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diff --git a/doc/rfc/rfc5760.txt b/doc/rfc/rfc5760.txt new file mode 100644 index 0000000..81302db --- /dev/null +++ b/doc/rfc/rfc5760.txt @@ -0,0 +1,3699 @@ + + + + + + +Internet Engineering Task Force (IETF) J. Ott +Request for Comments: 5760 Aalto University +Category: Standards Track J. Chesterfield +ISSN: 2070-1721 University of Cambridge + E. Schooler + Intel + February 2010 + + + RTP Control Protocol (RTCP) Extensions for + Single-Source Multicast Sessions with Unicast Feedback + +Abstract + + This document specifies an extension to the Real-time Transport + Control Protocol (RTCP) to use unicast feedback to a multicast + sender. The proposed extension is useful for single-source multicast + sessions such as Source-Specific Multicast (SSM) communication where + the traditional model of many-to-many group communication is either + not available or not desired. In addition, it can be applied to any + group that might benefit from a sender-controlled summarized + reporting mechanism. + +Status of This Memo + + This is an Internet Standards Track document. + + This document is a product of the Internet Engineering Task Force + (IETF). It represents the consensus of the IETF community. It has + received public review and has been approved for publication by the + Internet Engineering Steering Group (IESG). Further information on + Internet Standards is available in Section 2 of RFC 5741. + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + http://www.rfc-editor.org/info/rfc5760. + +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 + + + +Ott, et al. Standards Track [Page 1] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + 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. + +Table of Contents + + 1. Introduction ....................................................3 + 2. Conventions and Acronyms ........................................4 + 3. Definitions .....................................................5 + 4. Basic Operation .................................................6 + 5. Packet Types ...................................................10 + 6. Simple Feedback Model ..........................................11 + 7. Distribution Source Feedback Summary Model .....................13 + 8. Mixer/Translator Issues ........................................36 + 9. Transmission Interval Calculation ..............................37 + 10. SDP Extensions ................................................39 + 11. Security Considerations .......................................41 + 12. Backwards Compatibility .......................................51 + 13. IANA Considerations ...........................................51 + 14. References ....................................................53 + Appendix A. Examples for Sender-Side Configurations ...............57 + Appendix B. Distribution Report Processing at the Receiver ........60 + + + + + + + + + + + + + + + + + +Ott, et al. Standards Track [Page 2] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +1. Introduction + + The Real-time Transport Protocol (RTP) [1] provides a real-time + transport mechanism suitable for unicast or multicast communication + between multimedia applications. Typical uses of RTP are for real- + time or near real-time group communication of audio and video data + streams. An important component of the RTP protocol is the control + channel, defined as the RTP Control Protocol (RTCP). RTCP involves + the periodic transmission of control packets between group members, + enabling group size estimation and the distribution and calculation + of session-specific information such as packet loss and round-trip + time to other hosts. An additional advantage of providing a control + channel for a session is that a third-party session monitor can + listen to the traffic to establish network conditions and to diagnose + faults based on receiver locations. + + RTP was designed to operate in either a unicast or multicast mode. + In multicast mode, it assumes an Any Source Multicast (ASM) group + model, where both one-to-many and many-to-many communication are + supported via a common group address in the range 224.0.0.0 through + 239.255.255.255. To enable Internet-wide multicast communication, + intra-domain routing protocols (those that operate only within a + single administrative domain, e.g., the Distance Vector Multicast + Routing Protocol (DVMRP) [16] and Protocol Independent Multicast + (PIM) [17][18]) are used in combination with inter-domain routing + protocols (those that operate across administrative domain borders, + e.g., Multicast BGP (MBGP) [19] and the Multicast Source Discovery + Protocol (MSDP) [20]). Such routing protocols enable a host to join + a single multicast group address and send data to or receive data + from all members in the group with no prior knowledge of the + membership. However, there is a great deal of complexity involved at + the routing level to support such a multicast service in the network. + + Many-to-many communication is not always available or desired by all + group applications. For example, with Source-Specific Multicast + (SSM) [8][9] and satellite communication, the multicast distribution + channel only supports source-to-receiver traffic. In other cases, + such as large ASM groups with a single active data source and many + passive receivers, it is sub-optimal to create a full routing-level + mesh of multicast sources just for the distribution of RTCP control + packets. Thus, an alternative solution is preferable. + + Although a one-to-many multicast topology may simplify routing and + may be a closer approximation to the requirements of certain RTP + applications, unidirectional communication makes it impossible for + receivers in the group to share RTCP feedback information with other + group members. In this document, we specify a solution to that + problem. We introduce unicast feedback as a new method to distribute + + + +Ott, et al. Standards Track [Page 3] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + RTCP control information amongst all session members. This method is + designed to operate under any group communication model, ASM or SSM. + The RTP data stream protocol itself is unaltered. + + Scenarios under which the unicast feedback method can provide benefit + include but are not limited to: + + a) SSM groups with a single sender. + + The proposed extensions allow SSM groups that do not have many-to- + many communication capability to receive RTP data streams and to + continue to participate in the RTP control protocol (RTCP) by + using multicast in the source-to-receiver direction and unicast to + send receiver feedback to the source on the standard RTCP port. + + b) One-to-many broadcast networks. + + Unicast feedback may also be beneficial to one-to-many broadcast + networks, such as a satellite network with a terrestrial low- + bandwidth return channel or a broadband cable link. Unlike the + SSM network, these networks may have the ability for a receiver to + multicast return data to the group. However, a unicast feedback + mechanism may be preferable for routing simplicity. + + c) ASM with a single sender. + + A unicast feedback approach can be used by an ASM application with + a single sender to reduce the load on the multicast routing + infrastructure that does not scale as efficiently as unicast + routing does. Because this is no more efficient than a standard + multicast group RTP communication scenario, it is not expected to + replace the traditional mechanism. + + The modifications proposed in this document are intended to + supplement the existing RTCP feedback mechanisms described in Section + 6 of [1]. + +2. Conventions and Acronyms + + 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 [13]. + + The following acronyms are used throughout this document: + + ASM Any Source Multicast + SSM Source-Specific Multicast + + + + +Ott, et al. Standards Track [Page 4] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +3. Definitions + + Distribution Source: + In an SSM context, only one entity distributes RTP data and + redistributes RTCP information to all receivers. This entity is + called the Distribution Source. It is also responsible for + forwarding RTCP feedback to the Media Senders and thus creates a + virtual multicast environment in which RTP and RTCP can be + applied. + + Note that heterogeneous networks consisting of ASM multiple-sender + groups, unicast-only clients, and/or SSM single-sender receiver + groups MAY be connected via translators or mixers to create a + single-source group (see Section 8 for details). + + Media Sender: + A Media Sender is an entity that originates RTP packets for a + particular media session. In RFC 3550, a Media Sender is simply + called a source. However, as the RTCP SSM system architecture + includes a Distribution Source, to avoid confusion, in this + document a media source is commonly referred to as a Media Sender. + There may often be a single Media Sender that is co-located with + the Distribution Source. But although there MUST be only one + Distribution Source, there MAY be multiple Media Senders on whose + behalf the Distribution Source forwards RTP and RTCP packets. + + RTP and RTCP Channels: + The data distributed from the source to the receivers is referred + to as the RTP channel and the control information as the RTCP + channel. With standard RTP/RTCP, these channels typically share + the same multicast address but are differentiated via port numbers + as specified in [1]. In an SSM context, the RTP channel is + multicast from the Distribution Source to the receivers. In + contrast, the RTCP or feedback channel is actually the collection + of unicast channels between the receivers and the Distribution + Source via the Feedback Target(s). Thus, bidirectional + communication is accomplished by using SSM in the direction from + Distribution Source to the receivers and using the unicast + feedback channel in the direction from receivers to Distribution + Source. As discussed in the next section, the nature of the + channels between the Distribution Source and the Media Sender(s) + may vary. + + (Unicast RTCP) Feedback Target: + The Feedback Target is a logical function to which RTCP unicast + feedback traffic is addressed. The functions of the Feedback + Target and the Distribution Source MAY be co-located or integrated + in the same entity. In this case, for a session defined as having + + + +Ott, et al. Standards Track [Page 5] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + a Distribution Source A, on ports n for the RTP channel and k for + the RTCP channel, the unicast RTCP Feedback Target is identified + by an IP address of Distribution Source A on port k, unless + otherwise stated in the session description. See Section 10 for + details on how the address is specified. The Feedback Target MAY + also be implemented in one or more entities different from the + Distribution Source, and different RTP receivers MAY use different + Feedback Target instances, e.g., for aggregation purposes. In + this case, the Feedback Target instance(s) MUST convey the + feedback received from the RTP receivers to the Distribution + Source using the RTCP mechanisms specified in this document. If + disjoint, the Feedback Target instances MAY be organized in + arbitrary topological structures: in parallel, hierarchical, or + chained. But the Feedback Target instance(s) and Distribution + Source MUST share, e.g., through configuration, enough information + to be able to provide coherent RTCP information to the RTP + receivers based upon the RTCP feedback collected by the Feedback + Target instance(s) -- as would be done if both functions were part + of the same entity. + + In order for unicast feedback to work, each receiver MUST direct + its RTCP reports to a single specific Feedback Target instance. + + SSRC: + Synchronization source as defined in [1]. This 32-bit value + uniquely identifies each member in a session. + + Report blocks: + Report block is the standard terminology for an RTCP reception + report. RTCP [1] encourages the stacking of multiple report + blocks in Sender Report (SR) and Receiver Report (RR) packets. As + a result, a variable-size feedback packet may be created by one + source that reports on multiple other sources in the group. The + summarized reporting scheme builds upon this model through the + inclusion of multiple summary report blocks in one packet. + However, stacking of reports from multiple receivers is not + permitted in the Simple Feedback Model (see Section 6). + +4. Basic Operation + + As indicated by the definitions of the preceding section, one or more + Media Senders send RTP packets to the Distribution Source. The + Distribution Source relays the RTP packets to the receivers using a + source-specific multicast arrangement. In the reverse direction, the + receivers transmit RTCP packets via unicast to one or more instances + of the Feedback Target. The Feedback Target sends either the + original RTCP reports (the Simple Feedback Model) or summaries of + these reports (the Summary Feedback Model) to the Distribution + + + +Ott, et al. Standards Track [Page 6] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Source. The Distribution Source in turn relays the RTCP reports + and/or summaries to the Media Senders. The Distribution Source also + transmits the RTCP Sender Reports and Receiver Reports or summaries + back to the receivers, using source-specific multicast. + + When the Feedback Target(s) are co-located (or integrated) with the + Distribution Source, redistribution of original or summarized RTCP + reports is trivial. When the Feedback Target(s) are physically + and/or topologically distinct from the Distribution Source, each + Feedback Target either relays the RTCP packets to the Distribution + Source or summarizes the reports and forwards an RTCP summary report + to the Distribution Source. Coordination between multiple Feedback + Targets is beyond the scope of this specification. + + The Distribution Source MUST be able to communicate with all group + members in order for either mechanism to work. The general + architecture is displayed below in Figure 1. There may be a single + Media Sender or multiple Media Senders (Media Sender i, 1<=i<=M) on + whose behalf the Distribution Source disseminates RTP and RTCP + packets. The base case, which is expected to be the most common + case, is that the Distribution Source is co-located with a particular + Media Sender. A basic assumption is that communication is multicast + (either SSM or ASM) in the direction of the Distribution Source to + the receivers (R(j), 1<=j<=N) and unicast in the direction of the + receivers to the Distribution Source. + + Communication between Media Sender(s) and the Distribution Source may + be performed in numerous ways: + + i. Unicast only: The Media Sender(s) MAY send RTP and RTCP via + unicast to the Distribution Source and receive RTCP via unicast. + + ii. Any Source Multicast (ASM): The Media Sender(s) and the + Distribution Source MAY be in the same ASM group, and RTP and + RTCP packets are exchanged via multicast. + + iii. Source-Specific Multicast (SSM): The Media Sender(s) and the + Distribution Source MAY be in an SSM group with the source being + the Distribution Source. RTP and RTCP packets from the Media + Senders are sent via unicast to the Distribution Source, while + RTCP packets from the Distribution Source are sent via multicast + to the Media Senders. + + Note that this SSM group MAY be identical to the SSM group used + for RTP/RTCP delivery from the Distribution Source to the + receivers or it MAY be a different one. + + + + + +Ott, et al. Standards Track [Page 7] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Note that Figure 1 below shows a logical diagram and, therefore, no + details about the above options for the communication between Media + Sender(s), Distribution Source, Feedback Target(s), and receivers are + provided. + + Configuration information needs to be supplied so that (among other + reasons): + + o Media Sender(s) know the transport address of the Distribution + Source or the transport address of the (ASM or SSM) multicast + group used for the contribution link; + + o the Distribution Source knows either the unicast transport + address(es) or the (ASM or SSM) multicast transport address(es) to + reach the Media Sender(s); + + o receivers know the addresses of their respectively responsible + Feedback Targets; and + + o the Feedback Targets know the transport address of the + Distribution Source. + + The precise setup and configuration of the Media Senders and their + interaction with the Distribution Source is beyond the scope of this + document (appropriate Session Description Protocol (SDP) descriptions + MAY be used for this purpose), which only specifies how the various + components interact within an RTP session. Informative examples for + different configurations of the Media Sources and the Distribution + Source are given in Appendix A. + + Future specifications may be defined to address these aspects. + + + + + + + + + + + + + + + + + + + + +Ott, et al. Standards Track [Page 8] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Source-specific + +--------+ +-----+ Multicast + |Media | | | +----------------> R(1) + |Sender 1|<----->| D S | | | + +--------+ | I O | +--+ | + | S U | | | | + +--------+ | T R | | +-----------> R(2) | + |Media |<----->| R C |->+ +---- : | | + |Sender 2| | I E | | +------> R(n-1) | | + +--------+ | B | | | | | | + : | U | +--+--> R(n) | | | + : | T +-| | | | | + | I | |<---------+ | | | + +--------+ | O |F|<---------------+ | | + |Media | | N |T|<--------------------+ | + |Sender M|<----->| | |<-------------------------+ + +--------+ +-----+ Unicast + + FT = Feedback Target + Transport from the Feedback Target to the Distribution + Source is via unicast or multicast RTCP if they are not + co-located. + + Figure 1: System Architecture + + The first method proposed to support unicast RTCP feedback, the + 'Simple Feedback Model', is a basic reflection mechanism whereby all + Receiver RTCP packets are unicast to the Feedback Target, which + relays them unmodified to the Distribution Source. Subsequently, + these packets are forwarded by the Distribution Source to all + receivers on the multicast RTCP channel. The advantage of using this + method is that an existing receiver implementation requires little + modification in order to use it. Instead of sending reports to a + multicast address, a receiver uses a unicast address yet still + receives forwarded RTCP traffic on the multicast control channel. + This method also has the advantage of being backwards compatible with + standard RTP/RTCP implementations. The Simple Feedback Model is + specified in Section 6. + + The second method, the 'Distribution Source Feedback Summary Model', + is a summarized reporting scheme that provides savings in bandwidth + by consolidating Receiver Reports at the Distribution Source, + optionally with help from the Feedback Target(s), into summary + packets that are then distributed to all the receivers. The + Distribution Source Feedback Summary Model is specified in Section 7. + + + + + + +Ott, et al. Standards Track [Page 9] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The advantage of the latter scheme is apparent for large group + sessions where the basic reflection mechanism outlined above + generates a large amount of packet forwarding when it replicates all + the information to all the receivers. Clearly, this technique + requires that all session members understand the new summarized + packet format outlined in Section 7.1. Additionally, the summarized + scheme provides an optional mechanism to send distribution + information or histograms about the feedback data reported by the + whole group. Potential uses for the compilation of distribution + information are addressed in Section 7.4. + + To differentiate between the two reporting methods, a new SDP + identifier is created and discussed in Section 10. The reporting + method MUST be decided prior to the start of the session. A + Distribution Source MUST NOT change the method during a session. + + In a session using SSM, the network SHOULD prevent any multicast data + from the receiver being distributed further than the first hop + router. Additionally, any data heard from a non-unicast-capable + receiver by other hosts on the same subnet SHOULD be filtered out by + the host IP stack so that it does not cause problems with respect to + the calculation of the receiver RTCP bandwidth share. + +5. Packet Types + + The RTCP packet types defined in [1], [26], and [15] are: + + Type Description Payload number + ------------------------------------------------------- + SR Sender Report 200 + RR Receiver Report 201 + SDES Source Description 202 + BYE Goodbye 203 + APP Application-Defined 204 + RTPFB Generic RTP feedback 205 + PSFB Payload-specific feedback 206 + XR RTCP Extension 207 + + This document defines one further RTCP packet format: + + Type Description Payload number + --------------------------------------------------------- + RSI Receiver Summary Information 209 + + Within the Receiver Summary Information packet, there are various + types of information that may be reported and encapsulated in + optional sub-report blocks: + + + + +Ott, et al. Standards Track [Page 10] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Name Long Name Value + ------------------------------------------------------------------ + IPv4 Address IPv4 Feedback Target Address 0 + IPv6 Address IPv6 Feedback Target Address 1 + DNS Name DNS name indicating Feedback Target Address 2 + Reserved Reserved for Assignment by Standards Action 3 + Loss Loss distribution 4 + Jitter Jitter distribution 5 + RTT Round-trip time distribution 6 + Cumulative loss Cumulative loss distribution 7 + Collisions SSRC collision list 8 + Reserved Reserved for Assignment by Standards Action 9 + Stats General statistics 10 + RTCP BW RTCP bandwidth indication 11 + Group Info RTCP group and average packet size 12 + - Unassigned 13 - 255 + + As with standard RTP/RTCP, the various reports MAY be combined into a + single RTCP packet, which SHOULD NOT exceed the path MTU. Packets + continue to be sent at a rate that is inversely proportional to the + group size in order to scale the amount of traffic generated. + +6. Simple Feedback Model + +6.1. Packet Formats + + The Simple Feedback Model uses the same packet types as traditional + RTCP feedback described in [1]. Receivers still generate Receiver + Reports with information on the quality of the stream received from + the Distribution Source. The Distribution Source still MUST create + Sender Reports that include timestamp information for stream + synchronization and round-trip time calculation. Both Media Senders + and receivers are required to send SDES packets as outlined in [1]. + The rules for generating BYE and APP packets as outlined in [1] also + apply. + +6.2. Distribution Source Behavior + + For the Simple Feedback Model, the Distribution Source MUST provide a + basic packet-reflection mechanism. It is the default behavior for + any Distribution Source and is the minimum requirement for acting as + a Distribution Source to a group of receivers using unicast RTCP + feedback. + + The Distribution Source (unicast Feedback Target) MUST listen for + unicast RTCP data sent to the RTCP port. All valid unicast RTCP + packets received on this port MUST be forwarded by the Distribution + Source to the group on the multicast RTCP channel. The Distribution + + + +Ott, et al. Standards Track [Page 11] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Source MUST NOT stack report blocks received from different receivers + into one packet for retransmission to the group. Every RTCP packet + from each receiver MUST be reflected individually. + + If the Media Sender(s) are not part of the SSM group for RTCP packet + reflection, the Distribution Source MUST also forward the RTCP + packets received from the receivers to the Media Sender(s). If there + is more than one Media Sender and these Media Senders do not + communicate via ASM with the Distribution Source and each other, the + Distribution Source MUST forward each RTCP packet originated by one + Media Sender to all other Media Senders. + + The Distribution Source MUST forward RTCP packets originating from + the Media Sender(s) to the receivers. + + The reflected or forwarded RTCP traffic SHOULD NOT be counted as its + own traffic in the transmission interval calculation by the + Distribution Source. In other words, the Distribution Source SHOULD + NOT consider reflected packets as part of its own control data + bandwidth allowance. However, reflected packets MUST be processed by + the Distribution Source and the average RTCP packet size, RTCP + transmission rate, and RTCP statistics MUST be calculated. The + algorithm for computing the allowance is explained in Section 9. + +6.3. Disjoint Distribution Source and Feedback Target(s) + + If the Feedback Target function is disjoint from the Distribution + Source, the Feedback Target(s) MUST forward all RTCP packets from the + receivers or another Feedback Target -- directly or indirectly -- to + the Distribution Source. + +6.4. Receiver Behavior + + Receivers MUST listen on the RTP channel for data and on the RTCP + channel for control. Each receiver MUST calculate its share of the + control bandwidth R/n, in accordance with the profile in use, so that + a fraction of the RTCP bandwidth, R, allocated to receivers is + divided equally between the number of unique receiver SSRCs in the + session, n. R may be rtcp_bw * 0.75 or rtcp_bw * 0.5 (depending on + the ratio of senders to receivers as per [1]) or may be set + explicitly by means of an SDP attribute [10]. See Section 9 for + further information on the calculation of the bandwidth allowance. + When a receiver is eligible to transmit, it MUST send a unicast + Receiver Report packet to the Feedback Target following the rules + defined in Section 9. + + + + + + +Ott, et al. Standards Track [Page 12] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + When a receiver observes either RTP packets or RTCP Sender Reports + from a Media Sender with an SSRC that collides with its own chosen + SSRC, it MUST change its own SSRC following the procedures of [1]. + The receiver MUST do so immediately after noticing and before sending + any (further) RTCP feedback messages. + + If a receiver has out-of-band information available about the Media + Sender SSRC(s) used in the media session, it MUST NOT use the same + SSRC for itself. Even if such out-of-band information is available, + a receiver MUST obey the above collision-resolution mechanisms. + + Further mechanisms defined in [1] apply for resolving SSRC collisions + between receivers. + +6.5. Media Sender Behavior + + Media Senders listen on a unicast or multicast transport address for + RTCP reports sent by the receivers (and forwarded by the Distribution + Source) or other Media Senders (forwarded by the Distribution Source + if needed). Processing and general operation follows [1]. + + A Media Sender that observes an SSRC collision with another entity + that is not also a Media Sender MAY delay its own collision- + resolution actions as per [1], by 5 * 1.5 * Td, with Td being the + deterministic calculated reporting interval, for receivers to see + whether the conflict still exists. SSRC collisions with other Media + Senders MUST be acted upon immediately. + + Note: This gives precedence to Media Senders and places the burden + of collision resolution on the RTP receivers. + + Sender SSRC information MAY be communicated out-of-band, e.g., by + means of SDP media descriptions. Therefore, senders SHOULD NOT + change their own SSRC aggressively or unnecessarily. + +7. Distribution Source Feedback Summary Model + + In the Distribution Source Feedback Summary Model, the Distribution + Source is required to summarize the information received from all the + Receiver Reports generated by the receivers and place the information + into summary reports. The Distribution Source Feedback Summary Model + introduces a new report block format, the Receiver Summary + Information (RSI) report, and a number of optional sub-report block + formats, which are enumerated in Section 7.1. As described in + Section 7.3, individual instances of the Feedback Target may provide + preliminary summarization to reduce the processing load at the + Distribution Source. + + + + +Ott, et al. Standards Track [Page 13] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Sub-reports appended to the RSI report block provide more detailed + information on the overall session characteristics reported by all + receivers and can also convey important information such as the + feedback address and reporting bandwidth. Which sub-reports are + mandatory and which ones are optional is defined below. + + From an RTP perspective, the Distribution Source is an RTP receiver, + generating its own Receiver Reports and sending them to the receiver + group and to the Media Senders. In the Distribution Source Feedback + Summary Model, an RSI report block MUST be appended to all RRs the + Distribution Source generates. + + In addition, the Distribution Source MUST forward the RTCP SR reports + and SDES packets of Media Senders without alteration. If the + Distribution Source is actually a Media Sender, even if it is the + only session sender, it MUST generate its own Sender Report (SR) + packets for its role as a Media Sender and its Receiver Reports in + its role as the Distribution Source. + + The Distribution Source MUST use its own SSRC value for transmitting + summarization information and MUST perform proper SSRC collision + detection and resolution. + + The Distribution Source MUST send at least one Receiver Summary + Information packet for each reporting interval. The Distribution + Source MAY additionally stack sub-report blocks after the RSI packet. + If it does so, each sub-report block MUST correspond to the RSI + packet and constitutes an enhancement to the basic summary + information required by the receivers to calculate their reporting + time interval. For this reason, additional sub-report blocks are not + required but recommended. The compound RTCP packets containing the + RSI packet and the optional corresponding sub-report blocks MUST be + formed according to the rules defined in [1] for receiver-issued + packets, e.g., they MUST begin with an RR packet, contain at least an + SDES packet with a CNAME, and MAY contain further RTCP packets and + SDES items. + + Every RSI packet MUST contain either a Group and Average Packet Size + sub-report or an RTCP Bandwidth sub-report for bandwidth indications + to the receivers. + +7.1. Packet Formats + + All numeric values comprising multiple (usually two or four) octets + MUST be encoded in network byte order. + + + + + + +Ott, et al. Standards Track [Page 14] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +7.1.1. RSI: Receiver Summary Information Packet + + The RSI report block has a fixed header size followed by a variable + length report: + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |V=2|P|reserved | PT=RSI=209 | length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SSRC | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Summarized SSRC | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | NTP Timestamp (most significant word) | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | NTP Timestamp (least significant word) | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : Sub-report blocks : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + The RSI packet includes the following fields: + + Length: 16 bits + As defined in [1], the length of the RTCP packet in 32-bit words + minus one, including the header and any padding. + + SSRC: 32 bits + The SSRC of the Distribution Source. + + Summarized SSRC: 32 bits + The SSRC (of the Media Sender) of which this report contains a + summary. + + Timestamp: 64 bits + Indicates the wallclock time when this report was sent. Wallclock + time (absolute date and time) is represented using the timestamp + format of the Network Time Protocol (NTP), which is in seconds + relative to 0h UTC on 1 January 1900 [1]. The wallclock time MAY + (but need not) be NTP-synchronized but it MUST provide a + consistent behavior in the advancement of time (similar to NTP). + The full-resolution NTP timestamp is used, which is a 64-bit, + unsigned, fixed-point number with the integer part in the first 32 + bits and the fractional part in the last 32 bits. This format is + similar to RTCP Sender Reports (Section 6.4.1 of [1]). The + timestamp value is used to enable detection of duplicate packets, + reordering, and to provide a chronological profile of the feedback + reports. + + + +Ott, et al. Standards Track [Page 15] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +7.1.2. Sub-Report Block Types + + For RSI reports, this document also introduces a sub-report block + format specific to the RSI packet. The sub-report blocks are + appended to the RSI packet using the following generic format. All + sub-report blocks MUST be 32-bit aligned. + + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SRBT | Length | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SRBT-specific data + + | | + : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + SRBT: 8 bits + Sub-Report Block Type. The sub-report block type identifier. The + values for the sub-report block types are defined in Section 5. + + Length: 8 bits + The length of the sub-report in 32-bit words. + + SRBT-specific data: <length * 4 - 2> octets + This field may contain type-specific information based upon the + SRBT value. + +7.1.3. Generic Sub-Report Block Fields + + For the sub-report blocks that convey distributions of values (Loss, + Jitter, RTT, Cumulative Loss), a flexible 'data bucket'-style report + is used. This format divides the data set into variable-size buckets + that are interpreted according to the guide fields at the head of the + report block. + + 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 + +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ + | SRBT | Length | NDB | MF | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Minimum Distribution Value | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Maximum Distribution Value | + +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ + | Distribution Buckets | + | ... | + | ... | + +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ + + + + +Ott, et al. Standards Track [Page 16] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The SRBT and length fields are calculated as explained in Section + 7.1.2. + + Number of distribution buckets (NDB): 12 bits + The number of distribution buckets of data. The size of each + bucket can be calculated using the formula + ((length * 4) - 12) * 8 / NDB number of bits. The calculation is + based on the length of the whole sub-report block in octets + (length * 4) minus the header of 12 octets. Providing 12 bits for + the NDB field enables bucket sizes as small as 2 bits for a full- + length packet of MTU 1500 bytes. The bucket size in bits must + always be divisible by 2 to ensure proper byte alignment. A + bucket size of 2 bits is fairly restrictive, however, and it is + expected that larger bucket sizes will be more practical for most + distributions. + + Multiplicative Factor (MF): 4 bits + 2^MF indicates the multiplicative factor to be applied to each + distribution bucket value. Possible values of MF are 0 - 15, + creating a range of values from MF = 1, 2, 4 ... 32768. Appendix + B gives an example of the use of the multiplicative factor; it is + meant to provide more "bits" without having them -- the bucket + values get scaled up by the MF. + + Length: 8 bits + The length field tells the receiver the full length of the sub- + report block in 32-bit words (i.e., length * 4 bytes) and enables + the receiver to identify the bucket size. For example, given no + MTU restrictions, the data portion of a distribution packet may be + only as large as 1008 bytes (255 * 4 - 12), providing up to 4032 + data buckets of length 2 bits, or 2016 data buckets of length 4 + bits, etc. + + Minimum distribution value (min): 32 bits + The minimum distribution value, in combination with the maximum + distribution value, indicates the range covered by the data bucket + values. + + Maximum distribution value (max): 32 bits + The maximum distribution value, in combination with the minimum + distribution value, indicates the range covered by the data bucket + values. The significance and range of the distribution values is + defined in the individual subsections for each distribution type + (DT). + + + + + + + +Ott, et al. Standards Track [Page 17] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Distribution buckets: each bucket is ((length * 4) - 12) * 8 / NDB + bits + The size and number of buckets is calculated as outlined above + based upon the value of NDB and the length of the packet. The + values for distribution buckets are equally distributed; according + to the following formula, distribution bucket x (with 0 <= x < + NDB) covers the value range: + + x = 0; [min, min + (max - min) / NDB] + x > 0; [min + (x) * (max - min) / NDB, + min + (x + 1) * (max - min) / NDB] + + Interpretation of the minimum, maximum, and distribution values in + the sub-report block is sub-report-specific and is addressed + individually in the sections below. The size of the sub-report block + is variable, as indicated by the packet length field. + + Note that, for any bucket-based reporting, if the Distribution Source + and the Feedback Target(s) are disjoint entities, out-of-band + agreement on the bucket-reporting granularity is recommended to allow + the Distribution Source to forward accurate information in these + kinds of reports to the receivers. + +7.1.4. Loss Sub-Report Block + + The Loss sub-report block allows a receiver to determine how its own + reception quality relates to the other recipients. A receiver may + use this information, e.g., to drop out of a session (instead of + sending lots of error repair feedback) if it finds itself isolated at + the lower end of the reception quality scale. + + The Loss sub-report block indicates the distribution of losses as + reported by the receivers to the Distribution Source. Values are + expressed as a fixed-point number with the binary point at the left + edge of the field similar to the "fraction lost" field in SR and RR + packets, as defined in [1]. The Loss sub-report block type (SRBT) is + 4. + + Valid results for the minimum distribution value field are 0 - 254. + Similarly, valid results for the maximum distribution value field are + 1 - 255. The minimum distribution value MUST always be less than the + maximum. + + For examples on processing summarized loss report sub-blocks, see + Appendix B. + + + + + + +Ott, et al. Standards Track [Page 18] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +7.1.5. Jitter Sub-Report Block + + A Jitter sub-report block indicates the distribution of the estimated + statistical variation of the RTP data packet inter-arrival time + reported by the receivers to the Distribution Source. This allows + receivers both to place their own observed jitter values in context + with the rest of the group and to approximate dynamic parameters for + playout buffers. See [1] for details on the calculation of the + values and the relevance of the jitter results. Jitter values are + measured in timestamp units with the rate used by the Media Sender + and expressed as unsigned integers. The minimum distribution value + MUST always be less than the maximum. The Jitter sub-report block + type (SRBT) is 5. + + Since timestamp units are payload-type specific, the relevance of a + jitter value is impacted by any change in the payload type during a + session. Therefore, a Distribution Source MUST NOT report jitter + distribution values for at least 2 reporting intervals after a + payload type change occurs. + +7.1.6. Round-Trip Time Sub-Report Block + + A Round-Trip Time sub-report indicates the distribution of round-trip + times from the Distribution Source to the receivers, providing + receivers with a global view of the range of values in the group. + The Distribution Source is capable of calculating the round-trip time + to any other member since it forwards all the SR packets from the + Media Sender(s) to the group. If the Distribution Source is not + itself a Media Sender, it can calculate the round-trip time from + itself to any of the receivers by maintaining a record of the SR + sender timestamp and the current time as measured from its own system + clock. The Distribution Source consequently calculates the round- + trip time from the Receiver Report by identifying the corresponding + SR timestamp and subtracting the RR advertised holding time as + reported by the receiver from its own time difference measurement, as + calculated by the time the RR packet is received and the recorded + time the SR was sent. + + The Distribution Source has the option of distributing these round- + trip time estimations to the whole group, uses of which are described + in Section 7.4. The round-trip time is expressed in units of 1/65536 + seconds and indicates an absolute value. This is calculated by the + Distribution Source, based on the Receiver Report responses declaring + the time difference since an original Sender Report and on the + holding time at the receiver. The minimum distribution value MUST + always be less than the maximum. The Round-Trip Time sub-report + block type (SRBT) is 6. + + + + +Ott, et al. Standards Track [Page 19] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Note that if the Distribution Source and the Feedback Target + functions are disjoint, it is only possible for the Distribution + Source to determine RTT if all the Feedback Targets forward all + RTCP reports from the receivers immediately (i.e., do not perform + any preliminary summarization) and include at least the RR packet. + +7.1.7. Cumulative Loss Sub-Report Block + + The cumulative loss field in a Receiver Report [1], in contrast to + the fraction lost field, is intended to provide some historical + perspective on the session performance, i.e., the total number of + lost packets since the receiver joined the session. The cumulative + loss value provides a longer-term average by summing over a larger + sample set (typically the whole session). The Distribution Source + MAY record the values as reported by the receiver group and report a + distribution of values. Values are expressed as a fixed-point number + with the binary point at the left edge of the field, in the same + manner as the Loss sub-report block. Since the individual Receiver + Reports give the cumulative number of packets lost but not the + cumulative number sent, which is required as a denominator to + calculate the long-term fraction lost, the Distribution Source MUST + maintain a record of the cumulative number lost and extended highest + sequence number received, as reported by each receiver at some point + in the past. Ideally, the recorded values are from the first report + received. Subsequent calculations are then based on (<the new + cumulative loss value> - <the recorded value>) / (<new extended + highest sequence number> - <recorded sequence number>). + + Valid results for the minimum distribution value field are 0 - 254. + Similarly, valid results for the maximum distribution value field are + 1 - 255. The minimum distribution value MUST always be less than the + maximum. The Cumulative Loss sub-report block type (SRBT) is 7. + +7.1.8. Feedback Target Address Sub-Report Block + + The Feedback Target Address block provides a dynamic mechanism for + the Distribution Source to signal an alternative unicast RTCP + feedback address to the receivers. If a block of this type is + included, receivers MUST override any static SDP address information + for the session with the Feedback Target address provided in this + sub-report block. + + If a Feedback Target Address sub-report block is used, it MUST be + included in every RTCP packet originated by the Distribution Source + to ensure that all receivers understand the information. In this + manner, receiver behavior should remain consistent even in the face + of packet loss or when there are late session arrivals. + + + + +Ott, et al. Standards Track [Page 20] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SRBT={0,1,2} | Length | Port | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + : Address : + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + SRBT: 8 bits + The type of sub-report block that corresponds to the type of + address is as follows: + + 0: IPv4 address + 1: IPv6 address + 2: DNS name + + Length: 8 bits + The length of the sub-report block in 32-bit words. For an IPv4 + address, this should be 2 (i.e., total length = 4 + 4 = 2 * 4 + octets). For an IPv6 address, this should be 5. For a DNS name, + the length field indicates the number of 32-bit words making up + the string plus 1 byte and any additional padding required to + reach the next word boundary. + + Port: 2 octets + The port number to which receivers send feedback reports. A port + number of 0 is invalid and MUST NOT be used. + + Address: 4 octets (IPv4), 16 octets (IPv6), or n octets (DNS name) + The address to which receivers send feedback reports. For IPv4 + and IPv6, fixed-length address fields are used. A DNS name is an + arbitrary-length string that is padded with null bytes to the next + 32-bit boundary. The string MAY contain Internationalizing Domain + Names in Applications (IDNA) domain names and MUST be UTF-8 + encoded [11]. + + A Feedback Target Address block for a certain address type (i.e., + with a certain SRBT of 0, 1, or 2) MUST NOT occur more than once + within a packet. Numerical Feedback Target Address blocks for IPv4 + and IPv6 MAY both be present. If so, the resulting transport + addresses MUST point to the same logical entity. + + If a Feedback Target address block with an SRBT indicating a DNS name + is present, there SHOULD NOT be any other numerical Feedback Target + Address blocks present. + + + + +Ott, et al. Standards Track [Page 21] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The Feedback Target Address presents a significant security risk if + accepted from unauthenticated RTCP packets. See Sections 11.3 and + 11.4 for further discussion. + +7.1.9. Collision Sub-Report Block + + The Collision SSRC sub-report provides the Distribution Source with a + mechanism to report SSRC collisions amongst the group. In the event + that a non-unique SSRC is discovered based on the tuple [SSRC, + CNAME], the collision is reported in this block and the affected + nodes must reselect their respective SSRC identifiers. + + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SRBT=8 | Length | Reserved | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + : Collision SSRC : + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Collision SSRC: n * 32 bits + This field contains a list of SSRCs that are duplicated within the + group. Usually this is handled by the hosts upon detection of the + same SSRC; however, since receivers in an SSM session using the + Distribution Source Feedback Summary Model no longer have a global + view of the session, the collision algorithm is handled by the + Distribution Source. SSRCs that collide are listed in the packet. + Each Collision SSRC is reported only once for each collision + detection. If more Collision SSRCs need to be reported than fit + into an MTU, the reporting is done in a round robin fashion so + that all Collision SSRCs have been reported once before the second + round of reporting starts. On receipt of the packet, receiver(s) + MUST detect the collision and select another SSRC, if the + collision pertains to them. + + The Collision sub-report block type (SRBT) is 8. + + Collision detection is only possible at the Distribution Source. If + the Distribution Source and Feedback Target functions are disjoint + and collision reporting across RTP receiver SSRCs shall be provided, + the Feedback Targets(s) MUST forward the RTCP reports from the RTP + receivers, including at least the RR and the SDES packets to the + Distribution Source. + + + + + + +Ott, et al. Standards Track [Page 22] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + In system settings in which, by explicit configuration or + implementation, RTP receivers are not going to act as Media Senders + in a session (e.g., for various types of television broadcasting), + SSRC collision detection MAY be omitted for RTP receivers. In system + settings in which an RTP receiver MAY become a Media Sender (e.g., + any conversational application), SSRC collision detection MUST be + provided for RTP receivers. + + Note: The purpose of SSRC collision reporting is to ensure unique + identification of RTCP entities. This is of particular relevance + for Media Senders so that an RTP receiver can properly associate + each of the multiple incoming media streams (via the Distribution + Source) with the correct sender. Collision resolution for Media + Senders is not affected by the Distribution Source's collision + reporting because all SR reports are always forwarded among the + senders and to all receivers. Collision resolution for RTP + receivers is of particular relevance for those receivers capable + of becoming a Media Sender. RTP receivers that will, by + configuration or implementation choice, not act as a sender in a + particular RTP session, do not necessarily need to be aware of + collisions as long as the those entities receiving and processing + RTCP feedback messages from the receivers are capable of + disambiguating the various RTCP receivers (e.g., by CNAME). + + Note also that RTP receivers should be able to deal with the + changing SSRCs of a Media Sender (like any RTP receiver has to + do.) and, if possible, be prepared to continuously render a media + stream nevertheless. + +7.1.10. General Statistics Sub-Report Block + + The General Statistics sub-report block is used if specifying buckets + is deemed too complex. With the General Statistics sub-report block, + only aggregated values are reported back. The rules for the + generation of these values are provided in point b of Section 7.2.1. + + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SRBT=10 | Length | Reserved | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | MFL | HCNL | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Median inter-arrival jitter | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + + + + + +Ott, et al. Standards Track [Page 23] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Median fraction lost (MFL): 8 bits + The median fraction lost indicated by Receiver Reports forwarded + to this Distribution Source, expressed as a fixed-point number + with the binary point at the left edge of the field. A value of + all '1's indicates that the MFL value is not provided. + + Highest cumulative number of packets lost (HCNL): 24 bits + Highest 'cumulative number of packets lost' value out of the most + recent RTCP RR packets from any of the receivers. A value of all + '1's indicates that the HCNL value is not provided. + + Median inter-arrival jitter: 32 bits + Median 'inter-arrival jitter' value out of the most recent RTCP RR + packets from the receiver set. A value of all '1's indicates that + this value is not provided. + + The General Statistics sub-report block type (SRBT) is 10. + + Note that, in case the Distribution Source and the Feedback Target + functions are disjoint, it is only possible for the Distribution + Source to determine the median of the inter-arrival jitter if all the + Feedback Targets forward all RTCP reports from the receivers + immediately and include at least the RR packet. + +7.1.11. RTCP Bandwidth Indication Sub-Report Block + + This sub-report block is used to inform the Media Senders and + receivers about either the maximum RTCP bandwidth that is supposed to + be used by each Media Sender or the maximum bandwidth allowance to be + used by each receiver. The value is applied universally to all Media + Senders or all receivers. Each receiver MUST base its RTCP + transmission interval calculation on the average size of the RTCP + packets sent by itself. Conversely, each Media Sender MUST base its + RTCP transmission interval calculation on the average size of the + RTCP packets sent by itself. + + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SRBT=11 | Length |S|R| Reserved | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | RTCP Bandwidth | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + + + + + + + +Ott, et al. Standards Track [Page 24] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Sender (S): 1 bit + The contained bandwidth value applies to each Media Sender. + + Receivers (R): 1 bit + The contained bandwidth value applies to each RTP receiver. + + Reserved: 14 bits + MUST be set to zero upon transmission and ignored upon reception. + + RTCP Bandwidth: 32 bits + If the S bit is set to 1, this field indicates the maximum + bandwidth allocated to each individual Media Sender. This also + informs the receivers about the RTCP report frequency to expect + from the senders. This is a fixed-point value with the binary + point in between the second and third bytes. The value represents + the bandwidth allocation per receiver in kilobits per second, with + values in the range 0 <= BW < 65536. + + If the R bit is set to 1, this field indicates the maximum + bandwidth allocated per receiver for sending RTCP data relating to + the session. This is a fixed-point value with the binary point in + between the second and third bytes. The value represents the + bandwidth allocation per receiver in kilobits per second, with + values in the range 0 <= BW < 65536. Each receiver MUST use this + value for its bandwidth allowance. + + This report block SHOULD only be used when the Group and Average + Packet Size sub-report block is NOT included. The RTCP Bandwidth + Indication sub-report block type (SRBT) is 11. + +7.1.12. RTCP Group and Average Packet Size Sub-Report Block + + This sub-report block is used to inform the Media Senders and + receivers about the size of the group (used for calculating feedback + bandwidth allocation) and the average packet size of the group. This + sub-report MUST always be present, appended to every RSI packet, + unless an RTCP Bandwidth Indication sub-report block is included (in + which case it MAY, but need not, be present). + + Note: The RTCP Bandwidth Indication sub-report block allows the + Distribution Source to hide the actual group size from the + receivers while still avoiding feedback implosion. + + + + + + + + + +Ott, et al. Standards Track [Page 25] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | SRBT=12 | Length | Average Packet Size | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Receiver Group Size | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Group size: 32 bits + This field provides the Distribution Source's view of the number + of receivers in a session. Note that the number of Media Senders + is not explicitly reported since it can be derived by observing + the RTCP SR packets forwarded by the Distribution Source. The + group size is calculated according to the rules outlined in [1]. + When this sub-report block is included, this field MUST always be + present. + + Average RTCP packet size: 16 bits + This field provides the Distribution Source's view of the average + RTCP packet size as locally calculated, following the rules + defined in [1]. The value is an unsigned integer, counting + octets. When this sub-report block is included, this field MUST + always be present. + + The Group and Average Packet Size sub-report block type (SRBT) is 12. + +7.2. Distribution Source Behavior + + In the Distribution Source Feedback Summary Model, the Distribution + Source (the unicast Feedback Target) MUST listen for unicast RTCP + packets sent to the RTCP port. All RTCP packets received on this + port MUST be processed by the Distribution Source, as described + below. The processing MUST take place per Media Sender SSRC for + which Receiver Reports are received. + + The Distribution Source acts like a regular RTCP receiver. In + particular, it receives an RTP stream from each RTP Media Sender(s) + and MUST calculate the proper reception statistics for these RTP + streams. It MUST transmit the resulting information as report blocks + contained in each RTCP packet it sends (in an RR packet). + + Note: This information may help to determine the transmission + characteristics of the feed path from the RTP sender to the + Distribution Source (if these are separate entities). + + The Distribution Source is responsible for accepting RTCP packets + from the receivers and for interpreting and storing per-receiver + information, as defined in [1]. The importance of providing these + + + +Ott, et al. Standards Track [Page 26] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + functions is apparent when creating the RSI and sub-report block + reports since incorrect information can have serious implications. + Section 11 addresses the security risks in detail. + + As defined in [1], all RTCP reports from the Distribution Source MUST + start with an RR report, followed by any relevant SDES fields. Then + the Distribution Source MUST append an RSI header and sub-reports + containing any summarization-specific data. In addition, either the + Group and Average Packet Size sub-report or the Receiver RTCP + Bandwidth sub-report block MUST be appended to the RSI header. + + A Distribution Source has the option of masking the group size by + including only an RTCP Bandwidth sub-report. If both sub-reports are + included, the receiver is expected to give precedence to the + information contained in the Receiver RTCP Bandwidth sub-report. + + The Receiver RTCP Bandwidth sub-report block provides the + Distribution Source with the capability to control the amount of + feedback from the receivers, and the bandwidth value MAY be increased + or decreased based upon the requirements of the Distribution Source. + Regardless of the value selected by the Distribution Source for the + Receiver RTCP Bandwidth sub-report block, the Distribution Source + MUST continue to forward Sender Reports and RSI packets at the rate + allowed by the total RTCP bandwidth allocation. See Section 9 for + further details about the frequency of reports. + + A Distribution Source MAY start out reporting group size and switch + to Receiver RTCP Bandwidth reporting later on and vice versa. If the + Distribution Source does so, it SHOULD ensure that the + correspondingly indicated values for the Receiver RTCP Bandwidth sub- + report roughly match, i.e., are at least the same order of magnitude. + + In order to identify SSRC collisions, the Distribution Source is + responsible for maintaining a record of each SSRC and the + corresponding CNAME within at least one reporting interval by the + group, in order to differentiate between clients. It is RECOMMENDED + that an updated list of more than one interval be maintained to + increase accuracy. This mechanism should prevent the possibility of + collisions since the combination of SSRC and CNAME should be unique, + if the CNAME is generated correctly. If collisions are not detected, + the Distribution Source will have an inaccurate impression of the + group size. Since the statistical probability is very low that + collisions will both occur and be undetectable, this should not be a + significant concern. In particular, the clients would have to + randomly select the same SSRC and have the same username + IP address + (e.g., using private address space behind a NAT router). + + + + + +Ott, et al. Standards Track [Page 27] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +7.2.1. Receiver Report Aggregation + + The Distribution Source is responsible for aggregating reception- + quality information received in RR packets. In doing so, the + Distribution Source MUST consider the report blocks received in every + RR packet and MUST NOT consider the report blocks of an SR packet. + + Note: the receivers will get the information contained in the SR + packets anyway by packet forwarding, so duplication of this + information should be avoided. + + For the optional sub-report blocks, the Distribution Source MAY + decide which are the most significant feedback values to convey and + MAY choose not to include any. The packet format provides + flexibility in the amount of detail conveyed by the data points. + There is a tradeoff between the granularity of the data and the + accuracy of the data based on the multiplicative factor (MF), the + number of buckets, and the min and max values. In order to focus on + a particular region of a distribution, the Distribution Source can + adjust the minimum and maximum values and either increase the number + of buckets, and possibly the factorization, or decrease the number of + buckets and provide more accurate values. See Appendix B for + detailed examples on how to convey a summary of RTCP Receiver Reports + as RSI sub-report block information. + + For each value the Distribution Source decides to include in an RSI + packet, it MUST adhere to the following measurement rules. + + a) If the Distribution Source intends to use a sub-report to convey + a distribution of values (Sections 7.1.3 to 7.1.7), it MUST keep + per-receiver state, i.e., remember the last value V reported by + the respective receiver. If a new value is reported by a + receiver, the existing value MUST be replaced by the new one. + The value MUST be deleted (along with the entire entry) if the + receiver is timed out (as per Section 6.3.5 of [1]) or has sent a + BYE packet (as per Section 6.3.7 of [1]). + + All the values collected in this way MUST be included in the + creation of the subsequent Distribution sub-report block. + + The results should correspond as closely as possible to the + values received during the interval since the last report. The + Distribution Source may stack as many sub-report blocks as + required in order to convey different distributions. If the + distribution size exceeds the largest packet length (1008 bytes + data portion), more packets MAY be stacked with additional + information (but in total SHOULD NOT exceed the path MTU). + + + + +Ott, et al. Standards Track [Page 28] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + All stacked sub-reports MUST be internally consistent, i.e., + generated from the same session data. Overlapping of + distributions is therefore allowed, and variation in values + should only occur as a result of data set granularity, with the + more accurate bucket sizes taking precedence in the event that + values differ. Non-divisible values MUST be rounded up or down + to the closest bucket value, and the number of data buckets must + always be an even number, padding where necessary with an + additional zero bucket value to maintain consistency. + + Note: This intentionally provides persistent full coverage of the + entire session membership to avoid oscillations due to changing + membership samples. + + Scheduling the transmission of summarization reports is left to + the discretion of the Distribution Source. However, the + Distribution Source MUST adhere to the bandwidth limitations for + Receiver Reports as defined for the respective AV profile in use. + + b) If the Distribution Source intends to report a short summary + using the General Statistics sub-report block format, defined in + Section 7.1.10, for EACH of the values included in the report + (MFL, HCNL, average inter-arrival jitter), it MUST keep a timer + T_summary as well as a sufficient set of variables to calculate + the summaries for the last three reporting intervals. This MAY + be achieved by keeping per-receiver state (i.e., remember the + last value V reported by the respective receiver) or by + maintaining summary variables for each of these intervals. + + The summary values are included here to reflect the current group + situation. By recording the last three reporting intervals, the + Distribution Source incorporates reports from all members while + allowing for individual RTCP Receiver Report packet losses. The + process of collecting these values also aims to avoid resetting + any of the values and then having to send out an RSI report based + upon just a few values collected. If data is available for less + than three reporting intervals (as will be the case for the first + two reports sent), only those values available are considered. + + The timer T_summary MUST be initialized as T_summary = 1.5 * Td, + where Td is the deterministic reporting interval, and MUST be + updated following timer reconsideration whenever the group size + or the average RTCP size ("avg_rtcp_size") changes. This choice + should allow each receiver to report once per interval. + + + + + + + +Ott, et al. Standards Track [Page 29] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Td + __^__ + t0/ \ t1 t2 t3 t4 t5 + ---+---------+---------+---------+---------+---------+-------> + \____ ____/ : : : : + : V : : : : : + :T_summary: : : : : + :=1.5 * Td: : : : : + \______________ ______________/ : : + : V : : : + : 3 * T_summary : : + : : : : + \______________ ______________/ : + : V : + : 3 * T_summary : + : : + \______________ ______________/ + V + 3 * T_summary + + Figure 2: Overview of Summarization Reporting + + Figure 2 depicts how the summarization reporting shall be performed. + At time t3, the RTCP reports collected from t0 through t3 are + included in the RSI reporting; at time t4, those from t1 through t4; + and so on. The RSI summary report sent MUST NOT include any RTCP + report from more than three reporting intervals ago, e.g., the one + sent at time t5, must not include reports received at the + Distribution Source prior to t2. + +7.2.2. Handling Other RTCP Packets from RTP Receivers + + When following the Feedback Summary Model, the Distribution Source + MAY reflect any other RTCP packets of potential relevance to the + receivers (such as APP, RTPFB, PSFB) to the receiver group. Also, it + MAY decide not to forward other RTCP packets not needed by the + receivers such as BYE, RR, SDES (because the Distribution Source + performs collision resolution), group size estimation, and RR + aggregation. The Distribution Source MUST NOT forward RR packets to + the receiver group. + + If the Distribution Source is able to interpret and aggregate + information contained in any RTCP packets other than RR, it MAY + include the aggregated information along with the RSI packet in its + own compound RTCP packet. + + + + + + +Ott, et al. Standards Track [Page 30] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Aggregation MAY be a null operation, i.e., the Distribution Source + MAY simply append one or more RTCP packets from receivers to the + compound RTCP packet (containing at least RR, SDES, and RSI from the + Distribution Source). + + Note: This is likely to be useful only for a few cases, e.g., to + forward aggregated information from RTPFB Generic NACK packets and + thereby maintain the damping property of [15]. + + Note: This entire processing rule implies that the flow of + information contained in non-RR RTCP packets may terminate at the + Distribution Source, depending on its capabilities and + configuration. + + The configuration of the RTCP SSM media session (expressed in SDP) + MUST specify explicitly which processing the Distribution Source will + apply to which RTCP packets. See Section 10.1 for details. + +7.2.3. Receiver Report Forwarding + + If the Media Sender(s) are not part of the SSM group for RTCP packet + reflection, the Distribution Source MUST explicitly inform the Media + Senders of the receiver group. To achieve this, the Distribution + Source has two options: 1) it forwards the RTCP RR and SDES packets + received from the receivers to the Media Sender(s); or 2) if the + Media Senders also support the RTCP RSI packet, the Distribution + Source sends the RSI packets not just to the receivers but also to + the Media Senders. + + If the Distribution Source decides to forward RR and SDES packets + unchanged, it MAY also forward any other RTCP packets to the senders. + + If the Distribution Source decides to forward RSI packets to the + senders, the considerations of Section 7.2.2 apply. + +7.2.4. Handling Sender Reports + + The Distribution Source also receives RTCP (including SR) packets + from the RTP Media Senders. + + The Distribution Source MUST forward all RTCP packets from the Media + Senders to the RTP receivers. + + If there is more than one Media Sender and these Media Senders do not + communicate via ASM with the Distribution Source and each other, the + Distribution Source MUST forward each RTCP packet from any one Media + Sender to all other Media Senders. + + + + +Ott, et al. Standards Track [Page 31] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +7.2.5. RTCP Data Rate Calculation + + As noted above, the Distribution Source is a receiver from an RTP + perspective. The Distribution Sources MUST calculate its + deterministic transmission interval Td as every other receiver; + however, it MAY adjust its available data rate depending on the + destination transport address and its local operation: + + 1. For sending its own RTCP reports to the SSM group towards the + receivers, the Distribution Source MAY use up to the joint share + of all receivers as it is forwarding summaries on behalf of all of + them. Thus, the Distribution Source MAY send its reports up to + every Td/R time units, with R being the number of receivers. + + 2. For sending its own RTCP reports to the Media Senders only (i.e., + if the Media Senders are not part of the SSM group), the allocated + rate depends on the operation of the Distribution Source: + + a) If the Distribution Source only sends RSI packets along with + its own RTCP RR packets, the same rate calculation applies as + for #1 above. + + b) If the Distribution Source forwards RTCP packets from all other + receivers to the Media Senders, then it MUST adhere to the same + bandwidth share for its own transmissions as all other + receivers and use the calculation as per [1]. + +7.2.6. Collision Resolution + + A Distribution Source observing RTP packets from a Media Sender with + an SSRC that collides with its own chosen SSRC MUST change its own + SSRC following the procedures of [1] and MUST do so immediately after + noticing. + + A Distribution Source MAY use out-of-band information about the Media + Sender SSRC(s) used in the media session when available to avoid SSRC + collisions with Media Senders. Nevertheless, the Distribution Source + MUST monitor Sender Report (SR) packets to detect any changes, + observe collisions, and then follow the above collision-resolution + procedure. + + For collision resolution between the Distribution Source and + receivers or the Feedback Target(s) (if a separate entity, as + described in the next subsection), the Distribution Source and the + Feedback Target (if separate) operate similar to ordinary receivers. + + + + + + +Ott, et al. Standards Track [Page 32] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +7.3. Disjoint Distribution Source and Feedback Target + + If the Distribution Source and the Feedback Target are disjoint, the + processing of the Distribution Source is limited by the amount of + RTCP feedback information made available by the Feedback Target. + + The Feedback Target(s) MAY simply forward all RTCP packets incoming + from the RTP receivers to the Distribution Source, in which case the + Distribution Source will have all the necessary information available + to perform all the functions described above. + + The Feedback Target(s) MAY also perform aggregation of incoming RTCP + packets and send only aggregated information to the Distribution + Source. In this case, the Feedback Target(s) MUST use correctly + formed RTCP packets to communicate with the Distribution Source and + they MUST operate in concert with the Distribution Source so that the + Distribution Source and the Feedback Target(s) appear to be operating + as a single entity. The Feedback Target(s) MUST report their + observed receiver group size to the Distribution Source, either + explicitly by means of RSI packets or implicitly by forwarding all RR + packets. + + Note: For example, for detailed statistics reporting, the + Distribution Source and the Feedback Target(s) may need to agree + on a common reporting granularity so that the Distribution Source + can aggregate the buckets incoming from various Feedback Targets + into a coherent report sent to the receivers. + + The joint behavior of the Distribution Source and Feedback Target(s) + MUST be reflected in the (SDP-based) media session description as per + Section 7.2.2. + + If the Feedback Target performs summarization functions, it MUST also + act as a receiver and choose a unique SSRC for its own reporting + towards the Distribution Source. The collision-resolution + considerations for receivers apply. + +7.4. Receiver Behavior + + An RTP receiver MUST process RSI packets and adapt session + parameters, such as the RTCP bandwidth, based on the information + received. The receiver no longer has a global view of the session + and will therefore be unable to receive information from individual + receivers aside from itself. However, the information conveyed by + the Distribution Source can be extremely detailed, providing the + receiver with an accurate view of the session quality overall, + without the processing overhead associated with listening to and + analyzing all Receiver Reports. + + + +Ott, et al. Standards Track [Page 33] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The RTP receiver MUST process the report blocks contained in any RTP + SR and RR packets to complete its view of the RTP session. + + The SSRC collision list MUST be checked against the SSRC selected by + the receiver to ensure there are no collisions as MUST be incoming + RTP packets from the Media Senders. A receiver observing RTP packets + from a Media Sender with an SSRC that collides with its own chosen + SSRC MUST change its own SSRC following the procedures of [1]. The + receiver MUST do so immediately after noticing and before sending any + (further) RTCP feedback messages. + + A Group and Average Packet Size sub-report block is most likely to be + appended to the RSI header (either a Group Size sub-report or an RTCP + Bandwidth sub-report MUST be included). The group size n allows a + receiver to calculate its share of the RTCP bandwidth, r. Given R, + the total available RTCP bandwidth share for receivers (in the SSM + RTP session) r = R/(n). For the group size calculation, the RTP + receiver MUST NOT include the Distribution Source, i.e., the only RTP + receiver sending RSI packets. + + The receiver RTCP bandwidth field MAY override this value. If the + receiver RTCP bandwidth field is present, the receiver MUST use this + value as its own RTCP reporting bandwidth r. + + If the RTCP bandwidth field was used by the Distribution Source in an + RTCP session but this field was not included in the last five RTCP + RSI reports, the receiver MUST revert to calculating its bandwidth + share based upon the group size information. + + If the receiver has not received any RTCP RSI packets from the + Distribution Source for a period of five times the sender reporting + interval, it MUST cease transmitting RTCP Receiver Reports until the + next RTCP RSI packet is received. + + The receiver can use the summarized data as desired. This data is + most useful in providing the receiver with a more global view of the + conditions experienced by other receivers and enables the client to + place itself within the distribution and establish the extent to + which its reported conditions correspond to the group reports as a + whole. Appendix B provides further information and examples of data + processing at the receiver. + + The receiver SHOULD assume that any sub-report blocks in the same + packet correspond to the same data set received by the Distribution + Source during the last reporting time interval. This applies to + packets with multiple blocks, where each block conveys a different + range of values. + + + + +Ott, et al. Standards Track [Page 34] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + A receiver MUST NOT rely on all of the RTCP packets it sends reaching + the Media Senders or any other receiver. While RR statistics will be + aggregated, BYE packets will be processed, and SSRC collisions will + usually be announced, processing and/or forwarding of further RTCP + packets is up to the discretion of the Distribution Source and will + be performed as specified in the session description. + + If a receiver has out-of-band information available about the Media + Sender SSRC(s) used in the media session, it MUST NOT use the same + SSRC for itself. The receiver MUST be aware that such out-of-band + information may be outdated (i.e., that the sender SSRC(s) may have + changed) and MUST follow the above collision-resolution procedure if + necessary. + + A receiver MAY use such Media Sender SSRC information when available + but MUST beware of potential changes to the SSRC (which can only be + learned from Sender Report (SR) packets). + +7.5. Media Sender Behavior + + Media Senders listen on a unicast or multicast transport address for + RTCP reports sent by the receivers (and forwarded by the Distribution + Source) or other Media Senders (optionally forwarded by the + Distribution Source). + + Unlike in the case of the simple forwarding model, Media Senders MUST + be able to process RSI packets from the Distribution Source to + determine the group size and their own RTCP bandwidth share. Media + Senders MUST also be capable of determining the group size (and their + corresponding RTCP bandwidth share) from listening to (forwarded) + RTCP RR and SR packets (as mandated in [1]). + + As long as they send RTP packets, they MUST also send RTCP SRs, as + defined in [1]. + + A Media Sender that observes an SSRC collision with another entity + that is not also a Media Sender MAY delay its own collision- + resolution actions, as per [1], by 5 * 1.5 * Td, with Td being the + deterministic calculated reporting interval, for receivers to see + whether the conflict still exists. SSRC collisions with other Media + Senders MUST be acted upon immediately. + + Note: This gives precedence to Media Senders and places the burden + of collision resolution on RTP receivers. + + Sender SSRC information MAY be communicated out-of-band, e.g., by + means of SDP media descriptions. Therefore, senders SHOULD NOT + change their own SSRC eagerly or unnecessarily. + + + +Ott, et al. Standards Track [Page 35] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +8. Mixer/Translator Issues + + The original RTP specification allows a session to use mixers and + translators to help connect heterogeneous networks into one session. + There are a number of issues, however, which are raised by the + unicast feedback model proposed in this document. The term 'mixer' + refers to devices that provide data stream multiplexing where + multiple sources are combined into one stream. Conversely, a + translator does not multiplex streams but simply acts as a bridge + between two distribution mechanisms, e.g., a unicast-to-multicast + network translator. Since the issues raised by this document apply + equally to either a mixer or translator, the latter are referred to + from this point onwards as mixer-translator devices. + + A mixer-translator between distribution networks in a session must + ensure that all members in the session receive all the relevant + traffic to enable the usual operation by the clients. A typical use + may be to connect an older implementation of an RTP client with an + SSM distribution network, where the client is not capable of + unicasting feedback to the source. In this instance, the mixer- + translator must join the session on behalf of the client and send and + receive traffic from the session to the client. Certain hybrid + scenarios may have different requirements. + +8.1. Use of a Mixer-Translator + + The mixer-translator MUST adhere to the SDP description [5] for the + single-source session (Section 11) and use the feedback mechanism + indicated. Implementers of receivers SHOULD be aware that when a + mixer-translator is present in the session, more than one Media + Sender may be active, since the mixer-translator may be forwarding + traffic to the SSM receivers either from multiple unicast sources or + from an ASM session. Receivers SHOULD still forward unicast RTCP + reports in the usual manner to their assigned Feedback + Target/Distribution Source, which in this case -- by assumption -- + would be the mixer-translator itself. It is RECOMMENDED that the + simple packet-reflection mechanism be used under these circumstances, + since attempting to coordinate RSI + summarization reporting between + more than one source may be complicated unless the mixer-translator + is capable of summarization. + +8.2. Encryption and Authentication Issues + + Encryption and security issues are discussed in detail in Section 11. + A mixer-translator MUST be able to follow the same security policy as + the client in order to unicast RTCP feedback to the source, and it + therefore MUST be able to apply the same authentication and/or + encryption policy required for the session. Transparent bridging and + + + +Ott, et al. Standards Track [Page 36] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + subsequent unicast feedback to the source, where the mixer-translator + is not acting as the Distribution Source, is only allowed if the + mixer-translator can conduct the same source authentication as + required by the receivers. A translator MAY forward unicast packets + on behalf of a client but SHOULD NOT translate between multicast-to- + unicast flows towards the source without authenticating the source of + the feedback address information. + +9. Transmission Interval Calculation + + The Control Traffic Bandwidth referred to in [1] is an arbitrary + amount that is intended to be supplied by a session-management + application (e.g., SDR [21]) or decided based upon the bandwidth of a + single sender in a session. + + The RTCP transmission interval calculation either remains the same as + in the original RTP specification [1] or uses the algorithm in [10] + when bandwidth modifiers have been specified for the session. + +9.1. Receiver RTCP Transmission + + If the Distribution Source is operating in Simple Feedback Model + (which may be indicated in the corresponding session description for + the media session but which the receiver also notices from the + absence of RTCP RSI packets), a receiver MUST calculate the number of + other members in a session based upon its own SSRC count, derived + from the forwarded Sender and Receiver Reports it receives. The + receiver MUST calculate the average RTCP packet size from all the + RTCP packets it receives. + + If the Distribution Source is operating in Distribution Source + Feedback Summary Model, the receiver MUST use either the group size + field and the average RTCP packet size field or the Receiver + Bandwidth field from the respective sub-report blocks appended to the + RSI packet. + + A receiver uses these values as input to the calculation of the + deterministic calculated interval as per [1] and [10]. + +9.2. Distribution Source RTCP Transmission + + If operating in Simple Feedback Model, the Distribution Source MUST + calculate the transmission interval for its Receiver Reports and + associated RTCP packets, based upon the above control traffic + bandwidth, and MUST count itself as RTP receiver. Receiver Reports + will be forwarded as they arrive without further consideration. The + Distribution Source MAY choose to validate that all or selected + receivers roughly adhere to the calculated bandwidth constraints and + + + +Ott, et al. Standards Track [Page 37] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + MAY choose to drop excess packets for receivers that do not. In all + cases, the average RTCP packet size is determined from the forwarded + Media Senders' and receivers' RTCP packets and from those originated + by the Distribution Source. + + If operating in Distribution Source Feedback Summary Model, the + Distribution Source does not share the forward RTCP bandwidth with + any of the receivers. Therefore, the Distribution Source SHOULD use + the full RTCP bandwidth for its Receiver Reports and associated RTCP + packets, as well as RTCP RSI packets. In this case, the average RTCP + packet size is only determined from the RTCP packets originated by + the Distribution Source. + + The Distribution Source uses these values as input to the calculation + of the deterministic calculated interval as per [1] and [10]. + +9.3. Media Senders RTCP Transmission + + In Simple Feedback Model, the Media Senders obtain all RTCP SRs and + RRs as they would in an ASM session, except that the packets are + forwarded by the Distribution Source. They MUST perform their RTCP + group size estimate and calculation of the deterministic transmission + interval as per [1] and [10]. + + In Distribution Source Feedback Summary Model, the Media Senders + obtain all RTCP SRs as they would in an ASM session. They receive + either RTCP RR reports as in ASM (in case these packets are forwarded + by the Distribution Source) or RSI packets containing summaries. In + the former case, they MUST perform their RTCP group size estimate and + calculation of the deterministic transmission interval as per [1] and + [10]. In the latter case, they MUST combine the information obtained + from the Sender Reports and the RSI packets to create a complete view + of the group size and the average RTCP packet size and perform the + calculation of the deterministic transmission interval, as per [1] + and [10], based upon these input values. + +9.4. Operation in Conjunction with AVPF and SAVPF + + If the RTP session is an AVPF session [15] or an SAVPF session [28] + (as opposed to a regular AVP [6] session), the receivers MAY modify + their RTCP report scheduling, as defined in [15]. Use of AVPF or + SAVPF does not affect the Distribution Source's RTCP transmission or + forwarding behavior. + + It is RECOMMENDED that a Distribution Source and possible separate + Feedback Target(s) be configured to forward AVPF/SAVPF-specific RTCP + packets in order to not counteract the damping mechanism built into + AVPF; optionally, they MAY aggregate the feedback information from + + + +Ott, et al. Standards Track [Page 38] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + the receivers as per Section 7.2.2. If only generic feedback packets + that are understood by the Distribution Source and that can easily be + aggregated are in use, the Distribution MAY combine several incoming + RTCP feedback packets and forward the aggregate along with its next + RTCP RR/RSI packet. In any case, the Distribution Source and + Feedback Target(s) SHOULD minimize the extra delay when forwarding + feedback information, but the Distribution Source MUST stay within + its RTCP bandwidth constraints. + + In the event that specific APP packets without a format and + summarization mechanism understood by the Feedback Target(s) and/or + the Distribution Source are to be used, it is RECOMMENDED that such + packets are forwarded with minimal delay. Otherwise, the capability + of the receiver to send timely feedback messages is likely to be + affected. + +10. SDP Extensions + + The Session Description Protocol (SDP) [5] is used as a means to + describe media sessions in terms of their transport addresses, + codecs, and other attributes. Signaling that RTCP feedback will be + provided via unicast, as specified in this document, requires another + session parameter in the session description. Similarly, other SSM + RTCP feedback parameters need to be provided, such as the + summarization model at the sender and the target unicast address to + which to send feedback information. This section defines the SDP + parameters that are needed by the proposed mechanisms in this + document (and that have been registered with IANA). + +10.1. SSM RTCP Session Identification + + A new session-level attribute MUST be used to indicate the use of + unicast instead of multicast feedback: "rtcp-unicast". + + This attribute uses one parameter to specify the model of operation. + An optional set of parameters specifies the behavior for RTCP packet + types (and subtypes). + + rtcp-unicast:reflection + + This attribute MUST be used to indicate the "Simple Feedback" + model of operation where packet reflection is used by the + Distribution Source (without further processing). + + + + + + + + +Ott, et al. Standards Track [Page 39] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + rtcp-unicast:rsi *(SP <processing>:<rtcp-type>]) + + This attribute MUST be used to indicate the "Distribution Source + Feedback Summary" model of operation. In this model, a list or + parameters may be used to explicitly specify how RTCP packets + originated by receivers are handled. Options for processing a + given RTCP packet type are: + + aggr: The Distribution Source has means for aggregating the + contents of the RTCP packets and will do so. + + forward: The Distribution Source will forward the RTCP packet + unchanged. + + term: The Distribution Source will terminate the RTCP packet. + + The default rules applying if no parameters are specified are as + follows: + + RR and SDES packets MUST be aggregated and MUST lead to RSI + packets being generated. All other RTP packets MUST be terminated + at the Distribution Source (or Feedback Target(s)). + + The SDP description needs only to specify deviations from the + default rules. Aggregation of RR packets and forwarding of SR + packets MUST NOT be changed. + + The token for the new SDP attribute is "rtcp-unicast" and the formal + SDP ABNF syntax for the new attribute value is as follows: + + att-value = "reflection" + / "rsi" *(SP rsi-rule) + + rsi-rule = processing ":" rtcp-type + + processing = "aggr" / "forward" / "term" / token ;keep it extensible + + rtcp-type = 3*3DIGIT ;the RTCP type (192, 193, 202--209) + +10.2. SSM Source Specification + + In a Source-Specific Multicast RTCP session, the address of the + Distribution Source needs to be indicated both for source-specific + joins to the multicast group and for addressing unicast RTCP packets + on the backchannel from receivers to the Distribution Source. + + + + + + +Ott, et al. Standards Track [Page 40] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + This is achieved by following the proposal for SDP source filters + documented in [4]. According to the specification, only the + inclusion model ("a=source-filter:incl") MUST be used for SSM RTCP. + + There SHOULD be exactly one "a=source-filter:incl" attribute listing + the address of the Distribution Source. The RTCP port MUST be + derived from the m= line of the media description. + + An alternative Feedback Target Address and port MAY be supplied using + the SDP RTCP attribute [7], e.g., a=rtcp:<port> IN IP4 192.0.2.1. + This attribute only defines the transport address of the Feedback + Target and does not affect the SSM group specification for media + stream reception. + + Two "source-filter" attributes MAY be present to indicate an IPv4 and + an IPv6 representation of the same Distribution Source. + +10.3. RTP Source Identification + + The SSRC information for the Media Sender(s) MAY be communicated + explicitly out of band (i.e., outside the RTP session). One option + for doing so is the Session Description Protocol (SDP) [5]. If such + an indication is desired, the "ssrc" attribute [12] MUST be used for + this purpose. As per [12], the "cname" Source Attribute MUST be + present. Further Source Attributes MAY be specified as needed. + + If used in an SDP session description of an RTCP-SSM session, the + ssrc MUST contain the SSRC intended to be used by the respective + Media Sender and the cname MUST equal the CNAME for the Media Sender. + If present, the role SHOULD indicate the function of the RTP entity + identified by this line; presently, only the "media-sender" role is + defined. + + Example: + + a=ssrc:314159 cname:iptv-sender@example.com + + In the above example, the Media Sender is identified to use the SSRC + identifier 314159 and the CNAME iptv-sender@example.com. + +11. Security Considerations + + The level of security provided by the current RTP/RTCP model MUST NOT + be diminished by the introduction of unicast feedback to the source. + This section identifies the security weaknesses introduced by the + feedback mechanism, potential threats, and level of protection that + MUST be adopted. Any suggestions on increasing the level of security + + + + +Ott, et al. Standards Track [Page 41] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + provided to RTP sessions above the current standard are RECOMMENDED + but OPTIONAL. The final section outlines some security frameworks + that are suitable to conform to this specification. + +11.1. Assumptions + + RTP/RTCP is a protocol that carries real-time multimedia traffic, and + therefore a main requirement is for any security framework to + maintain as low overhead as possible. This includes the overhead of + different applications and types of cryptographic operations as well + as the overhead to deploy or to create security infrastructure for + large groups. + + Although the distribution of session parameters (typically encoded as + SDP objects) through the Session Announcement Protocol (SAP) [22], + email, or the web is beyond the scope of this document, it is + RECOMMENDED that the distribution method employs adequate security + measures to ensure the integrity and authenticity of the information. + Suitable solutions that meet the security requirements outlined here + are included at the end of this section. + + In practice, the multicast and group distribution mechanism, e.g., + the SSM routing tree, is not immune to source IP address spoofing or + traffic snooping; however, such concerns are not discussed here. In + all the following discussions, security weaknesses are addressed from + the transport level or above. + +11.2. Security Threats + + Attacks on media distribution and the feedback architecture proposed + in this document may take a variety of forms. A detailed outline of + the types of attacks follows: + + a) Denial of Service (DoS) + + DoS is a major area of concern. Due to the nature of the + communication architecture, a DoS can be generated in a number of + ways by using the unicast feedback channel to the attacker's + advantage. + + b) Packet Forgery + + Another potential area for attack is packet forgery. In + particular, it is essential to protect the integrity of certain + influential packets since compromise could directly affect the + transmission characteristics of the whole group. + + + + + +Ott, et al. Standards Track [Page 42] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + c) Session Replay + + The potential for session recording and subsequent replay is an + additional concern. An attacker may not actually need to + understand packet content but simply have the ability to record + the data stream and, at a later time, replay it to any receivers + that are listening. + + d) Eavesdropping on a Session + + The consequences of an attacker eavesdropping on a session already + constitutes a security weakness; in addition, eavesdropping might + facilitate other types of attacks and is therefore considered a + potential threat. For example, an attacker might be able to use + the eavesdropped information to perform an intelligent DoS attack. + +11.3. Architectural Contexts + + To better understand the requirements of the solution, the threats + outlined above are addressed for each of the three communication + contexts: + + a) Source-to-Receiver Communication + + The downstream communication channel, from the source to the + receivers, is critical to protect since it controls the behavior + of the group; it conveys the bandwidth allocation for each + receiver, and hence the rate at which the RTCP traffic is unicast, + directly back to the source. All traffic that is distributed over + the downstream channel is generated by a single source. Both the + RTP data stream and the RTCP control data from the source are + included in this context, with the RTCP data generated by the + source being indirectly influenced by the group feedback. + + The downstream channel is vulnerable to the four types of attack + outlined above. The denial of service attack is possible but less + of a concern than the other types. The worst case effect of DoS + would be the transmission of large volumes of traffic over the + distribution channel, with the potential to reach every receiver + but only on a one-to-one basis. Consequently, this threat is no + more pronounced than the current multicast ASM model. The real + danger of denial of service attacks in this context comes + indirectly via compromise of the source RTCP traffic. If + receivers are provided with an incorrect group size estimate or + bandwidth allowance, the return traffic to the source may create a + distributed DoS effect on the source. Similarly, an incorrect + feedback address -- whether as a result of a malicious attack or + + + + +Ott, et al. Standards Track [Page 43] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + by mistake, e.g., an IP address configuration error (e.g., typing) + -- could directly create a denial of service attack on another + host. + + An additional concern relating to Denial of service attacks would + come indirectly through the generation of fake BYE packets, + causing the source to adjust the advertised group size. A + Distribution Source MUST follow the correct rules for timing out + members in a session prior to reporting a change in the group + size, which allows the authentic SSRC sufficient time to continue + to report and, consequently, cancel the fake BYE report. + + The danger of packet forgery in the worst case may be to + maliciously instigate a denial of service attack, e.g., if an + attacker were capable of spoofing the source address and injecting + incorrect packets into the data stream or intercepting the source + RTCP traffic and modifying the fields. + + The replay of a session would have the effect of recreating the + receiver feedback to the source address at a time when the source + is not expecting additional traffic from a potentially large + group. The consequence of this type of attack may be less + effective on its own, but in combination with other attacks might + be serious. + + Eavesdropping on the session would provide an attacker with + information on the characteristics of the source-to-receiver + traffic, such as the frequency of RTCP traffic. If RTCP traffic + is unencrypted, this might also provide valuable information on + characteristics such as group size, Media Source SSRC(s), and + transmission characteristics of the receivers back to the source. + + b) Receiver-to-Distribution-Source Communication + + The second context is the return traffic from the group to the + Distribution Source. This traffic should only consist of RTCP + packets and should include Receiver Reports, SDES information, BYE + reports, extended reports (XR), feedback messages (RTPFB, PSFB) + and possibly application-specific packets. The effects of + compromise on a single or subset of receivers are not likely to + have as great an impact as in context (a); however, much of the + responsibility for detecting compromise of the source data stream + relies on the receivers. + + The effects of compromise of critical Distribution Source control + information can be seriously amplified in the present context. A + large group of receivers may unwittingly generate a distributed + + + + +Ott, et al. Standards Track [Page 44] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + DoS attack on the Distribution Source in the event that the + integrity of the source RTCP channel has been compromised and the + compromise is not detected by the individual receivers. + + An attacker capable of instigating a packet forgery attack could + introduce false RTCP traffic and create fake SSRC identifiers. + Such attacks might slow down the overall control channel data rate + since an incorrect perception of the group size may be created. + Similarly, the creation of fake BYE reports by an attacker would + cause some group size instability, but should not be effective as + long as the correct timeout rules are applied by the source in + removing SSRC entries from its database. + + A replay attack on receiver return data to the source would have + the same implications as the generation of false SSRC identifiers + and RTCP traffic to the source. Therefore, ensuring authenticity + and freshness of the data source is important. + + Eavesdropping in this context potentially provides an attacker + with a great deal of potentially personal information about a + large group of receivers available from SDES packets. It would + also provide an attacker with information on group traffic- + generation characteristics and parameters for calculating + individual receiver bandwidth allowances. + + c) Receiver-to-Feedback-Target Communication + + The third context is the return traffic from the group to the + Feedback Target. It suffers from the same threats as the + receiver-to-source context, with the difference being that now a + large group of receivers may unwittingly generate a distributed + DoS (DDos) attack on the Feedback Target, where it is impossible + to discern if the DDoS is deliberate or due merely to a + misconfiguration of the Feedback Target Address. While deliberate + attacks can be mitigated by properly authenticating messages that + communicate the Feedback Target Address (i.e., the SDP session + description and the Feedback Target Address sub-report block + carried in RTCP), a misconfigured address will originate from an + authenticated source and hence cannot be prevented using security + mechanisms. + + Furthermore, the Feedback Target is unable to communicate its + predicament with either the Distribution Source or the session + receivers. From the feedback packets received, the Feedback + Target cannot tell either which SSM multicast group the feedback + belongs to or the Distribution Source, making further analysis and + suppression difficult. The Feedback Target may not even support + RTCP or listen on the port number in question. + + + +Ott, et al. Standards Track [Page 45] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Note that because the DDoS occurs inside of the RTCP session and + because the unicast receivers adhere to transmission interval + calculations ([1], [10]), the bandwidth misdirected toward the + Feedback Target in the misconfigured case will be limited to a + percentage of the session bandwidth, i.e., the Control Traffic + Bandwidth established for the session. + +11.4. Requirements in Each Context + + To address these threats, this section presents the security + requirements for each context. + + a) The main threat in the source-to-receiver context concerns denial + of service attacks through possible packet forgery. The forgery + may take the form of interception and modification of packets from + the source, or it may simply inject false packets into the + distribution channel. To combat these attacks, data integrity and + source authenticity MUST be applied to source traffic. Since the + consequences of eavesdropping do not affect the operation of the + protocol, confidentiality is not a requirement in this context. + However, without confidentiality, access to personal and group + characteristics information would be unrestricted to an external + listener. Therefore, confidentiality is RECOMMENDED. + + b) The threats in the receiver-to-source context concern the same + kinds of attacks, but are considered less important than the + downstream traffic compromise. All the security weaknesses are + also applicable to the current RTP/RTCP security model, and + therefore only recommendations towards protection from compromise + are made. Data integrity is RECOMMENDED to ensure that + interception and modification of an individual receiver's RTCP + traffic can be detected. This would protect against the false + influence of group control information and the potentially more + serious compromise of future services provided by the distribution + functionality. In order to ensure security, data integrity and + authenticity of receiver traffic is therefore also RECOMMENDED. + With respect to data confidentiality, the same situation applies + as in the first context, and it is RECOMMENDED that precautions be + taken to protect the privacy of the data. + + c) The threats to the receiver-to-feedback-target context are similar + to those in the receiver-to-source context, and thus the + recommendations to protect against them are similar. + + However, there are a couple situations with broader issues to + solve, which are beyond the scope of this document. + + + + + +Ott, et al. Standards Track [Page 46] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + 1. An endpoint experiencing DDoS or the side effects of a + misconfigured RTCP session may not even be a participant in the + session, i.e., may not be listening on the respective port + number and may even support RTCP, so it will be unable to react + within RTCP. Determining that there is a problem will be up to + network administrators and, possibly, anti-malware software + that can perform correlation across receiver nodes. + + 2. With misconfiguration, unfortunately the normally desirable + usage of SRTP and SRTCP becomes undesirable. Because the + packet content is encrypted, neither the misconfigured Feedback + Target nor the network administrator have the ability to + determine the root cause of the traffic. + + In the case where the misconfigured Feedback Target happens to be + a node participating in the session or is an RTCP-enabled node, + the Feedback Target Address block provides a dynamic mechanism for + the Distribution Source to signal an alternative unicast RTCP + feedback address to the receivers. As this type of packet MUST be + included in every RTCP packet originated by the Distribution + Source, all receivers would be able to obtain the corrected + Feedback Target information. In this manner, receiver behavior + should remain consistent even in the face of packet loss or when + there are late-session arrivals. The only caveat is that the + misconfigured Feedback Target is largely uninvolved in the repair + of this situation and thus relies on others for the detection of + the problem. + + An additional security consideration, which is not a component of + this specification but which has a direct influence upon the general + security, is the origin of the session-initiation data. This + involves the SDP parameters that are communicated to the members + prior to the start of the session via channels such as an HTTP + server, email, SAP, or other means. It is beyond the scope of this + document to place any strict requirements on the external + communication of such information; however, suitably secure SDP + communication approaches are outlined in Section 11.7. + +11.5. Discussion of Trust Models + + As identified in the previous sections, source authenticity is a + fundamental requirement of the protocol. However, it is important to + also clarify the model of trust that would be acceptable to achieve + this requirement. There are two fundamental models that apply in + this instance: + + + + + + +Ott, et al. Standards Track [Page 47] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + a) The shared-key model, where all authorized group members share the + same key and can equally encrypt/decrypt the data. This method + assumes that an out-of-band method is applied to the distribution + of the shared group key, ensuring that every key-holder is + individually authorized to receive the key and, in the event of + member departures from the group, a re-keying exercise can occur. + The advantage of this model is that the costly processing + associated with one-way key-authentication techniques is avoided, + as well as the need to execute additional cipher operations with + alternative key sets on the same data set, e.g., in the event that + data confidentiality is also applied. The disadvantage is that, + for very large groups where the receiver set becomes effectively + untrusted, a shared key does not offer much protection. + + b) The public-key authentication model, using cryptosystems such as + RSA-based or PGP authentication, provides a more secure method of + source authentication at the expense of generating higher + processing overhead. This is typically not recommended for real- + time data streams but, in the case of RTCP reports, which are + distributed with a minimum interval of 5 seconds, this may be a + viable option (the processing overhead might still be too great + for small, low-powered devices and should therefore be considered + with caution). Wherever possible, however, the use of public key + source authentication is preferable to the shared key model + identified above. + + As concerns requirements for protocol acceptability, either model is + acceptable although it is RECOMMENDED that the more secure public- + key-based options be applied wherever possible. + +11.6. Recommended Security Solutions + + This section presents some existing security mechanisms that are + RECOMMENDED to suitably address the requirements outlined in Section + 11.5. This is only intended as a guideline and it is acknowledged + that there are other solutions that would also be suitable to comply + with the specification. + +11.6.1. Secure Distribution of SDP Parameters + + a) SAP, HTTPS, Email -- Initial distribution of the SDP parameters + for the session SHOULD use a secure mechanism such as the SAP + authentication framework, which allows an authentication + certificate to be attached to the session announcements. Other + methods might involve HTTPS or signed email content from a trusted + source. However, some more commonly used techniques for + distributing session information and starting media streams are + the Real-Time Streaming Protocol (RTSP) [25] and SIP [14]. + + + +Ott, et al. Standards Track [Page 48] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + b) RTSP -- RTSP provides a client- or server-initiated stream control + mechanism for real-time multimedia streams. The session + parameters are conveyed using SDP syntax and may adopt standard + HTTP authentication mechanisms in combination with suitable + network (e.g., IPsec)- or transport (e.g., Transport Layer + Security (TLS))-level security. + + c) SIP -- A typical use of SIP involving a unicast feedback + identifier might be a client wishing to dynamically join a multi- + party call on a multicast address using unicast RTCP feedback. + The client would be required to authenticate the SDP session + descriptor information returned by the SIP server. The + recommended method for this, as outlined in the SIP specification + [14], is to use an S/MIME message body containing the session + parameters signed with an acceptable certificate. + + For the purposes of this profile, it is acceptable to use any + suitably secure authentication mechanism that establishes the + identity and integrity of the information provided to the client. + +11.6.2. Suitable Security Solutions for RTP Sessions with Unicast + Feedback + + a) SRTP -- SRTP [3] is the recommended Audio/Video Transport (AVT) + security framework for RTP sessions. It specifies the general + packet formats and cipher operations that are used and provides + the flexibility to select different stream ciphers based on + preference/requirements. It can provide confidentiality of the + RTP and RTCP packets as well as protection against integrity + compromise and replay attacks. It provides authentication of the + data stream using the shared-key trust model. Any suitable key- + distribution mechanism can be used in parallel to the SRTP + streams. + + b) IPSEC -- A more general group security profile that might be used + is the Group Domain of Interpretation [23], which defines the + process of applying IPSec mechanisms to multicast groups. This + requires the use of the Encapsulating Security Payload (ESP) in + tunnel mode as the framework and it provides the capability to + authenticate -- either using a shared key or individually through + public-key mechanisms. It should be noted that using IPSec would + break the 'transport-independent' condition of RTP and would + therefore not be useable for anything other than IP-based + communication. + + c) TESLA - Timed Efficient Stream Loss-Tolerant Authentication + (TESLA) [24] is a scheme that provides a more flexible approach to + data authentication using time-based key disclosure. The + + + +Ott, et al. Standards Track [Page 49] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + authentication uses one-way, pseudo-random key functions based on + key chain hashes that have a short period of authenticity based on + the key disclosure intervals from the source. As long as the + receiver can ensure that the encrypted packet is received prior to + the key disclosure by the source, which requires loose time + synchronization between source and receivers, it can prove the + authenticity of the packet. The scheme does introduce a delay + into the packet distribution/decryption phase due to the key + disclosure delay; however, the processing overhead is much lower + than other standard public-key mechanisms and therefore may be + more suited to small or energy-restricted devices. + +11.6.3. Secure Key Distribution Mechanisms + + a) MIKEY -- Multimedia Internet KEYing (MIKEY) [29] is the preferred + solution for SRTP sessions providing a shared group-key + distribution mechanism and intra-session rekeying facilities. If + a partly protected communication channel exists, keys may also be + conveyed using SDP as per [27]. + + b) GSAKMP -- The Group Secure Association Key Management Protocol + (GSAKMP) is the general solution favored for Multicast Secure + group-key distribution. It is the recommended key distribution + solution for Group Domain of Interpretation (GDOI) [RFC3547] + sessions. + +11.7. Troubleshooting Misconfiguration + + As noted above, the security mechanisms in place will not help in + case an authorized source spreads properly authenticated and + integrity-protected yet incorrect information about the Feedback + Target. In this case, the accidentally communicated Feedback Target + will receive RTCP packets from a potentially large group of receivers + -- the RTCP rate fortunately limited by the RTCP timing rules. + + Yet, the RTCP packets do not provide much context information and, if + encrypted, do not provide any context, making it difficult for the + entity running (the network with) the Feedback Target to debug and + correct this problem, e.g., by tracking down and informing the origin + of the misconfiguration. + + One suitable approach may be to provide explicit context information + in RTCP packets that would allow determining the source. While such + an RTCP packet could be defined in this specification, it would be of + no use when using SRTP/SRTCP and encryption of RTCP reports. + Therefore, and because the extensions in this document may not be the + + + + + +Ott, et al. Standards Track [Page 50] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + only case that may face such a problem, it is desirable to find a + solution that is applicable to RTP at large. Such mechanisms are for + further study in the AVT WG. + +12. Backwards Compatibility + + The use of unicast feedback to the source should not present any + serious backwards compatibility issues. The RTP data streams should + remain unaffected, as should the RTCP packets from the Media + Sender(s) that continue to enable inter-stream synchronization in the + case of multiple streams. The unicast transmission of RTCP data to a + source that does not have the ability to redistribute the traffic + either by simple reflection or through summaries could have serious + security implications, as outlined in Section 11, but would not + actually break the operation of RTP. For RTP-compliant receivers + that do not understand the unicast mechanisms, the RTCP traffic may + still reach the group in the event that an ASM distribution network + is used, in which case there may be some duplication of traffic due + to the reflection channel, but this should be ignored. It is + anticipated, however, that typically the distribution network will + not enable the receiver to multicast RTCP traffic, in which case the + data will be lost and the RTCP calculations will not include those + receivers. It is RECOMMENDED that any session that may involve non- + unicast-capable clients should always use the simple packet- + reflection mechanism to ensure that the packets received can be + understood by all clients. + +13. IANA Considerations + + The following contact information shall be used for all registrations + included here: + + Contact: Joerg Ott + mail: jo@acm.org + tel: +358-9-470-22460 + + Based on the guidelines suggested in [2], a new RTCP packet format + has been registered with the RTCP Control Packet type (PT) Registry: + + Name: RSI + Long name: Receiver Summary Information + Value: 209 + Reference: This document. + + This document defines a substructure for RTCP RSI packets. A new + sub-registry has been set up for the sub-report block type (SRBT) + values for the RSI packet, with the following registrations created + initially: + + + +Ott, et al. Standards Track [Page 51] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Name: IPv4 Address + Long name: IPv4 Feedback Target Address + Value: 0 + Reference: This document. + + Name: IPv6 Address + Long name: IPv6 Feedback Target Address + Value: 1 + Reference: This document. + + Name: DNS Name + Long name: DNS Name indicating Feedback Target Address + Value: 2 + Reference: This document. + + Name: Loss + Long name: Loss distribution + Value: 4 + Reference: This document. + + Name: Jitter + Long name: Jitter Distribution + Value: 5 + Reference: This document. + + Name: RTT + Long name: Round-trip time distribution + Value: 6 + Reference: This document. + + Name: Cumulative loss + Long name: Cumulative loss distribution + Value: 7 + Reference: This document. + + Name: Collisions + Long name: SSRC Collision list + Value: 8 + Reference: This document. + + Name: Stats + Long name: General statistics + Value: 10 + Reference: This document. + + + + + + + +Ott, et al. Standards Track [Page 52] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + Name: RTCP BW + Long name: RTCP Bandwidth indication + Value: 11 + Reference: This document. + + Name: Group Info + Long name: RTCP Group and Average Packet size + Value: 12 + Reference: This document. + + The value 3 shall be reserved as a further way of specifying a + Feedback Target Address. The value 3 MUST only be allocated for a + use defined in an IETF Standards Track document. + + Further values may be registered on a first come, first served + basis. For each new registration, it is mandatory that a + permanent, stable, and publicly accessible document exists that + specifies the semantics of the registered parameter as well as the + syntax and semantics of the associated sub-report block. The + general registration procedures of [5] apply. + + In the registry for SDP parameters, the attribute named "rtcp- + unicast" has been registered as follows: + + SDP Attribute ("att-field"): + + Attribute Name: rtcp-unicast + Long form: RTCP Unicast feedback address + Type of name: att-field + Type of attribute: Media level only + Subject to charset: No + Purpose: RFC 5760 + Reference: RFC 5760 + Values: See this document. + +14. References + +14.1. Normative References + + [1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, + "RTP: A Transport Protocol for Real-Time Applications", STD 64, + RFC 3550, July 2003. + + [2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA + Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. + + + + + + +Ott, et al. Standards Track [Page 53] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + [3] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. + Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC + 3711, March 2004. + + [4] Quinn, B. and R. Finlayson, "Session Description Protocol (SDP) + Source Filters", RFC 4570, July 2006. + + [5] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session + Description Protocol", RFC 4566, July 2006. + + [6] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video + Conferences with Minimal Control", STD 65, RFC 3551, July 2003. + + [7] Huitema, C., "Real Time Control Protocol (RTCP) attribute in + Session Description Protocol (SDP)", RFC 3605, October 2003. + + [8] Meyer, D., Rockell, R., and G. Shepherd, "Source-Specific + Protocol Independent Multicast in 232/8", BCP 120, RFC 4608, + August 2006. + + [9] Holbrook, H., Cain, B., and B. Haberman, "Using Internet Group + Management Protocol Version 3 (IGMPv3) and Multicast Listener + Discovery Protocol Version 2 (MLDv2) for Source-Specific + Multicast", RFC 4604, August 2006. + + [10] Casner, S., "Session Description Protocol (SDP) Bandwidth + Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, + July 2003. + + [11] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD + 63, RFC 3629, November 2003. + + [12] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media + Attributes in the Session Description Protocol (SDP)", RFC 5576, + June 2009. + + [13] Bradner, S., "Key words for use in RFCs to Indicate Requirement + Levels", BCP 14, RFC 2119, March 1997. + +14.2. Informative References + + [14] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., + Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: + Session Initiation Protocol", RFC 3261, June 2002. + + [15] 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. + + + +Ott, et al. Standards Track [Page 54] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + [16] Pusateri, T., "Distance Vector Multicast Routing Protocol", Work + in Progress, October 2003. + + [17] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, + "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol + Specification (Revised)", RFC 4601, August 2006. + + [18] Adams, A., Nicholas, J., and W. Siadak, "Protocol Independent + Multicast - Dense Mode (PIM-DM): Protocol Specification + (Revised)", RFC 3973, January 2005. + + [19] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol + Extensions for BGP-4", RFC 4760, January 2007. + + [20] Fenner, B., Ed., and D. Meyer, Ed., "Multicast Source Discovery + Protocol (MSDP)", RFC 3618, October 2003. + + [21] Session Directory Rendez-vous (SDR), developed at University + College London by Mark Handley and the Multimedia Research + Group, http://www-mice.cs.ucl.ac.uk/multimedia/software/sdr/. + + [22] Handley, M., Perkins, C., and E. Whelan, "Session Announcement + Protocol", RFC 2974, October 2000. + + [23] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group + Domain of Interpretation", RFC 3547, July 2003. + + [24] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, + "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): + Multicast Source Authentication Transform Introduction", RFC + 4082, June 2005. + + [25] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming + Protocol (RTSP)", RFC 2326, April 1998. + + [26] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., "RTP + Control Protocol Extended Reports (RTCP XR)", RFC 3611, November + 2003. + + [27] Andreasen, F., Baugher, M., and D. Wing, "Session Description + Protocol (SDP) Security Descriptions for Media Streams", RFC + 4568, July 2006. + + [28] Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real- + time Transport Control Protocol (RTCP)-Based Feedback + (RTP/SAVPF)", RFC 5124, February 2008. + + + + + +Ott, et al. Standards Track [Page 55] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + [29] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. + Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August + 2004. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Ott, et al. Standards Track [Page 56] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +Appendix A. Examples for Sender-Side Configurations + + This appendix describes a few common setups, focusing on the + contribution side, i.e., the communications between the Media + Sender(s) and the Distribution Source. In all cases, the same + session description may be used for the distribution side as defined + in the main part of this document. This is because this + specification defines only the media stream setup between the + Distribution Source and the receivers. + +A.1. One Media Sender Identical to the Distribution Source + + In the simplest case, the Distribution Source is identical to the + Media Sender as depicted in Figure 3. Obviously, no further + configuration for the interaction between the Media Sender and the + Distribution Source is necessary. + + Source-specific + +--------+ Multicast + | | +----------------> R(1) + |M D S | | | + |E I O | +--+ | + |D S U | | | | + |I T R | | +-----------> R(2) | + |A R C |->+----- : | | + | = I E | | +------> R(n-1) | | + |S B | | | | | | + |E U | +--+--> R(n) | | | + |N T | | | | | + |D I |<---------+ | | | + |E O |<---------------+ | | + |R N |<--------------------+ | + | |<-------------------------+ + +--------+ Unicast + + Figure 3: Media Source == Distribution Source + +A.2. One Media Sender + + In a slightly more complex scenario, the Media Sender and the + Distribution Source are separate entities running on the same or + different machines. Hence, the Media Sender needs to deliver the + media stream(s) to the Distribution Source. This can be done either + via unicasting the RTP stream, via ASM multicast, or via SSM. In + this case, the Distribution Source is responsible for forwarding the + RTP packets comprising the media stream and the RTCP Sender Reports + towards the receivers and conveying feedback from the receivers, as + well as from itself, to the Media Sender. + + + +Ott, et al. Standards Track [Page 57] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + This scenario is depicted in Figure 4. The communication setup + between the Media Sender and the Distribution Source may be + statically configured or SDP may be used in conjunction with some + signaling protocol to convey the session parameters. Note that it is + a local configuration matter of the Distribution Source how to + associate a session between the Media Sender and itself (the + Contribution session) with the corresponding session between itself + and the receivers (the Distribution session). + + Source-specific + +-----+ Multicast + | | +----------------> R(1) + | D S | | | + | I O | +--+ | + | S U | | | | + +--------+ | T R | | +-----------> R(2) | + | Media |<---->| R C |->+----- : | | + | Sender | | I E | | +------> R(n-1) | | + +--------+ | B | | | | | | + | U | +--+--> R(n) | | | + | T | | | | | + | I |<---------+ | | | + | O |<---------------+ | | + | N |<--------------------+ | + | |<-------------------------+ + +-----+ Unicast + + Contribution Distribution + Session Session + (unicast or ASM) (SSM) + + Figure 4: One Media Sender Separate from Distribution Source + +A.3. Three Media Senders, Unicast Contribution + + Similar considerations apply if three Media Senders transmit to an + SSM multicast group via the Distribution Source and individually send + their media stream RTP packets via unicast to the Distribution + Source. + + In this case, the responsibilities of the Distribution Source are a + superset to the previous case; the Distribution Source also needs to + relay media traffic from each Media Sender to the receivers and to + forward (aggregated) feedback from the receivers to the Media + Senders. In addition, the Distribution Source relays RTCP packets + (SRs) from each Media Sender to the other two. + + + + + +Ott, et al. Standards Track [Page 58] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The configuration of the Media Senders is identical to the previous + case. It is just the Distribution Source that must be aware that + there are multiple senders and then perform the necessary relaying. + The transport address for the RTP session at the Distribution Source + may be identical for all Media Senders (which may make correlation + easier) or different addresses may be used. + + This setup is depicted in Figure 5. + + Source-specific + +-----+ Multicast + +--------+ | | +----------------> R(1) + | Media |<---->| D S | | | + |Sender 1| | I O | +--+ | + +--------+ | S U | | | | + | T R | | +-----------> R(2) | + +--------+ | R C |->+----- : | | + | Media |<---->| I E | | +------> R(n-1) | | + |Sender 2| | B | | | | | | + +--------+ | U | +--+--> R(n) | | | + | T | | | | | + +--------+ | I |<---------+ | | | + | Media |<---->| O |<---------------+ | | + |Sender 3| | N |<--------------------+ | + +--------+ | |<-------------------------+ + +-----+ Unicast + + Contribution Distribution + Session Session + (unicast) (SSM) + + Figure 5: Three Media Senders, Unicast Contribution + +A.4. Three Media Senders, ASM Contribution Group + + In this final example, the individual unicast contribution sessions + between the Media Senders and the Distribution Source are replaced by + a single ASM contribution group (i.e., a single common RTP session). + Consequently, all Media Senders receive each other's traffic by means + of IP-layer multicast and the Distribution Source no longer needs to + perform explicit forwarding between the Media Senders. Of course, + the Distribution Source still forwards feedback information received + from the receivers (optionally as summaries) to the ASM contribution + group. + + + + + + + +Ott, et al. Standards Track [Page 59] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The ASM contribution group may be statically configured or the + necessary information can be communicated using a standard SDP + session description for a multicast session. Again, it is up to the + implementation of the Distribution Source to properly associate the + ASM contribution session and the SSM distribution sessions. + + Figure 6 shows this scenario. + + _ Source-specific + / \ +-----+ Multicast + +--------+ | | | | +----------------> R(1) + | Media |<->| A | | D S | | | + |Sender 1| | S | | I O | +--+ | + +--------+ | M | | S U | | | | + | | | T R | | +-----------> R(2) | + +--------+ | G | | R C |->+----- : | | + | Media |<->| r |<->| I E | | +------> R(n-1) | | + |Sender 2| | o | | B | | | | | | + +--------+ | u | | U | +--+--> R(n) | | | + | p | | T | | | | | + +--------+ | | | I |<---------+ | | | + | Media |<->| | | O |<---------------+ | | + |Sender 3| \_/ | N |<--------------------+ | + +--------+ | |<-------------------------+ + +-----+ Unicast + + Contribution Distribution + Session Session + (ASM) (SSM) + + Figure 6: Three Media Senders in ASM Group + +Appendix B. Distribution Report Processing at the Receiver + +B.1. Algorithm + + Example processing of Loss Distribution Values + + X values represent the loss percentage. + Y values represent the number of receivers. + + Number of x values is the NDB value + + xrange = Max Distribution Value(MaDV) - Min Distribution Value(MnDV) + + + + + + + +Ott, et al. Standards Track [Page 60] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + First data point = MnDV,first ydata + + then + + For each ydata => xdata += (MnDV + (xrange / NDB)) + +B.2. Pseudo-Code + + Packet Variables -> factor,NDB,MnDVL,MaDV + Code variables -> xrange, ydata[NDB],x,y + + xrange = MaDV - MnDV + x = MnDV; + + for(i=0; i<NDB; i++) { + y = (ydata[i] * factor); + /*OUTPUT x and y values*/ + x += (xrange / NDB); + } + +B.3. Application Uses and Scenarios + + Providing a distribution function in a feedback message has a number + of uses for different types of applications. Although this appendix + enumerates potential uses for the distribution scheme, it is + anticipated that future applications might benefit from it in ways + not addressed in this document. Due to the flexible nature of the + summarization format, future extensions may easily be added. Some of + the scenarios addressed in this section envisage potential uses + beyond a simple SSM architecture, for example, single-source group + topologies where every receiver may in fact also be capable of + becoming the source. Another example may be multiple SSM topologies, + which, when combined, make up a larger distribution tree. + + A distribution of values is useful as input into any algorithm, + multicast or otherwise, that could be optimized or tuned as a result + of having access to the feedback values for all group members. + Following is a list of example areas that might benefit from + distribution information: + + - The parameterization of a multicast Forward Error Correction (FEC) + algorithm. Given an accurate estimate of the distribution of + reported losses, a source or other distribution agent that does not + have a global view would be able to tune the degree of redundancy + built into the FEC stream. The distribution might help to identify + whether the majority of the group is experiencing high levels of + loss, or whether in fact the high loss reports are only from a + small subset of the group. Similarly, this data might enable a + + + +Ott, et al. Standards Track [Page 61] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + receiver to make a more informed decision about whether it should + leave a group that includes a very high percentage of the worst- + case reporters. + + - The organization of a multicast data stream into useful layers for + layered coding schemes. The distribution of packet losses and + delay would help to identify what percentage of members experience + various loss and delay levels, and thus how the data stream + bandwidth might be partitioned to suit the group conditions. This + would require the same algorithm to be used by both senders and + receivers in order to derive the same results. + + - The establishment of a suitable feedback threshold. An application + might be interested to generate feedback values when above (or + below) a particular threshold. However, determining an appropriate + threshold may be difficult when the range and distribution of + feedback values is not known a priori. In a very large group, + knowing the distribution of feedback values would allow a + reasonable threshold value to be established, and in turn would + have the potential to prevent message implosion if many group + members share the same feedback value. A typical application might + include a sensor network that gauges temperature or some other + natural phenomenon. Another example is a network of mobile devices + interested in tracking signal power to assist with hand-off to a + different distribution device when power becomes too low. + + - The tuning of Suppression algorithms. Having access to the + distribution of round-trip times, bandwidth, and network loss would + allow optimization of wake-up timers and proper adjustment of the + Suppression interval bounds. In addition, biased wake-up functions + could be created not only to favor the early response from more + capable group members but also to smooth out responses from + subsequent respondents and to avoid bursty response traffic. + + - Leader election among a group of processes based on the maximum or + minimum of some attribute value. Knowledge of the distribution of + values would allow a group of processes to select a leader process + or processes to act on behalf of the group. Leader election can + promote scalability when group sizes become extremely large. + +B.4. Distribution Sub-Report Creation at the Source + + The following example demonstrates two different ways to convey loss + data using the generic format of a Loss sub-report block (Section + 7.1.4). The same techniques could also be applied to representing + other distribution types. + + + + + +Ott, et al. Standards Track [Page 62] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + 1) The first method attempts to represent the data in as few bytes as + possible. + + 2) The second method conveys all values without providing any savings + in bandwidth. + + Data Set + X values indicate loss percentage reported; Y values indicate the + number of receivers reporting that loss percentage. + + X - 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 + Y - 1000| 800 | 6 | 1800 | 2600 | 3120 | 2300 | 1100 | 200 | 103 + + X - 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 + Y - 74 | 21 | 30 | 65 | 60 | 80 | 6 | 7 | 4 | 5 + + X - 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 + Y - 2 | 10 | 870 | 2300 | 1162 | 270 | 234 | 211 | 196 | 205 + + X - 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 + Y - 163 | 174 | 103 | 94 | 76 | 52 | 68 | 79 | 42 | 4 + + Constant value + Due to the size of the multiplicative factor field being 4 bits, the + maximum multiplicative value is 15. + + The distribution type field of this packet would be value 1 since it + represents loss data. + + Example: 1st Method + + Description + The minimal method of conveying data, i.e., small amount of bytes + used to convey the values. + + Algorithm + Attempt to fit the data set into a small sub-report size, selected + length of 8 octets + + Can we split the range (0 - 39) into 16 4-bit values? The largest + bucket value would, in this case, be the bucket for X values 5 - + 7.5, the sum of which is 5970. An MF value of 9 will generate a + multiplicative factor of 2^9, or 512 -- which, multiplied by the + max bucket value, produces a maximum real value of 7680. + + + + + + + +Ott, et al. Standards Track [Page 63] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The packet fields will contain the values: + + Header distribution Block + Distribution Type: 1 + Number of Data Buckets: 16 + Multiplicative Factor: 9 + Packet Length field: 5 (5 * 4 => 20 bytes) + Minimum Data Value: 0 + Maximum Data Value: 39 + Data Bucket values: (each value is 16-bits) + + Results, 4-bit buckets: + + X - 0 - 2.5 | 2.5 - 5 | 5 - 7.5 | 7.5 - 10 + (Y - 1803 | 4403 | 5970 | 853 ) ACTUAL + Y - 4 | 9 | 12 | 2 + + X - 10 - 12.5 | 12.5 - 15 | 15 - 17.5 | 17.5 - 20 + (Y - 110 | 140 | 89.5 | 12.5) ACTUAL + Y - 0 | 0 | 0 | 0 + + X - 20 - 22.5 | 22.5 - 25 | 25 - 27.5 | 27.5 - 30 + (Y - 447 | 3897 | 609.5 | 506.5) ACTUAL + Y - 1 | 8 | 1 | 1 + + X - 30 - 32.5 | 32.5 - 35 | 35 - 37.5 | 37.5 - 40 + (Y - 388.5 | 221.5 | 159.5 | 85.5) ACTUAL + Y - 1 | 0 | 0 | 0 + + Example: 2nd Method + + Description + This demonstrates the most accurate method for representing the + data set. This method doesn't attempt to optimise any values. + + Algorithm + Identify the highest value and select buckets large enough to + convey the exact values, i.e., no multiplicative factor. + + The highest value is 3120. This requires 12 bits (closest 2 bit + boundary) to represent, therefore it will use 60 bytes to + represent the entire distribution. This is within the max packet + size; therefore, all data will fit within one sub-report block. + The multiplicative value will be 1. + + + + + + + +Ott, et al. Standards Track [Page 64] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + + The packet fields will contain the values: + + Header Distribution Block + Distribution Type: 1 + Number of Data Buckets: 40 + Multiplicative Factor: 0 + Packet Length field: 18 (18 * 4 => 72 bytes) + Minimum Loss Value: 0 + Maximum Loss Value: 39 + + Bucket values are the same as the initial data set. + + Result + Selecting one of the three methods outlined above might be done by + a congestion parameter or by user preference. The overhead + associated with processing the packets is likely to differ very + little between the techniques. The savings in bandwidth are + apparent, however, using 20, 52, and 72 octets respectively. + These values would vary more widely for a larger data set with + less correlation between results. + +Acknowledgements + + The authors would like to thank Magnus Westerlund, Dave Oran, Tom + Taylor, and Colin Perkins for detailed reviews and valuable comments. + + + + + + + + + + + + + + + + + + + + + + + + + + +Ott, et al. Standards Track [Page 65] + +RFC 5760 RTCP with Unicast Feedback February 2010 + + +Authors' Addresses + + Joerg Ott + Aalto University + School of Science and Technology + Department of Communications and Networking + PO Box 13000 + FIN-00076 Aalto + + EMail: jo@acm.org + + + Julian Chesterfield + University of Cambridge + Computer Laboratory, + 15 JJ Thompson Avenue + Cambridge + CB3 0FD + UK + + EMail: julianchesterfield@cantab.net + + + Eve Schooler + Intel Research / CTL + MS RNB6-61 + 2200 Mission College Blvd. + Santa Clara, CA, USA 95054-1537 + + EMail: eve_schooler@acm.org + + + + + + + + + + + + + + + + + + + + + +Ott, et al. Standards Track [Page 66] + |