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rfc:rfc5760

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:
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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]

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