Amogh Dhamdhere

Open issues in router buffer sizing

Recent research results suggest that the buffers of router interfaces can be made very small, much less than the links and width-delay product, without causing a utilization loss, as long as the link carries many TCP flows. In this letter we raise some concerns about the previous recommendation. We show that the use of such small buffers can lead to excessively high loss rates (up to 5%-15%in our simulations) in congested access links that carry many flows. Even if the link is fully utilized, small buffers lead to lower throughput for most large TCP flows, and significant variability in the per-flow throughput and transfer latency. We also discuss some important issues in router buffer sizing that are often ignored

The Internet is flat: modeling the transition from a transit hierarchy to a peering mesh

Recent measurements and anecdotal evidence indicate that the Internet ecosystem is rapidly evolving from a multi-tier hierarchy built mostly with transit (customer-provider) links to a dense mesh formed with mostly peering links. This transition can have major impact on the global Internet economy as well as on the traffic flow and topological structure of the Internet. In this paper, we study this evolutionary transition with an agent-based network formation model that captures key aspects of the interdomain ecosystem, viz., interdomain traffic flow and routing, provider and peer selection strategies, geographical constraints, and the economics of transit and peering interconnections. The model predicts several substantial differences between the Hierarchical Internet and the Flat Internet in terms of topological structure, path lengths, interdomain traffic flow, and the profitability of transit providers. We also quantify the effect of the three factors driving this evolutionary transition. Finally, we examine a hypothetical scenario in which a large content provider produces more than half of the total Internet traffic.

Buffer sizing for congested Internet links

Packet buffers in router/switch interfaces constitute a central element of packet networks. The appropriate sizing of these buffers is an important and open research problem. Much of the previous work on buffer sizing modeled the traffic as an exogenous process, i.e., independent of the network state, ignoring the fact that the offered load from TCP flows depends on delays and losses in the network. In TCP-aware work, the objective has often been to maximize the utilization of the link, without considering the resulting loss rate. Also, previous TCP-aware buffer sizing schemes did not distinguish between flows that are bottlenecked at the given link and flows that are bottlenecked elsewhere, or that are limited by their size or advertised window. In this work, we derive the minimum buffer requirement for a drop-tail link, given constraints on the minimum utilization, maximum loss rate, and maximum queueing delay, when it is feasible to achieve all three constraints. Our results are applicable when most of the traffic (80-90%) at the given link is generated by large TCP flows that are bottlenecked at that link. For heterogeneous flows, we show that the buffer requirement depends on the harmonic mean of their round-trip times, and on the degree of loss synchronization. To limit the maximum loss rate, the buffer should be proportional to the number of flows that are bottlenecked at that link, when that number exceeds a certain threshold. The maximum queueing delay constraint, on the other hand, provides a simple upper bound on the buffer requirement. We also describe how to estimate the parameters of our buffer sizing formula from packet and loss traces, evaluate the proposed model with simulations, and compare it with two other buffer provisioning schemes.

NetDiagnoser: troubleshooting network unreachabilities using end-to-end probes and routing data

The distributed nature of the Internet makes it difficult for a single service provider to troubleshoot the disruptions experienced by its customers. We propose NetDiagnoser, a troubleshooting algorithm to identify the location of failures in an internetwork environment. First, we adapt the well-known Boolean tomography technique to work in this environment. Then, we significantly extend this technique to improve the diagnosis accuracy in the presence of multiple link failures, logical failures (for instance, misconfigurations of route export filters), and incomplete topology inference. In particular, NetDiagnoser takes advantage of rerouted paths, routing messages collected at one providers network and Looking Glass servers. We evaluate each feature of Net-Diagnoser separately using C-BGP simulations on realistic topologies. Our results show that NetDiagnoser can successfully identify a small set of links, which almost always includes the actually failed/misconfigured links.

AS relationships, customer cones, and validation

Business relationships between ASes in the Internet are typically confidential, yet knowledge of them is essential to understand many aspects of Internet structure, performance, dynamics, and evolution. We present a new algorithm to infer these relationships using BGP paths. Unlike previous approaches, our algorithm does not assume the presence (or seek to maximize the number) of valley-free paths, instead relying on three assumptions about the Internets inter-domain structure: (1) an AS enters into a provider relationship to become globally reachable; and (2) there exists a peering clique of ASes at the top of the hierarchy, and (3) there is no cycle of p2c links. We assemble the largest source of validation data for AS-relationship inferences to date, validating 34.6% of our 126,082 c2p and p2p inferences to be 99.6% and 98.7% accurate, respectively. Using these inferred relationships, we evaluate three algorithms for inferring each ASs customer cone, defined as the set of ASes an AS can reach using customer links. We demonstrate the utility of our algorithms for studying the rise and fall of large transit providers over the last fifteen years, including recent claims about the flattening of the AS-level topology and the decreasing influence of tier-1 ASes on the global Internet.

Twelve years in the evolution of the internet ecosystem

Our goal is to understand the evolution of the autonomous system (AS) ecosystem over the last 12 years. Instead of focusing on abstract topological properties, we classify ASs into a number of types depending on their function and business type. Furthermore, we consider the semantics of inter-AS links: customer-provider versus peering relations. We find that the available historic datasets from RouteViews and RIPE are not sufficient to infer the evolution of peering links, and so we restrict our focus to customer-provider links. Our findings highlight some important trends in the evolution of the Internet over the last 12 years and hint at what the Internet is heading toward. After an exponential increase phase until 2001, the Internet has settled into a slower exponential growth in terms of both ASs and inter-AS links. The growth is mostly due to enterprise networks and content/access providers at the periphery of the Internet. The average path length remains almost constant, mostly due to the increasing multihoming degree of transit and content/access providers. The AS types differ significantly from each other with respect to their rewiring activity; content/access providers are the most active. A few large transit providers act as “attractors” or “repellers” of customers. For many providers, strong attractiveness precedes strong repulsiveness by 3-9 months. Finally, in terms of regional growth, we find that the AS ecosystem is now larger and more dynamic in Europe than in North America.

Measuring the deployment of IPv6: topology, routing and performance

We use historical BGP data and recent active measurements to analyze trends in the growth, structure, dynamics and performance of the evolving IPv6 Internet, and compare them to the evolution of IPv4. We find that the IPv6 network is maturing, albeit slowly. While most core Internet transit providers have deployed IPv6, edge networks are lagging. Early IPv6 network deployment was stronger in Europe and the Asia-Pacific region, than in North America. Current IPv6 network deployment still shows the same pattern. The IPv6 topology is characterized by a single dominant player -- Hurricane Electric -- which appears in a large fraction of IPv6 AS paths, and is more dominant in IPv6 than the most dominant player in IPv4. Routing dynamics in the IPv6 topology are largely similar to those in IPv4, and churn in both networks grows at the same rate as the underlying topologies. Our measurements suggest that performance over IPv6 paths is comparable to that over IPv4 paths if the AS-level paths are the same, but can be much worse than IPv4 if the AS-level paths differ.

Bootstrapping in Gnutella: A Measurement Study

To join an unstructured peer-to-peer network like Gnutella, peers have to execute a bootstrapping function in which they discover other on-line peers and connect to them. Until this bootstrapping step is complete, a peer cannot participate in file sharing activities. Once completed, a peer’s search and download experience is strongly influenced by the choice of neighbor peers resulting from the bootstrapping step. Despite its importance, there has been very little attention devoted to understanding the behavior of this bootstrapping function. In this paper, we study the bootstrapping process of a peer in the Gnutella network. We find that (1) there is considerable variation among various servent implementations, and hence in their bootstrapping performance. (2) The neighbors of a peer, which are the outcome of the bootstrapping process, play a very important role in the peer’s search and download performance. (3) Even though the GWebCache system for locating peers is designed to operate as a truly distributed caching system, it actually operates more like a centralized infrastructure function, with significant load imbalance. (4) The GWebCache system is subject to significant misreporting of peer and GWebCache availability, due to stale data and absence of validity checks.

Toward topology dualism: improving the accuracy of AS annotations for routers

To describe, analyze, and model the topological and structural characteristics of the Internet, researchers use Internet maps constructed at the router or autonomous system (AS) level. Although progress has been made on each front individually, a dual graph representing connectivity of routers with AS labels remains an elusive goal. We take steps toward merging the router-level and AS-level views of the Internet. We start from a collection of traces, i.e. sequences of IP addresses obtained with large-scale traceroute measurements from a distributed set of vantage points. We use state-of-the-art alias resolution techniques to identify interfaces belonging to the same router. We develop novel heuristics to assign routers to ASes, producing an AS-router dual graph. We validate our router assignment heuristics using data provided by tier-1 and tier-2 ISPs and five research networks, and show that we successfully assign 80% of routers with interfaces from multiple ASes to the correct AS. When we include routers with interfaces from a single AS, the accuracy drops to 71%, due to the 24% of total inferred routers for which our measurement or alias resolution fails to find an interface belonging to the correct AS. We use our dual graph construct to estimate economic properties of the AS-router dual graph, such as the number of internal and border routers owned by different types of ASes. We also demonstrate how our techniques can improve IP-AS mapping, including resolving up to 62% of false loops we observed in AS paths derived from traceroutes.

GENESIS: An agent-based model of interdomain network formation, traffic flow and economics

We propose an agent-based network formation model for the Internet at the Autonomous System (AS) level. The proposed model, called GENESIS, is based on realistic provider and peering strategies, with ASes acting in a myopic and decentralized manner to optimize a cost-related fitness function. GENESIS captures key factors that affect the network formation dynamics: highly skewed traffic matrix, policy-based routing, geographic co-location constraints, and the costs of transit/peering agreements. As opposed to analytical game-theoretic models, which focus on proving the existence of equilibria, GENESIS is a computational model that simulates the network formation process and allows us to actually compute distinct equilibria (i.e., networks) and to also examine the behavior of sample paths that do not converge. We find that such oscillatory sample paths occur in about 10% of the runs, and they always involve tier- 1 ASes, resembling the tier-1 peering disputes often seen in practice. GENESIS results in many distinct equilibria that are highly sensitive to initial conditions and the order in which ASes (agents) act. This implies that we cannot predict the properties of an individual AS in the Internet. However, certain properties of the global network or of certain classes of ASes are predictable. We also examine whether the underlying game is zero-sum, and identify three sufficient conditions for that property. Finally, we apply GENESIS in a specific “what-if” question, asking how the openness towards peering affects the resulting network in terms of topology, traffic flow and economics. Interestingly, we find that the peering openness that maximizes the fitness of different network classes (tier-1, tier-2 and tier-3 providers) closely matches that seen in real-world peering policies.