ability and durability of data stored in a P2P system. Even in the absence of attacks, ensuring availability can prove difficult due to churn. For a data object to be available, at least one node that stores a replica must be online at all times. To make sure an object remains available under churn, a system must constantly move replicas to live nodes, which can require significant network bandwidth. For this reason, a practical P2P storage system cannot simultaneously achieve all three goals of scalable storage, high availability, and resilience to churn. 6

Another challenge is that the long-term membership of a P2P storage system (that is, the set of nodes that periodically come online) must be non-decreasing to ensure the durability of stored data. Otherwise, the system may lose data permanently, since the storage space available among the remaining members may fall below that required to store all the data.

Incentives. Participants in a P2P system are expected to contribute resources for the common good of all peers. However, users don’t necessarily have an incentive to contribute if they can access the service for free. Such users, called free riders, may wish to save their own disk space, bandwidth, and compute cycles, or they may prefer not to contribute any content in a file-sharing system.

Free riding is reportedly widespread in many P2P systems. For instance, in 2000 and 2001, studies of the Gnutella system found a large fraction of free riders. 2, 28 More recently, a study of a DHT used in the eMule file-sharing system found large clusters of peers (with more than 10,000 nodes) that had modified their client software to produce the same node identifier for all nodes, which means these nodes are not responsible for any keys. 31

The presence of many free riders reduces the resources available to a P2P system, and can deteriorate the quality of the service the system is able to provide to its users. To address this issue, incentive schemes have been incorporated in the design of P2P systems.

BitTorrent uses a tit-for-tat strategy, where to be able to download a file from a peer, a peer must upload another part of the same file in return, or risk being disconnected from that peer. 10

This provides a strong incentive for users to share their upload bandwidth, since a peer that does not upload data will have poor download performance. A number of other incentive mechanisms have been proposed, which all try to tie the quality of the service a peer receives to how much that peer contributes. 12, 25

Managing P2P systems. Whether P2P systems are easier to manage than other distributed systems is an open question.

On the one hand, P2P systems adapt to a wide range of conditions with respect to workload and resource availability, they automatically recover from most node failures, and participating users look after their hardware independently. As a result, the burden associated with the day-to-day operation of P2P systems appears to be low compared to server-based solutions, as evidenced by the fact that graduate students have been able to deploy and manage P2P systems that attract millions of users. 16

On the other hand, there is evidence that P2P systems can experience widespread disruptions that are difficult to manage. For instance, on Aug. 16, 2007, the Skype overlay network collapsed and remained unavailable for several days. The problem was reportedly triggered by a Microsoft Windows Update patch that caused many of the peers to reboot around the same time, causing a lack of resources that, combined with a software bug, prevented the overlay from recovering. 30 This type of problem may indicate the lack of centralized control over available resources and participating nodes makes it difficult to manage systemwide disruptions when they occur. However, more research and long-term practical experience with deployed systems is needed to settle this question.

Some of the challenges P2P systems
face (for example, data integrity and
authenticity) are largely solved, while
others (for example, membership con-
trol and incentives) have partial solu-
tions that are sufficient for important
applications. However, some problems
remain wide open (for example, data
durability and management issues).
Progress on these problems may be
necessary to further expand the range
of applications of P2P technology.

Peer-to-Peer and iSPs

Internet service providers have witnessed the success of P2P applications with mixed feelings. On one hand, P2P is fueling demand for network bandwidth. Indeed, P2P accounts for the majority of bytes transferred on the Internet. 29 On the other hand, P2P traffic patterns are challenging certain assumptions that ISPs have made when engineering their networks and when pricing their services.

To understand this tension, we must consider the Internet’s structure and pricing. The Internet is a roughly hierarchical conglomeration of independent network providers. Local ISPs typically connect to regional ISPs, who in turn connect to (inter-)national backbone providers. ISPs at the same level of the hierarchy (so-called peer ISPs) may also exchange traffic directly. In particular, the backbone providers are fully interconnected.

Typically, peer ISPs do not charge each other for traffic they exchange directly, but customers pay for the bits they send to their providers. An exception is residential Internet connections that are usually offered at a flat rate by ISPs.

This pricing model originated at a time when client-server applications dominated the traffic in the Internet. Commercial server operators pay their ISPs for the bandwidth used, who in turn pay their respective providers. Since residential customers rarely operate servers (in fact, their terms of use do not allow them to operate commercial servers), it was reasonable to assume they generate little upstream traffic, keeping costs low for local ISPs and enabling them to offer flat-rate pricing.

With P2P content distribution applications, however, residential P2P nodes upload content to each other. Unless the P2P nodes happen to connect to the same ISP or to two ISPs that peer directly with each other, the uploading node’s ISP must forward the data to its own provider. This incurs costs that the ISP cannot pass on to its flat-rate customers. 20 As a result of this tension, some ISPs have started to traffic shape and even block Bit Torrent traffic. 14 Whether network operators