EVALUATING THE PERCEIVED QUALITY OF INFRASTRUCTURE-LESS VOIP. Kun-chan Lan and Tsung-hsun Wu

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1 EVALUATING THE PERCEIVED QUALITY OF INFRASTRUCTURE-LESS VOIP Kun-chan Lan and Tsung-hsun Wu Natonal Cheng Kung Unversty ABSTRACT Voce over IP (VoIP) s one of the most popular Internet applcatons. Many VoIP protocols (e.g. SIP) are based on a central-server scheme n whch a caller needs to query the central server frst for the callee s nformaton (e.g. IP address) before t can connect to them. However, such a centralzed archtecture has some drawbacks, such as an unbalanced load and a sngle pont of the falure. As a result, some researchers have proposed the use of peer-to-peer (P2P) technques for VoIP communcaton. However, compared to a centralzed approach, settng up a VoIP connecton over multple hops could potentally take a longer tme and mght dscourage users from utlzng ths servce. In ths paper, we set out to study the performance of exstng P2P protocols for VoIP applcatons n a more realstc settng based on traces collected from the Skype network. The Skype traces are used as an nput to model the VoIP network topology. We evaluated the performance of three types of dstrbute hash table(dht) protocols, namely, flat DHT, herarchcal DHT and proxmty DHT, n terms of ther lookup latency for the connecton setup phase of VoIP applcatons. In addton, to understand the feasblty of usng nfrastructure-less VoIP n the real world, we adopted par comparson technques to evaluate the qualty of the user experence. Fnally, we proposed a new hybrd protocol for the nfrastructure-less VoIP communcaton. We concluded that current P2P protocols for VoIP are not satsfactory as compared to the tradtonal centralzed approach and there s stll a lot of room for mprovement. Index Terms Skype, peer-to-peer, pared comparson 1. INTRODUCTION Voce over IP (VoIP) s one of the most popular Internet applcatons, wth many people makng VoIP calls for busness or personal purposes on a daly bass. VoIP nvolves sendng voce transmssons as data packets usng the Internet Protocol. The user's voce s frst converted nto a dgtal sgnal, compressed and splt nto a seres of packets. The packets are then transported over IP networks and reassembled and decoded at the recevng sde. VoIP communcaton typcally conssts of two phases. The frst s the sgnalng phase and the second s the communcaton phase. The sgnalng phase s generally for call setup, such as obtanng the callee s IP address. After a call s set up, the caller then starts communcatng wth the callee over the IP network. Tradtonal VoIP protocols, such as Sesson Intal Protocol (SIP) [1] and H.323 (ITU recommendaton), work n a centralzed manner. For example, SIP employs a central server for storng the caller s and callee s nformaton. Such a centralzed approach s generally consdered to have several drawbacks, such as load mbalance, lack of scalablty, and the potental to develop a sngle pont of falure. Some decentralzed VoIP systems have proposed the use of peer-to-peer technques for the sgnalng phase. However, a P2P approach could potentally ncrease the call setup tme, an mportant factor n people's decsons whether or not to use a VoIP servce, snce the query nformaton mght have to traverse multple hops. A long lookup latency could potentally dscourage users from usng a decentralzed VoIP system. In ths work we set out to study ths problem by surveyng exstng P2P approaches and compare ther performance aganst the centralzed approach usng real world traces. We collected Skype traces and used them to smulate a real-world VoIP network topology. We then compared the performance of three exstng P2P technques, namely, flat DHT, herarchcal DHT, and proxmty DHT. Furthermore, based on our smulaton results, we proposed a new hybrd protocol for nfrastructure-less VoIP communcaton. Fnally, we utlzed the pared comparson technque [2] to understand the userperceved qualty of the experence wth these technques, as compared to the centralzed approach. The rest of ths paper s organzed as follows. In secton 2, we provde an overvew of the related works. We descrbe the collecton and analyss of Skype data n Secton 3. In Secton 4, we compare the lookup latency of dfferent P2P technques based on the collected Skype topology. In Secton 5, we use the pared comparson technque to evaluate the feasblty of usng exstng P2P approaches to replace the centralzed approach. We offer the conclusons and dscuss drectons for future work n Secton 6.

2 2.1 Skype 2. RELATED WORK Skype s currently the most popular VoIP software n the world. Skype allows two users to establsh audo/vdeo streams wth each other. Some of Skype's servces are smlar to those offered wth MSN and Yahoo messenger, but ther underlyng technques and protocols are dfferent. In the Skype overlay network, there are two knds of nodes, ordnary and super nodes. An ordnary node s a normal Skype clent that can make voce calls and send text messages. When an ordnary node has a publc IP address and s equpped wth rch computng resources, such as powerful CPU, large memory and suffcent network bandwdth, t could be randomly selected to become a super node. A super node wll share some of ts resources to help mantan the Skype overlay network. Each ordnary node has to connect to at least one super node. Therefore, each node mantans a table of some Skype super nodes called a host cache. The host cache fle lsts a set of IP addresses and port numbers of super nodes. In ts newer versons, Skype has encrypted the IP addresses n the host cache. 2.2 Peer-to-Peer network P2P overlay networks [3] are dstrbuted systems wthout centralzed control. In a P2P network, peers form selforganzng networks that are overlad on top of the IP networks.p2p overlay systems are not lke tradtonal clentserver systems. An mportant ssue n a P2P system s to assgn and locate objects among nodes, and ths s typcally acheved by a P2P lookup servce. In the current P2P research, there are two classes of P2P overlay network: structured and unstructured. In a structured P2P network, the topology s tghtly controlled and the contents are placed at some specfc nodes followng certan rules to make subsequent queres more effcent. A structured P2P system typcally uses a Dstrbuted Hash Table (DHT). There are numerous exstng DHT works, such as Chord [3], Pastry [4], CAN [5], and Tapestry [6]. An unstructured P2P system, on the other hand, s composed of nodes that jon the network wth some loose rules wthout any pror knowledge of the topology. Nodes use a floodng mechansm to send queres across the overlay wth a lmted scope. The unstructured approach cannot fnd a specfc object effcently due to the use of floodng and ts performance s O(n), where n s the number of nodes n the overlay system. In addton, n an unstructured network, there s no guarantee that a specfc object can eventually be found, because there s no rule for placng/locatng an object. On the other hand, n a structured network, t s guaranteed that a specfc object can be found n a bounded tme O(log n). Therefore, we conclude that a structured P2P system mght be more sutable for VoIP. 2.3 Infrastructure-less VoIP Some prevous studes have suggested runnng a VoIP applcaton on top of a P2P network. Kundan Sngh [7] proposed a pure P2P archtecture for the SIP-based IP telephony systems. SIP s a sgnalng protocol, and generally used to control multmeda communcaton sessons lke VoIP. Ther P2P-SIP archtecture supports basc user regstraton and call setup, as well as advanced servces such as offlne message delvery, voce/vdeo mals and multparty conferencng. SOSIMPLE [8] s a fully decentralzed, P2P, standards-based approach to communcatons. By buldng on top of the exstng SIP/SIMPLE nfrastructure for VoIP and IM, SOSIMPLE avods the tradtonal centralzed archtectures, elmnates dependency on constant Internet connectvty, and supports ad hoc groups. SOSIMPLE s mplemented as a DHT overlay based on Chord usng SIP messages. In addton, t replcates locaton nformaton for the purpose of relablty. The DHT s used only for lookups, and actual communcaton s passed drectly between clents. However, the feasblty of the above approaches when used n a real world scenaro where there are a large number of nodes s not yet clear. 2.4 Qualty of Experence For evaluatng the perceved qualty of experence, Mean Opnon Score (MOS) s the most wdely used technque for multmeda, such as audo, vdeo, and voce telephony. MOS has a numercal ndcaton of the qualty of receved meda. The reason why MOS scorng has been wdely adopted s because t s smple and ntutve, but t has many problems [9]. In ths work, we thus adopt the par comparson technque [10] to evaluate the user-perceved qualty of VoIP for dfferent P2P archtectures. The pared comparson method can generate a rato-scale score and the executon of such a method s easy for partcpants. The only thng that the partcpants need to do s to compare two optons and decde whch one has the better qualty. Unlke MOS, the partcpants wll not have dffculty n decdng whch ratng they should gve when usng the pared comparson methodology. After fnshng the comparson of all pars, the Bradley-Terry-Luce (BTL) model s then used to generate rato-scale scores based on the results of par comparson, as shown n the equaton. Here the T stands for the opton. The π(t ) s the estmated score of T and the P j s the probablty of choosng T over T j. P j ( T ): j P : = π ( T ) ( T ) + π( T ) π T : th stmul π the estmated score of T probablt y j of selectng T over T j (1)

3 The advantages of the pared comparson methodology over the tradtonal MOS ratngs are 1) the ratng procedure s smpler thus less burden s on experment partcpants, 2) t derves rato-scale scores, and 3) t enables systematc verfcaton of partcpants nputs. Snce f a>b and b>c, then we can obtan a>c. We verfy do the nputs have contradctory. 3. COLLECTION AND ANALYSIS OF SKYPE TRACES In order to understand the performance of dfferent P2P protocols n a more realstc settng, we collected traces from the Skype network and used the collected data to model the topology of a VoIP network. In ths secton, we frst descrbe how we collected Skype data. We then provde an analyss of Skype topology and user sesson tme. The results of our work mght be useful to people who want to mplement a realstc topology generator for ther VoIP smulatons. 3.1 Data Collecton We collected traces from the Skype network because t s currently the most popular VoIP software and has the largest user populaton n the world. As mentoned prevously, the Skype overlay network has two types of nodes, ordnary and super. All the ordnary nodes need to connect to a super node before they ntate/receve a call. Therefore, every ordnary node has a lst of IP addresses of super nodes. Snce a Skype super node s also a normal Skype user, by collectng the lsts from ordnary nodes, we could obtan a lst of actve users on the Skype network Detals of the Experment In ths secton, we descrbe the process of collectng the Skype user topology. When a Skype node s actve, t mantans a table (called host cache) of other Skype nodes. The host cache conssts of IP addresses and port numbers of a set of super nodes, and t refreshes constantly. The host cache s mplemented as an XML fle named shared.xml on the Lnux system. Note that we used an older verson of the Skype software for our experment, snce that the current verson of Skype software encrypts the IP nformaton of the super nodes. Because the host cache s only refreshed when one starts the Skype software, we perodcally re-started the Skype program to get the lst of currently actve nodes. We contnuously collected the Skype traces for one week and dscovered 164,377 super nodes, whch s by far the largest Skype traces as far as we know [10]. We then used MaxMnd GeoIP [11] to determne the lattudes and longtudes of the collected nodes. However, we found only 12,372 unque (lattude, longtude) locatons due to the lmtatons of GeoIP, n whch the IPs belongng to a physcal subnet could have the same lattude and longtude nformaton. Most of the Skype nodes are concentrated n Amercan and Europe, whch s consstent wth the results of a prevous study of Skype network dstrbuton [12]. 4. COMPARING DIFFERENT DHT TECHNOLOGIES FOR VOIP COMMUNICATION In ths secton, we compare the performance of usng varous DHT technologes for the sgnalng phase of VoIP communcaton n a trace-based settng. The performance metrc we consder n ths work s the lookup latency for a caller to locate ts callee IP n a DHT overlay network. We wrote a program usng our Skype-trace-based topology to smulate the query routes of dfferent DHT methodologes as descrbed below. In a DHT network, the nformaton s evenly dstrbuted among all the overlay nodes. To locate the nformaton, a lookup query mght need to traverse multple nodes n the DHT overlay. Here we assume the lookup latency s manly contrbuted by the propagaton delay and queung delay that a packet mght ncur when t travels from the source (.e. the node ntatng the query) to the destnaton (.e. the node n whch the quered nformaton s stored). We model the queung delay by adoptng a smlar approach as n a pror work [13]. We estmate the propagaton delay based on our collected Skype traces. Usng our traces, we can estmate the geographc dstance between any two nodes and ther correspondng propagaton delay. More specfcally, we frst collected IPs of Skype nodes and then obtaned (lattude, longtude) of collected IPs from Maxmnd GeoIP database. The geographc dstance of any two nodes s calculated based on ther (lattude, longtude) nformaton and ther propagaton delay s calculated by dvdng the dstance by the speed of lght. Note that the way we calculated the propagaton delay s just an approxmaton snce t s dffcult to nfer the exact route a packet actually travels between any two arbtrary nodes due to the lmtaton of our traces. In a DHT-based VoIP network, to locate the IP address of the callee, the caller frst ntates a query through the DHT overlay. The query packet mght travel several hops to reach the node where the callee s IP nformaton s stored. For example, for a query route as shown n Fg. 1, the query packet ntated by the caller (.e. node A) mght have to frst travel through node B and C before t fnally arrves the node where the callee s nformaton s stored (.e. node D). In ths example, the lookup latency for node A to locate ts callee s P1+Q1+P2+Q2+P3+Q3+P4, where P1, P2, P3, and P4 are the propagaton delays and Q1, Q2, and Q3 are the queung delays. Here we assume the destnaton (.e. node D) can drectly return the callee s IP nformaton to the source (.e. node A) [4].

4 Fg. 1. Lookup latency of a query route Intutvely, the lookup latency of a centralzed approach wll be shorter than the dstrbuted approaches based on the P2P protocols, gven that a query does not need to traverse multple hops when a centralzed approach s used. Therefore, we frst compute the lookup latency of the centralzed approach as a baselne before we compare dfferent DHT technologes for VoIP communcaton. We pcked a Skype server ( ) whch s located n Denmark [14] and computed the propagaton delay of every node (n our collected traces) to ths server. The results are shown, as the lne ndcated as Centralzed, n Fg. 2. We fnd that half of the nodes have a lookup latency of less than 30ms. Usng ths as a baselne, we then compare how far the performance of three dfferent DHT technologes, namely, flat DHT (FDHT), herarchcal DHT (HDHT) and proxmty DHT (PDHT), s from the centralzed approach s. 4.1 Flat DHT We select Chord as an example of flat DHT and use the SHA-1 hash functon to hash the IP addresses nto ther correspondng IDs. All the nodes are arranged nto a rng ordered by ther IDs. We randomly choose a caller and a callee from the collected Skype traces, and use the Chord routng protocol to fnd the callee s IP address. We repeat the experment 10,000 tmes, and the results are shown, as the lne ndcated as FDHT, n Fg. 2. Clearly, the lookup latency usng FDHT s sgnfcantly worse than that of the centralzed approach. 4.2 Herarchcal DHT In herarchcal DHT [15], nodes are organzed nto groups, and each group has ts own ntra-group overlay network. The groups are organzed as a top-level overlay network. To locate the node whch s responsble for a key, one frst fnds the group responsble for the key n the top-level overlay. The selected group then uses ts ntra-group overlay to fnd the node whch s responsble for the key. In the top-level overlay network, some nter-group routng can be used to locate the responsble group. In the lower-level overlay network, some ntra-group routng s ssued to fnd the node that has the key. In our experment, we used Chord to mplement both the nter-group routng and the ntra-group routng. The total number of nodes s 164,377, dvded nto 410 groups wth a group sze of 400 nodes. Node IDs are assgned sequentally for each group (e.g. the frst group wll have nodes wth ID from 1 to 400, and the second group wll have nodes wth ID from 401 to 800). The other experment settngs are smlar to those we prevously used for the flat DHT approach. We randomly select (caller, callee) pars and repeated the experments for tmes. The results are shown, as the lne ndcated as HDHT, n Fg. 2. The mprovement from FDHT s very small when compared to the results for the flat DHT approach. As we descrbed prevously, the routng path selected by HDHT conssts of two parts: nter-group routng and ntra-group routng. However, n ths case, we assgn the node IDs sequentally for each group. We do not utlze the proxmty property, snce there s no physcal correlaton for nodes n the same group (e.g. the dstance between two nodes n the same group mght be greater than the dstance between two nodes n dfferent groups). 4.3 Proxmty DHT We next consder the property of node proxmty nto the desgn of HDHT. Specfcally, we group nodes whch have the same IP prefx (e.g x.x) nto the same group. Here we assume that nodes whch are n the same subnet mght be physcally close to each other. As shown, as the lne ndcated as PDHT, n Fg. 2, consderng the proxmty of nodes does mprove the performance of HDHT to some extent. On the other hand, the lookup latences of three DHT technologes are stll sgnfcantly longer than that obtaned when usng the centralzed approach. In PDHT, we fnd that a sgnfcant porton of the lookup latency s contrbuted by the tme when a packet travels across dfferent groups n the top-level overlay network. In practce, the nodes n the top-level overlay are typcally selected from nodes that are more persstent (.e. have a longer sesson tme), so that the topology wll reman more stable and result n better network performance. 4.4 Hybrd approach Based on the above observaton, we propose a new desgn by modfyng the routng mechansm of the top-level nodes. Specfcally, smlar to some pror work, such as the OneHop approach [16], each node n the top-level overlay mantans a routng table that contans complete nformaton to reach any other nodes n the top-level overlay. Therefore, each lookup n the top-level overlay takes only one hop. The overhead here s manly the cost of mantanng a routng table that has the nformaton of every node n the top-level overlay. If the network churn s hgher, t wll consume more bandwdth to keep the routng table up to date. However, n the top-level overlay network, the nodes are selected based on ther stablty. For nodes n the traces we collected, the probablty of havng a sesson tme greater than 1 hour was 90% and average sesson tme was 30 hours, whch suggests that t s feasble to mplement an OneHop-lke technque for the top-level overlay. The smulaton results are shown n Fg. 2, n whch the top-level overlay s mplemented usng OneHop and the lower-level

5 overlay s mplemented wth Chord, and we thus call our proposed approach a Hybrd one. In our smulaton, the number of super-nodes n the top-level overlay s 410. As shown n Fg. 2, the lookup latency s sgnfcantly mproved when the Hybrd approach s used as compared to other DHT technques. Note that, although the hybrd approach could sgnfcantly mprove the lookup latency as compared to other DHT technologes. It mght potentally ntroduce more control traffc snce t has to mantan the full topology nformaton of the upper overlay. On the other hand, for the herarchy-based DHTs (lke PDHT and HDHT) the routng table sze could be sgnfcantly smaller than a flat DHT (FDHT). Ths mght become an mportant ssue when deployng a large-scale P2P VoIP network. For example, the sze of routng table of FDHT n our case s around log 2 (164337) whle the routng table sze for HDHT and PDHT s only half of that (.e. around log 2 (400)) for 10 tmes by 15 dfferent people. In ths experment, to avod fatgue factor, people were allowed to take a break f they want. Fg. 3 shows an example of the resultng comparson matrx for one ndvdual. The a j ndcates the number of tmes when approach was rated better than approach j. For nstance, a 12 s 18 and a 21 s 2, whch ndcates 90% (.e. 18/(18+2)) of the tme the centralzed approach was consdered of havng a better response tme than the FDHT, and 10% of the tme FDHT was consdered better than the centralzed approach. After collectng all 15 people s measurements, we then used the BTL model to generate average rato-scale QoE scores, as shown n Fg. 4. Fg. 3. Comparson matrx The score s normalzed from 0 to 1, wth the score of the centralzed approach set to 1. As shown n Fg. 4, our results suggest there s no dfference n the perceve qualty between FDHT and HDHT. In addton, the perceved qualty of experence of all DHT approaches s sgnfcantly worse than that of the centralzed approach. Fg. 2. Lookup latency (Centralzed vs. FDHT vs. HDHT vs. PDHT vs. Hybrd) 5. QUALITY OF EXPERIENCE In the prevous secton we showed that the lookup latency of exstng DHT technologes s sgnfcantly worse than that of the centralzed approach. In ths secton, we set out to understand f ths observaton s consstent wth the human percepton of dfferent technologes, snce t mght be possble that human users wll not be able to notce dfferences that are n the order of a couple of hundred mllseconds. We used a pared comparson technque to evaluate the user-perceved qualty of call setup tme when dfferent technologes were employed. We assume that the lookup latency s the major delay n the call setup tme. We wrote a program smulatng the behavor of VoIP software. Specfcally, we frst made a call and then wated for a response. In our smulaton, the response/watng tme s derved from the probablty dstrbutons shown n Fg. 2. We used a par comparson technque to evaluate f approach s better than approach j for the fve dfferent approaches we dscussed. There were C(5,2)=10 pars n our comparson. Each par was compared Fg. 4. QoE score (Centralzed, FHDT, HDHT, PDHT, Hybrd) 6. CONCLUSIONS AND FUTURE WORK Some pror studes have proposed usng P2P approaches for the sgnalng phase of VoIP communcaton. However, a P2P approach may ncrease the call setup tme, an mportant factor when people decde whether or not they wll use a VoIP servce, gven the query nformaton mght have to traverse multple hops n a VoIP network. In ths paper, we set out to study the feasblty of usng exstng P2P

6 approaches for VoIP communcaton based on real-world Skype traces. We compared the performance of three exstng P2P technques, namely, flat DHT, herarchcal DHT, and proxmty DHT, under a trace-based VoIP network topology. In addton, we proposed a new hybrd protocol for nfrastructure-less VoIP communcaton. We also utlzed the pared comparson technque to understand the user-perceved qualty of experence for the above technques, as compared to the centralzed approach. Unfortunately, we found that current DHT-based protocols are not satsfactory for VoIP communcaton when compared to the tradtonal centralzed approach. On the other hand, ths suggests that there s stll a lot of room to mprove the current P2P protocols for VoIP communcaton. Fnally, the results we obtaned n ths paper are mostly based on the use of Chord. However, there are other DHT protocols such as CAN, Pastry, etc. We plan to evaluate the performance of usng other DHT protocols for nfrastructure-less VoIP communcaton n future research. Note that due to the lmtaton of our trace collecton, we do not have the complete nformaton of the whole Skype network and the detaled node behavor. Although ths mght ntroduce some naccuracy, we d lke to argue that 1) the number of super nodes we collected s, as far as we know, by far the largest Skype traces as compared to the pror work. 2) the topology analyss based on our traces s consstent wth the results of the pror work. 3) we only use the collected traces as an approxmaton to model a VoIP network to understand the feasblty of mplementng exstng DHT-based P2P technologes for VoIP communcaton. Intutvely, a more complete Skype network map (as compared to our partal nformaton) wll suggest a larger DHT overlay and possbly even hgher lookup latency. Therefore, we beleve that, although we can not obtan the detaled behavor of Skype network due to the lmtaton of our traces, our work s stll useful as a frst step toward understandng the feasblty of mplementng an nfrastructure-less VoIP system usng DHT-based technologes. 7. REFERENCES [1] M. Handley, et al., SIP: Sesson Intaton Protocol: RFC Edtor, [2] K.-T. Chen, et al., "A crowdsourceable QoE evaluaton framework for multmeda content," presented at the Proceedngs of the seventeen ACM nternatonal conference on Multmeda, Bejng, Chna, [3] I. Stoca, et al., "Chord: a scalable peer-to-peer lookup protocol for Internet applcatons," Networkng, IEEE/ACM Transactons on, vol. 11, pp , [4] A. Rowstron and P. Druschel, "Pastry: Scalable, Dstrbuted Object Locaton and Routng for Largescale Peer-to-peer Systems," Proc. Mddleware, [5] S. Ratnasamy, et al., "A scalable content-addressable network," presented at the Proceedngs of the 2001 conference on Applcatons, technologes, archtectures, and protocols for computer communcatons, San Dego, Calforna, Unted States, [6] B. Y. Zhao, et al., "Tapestry: a reslent global-scale overlay for servce deployment," Selected Areas n Communcatons, IEEE Journal on, vol. 22, pp , [7] K. Sngh and H. Schulzrnne, "Peer-to-peer nternet telephony usng SIP," presented at the Proceedngs of the nternatonal workshop on Network and operatng systems support for dgtal audo and vdeo, Stevenson, Washngton, USA, [8] D. A. Bryan, et al., "SOSIMPLE: A Serverless, Standards-based, P2P SIP Communcaton System," n Advanced Archtectures and Algorthms for Internet Delvery and Applcatons, AAA-IDEA Frst Internatonal Workshop on, 2005, pp [9] Y.-C. Chang, et al., "Onlne Game QoE Evaluaton usng Pared Comparsons," Proceedngs of IEEE CQR [10] Y. R. Y. Hayong Xe, "A Measurement-based Study of the Skype Peer-to-Peer VoIP Performance," presented at the In Proceedngs of IPTPS, Bellevue, WA,, [11] M. GeoIP. [12] S. Guha, et al., "An expermental study of the skype peer-to-peer vop system," presented at the Proceedngs of IPTPS,, [13] M. Garetto and D. Towsley, "Modelng, smulaton and measurements of queung delay under long-tal nternet traffc," presented at the Proceedngs of the 2003 ACM SIGMETRICS nternatonal conference on Measurement and modelng of computer systems, San Dego, CA, USA, [14] S. A. Baset and H. G. Schulzrnne, "An Analyss of the Skype Peer-to-Peer Internet Telephony Protocol," n INFOCOM th IEEE Internatonal Conference on Computer Communcatons. Proceedngs, 2006, pp [15] L. Garces-Erce, et al., "Herarchcal peer-to-peer systems," presented at the Parallel Processng Letters, [16] P. Fonseca, et al., "Full-Informaton Lookups for Peerto-Peer Overlays," IEEE Trans. Parallel Dstrb. Syst., vol. 20, pp , 2009.

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