Ubiquitous availability of web-based communications and presence information through portable data and GPS enabled mobile phones makes it possible to envision a system for dynamic carpooling to enhance flexibility in making carpool arrangements successful. However, a critical challenge is to design an algorithm that appropriately matches all travelers (riders and drivers). This paper introduces a dynamic ridesharing system that seeks to locate and optimize as many matches as possible, and then rank them by quality. This is a work-in-progress and current efforts address globally matching travelers so as to minimize the number of passengers without rides.

It has been claimed that multicast state cannot be aggregated. In this paper, we will debunk this myth and present a simple technique that can be used to aggregate multicast forwarding state. In particular, we present an interface-centric data structure model which allows aggregation of ranges of multicast addresses in the forwarding table. Understanding the limits of possible aggregation is critical to our knowledge of how IP multicast will scale as it becomes widely deployed. We show through analysis and simulation that some aggregation is possible, even under purely random address allocation and purely random group membership distribution. We further show how other methods of allocation can significantly improve the ability to aggregate, and how non-random distributions of membership can affect aggregation both positively and negatively.

Current multicast routing protocols can be classified into three types according to how the multicast tree is established: broadcast and prune (e.g., DVMRP, PIM-DM), membership advertisement (e.g., MO-SPF), and rendezvous-based (e.g., CBT, PIM-SM). Rendezvous-based protocols associate with each logical multicast group address, a physical unicast address, referred to as the 'core' or 'rendezvous point' (RP). Members first join a multicast tree rooted at this rendezvous point in order to receive data packets sent to the group. Rendezvous mechanisms are well suited to large wide-area networks because they distribute group-specific data and membership information only to those routers that are on the multicast distribution tree. However, rendezvous protocols require a bootstrap mechanism to map each logical multicast address to its current physical rendezvous point address. The bootstrap mechanism must adapt to network and router failures but should minimize unnecessary changes in the group-to-RP mapping. In addition, the bootstrap mechanism should be transparent to the hosts. This paper describes and analyzes the bootstrap mechanism developed for PIM-SM. The mechanism employs an algorithmic mapping of multicast group to rendezvous point address, based on a set of available RPs distributed throughout a multicast domain. The primary evaluation measures are convergence time, message distribution overhead, balanced assignment of groups to RPs, and host impact. The mechanism as a whole, and the design lessons in particular, are applicable to other rendezvous-based multicast routing protocols as well.

Hierarchies provide scalability in large networks and are integral to many widely-used protocols and applications. Previous approaches to constructing hierarchies have typically either assumed static hierarchy configuration, or have used bottom-up construction methods. We describe how to construct hierarchies in a top-down fashion, and show that our method is much more efficient than bottom-up methods. We also show that top-down hierarchy construction is a better choice when administrative policy constraints are imposed on hierarchy formation.

Multicast routing enables efficient data distribution to multiple recipients. However, existing work has concentrated on extending single-domain techniques to wide-area networks, rather than providing mechanisms to realize inter-domain multicast on a global scale in the Internet. We describe an architecture for inter-domain multicast routing that consists of two complementary protocols. The Multicast Address-Set Claim (MASC) protocol forms the basis for a hierarchical address allocation architecture. It dynamically allocates to domains multicast address ranges from which groups initiated in the domain get their multicast addresses. The Border-Gateway Multicast Protocol (BGMP), run by the border routers of a domain, constructs inter-domain bidirectional shared trees, while allowing any existing multicast routing protocol to be used within individual domains. The resulting shared tree for a group is rooted at the domain whose address range covers the group's address; this domain is typically the group initiator's domain. We demonstrate the feasibility and performance of these complementary protocols through simulation. This architecture, together with existing protocols operating within each domain, is intended as a framework in which to solve the problems facing the current multicast addressing and routing infrastructure.

Clusters of identical intermediate servers are often created to improve availability and robustness in many domains. The use of proxy servers for the World Wide Web (WWW) and of rendezvous points in multicast routing are two such situations. However, this approach can be inefficient if identical requests are received and processed by multiple servers. We present an analysis of this problem, and develop a method called the highest random weight (HRW) mapping that eliminates these difficulties. Given an object name and a set of servers, HRW maps a request to a server using the object name, rather than any a priori knowledge of server states. Since HRW always maps a given object name to the same server within a given cluster, it may be used locally at client sites to achieve consensus on object-server mappings. We present an analysis of HRW and validate it with simulation results showing that it gives faster service times than traditional request allocation schemes such as round-robin or least-loaded, and adapts well to changes in the set of servers. HRW is particularly applicable to domains in which there are a large number of requestable objects, there is a significant probability that a requested object will be requested again, and the CPU load due to any single object can be handled by a single server. HRW has now been adopted by the multicast routing protocols PIMv2 and CBTv2 as its mechanism for routers to identify rendezvous points/cores.

We explore the constraints of a new problem: that of coordinating network troubleshooting among peer administrative domains and untrusted observers. Allowing untrusted observers permits any entity to report problems, whether it is a network operations center (NOC), end-user, or application. Our goals are to define the inter-domain coordination problem clearly, and to develop an architecture which allows observers to report problems and receive timely feedback, regardless of their own locations and identities. By automating this process, we also relieve human bottlenecks at help desks and NOCs whenever possible. We present a doubleshooting methodology for coordinating problem diagnosis, and describe GDT, a distributed protocol which realizes this methodology.

Recent multicast routing protocol proposals such as protocol independent multicast (PIM) and core-based trees (CBT) have been based on the notion of group-shared trees. Since construction of a minimal-cost tree spanning for all members of a group is difficult, they rely on center-based trees and distribute packets from all sources over a single shortest-path tree rooted at some center. PIM and CBT provisionally use administrative selection or simple heuristics for locating the center of a group but do not preclude the use of other methods that provide an ordered list of centers. Other previously proposed heuristics typically require knowledge of the complete network topology, a requirement which is not always practical for a distributed problem such as Internet routing. We investigate the problem of finding a good center in a distributed fashion, study various heuristics for automating center selection, and examine their applicability to real-world networks. We also propose several new algorithms which we feel to be more practical than existing methods. We present simulation results on hierarchical and nonhierarchical networks showing that of the methods potentially feasible in the Internet multicast backbone, ours offer the best results in terms of cost and delay, and they incur low overhead.

Multicast routing protocol proposals such as PIM and CBT have been based on the notion of group-shared trees. Since the construction of a minimal-cost tree spanning all members of a group is difficult, they rely on center-based trees, and distribute packets from all sources over a single shortest-path tree rooted at some center. While PIM and CBT provisionally use administrative selection of centers or trivial heuristics for locating the center of a group, they do not preclude the use of other methods as long as they provide an ordered list of centers. Other previously proposed heuristics typically require knowledge of the complete network topology, a requirement which is not always practical for a distributed problem such as Internet routing. In this paper we investigate the problem of finding a good center in distributed fashion, study various heuristics for automating center selection, and examine their applicability to real-world networks. We also propose several new algorithms which we feel to be more practical than existing methods. We present simulation results showing that of the methods potentially feasible in the Internet multicast backbone, ours offer the best results in terms of cost and delay.