QoS Routing Mechanisms and OSPF Extensions Roch A. Gu´ erin, Ariel Orda, and Douglas Williams Abstract—This paper presents and discusses path selection algorithms to support QoS routes in IP networks. The work is carried out in the con- text of extensions to the OSPF protocol, and the initial focus is on unicast flows, although some of the proposed extensions are also applicable to mul- ticast flows. We first review the metrics required to support QoS, and then present and compare several path selection algorithms, which represent dif- ferent trade-offs between accuracy and computational complexity. We also describe and discuss the associated link advertisement mechanisms, and investigate some options in balancing the requirements for accurate and timely information with the associated control overhead. The overall goal of this study is to identify a framework and possible approaches to allow deployment of QoS routing capabilities with the minimum possible impact to the existing routing infrastructure. Keywords—Routing, QoS, OSPF, Bandwidth, Delay. I. I NTRODUCTION This paper deals with QoS routing, however rather than dis- cussing its generic issues at large, it focuses on a concrete pro- posal, based on top of an existing routing protocol. Specifically, we describe a set of proposed additions to the OSPF routing pro- tocol 1 to support Quality-of-Service (QoS) routing in IP. Focus- ing on unicast flows, we discuss the metrics required to support QoS, the associated link advertisement mechanisms, the path se- lection algorithm, as well as aspects of route establishment. Our goal is to define an approach which, while achieving the target of improving performance for QoS flows (likelihood to be routed on a path capable of providing the requested QoS), does so with the least possible impact on the existing OSPF protocol. Given the inherent complexity of QoS routing, achieving this goal ob- viously implies trading-off “optimality” for “simplicity”, but we believe this to be required in order to facilitate deployment of QoS routing capabilities. A. Why QoS-based Routing? The current routing protocols used in IP networks are typi- cally transparent to any particular Quality-of-Service (QoS) that different packets/flows may have. As a result, and although some protocols have “hooks,” routing decisions are currently made without any awareness of resources availability and re- quirements. This means that flows are often routed over paths that are unable to support their requirements, while alternate paths with sufficient resources are available. This may result in significant deterioration in performance, i.e., in terms of call blocking probability. The goal of QoS routing is to provide rout- ing algorithms that are capable of identifying such paths so as to satisfy the maximum possible number of flows with QoS re- quirements. In addition, such enhancements should be as syn- ergetic as possible with existing routing protocols, so as to fa- R. A. Gu´ erin is with the IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598. guerin@watson.ibm.com. A. Orda is with the Dept. Elec. Eng., Technion - I.I.T., Haifa, 32000 - IS- RAEL. ariel@ee.technion.ac.il. Part of this work was done while he was visiting the IBM T.J. Watson Research Center. D. Williams is with the IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598. dougw@watson.ibm.com. The additions are built on top of OSPF V2 [1]. cilitate their introduction. Accordingly, QoS routing in the con- text of the Internet has been gaining increasing attention in the recent years e.g., [2], [3]. In parallel, an Internet resource reser- vation protocol, RSVP [4], has been proposed, that enables ap- plications to request specific QoS guarantees across an Internet. However, RSVP has been careful to position itself as not being a routing protocol, and has left the issue of its interactions with QoS routing protocols for further research. B. Overall Framework We consider a network that supports both best-effort packets and packets with QoS guarantees. The way in which the net- work resources are split between the two classes is irrelevant to our proposal, except for the assumption that each QoS capable router in the network is able to dedicate some of its resources to satisfy the requirements of QoS packets. QoS capable routers are also assumed able to identify and advertise the amount of their resources that remain available for additional QoS flows. In addition, we limit ourselves to the case where all the routers involved support the QoS extensions described in this proposal, i.e., we do not consider the problem of establishing a route in an heterogeneous environment with routers that are QoS-capable and others that are not. Finally, as mentioned before, we focus on unicast flows, although many of the additions we define are applicable to multicast flows as well. We assume that a flow with QoS requirements specifies them in some fashion accessible to the routing protocol. For example, this could correspond to the arrival of an RSVP PATH message, whose TSpec is passed to routing together with the destination address. After processing such a request, the routing protocol returns a path that it deems the most suitable given the flow’s requirements. Depending on the scope of the path selection pro- cess, this returned path could range from simply identifying the best next hop, i.e., a traditional hop-by-hop routing, to spec- ifying all intermediate nodes to the destination, i.e., a source route. Note that this decision impacts the operation of the path selection algorithm as it translates into different requirements to construct and return the appropriate path information. Once a suitable path has been identified, the flow is assigned to it (pin- ning) and remains assigned to it until it either releases the path (unpinning) or deems that it has become unsuitable. The focus of this paper is the selection of an appropriate path based on link metrics information and flow requirements. Ob- viously, a complete proposal for QoS routing requires the spec- ification of many other aspects, as mentioned above, and the reader is referred to [5], [6] for more details. C. Simplifying Assumptions In order to achieve our goal of minimum impact to the exist- ing routing infrastructure, we impose certain restrictions on the range of requirements the QoS path selection algorithm needs to deal with directly. Specifically, the path selection algorithm