CHAPTER 2 QoS ROUTING AND ITS ROLE IN QOS PARADIGM 22
QoS ROUTING AND ITS ROLE IN QOS PARADIGM 2.1 INTRODUCTION As the main emphasis of the present research work is on achieving QoS in routing, hence this chapter presents a brief introduction about QoS architecture and role of QoS routing in QoS framework. The chapter first introduces current routing paradigm, the Best-Effort paradigm and defines the need of QoS. Then it presents the concepts and architecture related to Quality of Service. Afterwards, the QoS routing and its objectives are presented. The rest of the chapter discusses QoS routing s position in the QoS framework, relative to other QoS related mechanisms. 2.2 BEST-EFFORT PARADIGM In packet switched networks, such as the Internet, routing can be defined as the process of delivering packets to their ultimate destination through intermediary nodes, called routers, via the most appropriate path. Today Internet offers Best Effort service. Best effort means that each user is provided services and gets a fair share of the available network resources. In best-effort networks, routing protocols usually characterize the network with a single metric such as hop-count or delay, and use shortest path algorithms for path computation. Traffic is processed as quickly as possible, but there is no guarantee of actual delivery. The network itself does not actively distinguish in its treatment of services that travel the network. In a Best-Effort network, all packets are treated in the same fashion. The network undertakes its Best-Effort to deliver every packet as quickly as it can, but no promises can be made concerning QoS parameters like an upper bound on end to end delay, minimum throughput and delay jitter [73]. Internet routing protocols OSPF and RIP are opportunistic, providing the current shortest path to a destination. Alternate paths with acceptable but non-optimal cost cannot be used to route traffic. Shortest path routing protocols do allow a router to alternate among several equal cost paths to a destination. Traffic shifts from one path to another as soon as a better path is found even if the existing path can meet the service requirements. [23]
The Best-Effort is a fair approach, but it is considered insufficient when handling multiple types of traffic with diverse requirements. Applications differ in their QoS requirements. Most applications are loss-sensitive; while data applications can recover from packet loss via retransmission. Moreover data applications such as file transfer are not generally delay sensitive. However applications such as FTP (File Transfer Protocol) and email applications tend to have zero tolerance for packet loss and high tolerances for delay and jitter. Continuous media applications such as streaming audio and video generally require a fixed bandwidth. These applications demand high bandwidth, low delay and jitter, but are less sensitive to packet loss. Acceptable response time for FTP may be few seconds whereas it may be hours for email. The following table shows the several current common applications and the sensitivity of their QoS requirements [6]. Table 2.1 Common applications and the sensitivity of their QoS requirement This diversity of applications makes the current Internet approach of offering the same besteffort service to all applications, insufficient. Such architecture has significant technical advantages in terms of simplicity and high link utilization, may be suited for classical Internet applications like e-mail, FTP and Telnet, but lacks the facilities to provide service assurance. Since, the best-effort paradigm is not sufficient to handle the vast requirement of new emerging applications and therefore a new paradigm must be introduced to provide these applications with the required levels of Quality of Service. In the following sections, an overview of this new approach will be explained and the focus will be on QoS routing, which is a very important component in the QoS formation.
2.3 QUALITY OF SERVICE Quality of Service (QoS) for networks is an industry-wide set of standards and mechanisms for ensuring high-quality performance for critical applications. More formally: there are two commonly used definitions of QoS: Totality of characteristics of a telecommunications service that bear on its ability to satisfy stated and implied needs of the user of the service... (ITU-T: International Telecommunication Union - Telecommunication Standardization Sector) [w3]. A set of service requirements, to be met by the network, while transporting a flow. (IETF: Internet Engineering Task Force) [23] The goal of QoS is to provide preferential delivery service for the applications by ensuring sufficient bandwidth, controlling latency and jitter, and reducing data loss. The following table describes these network characteristics. [w1] Table 2.2 Network Characteristics Managed by QoS Network Characteristic Description Bandwidth The rate at which traffic is carried by the network. Latency The delay in data transmission from source to destination. Jitter The variation in latency. Reliability The percentage of packets discarded by a router Quality of Service covers several mechanisms that were designed to support flows which require some performance guarantees. 2.4 QOS ARCHITECTURE In order to provide support for QoS in an overall framework, a general QoS architecture is needed. The objectives of QoS architecture is, to define a set of quality of service configurable interfaces that formalize Quality of Service in the end-system as well as network, providing a framework for the integration of quality of service control and management mechanisms. The QoS service can be applied to individual applications on the basis of either Per flow or flow aggregate. Per Flow means - an individual, uni-
directional, data stream between two applications (sender and receiver) and Per Aggregate is simply two or more flows where the flows will have something in common [w6]. Internet Engineering Task Force (IETF) has defined two major models for QoS architecture: Integrated Services (Intserv) and Differentiated Services (Diffserv). The steps to end-to-end QoS support over the Internet are as follows [41]- 1. Define the service class of a packet received at each switch. 2. Allocate a certain amount of resources to each class. 3. Sort the incoming packets according to their respective classes. 4. Control the amount of traffic admitted to each class. 5. Apply the above four steps to each switch, or at least to all bottleneck routers. Integrated Services, or IntServ, was the first effort to provide QoS in the Internet. To perform these activities, IntServ has defined a framework, which consists of four components: the packet classifier, the packet scheduler, the reservation set up protocol, and the admission control routine. The packet classifier maps the incoming packets into classes that will be handled by the packet scheduler. The packet scheduler manages the forwarding of packet streams at the router using a set of queues and other mechanisms. The reservation set up protocol is used to set up the reservation. The admission control routine determines whether a flow can be granted for the requested QoS[8]. Specifically, IntServ was designed to support- (1) Guaranteed services, and (2) Controlled load services. IntServ guaranteed service provides guaranteed network bandwidth and strict bounds on endto-end queuing delay to the traffic flows.this service is intended for those applications that require high assurance on bandwidth and delay. On the other hand, the controlled load service does not provide any quantitative guarantee on bandwidth and delay. It tries to follow a lightly loaded network. This service attempts to provide several delays which the application can choose from [8].There were concerns about IntServ s complexity and scalability.
The other model, Differentiated Services, or DiffServ, separates the two sides of IntServ, providing forwarding on per-hop behaviour (PHB) basis with queue management and queue service disciplines, but leaves the admission control and end-to-end concerns outside its scope. The Diffserv model can specify and control network traffic by class in such a way that certain types of traffic get priority. Traffic is classified and possibly conditioned while entering a network. Based on the result of this classification, classes of traffic are then attributed to different behaviour aggregates. Each of these traffic classes is identified by a field, called a Diffserv Codepoint (DSCP), in the IP header. Within a Diffserv network, packets are forwarded according to the specific queuing behaviour (known as per-hop behaviour or PHB) associated with their DSCP. Per-hop behaviour defines how an individual router treats an individual packet when sending it over the next hop through the network. In addition, traffic classes from many flows having similar QoS requirements are marked with the same DSCP. These flows can then be aggregated to a common queue. Since the DS field is set at the network boundaries, no per-flow state is needed at the network core[7]. In order for a customer to receive Differentiated Services from its Internet Service Provider (ISP), it must have a Service Level Agreement (SLA) with its ISP. A SLA basically specifies the service classes supported and the amount of traffic allowed in each class.. Diffserv enables ISPs to define classes of service (CoS) to support particular traffic requirements.using the classification, policing, shaping and scheduling mechanisms, many services can be provided, for example, 1) Premium Service for applications requiring low delay and low jitter service; 2) Assured Service for applications requiring better reliability than Best Effort Service; and 3) Olympic Service, which provides three tiers of services: Gold, Silver and Bronze, with decreasing quality. Note that the Differentiated Services only defines DS fields and PHBs. It is the ISPs responsibility to decide what services to provide. [97] The characteristics of DiffServ can be summarized as [w4]-
1. While Intserv classifies traffic based on the individual flow, DiffServ aims to classify traffic by aggregate, and resources are allocated to the individual classes. This is intended to ease the complexity and scalability issues. 2. Intserv performs traffic classification on the basis of hop-by-hop i.e. on all routers, while DiffServ performs it at the edge of the network. 3. IntServ supports controlled-load services and guaranteed services, while DiffServ provides a wide range of services with respect to requirements. 4. In the DiffServ architecture, services are defined in the form of a Service Level Agreement (SLA) between a customer and the service provider. Resource assurance is provided through provisioning, rather than dynamic per flow reservation as in IntServ. 5. One important element in an SLA is the traffic conditioning agreement (TCA), which details the service parameters for traffic profiles and policing actions. To ease the complexity, traffic policing is done only at the network boundary. 6. DiffServ aims at minimal standardization and flexible set of building blocks. It uses the existing field in the IP protocol header to define the different services (or aggregates) desired, called DS code point (DSCP). 7. DiffServ specifies a small set of behaviour, called per-hop behaviour (PHB), that the nodes should follow in response to DSCP. 8. The IntServ is for end-to-end services, which may span across many domains. DiffServ forwarding classes can be defined for a single domain. If an end-to-end service extends through several domains, the service providers may extend their definition through bilateral agreements. All the mentioned reasons made adopting IntServ difficult and it was never really adopted. These models encompass several categories of mechanisms that provide preferential treatment to specified traffic. We can categorize QoS mechanism into three general categories. Category of QoS Mechanisms Admission control Table 2.3 Categories of QoS Mechanism Description Determines which applications and users are entitled to network resources. These mechanisms specify how, when, and by whom network resources on a network segment (subnet) can be used. Regulates data flows by classifying, scheduling, and marking packets based on
priority and by shaping traffic (smoothing bursts of traffic by limiting the rate of flow). Traffic control mechanisms segregate traffic into service classes and Traffic control control delivery to the network. The service class assigned to a traffic flow determines the QoS treatment, the traffic receives. Resource Reservation Protocol (RSVP) is a signalling technique used to guarantee Quality of Service (QoS) by reserving bandwidth for RSVP-capable Resource Reservation data flows. All nodes in the data path must be RSVP compliant for a guaranteed QoS. The Intserv model integrates resource reservation and admission control mechanisms to support special handling of individual traffic flows. The Diffserv model uses traffic control to support special handling of aggregated traffic flows. [w1] 2.5 QOS ROUTING AND ITS OBJECTIVES OSPF and other dynamic routing protocols always forward packets to the shortest path. This can cause problems for flows with a need of QoS guarantees, if the shortest path does not have enough resources to meet the requirements. IntServ is supposed to reserve resources for the flow, but cannot make the reservation if there are not sufficient resources along the path to begin with. DiffServ can be utilised better if the path with the best chance to provide the required service, is found somehow. The missing piece in the framework therefore seems to be a mechanism that can find a path, if one exists, which has the requested resources available. Only then, it is possible to utilize DiffServ or IntServ techniques efficiently. QoS routing is a routing scheme that considers the quality of service requirements of a flow when making routing decisions. As opposed to traditional shortest path routing, which considers only one parameter, QoS routing is designed to find paths that satisfy multiple constraints. Quality of Service (QoS) based routing is defined in RFC 2386 Crawley et al. [23] as a: Routing mechanism under which paths for flows are determined based on some knowledge of resource availability in the network as well as the QoS requirement of flows Objectives: QoS routing uses information about network state and resource availability as well as the QoS requirements of the flow to make routing decisions. The objectives of QoS routing are:
Dynamic determination of feasible paths: That is, to find a feasible path for the flow in question that can accommodate or at least has a good chance of accommodating the QoS requirements of flow. Optimization of resource usage: QoS-based routing can be used to help balancing the load of the network by efficient utilization of resources, and thus improving the total throughput of the network. Graceful performance degradation: In overload situations, QoS routing should be able to provide better throughput in the network than best effort routing or any state-insensitive routing scheme, and more graceful performance degradation. The issues that must be considered for providing QoS based routing in the Internet are summarized as: 2 The determination of QoS capability of each outgoing link by a router and reservation of link resources. 3 The type of routing decision to be taken i.e. destination based, source-destination based or flow based. 4 The type of routing metrics used. 5 The computation of QoS accommodating paths for unicast flows and multicast flows with different reservation styles and receiver heterogeneity. Different aspects related to these issues have been discussed in [23]. 2.6 POSITION OF QOS ROUTING IN THE QOS FRAMEWORK This section discusses the relationships between QoS routing and other QoS related mechanisms. The following table shows the relative position of QoS routing with different components of QoS in QoS framework and their relative layer. Table2.4: The relative position of the components in the QoS framework [97] Application Layer Transport Layer IntServ/RSVP, DiffServ Network Layer Constraint based routing/qos Routing Link Layer MPLS
2.6.1 Transport Layer DiffServ and QoS Routing Originally, the DiffServ scheme is intentionally decoupled from IP routing, so all traffic between a source-destination pair may follow the same path no matter which service class it belongs to, and DiffServ itself has no effect on routing decisions. This means congestion can occur in DiffServ. For example, aggregation of traffic in the core of the network could cause congestions. This is not a problem when traffic from boundary routers aggregate to a core router, since the link from the core router to the next core router is faster than the links from the boundary routers. On the other hand, when, among the core routers, traffic is routed so that it aggregates into one router, the link to which that router forwards the traffic may not be fast enough. DiffServ cannot solve this problem. QoS routing could be used to avoid this kind of a situation. Within a DiffServ domain, QoS routing is used for finding paths that are able to accommodate the flows and avoid congestion. [95][97] QoS Routing With Resource Reservation The QoS routing and the resource reservation are two important, closely related network components. In order to provide the guaranteed services, the required resources (CPU time, buffer, bandwidth etc.) must be reserved when a QoS connection is established. Hence, the data transmission of the connection will not be affected by the traffic dynamics of other connections sharing the common links. Before the reservation can be done, a path with the best chance to satisfy the resource requirement must be selected. That is the job of routing. [19] Resource reservation and QoS routing are independent mechanisms but complement each other well. The protocol most often suggested in papers concerning QoS routing and resource reservation is RSVP. RSVP is a reservation protocol in the Internet suite, which can be used in conjunction with QoS routing. It is receiver oriented, which means that the receiver of the data flow is responsible for initiation of resource reservation. RSVP messages can serve as the trigger to query QoS routing. During the processing of RSVP messages, RSVP queries
QoS routing to obtain the next hop for forwarding the message. The message is then forwarded on the interface returned by QoS routing.[12] When the source node initiates a flow, it sends a PATH message to the destination node identifying the characteristics of the flow for which resources are requested. Every intermediate router along the path forwards the PATH Message to the next hop determined by the routing protocol. Upon receiving a PATH Message, the receiver responds with a RESV Message to request resources for the flow. Every intermediate router along the path can reject or accept the request of the RESV Message. If the request is rejected, the router will send an error message to the receiver, and the signalling process will terminate. If the request is accepted, QoS resources are allocated for the flow and the related flow state information will be installed in the router. [97] 2.6.2 Network Layer Qos With Constraint-Based Routing Constraint-based routing evolves from QoS routing. Although the terms are sometimes used almost interchangeably, constraint-based routing is actually a more general term, which combines QoS routing and policy routing. Constraint- Based routing can be used to compute the routes subject to QoS and policy constraints. The goal is to meet the QoS requirements of traffic and to improve utilization of the networks. [97] 2.6.3 Link Layer QoS With MPLS Multiprotocol label switching is a forwarding scheme where packets are routed based on a short label, which makes forwarding faster than when dealing with IP addresses, and allows policy routing within a MPLS-capable domain. MPLS can be used together with differentiated services to provide QoS. Since MPLS is a forwarding scheme and QoS routing is a routing scheme, they can be used together for traffic engineering purposes. In fact, MPLS can provide more accurate information about traffic loads in the domain than traditional IP routing, thus facilitating QoS routing to compute better routes for setting up the label switched paths. Furthermore, it is relatively easy to combine a QoS routing framework with MPLS [2].
The aim of QoS routing in the MPLS network is, to route the traffic trunks along the network in a way that satisfies the given constraints and establish a more balanced traffic load distribution. It is also possible to re-route existing label switched paths to prevent congestion. An MPLS traffic trunk is an aggregate of flows that belong to the same class, for example all the traffic between specific ingress and egress routers. Traffic trunks are routable objects. Based on information about the traffic trunks, network topology and resources, QoS routing calculates explicit routes for each traffic trunk. The explicit route in this case is a specification of a label switched path, LSP, satisfying the requirements of the traffic trunk. Given the routes, MPLS sets up the LSP s using its label distribution protocol. It makes no difference to MPLS whether the routes are computed by QoS routing or traditional dynamic routing, where paths are selected based on some dynamic criteria, available bandwidth perhaps, but QoS requirements of the flows are not considered. [4] In MPLS, explicit routing may be needed in order to allow each stream to be individually routed, and to eliminate the need for each switch along the path of a stream to compute the route for each stream. Given that MPLS allows efficient explicit routing, it follows that MPLS also facilitates QoS routing. MPLS allows the explicit route to be carried only at the time that the label switched path is set up, and not with each packet. This implies that MPLS makes explicit routing practical. This in turn implies that MPLS can make possible a number of advanced routing features which depend upon explicit routing.[77] 2.7 SUMMARY The current applications of Internet require QoS for the victorious transmission. The two major models to provide QoS mechanism are IntServ and Diffserv. IntServ is supposed to reserve resources for the flow, but it would not be able to do that unless and until there are sufficient resources available to be reserved. DiffServ is also better utilized if there exists a path that can provide the required service. Thus, we can say we can achieve QoS, if and only if we are able to find a path that can satisfy QoS parameters. Only then, it is possible to utilize DiffServ or IntServ techniques efficiently. Finding a path that can satisfy QoS criteria is called QoS routing. So the attention of the thesis is on QoS routing in QoS framework. The next chapter presents the characterization of QoS routing.