Adaptive Lookup for Unstructured Peer-to-Peer Overlays K Haribabu, Dayakar Reddy, Chittaranjan Hota Antii Ylä-Jääski, Sasu Tarkoma Computer Science & Information Systems Telecommunication Software and Multimedia Laboratory Birla Institute of Technology & Science Helsinki University of Technology Pilani, Rajasthan, 333031, INDIA TKK, P.O. Box 5400, Helsinki, FINLAND {khari, f2005462, c_hota}@bits-pilani.ac.in {[email protected], [email protected]}.fi Abstract— Scalability and efficient global search in unstructured random peers but at specified locations that will make peer-to-peer overlays have been extensively studied in the subsequent queries more efficient. Most of the structured P2P literature. The global search comes at the expense of local overlays are Distributed Hash Table (DHT) based. Content interactions between peers. Most of the unstructured peer-to- Addressable Network (CAN) [6], Tapestry [7], Chord [8], peer overlays do not provide any performance guarantee. In this Pastry [9], Kademlia [10] and Viceroy [11] are some examples work we propose a novel Quality of Service enabled lookup for of structured P2P overlay networks. unstructured peer-to-peer overlays that will allow the user’s query to traverse only those overlay links which satisfy the given In unstructured P2P network, lookup is based on constraints. Additionally, it also improves the scalability by forwarding the queries [12]. At each node the query is judiciously using the overlay resources. Our approach selectively forwarded to neighbors. Unless the peer finds the item or the forwards the queries using QoS metrics like latency, bandwidth, hop count of the query reaches zero, query is forwarded to and overlay link status so as to ensure improved performance in neighbors. In this lookup approach, there are variations on how a scenario where the degree of peer joins and leaves are high. the query forwarding can be controlled without decreasing the User is given only those results which can be downloaded with the chance of finding an item. The query forwarding is controlled given constraints. Also, the protocol aims at minimizing the by selectively choosing the neighbors. The selection is based message overhead over the overlay network. on the information stored at the peer about its neighbors. The information is either past history or the indexes of the content Keywords- Peer-to-peer; Overlays; QoS; Lookup available of neighbors. The controlled forwarding also happens by randomly selecting the neighbors which reduces the chance I. INTRODUCTION of finding an item. Peer-to-peer (P2P) overlay networks are widely used as Unstructured peer-to-peer overlay networks mostly consist public file sharing networks. Data sharing P2P systems are of nodes which are home PCs. They are connected to network capable of sharing huge amounts of data. For example, in April by a weak bandwidth connection. In this paper we present an 2003 the KaZaA [5] P2P data sharing system reported over 4.5 approach to give freedom to the user to specify the constraints million users sharing a total of 7 petabytes of data. Such a huge that should be satisfied for the results obtained. The results are collection of data will be unusable without efficient lookup of ranked using a composite function that is expressed as a the object being looked for. function of QoS metrics defined in the later sections. The result P2P overlay networks are application-level logical with the highest rank will be from the node that can satisfy the networks built on top of the Internet. These networks maintain users constraints to the maximum. routing tables to enable efficient search and data exchange The type of Quality of Service (QoS) introduces several between peers. They don’t require any special administrative or factors that need to be taken into account. In this paper, we financial arrangement. They are self-organizing and adaptive, consider two parameters, bandwidth and link latency, at the distributed and decentralized. They can support the distribution link level. We consider one parameter past response or past of storage and computational problems. P2P overlay networks interactions with the peer as the node level constraint. This are categorized as unstructured and structured [1]. An paper presents a scalable and adaptive lookup approach that unstructured P2P system is composed of peers joining the takes user preferences into account in choosing the best overlay network with some loose rules, without any prior knowledge of route to fetch the object among the multiple locations. the topology. Freenet [2], Gnutella [3], FastTrack [4], and KaZaA [5] are examples of unstructured P2P overlay networks. These networks are typically power law networks (or scale free II. RELATED WORK networks). Gnutella [3] is a traditional example of a power law The lookup problem in a P2P overlay refers to finding any network, where search has a high cost due to many connections given data item in a scalable manner. More specifically, given a between peers. In structured P2P overlay networks, network data item stored at some dynamic set of nodes in the overlay, topology is tightly controlled and content is placed not at we need to locate it [13]. The unstructured overlays commonly use flooding [3], random walks [14], iterative deepening search probability of the neighbor being selected for querying that [15], directed breadth first search (BFS) [15] to lookup content object. In ant based search algorithm [21], the goodness of a stored on other overlay peers. neighbor is judged by number of documents and path length of the neighbor. Also the goodness updation algorithm Freenet [2] uses a symmetric lookup search where queries dynamically finds out optimal path for a particular query. But are forwarded from node to node based on the routing table this approach fails in case of high churn. In directed BFS entries that are built-up dynamically. It ensures anonymity by search [15], the query is forwarded to neighbors who have not forming any predictable topology and also by not good statistics. This is done only for the first hop and for the associating an object with any server. Because of anonymity, rest of the hops the query is forwarded to all neighbors. search for an object needs to visit large fraction of nodes that is time consuming. In flooding technique [3], the query is To reduce response time and bandwidth, approaches in [22, forwarded to all the neighbors. To improve the scalability, it 23] specify selection of a flooding or DHT based lookup based uses small time-to-live (TTL) counters. Though it reduces on the popularity of the content. It is computed using a global network traffic and load on peers, it also reduces the chances of collection algorithm. It is based on the observation that finding a match. In k-walker random walks [14] the query is flooding is efficient for finding a popular item, but to find a forwarded to k randomly selected neighbors. Those neighbors rare item DHT based lookup is used. in turn forward to k randomly selected neighbors. Although this search method reduces the network load but massively Our technique is similar to the search technique described increases the search latency. In iterative deepening search [15], in [19]. But our technique differs from this in two ways, one is consecutive BFS at increasing depths is performed to locate an that the criteria to be used as performance metric in forwarding object in the P2P overlay. This search method also increases the query is given by the user and the second is that we use network load and duplicate query messages. In this technique, composite function to compute a cost that gives us the best at every node the query is forwarded to all neighbors except the possible route in the churn scenario. one who sent the query. In GUESS (Gnutella UDP Extension for Scalable Searches), a hybrid peer-to-peer overlay builds III. LOOKUP IN UNSTRUCTURED OVERLAYS upon the notion of ultra-peers [16]. A search is conducted by Lookup in unstructured P2P overlay networks happens by iteratively contacting different ultra-peers for their leaf nodes forwarding messages to neighbors. In figure 1, let the until a number of objects are found. These ultra-peers need not requesting node be A, and responding node be H i.e. say user at be the neighboring nodes and also the order with which ultra- node A wants a video file that is stored at node H. As shown in peers are chosen is not specified. the figure, peer A sends the requests to its neighbors B and C In [17], author has studied the minimum delay P2P video and they in turn forward it to their neighbors D and E. This streaming problem. For a delay sensitive application, the happens until either the TTL becomes zero or item is found. standard uploading bandwidth of a peer cannot be utilized to When TTL reaches zero, the query is no more forwarded. Here, upload a piece of content until it completes the download of peer H has found a match for the query. When a peer finds a that content. He proposed minimum delay bound for real-time matching item, it sends a query hit message traversing the same P2P systems. He has shown that the bandwidth heterogeneity route as taken by the query. amongst peers can be exploited to significantly improve the Techniques discussed in [14, 15] for keyword search focus delay performance amongst peers. on efficient and partial search but not on comprehensive search. In routing indices [18] based search, each node keeps The partial search is acceptable in case of finding a single file information of topics and number of documents in each topic over the overlay. But that is not sufficient while searching for available in the neighbors.