Unveiling the MPLS Structure on Internet Topology

Transcription

1 Unveiling the MPLS Stuctue on Intenet Topology Gabiel Davila Revelo, Mauicio Andeson Ricci, Benoit Donnet, José Ignacio Alvaez-Hamelin INTECIN, Facultad de Ingenieía, Univesidad de Buenos Aies Agentina {gdavila, andeson, CONICET Agentina Univesité de Liège, Liège Belgium, Abstact Recently, eseaches have been conducted to discove and assess the usage of MPLS tunnels. Indeed, ecent developments in the ICMP potocol make cetain categoies of MPLS tunnels tanspaent to taceoute pobing. Additional techniques have been poposed to eveal the pesence of MPLS tunnels when they do not explicitly appea in taceoute. It has been shown that MPLS is a vey well deployed technology whose usage (i.e., Taffic Engineeing, load balancing, etc.) vaies in time and accoding to ASes. Howeve, the MPLS stuctue on the Intenet achitectue has not been studied yet. In this pape, we follow this path by poviding two contibutions to the state of the at: (i) we evaluate the biases involved on MPLS tunnel detection when they ae not diectly evealed though taceoute. (ii), we povide some popeties and achitectual details elated with MPLS deployment on oute topology based on a k-coe decomposition. I. INTRODUCTION Intenet topology efes to the study of the vaious types of connectivity stuctues and epesentations between diectly connected nodes on the Intenetq achitectue. This epesentation aims at modeling the Intenet with the geatest possible accuacy in ode to test new communications potocols, algoithms, QoS policies, taffic engineeing, etc. The Intenet topology can be seen at seveal abstaction levels i.e., IP inteface, oute, subnetwok, PoP, and Autonomous System (AS) levels. All these models have been widely studied in the past [1]. Howeve, the cuent state of the at of Intenet deployment involves a geat numbe of technologies impacting the Intenet Topology. And those technologies deseve a deep study in ode to include them in the cuent Intenet models. Fo instance, Multipotocol Label Switching (MPLS) [2] has been ecently the focus of seveal studies [3], [4], [5]. It has been demonstated that MPLS is a matue technology widely deployed fo (mainly) load balancing easons o taffic engineeing puposes. A few studies have patially questioned its impact on Intenet topology [6], [7]. Howeve, the MPLS stuctue on the Intenet achitectue has not been studied yet. The impotance to study the achitectual details of MPLS usage would help to know the way in which the Intenet Sevice Povides (ISPs) use thei netwoks o apply thei policies fo taffic engineeing as well as to bette undestand the Intenet achitectue moe accuately. This wok mainly povides a study aound the stuctue of MPLS usage ove Intenet Topology. Fistly, we focus on evaluating the accuacy of MPLS tunnel detection methods. Paticulaly, we povide a quantification of the biases elated with MPLS tunnels that ae not evealed explicitly by taceoute. Secondly, we study the MPLS stuctue based on the way MPLS outes and netwoks inteact with non- MPLS capable outes. In ode to do it, we define a new abstaction level on Intenet gaph, distinguishing oute-level and MPLS-level links. We also identified non-mpls capable outes and MPLS clustes. In this way, we contibute to the taditional Intenet topology with new details elated to MPLS usage. We mainly use k-coe decomposition [8] based tools to eveal the fingepints closely elated to MPLS pesence. It has been shown peviously that k-coe decomposition is a elevant tool to descibe Intenet Topology [9], [], [11]. Ou main findings eveal that the MPLS stuctue vaies depending on the type of MPLS tunnels that pevails fo a given AS and that MPLS deployment plays an impotant ole on the Intenet Backbone. Specifically, we find that local obustness of Intenet topology incease due to MPLS pesence. The emainde of this pape is oganized as follows: Sec. II povides the state of the at and the backgound elated to MPLS tunnels discovey. In paticula, it descibes how MPLS tunnels can be evealed though active measuements; Sec. III explains how we collected data fo this wok; Sec. IV pesents ou esults elated to mpls signatues accuacy; Sec. V pesents the main contibutions of this pape with a detailed study aound the behavio of MPLS netwoks on the Intenet Topology and achitectual details of some ASes with most MPLS usage; Finally, Sec. VI concludes this pape by summaizing its main achievements. II. RELATED WORK In this section, we fist povide an oveview of MPLS (Sec. II-A) befoe explaining how MPLS tunnels can be evealed though active measuements (Sec. II-B). We also position this wok egading the state of the at. A. MPLS Oveview The Multipotocol Label Switching (MPLS) [2] was oiginally designed to speed up the fowading pocess. In pactice, this was done with one o moe 32 bits label stack enties (LSE) inseted between the fame heade (Data-link laye) and the IP packet (Netwok laye). A given packet can manage seveal LSEs at the same time. In this case, the packet caies a stack of labels. Each LSE is made of fou fields: a 2-bit label value used fo fowading the packet to the next oute, a 3-bit Taffic Class field fo quality of sevice (QoS), pioity, and Explicit Congestion Notification (ECN) [12], a 1-bit bottom

2 of stack flag (when set the cuent label is the last in the stack [13]), and an 8-bit time-to-live (LSE-TTL) field having the same pupose as the IP-TTL field [14]. MPLS outes, called Label Switching Routes (LSRs), exchange labelled packets ove Label Switched Paths (LSPs). The fist MPLS oute (Ingess Label Edge Route, o Ingess LER, i.e., the tunnel enty point) adds the label stack, while the last MPLS oute (Egess Label Edge Route, o Egess LER, i.e., the tunnel exit point) emoves the label stack. In some cases, fo pefomance easons, the LSE stack may be emoved by the penultimate MPLS oute (penultimate hop popping, PHP). The Egess LER then pefoms a classic IP lookup and fowads the taffic, educing so the load on the Egess LER (specially if the Egess LER is shaed among seveal LSPs). This means that, when using PHP, the tunnel exit is one hop befoe the Egess LER. B. Revealing MPLS Tunnels MPLS outes may send ICMP time-exceeded messages when the LSE-TTL expies. In ode to debug netwoks whee MPLS is deployed, outes may also implement RFC495 [15], an extension to ICMP allowing a oute to embed an MPLS LSE in an ICMP time-exceeded message. In that case, the oute simply quotes the MPLS LSE (o the LSE stack) of the eceived packet in the ICMP time-exceeded message. RFC495 is paticulaly useful fo opeatos as it allows them to veify the coectness of thei MPLS tunnels and taffic engineeing policy. If the Ingess LER copies the IP-TTL value to the LSE- TTL field athe than setting the LSE-TTL to an abitay value such as 255, LSRs along the LSP will eveal themselves when using taceoute via ICMP messages even if they do not implement RFC495. Opeatos can configue this action using the ttl-popagate option povided by the oute manufactue [14] (while, to the best of ou knowledge, the RFC495 is just a matte of implementation and cannot be deactivated on ecent outes suppoting it). Using those two featues, Sommes et al. [3] povide an extensive study of MPLS tunnels as obseved in CAIDA s topology data. In this data, they find tunnels in 7% of ASes, and the faction is constant ove the yeas of data consideed. Recently, Vanaubel et al. [16] focus on MPLS deployment and usage unde IPv6. Vanaubel et al. [5] popose a classification of path divesity accoding to MPLS deployment. Thei classification eveals the actual usage of MPLS (e.g., load balancing, taffic engineeing) accoding to the infeed label distibution potocol. Finally, it has also been demonstated that MPLS tunnels may have an impact on Intenet topology discovey tools. Fo instance, the pesence of MPLS tunnels may intefee with load balancing detection [6] o violate the destination-based fowading [7]. Donnet et al. [4] popose a taxonomy of MPLS tunnels based on how they eact to taceoute pobes accoding to thei compliance (o not) to RFC495 fo MPLS and the ttl-popagate option. The classes poposed ae: explicit tunnels (i.e., ttl-popagate and RFC495 ae enabled), implicit tunnels (i.e., the oute that pushes the MPLS label enables the ttl-popagate option but LSRs do not implement RFC495), opaque tunnels (i.e., the LH implements RFC495 but the ingess LER does not enable the ttl-popagate option), and, finally, invisible tunnels (i.e., the ingess LER does not enable the ttl-popagate option and RFC495 is not implemented by the LH oute). Implicit and opaque tunnels can be evealed as follows: 1) a quoted IP-TTL (qttl) in ICMP time-exceeded messages > 1 will likely eveal the ttl-popagate option at the ingess LER of an LSP. Fo each subsequent taceoute pobe within an LSP, the qttl will be one geate esulting in an inceasing sequence of qttl values in taceoute. This is illustated in Fig. 1(a); 2) #hops diffeences with the IP-TTL in echo-eply messages (u-tun). It elies on the fact that LSRs along an LSP pesent an oiginal label stack default outing behavio: when the LSE-TTL expies, an LSR fist sends the time-exceeded eply to the Egess LER which then fowads the eply on its own to the pobing souce, while an LSR eplies to othe pobes using its own IP outing table if available. Theeby, u-tun is the signatue elated with the diffeence in these values. Summaizing, u-tun = TTL echo-eply TTL time-exceeded. The expected u- tun value is in the fom [2L, 2L 2, 2L 4,..., 2] whee L is the LSP length and the aay position coesponds to the LSR position within the LSP. 3) opaque tunnels ae evealed though the quoted LSE- TTL etuned by the LH in the time-exceeded eply. By compaing this quoted LSE-TTL and 255 (which is the standad initial LSE-TTL value), one can eveal the pesence of an opaque tunnel and its length [4]. Additional study by Vanaubel et al. [17] shows that the pobing heuistic to detect implicit tunnels seems quite eliable. Howeve, u-tun signatues ae by definition moe subject to false positives than qttl ones. This is exactly what we tackle in this pape (and, consequently, ou wok is complementay to Vanaubel et al. [17]): we want to test u-tun signatue accuacy. III. DATA COLLECTION In ode to collect MPLS data, we develop a tool called MAGALLANES [18] allowing us to easily un and manage scampe [19] based pobes though the PlanetLab (PL) infastuctue. MAGALLANES stats by andomly allocating seveal vantage points (VP) within the available set of PL nodes. It next distibutes, among those VPs, a given numbe of destinations (o pobe tagets ). To achieve some geogaphical unifomity in taget selection, MAGALLANES select andomly tagets fom data povided by IP geolocation database maxmind. 1 Additionally, MAGALLANES allows one to stoe an expeiment esults on a centalized database and to pefom alias esolution using MIDAR [2]. 1 See

3 u-tun 2L 2L n-1 n n-position (a) qttl signatue. (b) u-tun signatue. Fig. 1. Signatues behavio fo implicit MPLS tunnels. L=1 L=2 L=3 L= LSP We an MAGALLANES on Octobe 31 st, 215. We chose VPs and selected, tagets pe VP 2. Each VP managed its own set of tagets, meaning that pobes tagets ae disjoint sets between VPs. scampe was configued to un ICMP Pais taceoute [21]. To get the u-tun signatue, we sent a ping to each hop evealed by Pais taceoute. We sent six ICMP echo-equest packets fom the same VP. Six ICMP echo-eply allow us to infe with 95% confidence if thee is a single etun path and, theefoe, educe measuement eos caused by a evese path containing loadbalanced segments of diffeent lengths [6]. As a esult we discoveed aound 27, IP intefaces, 52, links, 42% of which wee available to un MIDAR and we found aliases successfully on 19% of them. To match IP intefaces to ASes, we used the CAIDA dataset [22] deived fom Routeviews 3 and collected the same day as the exploation. Additionally, we found that 44% of taces collected tavese at least one MPLS tunnel. The amount of explicit tunnels is highly supeio to implicit ones. We discoveed explicit tunnels on 34% of taceoutes and at least one implicit tunnel on 16%. Supisingly, we found moe implicit tunnels evealed though u-tun signatue (12%) athe than qttl signatue (4%). Howeve, the qttl signatue matched with at least 63% of the explicit tunnels. We discuss these esults in the next sections. Finally, we did not found opaque tunnels, confiming so thei aity [17]. IV. MPLS SIGNATURES VALIDATION In this section, we expose ou methodology fo validating the MPLS signatues used to eveal implicit MPLS tunnels (see Sec. II-B). Basically, we compae the LSR position within an MPLS tunnel, called the MPLS position, with the diffeent signatues values. Ou main goal hee is to assess the u-tun accuacy. The MPLS position of an LSR is obtained based on its appeaance ode in an LSR, as evealed by taceoute. The appeaance ode is called n-position, i.e., the fist LSR evealed by a taceoute pobe should be the fist LSR within the LSP (1-position), the LSR evealed by the next consecutive taceoute pobe should be the second LSR within the LSP (2-position), etc. Given that MPLS tunnels can 2 The collected dataset is available at Mat_TMA_216/. 3 See Ingess LER Non-RFC 495 Non-evealed qttl=1 LSR1 1-position qttl=2 LSR2 LSR3 2-position qttl=3 LH LSR Egess LER 3-position qttl=4 Fig. 2. Example of biased n-position. It shows a scenaio whee, due to pe-packet load balancing issues and the absence of RFC495 on LSR 1, the n-position could be eoneously infeed: Fist, taceoute eveals LSR 2 (and associates it the 1-position), the next taceoute pobe eveals the LH LSR (and associates it the 2-position). Finally, due to load balancing issues on LSR 2 the LH LSR is evealed twice (with diffeent qttl value). Fequency Tunnel Length qttl 1. obtained value.9 8 expected value n 6 8. (a) Tunnel length distibution (b) qttl and n-position compaison Fig. 3. Compaison between obtained and expected values fo qttl and u-tun. be configued to pefom load balancing (this is quite common, as shown by Vanaubel et al. [5]), the n-position evealed by taceoute might lead to a bias with espect to the actual MPLS position. This is illustated in Fig. 2. Implicit tunnels ae based eithe on qttl o u-tun signatues. Both of them ae diectly elated with MPLS position. Indeed, fist, the qttl value efes to the IP-TTL of the echo-equest packet when it entes the MPLS tunnel. Theefoe, a qttl of n in the esulting ICMP time-exceeded means that the sent pobe expied n hops late than the Ingess LER of the LSP, i.e., an LSR eply with qttl= n means that the LSR appeas in the n-position in the LSP. This is illustated in Fig. 1(a). Fom the Ingess LER, the qttl stats to gow linealy with the LSP length. We theefoe expect obseving a qttl=1 on the fist LSR in the LSP, a qttl= 2 on the second LSR in the LSP, etc. Second, a u-tun value is elated to the tunnel length, L, and the n-position of the LSR within the tunnel (see Sec. II-B) as is shown on Fig. 1(b). A. qttl Signatue Ou signatue validation elies on the hypothesis that the actual MPLS position matches with the n-position, i.e., the n- position of the LSR within the LSP coesponds to the qttl value geneated by that LSR. Said diffeently qttl = n. In ode to validate this assumption, we use the dataset descibed in Sec. III and compae the qttl with n-position fo explicit and implicit tunnels. The esults ae shown in Fig. 3. In paticula, Fig. 3(a) povides the MPLS tunnel length distibution computed as the numbe of LSRs in the tunnel.

4 We obseve, cooboating so pevious studies [3], [5], [4], that most of tunnels ae athe shot (length < 3 in moe than 8% of the cases). Fig. 3(a) also povides, by extension, possible values fo qttl (X-axis). This suggests thus that qttl values should oscillates between 1 and 8, with a stong pedominance fo shot values (i.e., between 1 and 3). Fig. 3(b) epesents a scatte plot showing the elationship between the qttl (Y-axis) and the n-position (X-axis). The cicle size in the scatte plot is elated with the occuence fequency of Y-axis values egading each n-position. The tanspaency of the cicle is elated with occuence fequency of the n-position egading each Y-axis value. Fo instance, on Fig. 3(b) fo values whee n > 1, the biggest cicles ae mainly located on qttl= 1 and qttl= n. So, this suggests that, fo a given n-position, the qttl value usually takes eithe the value 1 o n. Howeve, we notice, on Fig. 3(b) that the qttl signatue highly matches with n. The bias qttl = n±ɛ could occu due to two causes: one is the limitation in ou method to eveal the fist LSR in the LSP when RFC495 is not implemented (by definition of implicit tunnel); and the second cause could occu due to load balances (using Pais taceoute should avoid load balancing issues, except fo pe packet load balances), as suggest Fig. 2 suggests. Fig. 3(b) also shows that qttl fequently takes the value of 1, even fo n > 1, which means that the LSR implements the RFC495 but do not match with the qttl signatue. We also find that aound 2% of LSRs do not eact to qttl signatue, even if thei neighbos does, i.e., some LSRs intefaces located at i n±1 tunnel positions eact popely to qttl signatues but the LSR inteface located at i n position does not. Nevetheless, the n-position is highly eliable and theefoe, the potential load balance pesence on LSPs is not a common issue. Indeed, we find that in 58% of the cases the n-position matches with the qttl value while in 36, 3% of cases the qttl signatue is not pesent on explicit tunnels and takes the value of 1, and just 6, 7% of the cases pesented have some bias aound the expected value n. Those esults suppot ou hypothesis: the MPLS tunnel position highly matches with the n-position. Theeby, we use n-position as a efeence value to validate the u-tun signatues. B. u-tun Signatue As explained in Sec. II-B, the expected u-tun value is of the fom [2L, 2L 2, 2L 4,..., 2] whee L is the tunnel length and the aay position coesponds to the LSR position within the LSP, i.e., n (see Fig. 1(b)). The elationship between L, n, and the expected value can thus be witten u tun = 2 (L (n 1)). Because u-tun is commonly pesent in almost all LSRs, fist, we compae n with u-tun on LSRs evealed eithe explicitly o qttl-based using the dataset pesented in Sec. III (Fig. 4(a)). We also study n value on LSRs whee u-tun was the only detected signatue (Fig. 4(b)). We use the filte u-tun obtained value.2 expected value n L=5 u-tun obtained value.3.2 expected value n L=5 (a) u-tun on LSRs evealed though (b) u-tun on LSRs whee no othe RFC495 and qttl signatue was found Fig. 4. Compaison between obtained and expected values fo u-tun signatue. u-tun > 3 (i.e., avoiding shot tunnels whee biases ae moe likely to appea) to avoid false positives. The esults fo a given tunnel length L = 5 ae shown on Fig. 4, a scatte plot that must be ead the same way as Fig. 3(b). Quickly said, Fig. 4 suggests that u-tun is usually oveestimated. Simila esults wee obseved fo othe tunnel lengths. We notice that obtained u-tun values ae close to expected ones when the LSR was eithe explicitly evealed o when qttl is pesent (Fig. 4(a)). Howeve, fo LSRs evealed only by u-tun signatues (Fig. 4(b)) the obtained and expected u-tun values commonly do not match. If we accept a bias of ±2 aound the expected u-tun value ove ou whole dataset, we notice that, on LSRs explicitly evealed o qttl based, the 6% of obtained u-tun signatues match with the expected values. Howeve, fo LSRs evealed only though u- tun signatue, the obtained u-tun signatue just match in less than 25% with the expected values. Theefoe, LSRs evealed only though u-tun ae highly inaccuate, mainly, because MPLS tunnels ae not the only esponsible fo u-tun signatue. Indeed, it is also elated with load balancing on the etun path, whee ICMP echo-eply and ICMP time-exceeded at diffeent hops may be load balanced [21]. Up to now, we showed u-tun signatue s inaccuacy. We also believe it is impotant to know whethe u-tun signatue pesence has an impact on the Intenet achitectue. We tackle this (and othe inteesting questions) in Sec. V. V. LSRS AND MPLS CLUSTERS This section aims at bette undestanding the impact of MPLS deployment ove Intenet, specifically ove oute level topology. To achieve ou pupose, we study how LSRs and MPLS clustes inteacts with non MPLS outes. Geneally speaking, we compae the fingepints on k-coe decomposition that MPLS pesence causes ove the Intenet Topology. We popose two main studies: the fist one aims at locating the LSRs and MPLS clustes ove the entie oute topology; and a second one aims at bette undestanding the MPLS stuctue fo a given AS. A. Definitions and Backgound We define seveal gaphs at diffeent abstaction levels as follows: fist, the IP level gaph G ip is built with the IP

5 1 2 3 Degee 4 (a) Degee Distibution Pobability Fig Gip G G\ls Degee 4 Neaest Neighbo Degee Gip G G\ls Clusteing Coeficient Degee Distibution Pobability Gip G G\ls Degee 4 (b) Clusteing Coefficient (c) Neighbo Degee Distibution Metics fo IP, oute and MPLS cluste inteconnection topologies. addesses and links found though taceoute. Second, the oute level gaph G is obtained afte solving alias esolution pocess though MIDAR [2]). Thid, the MPLS oute level gaph Gmpls is fomed by MPLS links and outes in which at least one IP inteface belongs to an LSP. The ASes induced gaph G (as) is a subgaph of G whee each vetex has an inteface belonging to the same Autonomous System, namely as. In paticula, the induced (as). A connected component Cimpls is Gmpls gaph of Gmpls in a Gmpls (as) is called MPLS cluste. Finally, the MPLS cluste inteconnection gaph is a hybid oute level gaph, G\ls, whee all the MPLS clustes Cimpls ae gatheed togethe in a single node, while non-mpls capable outes emain unchanged. Boadly speaking, an MPLS cluste inteconnection gaph efes to a oute level gaph whee all MPLS clustes ae teated as a single node. Additionally, we call G\ls (as) the subgaphs of G\ls induced by outes having at least one inteface in the Autonomous System as. This section mainly focuses on MPLS clustes inteconnection gaph G\ls and thei espective ASes induced gaphs G\ls (as). In this way, we study how MPLS clustes ae connected to non-mpls capable outes. Paticulaly, as MPLS clustes inteconnection gaph analysis is mainly based on kcoe decomposition, we pesent the following definitions: k-coe: Given a gaph G = (V, E), then the subgaph H = (C, E C) induced by the set C V is a k-coe of ode k if f v C : degeeh (v) k and H is the maximum subgaph with this popety. Shell index. A vetex i has a shell index c if it belongs to the c-coe but not to (c + 1)-coe. We denote by Ci the shell index of vetex i. A shell Cc consists of all the vetices whose shell index is c. The maximum value c such that Cc is not empty is denoted by Cmax. Theefoe, the k-coe is thus the union of all shells Cc with c k. Coe-connectivity [11]. Let a coe-connected gaph, then vetices (i, j) having shell-index a and b espectively, has at least k = min(a, b) diffeent paths to join i to j. To etieve the k-coe decomposition of a gaph G, we use L A N ET- VI [9], [11]. This tool etuns a two dimensional plot, whee the position of each vetex is aanged into a cicle depending on its shell index and its neighbos index. A colo code allows fo the identification of shell indices, and diamete Fig. 6. k-coe visualization of oute level topology G. of the sphees epesent vetex s degee in a logaithmic scale. The k-coe decomposition can beak the oiginal netwok into vaious connected components which ae displayed as independent cicles. B. MPLS on Intenet Topology In this section, we study the LSRs and MPLS cluste stuctue ove the Intenet topology. We focus on the oute level topology fo ou analysis because it is close to a ealistic Intenet one and because we do not notice any stong diffeence between IP and oute level topology as Fig. 5 suggests. Indeed, we only emak that oute level topology has a slightly stonge clusteing coefficient (see Fig. 5(b)), due to alias esolution pocess. In this way, Fig. 6 4 shows the k-coe visualization of G. The figue is divided in fou pats, the main pat being in the uppe left while the thee othes ae a zoom on the main one. The main pat is composed of two scales, the one on the left is the node degee in a logaithmic scale, while the one on the ight is a gay scale 4 Highe esolution figues available at Sup_Mat_TMA_216/.

6 with each shell index C i. Between the two scales, we see the shell index with C max in the cente, the othe shells being located concentically aound it. Note that C max -coe is made of seveal components with one having the most significant pat, and it is shown at the left of the cente (black nodes). We also see that all the shell indexes ae highly populated and that the node degee is not elated with the shell index, i.e., thee ae many outes with high degee in the oute (lowe) shells. Anothe typical featue of oute level topology is that the links between outes mainly occu between outes belonging to neighbos shells (links have been omitted fo the sake of visualization), e.g., the outes on the oute shells ae not usually connected to the outes located on the C max -coe, as it is in the Autonomous Systems maps [9]. In ode to locate LSRs-outes with MPLS capabilities into the shell indexes ove k-coe decomposition, we paint in black the non-mpls outes and in gay the LSRs. The esults ae shown in Fig. 7(a). 4 We notice that the LSRs ae commonly distibuted aound the diffeent shells of Intenet, with slightly low density in the lowe ones. Additionally, we apply the same methodology fo the MPLS inteconnection cluste level gaph G \ls (Fig. 7(b)): MPLS clustes (gay nodes) ae distinguished fom the non-mpls capable outes (black nodes). In this case, MPLS clustes degee is coelated with shell index: highe the shell cluste, highe its degee. This behavio is obseved in hieachical netwoks, e.g., the Autonomous System netwok. Finally, we evaluate G \ls using metics such as degee distibution, local clusteing coefficient, and neaest neighbo degee. These metics ae widely used by the eseach community to descibe netwok popeties [23, p ]. Local clusteing coefficient is a measue of how connected ae the vetex s neighbos between them. Neaest neighbo degee is the aveage of the neighbo s degee fo a given vetex. The esults ae shown on Fig. 5. The x-axis values have been logaithmically binned. We notice that MPLS clustes C mpls i highly impact the oute level topology (notice that C mpls i is epesented by a single node). On one hand, the neaest neighbo degee is highe fo lowe degee nodes on G \ls, suggesting that outes with low degee ae highly connected to MPLS clustes (the highest degee nodes as Fig. 7(b) shows) and theeby to LSRs. On the othe hand, the clusteing coefficient of G \ls is highly inceased fo vetices having degee 5 to 2. It means that thei neighbos ae highly connected between them due to the MPLS clustes apaition. It also implies that MPLS clustes plays an impotant ole on Intenet obustness: the moe connected the vetex s neighbos ae, the moe disjoint paths exist between them. Moeove, this netwok veify the coe-connectivity popety [11] fo the highest coes. C. MPLS clustes on Autonomous Systems Although, the pevious esults give us a geneal oveview about MPLS deployment, we believe that the study of MPLS stuctue equies going deepe into the individual AS topology. Indeed, we found that aound 89.9% of MPLS links ae inta-domain. Theeby, we focus on the top ASes in tems of total numbe of discoveed links. On this set of ASes, we discad those having less than 5 MPLS links. The summay of this top ASes is shown in Fig. 8. Additionally, we identify the amount of discoveed MPLS links by AS, distinguishing the type of MPLS tunnel as follows: given a link between two MPLS intefaces i n 1 and i n discoveed by taceoute at n 1 and n position, we define : explicit MPLS link: as links whee i n belongs to an explicit MPLS tunnel; qttl MPLS link: as links whee i n belongs to an implicit MPLS tunnel qttl based; u-tun MPLS link: as links whee i n belongs to an implicit MPLS tunnel u-tun based. Regading Fig. 8, we notice that the atio mpls = (as) / E (as) is geate when moe explicit MPLS links have been discoveed. Inteestingly, we also see that the ASes with moe IP links discoveed have the lowest atio mpls. Fo ou puposes, we select the most epesentatives ASes fom those in Fig. 8. In this way, we analyze the gaphs G (as) and G \ls (as) fo AS1299, AS174, AS6762, AS2914, AS718, and AS1273 using k-coe decomposition (see Fig. 9). We obseve that k-coe decomposition stuctue vaies accoding the type of MPLS tunnels that pevails in the AS. Paticulaly, fo AS1299 (Teliasonea AB), AS174 (Cogent Communication), and AS6762 (Telecom Italia) whee pevails u-tun MPLS links, we show that MPLS clustes (epesented as gay nodes) ae spead out ove diffeent shells. These k-coe stuctues ae simila in ou top five ASes (fist five ASes with most links discoveed on Fig. 8) whee u-tun signatue was mostly discoveed, i.e., between 3% and 8% ove the total amount of MPLS links. Howeve, fo AS2914 (NTT Ameica Inc.), AS718 (AT&T), and AS1273 (Cable and Wieless Woldwide plc) whee explicit MPLS links pevail, we find a highly diffeent k-coe stuctue, i.e., most vetices ae diectly connected to a pedominant MPLS cluste located at C max -coe. The emaining ASes in Fig. 8 with high pecentage of explicit tunnels have the same stuctue. In summay, we notice that ASes whee explicit MPLS links pevail have a pedominant MPLS cluste while ASes whee u-tun MPLS links pevail have seveal MPLS cluste spead out ove the shells. Additionally, We believe that k- coe decomposition on the top five ASes could have diffeent stuctue due to eithe the u-tun signatue inaccuacy o due to some paticula MPLS deployment. Indeed, we emak only on these ASes a low atio mpls and an unusually high u-tun links pesence. Anothe emakable obsevation elies on the fact that the maximum degee eached by MPLS clustes is consideably high with espect to the netwok size. Indeed, except fo AS174, the est of ASes suggest that moe than 5% of non- MPLS outes ae connected to at least one LSR. Actually, even the oute shells of the k-coe decomposition ae linked diectly with the MPLS clustes located in the C max -coe. This E mpls

7 (a) The k-coe visualization of oute level topology G. (b) The k-coe visualization of MPLS cluste level topology G \ls. Fig. 7. k-coe visualization of G and G \ls. On Fig. 7(a), black nodes efe to non MPLS capable outes and gay nodes efe to LSRs. On Fig. 7(b), black nodes efe to non MPLS capable outes and gay nodes efe to MPLS clustes. behavio matches with ou obsevation of neaest neighbo degee and clusteing coefficient discussed in Sec. V-B. Additionally, because MPLS clustes ae mainly located on C max - coe (even on ASes with high pecentage of u-tun MPLS links), we believe that MPLS plays an impotant ole in ISPs backbone. Fequency (%) Fig. 8. explicit mpls links u-tun mpls links qttl mpls links non mpls links 64.9K 17.6K 14.2K.2K 7.1K 3.9K 3.5K 2.6K 2.5K 2.5K 2.4K 1.9K 1.7K AS1299 AS3356 AS3257 AS174 AS6762 AS2914 AS718 AS6453 AS1273 AS6461 AS683 AS15169 AS9498 Top of ASes with most links discoveed VI. CONCLUSION MPLS usage has an impotant ole on Intenet. Indeed, we found that MPLS tunnels ae tavesed by aound 34% (explicit tunnels) and 42% (explicit and implicit tunnels) of taceoute pobes in ou dataset. In this pape, we tested the detection methods of implicit MPLS tunnels. Ou esults suggested that although Pais taceoute woks popely (avoiding pe-packet load balances) fo MPLS tunnels discovey, u-tun signatues ae commonly biased due to peflow load balancing issues in the etun path. We also povided in this wok a fist and novel oveview about MPLS stuctue. On one hand, ou findings highlight the impotance of MPLS in the Intenet obustness; on the othe hands we showed that MPLS deployment plays an impotant ole in the ISP s backbone. Additionally, ou methodology based on k-coe decomposition allowed us to eveal the fingepints elated with the type of MPLS tunnels that pevails fo a given AS. Howeve, we believe that it is necessay to continue the cuent studies by adding new methods and mechanisms to eveal MPLS pesence. Mainly, because it is necessay to coect the u-tun biases and infe tunnels not evealed by taceoute (invisible tunnels). Additionally, this wok did not study the taffic behavio and pattens elated with MPLS pesence, this infomation could allow one to infe LSR not evealed by the cuent methods. Finally, we plan to pefom long time measuement campaigns in ode to exploe the impact of time on MPLS infastuctues. ACKNOWLEDGMENTS This wok is patially funded by the Euopean Commission funded mplane ICT poject, and also by UBACyT 214 ( BA). REFERENCES [1] B. Donnet, Intenet topology discovey, in Data Taffic Monitoing and Analysis: Fom Measuement, Classification and Anomaly Detection to Quality of Expeience, M. M. E. Biesack, C. Callegai, Ed. Spinge, 213, pp [2] E. Rosen, A. Viswanathan, and R. Callon, Multipotocol label switching achitectue, Intenet Engineeing Task Foce, RFC 331, Januay 21. [3] J. Sommes, B. Eiksson, and P. Bafod, On the pevalence and chaacteistics of MPLS deployments in the open Intenet, in Poc. ACM Intenet Measuement Confeence (IMC), Novembe 211. [4] B. Donnet, M. Luckie, P. Méindol, and J.-J. Pansiot, Revealing MPLS tunnels obscued by taceoute, ACM SIGCOMM Compute Communication Review, vol. 42, no. 2, pp , Apil 212. [5] Y. Vanaubel, P. Méindol, J.-J. Pansiot, and B. Donnet, MPLS unde the micoscope: Revealing actual tansit path divesity, in Poc. ACM Intenet Measuement Confeence (IMC), Octobe 215. [6] B. Augustin, R. Teixeia, and T. Fiedman, Measuing load-balanced paths in the intenet, in Poc. ACM Intenet Measuement Confeence (IMC), Novembe 27. [7] T. Flach, E. Katz-Bassett, and R. Govindan, Quantifying violations of destination-based fowading on the Intenet, in Poc. ACM Intenet Measuement Confeence (IMC), Novembe 212.

8 (a) AS1299 Teliasonea AB, Cmax = 21, Degeemax = 2781, V\ls = 4128, E\ls = (d) AS2914 NTT Ameica Inc., Cmax = 4, Degeemax = 19, V\ls = 187, E\ls = 236 (b) AS174 Cogent Communication, Cmax = 8, Degeemax = 751, V\ls = 4421, E\ls = 8611 (e) AS718 AT&T, Cmax = 3, Degeemax = 745, V\ls = 136, E\ls = 1441 (c) AS6762 Telecom Italia, Cmax =, Degeemax = 564, V\ls = 75, E\ls = 154 (f) AS1273 Cable and Wieless Woldwide plc, Cmax = 3, Degeemax = 86, V\ls = 1127, E\ls = 1215 Fig. 9. k-coe visualization of MPLS cluste inteconnection Gaph G\ls (as). On the top, the ASes show seveal MPLS clustes spead out aound the shells. On the bottom, the ASes show a pedominant MPLS cluste located on Cmax. [8] V. Batagelj and M. Zaves nik, Fast algoithms fo detemining (genealized) coe goups in social netwoks, Advances in Data Analysis and Classification, vol. 5, no. 2, pp , July 211. [9] J. I. Alvaez-Hamelin, A. Baat, and A. Vespignani, Lage-scale netwoks fingepinting and visualization using the k-coe decomposition, in Poc. Advances in Neual Infomation Pocessing Systems, Decembe 26. [] M. A ngeles Seano, M. Boguna, and A. Dı az-guilea, Modeling the Intenet, Eu. Phys. J. B., vol. 5, no. 1 2, pp , Febuay 26. [11] M. G. Beio, J. I. Alvaez-Hamelin, and J. R. Busch, A low complexity visualization tool that helps to pefom complex systems analysis, New J. Phys, vol., no. 12, p. 1253, 28. [12] L. Andesson and R. Asati, Multipocotol label switching (MPLS) label stack enty: EXP field enamed to taffic class field, Intenet Engineeing Task Foce, RFC 5462, Febuay 29. [13] E. Rosen, D. Tappan, G. Fedokow, Y. Rekhte, D. Fainacci, T. Li, and A. Conta, MPLS label stack encoding, Intenet Engineeing Task Foce, RFC 332, Januay 21. [14] P. Agawal and B. Akyol, Time-to-live (TTL) pocessing in multipotocol label switching (MPLS) netwoks, Intenet Engineeing Task Foce, RFC 3443, Januay 23. [15] R. Bonica, D. Gan, D. Tappan, and C. Pignatao, ICMP extensions fo multipotocol label switching, Intenet Engineeing Task Foce, RFC 495, August 27. [16] Y. Vanaubel, P. Me indol, J.-J. Pansiot, and B. Donnet, A bief histoy of MPLS usage in IPv6, in Poc. Passive and Activement Measuement Confeence (PAM), Mach/Apil 216. [17] Y. Vanaubel, J.-J. Pansiot, P. Me indol, and B. Donnet, Netwok [18] [19] [2] [21] [22] [23] fingepinting: TTL-based oute signatue, in Poc. ACM Intenet Measuement Confeence (IMC), Octobe 213. CoNexDat-UBA, Magallanes: Lage scale intenet exploe tool, Decembe 215, M. Luckie, Scampe: a scalable and extensible packet pobe fo active measuement of the Intenet, in Poc. ACM Intenet Measuement Confeence, Novembe 2. K. Keys, Y. Hyun, M. Luckie, and k. claffy, Intenet-scale IPv4 alias esolution with MIDAR, IEEE/ACM Tansactions on Netwoking, vol. 21, no. 2, pp , Apil 211. B. Augustin, X. Cuvellie, B. Ogogozo, F. Vige, T. Fiedman, M. Latapy, C. Magnien, and R. Teixeia, Avoiding taceoute anomalies with Pais taceoute, in Poc. ACM Intenet Measuement Confeence (IMC), Octobe 26. T. C. UCSD, Routeviews Pefix-to-AS mappings (pfx2as) fo IPv4 and IPv6, Octobe 215, see outeviews-pefix2as/215//outeviews-v pfx2as. gz. R. Pasto-Satoas and A. Vespignani, Evolution and stuctue of the Intenet: A statistical physics appoach. Cambidge Univesity Pess, 27.