QoS (quality of service)

QoS (Quality of Service)

Content

• Introduction• IP QoS: Mechanisms• IP QoS Frameworks• QoS in Wireless Networks

Introduction

Point 2 is Imposibll

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1. Computer networking relies on sharing of limited network resources for communications.

2. If communications resources were unlimited, a direct point-to-point link of unlimited bandwidth would be possible between any two end-points that wished to communicate.

Agree..!!

In the real world, two end-points that wish to communicate, unless they are connected to the same LAN, need to communicate through a network of links that have the following characteristics:

1. The links are shared between traffic from multiple sources to multiple destinations

2. The links have limited bandwidth

3. The links have propagation latency (i.e., time taken for data to traverse the links)

4. The nodes directly connected to each link have limited processing power and memory capacity

Introduction

How should these limited resource be shared?

FIFO (first in first out)

Introduction

However, this simple treatment of packets is not sufficient for today’s multiservice networks.The different applications have different requirement on the communications services provided by the network.

Voice

Video

1. Voice and Video have very stringent delay and latency requirements, as already discussed in chapter 4.

2. File transfer, on the other hand, does not have the same delay and latency requirments.

Introduction

IntroductionThese kind of requirements, plus others such as “guaranteed” bandwidth requirements, are know as QoS requirements, and the whole field that is about these requirement and the ways they are met in networks, is known as:

QoS(Quality of Service)

IntroductionTable 1. shows some different categories of traffic and their QoS requirements

IP QoS Mechanisms

1.Introduction to IP QoS2.Resource Reservation3.Admission Control4.Packet Classification and Marking5.Queuing Disciplines6.Traffic Shaping7.Policing8.Routing Control and Traffic

Engineering

Introduction to IP QoS

The existing internet is largely a best-effort network.

Internet

No guarantees are provided

that packet will not loss

Introduction to IP QoSThe IP QoS solution is predicated on three fundamental requirements:

1.To Provide mechanisms to offer different levels of services to different users, even though some users might get better service than others. Thus, IP QoS is inherently “unfair”

2.The Service should not usually be completely denied to lower-priority traffic.

3.The mechanisms are efficient, not wasteful of valuable network resources.

IP QoS MechanismResource Reservation

Perhaps a natural first step in moving away from a purely best-effort-service Internet is to Provide away to reserve resources for selected users while still using traditional IP Routing. Resource reservation refers to dynamic arrangements for QoS provisioning that can be made end-to-end over one or more intermediate networks. Usually, a reservation is specified by a traffic profile, a description of the rate, burst size, and other feature of the reserved traffic.

IP QoS MechanismResource Reservation Protocol (RSVP)

RSVP is a signaling protocol that allows resource reservation to be coordinated in a network. RSVP signaling generally results in resources being reserved in rsvp capable routers in the transit path of a traffic flow. Each of these routers maintains a soft state that needs to be periodically refresh. RSVP is unidirectional-resources are reserved in one direction from source to destination only.

IP QoS MechanismResource Reservation Protocol (RSVP)

Resources are reserved using the following procedures. See figure 1

IP QoS MechanismAdmission

Admission control refers to the decision whether to admit a certain flow or aggregation of flows into a network, in response to a request for resource reservation. It makes sense for admission control decsions to be made by an entity with an overall view of network resources.

IP QoS MechanismBandwidth Brokers

There are two basic choices for dynamic allocation of bandwidth resources in the routers in a given network.1. Each router makes its own decisions, perhaps as

configured manually or by policy.2. A single entity makes the decisions for the network, taking

into account the dynamic conditions in the network. This entity is often called a bandwidth broker

By the nature of its functions, it is typical that each domain would have its ownbandwidth broker to control its bandwidth resources. It would be involved in negotiations(perhaps with another domain’s bandwidth broker) for resource allocationfor interdomain traffic, making decisions on

IP QoS Mechanism

Figure 2.shows an example of a bandwidth broker providing admission control services to administrative domain

Bandwidth Brokers

IP QoS MechanismPacket Classification and Marking

Packet classification is the separation of packets into different classes based on some criteria

Example:1. Classifacation may be port-based (the input and/or output port of

the packet)2. Transport protocol-based (TCP or UDP),3. Application-Based (such as HTTP-based)4. Address –Based (Based on the source or destination address of

the packet).

It is also possible that out-of-profile packet are reclassified, typically into a lower priority class.

IP QoS MechanismPacket Classification and Marking

Marking is the setting of bits in the packet header corresponding to the packet class. The bits marked could be the three bits in the type of service (ToS) field and the three bits in the precedence field of the IP header

Packet classification and marking allows differential treatment of different, packet classes using other, complementary QoS. Packet classification and marking may be handled by external sources like a customer or other network. When the packets enter a new network, that network can accept the classification or reclassify.The decision depends on the QoS architecture and policies used..

IP QoS MechanismQueuing Disciplines

Queuing is fundamental to IP QoS schemes because each router has one or more input queues and one or more output queues that may be where the bulk of “processing time” is spent. The use of various queuing disciplines can be used to change such functions as processing times and flow rates. Most of the queuing disciplines discussed here operate on router output queues, although priority queuing could be implemented to operate jointly on input/output queues.

IP QoS MechanismQueuing Disciplines

1. FIFO Queuing2. Priority Queuing3. Fair Queuing (FQ), Weighted Fair

Queuing (WFQ), and Class-Based Queuing (CBQ)

IP QoS MechanismFIFO Queuing

FIFO queuing is the standard, basic queuing discipline. All packets are treated equally without preference except that earlier arriving packets leave before later arriving packets.

Advantages :1. Highly optimized forwarding performance resulting from years

of experience by router manufacturers.2. For lightly loaded networks with sufficient transmission and

switching capacity, queuing is necessary only for smoothing intermittent traffic bursts. FIFO does this very efficiently.

Disadvantages :1. For networks that are not lightly loaded, FIFO queuing may

result in loss of packets through discarding when the queues are full.

2. FIFO does not allow certain packets to receive priority/preference over other packets.

IP QoS MechanismPriority Queuing (PQ)

PQ (see Figure 3) is a non-FIFO queuing discipline. The router examines the input queue, takes high-priority packets and places them in the output queue ahead of normal packets. This effectively reorders packets according to priority.

IP QoS MechanismFair Queuing (FQ), Weighted Fair Queuing (WFQ), and Class-BasedQueuing (CBQ)

Therefore, FQ results in preferential treatment to low-volume traffic flows.WFQ is a variation and generalization of FQ that weights the outputs of the per-flow queues (according to the IP ToS field, for example) as well.Advantages of WFQ include:1. It prevents a misbehaving TCP session from consuming a large fraction ofresources at the expense of other flows.2. The fairness aspect of this type of scheme prevents buffer starvation.

Disadvantages of WFQ include:1. WFQ is a way to approximate generalized processor sharing (GPS), wherethe link sharing between queues is accomplished by scheduling alone.2. It does not scale well. Computational overhead is high, and forwardingperformance may be affected

IP QoS MechanismComparison of Queuing Disciplines

IP QoS Mechanism7.2.8 Traffic Shaping

Traffic shaping refers to controlling the rate of traffic passing through a givenrouter. It is often used at the ingress points of a network as a form of soft admission control. 1. Leaky bucket traffic shaping constrains the rate to be less than a maximumvalue, while 2. token bucket traffic shaping constrains the average rate to be less than amaximum value.

IP QoS Mechanism7.2.9 Policing

There are multiple definitions of policing in the literature, which all have to do with treatment of packets that are nonconforming (with a profile). According to one definition, policing refers to the process of dropping packets from a flow that are nonconforming

IP QoS Mechanism7.2.10 Routing Control and Traffic Engineering

The basic functional requirements for traffic engineering are :• Distribution of topology information, to allow nodes to build correct topology maps and to assist in path selection;• Path selection, to select a path based on some criteria, such as bandwidth,delay, shortest path;• Directing traffic along computed paths, using forwarding tables (computedindependently at each node using traditional IP routing protocols or signaledprotocols such as MPLS).

IP QoS Framework

The mechanisms discussed in Section 7.2 are often used in combination in networks, in order to provide end-to-end QoS, or to enforce some QoS policies in a network domain. Routers typically combine some of these mechanisms, for example as shown in Figures 7.6 and 7.7.

IP QoS Framework

There should be ways to more systematically coordinate the various QoSmechanisms used in a network according to some kind of framework to allow endto-end provision of QoS for a variety of network traffic, consistent with some idea of network QoS policy. IntServ and DiffServ are two such frameworks.

QoS in Wireless Networks

In wireless networks, there are a number of issues related to providing QoS. 1. there are the issues associated with all wireless networks regardless of host mobility, because of the characteristics of the wireless link, including relatively lower bandwidth, higher latency, and higher errors than comparable wired links. 2. there are the additional complications that come with mobility.

QoS in Wireless Networks

7.4.1 WLAN QoS Support

The 802.11 MAC consists of a distributed coordination function (DCF) and a point coordination function (PCF). The DCF is implemented in all stations and can be used in both ad hoc and infrastructure modes.

7.5 Summary

As introduced in this chapter, techniques are needed for providing differentiated QoS because limited resources in networks are shared, and because the network needs to be able to treat different classes of traffic differently based on issues including different application requirements and different subscriptions. We overview a number of different mechanisms for IP QoS, including resource reservation (using RSVP), admission control (e.g., using bandwidth brokers), packet classification and marking (e.g., using bits in the ToS field of the IP header), queuing (FIFO, priority queuing, WFQ, and CBQ), traffic shaping (leaky bucket and token bucket), policing (e.g., policing token bucket), and traffic engineering or QoS routing. We touch upon the two QoS frameworks for IP networks, the IntServ and DiffServ frameworks that provide ways to systematically apply QoS mechanisms. Then we explore QoS in wireless networks, looking at some length at QoS in 802.11, and the interactions of mobility protocols with QoS mechanisms.

References

Dn, T. Y., & Yyepg, T. (2005). TeAm YYePG Wireless Internet.

QoS (quality of service)

Technology

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