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RFC 2729 - Taxonomy of Communication Requirements for Large-scal

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


   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.

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 Packet Loss  . . . . . . . . . . . . . . . . 7 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 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).

   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.

      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

   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 . . . . . . . . . . . . . . . . . . . .
            Tolerated loss
            Semantic loss
         Component reliability . . . . . . . . . . . . . . .
            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
         Optimum bandwidth
         Tolerable bandwidth
         Required by time and tolerance
         Host performance
         Fair delay
         Frame size
         Content size
      Session Control  . . . . . . . . . . . . . . . . . . . . 3.2.4
         Start time
         End time
         Active time
         Session Burstiness
         Atomic join
         Late join allowed ?

         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)
      Security . . . . . . . . . . . . . . . . . . . . . . . . 3.2.7
         Authentication strength
         Non-repudiation strength
         Denial of service
         Action restriction
         Retransmit prevention strength
         Membership criteria
         Membership principals
         Collusion prevention
         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.

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.


   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)



      A label used to distinguish different parts of a communication


   Membership list/rule

      A small piece of executable code used to describe policies


3.2.2 Reliability Packet Loss


      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.


      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
                             Guaranteed - the communication will succeed
                             so long as all necessary components are
                             No guarantee - failure will not be
      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.

      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 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

      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

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

   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
                               Per Sender
                             Sequencing - the events are sequenced in
                             order of occurrence
                               Per Sender
                             Causality - the events form a graph
                             relating cause and effect
                               Per Sender
      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

      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


      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


      This is a measure of the variance of bandwidth requirements over

      Type:                  Fraction
      Meaning:               either:
                               Variation in b/w as fraction of b/w for
                               variable b/w communications
                               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

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


      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.


                             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)


                             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.

   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

                             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

   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


      Which initiation mechanism will be used.

      Type:                  Enumeration
      Meaning:               Announcement - session is publicly
                                 announced via a mass distribution
                             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

   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


      (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

   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
      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

   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

   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

      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

3.2.7. Directory

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


      Defines restrictions on when directory entries may be changed

      Type:                  Enumeration
      Meaning:               while entry is in use
                             while entry in unused
      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.

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


      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

      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

   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
      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 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

      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
                             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.
      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 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
      Strictest Requirement:  budget of attacker
      Scope:                 per stream
      Example Application:   Secure email -  value of transmitted

   Retransmit prevention strength

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

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

   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

      Type:                  {
                               Abstract Currency,
                               Abstract Currency,
                               Abstract Currency

      Meaning:               time race collusion - cost of colluding
                             key encryption key (KEK) sharing - cost of
                             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 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

      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

      Type:                  Enumeration
      Meaning:               warn but continue
      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). 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).

      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

      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

   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,

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

   [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.

6. Authors' Addresses

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

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

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

   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

   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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