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Service Specification

Roberto Bifulco — Fri, 01/25/2008 - 09:58

Visto che non è possibile prevedere a priori il traffico dati, poiché è composto da componenti eterogenee e variabili, è necessario fornire una specifica del servizio che si richiede alla rete sulla base di alcuni parametri:

  • Loss: probabilità che un pacchetto del flusso vada perso;

  • Delay: il tempo impiegato da un pacchetto per andare dalla sorgente alla destinazione;

  • Delay jitter: la massima differenza fra i ritardi subiti fra due pacchetti del flusso;

  • Bandwidth: la massima velocità a cui la sorgente può inviare dati.

 

Sulla base di questi parametri l'end system e la rete stabiliscono un contratto, il Service Level Agreement, in cui entrambe le parti si impegnano a dare delle garanzie. Chiaramente sono previsti anche meccanismi per verificare che il contratto, e dunque le garanzie, siano rispettati. Nel seguito sono riportati alcuni esempi di contratto:

  • Hard Real Time: servizio garantito

    • Contratto:

      • Dalla rete al client: è garantito un limite superiore deterministico al delay per ogni pacchetto della sessione;

      • Dal client alla rete: si garantisce che nella sessione non viene inviato più traffico di quanto specificato;

    • Supporto algoritmico:

      • Controllo di ammissione basato sull'analisi del caso peggiore;

      • Classificazione e scheduling del traffico per flusso a livello dei router;

  • Soft Real Time: servizio a carico controllato

    • Contratto:

      • Dalla rete al client: è garantito un livello di performance simile a quello di una rete best-effort con poco traffico;

      • Dal client alla rete: si garantisce che nella sessione non viene inviato più traffico di quanto specificato;

    • Supporto algoritmico:

      • Controllo di ammissione basato sulla misurazione di aggregati;

      • Scheduling per aggregati.

 

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QoS problems

  • II.1QoS problems
  • Service Specification
  • Traffic and Service Characterization
  • II.2Service Specification
  • Token Bucket
  • II.3Traffic and Service Characterization
    • II.3.1Token Bucket
    • II.3.2Leaky Bucket
    • II.3.3Queue management
    • II.3.4Scheduling
      • II.3.4.1Scheduling policies
        • II.3.4.1.1GPS
  • II.4Parekh-Gallager Theorem
  • II.5QoS Architectures
    • II.5.1Integrated Services (IntServ)
      • II.5.1.1RSVP
      • II.5.1.2IntServ today
    • II.5.2Differentiated Services (DiffServ)
      • II.5.2.1PHB: Expedited Forwarding
      • II.5.2.2PHB: Assured Forwarding
  • II.6QoS in Fast Interconnect
    • II.6.1Flow Control
      • II.6.1.1Flow Control in IBA
      • II.6.1.2Flow Control in ASI
      • II.6.1.3Flow Control in Ethernet
    • II.6.2Congestion control
      • II.6.2.1Congestion Control in IBA
      • II.6.2.2Congestion Control in ASI
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