Introduction to Peer-to-Peer Networks

What is a P2P network Uses the vast resource of the machines at the edge of the Internet to build a network that allows resource sharing without any central authority. Client-Server vs. Peer-to-peer. A peer is both a client and a server. Control is decentralized. Much more than a system for sharing pirated music.

Historical Perspective The Internet originally emphasized working in the P2P mode instead of the client-server mode. SRI, UCLA, UCSB and University of Utah had powerful host machines forming a league of equals. ARPANET arranged to integrate them in the late 1960’s.

Historical Perspective USENET was originally based on UUCP (Unix-to- Unix Copy Protocol). It allowed users on two different Unix machines to exchange messages and files.

Why does P2P need attention?

Overlay network A P2P network is an overlay network. Each link between peers consists of one or more IP links. Alice Bob Carol

Well-known P2P Systems Napster Gnutella KaZaA Limewire eDonkey Chord Tapestry CAN Pastry BitTorrent Kademlia Skype Various Social networks

Some important issues Search Storage Security Applications

A Distributed Storage Service Alice Bob Carol David

Promises Consider File Sharing as an Example –Available 24/7 –Durable despite machine failures –Information is protected –Resilient to Denial of Service

Additional Goals Massive scalability Anonymity Deniability Resistance to censorship

Challenges A P2P network must be self-organizing. Join and leave operations must be self-managed. The infrastructure is untrusted and the components are unreliable. The number of faulty nodes grows linearly with system size. Yet, the aggregate behavior has to be trustworthy.

Challenges Tolerance to failures and churn Efficient routing even if the structure of the network is unpredictable. Dealing with freeriders Load balancing Security issues

Looking up data How do you locate data/files/objects in a large P2P system built around a dynamic set of nodes in a scalable manner without any centralized server or hierarchy? Napster index servers used a central database. Questionable scalability and poor resilience. Check how names are looked up in internet’s DNS.

Napster Developed by Shawn Fanning in 1999, Shut down after 2 years for copyright infringement. Centralized directory servers were a bottleneck.. Root/ Redirector Directory server Directory server Directory server Users INTERNETINTERNET Stores indices of songs only

Gnutella Truly decentralized system. A search like where is Double Helix? is based on the flooding of the query on a graph of arbitrary topology. Obvious scalability problem, and the wastage of bandwidth caused serious inefficiencies.

Gnutella graph Client looking for “ double helix ” double helix

Unstructured vs. Structured Unstructured P2P networks allow resources to be placed at any node. The network topology is arbitrary, and the growth is spontaneous. Structured P2P networks simplify resource location and load balancing by defining a topology and defining rules for resource placement.

Distributed Hash Table (DHT) Object-to-machine mapping uses unique keys. H (object name) = key (H = hash function) H (machine name) = key Object name mapped to key k is placed in machine whose name is mapped to key k. Simplifies object location.

Distributed Hash Table (DHT) keyspace a c b 0 N-1 Machine name hashed to b Object name hashed to b Basic idea