We propose using coordinates-based mechanisms in a peer-to-peer architecture to predict Internet network distance (i.e. round-trip propagation and transmission delay). We study two mechanisms. The first is a previously proposed scheme, called the triangulated heuristic, which is based on relative coordinates that are simply the distances from a host to some special network nodes. We propose the second mechanism, called global network positioning (GNP), which is based on absolute coordinates computed from modeling the Internet as a geometric space. Since end hosts maintain their own coordinates, these approaches allow end hosts to compute their inter-host distances as soon as they discover each other. Moreover, coordinates are very efficient in summarizing inter-host distances, making these approaches very scalable. By performing experiments using measured Internet distance data, we show that both coordinates-based schemes are more accurate than the existing state of the art system IDMaps, and the GNP approach achieves the highest accuracy and robustness among them.

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Predicting Internet network distance with coordinates-based approaches

A number of large-scale distributed Internet applications could potentially benefit from some level of knowledge about the relative proximity between its participating host nodes. For example, the performance of large overlay networks could be improved if the application-level connectivity between the nodes in these networks is congruent with the underlying IP-level topology. Similarly, in the case of replicated Web content, client nodes could use topological information in selecting one of multiple available servers. For such applications, one need not find the optimal solution in order to achieve significant practical benefits. Thus, these applications, and presumably others like them, do not require exact topological information and can instead use sufficiently informative hints about the relative positions of Internet hosts. In this paper, we present a binning scheme whereby nodes partition themselves into bins such that nodes that fall within a given bin are relatively close to one another in terms of network latency. Our binning strategy is simple (requiring minimal support from any measurement infrastructure), scalable (requiring no form of global knowledge, each node only needs knowledge of a small number of well-known landmark nodes) and completely distributed (requiring no communication or cooperation between the nodes being binned). We apply this binning strategy to the two applications mentioned above: overlay network construction and server selection. We test our binning strategy and its application using simulation and Internet measurement traces. Our results indicate that the performance of these applications can be significantly improved by even the rather coarse-grained knowledge of topology offered by our binning scheme.

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Topologically-aware overlay construction and server selection

Consider a set of shared objects in a distributed network, where several copies of each object may exist at any given time. To ensure both fast access to the objects as well as efficient utilization of network resources, it is desirable that each access request be satisfied by a copy "close" to the requesting node. Unfortunately, it is not clear how to efficiently achieve this goal in a dynamic, distributed environment in which large numbers of objects are continuously being created, replicated, and destroyed. In this paper, we design a simple randomized algorithm for accessing shared objects that tends to satisfy each access request with a nearby copy. The algorithm is based on a novel mechanism to maintain and distribute information about object locations, and requires only a small amount of additional memory at each node. We analyze our access scheme for a class of cost functions that captures the hierarchical nature of wide-area networks. We show that under the particular cost model considered: (i) the expected cost of an individual access is asymptotically optimal, and (ii) if objects are sufficiently large, the memory used for objects dominates the additional memory used by our algorithm with high probability. We also address dynamic changes in both the network as well as the set of object copies.

Accessing nearby copies of replicated objects in a distributed environment

The ability to estimate network latencies between arbitrary Internet end hosts would enable new measurement studies and applications, such as investigating routing path inefficiencies on a wide-scale or constructing topologically sensitive overlay networks. In this paper we present King, a tool that accurately and quickly estimates the latency between arbitrary end hosts by using recursive DNS queries in a novel way. Compared to previous approaches, King has several advantages. Unlike IDMaps, King does not require the deployment of additional infrastructure, and unlike GNP, King does not require end hosts to agree upon a set of reference points. Unlike both IDMaps and GNP, King's estimates are based on direct online measurements rather than offline extrapolation. Because King uses existing DNS infrastructure, King scales naturally both in terms of the number of hosts that can be measured and in terms of the number of hosts performing measurements.After describing the techniques used in King, we present an extensive evaluation and analysis of the accuracy and consistency of our tool. Specifically, our evaluation shows that the accuracy of King is significantly better than the accuracy of IDMaps, and that King tends to preserve order among its latency estimates. Finally, we describe a variety of measurement studies and applications that could benefit from our tool, and present results from one such measurement study.

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King: estimating latency between arbitrary internet end hosts

Resource requests made by clients of origin servers in a network are intercepted by reflector mechanisms and selectively reflected to other servers called repeaters. The reflectors select a best repeater from a set of possible repeaters and redirect the client to the selected best repeater. The client then makes the request of the selected best repeater. The resource is possibly rewritten to replace at least some of the resource identifiers contained therein with modified resource identifiers designating the repeater instead of the origin server.

Optimized network resource location

In this paper we consider the problem of choosing among a collection of replicated servers, focusing on the question of how to make choices that segregate client/server traffic according to network topology. We explore the cost and effectiveness of a variety of approaches, ranging from those requiring routing layer support (e.g., anycast) to those that build location databases using application-level probe tools like traceroute. We uncover a number of tradeoffs between effectiveness, network cost, ease of deployment, and portability across different types of networks. We performed our experiments using a simulation parameterized by a topology collected from 7 survey sites across the United States, exploring a global collection of Network Time Protocol servers.

Locating nearby copies of replicated Internet servers

: The Network Time Protocol (NTP) is widely used to synchronize computer clocks throughout the Internet. Existing NTP clients and servers form a very large distributed system, and yet the tools available to observe and manage this system are fairly primitive. This paper describes our experiences with a prototype tool that attempts to discover relevant information about every NTP site on the Internet. The data produced by this tool can be used for a variety of purposes, including locating nearby accurate time servers and computing aggregate and long-term evaluations of the size and health of the NTP system. Importantly, our tool provides a means by which new NTP server administrators can make informed choices among the possible servers with which to synchronize, balancing the need for accurate time with the need to distribute server load. This is an important step towards improving global NTP system scalability, since at present our measurements indicate that the high-stratum servers are heavily overloaded.

/pdf/experiences-with-a-survey-tool-for-discovering-network-time-2vt3sx574j.pdf

James D. Guyton

Papers

Locating nearby copies of replicated Internet servers

Experiences with a Survey Tool for Discovering Network Time Protocol Servers.

Locating Nearby Copies of Replicated Internet Servers ; CU-CS-762-95

Experiences with a Survey Tool for Discovering Network Time Protocol ; CU-CS-704-94