Delivering end-to-end statistical QoS guarantees for expedited forwarding M. Listanti a , F. Ricciato a,b, * , S. Salsano b a Infocom Department, University of Rome, La Sapienza, Via Eudossiana 18, 00184 Rome, Italy b CoRiTeL, Via di Tor Vergata 135, 00133 Rome, Italy Received 6 November 2000; accepted 6 November 2000 Abstract This paper presents an admission control framework for Expedited Forwarding EF) traf®c in a Differentiated Service Network. The aim is to overcome the limitations, in terms of achievable ef®ciency, which are proper of a deterministic ªworst-caseº approach based on the zero- loss assumption. An admission control procedure is de®ned which provides quanti®able end-to-end QoS guarantees in terms of maximum delay and per-¯ow loss probability. The admission control scheme relies on the analytical derivation of a bound for the per-¯ow loss probability at a generic network node. The analytical approach is based on the insertion of a discarding device before the EF queue. The purpose of the dropper is to discard packets in order to avoid con¯icts at burst scale in the queue, and allows for simple analytical handling of the per-¯ow loss process. The degradation of the statistical characteristics of the ¯ow along its path are taken into account. Finally, a comparison between analytical bounds and actual performance results obtained by simulations is presented. The results show that the requested QoS targets are largely met and that the achievable ef®ciency is much higher than that derived from the worst-case allocation. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Quality-of-Service; Flow admission control 1. Introduction At present a growing number of Internet applications require some kind of Quality-of-Service QoS) guarantees, such as delay constraints or low packet loss. In the frame- work of the actual Internet architecture only the best effort service can be provided, so that additional mechanisms have to be de®ned in order to bring QoS into the Internet [5]. In this context, the Differentiated Services DiffServ) model is under study within the IETF [1,2]. The DiffServ model is aimed at providing QoS on a per-aggregate basis: a limited set of service classes called PHB in the DiffServ terminol- ogy) is supported, each one being associated to a speci®c QoS level. The internal routers handle packets according to a PHB identi®er DS code), and do not distinguish the indi- vidual ¯ows. Then resources are allocated on a per-class basis. In the DiffServ model the functional complexity is mostly located at the network edge, where traf®c control functions have to be enforced as admission control, traf®c policing and/or shaping, packets marking, etc. The set of PHB to be implemented is currently under discussion, at present only two PHB have been de®ned: Expedited Forwarding EF) [3] and Assured Forwarding AF) [4]. This paper deals with EF. The EF PHB is intended for supporting real-time applications and in general applications with stringent requirements in terms of delay and jitter and a strictly controlled peak emission rate. It is also to be used to provide the so-called Virtual Leased Line VLL) service. In other words, it should be considered as the `top' traf®c class. Inside the network, the packets marked EF should be served with higher priority over non-EF ones, so that competition versus lower classes is minimal. On the other hand, in order to deliver the target QoS, the network manager has to pro-actively control the input traf®c by means of an opportune ¯ow admission control FAC) enforced by policing/shaping actions at the ingress point. De®ning a suitable FAC scheme for EF is an actual major topic of interest in the DiffServ area: it should be able to ensure the target end-to-end QoS to the admitted ¯ows, but at the same time it should avoid unnecessary under-loading of the network. Any QoS driven admission control relies on the possibility of effectively evaluating the involved QoS performance parameters given the network load state, whose characterization depends on the adopted input traf®c parameters. Note that evaluating end-to-end QoS perfor- mance parameters in a multistage network is generally a more challenging task than doing the same at a single Computer Communications 24 2001) 822±832 www.elsevier.com/locate/comcom 0140-3664/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0140-366400)00355-8 * Correspondingauthor.Tel.: 139-06-4785-2302;fax: 139-06-4890-6114. E-mail addresses: ricciato@coritel.it F. Ricciato), salsano@coritel.it S. Salsano).