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Network Working Group                                         B. Adamson
Internet-Draft                                 Naval Research Laboratory
Intended status: Standards Track                              C. Bormann
Expires: October 4, 2008                         Universitaet Bremen TZI
                                                              M. Handley
                                               University College London
                                                               J. Macker
                                               Naval Research Laboratory
                                                           April 2, 2008

        Multicast Negative-Acknowledgment (NACK) Building Blocks

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
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on October 4, 2008.


   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
   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

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   operation.  It is anticipated that these building blocks will be
   useful in generating different instantiations of reliable multicast

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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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 . . . . . . . . . . . . . . . . . . . 14
       3.2.3.  NACK Content . . . . . . . . . . . . . . . . . . . . . 19
       3.2.4.  Sender Repair Response . . . . . . . . . . . . . . . . 23
     3.3.  Multicast Receiver Join Policies and Procedures  . . . . . 25
     3.4.  Reliable Multicast Member Identification . . . . . . . . . 26
     3.5.  Data Content Identification  . . . . . . . . . . . . . . . 26
     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 . . . . . . . . . 31
       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 . . . . . . . . . . . . . . . 33
     3.10. Router/Intermediate System Assistance  . . . . . . . . . . 34
     3.11. NACK-based Reliable Multicast Applicability  . . . . . . . 34
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 36
   6.  Changes from RFC3941 . . . . . . . . . . . . . . . . . . . . . 36
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 37
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
   Intellectual Property and Copyright Statements . . . . . . . . . . 40

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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 (See [RFC2357]).  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[ArchConsiderations], 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 [RFC3269].

   *Statement of Intent*

   This memo contains descriptions of building blocks that can be
   applied in the design of Reliable Multicast protocols utilizing
   Negative-Acknowledgement (NACK) feedback.  [RFC3941] contained a
   previous description of this specification.  RFC3941 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] and
   has been updated according to accumulated experience and growing
   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.

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   The differences between [RFC3941] and this document are listed in
   Section 6.

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
   [RFC5052] 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

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 [RFC1112] 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
   [NormFeedback].  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

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
   operating across a wide range of the networks to which general
   purpose IP service applies.  The bandwidth available on the links

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   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 [McastModel].  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
   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.

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   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:

   1.  Multicast Sender Transmission

   2.  NACK Repair Process

   3.  Multicast Receiver Join Policies

   1.  Node (member) Identification

   2.  Data Content Identification

   3.  Forward Error Correction (FEC)

   4.  Round-trip Timing Collection

   5.  Group Size Determination/Estimation

   6.  Congestion Control Operation

   7.  Router/Intermediate System Assistance

   8.  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
   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

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   applicable beyond NACK-based reliable multicast.  The most
   significant of these components are discussed in other building block
   documents such as the FEC Building Block [RFC5052].  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
      .---------------------.            .-----------------------.
      | 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 theFEC Building Block [RFC5052].  When
   congestion control mechanisms are needed (REQUIRED for general
   Internet operation), the sender transmission rate SHALL be controlled
   by the congestion control mechanism.  In any case, it is RECOMMENDED

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   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.


   1.  Application data and control

   2.  Sender node identifier

   3.  Data identifiers

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   4.  Segmentation and FEC parameters

   5.  Transmission rate

   6.  Application controls

   7.  Receiver feedback messages (e.g., NACKs)


   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

   1.  Receiver NACK process initiation,

   2.  NACK suppression,

   3.  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

   Alternatively, if the sender's transmission advertises the quantity

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   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.


   1.  Sender data content with sequencing identifiers from sender

   2.  History of content received from sender.


   1.  NACK process initiation decision

   2.  Recorded sender transmission sequence position.

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[SrmFramework].  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

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   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[McastFeedback].  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's GRTT assessment 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

   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

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

                               L = ln(R) + 1

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

                L                          L                   L
        --------------------  to --------------------  +  ----------

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       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) *
           }  // 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 the
   equations given in [McastFeedback]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:

   "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

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   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
   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

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   desirable for some applications.  The resolution of these trade-offs
   may be dependent upon the protocol's target application set or

   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

   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
   the ordinally lowest repair content needed from the sender.


   1.  NACK process initiation decision.

   2.  Recorded sender transmission sequence position.

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   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.


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

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.  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.
   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

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   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
   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

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   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 [RFC3453] 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.

   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

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   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.  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.

                                               [FEC Blocks]_

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

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

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   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.


   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.


   1.  NACK message with repair requests.

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

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   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

            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
   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

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   sender that can improve reliable multicast efficiency:

   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.

   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.


   1.  Receiver NACK messages

   2.  Group timing information


   1.  Repair messages (FEC and/or Data content retransmission)

   2.  Advertisement of current pending repair transmissions when
       unicast receiver feedback is detected.

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

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   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).


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


   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.

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

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   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
   (e.g., <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

   Since "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
   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 building block
   [I-D.ietf-rmt-bb-fec-basic-schemes-revised] 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.

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   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

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

   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
   repair process of NACK-oriented and other reliable multicast
   protocols [FecBroadcast], [RmFec], [RFC3453].  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
   [FecHybrid].  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

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   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 [RFC5052] 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 GRTT 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 each receiver in the group may need
   knowledge of its individual RTT.  In this case, an alternative RTT
   collection scheme may be utilized where receivers collect individual
   RTT measurements with respect to the sender(s) and advertise them to
   the group or sender(s).  Where it is 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 [DelayEstimation].  In some cases, there
   might be absolute time synchronization available among the
   participating 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 NACK
   timeouts used by receivers.

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3.7.1.  One-to-Many Sender GRTT Measurement

   The goal of this form of RTT measurement is for the sender to
   estimate 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>") isthe
   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
   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:

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                       GRTT = MAX(0.9*GRTT, RTT_peak)

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

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

   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 estimate of GRTT (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
   depend upon 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 given algorithm tracks the peak envelope
   of actual GRTT (including operating system effect as 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

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

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   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 senders "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
   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 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.

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       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);
               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) :

   Note that this function is useful for quantizing GRTT times in
   therange 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.
   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) [TfmccPaper] or Pragmatic General Multicast

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   Congestion Control (PGMCC) [PgmccPaper] techniques can 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 [RmComparison], [RmClasses] 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 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 [RFC1112],and are capable
   of scalable operation in asymmetric topologies such as Single-Source
   Multicast (SSM) [McastModel] 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 [RFC3453]and congestion control mechanisms such as those
   described in [TfmccPaper]and [PgmccPaper].  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

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   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

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.  However, unlike point-to-point (unicast)
   transport protocols, it is possible that one badly behaving
   participant can impact the transport service experience of others in
   the group.  For example, a malicious receiver node could
   intentionally transmit NACK messages to cause the sender(s) to
   unnecessarily transmit repairs instead of making forward progress
   with reliable transfer.  Also, group-wise messaging to support
   congestion control or other aspects of protocol operation may be
   subject to similar vulnerabilities.  Thus, it is highly RECOMMENDED
   that security such as authentication and encryption be applied for
   NACK-based reliable multicast deployments.  Protocol instantiations
   using this building block SHOULD identify approaches to security that
   can be used to address these and other security considerations.

   NACK-based reliable multicast is compatible with IP security (IPsec)
   authentication mechanisms [RFC4301] 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 could 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.  This MAY be necessary when IPsec implementations are used
   that do not provide multicast replay attack protection when multiple
   sources are present.

   For NACK-based multicast deployments with large receiver groups using
   IPsec, approaches might be developed that use shared, common keys for
   receiver-originated protocol messages to maintain a practical number
   of IPsec Security Associations (SAs).  However, such group-based
   authentication may not be sufficient unless one really can trust the
   receiver population.  Additionally, this can make identification of
   badly-behaving (but authenticated) receiver nodes problematic as such
   nodes could potentially masquerade as other receivers in the group.
   In deployments such as this, one SHOULD consider use of Specific-

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   Source Multicast (SSM) instead of Any-Source Multicast (ASM) models
   of multicast operation.  SSM can simplify security challenges with
   senders providing a centralized management point for secure group
   operation.  Related to this, implementations MAY also support
   policies to limit the impact of extremely or exceptionally poor-
   performing (due to bad behavior or otherwise) receivers upon overall
   group operation if this acceptable for the relevant application.

   The IETF Multicast Security (msec) Working Group is also developing
   automated group keying solutions which may be applicable to NACK-
   based reliable multicast security.  For example, to support IPsec or
   other security mechanisms, the Group Secure Association Key
   Management Protocol [RFC4535] MAY be used for automated group key

5.  IANA Considerations

   This document has no actions for IANA.

6.  Changes from RFC3941

   This section lists the changes between the Experimental version of
   this specification, [RFC3941], and this version:

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

   2.  Updated references to related, updated RMT Building Block

   3.  More detailed security considerations

7.  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.

8.  References

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8.1.  Normative References

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

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

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

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

              Gossink,  D. and J. Macker, "Reliable Multicast and
              Integrated Parity Retransmission with Channel Estimation",
              IEEE Globecomm 1998, 1998.

              Watson, M., "Basic Forward Error Correction (FEC)
              Schemes", draft-ietf-rmt-bb-fec-basic-schemes-revised-04
              (work in progress), November 2007.

              Nonnenmacher,  J. and E. Biersack, "Optimal Multicast
              Feedback", in IEEE Infocom p. 964, March/April 1998.

              Holbrook,  H., "A Channel Model for Multicast", PH.D.
              Dissertation Stanford University Department of Computer
              Science, August 2001.

              Adamson, B. and J. Macker, "Quantitative Prediction of
              Nack Oriented Reliable Multicast (NORM) Feedback", in IEEE
              MILCOM 2002, October 2002.

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

   [RFC2357]  Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
              Criteria for Evaluating Reliable Multicast Transport and
              Application Protocols", RFC 2357, June 1998.

   [RFC3269]  Kermode, R. and L. Vicisano, "Author Guidelines for
              Reliable Multicast Transport (RMT) Building Blocks and
              Protocol Instantiation documents", RFC 3269, April 2002.

   [RFC3453]  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.

   [RFC3941]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "Negative-Acknowledgment (NACK)-Oriented Reliable
              Multicast (NORM) Building Blocks", RFC 3941,
              November 2004.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, June 2006.

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052, August 2007.

              Levine,  B. and J. Garcia-Luna-Aceves, "A Comparison of
              Known Classes of Reliable Multicast Protocols", Proc.
              International Conference on Network Protocols (ICNP-
              96) Columbus, Ohio, October 1996.

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

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

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              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.

              Widmer, J. and M. Handley, "Extending Equation-Based
              Congestion Control to Multicast Applications", ACM
              SIGCOMM 2001, August 2001.

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
   University College London
   Gower Street
   London,   WC1E 6BT

   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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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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