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 .
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
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
NAT traversal A mechanism to create a connection to a host that is behind a
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
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." 
"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." 
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
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)ž . . . ž 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
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
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.
 D. Liben-Nowell , H. Balakrishnan , D. Karger. Analysis of the evolution of peer-to-peer systems, Proceedings of the twenty-first annual symposium on Principles of distributed computing, July 21-24, 2002, Monterey, California
 Rüdiger Schollmeier. A Definition of Peer-to-Peer Networking for the Classification of Peer-to-Peer Architectures and Applications. Peer-to-Peer Computing 2001
 S. Androutsellis-Theotokis and D. Spinellis. A Survey of Content Distribution Technologies. ACM Computing Surveys, Vol. 36, No. 4, December 2004.
 M Feldman, C Papadimitriou, J Chuang, I Stoica. Free-riding and whitewashing in peer-to-peer systems.
IEEE J. Selected Areas in Communications, Vol. 24, No. 5. (2006), pp. 1010-1019.