Prior measurement studies of the Internet have explored traffic and topology, but have largely ignored edge hosts. While the number of Internet hosts is very large, and many are hidden behind firewalls or in private address space, there is much to be learned from examining the population of visible hosts, those with public unicast addresses that respond to messages. In this paper we introduce two new approaches to explore the visible Internet. Applying statistical population sampling, we use censuses to walk the entire Internet address space, and surveys to probe frequently a fraction of that space. We then use these tools to evaluate address usage, where we find that only 3.6% of allocated addresses are actually occupied by visible hosts, and that occupancy is unevenly distributed, with a quarter of responsive /24 address blocks (subnets) less than 5% full, and only 9% of blocks more than half full. We show about 34 million addresses are very stable and visible to our probes (about 16% of responsive addresses), and we project from this up to 60 million stable Internet-accessible computers. The remainder of allocated addresses are used intermittently, with a median occupancy of 81 minutes. Finally, we show that many firewalls are visible, measuring significant diversity in the distribution of firewalled block size. To our knowledge, we are the first to take a census of edge hosts in the visible Internet since 1982, to evaluate the accuracy of active probing for address census and survey, and to quantify these aspects of the Internet.

Optical burst switching presents challenges to the design of optical routers. This paper considers how to dimension a router of N input data ports with an additional M fiber delay lines (N+M internal ports) in a hop-and-span constrained network. The router incorporates tunable FDLs that can vary their size to fit the burst being buffered. Tunable fiber delays achieve up to 20% higher throughput than static fiber delays at high input port load. Multiple recirculations are a critical requirement; when packets can circulate only once through the buffer, no measurable improvement is achieved after the number of as FDLs becomes equal to the number of data ports. When recirculation is permitted, throughput increases by up to 40%, depending on a combination of the number of FDLs added and the recirculation limit, which must increase in tandem. For a given number of FDLs, there is an optimal recirculation limit beyond which there is no measurable throughput benefit.

Optical CDMA local area networks allow shared access to a broadcast medium. Every node is assigned an optical orthogonal codeword (OOC) to transmit or receive on. Optical CDMA systems have low throughput under moderate to heavy offered load due to interference between codewords. Interference sensing is a media access architecture where nodes on the network sense the amount of interference on the line before transmission. Nodes defer transmissions if there is interference on the line. This paper discusses and analyzes three algorithms for interference sensing. Through simulation it is shown that these algorithms reduce or eliminate throughput degradation at high loads. The study shows that simple algorithms such as selfish or threshold based algorithms are sufficient to eliminate throughput degradation.

Deflection routing can be used in networks whose stations have the same number of input and output links. Fixed length packets arrive synchronously on the station's input links at the beginning of time slots, and each packet is routed via the output link that offers the shortest path to its destination. Since the number of packet buffers at each output link is finite, the simultaneous contention of two packets for the last buffer of a common output link must be resolved by "deflecting" one of the packets to another output link. Thus, the deflection of a packet could result in the packet following a route that is not a shortest path. The potentially unbounded number of routes that a given packet can take makes analyzing the performance of such networks difficult. In particular, there are no analytical models that can analyze multibuffer deflection-routing networks with nonuniform traffic. Using independence assumptions, the authors develop a performance model of deflection routing that allows to estimate accurately and efficiently the mean transport time and throughput in a network that has any given two-connected topology, multiple buffers at each output port, and an arbitrary traffic matrix.>

Deflection routing plays a prominent role in many optical network architectures, because it can be implemented with modest packet-buffering requirements. From the practical perspective, however, the implementation of deflection routing, which is normally based on global time slotting, might pose challenges. The authors develop approaches to the implementation of both slotted and unslotted deflection-routing optical networks. They analyse important tradeoffs that are inherent to the design of optical deflection-routing networks and compare the performance of slotted and unslotted networks. Under a reasonable set of assumptions about optical technology, the results suggest that the unslotted link protocol should be the preferred approach to building the optical deflection-routing network.>

The authors study the effect of channel sharing on the performance of multihop lightwave networks when channel sharing is achieved by using a time-division multiple-access (TDMA) technique. They present a result which gives an upper bound for the throughput that can be achieved with any virtual topology that can be established with N stations assuming that the traffic distribution is uniform and that all virtual links have the same capacity. Using this result they determine the optimal degree of channel sharing that maximizes throughput. They also determine the optimal degree of channel sharing when the criterion is to maximize the network power, which is defined as a ratio of throughput and delay.>

Given that many optical fibers can be economically packaged within a single cable, the design of communication networks that exploit multifiber topologies is considered. A network architecture that uses the multifiber tree topology to provide high-speed datagram and circuit-switching communication services to a large population of stations is presented. Using a combination of space-, wavelength-, and time-division multiplexing, this network architecture provides all-optical transmission media to its stations and can interconnect several thousand stations in a metropolitan region without the need for optical amplification or electrooptical signal regeneration.>

The use of independence assumptions has made it possible to derive an approximate model of deflection-routing networks that have multibuffer stations and two-connected topologies. A procedure is presented for estimating the mean transport time (and throughput) of these networks, even when the offered traffic is nonuniform. The proposed procedure is efficient and yields very accurate results, as comparisons with simulations have confirmed. The model is quite general and can be applied to deflection-routing networks that deflect packets according to destination-based priorities. The authors have modeled only a few of the several options for designing a deflection-routing network. In particular, they have modeled a network with no user input buffers, a specific admission-routed policy, and destination-based priorities.>

The twofold purpose of this research is to develop mathematical programming tools for the optimal or near-optimal design of a new type of network architecture called the wavelength-division optical network (WON) and to discover underlying design principles for such networks via experimentation using these tools. The WON, a multichannel, multihop lightwave network with tunable transceivers, is suitable for use as a metropolitan area network. A unique feature of the WON is that a great number of virtual topologies can be mapped onto a given physical topology. The problems of virtual- and physical-topology design in the WON are described, techniques for their solution are presented, and the interplay between these problems is studied.>

The authors present the framework for a downline load service to which users subscribe to be properly loaded with their required code images. They review the user's requirements and present the outline of a service definition and protocol that satisfies the requirements. The analysis of downline-load protocol performance is given from the point of view of the time required to downline load a large number of devices. The authors also examine the impact of the total traffic generated by downline loading. Two downline-loading algorithms are studied and their performance characteristics compared.>

Managing the communications resources of a computer network is critical to the successful operation of the network. A network management system is expected to manage a large collection of network nodes remotely. There is an obvious need for sophisticated network management application software in this process. Previous network management systems have been designed without particular regard for the application programmer's interface requirements. We propose that such software be written in a specialized Network Management Language (NML) that provides the programmer with high-level abstractions rather than low-level communication primitives. The NML would help reduce development costs, promote portability and accommodate multiple standards. We propose an NML which is an extension to the SQL database standard. An implementation of the NML within the Open Systems Interconnection (OSI) systems management framework is discussed, and examples of its functionality and expressiveness are presented.