Skip to main content

Quality of Service (QoS) Models

Image
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

 

 

 

 

Labels:

Comments

Post a Comment

Popular posts from this blog

What is Classification and marking? What is IP Precedence and DSCP Values? What is AF and EF?

By Edgar C Francis Internetworks
  Classification and Marking Classification and marking are the processes of identifying the priority of each packet. This is the first step of QOS control and should be done near the source hosts. Classification is the process of identifying and categorising traffic into classes, typically based upon incoming interface IP precedence, DSCP Source or Destination address. Application classification is the most fundamental QOS building block. Without classification, all packets are treated the same.   The table lists the criteria of classification                                  Marking  Marking is the QOS feature component that colours a packet so it can be identified and distinguished from other packets in QOS treatment. Commonly used markers: link-layer: COS (ISL, 802.1q), MPLS EXP bits,...
Post a Comment
Read more

OSPF default routing

By Edgar C Francis Internetworks
  OSPF Default Route Configuring OSPF Default route is not a big problem. We have couple of options when we want to OSPF default route. Here is an example R1(config)#router ospf 1  R1(config-router)#default-information originate ?   always       Always advertise default route   metric       OSPF default metric   metric-type  OSPF metric type for default routes   route-map    Route-map reference   <cr> We can change numbers of things for example metric or metric type but most of the people forget about the most important thing is the always keyword.  If we add the always command this feature advertises the default route even if we do not have in our routing table. It will look like that route is installed in routing table.   When we use the default information originate we can advertise a default route in OSPF. But OSPF won’t...
Post a Comment
Read more

BGP Local preference configuration

By Edgar C Francis Internetworks
BGP Local preference (Local_Pref) Path attribute gives the router inside a single autonomous system a value that they can set per-route and advertise to all iBGP routers inside the autonomous system, so that all routers in the autonomous system agree about which routers is the best exit point for packet destined for that prefix. Local preference (Local_Pref) is the second BGP attribute. We can use the local preference to choose the outbound external BGP path. Local preference is sent to all internal BGP (iBGP) routers in our autonomous system. Local preference (Local_Pref) is not exchanged between external BGP routers. It’s a well-known and discretionary BGP attribute. Default value is 100. The path with the highest local preference is desirable  Topology:- TASK: To reach 50.1.1.1, 15.0.0.1 network router 2 and router 3 using 3.1.1.2 path ( AS 65144) because it has less numbers of au...
Post a Comment
Read more