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Quality of Service (QoS) Models

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By Edgar C Francis Internetworks

 

Quality of Service (QoS) Models

QoS models define how network traffic is managed to meet performance requirements. The three primary models are:




1. Best-Effort Service

  • Description: No QoS guarantees; all traffic is treated equally ("first come, first served").

  • Characteristics:

    • No prioritization or resource reservation.

    • Suitable for non-critical applications (e.g., web browsing, email).

  • Pros: Simple, no overhead.

  • Cons: Poor performance under congestion (high latency, packet loss).

2. Integrated Services (IntServ)

  • Description: Provides per-flow QoS guarantees by reserving resources before transmission (like a virtual leased line).

  • Key Protocol: RSVP (Resource Reservation Protocol) signals bandwidth requirements.

  • Characteristics:

    • Guarantees strict latency, bandwidth, and jitter for critical apps (e.g., VoIP, video conferencing).

    • Uses admission control to reject flows if resources are insufficient.

  • Pros: High reliability for real-time traffic.

  • Cons: Scalability issues (each flow requires state maintenance).

3. Differentiated Services (DiffServ)

  • Description: Classifies traffic into priority groups (classes) rather than per-flow reservations.

  • Key Mechanism: Packets are marked with DSCP (Differentiated Services Code Point) in the IP header.

  • Characteristics:

    • Traffic is grouped (e.g., EF for Expedited Forwarding, AF for Assured Forwarding).

    • Routers apply Per-Hop Behaviors (PHB) based on class (e.g., priority queuing).

  • Pros: Scalable (no per-flow state), widely used in enterprise/ISP networks.

  • Cons: No absolute guarantees (relative prioritization only).

Comparison Table

ModelBest-EffortIntServ (RSVP)DiffServ (DSCP)
GranularityNonePer-flowPer-class
ScalabilityHighLowHigh
OverheadNoneHighModerate
Use CaseWeb, emailVoIP, telemedicineEnterprise/ISP traffic

Which Model to Use?

  • Best-Effort: Default for most internet traffic.

  • IntServ: Critical real-time apps (e.g., military comms, medical systems).

  • DiffServ: Balanced approach (e.g., prioritizing VoIP over HTTP in a corporate network).



Quality of service models:

Best-effort service

Integrated services

Differentiated services



Best-effort service model – The Internet was initially based on a best-effort packet delivery service. Best-effort is the default mode for all traffic. There is no differentiation among types of traffic.

The best-effort model is similar to standard mail service – it will get there when it arrives.





Advantages of the best-effort service model:

Highly scalable

No special mechanisms required



Disadvantages of the best-effort service model:

No service guarantees

No service differentiation

 

Integrated services (intserv model) are a QoS model that is older than QOS / Differentiated services model. IntServ model defines a signaling process by which an individual flow can request that the network reserve the bandwidth and delay needed for the flow.

 

The integrated services model uses RSVP for reserving bandwidth. RSVP stands for a resource reservation protocol. RSVP  is used for reserving bandwidth/QOS tools for a particular flow.

Here is a limitation of intserv model if reserved no other traffic can use it and it is not scalable each flow required separate admin configuration on the router.




 

 

The differentiated services model is designed to overcome the limitation of best-effort and intserv models. In Diffserv model network traffic is identified by classes and network QOS policy enforces differentiated treatment of traffic classes. You choose the level of service for each class.




Advantages of the Differentiated Service Model:

Highly scalable

Many levels of quality are possible

 

Disadvantages of the Differentiated Service Model:

No absolute service guarantee

Complex mechanisms

 

 

 

 

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