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RFC 2729

Taxonomy of Communication Requirements for Large-scale Multicast Applications

Pages: 27
Informational

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


                 Taxonomy of Communication Requirements
                 for Large-scale Multicast Applications

Status of this Memo

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

Copyright Notice

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

Abstract

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

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

1. Introduction

This taxonomy consists of a large number of parameters that are considered useful for describing the communication requirements of LSMAs. To describe a particular application, each parameter would be assigned a value. Typical ranges of values are given wherever possible. Failing this, the type of any possible values is given. The parameters are collected into ten or so higher level categories, but this is purely for convenience. The parameters are pitched at a level considered meaningful to application programmers. However, they describe communications not applications - the terms '3D virtual world', or 'shared TV' might imply communications requirements, but they don't accurately describe them. Assumptions about the likely mechanism to achieve each requirement are avoided where possible. While the parameters describe communications, it will be noticed that few requirements concerning routing etc. are apparent. This is because applications have few direct requirements on these second order aspects of communications. Requirements in these areas will have to be inferred from application requirements (e.g. latency).
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   The taxonomy is likely to be useful in a number of ways:

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

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

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

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

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

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

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

2. Definition of Sessions

The following terms have no agreed definition, so they will be defined for this document. Session a happening or gathering consisting of flows of information related by a common description that persists for a non-trivial time (more than a few seconds) such that the participants (be they humans or applications) are involved and interested at intermediate times. A session may be defined recursively as a super-set of other sessions. Secure session a session with restricted access
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   A session or secure session may be a sub and/or super set of a
   multicast group. A session can simultaneously be both a sub and a
   super-set of a multicast group by spanning a number of groups while
   time-sharing each group with other sessions.

3. Taxonomy

3.1 Summary of Communications Parameters

Before the communications parameters are defined, typed and given worst-case values, they are simply listed for convenience. Also for convenience they are collected under classification headings. Reliability . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Packet loss . . . . . . . . . . . . . . . . . . . . 3.2.1.1 Transactional Guaranteed Tolerated loss Semantic loss Component reliability . . . . . . . . . . . . . . . 3.2.1.2 Setup fail-over time Mean time between failures Fail over time during a stream Ordering . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Ordering type Timeliness . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Hard Realtime Synchronicity Burstiness Jitter Expiry Latency Optimum bandwidth Tolerable bandwidth Required by time and tolerance Host performance Fair delay Frame size Content size Session Control . . . . . . . . . . . . . . . . . . . . 3.2.4 Initiation Start time End time Duration Active time Session Burstiness Atomic join Late join allowed ?
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         Temporary leave allowed ?
         Late join with catch-up allowed ?
         Potential streams per session
         Active streams per sessions
      Session Topology . . . . . . . . . . . . . . . . . . . . 3.2.5
         Number of senders
         Number of receivers
      Directory  . . . . . . . . . . . . . . . . . . . . . . . 3.2.6
         Fail-over time-out (see Reliability: fail-over time)
         Mobility
      Security . . . . . . . . . . . . . . . . . . . . . . . . 3.2.7
         Authentication strength
         Tamper-proofing
         Non-repudiation strength
         Denial of service
         Action restriction
         Privacy
         Confidentiality
         Retransmit prevention strength
         Membership criteria
         Membership principals
         Collusion prevention
         Fairness
         Action on compromise
      Security dynamics  . . . . . . . . . . . . . . . . . . . 3.2.8
         Mean time between compromises
         Compromise detection time limit
         compromise recovery time limit
      Payment & Charging . . . . . . . . . . . . . . . . . . . 3.2.9
         Total Cost
         Cost per time
         Cost per Mb

3.2 Definitions, types and strictest requirements

The terms used in the above table are now defined for the context of this document. Under each definition, the type of their value is given and where possible worst-case values and example applications that would exhibit this requirement. There is no mention of whether a communication is a stream or a discrete interaction. An attempt to use this distinction as a way of characterizing communications proved to be remarkably unhelpful and was dropped.
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3.2.1 Types

Each requirement has a type. The following is a list of all the types used in the following definitions. Application Benchmark This is some measure of the processor load of an application, in some architecture neutral unit. This is non-trivial since the processing an application requires may change radically with different hardware, for example, a video client with and without hardware support. Bandwidth Measured in bits per second, or a multiple of. Boolean Abstract Currency An abstract currency is one which is adjusted to take inflation into account. The simplest way of doing this is to use the value of a real currency on a specific date. It is effectively a way of assessing the cost of something in "real terms". An example might be 1970 US$. Another measure might be "average man hours". Currency - current local Data Size Date (time since epoch) Enumeration Fraction Identifiers A label used to distinguish different parts of a communication Integer Membership list/rule Macro A small piece of executable code used to describe policies Time
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3.2.2 Reliability

3.2.2.1 Packet Loss
Transactional When multiple operations must occur atomically, transactional communications guarantee that either all occur or none occur and a failure is flagged. Type: Boolean Meaning: Transactional or Not transaction Strictest Requirement: Transactional Scope: per stream Example Application: Bank credit transfer, debit and credit must be atomic. NB: Transactions are potentially much more complex, but it is believed this is an application layer problem. Guaranteed Guarantees communications will succeed under certain conditions. Type: Enumerated Meaning: Deferrable - if communication fails it will be deferred until a time when it will be successful. Guaranteed - the communication will succeed so long as all necessary components are working. No guarantee - failure will not be reported. Strictest Requirement: Deferrable Example Application: Stock quote feed - Guaranteed Scope: per stream NB: The application will need to set parameters to more fully define Guarantees, which the middleware may translate into, for example, queue lengths. Tolerated loss This specifies the proportion of data from a communication that can be lost before the application becomes completely unusable.
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      Type:                  Fraction
      Meaning:               fraction of the stream that can be lost
      Strictest Requirement: 0%
      Scope:                 per stream
      Example Application:   Video - 20%

   Semantic loss

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

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

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

3.2.2.2. Component Reliability
Setup Fail-over time The time before a failure is detected and a replacement component is invoked. From the applications point of view this is the time it may take in exceptional circumstances for a channel to be set- up. It is not the "normal" operating delay before a channel is created. Type: Time Strictest Requirement: Web server - 1 second Scope: per stream Example Application: Name lookup - 5 seconds Mean time between failures The mean time between two consecutive total failures of the channel. Type: Time Strictest Requirement: Indefinite Scope: per stream Example Application: Telephony - 1000 hours
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   Fail over time during a stream

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

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

3.2.3. Ordering

Ordering type Specifies what ordering must be preserved for the application Type: { Enumeration timing, Enumeration sequencing, Enumeration causality } Meaning: Timing - the events are timestamped Global Per Sender none Sequencing - the events are sequenced in order of occurrence Global Per Sender none Causality - the events form a graph relating cause and effect Global Per Sender none Strictest Requirement: Global, Global, Global Scope: per stream Example Application: Game - { none, per sender, global } (to make sure being hit by bullet occurs after the shot is fired!)

3.2.4. Timeliness

Hard real- time There is a meta-requirement on timeliness. If hard real-time is required then the interpretation of all the other requirements changes. Failures to achieve the required timeliness must be
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      reported before the communication is made. By contrast soft real-
      time means that there is no guarantee that an event will occur in
      time. However statistical measures can be used to indicate the
      probability of completion in the required time, and policies such
      as making sure the probability is 95% or better could be used.

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

   Synchronicity

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

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

   Burstiness

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

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

   Jitter

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

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

   Expiry

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

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

   Latency

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

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

      Bandwidth required to complete communication in time

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

   Tolerable Bandwidth

      Minimum bandwidth that application can tolerate

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

   Required by time and tolerance

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

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

   Host performance

      Ability of host to create/consume communication

      Type:                  Application benchmark
      Meaning:               Level of resources required by Application
      Strictest Requirement: Full consumption
      Scope:                 per stream
      Example Application:   Video - consume 15 frames a second
      NB:                    Host performance is complex since load,
                             media type, media quality, h/w assistance,
                             and encoding scheme all affect the
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                             processing load. These are difficult to
                             predict prior to a communication starting.
                             To some extent these will need to be
                             measured and modified as the communication
                             proceeds.

   Frame size

      Size of logical data packets from application perspective

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

   Content size

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

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

3.2.5. Session Control

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

      Time sender starts sending!

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

   End Time

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

   Duration

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

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

   Active Time

      Total time session is active, not including breaks

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

   Session Burstiness

      Expected level of burstiness of the session

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

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

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

   Late join allowed ?

      Does joining a session after it starts make sense

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

   Temporary leave allowed ?

      Does leaving and then coming back make sense for session

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

   Late join with catch-up allowed ?

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

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

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

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

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

      Maximum number of streams that an application can consume
      simultaneously

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

3.2.6. Session Topology

Note: topology may be dynamic. One of the challenges in designing adaptive protocol frameworks is to predict the topology before the first join. Number of senders The number of senders is a result the middleware may pass up to the application Type: Integer Strictest Requirement: No upper limit Scope: per stream Example Application: network MUD - 100 Number of receivers The number of receivers is a results the middleware may pass up to the application Type: Integer Strictest Requirement: No upper limit Scope: per stream Example Application: video mcast - 100,000
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3.2.7. Directory

Fail-over timeout (see Reliability: fail-over time) Mobility Defines restrictions on when directory entries may be changed Type: Enumeration Meaning: while entry is in use while entry in unused never Strictest Requirement: while entry is in use Scope: per stream Example Application: voice over mobile phone, while entry is in use (as phone gets new address when changing cell).

3.2.8. Security

The strength of any security arrangement can be stated as the expected cost of mounting a successful attack. This allows mechanisms such as physical isolation to be considered alongside encryption mechanisms. The cost is measured in an abstract currency, such as 1970 UD$ (to inflation proof). Security is an orthogonal requirement. Many requirements can have a security requirement on them which mandates that the cost of causing the system to fail to meet that requirement is more than the specified amount. In terms of impact on other requirements though, security does potentially have a large impact so when a system is trying to determine which mechanisms to use and whether the requirements can be met security will clearly be a major influence. Authentication Strength Authentication aims to ensure that a principal is who they claim to be. For each role in a communication, (e.g. sender, receiver) there is a strength for the authentication of the principle who has taken on that role. The principal could be a person, organization or other legal entity. It could not be a process since a process has no legal representation. Type: Abstract Currency Meaning: That the cost of hijacking a role is in excess of the specified amount. Each role is a different requirement.
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      Strictest Requirement: budget of largest attacker
      Scope:                 per stream
      Example Application:   inter-governmental conference

   Tamper-proofing

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

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

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

   Non-repudiation strength

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

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

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

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

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

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

   Action restriction

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

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

   Privacy

      Privacy defines how well obscured a principals identity is. This
      could be for any interaction. A list of participants may be
      obscured, a sender may obscure their identity when they send.
      There are also different types of privacy. For example knowing two
      messages were sent by the same person breaks the strongest type of
      privacy even if the identity of that sender is still unknown. For
      each "level" of privacy there is a cost associated with violating
      it. The requirement is that this cost is excessive for the
      attacker.
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      Type:                  {
                               Abstract Currency,
                               Abstract Currency,
                               Abstract Currency,
                               Abstract Currency
                             }
      Meaning:               Level of privacy, expected cost to violate
                             privacy level for:-
                             openly identified - this is the unprotected
                                 case
                             anonymously identified  - (messages from
                                 the same sender can be linked)
                             unadvertised (but traceable) - meaning that
                                 traffic can be detected and traced to
                                 it's source or destination, this is a
                                 breach if the very fact that two
                                 specific principals are communicating
                                 is sensitive.
                             undetectable
      Strictest Requirement: All levels budget of attacker
      Scope:                 per stream
      Example Application:   Secret ballot voting system
                             openly identified - budget of any
                                 interested party
                             anonymously identified - zero
                             unadvertised - zero
                             undetectable - zero

   Confidentiality

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

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

   Retransmit prevention strength

      This is extremely hard at the moment. This is not to say it's not
      a requirement.
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      Type:                  Abstract Currency
      Meaning:               The cost of retransmitting a secure piece
                             of information should exceed the specified
                             amount.
      Strictest Requirement: Cost of retransmitting  value of
                             information
      Scope:                 per stream

   Membership Criteria

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

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

   Collusion prevention

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

      Type:                  {
                               Abstract Currency,
                               Abstract Currency,
                               Abstract Currency
Top   ToC   RFC2729 - Page 22
                             }
      Meaning:               time race collusion - cost of colluding
                             key encryption key (KEK) sharing - cost of
                             colluding
                             sharing of differential QoS (not strictly
                             collusion as across sessions not within
                             one) - cost of colluding
      Strictest Requirement: For all threats cost attackers
                             combined resources
      Scope:                 per stream
      Example Application:   A race where delay of the start signal may
                             be allowed for, but one participant may
                             fake packet delay while receiving the start
                             signal from another participant.
      NB:                    Time race collusion is the most difficult
                             one to prevent. Also note that while these
                             may be requirements for some systems this
                             does not mean there are necessarily
                             solutions. Setting tough requirements may
                             result in the middleware being unable to
                             create a valid channel.

   Fairness

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

      Type:                  Level of variance of the requirement that
                             needs to be fair. For example, if the
                             latency requirement states within 2
                             seconds, the level of fairness required may
                             be that variations in latency are not more
                             than 0.1s. This has in fact become an issue
                             in online gaming (e.g. Quake)
      Meaning:               The variance of performance with respect to
                             any other requirement is less than the
                             specified amount.
      Scope:                 per stream, per requirement
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      Example Application:   Networked game, latency to receive
                             positions of players must be within 5ms for
                             all players.

   Action on compromise

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

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

3.2.8.1. Security Dynamics
Security dynamics are the temporal properties of the security mechanisms that are deployed. They may affect other requirements such as latency or simply be a reflection of the security limitations of the system. The requirements are often concerned with abnormal circumstances (e.g. system violation). Mean time between compromises This is not the same as the strength of a system. A fairly weak system may have a very long time between compromises because it is not worth breaking in to, or it is only worth it for very few people. Mean time between compromises is a combination of strength, incentive and scale. Type: Time Scope: Per stream Strictest Requirement: indefinite Example Application: Secure Shell - 1500hrs Compromise detection time limit The average time it must take to detect a compromise (one predicted in the design of the detection system, that is).
Top   ToC   RFC2729 - Page 24
      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
Top   ToC   RFC2729 - Page 25
   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.
Top   ToC   RFC2729 - Page 26

6. Authors' Addresses

Peter Bagnall c/o B54/77 BT Labs Martlesham Heath Ipswich, IP5 3RE England EMail: pete@surfaceeffect.com Home page: http://www.surfaceeffect.com/people/pete/ Bob Briscoe B54/74 BT Labs Martlesham Heath Ipswich, IP5 3RE England Phone: +44 1473 645196 Fax: +44 1473 640929 EMail: bob.briscoe@bt.com Home page: http://www.labs.bt.com/people/briscorj/ Alan Poppitt B54/77 BT Labs Martlesham Heath Ipswich, IP5 3RE England Phone: +44 1473 640889 Fax: +44 1473 640929 EMail: apoppitt@jungle.bt.co.uk Home page: http://www.labs.bt.com/people/poppitag/
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7. Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.