    Network Working Group                                          P. Bagnall
    Request for Comments: 2729                                     R. Briscoe
    Category: Informational                                        A. Poppitt
                                                                           BT
                                                                December 1999

                     Taxonomy of Communication Requirements
                     for Large-scale Multicast Applications

    Status of this Memo

       This memo provides information for the Internet community.  It does
       not specify an Internet standard of any kind.  Distribution of this
       memo is unlimited.

    Copyright Notice

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

    Abstract

       The intention of this memo is to define a classification system for
       the communication requirements of any large-scale multicast
       application (LSMA). It is very unlikely one protocol can achieve a
       compromise between the diverse requirements of all the parties
       involved in any LSMA. It is therefore necessary to understand the
       worst-case scenarios in order to minimize the range of protocols
       needed. Dynamic protocol adaptation is likely to be necessary which
       will require logic to map particular combinations of requirements to
       particular mechanisms.  Standardizing the way that applications
       define their requirements is a necessary step towards this.
       Classification is a first step towards standardization.

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    Table of Contents

       1. Introduction . . . . . . . . . . . . . . . . . . . . . . 2
       2. Definitions of Sessions. . . . . . . . . . . . . . . . . 3
       3. Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . 4
         3.1. Summary of Communications Parameters . . . . . . . . 4
         3.2. Definitions, types and strictest requirements. . . . 5
           3.2.1. Types  . . . . . . . . . . . . . . . . . . . . . 6
           3.2.2. Reliability  . . . . . . . . . . . . . . . . . . 7
             3.2.2.1. Packet Loss  . . . . . . . . . . . . . . . . 7
             3.2.2.2. Component Reliability  . . . . . . . . . . . 8
           3.2.3. Ordering . . . . . . . . . . . . . . . . . . . . 9
           3.2.4. Timeliness . . . . . . . . . . . . . . . . . . . 9
           3.2.5. Session Control  . . . . . . . . . . . . . . . .13
           3.2.6. Session Topology . . . . . . . . . . . . . . . .16
           3.2.7. Directory  . . . . . . . . . . . . . . . . . . .17
           3.2.8. Security . . . . . . . . . . . . . . . . . . . .17
             3.2.8.1. Security Dynamics  . . . . . . . . . . . . .23
           3.2.9. Payment & Charging . . . . . . . . . . . . . . .24
       4. Security Considerations  . . . . . . . . . . . . . . . .25
       5. References   . . . . . . . . . . . . . . . . . . . . . .25
       6. Authors' Addresses . . . . . . . . . . . . . . . . . . .26
       7. Full Copyright Statement . . . . . . . . . . . . . . . .27

    1. Introduction

       This taxonomy consists of a large number of parameters that are
       considered useful for describing the communication requirements of
       LSMAs. To describe a particular application, each parameter would be
       assigned a value. Typical ranges of values are given wherever
       possible.  Failing this, the type of any possible values is given.
       The parameters are collected into ten or so higher level categories,
       but this is purely for convenience.

       The parameters are pitched at a level considered meaningful to
       application programmers. However, they describe communications not
       applications - the terms '3D virtual world', or 'shared TV' might
       imply communications requirements, but they don't accurately describe
       them.  Assumptions about the likely mechanism to achieve each
       requirement are avoided where possible.

       While the parameters describe communications, it will be noticed that
       few requirements concerning routing etc. are apparent. This is
       because applications have few direct requirements on these second
       order aspects of communications. Requirements in these areas will
       have to be inferred from application requirements (e.g. latency).

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       The taxonomy is likely to be useful in a number of ways:

       1. Most simply, it can be used as a checklist to create a
          requirements statement for a particular LSMA. Example applications
          will be classified [bagnall98] using the taxonomy in order to
          exercise (and improve) it

       2. Because strictest requirement have been defined for many
          parameters, it will be possible to identify worst case scenarios
          for the design of protocols

       3. Because the scope of each parameter has been defined (per session,
          per receiver etc.), it will be possible to highlight where
          heterogeneity is going to be most marked

       4. It is a step towards standardization of the way LSMAs define their
          communications requirements. This could lead to standard APIs
          between applications and protocol adaptation middleware

       5. Identification of limitations in current Internet technology for
          LSMAs to be added to the LSMA limitations memo [limitations]

       6. Identification of gaps in Internet Engineering Task Force (IETF)
          working group coverage

       This approach is intended to complement that used where application
       scenarios for Distributed Interactive Simulation (DIS) are proposed
       in order to generate network design metrics (values of communications
       parameters). Instead of creating the communications parameters from
       the applications, we try to imagine applications that might be
       enabled by stretching communications parameters.

    2. Definition of Sessions

       The following terms have no agreed definition, so they will be
       defined for this document.

       Session
          a happening or gathering consisting of flows of information
          related by a common description that persists for a non-trivial
          time (more than a few seconds) such that the participants (be they
          humans or applications) are involved and interested at
          intermediate times.  A session may be defined recursively as a
          super-set of other sessions.

       Secure session
          a session with restricted access

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       A session or secure session may be a sub and/or super set of a
       multicast group. A session can simultaneously be both a sub and a
       super-set of a multicast group by spanning a number of groups while
       time-sharing each group with other sessions.

    3. Taxonomy

    3.1 Summary of Communications Parameters

       Before the communications parameters are defined, typed and given
       worst-case values, they are simply listed for convenience. Also for
       convenience they are collected under classification headings.

          Reliability  . . . . . . . . . . . . . . . . . . . . . . 3.2.1
             Packet loss . . . . . . . . . . . . . . . . . . . . 3.2.1.1
                Transactional
                Guaranteed
                Tolerated loss
                Semantic loss
             Component reliability . . . . . . . . . . . . . . . 3.2.1.2
                Setup fail-over time
                Mean time between failures
                Fail over time during a stream
          Ordering . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2
             Ordering type
          Timeliness . . . . . . . . . . . . . . . . . . . . . . . 3.2.3
             Hard Realtime
             Synchronicity
             Burstiness
             Jitter
             Expiry
             Latency
             Optimum bandwidth
             Tolerable bandwidth
             Required by time and tolerance
             Host performance
             Fair delay
             Frame size
             Content size
          Session Control  . . . . . . . . . . . . . . . . . . . . 3.2.4
             Initiation
             Start time
             End time
             Duration
             Active time
             Session Burstiness
             Atomic join
             Late join allowed ?

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             Temporary leave allowed ?
             Late join with catch-up allowed ?
             Potential streams per session
             Active streams per sessions
          Session Topology . . . . . . . . . . . . . . . . . . . . 3.2.5
             Number of senders
             Number of receivers
          Directory  . . . . . . . . . . . . . . . . . . . . . . . 3.2.6
             Fail-over time-out (see Reliability: fail-over time)
             Mobility
          Security . . . . . . . . . . . . . . . . . . . . . . . . 3.2.7
             Authentication strength
             Tamper-proofing
             Non-repudiation strength
             Denial of service
             Action restriction
             Privacy
             Confidentiality
             Retransmit prevention strength
             Membership criteria
             Membership principals
             Collusion prevention
             Fairness
             Action on compromise
          Security dynamics  . . . . . . . . . . . . . . . . . . . 3.2.8
             Mean time between compromises
             Compromise detection time limit
             compromise recovery time limit
          Payment & Charging . . . . . . . . . . . . . . . . . . . 3.2.9
             Total Cost
             Cost per time
             Cost per Mb

    3.2 Definitions, types and strictest requirements

       The terms used in the above table are now defined for the context of
       this document. Under each definition, the type of their value is
       given and where possible worst-case values and example applications
       that would exhibit this requirement.

       There is no mention of whether a communication is a stream or a
       discrete interaction. An attempt to use this distinction as a way of
       characterizing communications proved to be remarkably unhelpful and
       was dropped.

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    3.2.1 Types

       Each requirement has a type. The following is a list of all the types
       used in the following definitions.

       Application Benchmark

          This is some measure of the processor load of an application, in
          some architecture neutral unit. This is non-trivial since the
          processing an application requires may change radically with
          different hardware, for example, a video client with and without
          hardware support.

       Bandwidth Measured in bits per second, or a multiple of.

       Boolean

       Abstract Currency
          An abstract currency is one which is adjusted to take inflation
          into account. The simplest way of doing this is to use the value
          of a real currency on a specific date. It is effectively a way of
          assessing the cost of something in "real terms". An example might
          be 1970 US$. Another measure might be "average man hours".

       Currency - current local

       Data Size

       Date (time since epoch)

       Enumeration

       Fraction

       Identifiers
          A label used to distinguish different parts of a communication

       Integer

       Membership list/rule

       Macro
          A small piece of executable code used to describe policies

       Time

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    3.2.2 Reliability

    3.2.2.1 Packet Loss

       Transactional

          When multiple operations must occur atomically, transactional
          communications guarantee that either all occur or none occur and a
          failure is flagged.

          Type:                  Boolean
          Meaning:               Transactional or Not transaction
          Strictest Requirement: Transactional
          Scope:                 per stream
          Example Application:   Bank credit transfer, debit and credit must
                                 be atomic.
          NB:                    Transactions are potentially much more
                                 complex, but it is believed this is
                                 an application layer problem.

       Guaranteed

          Guarantees communications will succeed under certain conditions.

          Type:                  Enumerated
          Meaning:               Deferrable - if communication fails it will
                                 be deferred until a time when it will be
                                 successful.
                                 Guaranteed - the communication will succeed
                                 so long as all necessary components are
                                 working.
                                 No guarantee - failure will not be
                                 reported.
          Strictest Requirement: Deferrable
          Example Application:   Stock quote feed - Guaranteed
          Scope:                 per stream
          NB:                    The application will need to set parameters
                                 to more fully define Guarantees, which the
                                 middleware may translate into, for example,
                                 queue lengths.

       Tolerated loss

          This specifies the proportion of data from a communication that
          can be lost before the application becomes completely unusable.

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          Type:                  Fraction
          Meaning:               fraction of the stream that can be lost
          Strictest Requirement: 0%
          Scope:                 per stream
          Example Application:   Video - 20%

       Semantic loss

          The application specifies how many and which parts of the
          communication can be discarded if necessary.

          Type:                  Identifiers, name disposable application
                                 level frames
          Meaning:               List of the identifiers of application
                                 frames which may be lost
          Strictest Requirement: No loss allowed
          Scope:                 per stream

          Example Application:   Video feed - P frames may be lost, I frames
                                 not

    3.2.2.2. Component Reliability

       Setup Fail-over time

          The time before a failure is detected and a replacement component
          is invoked. From the applications point of view this is the time
          it may take in exceptional circumstances for a channel to be set-
          up. It is not the "normal" operating delay before a channel is
          created.

          Type:                  Time
          Strictest Requirement: Web server - 1 second
          Scope:                 per stream
          Example Application:   Name lookup - 5 seconds

       Mean time between failures

          The mean time between two consecutive total failures of the
          channel.

          Type:                  Time
          Strictest Requirement: Indefinite
          Scope:                 per stream
          Example Application:   Telephony - 1000 hours

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       Fail over time during a stream

          The time between a stream breaking and a replacement being set up.

          Type:                  Time
          Strictest Requirement: Equal to latency requirement
          Scope:                 per stream
          Example Application:   File Transfer - 10sec

    3.2.3. Ordering

       Ordering type

          Specifies what ordering must be preserved for the application

          Type:                  {
                                   Enumeration timing,
                                   Enumeration sequencing,
                                   Enumeration causality
                                 }

          Meaning:               Timing - the events are timestamped
                                   Global
                                   Per Sender
                                   none
                                 Sequencing - the events are sequenced in
                                 order of occurrence
                                   Global
                                   Per Sender
                                   none
                                 Causality - the events form a graph
                                 relating cause and effect
                                   Global
                                   Per Sender
                                   none
          Strictest Requirement: Global, Global, Global
          Scope:                 per stream
          Example Application:   Game - { none, per sender, global } (to
                                 make sure being hit by bullet occurs
                                 after the shot is fired!)

    3.2.4. Timeliness

       Hard real- time

          There is a meta-requirement on timeliness. If hard real-time is
          required then the interpretation of all the other requirements
          changes.  Failures to achieve the required timeliness must be

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          reported before the communication is made. By contrast soft real-
          time means that there is no guarantee that an event will occur in
          time. However statistical measures can be used to indicate the
          probability of completion in the required time, and policies such
          as making sure the probability is 95% or better could be used.

          Type:                  Boolean
          Meaning:               Hard or Soft realtime
          Strictest Requirement: Hard
          Scope:                 per stream
          Example Application:   Medical monitor - Hard

       Synchronicity

          To make sure that separate elements of a session are correctly
          synchronized with respect to each other

          Type:                  Time
          Meaning:               The maximum time drift between streams
          Strictest Requirement: 80ms for human perception
          Scope:                 per stream pair/set
          Example Application:   TV lip-sync value 80ms
          NB:                    the scope is not necessarily the same as
                                 the session. Some streams may no need to be
                                 sync'd, (say, a score ticker in a football
                                 match

       Burstiness

          This is a measure of the variance of bandwidth requirements over
          time.

          Type:                  Fraction
          Meaning:               either:
                                   Variation in b/w as fraction of b/w for
                                   variable b/w communications
                                 or
                                   duty cycle (fraction of time at peak b/w)
                                   for intermittent b/w communications.
          Strictest Requirement: Variation = max b/w Duty cycle ~ 0
          Scope:                 per stream
          Example Application:   Sharing video clips, with chat channel -
                                 sudden bursts as clips are swapped.
                                 Compressed Audio - difference between
                                 silence and talking
          NB:                    More detailed analysis of communication
                                 flow (e.g. max rate of b/w change or

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                                 Fourier Transform of the b/w requirement) is
                                 possible but as complexity increases
                                 usefulness and computability decrease.

       Jitter

          Jitter is a measure of variance in the time taken for
          communications to traverse from the sender (application) to the
          receiver, as seen from the application layer.

          Type:                  Time
          Meaning:               Maximum permissible time variance
          Strictest Requirement: <1ms
          Scope:                 per stream
          Example Application:   audio streaming - <1ms
          NB:                    A jitter requirement implies that the
                                 communication is a real-time stream.  It
                                 makes relatively little sense for a file
                                 transfer for example.

       Expiry

                                 This specifies how long the information
                                 being transferred remains valid for.

          Type:                  Date
          Meaning:               Date at which data expires
          Strictest Requirement: For ever
          Scope:                 per stream
          Example Application:   key distribution - now+3600 seconds (valid
                                 for at least one hour)

       Latency

                                 Time between initiation and occurrence of
                                 an action from application perspective.

          Type:                  Time
          Strictest Requirement: Near zero for process control apps
          Scope:                 per stream
          Example Application:   Audio conference 20ms
          NB:                    Where an action consists of several
                                 distinct sequential parts the latency
                                 budget must be split over those parts. For
                                 process control the requirement may take
                                 any value.

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       Optimum Bandwidth

          Bandwidth required to complete communication in time

          Type:                  Bandwidth
          Strictest Requirement: No upper limit
          Scope:                 per stream
          Example Application:   Internet Phone 8kb/s

       Tolerable Bandwidth

          Minimum bandwidth that application can tolerate

          Type:                  Bandwidth
          Strictest Requirement: No upper limit
          Scope:                 per stream
          Example Application:   Internet phone 4kb/s

       Required by time and tolerance

          Time communication should complete by and time when failure to
          complete renders communication useless (therefore abort).

          Type:                  {
                                   Date - preferred complete time,
                                   Date - essential complete time
                                 }
          Strictest Requirement: Both now.
          Scope:                 per stream
          Example Application:   Email - Preferred 5 minutes & Essential in
                                 1 day
          NB:                    Bandwidth * Duration = Size; only two of
                                 these parameters may be specified. An API
                                 though could allow application authors to
                                 think in terms of any two.

       Host performance

          Ability of host to create/consume communication

          Type:                  Application benchmark
          Meaning:               Level of resources required by Application
          Strictest Requirement: Full consumption
          Scope:                 per stream
          Example Application:   Video - consume 15 frames a second
          NB:                    Host performance is complex since load,
                                 media type, media quality, h/w assistance,
                                 and encoding scheme all affect the

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                                 processing load. These are difficult to
                                 predict prior to a communication starting.
                                 To some extent these will need to be
                                 measured and modified as the communication
                                 proceeds.

       Frame size

          Size of logical data packets from application perspective

          Type:                  data size
          Strictest Requirement: 6 bytes (gaming)
          Scope:                 per stream
          Example Application:   video = data size of single frame update

       Content size

          The total size of the content (not relevant for continuous media)

          Type:                  data size
          Strictest Requirement: N/A
          Scope:                 per stream
          Example Application:   document transfer, 4kbytes

    3.2.5. Session Control

       Initiation

          Which initiation mechanism will be used.

          Type:                  Enumeration
          Meaning:               Announcement - session is publicly
                                     announced via a mass distribution
                                     system
                                 Invitation - specific participants are
                                     explicitly invited, e.g. my email
                                 Directive - specific participants are
                                     forced to join the session
          Strictest Requirement: Directive
          Scope:                 per stream
          Example Application:   Corporate s/w update - Directive

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       Start Time

          Time sender starts sending!

          Type:                  Date
          Strictest Requirement: Now
          Scope:                 per stream
          Example Application:   FTP - at 3am

       End Time

          Type:                  Date
          Strictest Requirement: Now
          Scope:                 per stream
          Example Application:   FTP - Now+30mins

       Duration

          (end time) - (start time) = (duration), therefore only two of
          three should be specified.

          Type:                  Time
          Strictest Requirement: - 0ms for discrete, indefinite for streams
          Scope:                 per stream
          Example Application:   audio feed - 60mins

       Active Time

          Total time session is active, not including breaks

          Type:                  Time
          Strictest Requirement: equals duration
          Scope:                 per stream
          Example Application:   Spectator sport transmission

       Session Burstiness

          Expected level of burstiness of the session

          Type:                  Fraction
          Meaning:               Variance as a fraction of maximum bandwidth
          Strictest Requirement: =bandwidth
          Scope:                 per stream
          Example Application:   commentary & slide show: 90% of max

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       Atomic join

          Session fails unless a certain proportion of the potential
          participants accept an invitation to join. Alternatively, may be
          specified as a specific numeric quorum.

          Type:                  Fraction (proportion required) or int
                                 (quorum)
          Strictest Requirement: 1.0 (proportion)
          Example Application:   price list update, committee meeting
          Scope:                 per stream or session
          NB:                    whether certain participants are essential
                                        is application dependent.

       Late join allowed ?

          Does joining a session after it starts make sense

          Type:                  Boolean
          Strictest Requirement: allowed
          Scope:                 per stream or session
          Example Application:   game - not allowed
          NB:                    An application may wish to define an
                                 alternate session if late join is not
                                 allowed

       Temporary leave allowed ?

          Does leaving and then coming back make sense for session

          Type:                  Boolean
          Strictest Requirement: allowed
          Scope:                 per stream or session
          Example Application:   FTP - not allowed

       Late join with catch-up allowed ?

          Is there a mechanism for a late joiner to see what they've missed

          Type:                  Boolean
          Strictest Requirement: allowed
          Scope:                 per stream or session
          Example Application:   sports event broadcast, allowed
          NB:                    An application may wish to define an
                                 alternate session if late join is not
                                 allowed

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       Potential streams per session

          Total number of streams that are part of session, whether being
          consumed or not

          Type:                  Integer
          Strictest Requirement: No upper limit
          Scope:                 per session
          Example Application:   football match mcast - multiple camera's,
                                 commentary, 15 streams

       Active streams per sessions  (i.e. max app can handle)

          Maximum number of streams that an application can consume
          simultaneously

          Type:                  Integer
          Strictest Requirement: No upper limit
          Scope:                 per session
          Example Application:   football match mcast - 6, one main video,
                                 four user selected, one audio commentary

    3.2.6. Session Topology

       Note: topology may be dynamic. One of the challenges in designing
       adaptive protocol frameworks is to predict the topology before the
       first join.

       Number of senders

          The number of senders is a result the middleware may pass up to
          the application

          Type:                  Integer
          Strictest Requirement: No upper limit
          Scope:                 per stream
          Example Application:   network MUD - 100

       Number of receivers

          The number of receivers is a results the middleware may pass up to
          the application

          Type:                  Integer
          Strictest Requirement: No upper limit
          Scope:                 per stream
          Example Application:   video mcast - 100,000

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    3.2.7. Directory

       Fail-over timeout (see Reliability: fail-over time)

       Mobility

          Defines restrictions on when directory entries may be changed

          Type:                  Enumeration
          Meaning:               while entry is in use
                                 while entry in unused
                                 never
          Strictest Requirement: while entry is in use
          Scope:                 per stream
          Example Application:   voice over mobile phone, while entry is in
                                 use (as phone gets new address when
                                 changing cell).

    3.2.8. Security

       The strength of any security arrangement can be stated as the
       expected cost of mounting a successful attack. This allows mechanisms
       such as physical isolation to be considered alongside encryption
       mechanisms.  The cost is measured in an abstract currency, such as
       1970 UD$ (to inflation proof).

       Security is an orthogonal requirement. Many requirements can have a
       security requirement on them which mandates that the cost of causing
       the system to fail to meet that requirement is more than the
       specified amount. In terms of impact on other requirements though,
       security does potentially have a large impact so when a system is
       trying to determine which mechanisms to use and whether the
       requirements can be met security will clearly be a major influence.

       Authentication Strength

          Authentication aims to ensure that a principal is who they claim
          to be.  For each role in a communication, (e.g. sender, receiver)
          there is a strength for the authentication of the principle who
          has taken on that role. The principal could be a person,
          organization or other legal entity. It could not be a process
          since a process has no legal representation.

          Type:                  Abstract Currency
          Meaning:               That the cost of hijacking a role is in
                                 excess of the specified amount. Each role
                                 is a different requirement.

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          Strictest Requirement: budget of largest attacker
          Scope:                 per stream
          Example Application:   inter-governmental conference

       Tamper-proofing

          This allows the application to specify how much security will be
          applied to ensuring that a communication is not tampered with.
          This is specified as the minimum cost of successfully tampering
          with the communication. Each non-security requirement has a
          tamper-proofing requirement attached to it.

          Requirement: The cost of tampering with the communication is in
          excess of the specified amount.

          Type:                  {
                                   Abstract Currency,
                                   Abstract Currency,
                                   Abstract Currency
                                 }
          Meaning:               cost to alter or destroy data,
                                 cost to replay data (successfully),
                                 cost to interfere with timeliness.
          Scope:                 per stream
          Strictest Requirement: Each budget of largest attacker
          Example Application:   stock price feed

       Non-repudiation strength

          The non-repudiation strength defines how much care is taken to
          make sure there is a reliable audit trail on all interactions. It
          is measured as the cost of faking an audit trail, and therefore
          being able to "prove" an untrue event. There are a number of
          possible parameters of the event that need to be proved. The
          following list is not exclusive but shows the typical set of
          requirements.

          1. Time 2. Ordering (when relative to other events) 3. Whom 4.
          What (the event itself)

          There are a number of events that need to be provable.  1. sender
          proved sent 2. receiver proves received 3. sender proves received.

          Type:                  Abstract Currency
          Meaning:               minimum cost of faking or denying an event
          Strictest Requirement:  Budget of largest attacker
          Scope:                 per stream
          Example Application:   Online shopping system

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       Denial of service

          There may be a requirement for some systems (999,911,112 emergency
          services access for example) that denial of service attacks cannot
          be launched. While this is difficult (maybe impossible) in many
          systems at the moment it is still a requirement, just one that
          can't be met.

          Type:                  Abstract Currency
          Meaning:               Cost of launching a denial of service
                                 attack is greater than specified amount.
          Strictest Requirement: budget of largest attacker
          Scope:                 per stream
          Example Application:   web hosting, to prevent individual hackers
                                 stalling system.

       Action restriction

          For any given communication there are a two actions, send and
          receive.  Operations like adding to members to a group are done as
          a send to the membership list. Examining the list is a request to
          and receive from the list. Other actions can be generalized to
          send and receive on some communication, or are application level
          not comms level issues.

          Type:                  Membership list/rule for each action.
          Meaning:               predicate for determining permission for
                                 role
          Strictest Requirement: Send and receive have different policies.
          Scope:                 per stream
          Example Application:   TV broadcast, sender policy defines
                                 transmitter, receiver policy is null.
          NB:                    Several actions may share the same
                                 membership policy.

       Privacy

          Privacy defines how well obscured a principals identity is. This
          could be for any interaction. A list of participants may be
          obscured, a sender may obscure their identity when they send.
          There are also different types of privacy. For example knowing two
          messages were sent by the same person breaks the strongest type of
          privacy even if the identity of that sender is still unknown. For
          each "level" of privacy there is a cost associated with violating
          it. The requirement is that this cost is excessive for the
          attacker.

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          Type:                  {
                                   Abstract Currency,
                                   Abstract Currency,
                                   Abstract Currency,
                                   Abstract Currency
                                 }
          Meaning:               Level of privacy, expected cost to violate
                                 privacy level for:-
                                 openly identified - this is the unprotected
                                     case
                                 anonymously identified  - (messages from
                                     the same sender can be linked)
                                 unadvertised (but traceable) - meaning that
                                     traffic can be detected and traced to
                                     it's source or destination, this is a
                                     breach if the very fact that two
                                     specific principals are communicating
                                     is sensitive.
                                 undetectable
          Strictest Requirement: All levels budget of attacker
          Scope:                 per stream
          Example Application:   Secret ballot voting system
                                 openly identified - budget of any
                                     interested party
                                 anonymously identified - zero
                                 unadvertised - zero
                                 undetectable - zero

       Confidentiality

          Confidentiality defines how well protected the content of a
          communication is from snooping.

          Type:                  Abstract Currency
          Meaning:               Level of Confidentiality, the cost of
                                 gaining illicit access to the content of a
                                 stream
          Strictest Requirement:  budget of attacker
          Scope:                 per stream
          Example Application:   Secure email -  value of transmitted
                                 information

       Retransmit prevention strength

          This is extremely hard at the moment. This is not to say it's not
          a requirement.

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          Type:                  Abstract Currency
          Meaning:               The cost of retransmitting a secure piece
                                 of information should exceed the specified
                                 amount.
          Strictest Requirement: Cost of retransmitting  value of
                                 information
          Scope:                 per stream

       Membership Criteria

          If a principal attempts to participate in a communication then a
          check will be made to see if it is allowed to do so. The
          requirement is that certain principals will be allowed, and others
          excluded. Given the application is being protected from network
          details there are only two types of specification available, per
          user, and per organization (where an organization may contain
          other organizations, and each user may be a member of multiple
          organizations). Rules could however be built on properties of a
          user, for example does the user own a key? Host properties could
          also be used, so users on slow hosts or hosts running the wrong OS
          could be excluded.

          Type:                  Macros
          Meaning:               Include or exclude
                                    users (list)
                                    organizations (list)
                                    hosts (list)
                                    user properties (rule)
                                    org properties (rule)
                                    hosts properties (rule)
          Strictest Requirement: List of individual users
          Scope:                 per stream
          Example Application:   Corporate video-conference - organization
                                 membership

       Collusion prevention

          Which aspects of collusion it is required to prevent. Collusion is
          defined as malicious co-operation between members of a secure
          session.  Superficially, it would appear that collusion is not a
          relevant threat in a multicast, because everyone has the same
          information, however, wherever there is differentiation, it can be
          exploited.

          Type:                  {
                                   Abstract Currency,
                                   Abstract Currency,
                                   Abstract Currency

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                                 }
          Meaning:               time race collusion - cost of colluding
                                 key encryption key (KEK) sharing - cost of
                                 colluding
                                 sharing of differential QoS (not strictly
                                 collusion as across sessions not within
                                 one) - cost of colluding
          Strictest Requirement: For all threats cost attackers
                                 combined resources
          Scope:                 per stream
          Example Application:   A race where delay of the start signal may
                                 be allowed for, but one participant may
                                 fake packet delay while receiving the start
                                 signal from another participant.
          NB:                    Time race collusion is the most difficult
                                 one to prevent. Also note that while these
                                 may be requirements for some systems this
                                 does not mean there are necessarily
                                 solutions. Setting tough requirements may
                                 result in the middleware being unable to
                                 create a valid channel.

       Fairness

          Fairness is a meta-requirement of many other requirements. Of
          particular interest are Reliability and Timeliness requirements.
          When a communication is first created the creator may wish to
          specify a set of requirements for these parameters. Principals
          which join later may wish to set tighter limits. Fairness enforces
          a policy that any improvement is requirement by one principal must
          be matched by all others, in effect requirements can only be set
          for the whole group. This increases the likelihood that
          requirements of this kind will fail to be met. If fairness if not
          an issue then some parts of the network can use more friendly
          methods to achieve those simpler requirements.

          Type:                  Level of variance of the requirement that
                                 needs to be fair. For example, if the
                                 latency requirement states within 2
                                 seconds, the level of fairness required may
                                 be that variations in latency are not more
                                 than 0.1s. This has in fact become an issue
                                 in online gaming (e.g. Quake)
          Meaning:               The variance of performance with respect to
                                 any other requirement is less than the
                                 specified amount.
          Scope:                 per stream, per requirement

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          Example Application:   Networked game, latency to receive
                                 positions of players must be within 5ms for
                                 all players.

       Action on compromise

          The action to take on detection of compromise (until security
          reassured).

          Type:                  Enumeration
          Meaning:               warn but continue
                                 pause
                                 abort
          Scope:                 Per stream
          Strictest Requirement: pause
          Example Application:   Secure video conference - if intruder
                                 alert, everyone is warned, but they can
                                 continue while knowing not to discuss
                                 sensitive matters (cf. catering staff
                                 during a meeting).

    3.2.8.1. Security Dynamics

          Security dynamics are the temporal properties of the security
          mechanisms that are deployed. They may affect other requirements
          such as latency or simply be a reflection of the security
          limitations of the system. The requirements are often concerned
          with abnormal circumstances (e.g. system violation).

       Mean time between compromises

          This is not the same as the strength of a system. A fairly weak
          system may have a very long time between compromises because it is
          not worth breaking in to, or it is only worth it for very few
          people. Mean time between compromises is a combination of
          strength, incentive and scale.

          Type:                  Time
          Scope:                 Per stream
          Strictest Requirement: indefinite
          Example Application:   Secure Shell - 1500hrs

       Compromise detection time limit

          The average time it must take to detect a compromise (one
          predicted in the design of the detection system, that is).

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          Type:                  Time
          Scope:                 Per stream
          Strictest Requirement: Round trip time
          Example Application:   Secure Shell - 2secs

       Compromise recovery time limit

          The maximum time it must take to re-seal the security after a
          breach.  This combined with the compromise detection time limit
          defines how long the system must remain inactive to avoid more
          security breaches. For example if a compromise is detected in one
          minute, and recovery takes five, then one minute of traffic is now
          insecure and the members of the communication must remain silent
          for four minutes after detection while security is re-established.

          Type:                  Time
          Scope:                 Per stream
          Strictest Requirement: 1 second
          Example Application:   Audio conference - 10 seconds

    3.2.9. Payment & Charging

       Total Cost

          The total cost of communication must be limited to this amount.
          This would be useful for transfer as opposed to stream type
          applications.

          Type:                  Currency
          Meaning:               Maximum charge allowed
          Scope:                 Per user per stream
          Strictest Requirement: Free
          Example Application:   File Transfer: comms cost must be < 1p/Mb

       Cost per Time

                                 This is the cost per unit time. Some
                                 applications may not be able to predict the
                                 duration of a communication. It may be more
                                 meaningful for those to be able to specify
                                 price per time instead.
          Type:                  Currency per timeS

          Scope:                 Per user per stream
          Strictest Requirement: Free
          Example Application:   Video Conference - 15p / minute

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       Cost per Mb

          This is the cost per unit of data. Some communications may be
          charged by the amount of data transferred. Some applications may
          prefer to specify requirements in this way.

          Type:                  Currency per data size
          Scope:                 Per user per stream
          Strictest Requirement: Free
          Example Application:   Email advertising - 15p / Mb

    4. Security Considerations

       See comprehensive security section of taxonomy.

    5. References

       [Bagnall98]   Bagnall Peter, Poppitt Alan, Example LSMA
                     classifications, BT Tech report,
                     <URL:http://www.labs.bt.com/projects/mware/>

       [limitations] Pullen, M., Myjak, M. and C. Bouwens, "Limitations of
                     Internet Protocol Suite for Distributed Simulation in
                     the Large Multicast Environment", RFC 2502, February
                     1999.

       [rmodp]       Open Distributed Processing Reference Model (RM-ODP),
                     ISO/IEC 10746-1 to 10746-4 or ITU-T (formerly CCITT)
                     X.901 to X.904. Jan 1995.

       [blaze95]     Blaze, Diffie, Rivest, Schneier, Shimomura, Thompson
                     and Wiener, Minimal Key Lengths for Symmetric Ciphers
                     to Provide Adequate Commercial Security, January 1996.

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    6. Authors' Addresses

       Peter Bagnall
       c/o B54/77 BT Labs
       Martlesham Heath
       Ipswich, IP5 3RE
       England

       EMail: pete@surfaceeffect.com
       Home page: http://www.surfaceeffect.com/people/pete/

       Bob Briscoe
       B54/74 BT Labs
       Martlesham Heath
       Ipswich, IP5 3RE
       England

       Phone: +44 1473 645196
       Fax:   +44 1473 640929
       EMail: bob.briscoe@bt.com
       Home page: http://www.labs.bt.com/people/briscorj/

       Alan Poppitt
       B54/77 BT Labs
       Martlesham Heath
       Ipswich, IP5 3RE
       England

       Phone: +44 1473 640889
       Fax:   +44 1473 640929
       EMail: apoppitt@jungle.bt.co.uk
       Home page: http://www.labs.bt.com/people/poppitag/

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    7.  Full Copyright Statement

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

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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