





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



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   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

















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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



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   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




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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



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      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



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   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.





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   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.




















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                                        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.






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   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:




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   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



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   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.






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   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.




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   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.






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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.



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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                     |
   |                             ...                               |
   |                             ...                               |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+




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   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).







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   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.






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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.




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      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.




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    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.




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   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.






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   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                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   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                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   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.









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    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



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   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).





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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).




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       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.







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            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.






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   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.




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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.






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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.



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   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.




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   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.



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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



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   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



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   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



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   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).








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   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.






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   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




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   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.





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   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




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      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




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      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.



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      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.





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      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:






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   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].



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   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



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      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





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   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:



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      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.







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      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.






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   [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.



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   [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.





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   [29] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
        Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August
        2004.
















































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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.



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   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.





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   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.







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   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)







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   First data point = MnDV,first ydata

   then

   For each ydata => xdata += (MnDV + (xrange / NDB))

B.2.  Pseudo-Code

   Packet Variables -> factor,NDB,MnDVL,MaDV
   Code variables -> xrange, ydata[NDB],x,y

   xrange = MaDV - MnDV
   x = MnDV;

   for(i=0; i<NDB; i++) {
        y = (ydata[i] * factor);
        /*OUTPUT x and y values*/
        x += (xrange / NDB);
   }

B.3.  Application Uses and Scenarios

   Providing a distribution function in a feedback message has a number
   of uses for different types of applications.  Although this appendix
   enumerates potential uses for the distribution scheme, it is
   anticipated that future applications might benefit from it in ways
   not addressed in this document.  Due to the flexible nature of the
   summarization format, future extensions may easily be added.  Some of
   the scenarios addressed in this section envisage potential uses
   beyond a simple SSM architecture, for example, single-source group
   topologies where every receiver may in fact also be capable of
   becoming the source.  Another example may be multiple SSM topologies,
   which, when combined, make up a larger distribution tree.

   A distribution of values is useful as input into any algorithm,
   multicast or otherwise, that could be optimized or tuned as a result
   of having access to the feedback values for all group members.
   Following is a list of example areas that might benefit from
   distribution information:

   - The parameterization of a multicast Forward Error Correction (FEC)
     algorithm.  Given an accurate estimate of the distribution of
     reported losses, a source or other distribution agent that does not
     have a global view would be able to tune the degree of redundancy
     built into the FEC stream.  The distribution might help to identify
     whether the majority of the group is experiencing high levels of
     loss, or whether in fact the high loss reports are only from a
     small subset of the group.  Similarly, this data might enable a



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     receiver to make a more informed decision about whether it should
     leave a group that includes a very high percentage of the worst-
     case reporters.

   - The organization of a multicast data stream into useful layers for
     layered coding schemes.  The distribution of packet losses and
     delay would help to identify what percentage of members experience
     various loss and delay levels, and thus how the data stream
     bandwidth might be partitioned to suit the group conditions.  This
     would require the same algorithm to be used by both senders and
     receivers in order to derive the same results.

   - The establishment of a suitable feedback threshold.  An application
     might be interested to generate feedback values when above (or
     below) a particular threshold.  However, determining an appropriate
     threshold may be difficult when the range and distribution of
     feedback values is not known a priori.  In a very large group,
     knowing the distribution of feedback values would allow a
     reasonable threshold value to be established, and in turn would
     have the potential to prevent message implosion if many group
     members share the same feedback value.  A typical application might
     include a sensor network that gauges temperature or some other
     natural phenomenon.  Another example is a network of mobile devices
     interested in tracking signal power to assist with hand-off to a
     different distribution device when power becomes too low.

   - The tuning of Suppression algorithms.  Having access to the
     distribution of round-trip times, bandwidth, and network loss would
     allow optimization of wake-up timers and proper adjustment of the
     Suppression interval bounds.  In addition, biased wake-up functions
     could be created not only to favor the early response from more
     capable group members but also to smooth out responses from
     subsequent respondents and to avoid bursty response traffic.

   - Leader election among a group of processes based on the maximum or
     minimum of some attribute value.  Knowledge of the distribution of
     values would allow a group of processes to select a leader process
     or processes to act on behalf of the group.  Leader election can
     promote scalability when group sizes become extremely large.

B.4.  Distribution Sub-Report Creation at the Source

   The following example demonstrates two different ways to convey loss
   data using the generic format of a Loss sub-report block (Section
   7.1.4).  The same techniques could also be applied to representing
   other distribution types.





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   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.







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      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.







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      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.


























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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





















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