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Versions: 00 02

RMT Working Group                                    B.  Adamson/Newlink
INTERNET-DRAFT                                      C.  Bormann/Tellique
draft-ietf-rmt-norm-bb-00.txt                            S.  Floyd/ACIRI
Expires: January 2001                                  M.  Handley/ACIRI
                                                          J.  Macker/NRL
                                                               July 2000


    NACK-Oriented Reliable Multicast (NORM) Protocol Building Blocks

Status of this Memo


     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC2026.

     Internet-Drafts are working documents of the Internet Engineering
     Task Force (IETF), its areas, and its working groups.  Note that
     other groups may also distribute working documents as Internet-
     Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other docu-
     ments at any time.  It is inappropriate to use Internet-Drafts as
     reference material or to cite them other than as "work in
     progress."

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.

     Copyright Notice

     Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

     This memo describes issues related to the creation of negative-
     acknowledgment (NACK) oriented reliable multicast (NORM) protocols.
     The general goals and  assumptions for NORM are defined.  The tech-
     nical challenges related to NACK-oriented (and in some cases gen-
     eral) reliable multicast protocol design are identified.  These
     challenges are resolved into a set of applicable "building blocks"
     which are described in further detail.  It is anticipated that
     these building blocks (as they are further refined and defined in
     future revisions of this document) will be useful in generating



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     different instantiations of reliable multicast protocols.

1.0 Background

     Reliable multicast transport is a desirable technology for the
     efficient and reliable distribution of data to a group on the
     Internet.  The complexities of group communication paradigms neces-
     sitate different protocol types and instantiations to meet the
     range of performance and scalability requirements of different
     potential reliable multicast applications and users [Mankin98].
     Properly designed negative-acknowledgement (NACK) oriented reliable
     multicast (NORM) protocols offer scalability advantages for appli-
     cations and/or network topologies where, for  various reasons, it
     is prohibitive to construct a higher order  delivery infrastructure
     above the basic Layer 3 IP multicast service (e.g.  unicast or
     hybrid unicast/multicast data distribution trees).  Additionally,
     the scalability property of NACK-oriented protocols [Pingali93,
     Levine96] may be applicable where broad "fanout" is expected for a
     single network hop (e.g.  cable-TV data delivery, satellite, or
     other broadcast communication communication services).  Further-
     more, the simplicity of a protocol based on "flat" group-wide mul-
     ticast distribution may offer advantages for a broad range of dis-
     tributed services or dynamic networks and applications.

     While different protocol instantiations may be required to meet
     specific application and network architecture demands [Clark90],
     there are a number of fundamental components which may be common to
     these different instantiations.  This document describes the frame-
     work and  common "building block" components relevant to multicast
     protocols based primarily on NACK operation for reliable transport.

2.0 Applicability

     Each potential protocol instantiation using the building blocks
     presented here (and other applicable building block documents) will
     have specific criteria which will 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 which 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)   Data content model
       2)   Group membership dynamics
       3)   Sender/receiver relationships
       4)   Group size



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       5)   Data delivery performance
       6)   Network topology
       3)   Router/intermediate system assistance

     In some cases, a building block may be designed to address a wide
     range of assumptions while in other cases there will be trade-offs
     required to meet different application needs.  Where necessary,
     building block features will designed to be parametric to meet dif-
     ferent requirements.  Of course, an underlying goal will be to min-
     imize design complexity and to at least recommend default values
     for any such parameters which meet a general purpose "bulk data
     transfer" requirement in a typical Internet environment.

2.1 Data content model

     The implicit goal of a reliable multicast protocol is the reliable
     delivery of "data" among a group of members communicating through
     IP multicast datagram service.  However, the nature of the data
     content and the service the application is attempting to provide
     can impact design decisions.  The service model may range from
     long-lived transfer sessions of bulk  quantities of data (file
     broadcast) to more interactive exhanges of  small messages (e.g.
     white-boarding, text chat).  And within those  different models
     there are other 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 dif-
     ferent 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 com-
     plete 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 drives the need for a number of
     different protocol instantiation designs.

     A significant challenge in developing generally useful building
     block mechanisms is accommodating even a limited range of these
     capabilities without defining specific application-level details.
     Note that this particular building block "guideline" may be gener-
     ally applicable beyond the realm of NACK-oriented reliable multi-
     cast.

2.2 Group membership dynamics

     Group communication can differ from point-to-point communications
     with respect to the fact that even if the composition of the group
     changes, the "thread" of communication can still exist.  This



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     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 communi-
     cation "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 or other timing, con-
     gestion 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 sin-
     gle  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

     Native IP multicast [Deering89] may scale to extremely large group
     sizes.  It may be desirable for some applications to be able to
     scale along with the multicast infrastructure's ability to scale.
     In its simplest form, there are limits to the group size to which a
     NACK-oriented protocol can apply without NACK implosion problems.
     However, the potential for router assistance or other non-linear
     NACK suppression mechanisms may enable these protocols to scale to
     very large group sizes.  In large scale cases, it may be pro-
     hibitive 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 generally appli-
     cable building blocks.

2.5 Data delivery performance

     There is a trade-off between scalability and data delivery latency
     when designing NACK-oriented protocols.  If NACK suppression is to
     be used, there will be some delays built into the NACK generation
     and repair transmission process to allow suppression to occur and
     for the sender of data to identify appropriate content for effi-
     cient repair transmission.  For example, backoff timeouts can be



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     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 scalabilty trade-off which is made when such
     bounds are applied.

2.6 Network topology

     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 oper-
     ating 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 wire-
     less communication services have begun to proliferate.   Many of
     these are inherently broadcast media with potentially large
     "fanouts" to which IP multicast service is highly applicable.

     Additionally, policy and/or technical issues may result in topolo-
     gies where multicast connectivity is limited to a single logical
     direction from a specific source or set of sources to the group at
     large.  Receivers in the group may be restricted to unicast feed-
     back for NACKs and other messages.  Consideration must be given, in
     building block development and protocol design, to the nature of
     the underlying networks over which the protocols may be operating.

2.7 Router/Intermediate System Assistance

     While intermediate assistance from devices/systems with direct
     knowledge of the underlying network topology may 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 should be capable of operating without
     such assistance but should also be capable of utilizing these fea-
     tures when available.

3.0 Building Block Areas

     The previous section has presented in general what building blocks
     are intended to be and some of the criteria which may affect build-
     ing block identification/design. This section describes different



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     building block areas applicable to  NACK-oriented reliable multi-
     cast protocols.  Some of these areas are  specific to NACK-oriented
     protocols.  Detailed descriptions of such  areas are provided
     below.  In other cases, the areas (e.g.  node  identifiers, FEC,
     etc) may be generally applicable to other forms of  reliable multi-
     cast.  In those cases, the discussion below describes requirements
     placed on those other general building block areas from  the stand-
     point of NACK-oriented reliable multicast.

     For the individual building blocks to be incorporated as part of a
     specific protocol instantiation, it is expected that a description
     of some notional "interface" to the building blocks' functionality
     be provided.  For example, a building block component may require
     some  form of "input" from another building block component or
     other source  in order to perform its function.  Any "inputs"
     required by each  building block component and/or any resultant
     "output" provided by the building block will be defined and
     described as the building  block component's interface definition.

     The following building block areas are described below:

     (NACK-Oriented Components)
       1)   Sender transmission
       2)   NACK-oriented Repair Process
       3)   "Late-joining" Receiver Policies and Mechanisms

     (Generally-applicable Components)
       4)   Node (member) Identification
       5)   Data Content Identification
       6)   Forward Error Correction
       7)   Round-trip Timing Collection
       8)   Group Size Determination/Estimation
       9)   Congestion Control Operation
       10)  Router/Intermediate System Assistance
       11)  Additional Protocol Mechanisms

     Figure 1 provides an pictoral overview of these building block
     areas and their relationships.  For example, the content of the
     data messages that sender initially transmits depends upon the
     "Node Identification", Data Content Identification", "FEC" , and
     "Congestion Control components (Note that the rate of message
     transmission will generally depend upon the "Congestion Control"
     component).  Subsequently, the receivers response to these trans-
     missions (e.g. NACKing for repair) will depend upon the content of
     these transmissions and inputs from other building block compo-
     nents.  Then, the sender's processing of these responses will feed
     back into its transmission strategy.




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                                           Application Data
                                                 |
                                                 V
    .---------------------.            .-----------------------.
    | Node Identification |----------->|  Sender Transmission  |<------.
    '---------------------'       _.-' '-----------------------'       |
    .---------------------.   _.-' .'            |                     |
    | Data Identification |--'   .''             | ("Join" Policy)     |
    '---------------------'    .' '              V                     |
    .---------------------.  .'  '     .------------------------.      |
 .->| Congestion Control  |-'   '      | Receiver NACK-oriented |      |
 |  '---------------------'   .'       | Repair Process         |      |
 |  .---------------------. .'         | .------------------.   |      |
 |  |        FEC          |'.          | | NACK Initiation  |   |      |
 |  '---------------------'. '._       | '------------------'   |      |
 |  .---------------------. ,   '-._   | .------------------.   |      |
 '--|    RTT Collection   |._'      '->| | NACK Content     |   |      |
    '---------------------' .''.       | '------------------'   |      |
    .---------------------.  ' ''-._   | .------------------.   |      |
    |    Group Size Est.  |---'-'---'->| | NACK Suppression |   |      |
    '---------------------''.  ' '     | '------------------'   |      |
    .---------------------.  '  ' '    '------------------------'      |
    |       Other         |   '  ' '             |                     |
    '---------------------'    '  ' '            | (Router Assistance) |
                                '. ' '           V                     |
                                  '.'' .-------------------------.     |
                                     '>| Sender NACK Processing  |_____/
                                       | and Repair Response     |
                                       '-------------------------'

                    ^                         ^
                    |                         |
                  .-----------------------------.
                  |         (Security)          |
                  '-----------------------------'

            Fig. 1 - NORM Building Block Framework

The components on the left side of this figure represent the components
which may be generally applicable beyond NORM and those on the right are
specific to NORM protocols.  Some components (e.g. "Security") will
impact to many aspects of the protocol and others such as "Router Assis-
tance" may be more transparent to the core protocol processing.  The
sections below discuss issues with regards to these building block com-
ponents and their relationships.  Where applicable, specific technical
recommendations are made for mechanisms which will properly satisfy the
goals of reliable multicast bulk transport for the Internet.




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3.1 Sender transmission

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 packet sizes determined by application and/or
network architecture requirements.  When congestion control mechanisms
are used (recommended), the transmission rate will be controlled by the
congestion control mechanism.  It is recommended that all data transmis-
sions from senders be subject to rate limitations determined by the
application or congestion control algorithm.  The sender's transmissions
should make good utlization of the available capacity (which may be lim-
ited 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.

In addition to data content, other sender messages or commands may be
employed as part of protocol operation.  For NACK-oriented operation,
the reliability of any such commands may depend upon redundant transmis-
sion.  Other factors related to NACK-oriented operation may determine
sender transmission formats and methods.  Some consideration needs to be
given to the sender's behavior during intermittent idle periods when it
has no data to transmit.  While many aspects may be protocol-specific,
there are techniques which may be generally applicable to NACK-oriented
reliable multicast.  For example, the periodicity of redundant transmis-
sion of command messages issued by a sender should be dependent upon the
greatest round trip timing estimate and the resultant NACK operation.
More specifically, 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 process described below.  For efficiency, the
sender should allow sufficient time between redundant transmissions to
receive any NACK-oriented responses from the receiver set to this com-
mand.  Other protocol components may benefit from a similar approach.

Inputs:

       1)   Data content
       2)   Segmentation size
       3)   Transmission rate
       4)   Application controls

     Outputs:

       1)   Rate-controlled stream of packets 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



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            control actions to be taken.

3.2 NACK-oriented repair process

     The most critical component of the NACK-oriented reliable multicast
     protocol building block is the NACK repair process.  There are four
     primary elements of a general NACK repair process:

       1)   Method for determining when receivers will initiate the NACK
            process in response to sender transmission for which they
            need repair.
       2)   NACK message content.
       3)   NACK suppression mechanisms to promote scalability of the
            protocol.
       4)   Sender NACK reception, aggregation, and repair response.

3.2.1 NACK Process Initiation

     The NACK process (cycle) will be initiated by receivers who detect
     they require repair transmissions from a specific sender at defined
     opportunities.  When FEC is applied, a NACK cycle should only be
     initiated when it is known by the receiver that its repair require-
     ments exceed the amount of FEC pending transmission for a given
     coding block of packets.  This may be determined by the receiver if
     the sender's transmission advertises the quantity of repair packets
     it is already planning to send for a block, and/or at the end of
     the current transmission block (if it is indicated) or at the start
     of subsequent coding block for packets transmitted within the con-
     text of a designed data content set (See object/stream data content
     identifier discussion below).

     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 use-
     ful to limit the initiation opportunities to specific events such
     as at the end-of-transmission of an FEC coding block (or alterna-
     tively at detection of subsequent coding blocks).  This can  allow
     receivers to aggregate NACK content into a smaller number of NACK
     messages.  In either case, the NACK cycle should begin with
     receivers observing backoff timeouts to facilitate NACK suppression
     as described below.

     Interface Description

     Inputs:

       1)   Object/stream data content and sequencing identifiers from
            sender transmissions.



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     Outputs:

       1)   NACK Process Initiation Decision

3.2.2 NACK Content

     The content of NACK messages generated by reliable multicast
     receivers will include information detailing the current repair
     needs of each receiver.  The specific information depends on the
     use and type of FEC in the NORM 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 data transport object (or stream) within the mul-
     ticast session which needs repair.  For the indicated transport
     entity, the NACK content will then identify the specific coding
     blocks and/or segments of coding blocks it needs to reconstruct the
     transmitted data.  This content may be one or more of the items
     described in the sections below.

3.2.2.1 FEC Block Erasure Counts

     Where FEC-based repair is used, the NACK message content will need
     to identify the coding block(s) for which repair is needed and the
     count of erasures (missing packets) in the coding block.  Note that
     this assumes the FEC algorithm is capable of repairing _any_ loss
     combination within the coding block and that the quantity of unique
     FEC parity packets the server has available to transmit is essen-
     tially unlimited (i.e. the server will always be able to provide
     new parity packets in response to anysubsequent repair requests for
     the same coding block).  In other cases, the NACK content will need
     to also _explicitly_ identify which packets (information and/or
     parity) the receiver needs to successfully reconstruct the content
     of the coding block.

3.2.2.2 Encoding of Explicit NACK Content

     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 packet 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.



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     For cases where the amount of loss is very small with respect to
     the coding block size, it may be efficient to simply provide a list
     of coding block vector identifiers.  However, in many cases, a bit
     mask marking the locations of missing packets may be significantly
     more efficient for communicating receiver repair needs.  And since
     the data content is logically divided into coding blocks, a system
     of hierarchical bit masks can be used to encode missing content at
     the object/stream, FEC block, and individual packet levels.  Hier-
     archical bit mask encoding can provide compact NACK messages even
     in high delay*bandwidth*loss conditions.  Bit mask based NACK con-
     tent can also be efficiently processed with logical operations dur-
     ing protocol operation.

3.2.2.3 Hybrid Approach

     When FEC is used and NACK content is designed to contain explicit
     repair requests, there is strategy where the receivers can NACK for
     specific content which 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 segments to complete
     reliable transfer.  Another assumption is that an FEC algorithm
     where any parity packet can fill any erasure within the coding
     block 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 a coding block to
     the group.

     In this approach, the sender transmits the data content of the cod-
     ing block (and optionally some quantity of parity packets) in its
     initial transmission.  Note that a coding block is considered to be
     logically made up of the contiguous set of data vectors plus parity
     vectors for the given FEC algorithm used.  The sender transmits
     parity vectors from the lowest ordinal part of the parity portion
     of the coding block.  When the receivers construct explicit NACK
     content, they request transmission of only the _upper_ ordinal por-
     tion, corresponding to the number of _data_ vectors, of the logical
     coding block required to fill their packet erasures (Note this
     _may_ include data vectors if there is a smaller number of parity
     vectors than data vectors for the selected code, but generally will
     consist solely of parity vectors).  The receivers can also provide
     a count of erasures as a convenience (saves processing time) to the
     sender.  Upon receipt of the NACK message, the sender will schedule
     transmission of fresh, new parity vectors based on the erasure
     count _if_ it has a sufficient quantity of vectors which were _not_
     previously transmitted and ignore the explicit content requested..
     Otherwise, the sender will resort to transmitting the set of repair



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     vectors requested.  With this approach, the sender needs to main-
     tain 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 this same algorithm to express their
     explict repair needs, NACK suppression among receivers is simpli-
     fied over the course of multiple repair cycles.  The receivers can
     simply compare NACKs heard from other receivers against their own
     calculated repair needs to determine whether they should transmit
     or suppress their pending NACK messages.

3.2.3 NACK Suppression Mechanisms

     A primary NACK suppression mechanism is the use of initial backoff
     timeouts by receivers wishing to transmit NACK messages[Floyd95].
     Upon expiration of the timeout, a receiver will transmit a NACK
     unless the content of the pending repair request is completely
     superseded by NACK messages heard from other receivers (when
     receivers are multicasting NACKs) or from some indicator from the
     sender.  (Note: When receivers are unicasting NACK messages, the
     sender may facilitate NACK suppression by forwarding appropriate
     NACKs it has received to the group at large or provide some other
     indicator of the repair information it will be subsequently trans-
     mitting).

     The backoff timeout periods used by receivers should be indepen-
     dently, randomly picked by receivers with an exponential distribu-
     tion [Nonnenmacher98].  This results in the bulk 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 dis-
     tribution should be determined as a function of the current esti-
     mate of sender<->group greatest round trip time (GRTT) and a group
     size estimate which determined by other mechanisms within the pro-
     tocol (See section below) 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) can also be controlled to
     allow the application or protocol tradeoff NACK latency versus vol-
     ume of feedback traffic.  A larger value of T will result in a
     lower density of feedback traffic for a given repair cycle.  A
     smaller value of T results in shorter latency which reduces the
     buffering requirements of senders and receivers for reliable trans-
     port.



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

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

                                L = ln(R) + 1

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

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

     3) Transform this random variate to generate the
        desired random backoff time (t) with the following
        equation:

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

     This is a C language function which can be used to perform this
     function:

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

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

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

     The number of expected NACKs generated (N) within the first round
     trip time for a single loss event can be approximately expected to
     be:

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



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     Thus the maximum backoff time (T) can be adjusted to tradeoff
     worst-case NACK feedback volume versus latency.  This is derived
     from [Nonnenmacher98] and assumes  T >= 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 protocol may
     work to reduce redundant NACK generation further.

     There are some secondary NACK suppression mechanisms which can also
     be  considered.  For example, the sender's transmissions may follow
     an ordinal sequence of transmission (observed through data/repair
     content <objectIds> and/or <segmentIds>) which is "rewound" during
     repair transmissions.  Receivers may wish to limit transmission of
     their NACKs only when the sender's current sequence of transmission
     passes the point at which the receiver has incomplete transmis-
     sions, thus reducing premature requests for repair of data the
     sender may be providing 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 mecha-
     nism (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
     advertisment of the number of FEC packets to be sent for the cur-
     rent 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 assumes there is some degree of correlation
     over successive intervals of time in the loss experienced by
     receivers.  This adjustment of backoff timeout selection may
     require the creation of an "early NACK" slot for these historical
     NACKers.  This additional slot in the NACK backoff window will
     result in a longer repair cycle process which may not be desirable
     for some applications.  The resolution of these trade-offs may be
     dependent upon the protocol's target application set or network.

     Interface Description

     Inputs:

       1)   Group greatest round trip time estimate (GRTT).
       2)   Group size estimate.
       3)   Application-defined bound on backoff timeout period.
       4)   NACKs from other receivers.
       5)   Pending repair indication from sender (may be forwarded
            NACKs).
       6)   Current sender transmission sequence position.



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     Outputs:

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

3.2.4 Sender NACK Processing and 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 effi-
     cient 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 pend-
     ing repair transmissions as part of the its current transmitted
     message content.  This can aid some NACK suppression mechanisms.
     Alternatively, in the case of unicast feedback from the receiver
     set, it may be useful for the sender to forward (via multicast)
     superseding NACK messages to the group to allow for NACK suppres-
     sion when there is not multicast connectivity among the receiver
     set.

     When FEC is used, it is beneficial that the sender transmit previ-
     ously untransmitted parity content whenever possible.  This maxi-
     mizes the receiving nodes' ability to reconstruct the entire trans-
     mitted content from their individual subsets of received messages.

     The transmitted object and/or stream content will be marked with
     monotonically increasing sequence numbers (within a reasonably
     large ordinal space).  If the sender observes the discipline of
     transmitting repair for the earliest content first, the receivers
     can use a strategy of witholding 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 tim-
     ing.  This also helps minimize the buffering requirements of
     receivers and senders and reduces redundant transmission of data to
     the group at large.

     Interface Description

     Inputs:

       1)   Receiver NACKs
       1)   Group timing information




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     Outputs:

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

3.3 Group "Join" Policies/ Procedures

     Consideration should be given to how new receivers join a group
     (perhaps where reliable transmission is already in progress) and
     beging NACKing for any repair needs. If this is unconstrained, the
     dynamics of group membership may impede the application's ability
     to meet it goals progressing the transmission of data.  Policies
     limiting the opportunities at which receivers begin participating
     in the NACK process may be used to achieve the desired behavior.
     For example, it may be beneficial if receivers only attempt reli-
     able reception from a newly-heard sender when upon non-repair
     transmissions of data in the first FEC block of an object or logi-
     cal portion of a stream.  The sender may also implement policies
     limiting which receivers from which it will accept NACK requests,
     but this may be prohibitive for scalability reasons in some situa-
     tions.  In some types of bulk transfer applications (and for poten-
     tial interactive applications), it may alternatively desirable to
     have a looser transport synchronization policy and rely upon ses-
     sion management mechanisms to control group dynamics which may
     result in poor behavior.

     Inputs:

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

     Outputs:

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

3.4 Reliable multicast member identification

     In a NORM protocol (or other multicast  protocols) where there is
     the potential for multiple sources of data, it is necessary to pro-
     vide some mechanism to uniquely identify the sources (and possibly
     some or all receivers in some cases) within the group.  Identity
     based on arriving packet source addresses is insufficient for sev-
     eral reasons.  These reasons include routing changes for hosts with
     multiple interfaces which result 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



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     session members.

3.5 Data content identification

     The data content transmitted by a NORM sender requires some form of
     identification in the protocol header fields.  This identification
     is required to facilitate the reliable NACK-oriented repair pro-
     cess.  These identifiers will be used in NACK messages generated.

     There are two very general types of data which may comprise bulk
     transfer session content.  These data types are static objects of
     finite size and continuous non-finite streams.  A given application
     may wish to reliably multicast data content using either one or
     both of these data models.  While it may be possible for some
     applications to further generalize this model and provide mecha-
     nisms to encapsulate static objects as content embedded within a
     stream, there are advantages to many applications to provide dis-
     tinct 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-realtime message broad-
     casts (e.g. stock ticker) or some content types which are part of
     collaborative tools or other more complex applications.  And, as
     indicated above, some applications may wish to encapsulate other
     bulk content (e.g. files) into one more streams within a multicast
     session.  Additionally, multiple streams can allow for parallized
     transmission within the context of a single multicast session.

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



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     Since the "bulk" object/stream content will generally require seg-
     mentation, some form of segment identification <segmentId> must
     also be  provided.  This segment identifier will be relative to any
     object or stream identifier which has been provided.  Thus, in some
     cases, NORM protocol instantiations may be able to receive trans-
     missions and request repair for multiple streams and one or more
     sets of static objects in parallel.

     Additionally, flags to determine the usage of the content identi-
     fier 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 individ-
     ual protocol instantiations.

     In summary, the following data content identifiers may be required
     for NORM protocol data content messages:

       1)   Object/Stream identifier (<objectId>)
       2)   Segment identifier (<segmentId>)
       3)   Flags to differentiate interpretation of identifier fields
            or identifier structure which implicitly indicates usage.

     These fields have been identified since any generated NACK messages
     will use these identifiers in requesting repair or retransmission
     of data.  NORM protocols are expected to greatly benefit from
     interaction with other reliable multicast building blocks (Generic
     Router Assist(GRA), in particular) and those other building blocks
     will need to appropriately consider these anticipated requirements.

3.6 Forward Error Correction

     Multiple forward error correction (FEC) approaches have been iden-
     tified which can provide great performance enhancements to the
     repair process of NACK-oriented and other reliable multicast proto-
     cols [Metzner84, Macker97].  NORM protocols can reap additional
     benefits since FEC-based repair does not _generally_ require
     explicit knowledge of repair content within the bounds of its cod-
     ing block size (in packets).

     Generally, repair packets generated using FEC algorithms with good
     erasure filling properties (e.g.  Reed-Solomon) will be transmitted
     only in response to NACK repair requests from receiving nodes.
     However, there are benefits in some network environments for trans-
     mitting some predetermined quantity of FEC repair packets multi-
     plexed with the regular data segment transmissions [Gossink98].
     This can reduce the amount of NACK traffic generated with rela-
     tively  little overhead cost when group sizes are very large or the
     network  connectivity has a large delay*bandwidth product with some



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     nominal level of expected packet loss.  While the application of
     FEC is not unique to NORM, these sorts of requirements may dictate
     the types of algorithms and protocol approaches which are applica-
     ble.

     A specific issue related to the use of FEC with NORM is the mecha-
     nism used to identify which 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 concate-
     nation of <sourceId/objectId/segmentId> during transport, it is
     expected that FEC packets will be identified in a similar manner.
     It may be sufficient to identify FEC packets using the identifier
     of the first segment of the block of data content packets for which
     the FEC repair packet is applicable and add an additional FEC iden-
     tifier <fecId> field to its position within the set of FEC packets
     for the block.  The size of the <fecId> field can potentially be
     small (8 or 16 bits) relative to other identifier fields since its
     size is limited by the FEC coding algorithm used.

3.7 Round-trip Timing Collection

     The measurement of packet propagation round-trip time (RTT) among
     members of the group is required to support NACK suppression algo-
     rithms, timing of sender commands or certain repair functions, and
     congestion control operation.  The nature of the round-trip infor-
     mation collected is dependent upon the type of interaction among
     the members of the group.  In the case where only "one-to-many"
     transmission is required, it may be necessary that only the sender
     require RTT knowledge of the greatest RTT (GRTT) among the receiver
     set and/or RTT knowledge of only a portion of the group.  Here, the
     GRTT information might be collected in a reasonably scalable man-
     ner.  For congestion control operation, it is possible that RTT
     information may be required by each receiver in the group.  In this
     case, an alternative RTT collection scheme may be utilized where
     receivers collect individual RTT measurements with respect to the
     sender and advertise them (or an competed subset) to the group or
     sender.  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 which might be employed for
     increased efficiency [Ozdemir99].  And in some cases, there might
     be absolute time synchronization among hosts which may simplify RTT
     measurement.  There are trade-offs in multicast congestion control
     design which need further consideration before a universal recom-
     mendation on RTT (or GRTT) measurement can be specified.  Regard-
     less of how the RTT information is collected (and more specifically
     GRTT) with respect to congestion control or other requirements, the



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     sender will need to advertise its current GRTT estimate to the
     group for timing of the

3.7.1 One-to-Many Sender GRTT Measurement

     The goal of this form of RTT measurement is for the sender to learn
     the GRTT among the receivers who are actively participating in NORM
     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 recep-
     tion 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 instantion specification), it will con-
     struct a "response" using the formula:

         <grttResponse> = <sendTime> + (<currentTime> - <recv_Time>)

     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 cur-
     rent RTT measurement for the receiver from whom the response was
     received using the following formula:

               <receiverRtt> = <currentTime> - <grttResponse>

     During the each periodic GRTT probing interval, the source keeps
     the peak round trip estimate from the set of responses it has
     received.  The GRTT estimate should be filtered to be conservative
     towards maintaining an estimate biased towards the greatest
     receiver RTT measurements received.  A conservative estimate of
     GRTT maximizes the efficiency 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 calculation is greater
            than the  current GRTT estimate AND current peak, the
            response value is immediately fed into the GRTT  update fil-
            ter given below.  In any case, the source records the "peak"



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            receiver RTT measurement for the current probe interval.

       2)   At the end of the response collection period (i.e. the GRTT
            probe interval), if the recorded "peak" response is less
            than the current GRTT estimate AND this is the third consec-
            utive collection period with a peak less than the current
            GRTT estimate the recorded peak is fed into the GRTT update.
            (Implicitly, Rule #1 was applied otherwise so no new update
            is required).

       3)   At the end of the response collection period, the peak
            tracking value is set to either ZERO if the "peak" is
            greater than or equal to the current GRTT estimate (i.e.
            Already incorporated into the filter under Rule #1) or kept
            the same if its value is less than the current GRTT estimate
            AND was not yet incorporated into the GRTT update filter
            according to Rule #2. Thus for decreases in the source's
            estimate of GRTT, the "peak" is tracked across three consec-
            utive probe intervals.  The current MDP implementation uses
            the following GRTT update filter to incorporate new peak
            responses into the the GRTT estimate:

          if (peak > current_estimate)
              current_estimate = 0.25 * current_estimate + 0.75 * peak;
          else
              current_estimate = 0.75 * current_estimate + 0.25 * peak;

     This update method is biased towards maintaining an estimate of the
     worst-case round trip delay.  The reason the GRTT estimate is
     reduced only after 3 consecutive collection periods with smaller
     response peaks is to be conservative where packet loss may have
     resulted in lost response messages.  And then the reduction is
     additionally conservatively weighted using the averaging filter
     from above.

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

     In summary, although NORM repair cycle timeouts are based on GRTT,



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     it should be noted that convergent operation of the protocol does
     not _strictly_ depend on accurate GRTT estimation.  The current
     mechanism has proved sufficient in simulations and in the environ-
     ments in which NORM-like protocols have been deployed to date.  The
     estimate provided by the algorithm tracks the peak envelope of
     actual GRTT (including operating system effect as well as network
     delays) even in relaitvely high loss connectivity.  The steady-
     state probing/update interval may potentially be varied to accommo-
     date different levels of expected network dynamics in different
     environments.

3.7.2 One-to-Many Receiver RTT Measurement

     (TBD - Receivers "ping" sender for RTT measurement, and then
     receivers competitively (worst case RTT metric) advertise their
     measurements to the sender and optionally group so the sender can
     determine GRTT ... Sender should still robustly advertise its cur-
     rent GRTT knowledge to the group so group can use appropriate tim-
     ing)

3.7.3 Many-to-Many RTT Measurement

     (TBD - Describe approach based on Ozdemir99, if appropriate/appli-
     cable?)

3.7.4 Sender GRTT Advertisement

     To facilitate determistic NORM 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 (com-
     pressing) the GRTT estimate to a single byte of information.  The
     following C-lanquage function algorithm allows this to be done over
     a wide range of GRTT values while maintaining a greater range of
     precision for small GRTT values and less precision for large val-
     ues:

          unsigned char QuantizeGrtt(double grtt)
          {
              if (grtt > 1.0e03)
                  grtt = 1.0e03;
              else if (grtt < 1.0e-06)
                  grtt = 1.0e-06;
              if (grtt < 3.3e-05)



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                  return ((unsigned char)(grtt * 1.0e06) - 1);
              else
                  return ((unsigned char)(ceil(255.0.-
                                          (13.0 * log(1.0e03/grtt)))));
          }

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

3.8 Group Size Determination/Estimation

     (TBD)

3.9 Congestion Control Operation

     (TBD - A NACK-oriented protocol may place limitations/requirements
     on collection of information to facilitate congestion control of
     senders.  There are a number of specific issues of TCP-Friendly
     Multicast Congestion Control (TFMCC)which must be addressed.)

3.10 Router/Intermediate System assistance

     (TBD - NACK-oriented protocols can potentially benefit greatly from
     router assistance.  In particular, additional NACK suppression
     would be beneficial (This may impact how synchronized receiver NACK
     cycles are, sender advertisement of NACK-cycle parameters (i.e.
     GRTT, group size, etc), NACK content, others)

3.11 Additional protocol mechanisms

     (TBD- e.g.  positive acknowledgement collection, performance
     statistics collection, session management, etc)

4.0 Security Considerations (TBD)

5.0 References

     [Mankin98] A. Mankin, A. Romanow, S.  Bradner, V.  Paxson, "IETF
     Criteria for Evaluating Reliable Multicast Transport and Applica-
     tion Protocols", RFC 2357, June 1998.

     [Pingali93] S. Pingali, D. Towsley, J.  Kurose.  "A Comparison of
     Sender-Initiated and Receiver-Initiated Reliable Multicast Proto-
     cols".  In Proc.  INFOCOM, San Francisco, CA, October 1993.

     [Levine96] B.N. Levine, J.J. Garcia-Luna-Aceves.  "A Comparison of



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     Known Classes of Reliable Multicast Protocols", Proc.  Interna-
     tional Conference on Network Protocols (ICNP-96), Columbus, Ohio,
     Oct 29--Nov 1, 1996.

     [Clark90] D. Clark, D. Tennenhouse, "Architectural Considerations
     for a New Generation of Protocols".  In Proc.  ACM SIGCOMM, pages
     201--208, September 1990.

     [Deering89] S.  Deering.  "Host Extensions for IP Multicasting".
     Internet RFC1112, August 1989.

     [Floyd95] S. Floyd, V.  Jacobson, S.  McCanne, C.  Liu, and L.
     Zhang.  "A Reliable Multicast Framework for Light-weight Sessions
     and Application Level Framing", Proc.  ACM SIGCOMM, August 1995.

     [Nonnenmacher98] J.  Nonnenmacher and E.  W.  Biersack, "Optimal
     multicast feedback," in IEEE Infocom , (San Francisco, California),
     p.  964, March/April 1998.

     [Gossink98] D. Gossink, J.  Macker, "Reliable Multicast and Inte-
     grated Parity Retransmission with Channel Estimation", IEEE GLOBE-
     COM 98'.

     [Metzner84] J. Metzner, "An Improved Broadcast Retransmission Pro-
     tocol", IEEE Transactions on Communications, Vol.  Com-32, No.6,
     June 1984.

     [Macker97a] J. Macker, "Integrated Erasure-Based Coding for Reli-
     able Multicast Transmission", IRTF Meeting presentation, March
     1997.

     [Macker97b] J. Macker, "Reliable Multicast Transport and Integrated
     Erasure-based Forward Error Correction", Proc. IEEE MILCOM 97,
     October  1997.

     [Ozdemir99]  V. Ozdemir, S. Muthukrishnan, I. Rhee, "Scalable, Low-
     Overhead Network Delay Estimation", NCSU/AT&T White Paper, February
     1999.



6.0 Authors' Addresses

     Brian Adamson
     adamson@itd.nrl.navy.mil
     Newlink Global Engineering Corporation
     8580 Cinder Bed Road, Suite 1000
     Newington, VA, USA, 22122



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     Carsten Bormann
     cabo@tellique.de
     Tellique Kommunikationstechnik GmbH
     Gustav-Meyer-Allee 25 Geb ude 12
     D-13355 Berlin, Germany

     Sally Floyd
     floyd@aciri.org
     1947 Center Street, Suite 600
     Berkeley, CA 94704

     Mark Handley
     mjh@aciri.org
     1947 Center Street, Suite 600
     Berkeley, CA 94704

     Joe Macker
     macker@itd.nrl.navy.mil
     Naval Research Laboratory
     Washington, DC, USA, 20375































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