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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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.
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.
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
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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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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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
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
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!)
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
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
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).
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.
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).
Security dynamics are the temporal properties of the security mechanisms that are deployed. They may affect other requirements such as latency or simply be a reflection of the security limitations of the system. The requirements are often concerned with abnormal circumstances (e.g. system violation).
Mean time between compromises
This is not the same as the strength of a system. A fairly weak system may have a very long time between compromises because it is not worth breaking in to, or it is only worth it for very few people. Mean time between compromises is a combination of strength, incentive and scale.
Type: Time Scope: Per stream Strictest Requirement: indefinite Example Application: Secure Shell - 1500hrs
Compromise detection time limit
The average time it must take to detect a compromise (one predicted in the design of the detection system, that is).
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Type: Time Scope: Per stream Strictest Requirement: Round trip time Example Application: Secure Shell - 2secs
Compromise recovery time limit
The maximum time it must take to re-seal the security after a breach. This combined with the compromise detection time limit defines how long the system must remain inactive to avoid more security breaches. For example if a compromise is detected in one minute, and recovery takes five, then one minute of traffic is now insecure and the members of the communication must remain silent for four minutes after detection while security is re-established.
Type: Time Scope: Per stream Strictest Requirement: 1 second Example Application: Audio conference - 10 seconds
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
[limitations] Pullen, M., Myjak, M. and C. Bouwens, "Limitations of Internet Protocol Suite for Distributed Simulation in the Large Multicast Environment", RFC 2502, February 1999.
[rmodp] Open Distributed Processing Reference Model (RM-ODP), ISO/IEC 10746-1 to 10746-4 or ITU-T (formerly CCITT) X.901 to X.904. Jan 1995.
[blaze95] Blaze, Diffie, Rivest, Schneier, Shimomura, Thompson and Wiener, Minimal Key Lengths for Symmetric Ciphers to Provide Adequate Commercial Security, January 1996.
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6. Authors' Addresses
Peter Bagnall c/o B54/77 BT Labs Martlesham Heath Ipswich, IP5 3RE England
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