A New Path Probing Strategy for Inter-domain Multicast Routing Ant´ onio Costa , Maria Jo˜ ao Nicolau , Alexandre Santos and Vasco Freitas Departamento de Inform´ atica, Universidade do Minho, Campus de Gualtar, 4710 Braga, Portugal Email: costa,alex,vf @uminho.pt Departamento de Sistemas de Informac ¸˜ ao, Universidade do Minho, Campus de Azur´ em, 4800 Guimar˜ aes, Portugal Email: joao@uminho.pt Abstract— Many already in use applications require the provision of QoS services from the underlying network infra-structure. This is particularly true for multicast, since it involves many participants at very sparse locations usually aiming to receive or send multimedia real-time streams. One way to provide QoS is through routing, since QoS aware multicast routing protocols can find feasible multicast trees. At large scale, scalability issues make the QoS multicast routing task a lot more difficult, since it is not possible to have a complete knowledge of the network topology and its path QoS metrics in a clear up-to-date way. The most promising proposals are therefore based on path probing strategies that evaluate a subset of the available connecting paths. Assuming that each member can express its require- ments as a combination of QoS metrics like available bandwidth, end-to-end loss probability and delay, it is up to QoS routing multicast protocols to build distribution trees connecting members through paths that can satisfy those requirements. In this paper an inter-domain QoS multicast routing protocol is presented, specifically designed for the hier- archical inter-domain scenario, where requirements like intra-domain independency and policy awareness should be met. Emphasis is given to the path probing mechanism used to connect new members to the multicast tree, stressing how it differs from others. Simulation results show that despite using a less aggres- sive and simplified probing mechanism - more suitable for inter-domain scenarios - the proposed routing strategy can build multicast trees with metrics similar to those build by more aggressive technics, with considerable less effort. I. I NTRODUCTION To support group applications, such as video- conference, distance learning and cooperative work ap- plications, multicast support is needed from the un- derlying network. Multicast routing protocols build a distribution tree (or a set of trees) to deliver data packets from sources to all receivers. When the network supports Quality of Service (QoS), applications specify their QoS requirements as a function of QoS metrics like delay, bandwidth or loss probability. In this case, multicast routing protocols should be modified in order to find feasible distribution trees. Such trees are distribution multicast trees which have enough resources to satisfy the QoS requirements specified by the applications. The problem of finding feasible trees is very difficult to treat due to different reasons. First the dynamism of metrics like bandwidth, delay or loss probability is extremely high, which turns almost impossible to maintain the accuracy of the network state information. Second, the communication overhead of such metrics as well as the large amount of state information that is necessary to maintain turn the use of link state protocols too expensive. Another approach is a method called ”path probing” that do not require the maintenance of global sate infor- mation in the nodes. Beyond the path probing strategy the path searching process is initiated by the new receiver which explores different alternative paths and evaluates them in terms of how well do they fulfill its requirements. We believe that this type of strategy is better suitable for large scale networks in particular to implement inter- domain QoS multicast routing.