=Paper=
{{Paper
|id=Vol-1256/paper8
|storemode=property
|title=A Derived Queueing Network Model for Structured P2P Architectures
|pdfUrl=https://ceur-ws.org/Vol-1256/paper8.pdf
|volume=Vol-1256
|dblpUrl=https://dblp.org/rec/conf/vecos/MordjiAA14
}}
==A Derived Queueing Network Model for Structured P2P Architectures==
76 Derivation of a Queuing Network Model for Structured P2P Architectures Zouweyna MORDJI Mourad AMAD Djamil AÏSSANI LaMOS research unit LaMOS research unit LaMOS research unit Faculty of Exact Sciences Faculty of Exact Sciences Faculty of Exact Sciences University of Béjaia University of Béjaia University of Béjaia 06000 Béjaia, Algeria 06000 Béjaia, Algeria 06000 Béjaia, Algeria mordji.zouweyna@gmail.com amad.mourad@gmail.com Djamil.AISSANI@gmail.com Peer to peer (P2P) networks have using commonly used for tasks such as file sharing or file distribution, and for building distributed applications in large scale network. Their performance measures are generally based on simulation methods software such as NS-2, P2PSim, OpenNet, etc. Hence, the absence of a validation of the simulation model is a critical issue. In this paper, we propose a new analytical model derived from (8), to evaluate the performance of HPM protocol (hierarchical Peer-to-Peer model). Performance is done principally in terms of total download time of requested resources in the P2P network, and then we analyze the impact of various parameters associated with the heterogeneity of nodes. P2P, Performance evaluation, Analytical model, HPM, Queuing model. 1. INTRODUCTION (8), (9), (2). In this paper, we follow mostly the approaches put forward in (8). The P2P paradigm has emerged as a solution to The goal of this paper is to propose an analytical some limitations of the classical methods of resource model for evaluating the performance of the HPM (hi- sharing based on the paradigm Client / Server. It is erarchical Peer-to-Peer model) (13). Our proposed a distributed system without or with minimal central model is different from those explored in (8), in authority and with varying computational power at their work, the structured P2P systems have not each machine. Its applications are varied, we cite as been addressed. Our work is then to complete the an example: the multicast application, parallel com- analytical model proposed in (8). puting, file sharing, IP telephony, instant messaging, We interest about structured P2P architecture, be- search engines, etc. This type of network is generally cause it is more efficient in terms of lookup and characterized by a good scalability (scaling) and download resources, and more complicated to im- a very high dynamic (churn rate), for more details plement. For this, the analytical model is slightly about this topic see (1). more interesting to take into account the system P2P networks have seen an unprecedented develop- characteristics and evaluate the performance of the ment that was accompanied by a significant increase corresponding architecture in a very short time, and in their complexity. So, we focus in this paper on give a good insight into the operation of the systems analytical models, because of the interest in their under study at low cost, compared to other perfor- high speed resolution. Indeed, when considering mance evaluation techniques of P2P networks like making a change to the system, decisions will lead measurement and simulation approaches. Analytical to very high cost, and it is very useful to have an- models are least expensive and give the modeler alytical solution with their much reduced computing deep insight into the main characteristics of the time. Analytical and mathematical frameworks let us system. to model and study the performance of the P2P In this work, we want, exactly, to answer the ques- networks, and several models have been proposed tion: using the HPM, how long does it take a re- in order to investigate the dynamics of this kind quested resource to lookup and download? To an- of systems, many researches are focused on the swer this question, we use a queuing model with Markov chain based modeling , (7), (3),(4), (10), and structured P2P system, we consider two types of recently the researchers exploit the queuing model nodes, the relay and the end peer nodes as de- for performance evaluation of the of P2P networks scribed in HPM, we use also a single class open 77 queuing network to evaluates the delays in a relay nodes, we model each relay node as G/G/1 queue (13) and the end peers as M/G/1/K processor shar- ing queues. The rest of this paper is organized as follows: next section gives a background and related work on Peer to Peer networks and existing performances evaluation methods, we describe and analysis the proposed models existing in the literature. Section 3 describes our proposed model. In section 4, we validate our model; finally, we conclude and give Figure 1: Classification of P2P systems some perspectives. Centralized Systems (CSy): consists of a single 2. BACKGROUND AND RELATED WORK server that is responsible to relate directly all connected peers and to identify the files offered by In this section we give a brief description of different clients. The advantage of this technique lies P2P network and different classes of associated in the centralized indexing all directories and titles mathematical models. of shared files by the subscribers in the network. 2.1. Background Clients send queries to the server, and it returns a list of peers currently connected to the service and P2P Systems offer an important opportunity for a who’s shared the desired files. The file transfer will large number (hundreds of thousands) of nodes be done between final users and not by the server. to cooperate in order to share resources via a Under these conditions and at any time, files are widely distributed network. Nodes of the network, found stored on the central server. Such centralized are an equal participant, and there are no nodes approach does not scale well and have a single with special facilitating or administrative roles. Since point of failure, (e.g., Napster ). the service is distributed to all participating nodes, the system is expected to scale well even when Decentralized Unstructured Systems (DUSy): it the network is very large. Avoiding bottlenecks and is called also the pure P2P systems in which all likely with good fault tolerance, the P2P paradigm nodes play equivalent roles, there are no servers is suitable for large-scale distributed environments or nodes privileged, each node has high degree where nodes (called also peers) can share their of autonomy. Nodes are organized arbitrarily using resources (eg. computing power, storage capacity, flooding (broadcast or random walks) for the content bandwidth) as an autonomous and decentralized. discovery. To find a resource, a request will be sent Due to its advantages, several areas have already from a peer to another until it reaches the client that taken advantage of this paradigm (eg. file sharing, has the desired object. To avoid flooding the network sharing computing capacity and exchange instant for too long, the system associates each request a messages). A peer can play the role of client timer TTL ”Time To Live”, the value assigned to the when it consuming resources, and the role of TTL is usually 7 (as in Http). When it reaches zero, server when it offered resources, router when it the query is not returned. The major disadvantage spreads requests received from other nodes in the of this mechanism comes from the expiration of TTL system, and host data source when sharing their before the course of the entire network, which can data with other peers. There are many different lead to failure of research although the desired file P2P systems, each one with various advantages is available on the P2P network, and usually, these and disadvantages. They differ both in their object networks do not scale well, because of the high query mechanism and in their logical topology, we amount of signaling traffic. On the other hand, they therefore distinguish three families of P2P systems: present a very high level of anonymity. The most centralized systems,decentralized (structured and known examples of these networks are Gnutella and unstructured) systems, and hybrid systems. FreeNet . Decentralized Structured Systems (DSSy): the emergence of structured P2P systems is due to the problem of large number of messages exchanged in unstructured P2P systems. Nodes should be structured in a precise geometrical form, and in general, they consist in some variant of a Distributed Hash Table (DHT) technology. These networks are 78 constructed in a structured overlay where each node models, fluid flow models, and queuing network maintains a specific set of contents (or a set of models. In most cases, these models are used to content location indexes), this information is often describe the performance of the whole system and used to guide the routing of the requests / messages stochastic characteristics of a peer, they are clearly in the system. Therefore, the content searches are able to reflect the effect of different parameters on deterministic and efficient. As examples of their P2P systems performance, they permit efficient systems, we can cote: Chord, HPM (11). and detailed exploration of the parameter space to evaluate the effect of not just only single parameter, Hybrid Systems (HSy): hybrid P2P network is but also the combined effect of variation of several more complex to implement, as it combines both parameters. However, many of these models are centralized and distributed network. A network of this based on unrealistic assumption like peers having type is based on a set of servers managing a group global information about the state of all peers, of users according to the centralized architecture. simplifying assumption on the underlying network Each server is then connected to other servers topology, and on the arrivals and departures of according to the distributed architecture. peers. In this way, if a user searched a file that is not indexed by the server to which it is attached, it 2.2. Related Work then forwards the request to another server. This architecture can benefit from better bandwidth while In the following, we present the different analytical reducing the query traffic. models used to evaluate the performance of P2P In this paper, we are interested in the structured networks: P2P architecture in general and particularly HPM (11). HPM is composed of a set of hierarchical rings, Fluid Flow Model: An homogenous branching which consist of the nodes that are neighboring process is used to study the service capacity in terms of physically proximity, without distinction of BitTorrent-like P2P file sharing network in the between nodes in different levels, and it is based on transient regime and a simple Markovian Model is a hash function and cryptographic for the identifiers presented in (3) to study the steady-state properties. of resources, the IP address and port number for They found that the capacity of such systems the identifiers of nodes. HPM routing objectives grows exponentially in transient and stabilizes are: provides the discovery/localization service, at steady state. Various techniques are studied based on a complete decentralized architecture, by that might help to improve P2P performance. determining with efficiency the node responsible for Multi-part combined with parallel uploading when storing the requested key’s value. properly optimized will generally improve system One of the main characteristics of HPM is the routing performance, particularly when peers exit in the optimization at IP level, as it takes into consideration system at a high rate. The above work is extended the physical proximity while minimizing the number in (4) where simple deterministic Fluid Model is of hops for lookup process (cost lookup). The derived from Markovian model proposed in (3), authors evaluated the performance of HPM with in order to study peer number temporal evolution simulation, the metric they are taken: cost lookup, and average downloading time in BitTorrent-like file size of data structure, number of rings at each level sharing systems. Furthermore, other features of and the number of rings and level for HPM with IPv6 BitTorrent networks such as downloading efficiency and IPv4 address format. However, in our work, we and incentives are discussed. evaluate the system with analytical model in term In (6), the authors develop an analytical model of total download time of requested resource, and allowing to study the effect of network characteristics capture the impact of nodes level characteristics on on P2P file sharing system performance. Particularly, the performance of HPM. they have focused on access link capacity and Mathematical models can be used to predict system heterogeneity. In (5), a simple fluid model, (an behavior. Among both techniques: simulation and extension of (4)) is proposed, the authors analyze analytic modeling that can be used at the design the effect of bandwidth heterogeneity on file transfer stage, this one is much less expensive. Also, dynamics and content diffusion process in detail. with the availability of very powerful and effective They compare the performance of heterogeneous general purpose modeling tools, analytic models networks and the equivalent homogeneous networks are becoming increasingly more cost effective than under different conditions of equivalence. Their simulation. Many researches are focused on this results show that heterogeneity bandwidth can have modeling method for evaluating the performance of a positive effect on content propagation. P2P systems; the most significant classification of the modeling techniques we found in the literature, Markov Chain Model: Birth and Death Markov they are classified into four models: Markov chain model based structured peer-to-peer networks are 79 studied on (7). The result shows that the structured 3. PROPOSITION peer-to-peer network is very suitable for networks with low dynamicity. The authors in (? ) study the In this section we develop our proposition model, dynamic and robustness properties of large-scale which consists of modeling the HPM structured peer-to-peer systems. They propose an analytical P2P system, and evaluate its performances with model of the local behavior of clusters, based on an analytical method. This work is an extension Markov chains to evaluate the impact of malicious of (8). The authors of (8) did not evaluate withe behaviors on the correctness of the system, and their analytical models the performance of structured analytically evaluation of the performance of the P2P systems, so we want to complement their work global system, allowing to characterize the global and study case not treated. For this, we use a behavior of the system with respect to its dynamics queuing model to model and evaluate the HPM. As and to the presence of malicious nodes. The focus described in HPM architecture, two types of nodes of these studies is primarily on the evolutionary is considered, the relay and the end peer, the relay dynamic of the system. These studies also do not node represents a gateway between rings. We use account for queuing effects and heterogeneities in a single class open queuing network to model and hosts and the network. evaluate the delays in relay nodes, we model each relay node as a GI/G/1 queue like in (13) and the Queuing Network Model: the famous model end peers as M/G/1/K processor sharing queues. of P2P file sharing systems is presented in (2), in which a multiple class Closed Queuing Model was 3.1. Functional Principal proposed to capture distinguishing characteristics HPM is organized as a set of hierarchical rings of P2P file sharing systems, This model is applied composed of nodes, we interested in two types of in three different types of architecture (Centralized nodes: relays and peers nodes. Each relay acts as Indexing: CIA, Distributed Indexing with Flooded a gateway to one or more rings, and peers are queries : DIF, and Distributed Indexing with Hashing the nodes residing in the various ring. We take directed queries: DIHA), and it is used for analyzing as example the scenario below: three relays (R1, important aspects regarding performance like R2 and R3) and four rings as shown in figure 2. system scaling, freeloaders, file popularity and When a peer P2 in ring 2 searches a file that is availability. This model does not capture the on peer P9 in ring4, the request (packet) should be significance of the physical underlying topology of transferred through the R1 and R2 relays to get the the considered P2P network. In (8), an analytic file that on P9, (the principal detailed of the lookup framework to evaluate the performance of peer to and download of resources in HPM is described peer networks is proposed. The authors used as a in (11)), so, how long does it take to lookup and metric: a time to download or replicate an arbitrary download a requested resource?, : to answer this file. Their proposed model captures the impact of question, we must first get query search time, the various networks and peer level characteristics on transmission time of the file being downloaded, and the performance of P2P network, and they propose a queuing delay at the intermediate relay, the sum of queuing model which evaluates the delay of routers the three quantities provides us the total resource using a single class open queuing network and transfer time. For this, in next section, we studied the peers as M/G/1/K processor sharing queues. and analyzed each point separately, and we use a An important abstraction unaddressed in is the queuing model to model relays and the peers and availability, and dynamism of nodes in term or churn evaluate the total delay to download a requested rate (disconnection/connection) for P2P network, resource in the HPM architecture. which can influence on total network latency. In (9), the same authors evaluate the file transfer delay at the peers, and contributed more compared to their previous work in (8), the case of online-offline transition of peers and gives the total peer latency. In this work, we are interested to apply the proposed analytical model (8) in order to give performance evaluation of structured P2P systems, a case study concerns HPM protocol (11). In table 1, we summarized the work discussed in this section, we gave the focus of analyses and result, and the weak point of each work. Figure 2: example of lookup and download in HPM 80 We use the notations bellow: Determine two parameters for each relay j: (i) the arrival rate λj can be obtained from the traffic • n : number of internal relays in the network. for rate equations: each j, j = 1,...,n • λ0j : expected external arrival rate at relay j ∑ n λ0j = [1/E(A0j )] λj = λ0j + λij , f orj = 1, ..n, (1) i=1 • Ca00j : squared coefficient of variation (scv) or variability of external interarrival time at relay j: where λij = pij λi , is the expected arrival rate at Ca20j = [V ar(A0j )]/[E(A0j )2 ] station j from station i. We also get : • µj : expected service rate at relay j: µj = 1/E(Sj ) ρj = λj /µj , 0 ≤ ρj ≤ 1, (2) • 0 CS0j : scv or variability of service time at relay j, CS00j = [V arSj ]/[E(Sj )2 ] (ii)the squared coefficient of variation (scv) for the arrival process or variability of interarrival time. • pij : for each pair (i,j), probability of a packet The expected (external) departure rate to station 0 going to relay j after completing service at relay from station j is given by : i ∑ n 3.2. Analytical Model of HPM λ0j = λj + (1 − pij ). (3) i=1 We focus on the network queuing delays and the delays at the end peers, using a queuing model. We Throughput or total external rate of traffic into the break up the system into two components: the relay relays : nodes, modeled with single class open network, GI/G/1 OQNS, and the end peers modeled with ∑ n ∑ n M/G/1/K Processor Shared. λ0 = λ0j or λj0 (4) j=1 j=1 . 3.2.1. Relay Network Model The expected number of visits In our proposition, we use a decomposition method (13),(14), that consists of decomposing a network vj = E(Kj ) = λj /λ0 . (5) relay nodes into smaller sub-networks and then analyzing each sub-network separately which it The s.c.v. of arrival time can be approximated by consist of individual queues. In this approach, a Traffic variability equation (8): network is approximated as a set of individual isolated GI/G/1 queues. Performance metrics at ∑ n λij each queue are computed using approximation 2 Caj = wj 2 Cai + 1 − wj (6) i=0 λj formulas for the GI/G/1 queue. We model each relay nodes by a GI/G/1 to allow , where for arbitrary arrival and service time distribution. 1 Traffic to a network relay can be rather irregular and wj = , does not necessarily follow a Poissonien distribution 1 + 4(1 − ρj )2 (xj − 1) (15),(16), and may vary from network to another. and Given this, we model our network with generalized 1 inter-arrival (GI) process. xj = ∑n λij 2 . i=0 ( λj ) To find the relay network delay, we should pass through three steps: Step 1. Analysis of interaction between relays of the Step 2. Evaluation of performance measures at networks, each relay Step 2. Evaluation of performance measures at each The expected number of packets in the j th relay, relay, including one in service, is given by using Little’s law: Step 3. Evaluation of performance measures for the whole network. E[NCj ] = ρj + λj E[WQj ] (7) The expected waiting time at the jth relay is given by Step 1. Analysis of interaction between relays of : the networks 81 E[Np ] represent the expected number of resource ρj (Ca2j + CS2j )g(ρj , Ca2j , CS2j ) transfers in progress at the end peer at any given E(wj ) = (8) 2µj (1 − ρj ) time, it is given by: ∞ ∑ where g(ρj , Ca2j , CS2j ) = E[Np ] = ipi (13) { } i=1 −2(1+ρj )(1−Ca2 ) 2 j 2 exp 3ρj (Ca2 −CSj )2 , Caj > 1), 3.2.3. Query Search Time j 2 1, Caj ≥1 The expression of query search time is the time taken for the entire search process to terminate; it differs from one architecture to another, depending Step 3. Evaluation of performance measures for on the research technique used. In this work, we the whole network consider a structured architecture to evaluate this The total number of packets in the network is: parameter, which is the HPM, so, the neighbor ∑ n relationship between peers and data locations is NC = NCi . (9) strictly defined. Searching in such systems is i=1 therefore determined by the particular network The relay delay per packet: architecture. The search technique employed to lookup the requesting resource in this architecture NC is defined as follow: E[TN r ] = (10) λ0 Each ring k at level i, used the ith part of data key for the lookup process, when a peer in ring k if the 3.2.2 The End Peers requestor node belongs to, the request succeeds on The HPM architecture is composed of peers and this ring k, if the resource does not exist on the active relays, each relay is attached to a number of rings; covered ring, the search is done on ring level i + 1, or in turn harbor the end peer. In the previous section, i−1, in a deterministic ∑m manner the cost of the search we provided a queuing delay at the intermediate process is O( i=1 log2 (ni )), where, ni is the number relay, and in this subsection, we give and develop of nodes on the covered ring at level i on which the a queuing model for the end peer and provide request succeeded. expression of the expected time takes to service a The query process terminates when the last of requesting resource. We model each end peer as the responses finds it’s way back to the source. M/G/1/m Processor sharing. The choice of this last is The expected time elapsed between the query motivated by the fact that, the service time depends generation and termination is thus: on the size of the resource being downloaded and resource size distribution is typically heavy-tailed, ∑ N E[TQS ] = [2C ∗ d (E[WQi ] + τi )]/NR , (14) so the service time cannot be modeled as an i exponential process. For this, we are motivated to choice an arbitrary distributions for the time services ∑N where [ i (E[WQi ] + τi )]/NR represent the average and taking generalized model for resource size, and queuing delay at a relay, and [WQi ] is given the arrival process follows the Poisson process with previously in Eq.(14) rate λdi , since the SCV of the arrival process at the The factor of 2 comes in since the query response end peers equals 1, the demonstration of this result traces the same forward path back to the query is given in (8). originator. State probabilities pk are given by: C is the cost of the lookup in HPM architecture, and is given by: k m ρ (1−ρ) ρ (1−ρ) pk = 1−ρm +1 , Pl = 1−ρm+1 , ρ = λdi X̂, (11) ∑m C = O( log2 (ni ), i=1 for k = 0, 1, ..., m. where Pl is the loss probability, d give the approximate distance for random graph, and X̂ is the average service time per request. it represent the shortest path between two random Using Little’s Law, the expected service time that a chosen nodes on the relay graph: user encounters can be expressed as: ln(NR − 1)(ẑ2 − ẑ1 ) − ln(ẑ12 ) d= , E[Np ] = λp (1 − Pl )E[wp ], (ln(ẑ2 /ẑ1 )) E[Np ] E[wp ] = (12) where ẑi is the average number of i hop neighbors λp (1 − Pl ) and NR is the total number of nodes in the relay 82 graph: The final expression for the overall waiting time, E[T ], gives : ∑ NR ∑ NR ẑ1 = [ Aij ]/NR , ẑ2 = [ IÂ (i, j)]/NR i,j=1 i,j=1,i̸=j E[T ] = E[TD ] + E[TQS ] (18) A is the relay adjacency matrix, Â = A2 and IA (i, j) defined as: 4. CONCLUSION AND FUTURE WORK { 1, if Âij > 0, In this paper, we have presented the performance IÂ (i, j) = 0, otherwise evaluation models for P2P systems, which of these P2P overlay networks is best suited depends on 3.2.4. Expected Download Time the application and its required functionalities and We arrived at the expression that gives the total performance metrics (e.g. scalability, network routing download time of resource, which is the time elapsed performance, location service, file sharing, content from when the query was generated until the entire distribution, and so on). resource is downloaded, with O(i) copies of the resource in the network, which is generated by the We have summarized various recent works on Zipf’s Law : performance modeling of such systems that have O(i) = Von /iθ Hθ (V ), been proposed in the literature, offering an insightful and useful overview of system properties, focus of analyses and results of each work studying. where Hθ (V ), is the harmonic number of order θ of V and defined as: A novel analytical model derived from work in (8) ∑ V is proposed in this paper, it consist of evaluating Hθ (V ) = 1/iθ . the performance of structured P2P network, we i=1 have take as an example the HPM architecture and as performance metrics the total download time of The given formula in Section 3.2.1. gives us the total requested resources. time it takes a packet takes to get the customer E[TN R ], so the download time is determined by the time spent by the last packet sent by the slower peer As future work, we envision to take into account the to reach the destination. behavior of nodes and we will study the online-offline The time when the last packet reaches the edge transition, which may influence the performance of of the network is when the slower peer is done the system, and especially on the download time of transmitting it’s allocated resource part i.e. after a resource. E[TW P ] seconds. The packet, then spends a further E[TN R ] in the network. The expected service time for data transfer at the slower peer is: B/O(i) E[TW P ] = (15) C/E[NW P ] B is the total file size, O(i) number of copies of the resource in the network E[NW P ] is the expected number of files serving at any point in time ∞ ∑ ∑ j E[NW P ] = [ i ∗ p(i)]P (m = j) (16) j=0 i=0 Thus, the total download time, E[TD ], is given by: E[TD ] = E[TW P ] + E[TNR ] (17) 83 Ref. Analytical Architecture Focus of Analyses and Re- Weak points Method sults (2) CQMo CIA,DIFA,DIHA Generality, flexibility of mod- Not capture the effect of the eling,analyze the effect of differences in the file size of freeloaders, files and user be- different request on the sys havior. performance, access rate and varying load on different peer not modeled, ignore the effect of the network topology. (3) MMo,Bpr DUSy (BitTor- Study service capacity and Performance of individual user rent) fairness. P2P system achieves not degrades significantly, the favorable scaling in terms of average delays scale well in average download delay with the offered load Not account increasing load. for queuing effects and hetero- geneities in the network. (4) FMo DUSy Study the scalability and in- Not account for queuing ef- HFflow centive mechanism, The sys- (BitTorrent- fects in the network, and like) tem scales well with the num- heterogeneities in hosts and ber of peers, the incentive the network. The assumption mechanisms are efficient, op- about global knowledge of all timistic unlocking may encour- peers for peer selection age free-riders. (5) HFMo DSSy Effect of Bandwidth Hetero- Not account for queuing ef- (BitTorrent- geneity on download perfor- fects in the network. like) mance. Heterogeneity band- width can have a positive ef- fect on content propagation. (8) QMo(OQN) CSy,DUsy,HSy Evaluate the expected time to Do not capture all aforemen- download a file, accounts for tioned phases of the sharing a host of network and peer process, namely flash crowd, level characteristics, interplay steady state. Not account the among various critical pa- query search time (propaga- rameters (external traffic rate, tion delay), not treated the service variability, file popu- structured architecture. larity...). queue type: router: GI/G/1, Peer M/G/1/K Table 1: comparative table of various work performance evaluation of P2P systems REFERENCES [5] : F. Lo Piccolo, G. Neglia, G. Bianchi, ”The Effect of Heterogeneou Link Capacities in BitTorrent- [1] : M. Amad, A. Meddahi, D. Aı̈ssani, ”P2P like File Sharing Systems”, International Work- Networks Management Survey”, International shop on Hot Topics in P2P Systems, pp. 40 47 , Journal of Computer Sciences Issues (IJCSI), 2004. Vol. 9, Issue 1, No 3, pp.193 148, January 2012. [6] : F. Lo Piccolo, G. 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