possibility is when there may have multiple distribution source placed between the media source and the receivers, the upstream distribution source MAY inform downstream distribution source of the detected packet loss using Feedback Suppression messages. In response, the distribution source forwards packet loss suppression report received from upstream to all the RTP receivers, over the multicast channel. This loss suppression report tells the receivers that the lost packet will either be forthcoming from distribution source, or it irretrievably lost such that there is nothing to be gained by the receiver sending a NACK to the media source. The distribution source then can (optionally) ask for the lost packets from the media source on behalf of all the RTP receivers. When there is only one distribution source with loss detection support between the media source and the receivers, redistribution of the feedback suppression report to all the receivers is trivial. When there are multiple distribution sources between the media source and the receiver, , each distribution source with loss detection support MAY create a Feedback Suppression Report using the similar format as conventional RTCP NACK packets at the RTP layer and send it to its downstream distribution source or forward one Feedback Suppression Report from upstream to its downstream distribution source or the receivers. Also each distribution source at the downstream of the other distribution source MAY also append non- identical packet loss information into the Feedback Suppression Report sent from upstream and form one summary report and send it to the receivers. The distribution source must be able to communicate with all group members in order for either mechanism to be effective at suppressing feedback. As outlined in the [RFC5760], there are two Unicast Feedback models that may be used for reporting, - the Simple Feedback model and the Wu, et al. Expires April 23, 2011 [Page 10] Internet-Draft Feedback Suppression October 2010 Distribution Source Feedback Summary Model. The RTCP Feedback Suppression report extension specified in the section 4 of this document will work in both Feedback models. Details of operation in each are specified below. 6.1.1. Simple Feedback Model In the simple Feedback Model, the Loss detection instance(s)are distributed into two different distribution sources. e.g., upstream distribution source with loss detection support may act as the loss detector of downstream distribution source while The downstream distribution source doesn't have loss detection support. The upstream distribution source MUST listen on the corresponding RTP session for data. When the upstream distribution source observes that a sequence of RTP packets from a media source contains gaps (by checking the sequence number of packets), the upstream distribution source MUST use the same packet types as traditional RTCP feedback described in [RFC3550] and create one new RTCP Feedback Report with information on the RTP sequence number of the lost packets and suppression early indication event. When the upstream distribution source is eligible to transmit, it MUST send this Report packet to the upstream distribution source. The downstream Distribution Source MUST listen for RTCP data sent to the RTCP port. Upon receiving the RTCP Feedback Report packet from the upstream distribution source, the downstream Distribution Source MUST forward it to the group on the multicast RTCP session. Every RTCP packet from each upstream distribution source MUST be reflected individually. This RTCP Feedback Report lets the receivers know that feedback for this packet loss is not needed and SHOULD NOT be sent to the media source(s). If the media source(s) are part of the SSM group for RTCP packet reflection, the Distribution Source MUST filter this packet out. If the media source(s) are not part of the SSM group for RTCP packets, the Distribution Source MUST not forward this RTCP packets received from the receivers to the media source(s). When the receiver receives the RTCP packet, if the receiver understands the message it will not send feedback (e.g., NACK) for the missing packets reported in the message and will accept a retransmission packet (if forthcoming) transmitted from the Distribution Source. If it did not understand the Feedback Suppression report the receiver MAY of course still send feedback messages to the specified media source. When the distribution source receives a feedback message reporting loss from one or more receivers after it has already detected packet loss via loss detection Wu, et al. Expires April 23, 2011 [Page 11] Internet-Draft Feedback Suppression October 2010 functionality, the distribution source MUST filter them out until proactive recovery is complete. 6.1.2. Distribution Source Feedback Summary Model In the distribution source feedback summary model, there may have multiple distribution sources and the Loss Detection instances are distributed into different distribution sources. In some cases, these Loss Detection instances for the same session can exist at the same time, e.g., one Loss Detection instance is implemented in the upstream distribution source A, another two Loss Detection instances for the same session is part of feedback target A and feedback target B respectively within the distribution source B. In this section, we focus on this generic case to discuss the distribution Source Feedback Summary Model. The distribution source A MUST listen on the RTP channel for data. When the distribution source A observes RTP packets from a media source are not consecutive by checking the sequence number of packets, the distribution source A generates the new RTCP Feedback Suppression Report packet described in the section 6, and then send it to the distribution source B. Two loss detection instances within the Distribution Source B MUST listen for RTCP data sent to the RTCP port. Upon receiving the RTCP Feedback Report packet from the Distribution Source A, the distribution source B needs to summarize the information received from all the RTCP Feedback Reports generated by the upstream distribution source together with the information generated by two loss detection instances witin the Distribution Source B and then create the summary report to include all these information. In order to reduce the processing load at the distribution source, each loss detection instance MAY provide preliminary summarization report. During the summary report creating, the Distribution Source B MUST use its own SSRC value for transmitting summarization information and MUST perform proper SSRC collision detection and resolution. In some case, the distribution source B MAY receive RTCP NACK messages from the receivers behind the Distribution Source before the distribution source detects the packet loss which may cause potential Feedback implosion. In such case, the distribution source MAY filter them out if it already sent a packet loss request for the missing packet to the media source. When the distribution source confirms packet loss reported by the receiver, the distribution source generates the summary report to include the packet loss information from the corresponding receiver or upstream distribution source. Wu, et al. Expires April 23, 2011 [Page 12] Internet-Draft Feedback Suppression October 2010 The distribution source MAY send this new RTCP summary report described in the section 6 to the group on the multicast RTCP channel and in the meanwhile sending a packet loss request to the media source. If there are a couple of distribution sources with loss detection support looking at the same RTP stream, then the loss may be identified by all and they will all send requests for the same packet loss. In this case, the distribution source MUST filter out the duplicated information from various distribution source and only append one copy of such information to the summary report. When the host receives the RTCP packet, if the host understands this message it will not send packet loss request (e.g., NACK) for the missing packets reported in the message. If it did not understand this new message, the host MAY send packet loss request(e.g., NACK messages) to the specified media source. When the distribution source receives the packet loss request from the hosts after it has already detected packet loss, the distribution source MUST filter it out until proactive recovery is complete. 6.2. Unicast based Rapid Acquisition of Multicast Stream (RAMS) use case In the typical RAMS architecture, there may have one distribution source placed between media source and BRS for relaying SSM stream from media source. The BRS will receive the SSM stream from the DS. Suppose there are several BRSes behind the distribution source or media source, there may be just one BRS that detects packet loss on its upstream link between the distribution source and BRS, but the others will perhaps not, as the packet loss took place on SSM tree branch that does not impact the other BRSes. In such case, the distribution source with loss detection functionality support can not detect packet loss at the downstream of itself, therefore the distribution source SHOULD NOT create new Feedback Suppression message and send it to all the BRS. If BRS impacted by packet loss has loss detection support, the BRS MAY choose to create new Feedback Suppression message and send it to the receivers behind this BRS. 6.3. RTP transport translator use case A Transport Translator (Topo-Trn-Translator), as defined in [RFC5117] is typically forwarding the RTP and RTCP traffic between RTP clients, for example converting between multicast and unicast for domains that do not support multicast. The translator can identify packet loss from the upstream and send the Feedback Suppression message to the unicast receivers. The translator can also serve as a loss reporter on the multicast side as described in the SSM case. Wu, et al. Expires April 23, 2011 [Page 13] Internet-Draft Feedback Suppression October 2010 6.4. Multipoint Control Unit (MCU) use case In point to multipoint topologies using video switching MCU (Topo- Video-switch-MCU) [RFC5117], the MCU typically forwards a single media stream to each participant, selected from the available input streams. The selection of the input stream is often based on voice activity in the audio-visual conference, but other conference management mechanisms (like presentation mode or explicit floor control) exist as well. In this case the MCU MAY detect packet loss from the sender or may decide to switch to a new source. In both cases the receiver MAY lose synchronization with the video stream and MAY send a FIR request. If the MCU itself can detect the mis-synchronization of the video, the MCU can send the FIR suppression message to the receivers and send a FIR request to the video source. 7. 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. Spoofed or maliciously created feedback messages of the type defined in this specification can have the following implications: Sending NACK Suppression Report with wrong sequence number of lost packet that makes missing RTP packets can not be compensated. Sending FIR Suppression Report with wrong sequence number of lost packet that makes missing RTP packets can not be compensated by update request mechanism. To prevent these attacks, there is a need to apply authentication and integrity protection of the feedback messages. This can be accomplished against threats external to the current RTP session using the RTP profile that combines Secure RTP [RFC3711] and AVPF into SAVPF [RFC5124]. Note that middleboxes that are not visible at the RTP layer that wish to send NACK/FIR suppression reports on behalf of the media source can only do so if they spoof the SSRC of the media source. This is difficult in case SRTP is in use. If the middlebox is visible at the RTP layer, this is not an issue, provided the middlebox is part of the security context for the session. Also note that endpoints that receive a NACK/FIR suppression request Wu, et al. Expires April 23, 2011 [Page 14] Internet-Draft Feedback Suppression October 2010 would be well-advised to ignore it, unless it is authenticated via SRTCP or similar. Accepting un-authenticated NACK/ FIR suppression requests can lead to a denial of service attack, where the endpoint accepts poor quality media that could be repaired. 8. IANA Consideration New feedback type and New parameters for RTCP FSS receiver feedback report are subject to IANA registration. For general guidelines on IANA considerations for RTCP feedback, refer to [RFC4585]. This document assigns one new feedback type value x in the RTCP feedback report registry to "Feedback Storm Suppression" with the following registrations format: Name: FSS Long Name: Feedback Storm Suppression Value: TBD Reference: This document. This document also assigns the parameter value y in the RTCP FSS feedback report Registry to "NACK Suppression Early Indication ", with the following registrations format: Name: NSEI Long name: NACK Suppression Early Indication Value: TBD Reference: this document. This document also assigns the parameter value z in the RTCP FSS feedback report Registry to "FIR Suppression Early Indication ", with the following registrations format: Name: FSEI Long name: FIR Suppression Early Indication Value: TBD Reference: this document. The contact information for the registrations is: Qin Wu sunseawq@huawei.com 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012, China Wu, et al. Expires April 23, 2011 [Page 15] Internet-Draft Feedback Suppression October 2010 9. Acknowledgement The authors would like to thank David R Oran, Ali C. Begen, Colin Perkins,Tom VAN CAENEGEM, Ingemar Johansson S, Bill Ver Steeg, WeeSan Lee for their valuable comments and suggestions on this document. 10. References 10.1. Normative References [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback", RFC 5760, February 2010. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 2006. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, January 2008. [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, July 2006. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", RFC 5104, February 2008. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. Wu, et al. Expires April 23, 2011 [Page 16] Internet-Draft Feedback Suppression October 2010 [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)", RFC 5124, February 2008. 10.2. Informative References [RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker, "NACK-Oriented Reliable Multicast (NORM) Transport Protocol", November 2009. [DVB-IPTV] ETSI Standard, "Digital Video Broadcasting(DVB); Transport of MPEG-2 TS Based DVB Services over IP Based Networks", ETSI TS 102 034, V1.4.1 , August 2009. [I-D.hunt-avt-monarch-01] Hunt, G. and P. Arden, "Monitoring Architectures for RTP", August 2008. [I-D.ietf-pmol-metrics-framework-02] Clark, A., "Framework for Performance Metric Development". Appendix A. Example scenarios for NACK Implosion In SSM RTP sessions as described in [RFC5760], there may have more than one distribution source between the media source and the receivers. In order for loss repair, the distribution source may choose to include the support for retransmission as part of the offered SDP and will expect such support from the media source.The following section outline several example scenarios for NACK Implosion. A.1. Scenario 1: One or more media source, One distribution source The scenario 1 is displayed below in Figure 3. In this scenario, one or more Media Sources send RTP packets to the RTP Receivers through the same Distribution Source. The Distribution Source relays the RTP packets to the receivers using a source-specific multicast channel. In the reverse direction, the receivers transmit RTCP packets via unicast to the distribution source. The Distribution Source in turn relays RTCP packets to the media souce and then transmits the RTCP packets back to the receivers, using source-specific multicast. Wu, et al. Expires April 23, 2011 [Page 17] Internet-Draft Feedback Suppression October 2010 +-------+ |---->|RTP_Rx1| +--------+ | +-------+ | | +--------------+ | | | | | | +-------+ | Media |-------| Distribution |-------|---->|RTP_Rx2| | | | Source | | +-------+ | Source | | | | . | | +--------------+ | . | | | . +--------+ | +-------+ |---->|RTP_Rxn| +-------+ Figure 3: One media Source, one Distribution Source A.2. Scenario 2:One media source, Two distribution sources in cascade +-------+ |---->|RTP_Rx1| | +-------+ +------+ | | | +------------+ +------------+ | +-------+ |Media |-+Distribution+--|Distribution+--|---->|RTP_Rx2| |Source| | Source1 | | Source2 | | +-------+ | | +------------+ +------------+ | . +------+ | . | . | +-------+ |---->|RTP_Rxn| +-------+ Figure 4: One media source, Two distribution sources in cascade The scenario 2 is displayed below in Figure 4. In this scenario, One media source passes through two distribution source in cascading and sends RTP packets to all the RTP receivers. In this case, the distribution source 2 is located in the downstream direction of distribution source 1. A.3. Scenario 3:One media source, Two distribution sources in parallel The scenario 3 is displayed below in Figure 5. In this scenario, the Media Sources send RTP packets to all the RTP receivers through two different path respectively. In each path, there is a distribution source. The distribution source1 is a neighboring node of distribution source 2. Wu, et al. Expires April 23, 2011 [Page 18] Internet-Draft Feedback Suppression October 2010 +--------+ |---->|RTP_Rx11| | +--------+ +--------------+ | | | | +--------+ |--->| Distribution |----|---->|RTP_Rx12| | | Source1 | | +--------+ | | | | . +--------+ | +--------------+ | . | | | | . | | | | +--------+ | Media | | |---->|RTP_Rx1k| | |---| +--------+ | Source | | +--------+ | | | |---->|RTP_Rx21| | | | | +--------+ +--------+ | +--------------+ | | | | | +--------+ | | Distribution |----|---->|RTP_Rx22| |--->| Source2 | | +--------+ | | | . +--------------+ | . | . | +--------+ |---->|RTP_Rx2j| +--------+ Figure 5: One Media Source, more distribution sources Appendix B. Applicability of Feedback Suppression This document defines new RTCP feedback Report, which we refer to as Feedback Suppression to deal with NACK Implosion mentioned above. Here we give two examples to show how this new RTCP feedback report is applied into three scenarios described in Appendix A. Applicability of Feedback Suppression in Scenario 1 described in Figure 3 is shown in the Figure 6. In this figure, the distribution source detect the packet loss before the receiver perceive it and ask for retransmission of the lost packets from the media source on behalf of all the RTP receivers. , in the meanwhile, the distribution source transmits the RTCP Feedback Suppression Indication back to the receivers using source-specific multicast channel. Upon receiving the lost packet via the RTP retransmission payload format, the distribution source forwards the retransmitted packet to all the receivers. The receiver will accept a retransmission stream transmitted from the Distrbituion Source. Wu, et al. Expires April 23, 2011 [Page 19] Internet-Draft Feedback Suppression October 2010 When the distribution source receives a feedback message reporting loss from one or more receivers after it has already detected packet loss, the distribution source MUST filter them out until the Retransmission stream is ready in the Distrbitution Source. In this way, the delay for the receiver to recover from the packet loss can be reduced and the risk of increasing network congestion can be mitigated. +------+ +--------------+ +--------+ |Media | | Distribution | | | |Source| | Source | | RTP_Rx | +--+---+ +------+-------+ +---+----+ | | | | | | |------------------->|------RTP Multicast---->| | | | | | | | +--------+----------+ | | |Detect Packet Loss | | | |and Identify the SN| | | |of missing Packets | | | +--------+----------+ | |<-----RTCP NACK-----| | | | | | +--Multicast RTCP FSS--->| | RTP Retransmission | | |------------------->| | | |------RTP Multicast---->| | | Retransmission | | | | | | | | | | Figure 6: Applicability of NACK Suppression Early Indication Applicability of Feedback Suppression in Scenario 2 or 3 described in Figure 4 and Figure 5 is shown in the Figure 7. The procedure in the Figure 7 is similar to the one in the figure Figure 6. The only difference is distribution source should not only notify all the receiver behind itself not to send NACK but also propagate the retransmission suppression indication to the neighboring distribution sources. In this way, all the receivers behind all the neighboring distribution source can avoid sending massive retransmission request to the media source. Wu, et al. Expires April 23, 2011 [Page 20] Internet-Draft Feedback Suppression October 2010 +------+ +--------+ +--------+ |Media | +-----+ | RTP_Rx | +-----+ | RTP_Rx | |Source| | DS1 | | served | | DS2 | | served | +--+---+ +-----+ | by DS1 | +-----+ | by DS2 | | | ----+----+ | ----+----+ | |RTP Multicast | | | |----------->|------------->| | | | | | | | | | | |RTP Multicast| |------------------------------------------->|------------>| | | | | | | +--------+------------+ | | | | |Detect Packet Loss | | | | | |and Identify the SN | | | | | |of the missing Packets | | | | +--------+------------+ | | | | | | | | |<-RTCP NACK-| Multicast RTCP FSS | | | |------------->| | | | | | | | | |-----Unicast RTCP FSS -------->|Multicast RTCP FSS | | | |------------>| |RTP Retransmission | | | |----------->| | | | | | | | | | | RTP Retransmission | | |------------+--------------+--------------->| | | | | | | | | RTP Multicast| | RTP Multicast | |Retransmission| |Retransmission | |------------->| |------------>| | | | | | DS1: Distribution Source 1 DS2: Distribution Source 2 Figure 7: Applicability of NACK Suppression Early Indication Authors' Addresses Qin Wu Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China Email: sunseawq@huawei.com Wu, et al. Expires April 23, 2011 [Page 21] Internet-Draft Feedback Suppression October 2010 Frank Xia Huawei 1700 Alma Dr. Suite 500 Plano, TX 75075 USA Phone: +1 972-509-5599 Email: xiayangsong@huawei.com Roni Even Huawei 14 David Hamelech Tel Aviv 64953 Israel Email: even.roni@huawei.com Wu, et al. Expires April 23, 2011 [Page 22]