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Multicast Negative-Acknowledgment (NACK) Building Blocks

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 5401.
Authors Joseph P. Macker , Mark J. Handley , Carsten Bormann , Brian Adamson
Last updated 2015-10-14 (Latest revision 2008-09-09)
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IESG IESG state Became RFC 5401 (Proposed Standard)
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Responsible AD Magnus Westerlund
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Network Working Group                                         B. Adamson
Internet-Draft                                 Naval Research Laboratory
Obsoletes: 3941 (if approved)                                 C. Bormann
Intended status: Standards Track                 Universitaet Bremen TZI
Expires: March 13, 2009                                       M. Handley
                                               University College London
                                                               J. Macker
                                               Naval Research Laboratory
                                                       September 9, 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
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on March 13, 2009.


   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
   protocols.  This document obsoletes RFC 3941.

Requirements Language

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Delivery Service Model . . . . . . . . . . . . . . . . . .  5
     2.2.  Group Membership Dynamics  . . . . . . . . . . . . . . . .  6
     2.3.  Sender/Receiver Relationships  . . . . . . . . . . . . . .  6
     2.4.  Group Size Scalability . . . . . . . . . . . . . . . . . .  6
     2.5.  Data Delivery Performance  . . . . . . . . . . . . . . . .  7
     2.6.  Network Environments . . . . . . . . . . . . . . . . . . .  8
     2.7.  Intermediate System Assistance . . . . . . . . . . . . . .  8
   3.  Functionality  . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Multicast Sender Transmission  . . . . . . . . . . . . . . 11
     3.2.  NACK Repair Process  . . . . . . . . . . . . . . . . . . . 13
     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
     3.8.  Group Size Determination/Estimation  . . . . . . . . . . . 33
     3.9.  Congestion Control Operation . . . . . . . . . . . . . . . 34
     3.10. Intermediate System Assistance . . . . . . . . . . . . . . 34
   4.  NACK-based Reliable Multicast Applicability  . . . . . . . . . 34
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 36
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   7.  Changes from RFC3941 . . . . . . . . . . . . . . . . . . . . . 38
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 38
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 39
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
   Intellectual Property and Copyright Statements . . . . . . . . . . 43

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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.  NACK-based protocols
   generally entail less frequent feedback messaging than reliability
   protocols based on positive acknowledgment (ACK).  The less frequent
   feedback messaging helps simplify the problem of feedback implosion
   as group size grows large.  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.  NACK-based Multicast sender transmission strategies,

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

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

   NACK-based reliable multicast implementations SHOULD make use of
   Forward Error Correction (FEC) erasure coding techniques as described
   in the FEC Building Block [RFC5052] document.  Packet-level erasure
   coding allows missing packets from a given FEC block to be recovered
   using the parity packets instead of classical, individualized re-
   transmission of original source data content.  For this reason, this
   document refers to the protocol mechanisms for reliability as a
   "repair process."  Note that NACK-based protocols can reactively
   provide the parity packets in response to receiver requests for
   repair rather than just proactively sending added FEC parity content
   as part of the original transmission.  Hybrid proactive/reactive use
   of FEC content is also possible with the mechanisms described in this
   document.  Some classes of FEC coding such as Maximal Separable
   Distance (MDS) codes allow senders to dynamically implement
   deterministic, highly efficient receiver group repair strategies as
   part of a NACK-based, selective automated repeat-request (ARQ)
   scheme.  This document describes approaches

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   The potential relationships to other reliable multicast transport
   building blocks (e.g., FEC, congestion control) and general issues
   with NACK-based reliable multicast protocols are also discussed.
   This document 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.

   The differences between [RFC3941] and this document are listed in
   Section 7.

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, and

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   6.  Network Environments,

   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.  The forms of "bulk data transfer" covered here
   include reliable transport of bulky, but fixed-length, a priori
   static content and also transmission of non-predetermined, perhaps
   streamed content of indefinite length.  Section 3.5 discusses these
   different forms of bulk data content in further detail.

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 service 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

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   these capabilities without defining specific application-level

   Another factor impacting the delivery service model is the potential
   for different receivers in the multicast group to have significantly
   differing quality of network connectivity.  This may involve
   receivers with very limited goodput due to connection rate or
   substantial packet loss.  NACK-based protocol implementations may
   wish to provide policies by which extremely poor-performing receivers
   are excluded from the main group or migrated to a separate delivery
   group.  Note that some application models may require that the entire
   group be constrained to the performance of the "weakest member" to
   satisfy operational requirements.  In either case, protocol designs
   should consider this aspect of the reliable multicast delivery
   service model.

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.

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

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   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 be applied without the potential for the volume of NACK
   feedback messages to overwhelm network capacity.  This is often
   referred to as "feedback implosion".  Research suggests that NACK-
   based reliable multicast group sizes on the order of tens of
   thousands of receivers may operate with acceptable levels of feedback
   to the sender using probabilistic, timer-based suppression techniques
   [NormFeedback].  Instead of receivers immediately transmitting
   feedback messages when loss is detected, these techniques specify use
   of purposefully-scaled, random back-off timeouts such that some
   potential NACKing receivers can self-suppress their feedback upon
   hearing messages from other receivers that have selected shorter
   random timeout intervals.  However, there may be additional NACK
   suppression heuristics that can be applied to enable these protocols
   to scale to even larger 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

   Intermediate assistance from devices/systems with direct knowledge of
   the underlying network topology may be used to increase the
   performance and scalability of NACK-based reliable multicast
   protocols.  Feedback aggregation and filtering of sender repair data
   may be possible with NACK-based protocols using FEC-based repair
   strategies as described in the and other reliable multicast transport
   building block documents.  However, there will continue to be a
   number of instances where intermediate system assistance is not
   available or practical.  Thus, building block components for based
   reliable multicast should be capable of operating without such

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, back-off 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.

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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
   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 Source-Specific Multicast (SSM) model
   from a specific source [RFC4607].  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.  Intermediate System Assistance

   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

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   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.
   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.  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

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   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
   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 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         |   \ . .           | +----------------+   |
      `---------------------'    \ \ \          | | Intermediate   |   |
                                  \ \ \         | | System Assist  |   |
                                   \ \ |        V +----------------+   |
                                    `-` >.-------------------------.   |
                                         | Sender NACK Processing  |___/
                                         | and Repair Response     |
                      ^                         ^
                      |                         |
                    |         (Security)          |

     Figure 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 the FEC Building Block [RFC5052].
   When congestion control mechanisms are needed (REQUIRED for general
   Internet operation), the sender transmission rate SHALL be controlled

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

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

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


   1.  Application data and control

   2.  Sender node identifier

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   3.  Data identifiers

   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

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   Alternatively, if the sender's transmission advertises the quantity
   of repair packets it is already planning to send for a block, the
   receiver may be able to initiate the NACK process 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-based 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

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   been completely superseded by NACK messages heard from other
   receivers (when receivers are multicasting NACKs) or from some
   indicator from the sender.  When receivers are unicasting NACK
   messages, the sender may facilitate NACK suppression by forwarding a
   representation of NACK content it has received to the group at large
   or provide some other indicator of the repair information it will be
   subsequently transmitting.

   For effective and scalable suppression performance, the backoff
   timeout periods used by receivers should be independently, randomly
   picked by receivers with a truncated exponential
   distribution[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

   In the functions below, the "log()" function specified refers to the
   "natural logarithm" and the "exp()" function is similarly based upon
   the mathematical constant 'e' (a.k.a.  Euler's number) where "exp(x)"
   corresponds to '"e"' raised to the power of '"x"'.  Given the
   receiver group size ("groupSize"), 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 "groupSize":

                           L = log(groupSize) + 1

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

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

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

       t' = T_maxBackoff/L * log(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 T_maxBackoff, double groupSize)
            double lambda = log(groupSize) + 1;
            double x = UniformRand(lambda/T_maxBackoff) +
                       lambda / (T_maxBackoff*(exp(lambda)-1));
            return ((T_maxBackoff/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 trade-off 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

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

   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

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   selection may require the creation of an "early NACK" slot for these
   historical NACKers.  This additional slot in the NACK backoff window
   will result in a longer repair cycle process that may not be
   desirable for some applications.  The resolution of these trade-offs
   may be dependent upon the protocol's target application set or

   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.


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   1.  NACK process initiation decision.

   2.  Recorded sender transmission sequence position.

   3.  Sender GRTT.

   4.  Sender group size estimate.

   5.  Application-defined bound on backoff timeout period.

   6.  NACKs from other receivers.

   7.  Pending repair indication from sender (may be forwarded NACKs).

   8.  Current sender transmission sequence position.


   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.

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   Considering that multiple repair rounds may be required to
   successfully complete repair, an erasure count also implies that the
   quantity of unique FEC parity packets the server has available to
   transmit is essentially unlimited (i.e., the server will always be
   able to provide new, unique, previously unsent parity packets in
   response to any subsequent repair requests for the same coding
   block).  Alternatively, the sender may "round-robin" transmit through
   its available set of FEC symbols for a given coding block, and
   eventually effect 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 providing 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

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   provides for 64 data symbols and 32 parity symbols per coding block
   would contain FEC symbol identifiers in the range of 0 to 95.

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

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

   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.

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

    Figure 2: Reliable Multicast Data Content Identification Hierarchy

   The format of NACK messages should enable the following:

   1.  Identification of transport data units required to repair the
       received content, whether this is an entire missing object/stream
       (or range), entire FEC coding block(s), or sets of symbols,

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   2.  Simple processing for NACK aggregation and suppression,

   3.  Inclusion of NACKs for multiple objects, FEC coding blocks and/or
       symbols in a single message, and

   4.  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


   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.

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3.2.4.  Sender Repair Response

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

            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 "hold-off" 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 "hold-off" time should be:

                           T_sndrHoldoff = 1*GRTT

   Recall that the receivers will also employ a "hold-off" 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 hold-off timeouts of:

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

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

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   not multicast connectivity among the receiver set.

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

   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

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   data transmission.  Policies limiting the opportunities when
   receivers begin participating in the NACK process may be used to
   achieve the desired behavior.  For example, it may be beneficial for
   receivers to attempt reliable reception from a newly-heard sender
   only upon non-repair transmissions of data in the first FEC block of
   an object or logical portion of a stream.  The sender may also
   implement policies limiting the receivers from which it will accept
   NACK requests, but this may be prohibitive for scalability reasons in
   some situations.  Alternatively, it may be desirable to have a looser
   transport synchronization policy and rely upon session management
   mechanisms to limit group dynamics that can cause poor performance,
   in some types of bulk transfer applications (or for potential
   interactive reliable multicast applications).


   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.  Receivers that send NACK messages to the group will need to
   identify the sender to which the NACK is intended.  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

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

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

   The components described within this building block document are
   envisioned to be applicable to both of these models with the
   potential for a mix of both types within a single multicast session.
   To support this requirement, the normal data content identification
   should include a field to uniquely identify the object or stream
   (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

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   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.

   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

3.6.  Forward Error Correction (FEC)

   Multiple forward error correction (FEC) approaches using erasure
   coding techniques 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

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   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 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 of "one-to-
   many" transmission, it may be that only the sender requires 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

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   requirements, the sender will need to advertise its current GRTT
   estimate to the group for various NACK timeouts used by receivers.

3.7.1.  One-to-Many Sender GRTT Measurement

   The goal of this form of RTT measurement is for the sender to
   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>") is
   the amount of time differential since that request was received until
   the receiver generated the response.

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

                   RTT_rcvr = currentTime - grttResponse

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

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

                       GRTT = MAX(0.9*GRTT, RTT_peak)

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

   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

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   measurement has aged most should have precedence over other
   receivers.  In turn the sender may have limited capacity to provide
   an "echo" of the receiver timestamps back to the group, and it could
   use this RTT "age" metric to determine which receivers get
   precedence.  The sender can determine the "GRTT" as described in
   3.7.1 if it provides sender timestamps to the group.  Alternatively,
   receivers who note their RTT is greater than the sender GRTT can
   compete in the feedback opportunity/suppression scheme to provide the
   sender and group with this information.

3.7.3.  Many-to-Many RTT Measurement

   For reliable multicast sessions that involve multiple senders, it may
   be useful to have RTT measurements occur on a true "many-to-many"
   basis rather than have each sender independently tracking RTT.  Some
   protocol efficiency can be gained when receivers can infer an
   approximation of their RTT with respect to a sender based on RTT
   information they have on another sender and that other sender's RTT
   with respect to the new sender of interest.  For example, for
   receiver "a" and 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 the
   range of 1 microsecond to 1000 seconds.  Of course, NACK-based
   reliable multicast protocol implementations may wish to further
   constrain advertised GRTT estimates (e.g., limit the maximum value)
   for practical reasons.

3.8.  Group Size Determination/Estimation

   When NACK-based reliable multicast protocol operation includes
   mechanisms that excite feedback from the group at large (e.g.,
   congestion control), it may be possible to roughly estimate the group
   size based on the number of feedback messages received with respect
   to the distribution of the probabilistic suppression mechanism used.
   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 any 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.  This conservative estimate
   (over-estimate) of group size in the algorithms described above will
   result in some added latency to the NACK repair process if the actual
   group size is smaller but with a guarantee of feedback implosion
   protection.  The study of the timer-based feedback suppression
   mechanism described in [McastFeedback] and [NormFeedback] showed that
   the group size estimate need only be with an order-of-magnitude to
   provide effective suppression performance.

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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
   Congestion Control (PGMCC) [PgmccPaper] techniques can be applied to
   NACK-based reliable multicast operation to meet this requirement.
   The former technique has been further documented in [RFC4654] and has
   been successfully applied in the NACK-Oriented Reliable Multicast
   Protocol [RFC3940].

3.10.  Intermediate System Assistance

   NACK-based multicast protocols may benefit from general purpose
   intermediate system assistance.  In particular, additional NACK
   suppression where 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.  Similarly, intermediate systems could
   monitor receiver NACKs and provide repair transmissions on-demand in
   response if sufficient state on the content being transmitted was
   being maintained.  This can reduce the latency and volume of repair
   transmissions when the intermediate system is associated with a
   network link that is particularly problematic with respect to packet
   loss.  These types of assist functions would require intermediate
   system interpretation of transport data unit content identifiers and
   flags.  NACK-based protocol designs should consider the potential for
   intermediate system assistance in the specification of protocol
   messages and operations.  It is likely that intermediate systems
   assistance will be more pragmatic if message parsing requirements are
   modest and if the amount of state an intermediate system is required
   to maintain is relatively small.

4.  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

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   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 Source-
   Specific Multicast (SSM) [RFC4607] 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.  It is possible that, with proper protocol design, the
   intermediate system assistance techniques mentioned in Section 2.4
   and described further in Section 3.10 can allow NACK-based approaches
   to scale to larger group sizes.  NACK-based reliable multicast
   operation is also governed by implementation buffering constraints.
   Buffering greater than that required for typical point-to-point
   reliable transport (e.g., TCP) is recommended to allow for disparity
   in the receiver group connectivity and to allow for the feedback
   delays required to attain group size scalability.

   Prior experimental work included various protocol instantiations that
   implemented some of the concepts described in this building block
   document.  This includes the Pragmatic General Multicast (PGM)
   protocol described in [RFC3208] among others that were documented or
   deployed outside of IETF activities.  While the PGM protocol
   specification and some other approaches encompassed many of the goals
   of bulk data delivery as described here, this NACK-based building
   block provides a more generalized framework so that different
   application needs can be met by different protocol instantiation
   variants.  The NACK-based building block approach described here
   includes compatiblity with the other protocol mechanisms including
   FEC and congestion control that are described in other IETF reliable
   multicast building block documents.  The NACK repair process
   described in this document can provide performance advantages as
   compared to PGM when both are deployed on a pure end-to-end basis
   without intermediate system assistance.  The round-trip timing
   estimation described here and its use in the NACK repair process
   allow protocol operation to more automatically adapt to different

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   network environments or operate within environments where
   connectivity is dynamic.  Use of the FEC payload identification
   techniques described in the FEC building block [RFC5052] and specific
   FEC instantiations allow protocol instantiations more flexibility as
   FEC techniques evolve than the specific sequence number data
   identification scheme described in the PGM specification.  Similar
   flexibility is expected if protocol instantiations are designed to
   modularly invoke (at design time, if not run-time) the appropriate
   congestion control building block for different application or
   deployment purposes.

5.  Security Considerations

   NACK-based reliable multicast protocols are expected to be subject to
   the same 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 techniques such as
   authentication and data integrity checks be applied for NACK-based
   reliable multicast deployments.  Protocol instantiations using this
   building block MUST 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.  The IETF Multicast Security (MSEC) Working Group has developed
   a set of recommendations in its Multicast Extensions to the Internet
   Protocol Security Architecture [I-D.ietf-msec-ipsec-extensions] that
   can be applied to appropriately extend IPsec mechanisms to multicast
   operation.  An appendix of this document specifically addresses the
   Nack-Oriented Reliable Multicast protocol service model.  As complete
   support for IPsec multicast operation may potentially follow reliable
   multicast deployment, 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

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   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 the receiver population
   can be completely trusted.  Additionally, this can make
   identification of badly-behaving (although 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 Source-Specific Multicast (SSM) instead of Any-Source
   Multicast (ASM) models of multicast operation.  SSM operation can
   simplify security challenges in a couple of ways:

   1.  A NACK-based protocol supporting SSM operation can eliminate
       direct receiver-to-receiver signaling.  This dramatically reduces
       the number of security associations that need to be established.

   2.  The SSM sender(s) can provide a centralized management point for
       secure group operation for its respective data flow with the
       sender alone required to conduct individual host authentication
       for each receiver when group-based authentication does not
       suffice or is not pragmatic to deploy.

   When individual host authentication is required, then it is possible
   receivers could use a digital signature on the IPsec Encapsulating
   Security Protocol (ESP) payload as described in [RFC4359].  Either an
   identity-based signature system or a group-specific public key
   infrastructure could avoid per-receiver state at the sender(s).
   Additionally, implementations MUST 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
   is acceptable for the relevant application.

   As described in Section 3.4, deployment of NACK-based reliable
   multicast in some network environments may require identification of
   group members beyond that of IP addressing.  If protocol-specific
   security mechanisms are developed, then it is RECOMMENDED that
   protocol group member identifiers are used as selectors (as defined
   in [RFC4301]) for the applicable security associations.  When IPsec
   is used, it is RECOMMENDED that the protocol implementation verify
   that the source IP address of received packets are valid for the
   given protocol source identifier in addition to usual IPsec
   authentication.  This would prevent a badly-behaving (although
   authorized) member spoofing messages from other legitimate members,
   providing that individual host authentication is supported.

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   The MSEC Working Group has also developed automated group keying
   solutions which are 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 management.  The
   technique it identifies for "Group Establishment for Receive-Only
   Members" may be application NACK-based reliable multicast SSM

6.  IANA Considerations

   This document has no actions for IANA.

7.  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

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

   3.  More detailed security considerations.

8.  Acknowledgements

   (and these are not Negative)

   The authors would like to thank George Gross, 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

9.  References

9.1.  Normative References

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

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

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.

9.2.  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.

              Weis, B., Gross, G., and D. Ignjatic, "Multicast
              Extensions to the Security Architecture for the Internet
              Protocol", draft-ietf-msec-ipsec-extensions-09 (work in
              progress), June 2008.

              Watson, M., "Basic Forward Error Correction (FEC)
              Schemes", draft-ietf-rmt-bb-fec-basic-schemes-revised-05
              (work in progress), July 2008.

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

              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.

   [RFC3208]  Speakman, T., Crowcroft, J., Gemmell, J., Farinacci, D.,
              Lin, S., Leshchiner, D., Luby, M., Montgomery, T., Rizzo,
              L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
              Sumanasekera, R., and L. Vicisano, "PGM Reliable Transport
              Protocol Specification", RFC 3208, December 2001.

   [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.

   [RFC3940]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "Negative-acknowledgment (NACK)-Oriented Reliable
              Multicast (NORM) Protocol", RFC 3940, November 2004.

   [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.

   [RFC4359]  Weis, B., "The Use of RSA/SHA-1 Signatures within
              Encapsulating Security Payload (ESP) and Authentication
              Header (AH)", RFC 4359, January 2006.

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

   [RFC4654]  Widmer, J. and M. Handley, "TCP-Friendly Multicast
              Congestion Control (TFMCC): Protocol Specification",
              RFC 4654, August 2006.

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error

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              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.

              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


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


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   Mark Handley
   University College London
   Gower Street
   London,   WC1E 6BT


   Joe Macker
   Naval Research Laboratory
   Washington, DC  20375


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