Abstract — The conventional wisdom has been that IP is the natural protocol layer for implementing multicast related functionality. However, more than a decade after its initial proposal, IP Multicast is still plagued with concerns pertaining to scalability, network management, deployment and support for higher layer functionality such as error, flow and congestion control. In this paper, we explore an alternative architecture that we term End System Multicast, where end systems implement all multicast related functionality including membership management and packet replication. This shifting of multicast support from routers to end systems has the potential to address most problems associated with IP Multicast. However, the key concern is the performance penalty associated with such a model. In particular, End System Multicast introduces duplicate packets on physical links and incurs larger end-to-end delays than IP Multicast. In this paper, we study these performance concerns in the context of the Narada protocol. In Narada, end systems selforganize into an overlay structure using a fully distributed protocol. Further, end systems attempt to optimize the efficiency of the overlay by adapting to network dynamics and by considering application level performance. We present details of Narada and evaluate it using both simulation and Internet experiments. Our results indicate that the performance penalties are low both from the application and the network perspectives. We believe the potential benefits of transferring multicast functionality from end systems to routers significantly outweigh the performance penalty incurred. I.

The IP multicast model allows scalable and efficient multi-party communication, particularly for groups of large size. However, deployment of IP multicast requires substantial infrastructure modifications and is hampered by a host of unresolved open problems. To circumvent this situation, we have designed and implemented ALMI, an application level group communication middleware, which allows accelerated application deployment and simplified network configuration, without the need of network infrastructure support. ALMI is tailored toward support of multicast groups of relatively small size (several I Os of members) with many to many semantics. Session participants are connected via a vir- tual multicast tree, which consists of unicast connections between end hosts and is formed as a minimum spanning tree (MST) using application-specific performance metric. Using simulation, we show that the performance penalties, introduced by this shift of multicast to end systems, is a relatively small increase in traffic load and that ALMI multicast trees approach the efficiency of IP multicast trees. We have also implemented ALMi as a Java based middleware package and performed experiments over the Internet. Experimental results show that ALMI is able to cope with network dynamics and keep the mul- ticast tree efficient.

We investigate cost-sharing algorithms for multicast transmission. Economic considerations point to two distinct mechanisms, marginal cost and Shapley value, as the two solutions most appropriate in this context. We prove that the former has a natural algorithm that uses only two messages per link of the multicast tree, while we give evidence that the latter requires a quadratic total number of messages. We also show that the welfare value achieved by an optimal multicast tree is NP-hard to approximate within any constant factor, even for bounded-degree networks. The lower-bound proof for the Shapley value uses a novel algebraic technique for bounding from below the number of messages exchanged in a distributed computation; this technique may prove useful in other contexts as well. 1

IP multicast offers scalable point-to-multipoint delivery necessary for using group communication applications on the Internet. However, the IP multicast service has seen slow commercial deployment by ISPs and carriers. The original service model was designed without a clear understanding of commercial requirements or a robust implementation strategy. The very limited number of applications and the complexity of the architectural design — which we believe is a consequence of the open service model — have deterred widespread deployment as well. We examine the issues that have limited the commercial deployment of IP-multicast from the viewpoint of carriers. We analyze where the model fails, what it does not offer, and we discuss requirements for successful deployment of multicast services. 1

In the IP multicast model, a set of hosts can be aggregated into a group of hosts with one address, to which any host can send. However, Internet TV, distance learning, file distribution and other emerging large-scale multicast applications strain the current realization of this model, which lacks a basis for charging, lacks access control, and is difficult to scale. This paper proposes an extension to IP multicast to support the channel model of multicast and describes a specific realization called EXPlicitly REquested SingleSource (EXPRESS) multicast. In this model, a multicast channel has exactly one explicitly designated source, and zero or more channel subscribers. A single protocol supports both channel subscription and efficient collection of channel information such as subscriber count. We argue that EXPRESS addresses the aforementioned problems, justifying this multicast service model in the Internet.

While the advantages of multicast delivery over multiple unicast deliveries is undeniable, the deployment of the IP multicast protocol has been limited to "islands" of network domains under single administrative control. Deployment of inter-domain multicast delivery has been slow due to both technical and administrative reasons. In this paper we propose a Host Multicast Tree Protocol (HMTP) that (1) automates the interconnection of IP-multicast enabled islands and (2) provides multicast delivery to end hosts where IP multicast is not available. With HMTP, end-hosts and proxy gateways of IP multicast-enabled islands can dynamically create shared multicast trees across different islands. Members of an HMTP multicast group self-organize into an efficient, scalable and robust multicast tree. The tree structure is adjusted periodically to accommodate changes in group membership and network topology. Simulation results show that the multicast tree has low cost, and data delivered over it experiences moderately low latency. I.

Without a doubt, multicast communication---the one-to-manyormany-to-many delivery of data---has become a hot topic. It is of interest in the research community, among standards groups, and to network service providers. For all the attention multicast has received, there are still issues that have not been completely resolved. One result is that protocols are still evolving and some standards are not yet finished. From a deployment perspective, the lackof standards has slowed progress, but efforts to deploymulticast as an experimental service are in fact gaining momentum. The question nowishow long it will be before multicast becomes a true Internet service. The goal of this paper is to describe the past, present, and future of multicast.

We propose a new multicast protocol called REUNITE. The key idea of REUNITE is to use recursive unicast trees to implement multicast service. REUNITE does not use class D IP addresses. Instead, both group identification and data forwarding are based on unicast IP addresses. Compared with existing IP multicast protocols, REUNITE has several unique properties. First, only routers that are acting as multicast tree branching points for a group need to keep multicast forwarding state of the group. All other non-branching-point routers simply forward data packets by unicast routing. In addition, REUNITE can be incrementally deployed in the sense that it works even if only a subset of the routers implement the protocol. Furthermore, REUNITE supports load balancing and graceful degradation such that when a router does not have resources (forwarding table entry, buffer space, processing power) to support additional multicast groups, the branching can be automatically migrated to other less load...

The IP Multicast service model extends the traditional best effort Internet datagram delivery service for efficient multi-point packet delivery. However, in spite of a decade of research on multicast protocols and applications, a globally deployed multicast service is nowhere in sight, hindered by multitudes of problems such as manageability, lack of a robust inter-domain multicast routing protocol, scalability, and heterogeneity. In this work, we propose a new model for Internet multicast where we view multi-point delivery not as a network primitive but rather as an application-level infrastructure service. Our architecture relies on a collection of strategically placed network agents that collaboratively provides the multicast service for a session. Clients locate a nearby agent and tap into the session via that agent. Agents organize themselves into an overlay network of unicast connections and build data distribution trees on top of this overlay structure. This model effectively pa...

Multicast services have been increasingly used in large scale continuous media applications. The quality-of-service (QoS) requirements of these continuous media applications prompt the necessity for QoS-driven, constraint-based multicast routing. This article provides a comprehensive overview of existing multicast routing algorithms, protocols, and their QoS extension. In particular, we classify multicast routing problems according to their optimization functions and performance constraints, present basic routing algorithms in each problem class, and discuss their strengths and weakness. We also categorize existing multicast routing protocols, outline the issues and challenges in providing QoS in multicast routing, and point out possible future research directions.