Kannan Varadhan

Advances in network simulation

Network researchers must test Internet protocols under varied conditions to determine whether they are robust and reliable. The paper discusses the Virtual Inter Network Testbed (VINT) project which has enhanced its network simulator and related software to provide several practical innovations that broaden the conditions under which researchers can evaluate network protocols.

Persistent Route Oscillations in Inter-Domain Routing

Hop-by-hop inter-domain routing protocols, such as border gateway protocol (BGP) and inter-domain routing protocol (IDRP), use independent route selection to realize domains’ local policies. A domain chooses its routes based on path attributes present in a route. It is widely believed that these inter-domain routing protocols always converge. We show that there exist domain policies that cause BGP/IDRP to exhibit persistent oscillations. In these oscillations, each domain repeatedly chooses a sequence of routes to a destination. Complex oscillation patterns can occur even in very simple topologies. We analyze the conditions for persistent route oscillations in a simple class of inter-domain topologies and policies. Using this analysis, we evaluate ways to prevent or avoid persistent oscillations in general topologies. We conclude that if a hop-by-hop inter-domain routing protocol allows unconstrained route selection at a domain, the protocol may be susceptible to route oscillations. Constraining route selection to a provably ‘‘safe’’ procedure (such as shortest path) can reduce the number of realizable policies. Alternatively, a routing policy registry can help detect unsafe policies. ” 2000 Elsevier Science B.V. All rights reserved.

IP-based access network infrastructure for next-generation wireless data networks

Next-generation wireless network standards are currently being defined. The access network architectures have several specialized components tailored for their respective wireless link technologies, even though the services provided by these different wireless networks are fairly similar. We propose a homogeneous IP-based network as a common access network for the different wireless technologies. The IP-based access network uses the Internet standard, Mobile IP, to support macro-mobility of mobile hosts, and HAWAII to support micro-mobility and paging functionality of current wireless networks. We also illustrate how the proposed IP-based solution can interwork with existing infrastructure so that deployment can be incremental.

Route servers for inter-domain routing

Internet transmission and switching facilities are partitioned into different administrative domains. To effect routing between domains, domain border routers establish pairwise peering sessions and exchange routing information at exchange points. An alternative arrangement, in which each border router at an exchange point peers only with a Route Ser˝er, provides some operational benefits. From a set-theoretic characterization of border router behavior, we derive a set-theoretic expression that completely and succinctly characterizes Route Server functionality. Performance measurements from our Route Server implementation reveal that the storage requirements of Route Servers can be much larger than that of a typical border router. We discuss a variety of techniques that can, in some cases, reduce Route Server storage requirements by a factor of five or more. q 1998 Elsevier Science B.V. All rights reserved.

Impact of network dynamics on end-to-end protocols: case studies in reliable multicast

End-to-end protocols measure network characteristics and react based on their estimates of network performance. Network dynamics can alter the topology significantly, and thereby affect protocol operation. Topology changes may result in routing pathologies (such as route loops, packet interleaving), changes to the end-to-end path characteristics, network partition etc., that then impact the performance of end-to-end protocols. This paper presents methodologies to evaluate an end-to-end protocol in the presence of network dynamics using a simulator. We evaluate a reliable multicast transport protocol over dynamic topologies and study its adaptivity to topology change. We present a systematic evaluation of the adaptive timer mechanisms in scalable reliable multicast (SRM). The timer mechanisms are evaluated under simple topology changes, as well as under network partition conditions. The paper concludes by posing a number of open research questions about the behaviour of different reliable multicast mechanisms when operating over dynamic topologies.