Network Working Group P. Bagnall

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

Taxonomy of Communication Requirements
for Large-scale Multicast Applications

Status of this Memo

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

Copyright Notice

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

Abstract

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

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Table of Contents

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

1. Introduction

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

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

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

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

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

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

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

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

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

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

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

2. Definition of Sessions

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

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

Secure session
a session with restricted access

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

3. Taxonomy

3.1 Summary of Communications Parameters

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

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

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

3.2 Definitions, types and strictest requirements

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

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

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3.2.1 Types

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

Application Benchmark

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

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

Boolean

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

Currency - current local

Data Size

Date (time since epoch)

Enumeration

Fraction

Identifiers
A label used to distinguish different parts of a communication

Integer

Membership list/rule

Macro
A small piece of executable code used to describe policies

Time

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3.2.2 Reliability

3.2.2.1 Packet Loss

Transactional

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

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

Guaranteed

Guarantees communications will succeed under certain conditions.

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

Tolerated loss

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

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

Semantic loss

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

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

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

3.2.2.2. Component Reliability

Setup Fail-over time

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

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

Mean time between failures

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

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

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Fail over time during a stream

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

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

3.2.3. Ordering

Ordering type

Specifies what ordering must be preserved for the application

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

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

3.2.4. Timeliness

Hard real- time

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

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

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

Synchronicity

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

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

Burstiness

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

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

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

Jitter

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

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

Expiry

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

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

Latency

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

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

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Optimum Bandwidth

Bandwidth required to complete communication in time

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

Tolerable Bandwidth

Minimum bandwidth that application can tolerate

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

Required by time and tolerance

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

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

Host performance

Ability of host to create/consume communication

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

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

Frame size

Size of logical data packets from application perspective

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

Content size

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

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

3.2.5. Session Control

Initiation

Which initiation mechanism will be used.

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

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Start Time

Time sender starts sending!

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

End Time

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

Duration

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

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

Active Time

Total time session is active, not including breaks

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

Session Burstiness

Expected level of burstiness of the session

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

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Atomic join

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

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

Late join allowed ?

Does joining a session after it starts make sense

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

Temporary leave allowed ?

Does leaving and then coming back make sense for session

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

Late join with catch-up allowed ?

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

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

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Potential streams per session

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

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

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

Maximum number of streams that an application can consume
simultaneously

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

3.2.6. Session Topology

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

Number of senders

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

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

Number of receivers

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

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

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3.2.7. Directory

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

Mobility

Defines restrictions on when directory entries may be changed

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

3.2.8. Security

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

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

Authentication Strength

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

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

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

Tamper-proofing

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

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

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

Non-repudiation strength

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

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

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

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

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Denial of service

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

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

Action restriction

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

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

Privacy

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

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

Confidentiality

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

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

Retransmit prevention strength

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

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

Membership Criteria

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

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

Collusion prevention

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

Type: {
Abstract Currency,
Abstract Currency,
Abstract Currency

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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

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

Action on compromise

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

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

3.2.8.1. Security Dynamics

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

Mean time between compromises

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

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

Compromise detection time limit

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

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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

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Cost per Mb

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

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

4. Security Considerations

See comprehensive security section of taxonomy.

5. References

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

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

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

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

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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: [email protected]
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: [email protected]
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: [email protected]
Home page: http://www.labs.bt.com/people/poppitag/

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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.

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