Matthew Luckie

Revealing MPLS tunnels obscured from traceroute

By: 
Benoit Donnet, Matthew Luckie, Pascal Mérindol, Jean-Jacques Pansiot
Appears in: 
CCR April 2012

Operators have deployed Multiprotocol Label Switching (MPLS) in the Internet for over a decade. However, its impact on Internet topology measurements is not well known, and it is possible for some MPLS configurations to lead to false router-level links in maps derived from traceroute data. In this paper, we introduce a measurement-based classification of MPLS tunnels, identifying tunnels where IP hops are revealed but not explicitly tagged as label switching routers, as well as tunnels that obscure the underlying path.

Public Review By: 
Yin Zhang

Multiprotocol Label Switching (MPLS) has been widely deployed in the Internet. It is well known that MPLS clouds may potentially lead to inaccurate and incomplete Internet maps when performing active measurements using traceroute. In particular, MPLS tunnels may either obscure the underlying path or be incorrectly classified as direct IP links (between IP routers that are not physically adjacent). However, the quantitative impact of MPLS tunnels on Internet topology measurements is not well understood. The paper uses two features to characterize MPLS tunnels: (i) the ttl-propagate option on ingress label edge routers (LERs) and (ii) RFC4950. The ttl-propagate option allows traceroute to reveal IP hops within an MPLS tunnel. RFC4950 implementation provides all information related to a label switched path (LSP). The paper provides a taxonomy of MPLS tunnels: (i) explicit tunnels, which use both ttl-propagate and RFC4950, (ii) implicit tunnels, which use only ttl-propagate, (iii) opaque tunnels, which use only RFC4950, and (iv) invisible tunnels, which use neither option. The paper then develops new inference mechanisms to identify different types of MPLS tunnels and thus reduce the impact of MPLS on IP topology discovery. The focus is on explicit, implicit and opaque tunnels. Based on the new inference techniques, the authors collected a largescale measurement dataset. According to this dataset, a significant fraction (at least 30%) of the paths traverse an MPLS tunnel. Moreover, a significant fraction of MPLS tunnels are not explicitly flagged, but fortunately most of these MPLS tunnels do not obscure IP-level topology discovery. Overall, a nice measurement paper. The topic is timely. The proposed measurement techniques are sound. The findings are interesting and shed light on the impact of MPLS tunnels on IPbased topology discovery. I also expect the techniques developed in this paper to be applied by others in future research on IP-based topology discovery.

Measured Impact of Crooked Traceroute

By: 
Matthew Luckie, Amogh Dhamdhere, kc claffy, and David Murrell
Appears in: 
CCR January 2011

Data collected using traceroute-based algorithms underpins research into the Internet’s router-level topology, though it is possible to infer false links from this data. One source of false inference is the combination of per-flow load-balancing, in which more than one path is active from a given source to destination, and classic traceroute, which varies the UDP destination port number or ICMP checksum of successive probe packets, which can cause per-flow load-balancers to treat successive packets as distinct flows and forward them along different paths.

Public Review By: 
R. Teixeira

The research community has applied traceroute-style probing to measure Internet topologies for more than a decade with systems such as Skitter/Ark, Dimes, or Rocketfuel. These topologies are the basis of many other research efforts. Unfortunately, recent studies showed that classic traceroute can report false links when a router in the path performs load balancing. Although new probing techniques correct measurement artifacts under per-flow load balancing, we cannot correct topologies that have already been collected using classic traceroute and no prior work has studied how these errors affect inferred topologies. A natural question is then: how accurate are the topologies that we have all been using in our research?
This paper gives us a mixed answer. Measurement artifacts due to per-flow load balancing introduce only few errors when traceroute is used to discover a macroscopic topology (i.e., an Internet-wide topology), but they introduce significant errors when discovering the topology of an ISP. Such a sharp difference in the fraction of false links between the macroscopic topology and the ISP topology suggests that the error really depends on the set of vantage points and the networks traversed. This paper studies only one source of errors in inferred Internet topologies. As the authors point out: "the state of the art in Internet topology measurement is essentially and necessarily a set of hacks, which introduce many sources of possible errors". Hopefully, new studies will follow to understand the caveats of measured Internet topologies and to measure more accurate topologies. In the mean time, this paper confirms that we should be cautions when using inferred Internet topologies.

The 2nd Workshop on Active Internet Measurements (AIMS-2) Report

By: 
kc claffy, Emile Aben, Jordan Auge, Robert Beverly, Fabian Bustamante, Benoit Donnet, Timur Friedman, Marina Fomenkov, Peter Haga, Matthew Luckie, and Yuval Shavitt
Appears in: 
CCR October 2010

On February 8-10, 2010, CAIDA hosted the second Workshop on Active Internet Measurements (AIMS-2) as part of our series of Internet Statistics and Metrics Analysis (ISMA) workshops. The goals of this workshop were to further our understanding of the potential and limitations of active measurement research and infrastructure in the wide-area Internet, and to promote cooperative solutions and coordinated strategies to addressing future data needs of the network and security research communities.

The workshop on active internet measurements (AIMS) report

By: 
k. c. claffy, Marina Fomenkov, Ethan Katz-Bassett, Robert Beverly, Beverly A. Cox, and Matthew Luckie
Appears in: 
CCR October 2009

Measuring the global Internet is a perpetually challenging task for technical, economic and policy reasons, which leaves scientists as well as policymakers navigating critical questions in their field with little if any empirical grounding. On February 12-13, 2009, CAIDA hosted the Workshop on Active Internet Measurements (AIMS) as part of our series of Internet Statistics and Metrics Analysis (ISMA) workshops which provide a venue for researchers, operators, and policymakers to exchange ideas and perspectives.

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