QoS Models

There are three QoS Models:

  1. Best-Effort
  2. Integrated Services Model
  3. Differentiated Services Model

Best-Effort Delivery/FIFO:
Switches and routers work to deliver packets as quickly as possible with no regard for the type of traffic or need for priority. There is no QoS policy applied to traffic.

If we use the firetruck example as in one of the books, sorry I can’t remember which one, firetrucks headed to a fire must stay within the normal flow of traffic. At an intersection it must wait in line like all other vehicles and gets no special treatment.

Benefits:

  • Scalable
  • Easy

Drawbacks:

  • No differentitation
  • No guarantees

Integrated Services (IntServ):
InServe is also called HardQoS because it guarantees bandwidth, delay and packet loss. IntServ applies QoS on a per-flow basis and requires that each node establish guaranteed bandwidth before a single packet is sent.† If each node cannot guarantee bandwidth the application does not start.

IntServ model involves setting up an end-to-end connection across the network using an ip based signaling protocol called Resource Reservation Setup Protocol (RSVP, RFCs 2205 – 2215). RSVP is used by the host to request bandwidth and it is used by routers in the path to establish and maintain state of the flow (RFC 2205). While the flow is active, the routers maintain the bandwidth reserved for that application from source to destination. If the reservation is not successful from source to destination, the application can not start.

RFC 1633 InServ describes two components that are key building blocks:

  • Resource Reservation
  • Admission Control

Resource Reservation:
Bandwidth must be managed because it is an important and finite resource and guarantees can not be achieved without reservation. Routers reserve the queuing preferences for a flow based on an RSVP request. RSVP reserves the bandwidth when the flow starts and continues while the flow is still active. RSVP can also request delay, but guaranteeing delay is not easy.

Admission Control:
Admission control is the algorithm that decides whether a flow can begin. Remember a flow will not start unless the network has met the requested guarantees. This is not to be confused with policing or enforcement, only whether or not the request has been met.

The ONT book also specifies Policing, Queuing and Scheduling:
Policing:
Ensure that clients do not exceed resource reservation by measuring and monitoring.

Queuing:
Routers must be able hold packets while processing others.

Scheduling:
Scheduling works in conjunction with queuing, the amount of data dequeued and sent is based queuing mechanism.

Using the firetruck example, the firetruck would radio ahead to each intersection before it left the firehouse to make sure the light will be green when it gets there. If all the lights can not be green, then the firetruck would not leave. This does not scale well.

Benefits:

  • Resource control
  • Per request control
  • Signaling of dynamic port numbers

Drawbacks:

  • Signaling requires overhead
  • Does not scale

Differentiated Services (DiffServ):
DiffServ is more commonly used than IntServ and is therefore covered in more depth within the ONT book and I presume on the test. Chapters 3, 4 and 5 all deal with DiffServ so this section will be short.

DiffServ is also called SoftQoS and is defined in RFC 2474 and 2475. There is no signaling by the client or router before sending data. Traffic is first classified and then marked. As marked traffic moves through the network it’s level of service depends upon it’s marking.

Marking is done in the headers of each packet. The fields in the packet headers used to classify traffic are defined in RFC 2474, the DS field in IPv4 and the Traffic Class octet in IPv6. The IPv4 DSCP superseded the ToS octet from RFC 791. Six bits of the DS field are used as a codepoint (DSCP) to select the Per Hop Behavior (PHB) a packet experiences at each node.

DiffServ applies QoS on a per-hop basis to a group of similar flows. Each device handles packets on an individual basis, which is called Per Hop Behavior. Packets should be classified as close to the source as possible. Each router or switch can be configured with QoS policies to follow and decisions are made accordingly. While providing flexibility, you have to configure each router, regardless, it scales more readily because you have less overhead from no use of RSVP. DiffServ requires no advance reservations, QoS is handled in a distributed fashion.

Continuing with the traffic example, police are stationed at every intersection, however, none of them knows a firetruck is coming until they hear the siren. At each intersection the decision on how to handle the fire truck is made based on the rules of that intersection.

DiffServ Benefits:

  • Scalability
  • Many service levels

DiffServ Drawbacks:

  • No service guarantee
  • Configuration throughout the network

Sources:
http://rfc.sunsite.dk/rfc/rfc1633.html
http://rfc.sunsite.dk/rfc/rfc2205.html
http://rfc.sunsite.dk/rfc/rfc2210.html
http://rfc.sunsite.dk/rfc/rfc2474.html
http://rfc.sunsite.dk/rfc/rfc2475.html
Cisco QoS Exam Certification Guide
CCNP ONT
Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.4T

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