Thực hiện chất lượng dịch vụ trong các mạng IP (P3)

3 Network Mechanisms for Multi-service Quality Support This chapter deals with service quality support mechanisms in the network. Of particular interest are mechanisms inside an Inter-net Protocol domain, including edge treatment and service qual-ity support mechanisms in the network core. Requirements for signalling between the endpoint and the network edge will be discussed in Chapter 5. The Internet today is based on the Best Effort (BE) service para-digm in which all IP traffic on a network link is treated alike, or in other words, no service quality support is provided when momentary traffic volume exceeds link capacity. Subsequently, to offer true multi-service support for critical traffic types such as VoIP in an IP-based network, the technical tasks to be carried out by an IP service provider seem challenging at first sight. Not only must mechanisms be provided for implementing the network sup-port for different service types, but also mechanisms are needed to map service requirements onto network resources. To make best use of network resources, the service mapping scheme may need to be revisited when resources or distribution of services change. The whole service quality support system should be manageable in a scalable way. Implementing Service Quality in IP Networks Vilho Raisanen  2003 John Wiley & Sons, Ltd ISBN: 0-470-84793-X 54 NETWORK MECHANISMS FOR MULTI-SERVICE QUALITY SUPPORT It turns out that implementation of multi-service quality support in an IP domain is possible and manageable. Moreover, this sup-port can be provided with a mechanism allowing the network element in the core of the network to be kept simple. Despite technical feasibility, multi-service support in IP network is not likely to be immediately adopted by all ISPs and other net-work operators. However, the ability to provide better-than-best effort service will become an important business proposition due to the progress of content digitalization to include also non-data type services. In implementating such support cost-efficiently, IP-based service quality support mechanisms are a viable alternative, as we shall see. In what follows, we shall discuss the basic multi-service quality support mechanisms and ways of implementing them with present-day technology in a cost-efficient way. Further, service level description mechanisms needed at the boundaries of operators’ domains are accounted for in this and subsequent chapters. In this chapter, we shall first briefly review issues relevant to network multi-service quality support, and discuss policing at the edge of the network and the effect of different protocol layers to service quality. Next, the generic ways of supporting service quality are reviewed, followed by a summary of service quality support means in ATM and IP. Routing control in IP networks and link layer service quality issues are discussed next, and the present chapter concludes with a summary. This chapter attempts to demonstrate that the building blocks for service quality support mostly exists already today without ATM. ATM-based service quality support is used as a point of reference in this chapter. Management of IP-based service quality will be discussed further in the next chapter, and the provision of network resources in Chapter 5. Routing control beyond the basic operation of IP routing protocols will be discussed in this chapter, and further in next chapter. 3.1 INTRODUCTION TO NETWORK QUALITY SUPPORT The adoption of dedicated multi-service quality support mecha-nisms in the network is only necessary when the following two conditions are met: 3.1 INTRODUCTION TO NETWORK QUALITY SUPPORT 55 1. Over-dimensioning of the network is not a feasible solution. 2. Providing of engineered service quality level for some service traffic types transported in the multi-service network such as low delay and/or packet loss rate is desirable. Above, over-dimensioning means designing the network in such a way that the network is at all times capable of transmitting the momentary traffic volumes so that the service quality require-ments of transported streams are satisfied. The latter condition amounts to delay requirement of all traffic being determined by the most delay-critical and loss-intolerant service type. An application example is in a corporate access network carry-ing Voice over IP, sharing the access network capacity with bursty HTTP traffic and Simple Mail Transfer Protocol (SMTP) traffic. It may not be economically feasible to dimension the network to handle the largest possible bandwidth bursts. This would neces-sitate providing the same delay to transport of HTTP and SMTP than VoIP. If over-provisioning is not a feasible alternative, either a sepa-rate capacity is provided for VoIP, or a prioritization mechanism needs to be built into the actual network transport to provide differentiated handling for distinct service quality classes. These main alternatives benefit from further support mechanisms, the most important of which are discussed below. Before that, how-ever, let us note that service quality support may be needed even with a single service quality type. An example of this could be providing pre-defined performance for browsing traffic. In such a case, a suitable subset of the service quality support machinery reviewed below could be used. Let us now discuss service quality support on a more general level. Discussing the bandwidth of a single link for concreteness, there are basically two alternatives to over-dimensioning in pro-viding service quality support in the presence of multiple service types: capacity reservation and differentiated treatment. Capac-ity reservation means that a part of the total capacity of a link is reserved solely for one or more traffic types. The remaining capac-ity can be used for implementing other reservations, or be shared by services on a best effort basis. Differentiated treatment, on the other hand, means not reserving any fixed capacity, but priori-tizing service types with respect to each other when momentary 56 NETWORK MECHANISMS FOR MULTI-SERVICE QUALITY SUPPORT 6 5 Urgent traffic 4 Non-urgent traffic 3 Reservation 2 Differentiation 1 Sum 0 1 6 11 16 21 26 31 36 41 46 Time Figure 3.1 The benefits of prioritization Note: Unit in the vertical axis is the maximum required capacity of urgent traffic offered traffic exceeds link capacity. In the prioritization approach, less traffic can be displaced in time to make room for momentary variations in the volume of more urgent traffic. Let us next compare the over-dimensioning approach against capacity reservation and differentiated treatment using a case study. Figure 3.1 below shows a case in which the average volume of non-urgent traffic is fivefold as compared to the volume of urgent traffic. Over-dimensioning according to the peak value would require capacity C of 6 units. Capacity reservation for urgent traffic would mean putting aside 1 unit of capacity, and dimensioning separate capacity according to the average volume of non-urgent traffic, leading to total capacity requirement C of 1 + 2.5 = 3.5 units. Finally, implementing prioritization mechanism in the network leads to dimensioning being based on the average volume of all traffic, i.e., approximately capacity of 3 units. In this case, differentiation based on urgency brings capacity-saving benefits of 50% compared to over-dimensioning, and more than 14% based on capacity reservation for non-urgent traffic. This requires that non-urgent traffic can be delayed. The above simple calculation was based on a “fluid flow” approximation, not taking into account discreteness of data. An exact analysis of the process requires application of queueing theory, potentially yielding corrections to the simple case when the size of largest packets is not insignificant with respect to total link capacity. The fact that packet length can vary gives rise to variation in the service time of a packet, causing variable amount of delay to the subsequent packets. 3.1 INTRODUCTION TO NETWORK QUALITY SUPPORT 57 A further advantage of multiplexing is that the relative burst-iness of a traffic aggregates is considered to decrease with the num-ber of flows, reducing the effects due to fractal traffic phenomena, for example [CCL +02]. As will be discussed in Section 3.2, engi-neering of network for bursty flows benefits from edge mecha-nisms. In general, the saving calculation figures will depend on the percentage of traffic types, which need priority treatment. If all of the traffic needs priority treatment and is delay-sensitive, over-dimensioning is the only alternative. As mentioned above, appli-cation of some service quality control means may be beneficial also in this case from the point of view of end user experienced qual-ity. At the other end of the spectrum, when priority traffic with variable bandwidth is sharing the link with bursty non-urgent traffic and the maximum volume of priority traffic is less than half of the link capacity, savings can be considerable. In what fol-lows, it is assumed that when service quality support is feasible, purely prioritization-based service quality support scheme is used to maximize utilization of installed capacity. The dimensioning of capacity reservations is a well-established discipline, and more information about that can be found in [McD00], for example. The above analysis calls our attention to the following important issues in considering the usefulness of differentiated treatment in the network: • Are savings from implementation of differentiated treatment high enough? Implementing support for differentiated treatment in the network makes the elements more complex and poses requirements for management system. • Can differences in service delivery time requirements be leveraged to implement differentiation? If the delay requirements of traffic aggregates are too close to each other, delay differentiation may not be practical. • Are relative volume shares of services known? Traffic volumes need to be known or limits need to be imposed to maintain per-node differentiation. It should be noted that relative volumes need to be computable in all routers which implement prioritization. • Are absolute volumes of traffic aggregates predictable? Even with differentiation, range of variation of different traffic types needs to be known. ... - tailieumienphi.vn