On the cohabitation of Adaptive Search Radius enabled peers with regular eMule peers

Ricardo Lopes Pereira Teresa Vaz˜ao Instituto Superior T´ecnico / Inesc-id Instituto Superior T´ecnico / Inesc-id Av. Professor Cavaco Silva Av. Professor Cavaco Silva 2744-016 Porto Salvo, Portugal 2744-0160 Porto Salvo, Portugal Email: [email protected] Email: [email protected]

Abstract— Adaptive Search Radius (ASR) is a peer selection restrictions to manage P2P traffic. In order for this conflicting method that can be used in Peer-to-Peer (P2P) file sharing goals to coexist, P2P file sharing applications must have a applications, such as eMule or BitTorrent, in order to reduce smaller impact on the Internet. the generated traffic. ASR enabled peers try to exchange file parts with the peers which are topologically closer, instead of P2P file sharing research has mostly focused on developing swapping parts indiscriminately with peers near and far. Traffic efficient content location protocols, capable of overcoming travels fewer hops, resulting in fewer congested links, less traffic the limitations imposed by the central index server used by through tier-1 ISP’s core links and less traffic exchanged among popular P2P file sharing applications. Distributed Hash Table ISPs. (DHT) algorithms such as Chord [4], Pastry [5] or CAN We believe it would be easier for ASR to be accepted if it was presented as an enhancement or modification to current [6] have shown that completely distributed content location P2P applications, instead of an entirely new application. In this indexes are possible. Recent implementations of eMule and paper we study the use of ASR with the eMule protocol and BitTorrent, the most popular P2P file sharing applications [2], the impact a progressive migration would have. We found out use the Kademlia [7] DHT as a way to overcome centralised that introduction of ASR enabled peers reduces the generated server/tracker dependency. However, what consumes the most traffic but also improve the performance of regular eMule peers, preventing users from adopting ASR. time and bandwidth is the file exchange itself. We propose modifications to the original ASR algorithm in Although eMule and BitTorrent use different algorithms to order for ASR peers to present superior performance when process the file exchange, they both contact all the peers they combined with regular eMule peers, prompting users to migrate know, regardless of topological distance. The same file part willingly. We also propose a migration plan which would enable that could be serviced by a peer a few hops away, may be ASR to be gradually introduced into the eMule population. We resort to simulation studies to backup our proposal. fetched from a peer across the world, crossing many hops, and contributing to congestion on all of them. Ignoring protocol I.INTRODUCTION overheads and retransmissions, regardless of the protocol used, Peer-to-Peer (P2P) file sharing applications were rapidly in order to distribute a file to n peers, that file must be adopted by users worldwide, becoming the de facto method transported over the network n times. What will determine for quick and convenient access to multimedia files. The the impact of the file transfers on the Internet is how many announced use of P2P technology by large content producers (and which) hops are crossed by each file part. as a distribution medium [1], leads us to believe that it its Adaptive Search Radius (ASR) is a peer selection algorithm popularity will continue to increase. where peers restrain from contacting distant peers. Only peers P2P applications such as eMule or BitTorrent are robust within the search radius (in network hops) may be contacted. and scalable. However, they were design to use as much The search radius is regularly updated as to guarantee that a bandwidth as possible, with no regard for network topology. minimum number of sources for each file part are reachable. These applications are often left working unattended, 24 hours ASR, which will be described in section II, significantly a day, creating traffic patterns very different from those of reduces network traffic and provides faster downloads. Hyper Text Transfer Protocol (HTTP), for which most ISPs The use of ASR could reduce the impact of P2P file sharing designed their networks. Measurement studies indicate that traffic on the Internet. For ASR to be widely spread it would P2P traffic represents from 35% to 60% of the traffic carried by have to be used in popular implementations of eMule or transit (tier 1) Internet Service Providers (ISPs) and is growing BitTorrent, as the deployment of a new protocol would be in significance [2], [3]. P2P file sharing applications do not difficult. In this paper we study the benefits of migrating eMule provide revenue to ISPs, while using as much capacity as clients to an ASR enabled version of the eMule protocol. But, possible thus affecting the performance of other applications. as will be shown in section III, the original ASR algorithm When P2P file sharing protocols are finally being considered only accomplishes all its goals when used by every peer. by content producers as a legitimate file distribution medium, Modifications to the ASR algorithm are proposed and most ISPs are forced to resort to Quality of Service (QoS) analysed in section IV. These allow ASR to provide faster download times than regular eMule, which is a powerful the search radius, information about the file parts shared is incentive for users to migrate. A migration plan, to go from recorded and, when possible, a download request is issued. todays eMule population to a 100% ASR enabled population, Two low overhead methods for calculating the hop distance is also proposed. to a peer are available, both using the time-to-live (TTL) Further motivation for this work is provided in the related field of the IP header. The first may be performed before work analyses, presented in chapter V. Chapter VI presents establishing the connection, and consists in performing an the conclusions. UDP ping. The resulting ICMP message will indicate the TTL value of the packet that reached the peer, which subtracted II. ASR DESCRIPTION to the original TTL, will indicate the distance. The second In this section we start by briefly describing the eMule method consists in comparing the TTL value of the arriving protocol as a motivation for the ASR algorithm. In this paper packets of the TCP connection with their original value. In ASR is evaluated using the eMule protocol, however we order to know their original value an extension to the eMule believe that it may also be applied to BitTorrent. protocol would be used, allowing peers to inform each other of which TTL value is used in the departing packets. A. eMule protocol Whenever new information about the file parts shared by The eMule protocol relies on the use of centralised servers a peer is available, the minimum file part availability is which know which peers share which files. In order to down- calculated. This is defined as the number of peers within the load a file, a peer connects to the server (TCP) and periodically search radius that share the rarest file part. The ASR peer asks for file sources (peers sharing the file). is configured with two constant thresholds: upper and lower Files are divided into 9.5MB parts, which represent the minimum file part availability. When the minimum file part minimum sharing granularity. In order to download from availability is larger than the upper threshold, the search radius another peer (uploader peer), a TCP connection is created to is decreased one hop if this allows the lower threshold not to the uploader. After asking which file parts the uploader shares, be crossed. In this situation, the peer will contact all the peers the downloader determines if it can provide any new file parts. now outside the search radius where it is queued waiting for a If so, the downloader asks for a chunk (180KB) of a file part, download slot, and give up the download request. Should the and will keep on asking for file chunks as long as the uploader minimum file part availability drop bellow the lower threshold, provides them. the search radius will be increased one hop. Only after the When an uploader receives a request for a file chunk, it newly reachable peers have all been contacted may the search determines what to do according to the score it assigns to each radius be increased again. ASR only limits the distance of the downloader. This score is proportional to the time the peer has peers one downloads from, not the ones we upload to, as this been waiting to download and the amount of data the uploader would hinder a solitary peer’s (or set of peers) capacity to has received from this peer. It is also inversely proportional to begin downloading a file. the amount of data already sent by the uploader. Each peer still downloads the same amount of information The uploader maintains a queue, sorting downloaders ac- but packets cross fewer links, resulting in fewer congested cording to their scores. Only the peers at the top few positions links. This frees up capacity for other protocols and for ASR may download. In order to avoid fluctuations, once a peer is itself. The fact that ASR peers try to download from fewer allowed to download, its score is doubled during 15m. If the peers is compensated by each peer having shorter upload peer which is to start downloading is no longer connected, a queues. Peers download from fewer sources but have to wait callback connection is performed. less time to start downloading. Each peer tries to download from every other peer it knows about. As such, the upload queues are usually very long. III. PROBLEM ANALYSES ASR’s main goal is to reduce the impact of P2P traffic on B. Adaptive Search Radius the Internet. But even if this goal is fully achieved, it will not ASR is a peer selection algorithm, which limits the peers be enough to motive users into migrating to an ASR enabled contacted to the ones closer, in network hops. Initially, an implementation of eMule. There must be a more immediate ASR peer behaves just like a regular eMule peer: after finding reward: the capacity to download faster. However, we found out which peers share the wanted file it begins to contact that although ASR contributes to reduce global traffic, it only every one of them. When it establishes contact with a peer, it provides its users with faster downloads when it is utilised by measures the distance to it in network hops. For each file being 100% of the users. downloaded, a search radius is maintained. The initial search We simulated the distribution of a 200MB file, seeded by radius is 32 hops, in order to encompass the entire Internet. For 6 hosts, to 554 other hosts. After completing the download, popular files, shared by many peers, the initial search radius the peer would continue to share the file for an average of need not encompass the entire Internet, preventing the peer 2000s (exponential distribution). Peer download start time was from contacting a large number of peers. Should the contacted uniformly distributed over the first 4 hours of simulation. peer be outside the search radius (further away), the connection We experimented with 5 different scenarios: every peer is closed and the peer forgotten. Should the peer be within using regular eMule, every peer using ASR enabled eMule, half the peers using each method, 90% using one method and they know. As there is more link capacity available and shorter the other 10% using the other. We intended to observe what upload queues they will experience faster downloads. would happen when more and more users migrated to an ASR When using an 100% ASR population, the short upload enabled implementation of eMule, until all were using it. queues compensate for fewer peers to download from. When Simulation was performed using the SSFNet 2.0 network eMule peers are present, upload queues become large. ASR simulator, where we implemented eMule. We utilised a transit- peers have to wait for their turn to download from a few peers stub network generated using the GT-ITM topology generator while eMule ones wait to download from a large set of peers. [8]. It has 2 transit networks (representing tier-1 ISPs), con- necting 28 stub networks (representing national or regional IV. SOLUTION AND MIGRATION PLAN ISPs) to which peer hosts were connected. This adds up to The behaviour of the currently installed eMule peers may 294 routers and 560 hosts, which are, on average, 14 hops not be changed, as such its up to ASR to take eMule’s way from each other. The maximum distance between two behaviour into account. The key to ASR’s success are small hosts is 18 hops. upload queues. ASR peers only contact peers within their Hosts are connected to their routers using 0.5 and 1Mb/s search radius and expect them to have small upload queues, links. Stub routers are connected among themselves using 5 which does not happen in the presence of eMule peers. and 10 Mb/s links. Stub networks are connected to each other We decided to modify the queue score calculation algorithm and to transit networks using 2 and 5 Mb/s links. Transit used by ASR peers in order to favor other ASR peers over routers are connected using 10, 20 and 50Mb/s links. Transit eMule peers. We tried to maintain the original score for ASR networks are connected using 10 and 20Mb/s links. peers but multiply it by a factor for eMule peers. A value The eMule server was run on one of the transit networks, as of 0.2 provided ASR peers with download times lower than to be close to every peer. The ASR algorithm was configured those of eMule, which increased compared to the use of the with a minimum file availability lower threshold of 3 and a original ASR, but still showed an improvement over a 100% upper threshold of 6. eMule population. This worked when ASR peers accounted Table I summarises the simulation results. Simulations were for 10% and 50% of the population. However, when ASR repeated using different random seeds. Values are reported peers became 90% of the population, they performed worse with a 95% confidence level and confidence intervals inferior than eMule peers, which would prevent the migration from to 5%. The same seed was used under the 5 scenarios, completing. Furthermore, the smaller the penalty factor the guaranteeing that for each seed the same peer asked the same more traffic was generated, as eMule peers were prevented file at the same time. from downloading as much from close by ASR peers. The average download time is reduced by the introduction The eMule scoring method introduces reciprocity: peers of more ASR peers. The same happens, as expected, to all the tend to upload to the peers they are allowed to download from. traffic types: transit, the sum of traffic crossing all links outside By favoring other ASR peers, ASR peers upload less to eMule stub networks; stub, the sum of traffic over all stub network peers and, as such, also get to download less from them. eMule links; inter-ISP, traffic crossing the links which connect stub peers within the search radius of a file should be allowed to networks to transit ones (subset of transit traffic). The average download the file, otherwise it will be difficult to download hops crossed, the traffic observed over every link divided from them. As ASR peers will never ask to download from by the file bytes exchanged among peers, estimates how far peers outside the search radius, there is little benefit to be file parts are obtained from. We can see that the more ASR gained from uploading to these peers. Nevertheless, ASR does peers the closer data comes from. The number of connections not discriminate ASR peers outside the search radius (other between peers (protocol overhead) also declines as the number than the low score by not downloading) in order for them to of ASR peers increases. have the chance to download a file. Likewise, eMule peers From the perspective of an ISP this is all good news, outside the search radius should not be completely stopped unfortunately peers using regular eMule experience faster from downloading. downloads that those using ASR. Users would never con- We tried several variations of the same base algorithm: sider using a slower application when the one they already providing full score to other ASR peers, give eMule peers use works better, especially during the initial stages of the within the search radius of a file a fraction of the score and migration (10% ASR), where ASR users experience longer an even smaller fraction to those outside the search radius. The download times than when there were only eMule peers. variation that provided the best performance with the lowest Furthermore, the introduction of ASR peers benefits eMule traffic consisted in providing eMule peers within the search users, who experience even faster downloads as the number radius with a penalty factor divided by their distance (in hops) of ASR peers increases. and those outside the search radius with a tenth of this. The faster average download times and reduced traffic are We also found that, as eMule peers will not be as coopera- explained by the behaviour of ASR peers, who only download tive to ASR peers as other ASR peers, the search radius should from nearby peers. As a consequence less capacity is used on be larger in order to encompass more peers. In counting the each link and the upload queues on each peer are shorter. On number of sources for each file part, ASR was modified to the other hand, eMule peers try to download from every peer account eMule sources as a fraction of a source, thus resulting TABLE I COHABITATION USING ORIGINAL ASR ALGORITHM

100% ASR 90% ASR 10% eMule 50% ASR 50% eMule 10% ASR 90% eMule 100% eMule Avg. Download time 3136.32 3262.95 2596.16 4627.36 3150.92 5807.59 4114.68 4400.12 Avg. search radius 7.4 7.29 7.2 7.24 Transit traffic 1.37E+11 1.69E+11 3.07E+11 4.20E+11 4.58E+11 Stub traffic 5.10E+11 5.29E+11 6.04E+11 6.58E+11 6.77E+11 Inter-ISP traffic 9.93E+10 1.11E+11 1.65E+11 2.06E+11 2.21E+11 Num. connections 102359 114415 313023 630575 747180 Avg. hops crossed 5.84 6.31 8.22 9.73 10.24

TABLE III in larger search radiuses. The same penalty factor used to COHABITATION WHEN REFUSING TO SERVICE EMULEPEERS modify the queue score was used to account eMule sources. Several penalty factors were tried. Figures 1(a) and 1(b) 90% ASR 10% eMule show how the different penalty factors impact the download times perceived by both ASR and eMule users. The lower Avg. Download time 3075.54 6253.65 Avg. search radius 7.63 this factor, the higher the generated traffic, but the better the Transit traffic 1.85E+11 performance of ASR, at the expense of eMule’s. A value of Stub traffic 5.34E+11 0.1 was chosen as the most appropriate to incentive users to Inter-ISP traffic 1.21E+11 migrate from eMule to ASR. Num. connections 167789 Avg. hops crossed 6.49 Table II summarises the results obtained with the revised ASR algorithm. The first users to migrate to ASR experience much faster download times, which decrease continuously as more users migrate and ASR is able to use shorter search ASR becomes common. When ASR enabled peers are the radius. Simultaneously, the introduction of ASR peers also dominant application type, newer versions of the ASR enabled provides eMule peers with faster download times, even though eMule clients will refuse to upload to legacy eMule peers. they are considered “second class citizens” by ASR peers. As users migrate to these newer versions, those using legacy The traffic saving properties of ASR are still maintained, eMule will start to experience ever increasing download times although savings are inferior than those of the original ASR and decide to migrate. This is a feasible migration plan as protocol, showed in table I. Everyone stands to gains from the most eMule clients are distributed as free or open software, introduction of the revised ASR peers. and migrating incurs in no cost to users. However, users are only compelled to migrate up to the The simulations shown up to now consisted of all peers point when ASR peers become dominant. When 90% of the downloading the same file. In table IV we show a more realist peers use ASR, those who stuck to eMule will observe much scenario, where peers continuously download new files. Each faster downloads. This is explained by ASR peers gathering peer starts a new download every 6 hours on average (uniform in small clusters, not asking to download from peers outside distribution). There is a total of 1000 files to download at a their search radius. The remainder eMule peers, by asking to given time, each shared by 4.85 peers and with sizes from download from everyone, are able to take advantage of ASR 3 to 900MB (uniform distribution) Every 30m, a new file to peers which are outside the search radius of other ASR peers. download is introduced. Its popularity will be defined by a As such, in this late stage, further favoring ASR peers provided zifp distribution. The least popular file is removed. Peers chose no results. This situation might prompt some ASR users to which file to download according to popularity using a zipf return to eMule, ending up with higher download times for distribution. About 2000 downloads are started during each everyone and more traffic. simulation, which lasts for a day. However, if ASR peers completely refuse to upload to As expected, the introduction of ASR peers allows traffic to eMule peers, these will experience long download times, drop. The larger amount of traffic when using a 100% ASR forcing users to move on to ASR. Table III shows what population is explained by more downloads being finished. happens in a 90% ASR, 10% eMule population when ASR The number of hops crossed by data is the smallest. peers refuse to upload to eMule ones. At the cost of higher The download times perceived by ASR users also drop traffic, eMule users experience a large enough download time and are always inferior to those of eMule, which provides a to prompt them to make the change to ASR. strong incentive to migrate. The ratio of completed download A migration plan is therefore necessary, to go from todays (the fraction of started downloads which finished during the 100% eMule population to a 100% ASR enabled eMule simulation) also improves as more ASR peers are introduced. population. We suggest that newer versions of eMule ap- This means that not only downloads are faster as more plication implement the revised ASR algorithm. As people downloads get finished. If we take into account the partial upgrade their software, compelled by the faster downloads, downloads, in the form of what fraction of the total file bytes 300 300

250 250

200 200

150 150

100 100 Num downloads completed Num downloads completed 50 0.5 50 0.5 0.2 0.2 0.1 0.1 0 0 2000 3000 4000 5000 6000 7000 2000 3000 4000 5000 6000 7000 Download time (s) Download time (s) (a) ASR peers (b) eMule peers

Fig. 1. Cumulative download time using revised ASR and a 50% eMule population

TABLE II COHABITATION USING REVISED ASR ALGORITHM

90% ASR 10% eMule 50% ASR 50% eMule 10% ASR 90% eMule Avg. Download time 3094.41 2850.9 3212.43 3325.66 3536.11 4121.88 Avg. search radius 7.46 8.47 10.21 Transit traffic 1.78E+11 3.14E+11 4.22E+11 Stub traffic 5.39E+11 6.17E+11 6.61E+11 Inter-ISP traffic 1.18E+11 1.76E+11 2.09E+11 Num. connections 136903 352786 651426 Avg. hops crossed 6.47 8.4 9.78

TABLE IV COHABITATION UNDER CONTINUOUS LOAD USING REVISED ASR AND THE MIGRATION PLAN

100% ASR 90% ASR 10% eMule 50% ASR 50% eMule 10% ASR 90% eMule 100% eMule Avg. Download time 9233.76 9404.81 17283.99 9949.1 11067.98 10680.76 11974.58 11352.49 Avg. search radius 9.61 9.97 11.71 13.33 Completed files 0.81 0.24 0.76 0.72 0.74 0.68 Completed bytes 0.86 0.19 0.83 0.75 0.82 0.75 Transit traffic 2.06E+12 1.97E+12 2.56E+12 2.70E+12 2.83E+12 Stub traffic 4.19E+12 3.89E+12 4.21E+12 4.10E+12 4.20E+12 Inter-ISP traffic 1.22E+12 1.14E+12 1.31E+12 1.31E+12 1.36E+12 Num. connections 407488 619102 1224932 1894222 1874260 Avg. hops crossed 7.97 8.17 9.47 9.94 10.1 Completed files 0.8 0.75 0.74 0.68 0.7 Completed bytes 0.87 0.8 0.79 0.76 0.77

have been downloaded, ASR is also a better choice. Protocol penalised. Nevertheless, those who migrated were benefited overhead (in the form of needed connections) also decreases and ISPs also gained as fewer hops are used. This should with the introduction of ASR peers. provide for an even faster migration. In the 50%-50% scenario When only 10% of the peers use the legacy eMule protocol, everyone gains, including those users still using eMule. these experience much slower downloads and lower download finish rates. This scenario is an extreme one, where all 90% Nowadays, many ISPs limit the bandwidth usable by P2P of the ASR enabled peers refuse to upload to eMule peers. protocols. As ASR is more ISP-friendly, allowing less traffic As the introduction of ASR peers refusing to upload to eMule to be exchanged with peers outside one’s ISP, ISPs might would be gradual, most users would abandon legacy eMule chose to handle ASR traffic differently. This would provide implementations before reaching this point. users with even better downloads when compared to eMule, Unlike the previous scenario, the introduction of only 10% prompting uses to migrate to ASR enabled implementation of of ASR peers does not benefit eMule users, in fact they were eMule faster, which would further benefit the ISP. V. RELATED WORK downloads than those using ASR enable implementations, The idea of only using the closest peers, ignoring the others, hindering the migration to an all ASR enabled population, has been used before, in Gnutella [9]. However, Gnutella hops which would benefit all users and ISPs. are counted over the overlay network and have no relation to We proposed a revised ASR algorithm which allows ASR the Internet topology [10]. peers to perform better than eMule ones in a mixed en- Cluster-based Architecture for P2P Systems (CAP) tries to vironment, while eMule peers are the dominant type. We structure Gnutella as to form overlay networks comprised of also propose a migration plan, were after ASR becomes the connected clusters of topologically close peers [11]. File ex- dominant peer type, newer versions of the ASR enabled eMule changes among peers in the cluster should result in less traffic clients will refuse to upload to non ASR enabled eMule peers, exchanged among ISPs if the clusters are contained within a thus maintaining the superior performance of ASR peers. single ISP. However, once a query leaves a cluster, there is Together, this revised version of ASR and the migration plan no guarantee that it reaches a close (topology-wise) cluster. enable P2P file sharing clients to download faster and decrease Furthermore, CAP requires the centralised server to know the the impact of P2P file sharing on the Internet. This might allow Internet topology in order to group close-by peers together. 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