Hyunseok Chang

The origin of power laws in Internet topologies revisited

C. Faloutsos et al. (see Proc. ACM SIGCOMM, 1999) found that the inter autonomous system (AS) topology exhibits a power-law vertex degree distribution. This result was quite unexpected in the networking community and stirred significant interest in exploring the possible causes of this phenomenon. The work of A.-L. Barabasi and R. Albert (see Science, p.509-512, 1999) and its application to network topology generation in the work of A. Medina et al. (see Proc. MASCOTS, 2001) have explored a promising class of models that yield strict power-law vertex degree distributions. We re-examine the BGP (border gateway protocol) measurements that form the basis for the results reported by Faloutsos et al. We find that by their very nature (i.e., being strictly BGP-based), the data provides a very incomplete picture of Internet connectivity at the AS level. The AS connectivity maps constructed from this data (original maps) typically miss 20-50% or even more of the physical links in AS maps constructed using additional sources (extended maps). Subsequently, we find that while the vertex degree distributions resulting from the extended maps are heavy-tailed, they deviate significantly from a strict power law. Finally, we show that available historical data does not support the connectivity-based dynamics assumed by Barabasi and Albert. Together, our results suggest that the Internet topology at the AS level may well have developed over time following a very different set of growth processes than those proposed by Barabasi and Albert.

To Peer or Not to Peer: Modeling the Evolution of the Internet's AS-Level Topology

Internet connectivity at the AS level, defined in terms of pairwise logical peering relationships, is constantly evolving. This evolution is largely a response to economic, political, and technological changes that impact the way ASs conduct their business. We present a new framework for modeling this evolutionary process by identifying a set of criteria that ASs consider either in establishing a new peering relationship or in reassessing an existing relationship. The proposed framework is intended to capture key elements in the decision processes underlying the formation of these relationships. We present two decision processes that are executed by an AS, depending on its role in a given peering decision, as a customer or a peer of another AS. When acting as a peer, a key feature of the AS’s corresponding decision model is its reliance on realistic inter-AS traffic demands. To reflect the enormous heterogeneity among customer or peer ASs, our decision models are flexible enough to accommodate a wide range of AS-specific objectives. We demonstrate the potential of this new framework by considering different decision models in various realistic “what if” experiment scenarios. We implement these decision models to generate and study the evolution of the resulting AS graphs over time, and compare them against observed historical evolutionary features of the Internet at the AS level.

Inferring AS-level Internet topology from router-level path traces

A number of recent studies characterize AS-level topology of the Internet by exploiting connectivity information contained in BGP routing tables. In this paper, we present an alternative method for discovering AS connectivity by inferring individual AS connections from the Internets router-level topology. This methodology has several advantages over using BGP routing tables. First, it allows us to obtain AS-level connectivity information at a finer granularity (e.g., multiple connections between a pair of ASs); second, we can discover ASs aggregated in BGP routing tables; and third, we can identify AS border routers, which may allow us to further characterize inter-AS connections. Since border routers have, by definition, multiple interfaces, each with an address in a potentially different AS, a major challenge of our approach is to properly map border routers to their corresponding ASs. To this end, we present in this paper several mapping rules and heuristics for inferring the ASs of border routers and report on results showing the effectiveness and validity of these rules and heuristics.

Towards capturing representative AS-level Internet topologies

For the past two years,there has been a significant increase in research activities related to studying and modeling the Internets topology, especially at the level of autonomous systems (ASs). A closer look at the measurements that form the basis for all these studies reveals that the data sets used consist of the BGP routing tables collected by the Oregon route server (henceforth, the Oregon route-views) [1]. So far, there has been anecdotal evidence and an intuitive understanding among researchers in the field that BGP-derived AS connectivity is not complete. However, as far as we know, there has been no systematic study on quantifying the completeness of currently known AS-level Internet topologies. Our main objective in this paper is to quantify the completeness of Internet AS maps constructed from the Oregon route-views and to attempt to capture more representative AS-level Internet topology. One of the main contributions of this paper is in developing a methodology that enables quantitative investigations into issues related to the (in)completeness of BGP-derived AS maps.

THE MANY FACETS OF INTERNET TOPOLOGY AND TRAFFIC

The Internets layered architecture and organizational structure give rise to a number of different topologies, with the lower layers defining more physical and the higher layers more virtual/logical types of connectivity structures. These structures are very different, and successful Internet topology modeling requires annotating the nodes and edges of the corresponding graphs with information that reflects their network-intrinsic meaning. These structures also give rise to different representations of the traffic that traverses the heterogeneous Internet, and a traffic matrix is a compact and succinct description of the traffic exchanges between the nodes in a given connectivity structure. In this paper, we summarize recent advances in Internet research related to (i) inferring and modeling the router-level topologies of individual service providers (i.e., the physical connectivity structure of an ISP, where nodes are routers/switches and links represent physical connections), (ii) estimating the intra-AS traffic matrix when the ASs router-level topology and routing configuration are known, (iii) inferring and modeling the Internets AS-level topology, and (iv) estimating the inter-AS traffic matrix. We will also discuss recent work on Internet connectivity structures that arise at the higher layers in the TCP/IP protocol stack and are more virtual and dynamic; e.g., overlay networks like the WWW graph, where nodes are web pages and edges represent existing hyperlinks, or P2P networks like Gnutella, where nodes represent peers and two peers are connected if they have an active network connection.

Characterizing guarded hosts in peer-to-peer file sharing systems

We call end-hosts behind network address translator (NAT) gateways or firewalls guarded hosts, and otherwise open hosts. In this paper, we empirically measure the prevalence of guarded hosts in two popular peer-to-peer file sharing systems, eDonkey and Gnutella, and study the characteristics of their shared files. By performing passive and active probes, we found that about 25-36% of eDonkey and Gnutella users reside on guarded hosts and that the ratio of files shared by guarded hosts is also non-trivial. When discounting guarded hosts, we found that a popular files availability, i.e., the number of copies available for download, decreases by 25-30%. Our measurement study testifies to the significant impact guarded hosts may have on the performance of current peer-to-peer file sharing systems, and points to a need to consider their presence when designing next generation peer-to-peer systems.

Difficulties Measuring the Internet's AS-Level Ecosystem

Compared to other large-scale, complex systems, one of the most appealing features of the Internet is that a thorough understanding of its component technologies combined with a unique ability to measure the network means that most of the models and theories proposed for explaining the Internets observed structure and behavior can be in general unambiguously validated, though perhaps not without substantial efforts. An example that requires such substantial effort is the structure and dynamics of the Internet at the level of autonomous systems (ASs). We show in this paper that the problem of inferring connectivity within the Internets AS-level ecosystem has been severely hampered by highly ambiguous measurements, with serious but often ignored implications for AS-level topology modeling and model validation. The situation is even worse with respect to inferring inter-AS traffic demands where the basic problem has been a complete lack of any sort of useful and publicly available measurements. We describe recent attempts that try to change this situation and discuss their implications for AS-level traffic matrix modeling and model validation.

Impacts of Peer Characteristics on P2PTV Networks Scalability

A P2PTV system allows users to watch live video streams redistributed by other users via a peer-to-peer (P2P) network. In an ideal world, each peer in a P2P network would be able to redistribute more bytes than it receives. A P2PTV system built from such peers can support a virtually unlimited number of peers; with only a single copy of content stream injected into the network, it can redistribute the content to all peers. Two factors in the development of the Internet prevented the realization of this scenario: the deployment of asymmetric access networks and the adoption of NAT boxes. For real-time live streaming, such peer asymmetry and incompatibility is a limiting factor on the P2P network scalability. We first develop a basic formal analysis of the effect of bandwidth asymmetry on P2P network scalability. Then we present several characteristics of peer asymmetry as measured on the Zattoo P2PTV network. Our simulation results, driven by the measured peer characteristics, confirm that we cannot rely on P2P network alone to distribute live streaming content on todays Internet.