A Case for Relative Differentiated Services and the Proportional Differentiation Model

Internet applications and users have very diverse quality of service expectations, making the same-service-to-all model of the current Internet inodequate and limiting. There i s a widespread consensus today that the Internet architecture has to be extended with service differentiation mechanisms so that certain users and applications can get better service than others at a higher cost. One approach, referred to as absolute differentiated services, is based on sophisticated admission control and resource reservation mechanisms in order to provide guarantees or statistical assurances for absolute performance measures, such as a minimum service rate or maximum end-to-end de lay . Another approach, which i s simpler in terms of implementation, deployment, and network manageability, is to offer relative differentiated services between a small number of service classes. These classes are ordered based on their pocket forwording quality, in terms of per-hop metrics for the queuing delays and packet losses, giving the assurance that higher classes are better than lower classes. The applications and users, in this context, can d namipriori guarantees for the actual performance level of each cfass. The relative differentiation approach can be further refined and quantified using the proportional differentiation model. This model aims to provide the network operator with the “tuning knobs” for adjusting the quality spacing between classes, independent of the class loads. When this spacin i s feasible in short timescales, it can lead to redictable and controllable class jifferentiation, which are two important features E r any relative differentiation model. The proportional differentiation model can be approximated in practice with simple forwardin mechanisms (packet scheduling and buffer management) that we briefly describetere. cally select the class that best meets their quality and pricin constraints, wit I out a he Internet i s being uscd by business and user communities with widely varicd scrvicc cxpcctations from the network infrastructure. For example, many companies rcly on thc Intcrnct for the day-to-day management of their global enterprise. These companics are willing to pay a substantially higher cost for the best possible service level from the Internet. Similarly, there are many uscrs who are willing to pay a higlicr Internet access fee in order to make u s e o f demanding applications, such as IP telephony and vidcoconfcrcncing. A t the same time, there are mill ions of users who want to pay as l i t t le as possible for more elemcntary scrviccs, likc cxchanging e-mails and/or surfing the Web. In addition to this variety of user expectations, there has also been a rapid evolution in thc set of Intcrnct applications. A few years ago the key Intcrnct applications wcrc only e-mail, ftp, or newsgroups. I n contrast, the present-day Internet applications have widely diversc servicc necds because thcy transfcr a wide range of information types, including voice, music, video, graphics, Java scripts, and hypcrtcxt links. As a result o f these changes in user expectations and I i i tc rnet applications, there i s a growing demand to replace the current same-seivice-to-all paradigm with a model in which users, applications, or individual packets are differentiated based on thcir service needs. Architectures for providing service differentiation in the lntcrnet have been the focus of extensive research in thc last few years. Thesc rcscarch efforts have identified two fundamentally different approaches for service differentiation: integrated services and diflerentiated services. The lntegrated Services Approach The integrated services (IntServ) approach [l] focuses on individual packct flows, that is, streams of IP packets between end hosts and applications which have thc samc sourcc and desti26 0~90-xn~4~9~/$tn,nn o 1999 IEEE IEEE Nctwork SeptemhcdOctoher 1999 nat ion addresscs , t he samc TCPiUDP port numbers, and the same protocol field. In this approach, cach flow can request specific levcls of service from the network. The lcvcls of service arc typically quantified as a minimum service ratc, or ii maximum tolerablc ctid-to-end dclay o r loss ratc. The network grants o r rcjccts the flow rcquests, bascd on availability of resourccs and the guarantees provided to othcr flows. The three major componenls of t h e IntScrv archi tccturc a r e the adniission control unit , which checks if the network can grant the scrvice requcst; the packet fonvarding mechanisms, which pcrform the per-packet operations of flow classification, shaping, schcduling, and buffer managcmcnt in the routers; and the Resource Reservation Protocol (RSVP), which sets up some flow .state (e.g. bandwidth reservat ions, f i l tcrs, accounting) in thc routers a flow gocs through. T h c IiitServ approach is bascd on a solid background of rcscarch in quality of service mcchanisms and protocols f o r packe t networks [2-41. However, thc acccptance of IntServ f rom network providers and router vendors has been quitc limited, a t lcast so far, mainly due Type of service dlfferentiation