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Versions: 00 01 02 03 04 05 06 07 RFC 5401

Reliable Multicast Transport (RMT)                            B. Adamson
Working Group                                                        NRL
Internet-Draft                                                C. Bormann
Expires: 02 September 2006                       Universitaet Bremen TZI
                                                              M. Handley
                                                                     UCL
                                                               J. Macker
                                                                     NRL
                                                              March 2006


        Multicast Negative-Acknowledgment (NACK) Building Blocks
                   draft-ietf-rmt-bb-norm-revised-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on March 17, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document discusses the creation of reliable multicast protocols
   utilizing negative-acknowledgment (NACK) feedback.  The rationale for
   protocol design goals and assumptions are presented.  Technical



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   challenges for NACK-based (and in some cases general) reliable
   multicast protocol operation are identified.  These goals and
   challenges are resolved into a set of functional "building blocks"
   that address different aspects of reliable multicast protocol
   operation.  It is anticipated that these building blocks will be
   useful in generating different instantiations of reliable multicast
   protocols.












































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                           Table of Contents


1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . .   4
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
 2.1. Delivery Service Model . . . . . . . . . . . . . . . . . . . .   6
 2.2. Group Membership Dynamics. . . . . . . . . . . . . . . . . . .   6
 2.3. Sender/Receiver Relationships. . . . . . . . . . . . . . . . .   7
 2.4. Group Size Scalability . . . . . . . . . . . . . . . . . . . .   7
 2.5. Data Delivery Performance. . . . . . . . . . . . . . . . . . .   7
 2.6. Network Environments . . . . . . . . . . . . . . . . . . . . .   7
 2.7. Router/Intermediate System Assistance. . . . . . . . . . . . .   8
3. Functionality . . . . . . . . . . . . . . . . . . . . . . . . . .   8
 3.1. Multicast Sender Transmission. . . . . . . . . . . . . . . . .  11
 3.2. NACK Repair Process. . . . . . . . . . . . . . . . . . . . . .  13
  3.2.1. Receiver NACK Process Initiation. . . . . . . . . . . . . .  13
  3.2.2. NACK Suppression. . . . . . . . . . . . . . . . . . . . . .  15
  3.2.3. NACK Content. . . . . . . . . . . . . . . . . . . . . . . .  19
   3.2.3.1. NACK and FEC Repair Strategies . . . . . . . . . . . . .  19
   3.2.3.2. NACK Content Format. . . . . . . . . . . . . . . . . . .  22
  3.2.4. Sender Repair Response. . . . . . . . . . . . . . . . . . .  24
 3.3. Multicast Receiver Join Policies and Proce-
 dures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
 3.4. Reliable Multicast Member Identification . . . . . . . . . . .  26
 3.5. Data Content Identification. . . . . . . . . . . . . . . . . .  27
 3.6. Forward Error Correction (FEC) . . . . . . . . . . . . . . . .  28
 3.7. Round-trip Timing Collection . . . . . . . . . . . . . . . . .  29
  3.7.1. One-to-Many Sender GRTT Measurement . . . . . . . . . . . .  30
  3.7.2. One-to-Many Receiver RTT Measurement. . . . . . . . . . . .  32
  3.7.3. Many-to-Many RTT Measurement. . . . . . . . . . . . . . . .  32
  3.7.4. Sender GRTT Advertisement . . . . . . . . . . . . . . . . .  32
 3.8. Group Size Determination/Estimation. . . . . . . . . . . . . .  33
 3.9. Congestion Control Operation . . . . . . . . . . . . . . . . .  34
 3.10. Router/Intermediate System Assistance . . . . . . . . . . . .  34
 3.11. NACK-based reliable multicast Applicability . . . . . . . . .  34
4. Security Considerations . . . . . . . . . . . . . . . . . . . . .  35
5. Changes from RFC3941. . . . . . . . . . . . . . . . . . . . . . .  35
6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . .  36
7. References. . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
 7.1. Normative References . . . . . . . . . . . . . . . . . . . . .  36
 7.2. Informative References . . . . . . . . . . . . . . . . . . . .  36
8. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . .  38
9. Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .  38








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1.  Introduction

   Reliable multicast transport is a desirable technology for efficient
   and reliable distribution of data to a group on the Internet.  The
   complexities of group communication paradigms necessitate different
   protocol types and instantiations to meet the range of performance
   and scalability requirements of different potential reliable
   multicast applications and users [3].  This document addresses the
   creation of reliable multicast protocols utilizing negative-
   acknowledgment (NACK) feedback.  While different protocol
   instantiations may be required to meet specific application and
   network architecture demands [5], there are a number of fundamental
   components that may be common to these different instantiations.
   This document describes the framework and common "building block"
   components relevant to multicast protocols based primarily on NACK
   operation for reliable transport.  While this document discusses a
   large set of reliable multicast components and issues relevant to
   NACK-based reliable multicast protocol design, it specifically
   addresses in detail the following building blocks which are not
   addressed in other IETF documents:

     1)   Multicast sender transmission strategies,

     2)   NACK repair process with timer-based feedback suppression, and

     3)   Round-trip timing for adapting NACK and other timers.

   The potential relationships to other reliable multicast transport
   building blocks (Forward Error Correction (FEC), congestion control)
   and general issues with NACK-based reliable multicast protocols are
   also discussed.  This document is a product of the IETF RMT WG and
   follows the guidelines provided in RFC 3269 [6].  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 BCP 14, RFC 2119 [1].

Statement of Intent

   This memo contains descriptions of building blocks that can be
   applied in the design of Reliable Multicast protocols utilizing
   Negative-Acknowledgemnet (NACK) feedback.  RFC3941 [4] contained a
   previous description of the this specification .  RF3941 was
   published in the "Experimental" category.  It was the stated intent
   of the RMT working group to re-submit this specifications as an IETF
   Proposed Standard in due course.

   This Proposed Standard specification is thus based on RFC3941 [4] and
   has been updated according to accumulated experience and growing



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   protocol maturity since the publication of RFC3941.  Said experience
   applies both to this specification itself and to congestion control
   strategies related to the use of this specification.

   The differences between RFC3941 [4] and this document are listed in
   Section 5.

2.  Rationale

   Each potential protocol instantiation using the building blocks
   presented here (and in other applicable building block documents)
   will have specific criteria that may influence individual protocol
   design.  To support the development of applicable building blocks, it
   is useful to identify and summarize driving general protocol design
   goals and assumptions.  These are areas that each protocol
   instantiation will need to address in detail.  Each building block
   description in this document will include a discussion of the impact
   of these design criteria.  The categories of design criteria
   considered here include:

     1)   Delivery Service Model,
     2)   Group Membership Dynamics,
     3)   Sender/receiver relationships,
     4)   Group Size Scalability,
     5)   Data Delivery Performance,
     6)   Network Environments, and
     7)   Router/Intermediate System Interactions.

   All of these areas are at least briefly discussed.  Additionally,
   other reliable multicast transport building block documents such as
   [10] have been created to address areas outside of the scope of this
   document.  NACK-based reliable multicast protocol instantiations may
   depend upon these other building blocks as well as the ones presented
   here.  This document focuses on areas that are unique to NACK-based
   reliable multicast but may be used in concert with the other building
   block areas.  In some cases, a building block may be able address a
   wide range of assumptions, while in other cases there will be trade-
   offs required to meet different application needs or operating
   environments.  Where necessary, building block features are designed
   to be parametric to meet different requirements.  Of course, an
   underlying goal will be to minimize design complexity and to at least
   recommend default values for any such parameters that meet a general
   purpose "bulk data transfer" requirement in a typical Internet
   environment.







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2.1.  Delivery Service Model

   The implicit goal of a reliable multicast transport protocol is the
   reliable delivery of data among a group of members communicating
   using IP multicast datagram service.  However, the specific service
   the application is attempting to provide can impact design decisions.
   A most basic service model for reliable multicast transport is that
   of "bulk transfer" which is a primary focus of this and other related
   RMT working group documents.  However, the same principles in
   protocol design may also be applied to other services models, e.g.,
   more interactive exchanges of small messages such as with white-
   boarding or text chat.  Within these different models there are
   issues such as the sender's ability  to cache transmitted data (or
   state referencing it) for retransmission or repair.  The needs for
   ordering and/or causality in the sequence of transmissions and
   receptions among members in the group may be different depending upon
   data content.  The group communication paradigm differs significantly
   from the point-to-point model in that, depending upon the data
   content type, some receivers may complete reception of a portion of
   data content and be able to act upon it before other members have
   received the content.  This may be acceptable (or even desirable) for
   some applications but not for others.  These varying requirements
   drive the need for a number of different protocol instantiation
   designs.  A significant challenge in developing generally useful
   building block mechanisms is accommodating even a limited range of
   these capabilities without defining specific application-level
   details.


2.2.  Group Membership Dynamics

   One area where group communication can differ from point-to-point
   communications is that even if the composition of the group changes,
   the "thread" of communication can still exist.  This contrasts with
   the point-to-point communication model where, if either of the two
   parties leave, the communication process (exchange of data) is
   terminated (or at least paused).  Depending upon  application goals,
   senders and receivers participating in a reliable multicast transport
   "session" may be able to join late, leave, and/or potentially rejoin
   while the ongoing group communication "thread"  still remains
   functional and useful.  Also note that this can impact protocol
   message content.  If "late joiners" are supported, some amount of
   additional information may be placed in  message headers to
   accommodate this functionality.  Alternatively, the information may
   be sent in its own message (on demand or intermittently) if the
   impact to the overhead of typical message transmissions is deemed too
   great.  Group dynamics can also impact other protocol mechanisms such
   as NACK timing, congestion control operation, etc.



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2.3.  Sender/Receiver Relationships

   The relationship of senders and receivers among group members
   requires consideration.  In some applications, there may be a single
   sender multicasting to a group of receivers.  In other cases, there
   may be more than one sender or the potential for everyone in the
   group to be a sender _and_ receiver of data may exist.


2.4.  Group Size Scalability

   Native IP multicast [2] may scale to extremely large group sizes.  It
   may be desirable for some applications to scale along with the
   multicast infrastructure's ability to scale.  In its simplest form,
   there are limits to the group size to which a NACK-based protocol can
   apply without NACK implosion problems.  Research suggests that NACK-
   based reliable multicast group sizes on the order of tens of
   thousands of receivers may operate with modest feedback to the sender
   using probabilistic, timer-based suppression techniques [8].
   However, the potential for router assistance and/or other NACK
   suppression heuristics may enable these protocols to scale to very
   large group sizes.  In large scale cases, it may be prohibitive for
   members to maintain state on all other members (in particular, other
   receivers) in the group.  The impact of group size needs to be
   considered in the development of applicable building blocks.


2.5.  Data Delivery Performance

   There is a trade-off between scalability and data delivery latency
   when designing NACK-oriented protocols.  If probabilistic, timer-
   based NACK suppression is to be used, there will be some delays built
   into the NACK process to allow suppression to occur and for the
   sender of data to identify appropriate content for efficient repair
   transmission.  For example, backoff timeouts can be used to ensure
   efficient NACK suppression and repair transmission, but this comes at
   a cost of increased delivery latency and increased buffering
   requirements for both senders and receivers.  The building blocks
   SHOULD allow applications to establish bounds for data delivery
   performance.  Note that application designers must be aware of the
   scalability trade-off that is made when such bounds are applied.


2.6.  Network Environments

   The Internet Protocol has historically assumed a role of providing
   service across heterogeneous network topologies.  It is desirable
   that a reliable multicast protocol be capable of effectively



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   operating across a wide range of the networks to which general
   purpose IP service applies.  The bandwidth available on the links
   between the members of a single group today may vary between low
   numbers of kbit/s for wireless links and multiple Gbit/s for high
   speed LAN connections, with varying degrees of contention from other
   flows.  Recently, a number of asymmetric network services including
   56K/ADSL modems, CATV Internet service, satellite and other wireless
   communication services have begun to proliferate.  Many of these are
   inherently broadcast media with potentially large "fan-out" to which
   IP multicast service is highly applicable.  Additionally, policy
   and/or technical issues may result in topologies where multicast
   connectivity is limited to a single source multicast (SSM) model from
   a specific source [9].  Receivers in the group may be restricted to
   unicast feedback for NACKs and other messages.  Consideration must be
   given, in building block development and protocol design, to the
   nature of the underlying networks.


2.7.  Router/Intermediate System Assistance

   While intermediate assistance from devices/systems with direct
   knowledge of the underlying network topology may be used to leverage
   the performance and scalability of reliable multicast protocols,
   there will continue to be a number of instances where this is not
   available or practical.  Any building block components for NACK-
   oriented reliable multicast SHALL be capable of operating without
   such assistance.  However, it is RECOMMENDED that such protocols also
   consider utilizing these features when available.


3.  Functionality

   The previous section has presented the role of protocol building
   blocks and some of the criteria that may affect NACK-based reliable
   multicast building block identification/design.  This section
   describes different building block areas applicable to NACK-based
   reliable multicast protocols.  Some of these areas are specific to
   NACK-based protocols.  Detailed descriptions of such  areas are
   provided.  In other cases, the areas (e.g., node identifiers, forward
   error correction (FEC), etc.) may be applicable to other forms of
   reliable multicast.  In those cases, the discussion below describes
   requirements placed on those other general building block areas from
   the standpoint of NACK-based reliable multicast.  Where applicable,
   other building block documents are referenced for possible
   contribution to NACK-based reliable multicast protocols.

   For each building block, a notional "interface description" is
   provided to illustrate any dependencies of one building block



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   component upon another or upon other protocol parameters.  A building
   block component may require some form of "input" from another
   building block component or other source to perform its function.
   Any "inputs" required by a building block component and/or any
   resultant "output" provided will be defined and described in each
   building block component's interface description.  Note that the set
   of building blocks presented here do not fully satisfy each other's
   "input" and "output" needs.  In some cases, "inputs" for the building
   blocks here must come from other building blocks external to this
   document (e.g., congestion control or FEC).  In other cases NACK-
   based reliable multicast building block "inputs" must be satisfied by
   the specific protocol instantiation or implementation (e.g.,
   application data and control).

   The following building block components relevant to NACK-based
   reliable multicast are identified:

   (Specific to NACK-based Reliable Multicast)
     1)   Multicast Sender Transmission
     2)   NACK Repair Process
     3)   Multicast Receiver Join Policies

   (General Purpose)

     4)   Node (member) Identification

     5)   Data Content Identification

     6)   Forward Error Correction (FEC)

     7)   Round-trip Timing Collection

     8)   Group Size Determination/Estimation

     9)   Congestion Control Operation

     10)  Router/Intermediate System Assistance

     11)  Ancillary Protocol Mechanisms

   Figure 1 provides a pictorial overview of these building block areas
   and some of their relationships.  For example, the content of the
   data messages that a sender initially transmits depends upon the
   "Node Identification", "Data Content Identification", and "FEC"
   components while the rate of message transmission will generally
   depend upon the "Congestion Control" component.  Subsequently, the
   receivers' response to these transmissions (e.g., NACKing for repair)
   will depend upon the data message content and inputs from other



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   building block components.  Finally, the sender's processing of
   receiver responses will feed back into its transmission strategy.

   The components on the left side of this figure are areas that may be
   applicable beyond NACK-based reliable multicast.  The most
   significant of these components are discussed in other building block
   documents such as [10].  A brief description of these areas and their
   role in NACK-based reliable multicast protocols is given below.  The
   components on the right are seen as specific to NACK-based reliable
   multicast protocols, most notably the NACK repair process.  These
   areas are discussed in detail below.  Some other components (e.g.,
   "Security") impact many aspects of the protocol, and others such as
   "Router Assistance" may be more transparent to the core protocol
   processing.  The sections below describe the "Multicast Sender
   Transmission", "NACK Repair Process", and "RTT Collection" building
   blocks in detail.  The relationships to and among the other building
   block areas are also discussed, focusing on issues applicable to
   NACK-based reliable multicast protocol design.  Where applicable,
   specific technical recommendations are made for mechanisms that will
   properly satisfy the goals of NACK-based reliable multicast transport
   for the Internet.






























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                                        Application Data and Control
                                                    |
                                                    V
       .---------------------.            .-----------------------.
       | Node Identification |----------->|  Sender Transmission  |<----.
       `---------------------'       _.-' `-----------------------'     |
       .---------------------.   _.-' .'           | .--------------.   |
       | Data Identification |--'   .''            | |  Join Policy |   |
       `---------------------'    .' '             V `--------------'   |
       .---------------------.  .'  '     .----------------------.      |
    .->| Congestion Control  |-'   '      | Receiver NACK        |      |
    |  `---------------------'   .'       | Repair Process       |      |
    |  .---------------------. .'         | .------------------. |      |
    |  |        FEC          |'.          | | NACK Initiation  | |      |
    |  `---------------------'` `._       | `------------------' |      |
    |  .---------------------. ``. `-._   | .------------------. |      |
    `--|    RTT Collection   |._` `    `->| | NACK Content     | |      |
       `---------------------'` `` `      | `------------------' |      |
       .---------------------.  ` ``-`._  | .------------------. |      |
       |    Group Size Est.  |---`-`---`->| | NACK Suppression | |      |
       `---------------------'`.  ` `     | `------------------' |      |
       .---------------------.  `  ` `    `----------------------'      |
       |       Other         |   `  ` `           | +-----------------+ |
       `---------------------'    `  ` `          | |Router Assistance| |
                                   `. ` `         V +-----------------+ |
                                     `-` >.-------------------------.   |
                                          | Sender NACK Processing  |___/
                                          | and Repair Response     |
                                          `-------------------------'
                       ^                         ^
                       |                         |
                     .-----------------------------.
                     |         (Security)          |
                     `-----------------------------'
      Fig. 1 - NACK-based Reliable Multicast Building Block Framework



3.1.  Multicast Sender Transmission

   Reliable multicast senders will transmit data content to the
   multicast session.  The data content will be application dependent.
   The sender will transmit data content at a rate, and with message
   sizes, determined by application and/or network architecture
   requirements.  Any FEC encoding of sender transmissions SHOULD
   conform with the guidelines of [10].  When congestion control
   mechanisms are needed (REQUIRED for general Internet operation), the
   sender transmission rate SHALL be controlled by the congestion



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   control mechanism.  In any case, it is RECOMMENDED that all data
   transmissions from multicast senders be subject to rate limitations
   determined by the application or congestion control algorithm.  The
   sender's transmissions SHOULD make good utilization of the available
   capacity (which may be limited by the application and/or by
   congestion control).  As a result, it is expected there will be
   overlap and multiplexing of new data content transmission with repair
   content.  Other factors related to application operation may
   determine sender transmission formats and methods.  For example, some
   consideration needs to be given to the sender's behavior during
   intermittent idle periods when it has no data to transmit.

   In addition to data content, other sender messages or commands may be
   employed as part of protocol operation.  These messages may occur
   outside of the scope of application data transfer.  In NACK-based
   reliable multicast protocols, reliability of such protocol messages
   may be attempted by redundant transmission when positive
   acknowledgement is prohibitive due to group size scalability
   concerns.  Note that protocol design SHOULD provide mechanisms for
   dealing with cases where such messages are not received by the group.
   As an example, a command message might be redundantly transmitted by
   a sender to indicate that it is temporarily (or permanently) halting
   transmission.  At this time, it may be appropriate for receivers to
   respond with NACKs for any outstanding repairs they require following
   the rules of the NACK procedure.  For efficiency, the sender should
   allow sufficient time between the redundant transmissions to receive
   any NACK responses from the receivers to this command.

   In general, when there is any resultant NACK or other feedback
   operation, the timing of redundant transmission of control messages
   issued by a sender and other NACK-based reliable multicast protocol
   timeouts should be dependent upon the group greatest round trip
   timing (GRTT) estimate and any expected resultant NACK or other
   feedback operation.  The sender GRTT is an estimate of the worst-case
   round-trip timing from a given sender to any receivers in the group.
   It is assumed that the GRTT interval is a conservative estimate of
   the maximum span (with respect to delay) of the multicast group
   across a network topology with respect to given sender.  NACK-based
   reliable multicast instantiations SHOULD be able to dynamically adapt
   to a wide range of multicast network topologies.











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Sender Transmission Interface Description

   Inputs:

     1)   Application data and control
     2)   Sender node identifier
     3)   Data identifiers
     4)   Segmentation and FEC parameters
     5)   Transmission rate
     6)   Application controls
     7)   Receiver feedback messages (e.g., NACKs)

   Outputs:

     1)   Controlled transmission of messages with headers uniquely
          identifying data or repair content within the context of the
          reliable multicast session.
     2)   Commands indicating sender's status or other transport control
          actions to be taken.

3.2.  NACK Repair Process

   A critical component of NACK-based reliable multicast protocols is
   the NACK repair process.  This includes the receiver's role in
   detecting and requesting repair needs, and the sender's response to
   such requests.  There are four primary elements of the NACK repair
   process:

     1)   Receiver NACK process initiation,

     3)   NACK suppression,

     2)   NACK message content,

     4)   Sender NACK processing and response.

3.2.1.  Receiver NACK Process Initiation

   The NACK process (cycle) will be initiated by receivers that detect a
   need for repair transmissions from a specific sender to achieve
   reliable reception.  When FEC is applied, a receiver should initiate
   the NACK process only when it is known its repair requirements exceed
   the amount of pending FEC transmission for a given coding block of
   data content.  This can be determined at the end of the current
   transmission block (if it is indicated) or upon the start of
   reception of a subsequent coding block or transmission object.  This
   implies the sender data content is marked to identify its FEC block
   number and that ordinal relationship is preserved in order of



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   transmission.

   Alternatively, if the sender's transmission advertises the quantity
   of repair packets it is already planning to send for a block, the
   receiver may be able to initiate the NACK processor earlier.
   Allowing receivers to initiate NACK cycles at any time they detect
   their repair needs have exceeded pending repair transmissions may
   result in slightly quicker repair cycles.  However, it may be useful
   to limit NACK process initiation to specific events such as at the
   end-of-transmission of an FEC coding block or upon detection of
   subsequent coding blocks.  This can allow receivers to aggregate NACK
   content into a smaller number of NACK messages and provide some
   implicit loose synchronization among the receiver set to help
   facilitate effective probabilistic suppression of NACK feedback.  The
   receiver MUST maintain a history of data content received from the
   sender to determine its current repair needs.  When FEC is employed,
   it is expected that the history will correspond to a record of
   pending or partially-received coding blocks.

   For probabilistic, timer-base suppression of feedback, the NACK cycle
   should begin with receivers observing backoff timeouts.  In
   conjunction with initiating this backoff timeout, it is important
   that the receivers record the current position in the sender's
   transmission sequence at which they initiate the NACK cycle.  When
   the suppression backoff timeout expires, the receivers should only
   consider their repair needs up to this recorded transmission position
   in making the decision to transmit or suppress a NACK.  Without this
   restriction, suppression is greatly reduced as additional content is
   received from the sender during the time a NACK message propagates
   across the network to the sender and other receivers.

Receiver NACK Process Initiation Interface Description

   Inputs:

     1)   Sender data content with sequencing identifiers from sender
          transmissions.

     2)   History of content received from sender.

   Outputs:

     1)   NACK process initiation decision

     2)   Recorded sender transmission sequence position.






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3.2.2.  NACK Suppression

   An effective feedback suppression mechanism is the use of random
   backoff timeouts prior to NACK transmission by receivers requiring
   repairs [11].  Upon expiration of the backoff timeout, a receiver
   will request repairs unless its pending repair needs have been
   completely superseded by NACK messages heard from other receivers
   (when receivers are multicasting NACKs) or from some indicator from
   the sender.  When receivers are unicasting NACK messages, the sender
   may facilitate NACK suppression by forwarding a representation of
   NACK content it has received to the group at large or provide some
   other indicator of the repair information it will be subsequently
   transmitting.

   For effective and scalable suppression performance, the backoff
   timeout periods used by receivers should be independently, randomly
   picked by receivers with a truncated exponential distribution [7].
   This results in the majority of the receiver set holding off
   transmission of NACK messages under the assumption that the smaller
   number of "early NACKers" will supersede the repair needs of the
   remainder of the group.  The mean of the distribution should be
   determined as a function of the current estimate of sender<->group
   GRTT and a group size estimate that is determined by other mechanisms
   within the protocol or preset by the multicast application.

   A simple algorithm can be constructed to generate random backoff
   timeouts with the appropriate distribution.  Additionally, the
   algorithm may be designed to optimize the backoff distribution given
   the number of receivers (R) potentially generating feedback.  This
   "optimization" minimizes the number of feedback messages (e.g., NACK)
   in the worst-case situation where all receivers generate a NACK. The
   maximum backoff timeout (T_maxBackoff) can be set to control reliable
   delivery latency versus volume of feedback traffic.  A larger value
   of T_maxBackoff will result in a lower density of feedback traffic
   for a given repair cycle.  A smaller value of T_maxBackoff results in
   shorter latency which also reduces the buffering requirements of
   senders and receivers for reliable transport.

   Given the receiver group size (R), and maximum allowed backoff
   timeout (T_maxBackoff), random backoff timeouts (t') with a truncated
   exponential distribution can be picked with the following algorithm:


     1)   Establish an optimal mean (L) for the exponential backoff
          based on the group size:

                                  L = ln(R) + 1




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     2)   Pick a random number (x) from a uniform distribution over a
          range of:

                   L                           L                   L
           --------------------  to   --------------------  +  ----------
          T_maxBackoff*(exp(L)-1)    T_maxBackoff*(exp(L)-1)  T_maxBackoff


     3)   Transform this random variate to generate the desired random
          backoff time (t') with the following equation:

          t' = T_maxBackoff/L * ln(x * (exp(L) - 1) * (T_maxBackoff/L))


   This C language function can be used to generate an appropriate
   random backoff time interval:

     double RandomBackoff(double maxTime, double groupSize)
     {
         double lambda = log(groupSize) + 1;
         double x = UniformRand(lambda/maxTime) +
                    lambda / (maxTime*(exp(lambda)-1));
         return ((maxTime/lambda) *
                 log(x*(exp(lambda)-1)*(maxTime/lambda)));
     }  // end RandomBackoff()

   where UniformRand(double max) returns random numbers with a uniform
   distribution from the range of 0..max.  For example, based on the
   POSIX "rand()" function, the following C code can be used:

     double UniformRand(double max)
     {
         return (max * ((double)rand()/(double)RAND_MAX));
     }

   The number of expected NACK messages generated (N) within the first
   round trip time for a single feedback event is approximately:

                 N = exp(1.2 * L / (2*T_maxBackoff/GRTT))

   Thus the maximum backoff time can be adjusted to tradeoff worst-case
   NACK feedback volume versus latency.  This is derived from [7] and
   assumes  T_maxBackoff >= GRTT, and L is the mean of the distribution
   optimized for the given group size as shown in the algorithm above.
   Note that other mechanisms within the protocol may work to reduce
   redundant NACK generation further.  It is suggested that T_maxBackoff
   be selected as an integer multiple of the sender's current advertised
   GRTT estimate such that:



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                   T_maxBackoff = K * GRTT ;where K >= 1

   For general Internet operation, a default value of K=4 is RECOMMENDED
   for operation with multicast (to the group at large) NACK delivery
   and a value of K=6 for unicast NACK delivery.  Alternate values may
   be used to for buffer utilization, reliable delivery latency and
   group size scalability tradeoffs.

   Given that (K*GRTT) is the maximum backoff time used by the receivers
   to initiate NACK transmission, other timeout periods related to the
   NACK repair process can be scaled accordingly.  One of those timeouts
   is the amount of time a receiver should wait after generating a NACK
   message before allowing itself to initiate another NACK
   backoff/transmission cycle (T_rcvrHoldoff).  This delay should be
   sufficient for the sender to respond to the received NACK with repair
   messages.  An appropriate value depends upon the amount of time for
   the NACK to reach the sender and the sender to provide a repair
   response.  This MUST include any amount of sender NACK aggregation
   period during which possible multiple NACKs are accumulated to
   determine an efficient repair response.  These timeouts are further
   discussed in the section below on "Sender NACK Processing and Repair
   Response".

   There are also secondary measures that can be applied to improve the
   performance of feedback suppression.  For example, the sender's data
   content transmissions can follow an ordinal sequence of transmission.
   When repairs for data content occur, the receiver can note that the
   sender has "rewound" its data content transmission position by
   observing the data object, FEC block number, and FEC symbol
   identifiers. Receivers SHOULD limit transmission of NACKs to only
   when the sender's current transmission position exceeds the point to
   which the receiver has incomplete reception. This reduces premature
   requests for repair of data the sender may be planning to provide in
   response to other receiver requests.  This mechanism can be very
   effective for protocol convergence in high loss conditions when
   transmissions of NACKs from other receivers (or indicators from the
   sender) are lost.  Another mechanism (particularly applicable when
   FEC is used) is for the sender to embed an indication of impending
   repair transmissions in current packets sent.  For example, the
   indication may be as simple as an advertisement of the number of FEC
   packets to be sent for the current applicable coding block.

   Finally, some consideration might be given to using the NACKing
   history of receivers to weight their selection of NACK backoff
   timeout intervals.  For example, if a receiver has historically been
   experiencing the greatest degree of loss, it may promote itself to
   statistically NACK sooner than other receivers.  Note this requires
   there is correlation over successive intervals of time in the loss



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   experienced by a receiver.  Such correlation MAY not always be
   present in multicast networks.  This adjustment of backoff timeout
   selection may require the creation of an "early NACK" slot for these
   historical NACKers.  This additional slot in the NACK backoff window
   will result in a longer repair cycle process that may not be
   desirable for some applications.  The resolution of these trade-offs
   may be dependent upon the protocol's target application set or
   network.

   After the random backoff timeout has expired, the receiver will make
   a decision on whether to generate a NACK repair request or not (i.e.,
   it has been suppressed).  The NACK will be suppressed when any of the
   following conditions has occurred:


     1)   The accumulated state of NACKs heard from other receivers (or
          forwarding of this state by the sender) is equal to or
          supersedes the repair needs of the local receiver.  Note that
          the local receiver should consider its repair needs only up to
          the sender transmission position recorded at the NACK cycle
          initiation (when the backoff timer was activated).


     2)   The sender's data content transmission position "rewinds" to a
          point ordinally less than that of the lowest sequence position
          of the local receiver's repair needs. (This detection of
          sender "rewind" indicates the sender has already responded to
          other receiver repair needs of which the local receiver may
          not have been aware).  This "rewind" event can occur any time
          between 1) when the NACK cycle was initiated with the backoff
          timeout activation and 2) the current moment when the backoff
          timeout has expired to suppress the NACK.  Another NACK cycle
          must be initiated by the receiver when the sender's
          transmission sequence position exceeds the receiver's lowest
          ordinal repair point.  Note it is possible that the local
          receiver may have had its repair needs satisfied as a result
          of the sender's response to the repair needs of other
          receivers and no further NACKing is required.


   If these conditions have not occurred and the receiver still has
   pending repair needs, a NACK message is generated and transmitted.
   The NACK should consist of an accumulation of repair needs from the
   receiver's lowest ordinal repair point up to the current sender
   transmission sequence position.  A single NACK message should be
   generated and the NACK message content should be truncated if it
   exceeds the payload size of single protocol message.  When such NACK
   payload limits occur, the NACK content SHOULD contain requests for



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   the ordinally lowest repair content needed from the sender.


NACK Suppression Interface Description

   Inputs:

     1)   NACK process initiation decision.
     2)   Recorded sender transmission sequence position.
     3)   Sender GRTT.
     4)   Sender group size estimate.
     5)   Application-defined bound on backoff timeout period.
     6)   NACKs from other receivers.
     7)   Pending repair indication from sender (may be forwarded
          NACKs).
     8)   Current sender transmission sequence position.

   Outputs:

     1)   Yes/no decision to generate NACK message upon backoff timer
          expiration.

3.2.3.  NACK Content

   The content of NACK messages generated by reliable multicast
   receivers will include information detailing their current repair
   needs.  The specific information depends on the use and type of FEC
   in the NACK repair process.  The identification of repair needs is
   dependent upon the data content identification (See Section 3.5
   below).  At the highest level the NACK content will identify the
   sender to which the NACK is addressed and the data transport object
   (or stream) within the sender's transmission that needs repair.  For
   the indicated transport entity, the NACK content will then identify
   the specific FEC coding blocks and/or symbols it requires to
   reconstruct the complete transmitted data.  This content may consist
   of FEC block erasure counts and/or explicit indication of missing
   blocks or symbols (segments) of data and FEC content.  It should also
   be noted that NACK-based reliable multicast can be effectively
   instantiated without a requirement for reliable NACK delivery using
   the techniques discussed here.

3.2.3.1.  NACK and FEC Repair Strategies

   Where FEC-based repair is used, the NACK message content will
   minimally need to identify the coding block(s) for which repair is
   needed and a count of erasures (missing packets) for the coding
   block.  An exact count of erasures implies the FEC algorithm is
   capable of repairing _any_ loss combination within the coding block.



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   This count may need to be adjusted for some FEC algorithms.
   Considering that multiple repair rounds may be required to
   successfully complete repair, an erasure count also implies that the
   quantity of unique FEC parity packets the server has available to
   transmit is essentially unlimited (i.e., the server will always be
   able to provide new, unique, previously unsent parity packets in
   response to any subsequent repair requests for the same coding
   block).  Alternatively, the sender may "round-robin" transmit through
   its available set of FEC symbols for a given coding block, and
   eventually affect repair.  For a most efficient repair strategy, the
   NACK content will need to also _explicitly_ identify which symbols
   (information and/or parity) the receiver requires to successfully
   reconstruct the content of the coding block.  This will be
   particularly true of small to medium size block FEC codes (e.g., Reed
   Solomon) that are capable of provided a limited number of parity
   symbols per FEC coding block.

   When FEC is not used as part of the repair process, or the protocol
   instantiation is required to provide reliability even when the sender
   has transmitted all available parity for a given coding block (or the
   sender's ability to buffer transmission history is exceeded by the
   delay*bandwidth*loss characteristics of the network topology), the
   NACK content will need to contain _explicit_  coding block and/or
   segment loss information so that the sender can provide appropriate
   repair packets and/or data retransmissions.  Explicit loss
   information in NACK content may also potentially serve other
   purposes.  For example, it may be useful for decorrelating loss
   characteristics among a group of receivers to help differentiate
   candidate congestion control bottlenecks among the receiver set.

   When FEC is used and NACK content is designed to contain explicit
   repair requests, there is a strategy where the receivers can NACK for
   specific content that will help facilitate NACK suppression and
   repair efficiency.  The assumptions for this strategy are that sender
   may potentially exhaust its supply of new, unique parity packets
   available for a given coding block and be required to explicitly
   retransmit some data or parity symbols to complete reliable transfer.
   Another assumption is that an FEC algorithm where any parity packet
   can fill any erasure within the coding block (e.g., Reed Solomon) is
   used.  The goal of this strategy is to make maximum use of the
   available parity and provide the minimal amount of data and repair
   transmissions during reliable transfer of data content to the group.

   When systematic FEC codes are used, the sender transmits the data
   content of the coding block (and optionally some quantity of parity
   packets) in its initial transmission.  Note that a systematic FEC
   coding block is considered to be logically made up of the contiguous
   set of source data vectors plus parity vectors for the given FEC



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   algorithm used.  For example, a systematic coding scheme that
   provides for 64 data symbols and 32 parity symbols per coding block
   would contain FEC symbol identifiers in the range of 0 to 95.

   Receivers then can construct NACK messages requesting sufficient
   content to satisfy their repair needs.  For example, if the receiver
   has three erasures in a given received coding block, it will request
   transmission of the three lowest ordinal parity vectors in the coding
   block. In our example coding scheme from the previous paragraph, the
   receiver would explicitly request parity symbols 64 to 66 to fill its
   three erasures for the coding block.  Note that if the receiver's
   loss for the coding block exceeds the available parity quantity
   (i.e., greater than 32 missing symbols in our example), the receiver
   will be required to construct a NACK requesting all (32) of the
   available parity symbols plus some additional portions of its missing
   data symbols in order to reconstruct the block.  If this is done
   consistently across the receiver group, the resulting NACKs will
   comprise a minimal set of sender transmissions to satisfy their
   repair needs.

   In summary, the rule is to request the lower ordinal portion of the
   parity content for the FEC coding block to satisfy the erasure repair
   needs on the first NACK cycle.  If the available number of parity
   symbols is insufficient, the receiver will also request the subset of
   ordinally highest missing data symbols to cover what the parity
   symbols will not fill.  Note this strategy assumes FEC codes such as
   Reed-Solomon for which a single parity symbol can repair any erased
   symbol.  This strategy would need minor modification to take into
   account the possibly limited repair capability of other FEC types.
   On subsequent NACK repair cycles where the receiver may have received
   some portion of its previously requested repair content, the receiver
   will use the same strategy, but only NACK for the set of parity
   and/or data symbols it has not yet received.  Optionally, the
   receivers could also provide a count of erasures as a convenience to
   the sender or intermediate systems assisting NACK operation.

   Other types of FEC schemes may require alteration to the NACK and
   repair strategy described here.  For example, some of the large block
   or expandable FEC codes described in [15]  may be less deterministic
   with respect to defining optimal repair requests by receivers or
   repair transmission strategies by senders.  For these types of codes,
   it may be sufficient for receivers to NACK with an estimate of the
   quantity of additional FEC symbols required to complete reliable
   reception and for the sender to respond accordingly.  This apparent
   disadvantage as compared to codes such as Reed Solomon may be offset
   by reduced computational requirements and/or ability to support large
   coding blocks for increased repair efficiency that these codes can
   offer.



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   After receipt and accumulation of NACK messages during the
   aggregation period, the sender can begin transmission of fresh
   (previously untransmitted) parity symbols for the coding block based
   on the highest receiver erasure count _if_ it has a sufficient
   quantity of parity symbols that were _not_ previously transmitted.
   Otherwise, the sender MUST resort to transmitting the explicit set of
   repair vectors requested.  With this approach, the sender needs to
   maintain very little state on requests it has received from the group
   without need for synchronization of repair requests from the group.
   Since all receivers use the same consistent algorithm to express
   their explicit repair needs, NACK suppression among receivers is
   simplified over the course of multiple repair cycles.  The receivers
   can simply compare NACKs heard from other receivers against their own
   calculated repair needs to determine whether they should transmit or
   suppress their pending NACK messages.

3.2.3.2.  NACK Content Format

   The format of NACK content will depend on the protocol's data service
   model and the format of data content identification the protocol
   uses.  This NACK format also depends upon the type of FEC encoding
   (if any) used.  Figure 2 illustrates a logical, hierarchical
   transmission content identification scheme, denoting that the notion
   of objects (or streams) and/or FEC blocking is optional at the
   protocol instantiation's discretion.  Note that the identification of
   objects is with respect to a given sender.  It is recommended that
   transport data content identification is done within the context of a
   sender in a given session. Since the notion of session "streams" and
   "blocks" is optional, the framework degenerates to that of typical
   transport data segmentation and reassembly in its simplest form.

   Session_
           \_
              Sender_
                     \_
                        [Object/Stream(s)]_
                                           \_
                                              [FEC Blocks]_
                                                           \_
                                                              Symbols

     Fig. 2: Reliable Multicast Data Content Identification Hierarchy

   The format of NACK messages should meet the following goals:


1)   Able to identify transport data unit transmissions required to
     repair a portion of the received content, whether it is an entire



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     missing object/stream (or range), entire FEC coding block(s), or
     sets of symbols,

2)   Be simple to process for NACK aggregation and suppression,

3)   Be capable of including NACKs for multiple objects, FEC coding
     blocks and/or symbols in a single message, and

4)   Have a reasonably compact format.

   If the reliable multicast transport object/stream is identified with
   an <objectId> and the FEC symbol being transmitted is identified with
   an <fecPayloadId>, the concatenation of <objectId::fecPayloadId>
   comprises a basic transport protocol data unit (TPDU) identifier for
   symbols from a given source.  NACK content can be composed of lists
   and/or ranges of these TPDU identifiers to build up NACK messages to
   describe the receivers repair needs.  If no hierarchical object
   delineation or FEC blocking is used, the TPDU is a simple linear
   representation of the data symbols transmitted by the sender.  When
   the TPDU represents a hierarchy for purposes of object/stream
   delineation and/or FEC blocking, the NACK content unit may require
   flags to indicate which portion of the TPDU is applicable.  For
   example, if an entire "object" (or range of objects) is missing in
   the received data, the receiver will not necessarily know the
   appropriate range of <sourceBlockNumbers> or <encodingSymbolIds> for
   which to request repair and thus requires some mechanism to request
   repair (or retransmission) of the entire unit represented by an
   <objectId>.  The same is true if entire FEC coding blocks represented
   by one or a range of <sourceBlockNumbers> have been lost.


NACK Content Interface Description

   Inputs:

     1)   Sender identification.
     2)   Sender data identification.
     3)   Sender FEC Object Transmission Information.
     4)   Recorded sender transmission sequence position.
     5)   Current sender transmission sequence position.  History of
          repair needs for this sender.

   Outputs:

     1)   NACK message with repair requests.






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3.2.4.  Sender Repair Response

   Upon reception of a repair request from a receiver in the group, the
   sender will initiate a repair response procedure.  The sender may
   wish to delay transmission of repair content until it has had
   sufficient time to accumulate potentially multiple NACKs from the
   receiver set.  This allows the sender to determine the most efficient
   repair strategy for a given transport stream/object or FEC coding
   block.  Depending upon the approach used, some protocols may find it
   beneficial for the sender to provide an indicator of pending repair
   transmissions as part of its current transmitted message content.
   This can aid some NACK suppression mechanisms.  The amount of time to
   perform this NACK aggregation should be sufficient to allow for the
   maximum receiver NACK backoff window ("T_maxBackoff" from Section
   3.2.2) and propagation of NACK messages from the receivers to the
   sender.  Note the maximum transmission delay of a message from a
   receiver to the sender may be approximately (1*GRTT) in the case of
   very asymmetric network topology with respect to transmission delay.
   Thus, if the maximum receiver NACK backoff time is T_maxBackoff =
   K*GRTT, the sender NACK aggregation period should be equal to at
   least:

           T_sndrAggregate = T_maxBackoff + 1*GRTT = (K+1)*GRTT

   Immediately after the sender NACK aggregation period, the sender will
   begin transmitting repair content determined from the aggregate NACK
   state and continue with any new transmission.  Also, at this time,
   the sender should observe a "holdoff" period where it constrains
   itself from initiating a new NACK aggregation period to allow
   propagation of the new transmission sequence position due to the
   repair response to the receiver group.  To allow for worst case
   asymmetry, this "holdoff" time should be:

                          T_sndrHoldoff = 1*GRTT

   Recall that the receivers will also employ a "holdoff" timeout after
   generating a NACK message to allow time for the sender's response.
   Given a sender <T_sndrAggregate> plus <T_sndrHoldoff> time of
   (K+1)*GRTT, the receivers should use holdoff timeouts of:

       T_rcvrHoldoff = T_sndrAggregate + T_sndrHoldoff = (K+2)*GRTT

   This allows for a worst-case propagation time of the receiver's NACK
   to the sender, the sender's aggregation time and propagation of the
   sender's response back to the receiver.  Additionally, in the case of
   unicast feedback from the receiver set, it may be useful for the
   sender to forward (via multicast) a representation of its aggregated
   NACK content to the group to allow for NACK suppression when there is



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   not multicast connectivity among the receiver set.

   At the expiration of the <T_sndrAggregate> timeout, the sender will
   begin transmitting repair messages according to the accumulated
   content of NACKs received.  There are some guidelines with regards to
   FEC-based repair and the ordering of the repair response from the
   sender that can improve reliable multicast efficiency:


1)   When FEC is used, it is beneficial that the sender transmit
     previously untransmitted parity content as repair messages whenever
     possible.  This  maximizes the receiving nodes' ability to
     reconstruct the entire transmitted content from their individual
     subsets of received messages.


2)   The transmitted object and/or stream data and repair content should
     be indexed with  monotonically increasing sequence numbers (within
     a reasonably large ordinal space).  If the sender observes the
     discipline of  transmitting repair for the earliest content (e.g.,
     ordinally lowest FEC blocks) first, the receivers can use a
     strategy of withholding repair requests for later content until the
     sender once again returns to that point in the object/stream
     transmission sequence.  This can increase overall message
     efficiency among the group and help work to keep repair cycles
     relatively synchronized without dependence upon strict time
     synchronization among the sender and receivers.  This also helps
     minimize the buffering requirements of receivers and senders and
     reduces redundant transmission of data to the group at large.


Sender Repair Response Interface Description

   Inputs:

     1)   Receiver NACK messages

     2)   Group timing information

   Outputs

     1)   Repair messages (FEC and/or Data content retransmission)
     2)   Advertisement of current pending repair transmissions when
          unicast receiver feedback is detected.







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3.3.  Multicast Receiver Join Policies and Procedures

   Consideration should be given to the policies and procedures by which
   new receivers join a group (perhaps where reliable transmission is
   already in progress) and begin requesting repair. If receiver joins
   are unconstrained, the dynamics of group membership may impede the
   application's ability to meet its goals for forward progression of
   data transmission.  Policies limiting the opportunities when
   receivers begin participating in the NACK process may be used to
   achieve the desired behavior.  For example, it may be beneficial for
   receivers to attempt reliable reception from a newly-heard sender
   only upon non-repair transmissions of data in the first FEC block of
   an object or logical portion of a stream.  The sender may also
   implement policies limiting the receivers from which it will accept
   NACK requests, but this may be prohibitive for scalability reasons in
   some situations.  Alternatively, it may be desirable to have a looser
   transport synchronization policy and rely upon session management
   mechanisms to limit group dynamics that can cause poor performance,
   in some types of bulk transfer applications (or for potential
   interactive reliable multicast applications).


Group Join Policy Interface Description

   Inputs:

     1)   Current object/stream data/repair content and sequencing
          identifiers from sender transmissions.

   Outputs:

     1)   Receiver yes/no decision to begin receiving and NACKing for
          reliable reception of data

3.4.  Reliable Multicast Member Identification

   In a NACK-based reliable multicast protocol (or other multicast
   protocols) where there is the potential for multiple sources of data,
   it is necessary to provide some mechanism to uniquely identify the
   sources (and possibly some or all receivers in some cases) within the
   group.  Identity based on arriving packet source addresses is
   insufficient for several reasons.  These reasons include routing
   changes for hosts with multiple interfaces that result in different
   packet source addresses for a given host over time, network address
   translation (NAT) or firewall devices, or other transport/network
   bridging approaches.  As a result, some type of unique source
   identifier <sourceId> field should be present in packets transmitted
   by reliable multicast session members.



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3.5.  Data Content Identification

   The data and repair content transmitted by a NACK-based reliable
   multicast sender requires some form of identification in the protocol
   header fields.  This identification is required to facilitate the
   reliable NACK-oriented repair process.  These identifiers will also
   be used in NACK messages generated.  This building block document
   assumes two very general types of data that may comprise bulk
   transfer session content.  One type is static, discrete objects of
   finite size and the other is continuous non-finite streams.  A given
   application  may wish to reliably multicast data content using either
   one or both of these paradigms.  While it may be possible for some
   applications to further generalize this model and provide mechanisms
   to encapsulate static objects as content embedded within a stream,
   there are advantages in many applications to provide distinct support
   for static bulk objects and messages with the context of a reliable
   multicast session.  These applications may include content caching
   servers, file transfer, or collaborative tools with bulk content.
   Applications with requirements for these static object types can then
   take advantage of transport layer mechanisms (i.e.,
   segmentation/reassembly, caching, integrated forward error correction
   coding, etc.) rather than being required to provide their own
   mechanisms for these functions at the application layer.

   As noted, some applications may alternatively desire to transmit bulk
   content in the form of one or more streams of non-finite size.
   Example streams include continuous quasi-real-time message broadcasts
   (e.g., stock ticker) or some content types that are part of
   collaborative tools or other applications.  And, as indicated above,
   some applications may wish to encapsulate other bulk content (e.g.,
   files) into one or more streams within a multicast session.

   The components described within this building block document are
   envisioned to be applicable to both of these models with the
   potential for a mix of both types within a single multicast session.
   To support this requirement, the normal data content identification
   should include a field to uniquely identify the object or stream
   <objectId> within some reasonable temporal or ordinal interval.  Note
   that it is _not_ expected that this data content identification will
   be globally unique.  It is expected that the object/stream identifier
   will be unique with respect to a given sender within the reliable
   multicast session and during the time that sender is supporting a
   specific transport instance of that object or stream.

   Since the "bulk" object/stream content usually requires segmentation,
   some form of segment identification must also be  provided.  This
   segment identifier will be relative to any object or stream
   identifier that has been provided.  Thus, in some cases, NACK-based



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   reliable multicast protocol instantiations may be able to receive
   transmissions and request repair for multiple streams and one or more
   sets of static objects in parallel.  For protocol instantiations
   employing FEC the segment identification portion of the data content
   identifier may consist of a logical concatenation of a coding block
   identifier <sourceBlockNumber> and an identifier for the specific
   data or parity symbol <encodingSymbolId> of the code block.  The FEC
   Basic Schemes document [12] and descriptions of additional FEC
   schemes that may be documented later provide a standard message
   format for identifying FEC transmission content. NACK-based reliable
   multicast protocol instantiations using FEC SHOULD follow such
   guidelines.

   Additionally, flags to determine the usage of the content identifier
   fields (e.g., stream vs. object) may be applicable.   Flags may also
   serve other purposes in data content identification.  It is expected
   that any flags defined will be dependent upon individual protocol
   instantiations.

   In summary, the following data content identification fields may be
   required for NACK-based reliable multicast protocol data content
   messages:

     1)   Source node identifier (<sourceId>)

     2)   Object/Stream identifier (<objectId>), if applicable.

     3)   FEC Block identifier (<sourceBlockNumber>), if applicable.

     4)   FEC Symbol identifier (<encodingSymbolId>)

     5)   Flags to differentiate interpretation of identifier fields or
          identifier structure that implicitly indicates usage.

     6)   Additional FEC transmission content fields per FEC Building
          Block

   These fields have been identified because any generated NACK messages
   will use these identifiers in requesting repair or retransmission of
   data.  NACK-based reliable multicast protocols that use these data
   content fields should also be compatible with support for
   intermediate system assistance to reliable multicast transport
   operation when available.

3.6.  Forward Error Correction (FEC)

   Multiple forward error correction (FEC) approaches have been
   identified that can provide great performance enhancements to the



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   repair process of NACK-oriented and other reliable multicast
   protocols [13], [14], [15].  NACK-based reliable multicast protocols
   can reap additional benefits since FEC-based repair does not
   _generally_ require explicit knowledge of repair content within the
   bounds of its coding block size (in symbols).  In NACK-based reliable
   multicast, parity repair packets generated will generally be
   transmitted only in response to NACK repair requests from receiving
   nodes.  However, there are benefits in some network environments for
   transmitting some predetermined quantity of FEC repair packets
   multiplexed with the regular data symbol transmissions [16].  This
   can reduce the amount of NACK traffic generated with relatively
   little overhead cost when group sizes are very large or the network
   connectivity has a large delay*bandwidth product with some nominal
   level of expected packet loss.  While the application of FEC is not
   unique to NACK-based reliable multicast, these sorts of requirements
   may dictate the types of algorithms and protocol approaches that are
   applicable.

   A specific issue related to the use of FEC with NACK-based reliable
   multicast is the mechanism used to identify the portion(s) of
   transmitted data content to which specific FEC packets are
   applicable.  It is expected that FEC algorithms will be based on
   generating a set of parity repair packets for a corresponding block
   of transmitted data packets.  Since data content packets are uniquely
   identified by the concatenation of
   <sourceId::objectId::sourceBlockNumber::encodingSymbolId> during
   transport, it is expected that FEC packets will be identified in a
   similar manner.  The FEC Building Block document [10] provides
   detailed recommendations concerning application of FEC and standard
   formats for related reliable multicast protocol messages.

3.7.  Round-trip Timing Collection

   The measurement of packet propagation round-trip time (RTT) among
   members of the group is required to support timer-based NACK
   suppression algorithms, timing of sender commands or certain repair
   functions, and congestion control operation.  The nature of the
   round-trip information collected is dependent upon the type of
   interaction among the members of the group.  In the case where only
   "one-to-many" transmission is required, it may be that only the
   sender require RTT knowledge of the greatest RTT (GRTT) among the
   receiver set and/or RTT knowledge of only a portion of the group.
   Here, the GRTT information might be collected in a reasonably
   scalable manner.  For congestion control operation, it is possible
   that RTT information may be required by each receiver in the group.
   In this case, an alternative RTT collection scheme may be utilized
   where receivers collect individual RTT measurements with respect to
   the sender and advertise them to the group or sender.  Where it is



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   likely that exchange of reliable multicast data will occur among the
   group on a "many-to-many" basis, there are alternative measurement
   techniques that might be employed for increased efficiency [17].  And
   in some cases, there might be absolute time synchronization among
   hosts that may simplify RTT measurement.  There are trade-offs in
   multicast congestion control design that require further
   consideration before a universal recommendation on RTT (or GRTT)
   measurement can be specified.  Regardless of how the RTT information
   is collected (and more specifically GRTT) with respect to congestion
   control or other requirements, the sender will need to advertise its
   current GRTT estimate to the group for various timeouts used by
   receivers.

3.7.1.  One-to-Many Sender GRTT Measurement

   The goal of this form of RTT measurement is for the sender to learn
   the GRTT among the receivers who are actively participating in NACK-
   based reliable multicast operation.  The set of receivers
   participating in this process may be the entire group or some subset
   of the group determined from another mechanism within the protocol
   instantiation.  An approach to collect this GRTT information follows.

   The sender periodically polls the group with a message (independent
   or "piggy-backed" with other transmissions) containing a <sendTime>
   timestamp relative to an internal clock at the sender.  Upon
   reception of this message, the receivers will record this <sendTime>
   timestamp and the time (referenced to their own clocks) at which it
   was received <recvTime>.  When the receiver provides feedback to the
   sender (either explicitly or as part of other feedback messages
   depending upon protocol instantiation specification), it will
   construct a "response" using the formula:

            grttResponse = sendTime + (currentTime - recvTime)

   where the <sendTime> is the timestamp from the last probe message
   received from the source and the (<currentTime> - <recvTime>) is the
   amount of time differential since that request was received until the
   receiver generated the response.

   The sender processes each receiver response by calculating a current
   RTT measurement for the receiver from whom the response was received
   using the following formula:

                   RTT_rcvr = currentTime - grttResponse

   During the each periodic GRTT probing interval, the source keeps the
   peak round trip timing measurement (RTT_peak) from the set of
   responses it has received.  A conservative estimate of GRTT is kept



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   to maximize the efficiency of redundant NACK suppression and repair
   aggregation.  The update to the source's ongoing estimate of GRTT is
   done observing the following rules:


     1)   If a receiver's response round trip time (RTT_rcvr) is greater
          than the current GRTT estimate, the GRTT is immediately
          updated to this new peak value:

                                 GRTT = RTT_rcvr


     2)   At the end of the response collection period (i.e., the GRTT
          probe interval), if the recorded "peak" response RTT_peak) is
          less than the current GRTT estimate, the GRTT is updated to:

                          GRTT = MAX(0.9*GRTT, RTT_peak)


     3)   If no feedback is received, the sender GRTT estimate remains
          unchanged.


     4)   At the end of the response collection period, the peak
          tracking value (RTT_peak) is reset to ZERO for subsequent peak
          detection.

   The GRTT collection period (i.e., period of probe transmission) could
   be fixed at a value on the order of that expected for group
   membership and/or network topology dynamics.  For robustness, more
   rapid probing could be used at protocol startup before settling to a
   less frequent, steady-state interval.  Optionally, an algorithm may
   be developed to adjust the GRTT collection period dynamically in
   response to the current GRTT estimate (or variations in it) and to an
   estimation of packet loss.  The overhead of probing messages could
   then be reduced when the GRTT estimate is stable and unchanging, but
   be adjusted to track more dynamically during periods of variation
   with correspondingly shorter GRTT collection periods.  GRTT
   collection may also be coupled with collection of other information
   for congestion control purposes.

   In summary, although NACK repair cycle timeouts are based on GRTT, it
   should be noted that convergent operation of the protocol does not
   _strictly_ depend on highly accurate GRTT estimation.  The current
   mechanism has proved sufficient in simulations and in the
   environments where NACK-based reliable multicast protocols have been
   deployed to date.  The estimate provided by the algorithm tracks the
   peak envelope of actual GRTT (including operating system effect as



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   well as network delays) even in relatively high loss connectivity.
   The steady-state probing/update interval may potentially be varied to
   accommodate different levels of expected network dynamics in
   different environments.

3.7.2.  One-to-Many Receiver RTT Measurement

   In this approach, receivers send messages with timestamps to the
   sender.  To control the volume of these receiver-generated messages,
   a suppression mechanism similar to that described for NACK
   suppression my be used.  The "age" of receivers' RTT measurement
   should be kept by receivers and used as a metric in competing for
   feedback opportunities in the suppression scheme.  For example,
   receiver who have not made any RTT measurement or whose RTT
   measurement has aged most should have precedence over other
   receivers.  In turn the sender may have limited capacity to provide
   an "echo" of the receiver timestamps back to the group, and it could
   use this RTT "age" metric to determine which receivers get
   precedence.  The sender can determine the GRTT as described in 3.7.1
   if it provides sender timestamps to the group.  Alternatively,
   receivers who note their RTT is greater than the sender GRTT can
   compete in the feedback opportunity/suppression scheme to provide the
   sender and group with this information.

3.7.3.  Many-to-Many RTT Measurement

   For reliable multicast sessions that involve multiple senders, it may
   be useful to have RTT measurements occur on a true "many-to-many"
   basis rather than have each sender independently tracking RTT.  Some
   protocol efficiency can be gained when receivers can infer an
   approximation of their RTT with respect to a sender based on RTT
   information they have on another sender and that other sender's RTT
   with respect to the new sender of interest.  For example, for
   receiver "a" and sender's "b" and "c", it is likely that:

                   RTT(a<->b) <= RTT(a<->c)) + RTT(b<->c)

   Further refinement of this estimate can be conducted if RTT
   information is available to a node concerning its own RTT to a small
   subset of other group members and RTT information among those other
   group members it learns during protocol operation.

3.7.4.  Sender GRTT Advertisement

   To facilitate deterministic protocol operation, the sender should
   robustly advertise its current estimation of GRTT to the receiver
   set.  Common, robust knowledge of the sender's current operating GRTT
   estimate among the group will allow the protocol to progress in its



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   most efficient manner.  The sender's GRTT estimate can be robustly
   advertised to the group by simply embedding the estimate into all
   pertinent messages transmitted by the sender.  The overhead of this
   can be made quite small by quantizing (compressing) the GRTT estimate
   to a single byte of information.  The following C-language functions
   allows this to be done over a wide range (RTT_MIN through RTT_MAX) of
   GRTT values while maintaining a greater range of precision for small
   GRTT values and less precision for large values.  Values of 1.0e-06
   seconds and 1000 seconds are RECOMMENDED for RTT_MIN and RTT_MAX
   respectively.  NACK-based reliable multicast applications may wish to
   place an additional, smaller upper limit on the GRTT advertised by
   senders to meet application data delivery latency constraints at the
   expense of greater feedback volume in some network environments.

     unsigned char QuantizeGrtt(double grtt)
     {
         if (grtt > RTT_MAX)
             grtt = RTT_MAX;
         else if (grtt < RTT_MIN)
             grtt = RTT_MIN;
         if (grtt < (33*RTT_MIN))
             return ((unsigned char)(grtt / RTT_MIN) - 1);
         else
             return ((unsigned char)(ceil(255.0 -
                                     (13.0 * log(RTT_MAX/grtt)))));
     }

     double UnquantizeRtt(unsigned char qrtt)
     {
          return ((qrtt <= 31) ?
                    (((double)(qrtt+1))*(double)RTT_MIN) :
                   (RTT_MAX/exp(((double)(255-qrtt))/(double)13.0)));
     }


   Note that this function is useful for quantizing GRTT times in the
   range of 1 microsecond to 1000 seconds.  Of course, NACK-based
   reliable multicast protocol implementations may wish to further
   constrain advertised GRTT estimates (e.g., limit the maximum value)
   for practical reasons.

3.8.  Group Size Determination/Estimation

   When NACK-based reliable multicast protocol operation includes
   mechanisms that excite feedback from the group at large (e.g.,
   congestion control), it may be possible to roughly estimate the group
   size based on the number of feedback messages received with respect
   to the distribution of the probabilistic suppression mechanism used.



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   Note the timer-based suppression mechanism described in this document
   does not require a very accurate estimate of group size to perform
   adequately.  Thus, a rough estimate, particularly if conservatively
   managed, may suffice.  Group size may also be determined
   administratively.  In absence of a group size determination mechanism
   a default group size value of 10,000 is RECOMMENDED for reasonable
   management of feedback given the scalability of expected NACK-based
   reliable multicast usage.

3.9.  Congestion Control Operation

   Congestion control that fairly shares available network capacity
   with other reliable multicast and TCP instantiations is REQUIRED for
   general Internet operation.  The TCP-Friendly Multicast Congestion
   Control (TFMCC) [18] or Pragmatic General Multicast Congestion
   Control (PGMCC) techniques [19] may be applied to NACK-based reliable
   multicast operation to meet this requirement.

3.10.  Router/Intermediate System Assistance

   NACK-based multicast protocols may benefit from general purpose
   router assistance.  In particular, additional NACK suppression where
   routers or intermediate systems can aggregate NACK content (or filter
   duplicate NACK content) from receivers as it is relayed toward the
   sender could enhance NORM group size scalability.  For NACK-based
   reliable multicast protocols using FEC, it is possible that
   intermediate systems may be able to filter FEC repair messages to
   provide an intelligent "subcast" of repair content to different legs
   of the multicast topology depending on the repair needs learned from
   previous receiver NACKs.  Both of these types of assist functions
   would require router interpretation of transport data unit content
   identifiers and flags.

3.11.  NACK-based reliable multicast Applicability

   The Multicast NACK building block applies to protocols wishing to
   employ negative acknowledgement to achieve reliable data transfer.
   Properly designed NACK-based reliable multicast protocols offer
   scalability advantages for applications and/or network topologies
   where, for various reasons, it is prohibitive to construct a higher
   order delivery infrastructure above the basic Layer 3 IP multicast
   service (e.g., unicast or hybrid unicast/multicast data distribution
   trees).  Additionally, the multicast scalability property of NACK-
   based protocols [20], [21] is applicable where broad "fan-out" is
   expected for a single network hop (e.g., cable-TV data delivery,
   satellite, or other broadcast communication services).  Furthermore,
   the simplicity of a protocol based on "flat" group-wide multicast
   distribution may offer advantages for a broad range of distributed



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   services or dynamic networks and applications.  NACK-based reliable
   multicast protocols can make use of reciprocal (among senders and
   receivers) multicast communication under the Any-Source Multicast
   (ASM) model defined in RFC 1112 [2], and are capable of scalable
   operation in asymmetric topologies such as Single-Source Multicast
   (SSM) [9] where there may only be unicast routing service from the
   receivers to the sender(s).

   NACK-based reliable multicast protocol operation is compatible with
   transport layer forward error correction coding techniques as
   described in [15] and congestion control mechanisms such as those
   described in [18] and [19].  A principal limitation of NACK-based
   reliable multicast operation involves group size scalability when
   network capacity for receiver feedback is very limited.  NACK-based
   reliable multicast operation is also governed by implementation
   buffering constraints.  Buffering greater than that required for
   typical point-to-point reliable transport (e.g., TCP) is recommended
   to allow for disparity in the receiver group connectivity and to
   allow for the feedback delays required to attain group size
   scalability.

4.  Security Considerations

   NACK-based reliable multicast protocols are expected to be subject to
   the same sort of security vulnerabilities as other IP and IP
   multicast protocols.  NACK-based reliable multicast is compatible
   with IP security (IPsec) authentication mechanisms [22] that are
   RECOMMENDED for protection against session intrusion and denial of
   service attacks.  A particular threat for NACK based protocols is
   that of NACK replay attacks that would prevent a multicast sender
   from making forward progress in transmission.  Any standard IPsec
   mechanisms that can provide protection against such replay attacks
   are RECOMMENDED for use.  Additionally, NACK-based reliable multicast
   protocol instantiations SHOULD consider providing support for their
   own NACK replay attack protection when network layer mechanisms are
   not available.  The IETF Multicast Security (msec) Working Group is
   also developing solutions which may be applicable to NACK-based
   reliable multicast in the future.

5.  Changes from RFC3941

   This section lists the changes between the Experimental version of
   this specification, [4], and this version:

     1)   Change of title to avoid confusion with NORM Protocol
          specification, and





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     2)   Updated references to related, updated RMT Building Block
          documents.

6.  Acknowledgements (and these are not Negative)

   The authors would like to thank Rick Jones, and Joerg Widmer for
   their valuable comments on this document.  The authors would also
   like to thank the RMT working group chairs, Roger Kermode and Lorenzo
   Vicisano, for their support in development of this specification, and
   Sally Floyd for her early inputs into this document.

7.  References

7.1.  Normative References

   [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
   Levels", BCP 14, RFC 2119, March 1997.

   [2] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
   1112, August 1989.

7.2.  Informative References

   [3] Mankin, A., Romanow, A., Bradner, S., and V. Paxson, "IETF
   Criteria for Evaluating Reliable Multicast Transport and Application
   Protocols", RFC 2357, June 1998.

   [4] Adamson, B., Bormann, C., Handley, M., and J. Macker, "Negative-
   Acknowledgement (NACK)-Oriented Reliable Multicast Building Blocks",
   RFC 3941, November 2004.

   [5] Clark, D. and D. Tennenhouse, "Architectural Considerations for a
   New Generation of Protocols".  In Proc.  ACM SIGCOMM, pages 201--208,
   September 1990.

   [6] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable
   Multicast Transport (RMT) Building Blocks and Protocol Instantiation
   documents", RFC 3269, April 2002.

   [7] Nonnenmacher, J. and E. Biersack, "Optimal Multicast Feedback,"
   in IEEE Infocom , San Francisco, California, p. 964, March/April
   1998.

   [8] Macker, J., and R. Adamson, "Quantitative Prediction of Nack
   Oriented Reliable Multicast (NORM) Feedback", Proc. IEEE MILCOM 2002,
   October 2002.





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   [9] Holbrook, H., "A Channel Model for Multicast", Ph.D.
   Dissertation, Stanford University, Department of Computer Science,
   Stanford, California, August 2001.

   [10] Watson, M., Luby, M., and L. Vicisano, "Forward Error Correction
   (FEC) Building Block", draft-ietf-rmt-fec-bb-revised-03, January
   2006.

   [11] Floyd, S., Jacobson, V., McCanne, S., Liu, C., and L. Zhang. "A
   Reliable Multicast Framework for Light-weight Sessions and
   Application Level Framing", Proc. ACM SIGCOMM, August 1995.

   [12] Watson, M., "Basic Forward Error Correction (FEC) Schemes",
   Internet-Draft draft-ietf-rmt-bb-fec-basic-schemes-revised-02, March
   2006.

   [13] Metzner, J., "An Improved Broadcast Retransmission Protocol",
   IEEE Transactions on Communications, Vol.  Com-32, No.6, June 1984.

   [14] Macker, J., "Reliable Multicast Transport and Integrated
   Erasure-based Forward Error Correction", Proc. IEEE MILCOM 97,
   October  1997.

   [15] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
   J. Crowcroft, "The Use of Forward Error Correction (FEC) in Reliable
   Multicast", RFC 3453, December 2002.

   [16] Gossink, D., and J. Macker, "Reliable Multicast and Integrated
   Parity Retransmission with Channel Estimation", IEEE GLOBECOM 98'.

   [17] Ozdemir, V., Muthukrishnan, S., and I. Rhee, "Scalable, Low-
   Overhead Network Delay Estimation", NCSU/AT&T White Paper, February
   1999.

   [18] Widmer J., and M. Handley, "Extending Equation-Based Congestion
   Control to Multicast Applications", Proc ACM SIGCOMM 2001, San Diego,
   August 2001.

   [19] Rizzo, L., "pgmcc: A TCP-Friendly Single-Rate Multicast
   Congestion Control Scheme", Proc ACM SIGCOMM 2000, Stockholm, August
   2000.

   [20] Pingali, S., Towsley, D., and J. Kurose, "A Comparison of
   Sender-Initiated and Receiver-Initiated Reliable Multicast
   Protocols".  In Proc.  INFOCOM, San Francisco, CA, October 1993.






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   [21] Levine, B., and J.J. Garcia-Luna-Aceves, "A Comparison of Known
   Classes of Reliable Multicast Protocols", Proc.  International
   Conference on Network Protocols (ICNP-96), Columbus, Ohio, Oct
   29--Nov 1, 1996.

   [22] Kent, S. and R. Atkinson, "Security Architecture for the
   Internet Protocol", RFC 2401, November 1998.

8.  Authors' Addresses

   Brian Adamson
   Naval Research Laboratory
   Washington, DC  20375

   EMail: adamson@itd.nrl.navy.mil


   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28334 Bremen, Germany

   EMail: cabo@tzi.org


   Mark Handley
   Department of Computer Science
   University College London
   Gower Street
   London
   WC1E 6BT
   UK

   EMail: M.Handley@cs.ucl.ac.uk


   Joe Macker
   Naval Research Laboratory
   Washington, DC  20375

   EMail: macker@itd.nrl.navy.mil










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9.  Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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Acknowledgement

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