Thực hiện chất lượng dịch vụ trong các mạng IP (P8)

8 Mechanisms for Dynamic Service Quality Control The means of dynamically controlling service quality in IP net-works is our next topic. There are two primary uses for such methods: dynamic control of resources within an IP domain, and implementation of dynamic SLAs between domains. The generic element handling both of these tasks is often called the “band-width broker” (BB), and this convention is followed also within this book. Admission control or service quality instantiation control is not part of the basic DiffServ framework [RFC2475]. Subsequently, the need for IP service quality support mechanisms for DiffServ networks other than edge provisioning and per-hop prioritization mechanisms in core networks has been widely acknowledged [RFC2638, Sch98, THD +99, TAP +01, JC01, SAF + 01, TIPHON-3]. The basic reason for the need of dynamic per-domain service quality control mechanism is the following: up to a limit, service quality for critical service types can be provided by mapping them to a proper service quality support aggregate such as EF. This works because of the prioritization mechanism of DiffServ. Alas, when there is too much traffic within the EF PSC, the flows sharing the PHB begin to suffer. The effect of EF PHB to other PSCs within the router can be controlled by using rate limiters, but basic DiffServ framework does not provide tools for Implementing Service Quality in IP Networks Vilho Raisanen  2003 John Wiley & Sons, Ltd ISBN: 0-470-84793-X 252 MECHANISMS FOR DYNAMIC SERVICE QUALITY CONTROL dynamic control within the aggregate. Similarly, the quality in AF PSCs suffers from too many service instances trying to share them. The end-user SLAs at the edge of the network are imposed by policing of traffic at the edge of the DiffServ domain. As we saw in Chapter 3, policing requires that adequate DiffServ classifier be placed at the edge of the network. With statically configured DiffServ classifiers only, it is not possible to perform dynamical resource control. Basically, two techniques can be used to improve on this: • Admission control to existing classifiers. In this case, the classi-fier can be per endpoint IP address, per prefix, or per traffic aggregate. This requires that there is an extra-DiffServ means of blocking flows. • Installment of Diffserv classifiers on demand. For the latter alternative, using of outsourced policy model is an enabling technology from the control architecture point of view [LV02]. Obviously, in addition to the machinery to distribute the classifiers, there has to be a policy existing as to when to limit service quality instantiation and intelligence to create the actual DiffServ classifiers. Per-domain Bandwidth Broker (BB) is a poten-tial tool for taking care of the former task, and for providing the data for the second task. Recent examples of application of band-width broker to manage the network resources in multi-service IP network domain can be found in [Lak02] and [MNV02]. In the former, the emphasis is on the algorithms used for controlling resources, whereas in the latter, also an architecture for manage-ment is needed. Between the domains, being limited solely to static SLAs limits the level of efficiency of network use that can be reached. This is basically due to statistical multiplexing of flows in traffic aggregates. If the SLA between two operator specifies that the delays and packet losses have to be within certain range, it may be beneficial for a transit network operator to make dimensioning based slightly on conservative multiplexing calculation. If the operator could indicate to the business parties of momentarily available extra capacity, this capacity could be utilized. This can be viewed as an enabling technology for the broker-type market models discussed shortly in Chapter 5. Another potential use of automated inter-domain communication between MECHANISMS FOR DYNAMIC SERVICE QUALITY CONTROL 253 bandwidth brokers is implementing a transport transit service, which performs admission control in the access domain solely based on inter-domain bandwidth availability information. This is one of the models discussed in end-to-end ETSI EP TIPHON service quality support model [TIPHON-3], and has been analysed in detail by Schelen [Sch98]. A bandwidth broker controlled DiffServ domain can also interface to a 3GPP domain with suitable interworking arrangements, as discussed in [MNV02]. The basic DiffServ framework is based on the edge provisioning principle, where policing and marking of traffic at the network edge are the determining components for per-flow quality sup-port. Using bandwidth brokers, per-flow service quality in Diff-Serv can be made more closely integrated with the resource avail-ability situation in the network domains relevant for the transport of individual service instances. The means that can be used to maintain up-to-date information about service quality support are discussed in Section 8.2.2. Policy-based management is one way of implementing control of service quality instantiation, others being discussed further below. On an architectural level, at least the following schemes are pos-sible to implement end-to-end support for service quality when multiple transport domains (ADs) are involved: • One or more service providers take care of providing trans-port resources for their own services, where transport resources may be under control of separate transport operators. The trans-port operators may obviously be used by other parties as well. The provision can be SLA-based. This is one of the scenarios analysed in the ETSI EP TIPHON QoS control reference model [TIPHON-3]. • Separateper-elemententitieshandledynamicservicequalitycon-trolbased ontransportservice qualitysupportresourceavailabil- ity. This is the bandwidth broker model analysed in [RFC2638, Sch98, THD99], to name but a few sources. • Hybrid model. In this approach, bandwidth brokers are used by providing up-to-date service quality support resource availabil- ity information to service providers. The two first models are illustrated in Figure 8.1 and Figure 8.2. Note that in the first case, the correspondence between service providers and transport domain does not need to be one-to-many. 254 MECHANISMS FOR DYNAMIC SERVICE QUALITY CONTROL Service domain Transport domain 1 End system Transport domain 2 End system Figure 8.1 An example of end-to-end provisioning on service layer Note: The media stream requiring service quality support is drawn in the thick line, signalling of end systems with service domain in the thin solid line, and interaction of service provider with transport domains in the dotted thin line BB BB Transport domain 1 End system Transport domain 2 End system Figure 8.2 An example of end-to-end provisioning performed by per-domain bandwidth brokers (BB) Note: The communication to bandwidth broker and between bandwidth brokers is drawn in solid thin lines, communication of BBs with transport domains in dotted thin lines, and user traffic in the thick black line Regarding the two basic scenarios, the present chapter is primar-ily concerned with service-independent resource management. As was mentioned in the previous chapter, it is also possible to make a link between bandwidth broker-type entity and service admission control. We shall discuss this topic further in Section 8.2.4. Let us next have a look at existing studies on bandwidth bro-kers, and then formulate a generic framework for application of bandwidth broker in the context of multi-service DiffServ network domains. The present chapter will be concluded with a summary. 8.1 PREVIOUS STUDIES Other bandwidth broker architecture models have been studied in the TEQUILA project [TAP +01], the AQUILA project [MNV02], 8.1 PREVIOUS STUDIES 255 the GARA Application Programming Interface (API) model [SAF +01] and other sources [JC01], to give examples. The technology repertoire analysed in these studies includes SLS negotiation, DiffServ, and MPLS. For the purposes of the present book, we will concentrate on intra-domain and inter-domain resource management in DiffServ specifically. MPLS is considered to be a traffic engineering technique that is used by the policy management to optimize the resource usage within a network domain, and as such is not directly related to bandwidth brokers. The information supplied by the bandwidth broker, of course, can be used by the policy management system as input in optimizing the domain resources. In what follows, the IETF two-bit DiffServ architecture, the QBone bandwidth broker architecture, and Schelen’s funnel concept are discussed. 8.1.1 Two-bit DiffServ architecture The “two-bit” DiffServ architecture described in [RFC2638] has been designed to support two service models, namely the “assured service” and “premium service”. The two-bit architecture basically accommodates both EF PHB and AF PHB groups within the same DiffServ network domain. The assured service, akin to the AF PHB group, provides statistical guarantees for in-profile traffic. The second service model, EF-like, is based on prioritization of “premium” flows with respect to other traffic in the network. In accord with the DiffServ framework, both service models make use of traffic conditioning at the edge of the domain. Thus, the two services are not in contrast with the AF and EF PHBs reviewed in earlier chapters. Implementing both services requires configuration of appropri-ate filters at the edge of the network to correspond to the required services. Referring to a case of a company with multiple users, the authors of [RFC2638] propose a bandwidth broker as an entity into which organizational policies can be configured, and which can use these – together with the knowledge of existing connec-tions – to make classification decisions for service instantiations. The reference model is based on requesting the decision for new flows from the bandwidth broker. Thefunctionofabandwidthbroker – asdefinedin[RFC2638] – is twofold: ... - tailieumienphi.vn