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rfc:rfc2729

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.

Bagnall, et al. Informational [Page 1] RFC 2729 Taxonomy of Communication Requirements December 1999

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).

Bagnall, et al. Informational [Page 2] RFC 2729 Taxonomy of Communication Requirements December 1999

 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

Bagnall, et al. Informational [Page 3] RFC 2729 Taxonomy of Communication Requirements December 1999

 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 ?

Bagnall, et al. Informational [Page 4] RFC 2729 Taxonomy of Communication Requirements December 1999

       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.

Bagnall, et al. Informational [Page 5] RFC 2729 Taxonomy of Communication Requirements December 1999

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

Bagnall, et al. Informational [Page 6] RFC 2729 Taxonomy of Communication Requirements December 1999

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.

Bagnall, et al. Informational [Page 7] RFC 2729 Taxonomy of Communication Requirements December 1999

    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

Bagnall, et al. Informational [Page 8] RFC 2729 Taxonomy of Communication Requirements December 1999

 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

Bagnall, et al. Informational [Page 9] RFC 2729 Taxonomy of Communication Requirements December 1999

    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

Bagnall, et al. Informational [Page 10] RFC 2729 Taxonomy of Communication Requirements December 1999

                           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.

Bagnall, et al. Informational [Page 11] RFC 2729 Taxonomy of Communication Requirements December 1999

 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

Bagnall, et al. Informational [Page 12] RFC 2729 Taxonomy of Communication Requirements December 1999

                           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

Bagnall, et al. Informational [Page 13] RFC 2729 Taxonomy of Communication Requirements December 1999

 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

Bagnall, et al. Informational [Page 14] RFC 2729 Taxonomy of Communication Requirements December 1999

 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

Bagnall, et al. Informational [Page 15] RFC 2729 Taxonomy of Communication Requirements December 1999

 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

Bagnall, et al. Informational [Page 16] RFC 2729 Taxonomy of Communication Requirements December 1999

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.

Bagnall, et al. Informational [Page 17] RFC 2729 Taxonomy of Communication Requirements December 1999

    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

Bagnall, et al. Informational [Page 18] RFC 2729 Taxonomy of Communication Requirements December 1999

 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.

Bagnall, et al. Informational [Page 19] RFC 2729 Taxonomy of Communication Requirements December 1999

    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.

Bagnall, et al. Informational [Page 20] RFC 2729 Taxonomy of Communication Requirements December 1999

    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

Bagnall, et al. Informational [Page 21] RFC 2729 Taxonomy of Communication Requirements December 1999

                           }
    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

Bagnall, et al. Informational [Page 22] RFC 2729 Taxonomy of Communication Requirements December 1999

    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).

Bagnall, et al. Informational [Page 23] RFC 2729 Taxonomy of Communication Requirements December 1999

    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

Bagnall, et al. Informational [Page 24] RFC 2729 Taxonomy of Communication Requirements December 1999

 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.

Bagnall, et al. Informational [Page 25] RFC 2729 Taxonomy of Communication Requirements December 1999

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/

Bagnall, et al. Informational [Page 26] RFC 2729 Taxonomy of Communication Requirements December 1999

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.

Bagnall, et al. Informational [Page 27]

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