Glossary of Peer-to-Peer Terminology

Unless otherwise noted, all definitions are from:

John Buford, Heather Yu, Eng K. Lua. P2P Networking and Applications. Morgan Kaufmann, 2009.

and are � 2009 Morgan Kaufmann.

Bootstrap (1) The process by which a peer which intends to join an overlay locates one or more peers that are connected to the overlay and through which the joining peer can perform the join protocol. (2) The process by which a new overlay is initially created.

Churn The arrival and departure of peers to and from the overlay, which changes the peer population of the overlay. A related concept is mobility-induced churn, which occurs when a roaming node departs and rejoins the overlay due to a network address change after a network-layer roaming transition.

Content-Addressable Network (CAN) A type of distributed object location and routing system in which objects can be located either by identifier or by some other representation of the content of the object.

Content locality In an overlay that spans multiple administrative domains, data inserted by peers within one administrative domain or in the same local area network are stored physically at peers within the same administrative domain or located on the same LAN.

Distributed Hash Table (DHT) A structured overlay that uses key-based routing for put and get index operations and in which each peer is assigned to maintain a portion of the DHT index.

Distributed Object Location and Routing (DOLR) A method for storing and finding an object placed arbitrarily in a distributed set of networked hosts using an identifier of the object. An example DOLR is key-based routing.

Eclipse attack An attack on an overlay network in which the attacker controls a large fraction of neighboring nodes.

Federated overlay An overlay that is implemented by multiple administrative domains, which may use different routing algorithms and addressing mechanisms in each domain.

Free rider A peer that uses resources of a P2P system significantly in excess of the amount of resources it contributes to the P2P system.

Gossip protocol A protocol by which a node randomly selects and exchanges information with other nodes in the network.

Half-life The time it takes for a peer-to-peer network to replace half its nodes through departures and arrivals [1].

Hierarchical overlay An overlay architecture that uses multiple overlays arranged in a nested fashion, and the nested overlays are interconnected in a tree. A message to a peer in a different overlay is forwarded to the nearest common parent overlay in the hierarchy.

Hop In the path in which an overlay message is carried from a source peer to destination peer, a hop is each direct transmission of the message from one peer to another peer in the path.

Internet Coordinate System (ICS) An ICS maps network position to an arbitrary dimension Euclidean space with a distance function such that the distance between two Internet hosts can be accurately estimated by the distance between their respective ICS coordinates.

Key-based routing A set of keys is associated with addresses in the address space such that the nearest peer to an address stores the values for the associated keys, and the routing algorithm treats keys as addresses.

Managed overlay An overlay that can be dynamically configured and continuously monitored for operational parameters so as to continuously enforce performance, administrative, and security policies, where such policies can be changed by the overlay operator.

Multihop structured overlay A structured overlay in which messages from source peer to destination peer on average require multiple hops. An important category of multihop overlays includes those in which the number of hops is bounded by O(log N) hops, where N is the number of peers in the overlay.

Multicast A means of point-to-multipoint communication in which the same message is sent to multiple destinations such that duplicate messages are carried by the network only when routing paths for some subset of the destinations diverge.

Network Address Translator (NAT) A device that converts network addresses in packets as they are transmitted between two different address domains. Typically a NAT is used to convert between a private address space and a public address.

NAT traversal A mechanism to create a connection to a host that is behind a NAT.

One-hop structured overlay A structured overlay in which messages from source peer to destination peer on average require a constant number, that is, O(1), of hops, regardless of the size N of the overlay.

Overlay address An identifier in an address space for the overlay in which each peer has at least one unique identifier. The overlay address is used to route messages in the overlay toward a destination peer.

Overlay Multicast (OM) (1) Hosts participating in a multicast session form an overlay network and only utilize Unicast among pairs of hosts for data dissemination. The hosts in overlay multicast exclusively handle group management, routing, and tree construction, without any support from Internet routers. This is also commonly known as Application Layer Multicast (ALM) or End System Multicast (ESM). (2) A backbone overlay is constructed by deploying special intermediate proxies that create multicast trees among themselves. End hosts communicate with proxies via Unicast or native multicast.

Overlay network or overlay An application layer virtual or logical network in which endpoints are addressable and that provides connectivity, routing, and messaging between endpoints. Overlay networks are frequently used as a substrate for deploying new network services or for providing a routing topology not available from the underlying physical network. Many peer-topeer systems are overlay networks that run on top of the Internet.

Overlay operator An entity that develops an application layer overlay to provide peer-to-peer services to end users and a service delivery platform for third parties.

Path locality In an overlay that spans multiple administrative domains, queries for objects that are available at peers in the same administrative domain are only routed to peers in the same administrative domain.

Peer An end system, node, or host that is a member of a peer-to-peer system.

Peer capability The available resources at a peer relevant to its role in a peerto- peer overlay, specified as a set of capacities and system attributes.

Peer software Application software provided by the overlay, which peers use to participate in the overlay. The peer software may be extendable by third parties. It may be instrumented to enable management of the peer function in the overlay.

Peer-to-peer (P2P) "A distributed network architecture may be called a peerto- peer network, if the participants share a part of their own hardware resources (processing power, storage capacity, network link capacity, printers). These shared resources are necessary to provide the Service and content offered by the network (e.g. file sharing or shared workspaces for collaboration). They are accessible by other peers." [2]

"Peer-to-peer systems are distributed systems consisting of interconnected nodes able to self-organize into network topologies with the purpose of sharing resources such as content, CPU cycles, storage and bandwidth, capable of adapting to failures and accommodating transient populations of nodes while maintaining acceptable connectivity and performance, without requiring the intermediation or support of a global centralized server or authority." [3]

Prefix-based routing Each peer in the overlay forwards messages to the next hop based on the next sequence of address elements (the prefix) in the overlay address. First used in PRR, Tapestry, Pastry, P-Grid, Cycloid, and Z-Grid are systems that use prefix-based addressing.

Proximity-based overlay Peers select adjacent nodes based on proximity of the nodes in the network.

Proximity Neighbor Selection (PNS) An overlay maintenance strategy in which each node biases its selection of nodes in its routing table to those with which it has low latency.

Proximity Route Selection (PRS) An overlay maintenance strategy in which a peer operates to increase the size of its routing tables so as to have as many nodes to choose from when routing.
Quarantine A mechanism to reduce churn in an overlay in which a joining peer remains only a client of the overlay until the quarantine period has passed, after which it is promoted to a normal peer. Quarantine benefits from the node lifetime following a heavy-tailed distribution.

Routing table A method to organize overlay routing information that contains a mapping between overlay addresses and native addresses and may contain other information such as the capabilities of the peer, distance from the peer, and time of last communication with the peer.

Small-world network A class of random graphs in which most nodes can be reached by every other node in a small number of hops.

Semantic overlay An overlay network in which routing topology is organized according to the semantic associations and relationship of information being stored in the overlay.

Service overlay The integration of an overlay with one or more network or application services, and which may include coupling the overlay with a service advertisement and discovery mechanism.

Service-Oriented Architecture (SOA) An information system architecture that uses service orientation as its basic design principle, including external interfaces defined as services with service description documents and typically service discovery and advertisement mechanisms.

Stealth node A mechanism to avoid mobility-induced churn in which a mobile peer can send outgoing overlay requests but does not receive incoming overlay requests.

Stretch An overlay performance metric that compares the network distance of the overlay route versus the direct underlay route between the endpoints. Specifically, if d(xi , xj) denotes the time to send a message from xi to xj, the stretch of that path is [d(x1,x2)� . . . � d(xn1,xn)]/d(x1,xn).

Structured overlay An overlay in which nodes cooperatively maintain routing information about how to reach all nodes in the overlay.

Superdistribution The distribution of digital content from user to user such that the content is freely distributed but usage and content changes are controlled by the content owner.

Superpeer A hierarchical overlay in which peers with public IP address perform the overlay operations on behalf of regular peers that are behind NATs

Sybil attack An attack on an overlay in which the attacker obtains multiple different overlay identities.

Unicast A means of point-to-point communication where a message is delivered from a sender node to a single other node. If a node wants to send the same information to many destinations using unicast, it must perform replicated unicast and send N copies of the data to each destination in separate messages.

Unstructured overlay An overlay in which a node relies only on its adjacent nodes for delivery of messages to other nodes in the overlay. Example message propagation strategies are flooding and random walk.

Variable-hop structured overlay A structured overlay adapts the hop-count performance of the overlay according to the peer�s network bandwidth budget so that at higher bandwidth budget the average hop count decreases and at lower bandwidth budget the average hop count increases.

Virtual node The assignment of multiple peer addresses to a single node in an overlay, proposed in the design of the Chord DHT, to improve the distribution of keys in the overlay.

Whitewasher A user who leaves a P2P system and rejoins it after acquiring a new identity to avoid reputational penalties.[4]