rfc2729.txt

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                             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