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A friend-to-friend (or F2F) computer network is a type of peer-to-peer network in which users only make direct connections with people they know. Passwords or digital signatures can be used for authentication.
Unlike other kinds of private P2P, users in a friend-to-friend network cannot find out who else is participating beyond their own circle of friends, so F2F networks can grow in size without compromising their users' anonymity. Retroshare, WASTE, GNUnet, Freenet and OneSwarm are examples of software that can be used to build F2F networks, though RetroShare is the only one of these configured for friend-to-friend operation by default.
Many F2F networks support indirect anonymous or pseudonymous communication between users who do not know or trust one another. For example, a node in a friend-to-friend overlay can automatically forward a file (or a request for a file) anonymously between two friends, without telling either of them the other's name or IP address. These friends can in turn automatically forward the same file (or request) to their own friends, and so on.
Potential applications of F2F 
- The Bouillon project uses a friend-to-friend network to assign trust ratings to messages.
F2F software with support for automatic anonymous forwarding 
- Freenet (Open Source, Windows/GNU+Linux/Mac OS X, from version 0.7 optionally F2F with the "opennet" option disabled)
- GNUnet (Open Source, GNU+Linux/Windows/Mac OS X, optionally F2F with the "F2F topology" option enabled)
- Retroshare (Open Source, Win/Lin/Mac, Friend management based on GPG)
- OneSwarm (Open Source, Win/Lin/Mac, backwards compatible with BitTorrent)
- Turtle F2F (Open Source, Linux only)
F2F software with support for automatic pseudonymous forwarding 
- anoNet (pseudonymous, based on standard VPN software)
- WASTE (Open Source, Win/Linux/Mac, optionally F2F with the "ping packets" option disabled)
- Cjdns (Open Source), internal IPv6 address used as a pseudonym
F2F software without support for automatic forwarding 
- FrostWire (Open Source, Win/Linux/Mac)
- LimeWire (Open Source, Win/Linux/Mac, from version 5.0) Limewire is no longer available.
- Tudzu (Freeware, Win/Linux)
- Alliance P2P
See also 
- B.C. Popescu, B. Crispo, and A.S. Tanenbaum. "Safe and Private Data Sharing with Turtle: Friends Team-Up and Beat the System." In 12th International Workshop on Security Protocols, Cambridge, UK, April 2004.
- T. Chothia and K. Chatzikokolakis. "A Survey of Anonymous Peer-to-Peer File-Sharing." In Proceedings of the IFIP International Symposium on Network-Centric Ubiquitous Systems (NCUS 2005), Nagasaki, Japan, volume 3823 of Lecture Notes in Computer Science, pages 744-755. Springer, 2005.
- J. Li and F. Dabek. "F2F: Reliable Storage in Open Networks." In 5th International Workshop on Peer-to-Peer Systems (IPTPS '06), Santa Barbara, CA, USA, February 2006.
- M. Rogers and S. Bhatti. "How to Disappear Completely: A Survey of Private Peer-to-Peer Networks." In 1st International Workshop on Sustaining Privacy in Collaborative Environments (SPACE 2007), Moncton, NB, Canada, July 2007.
- Numbered references:
- Friend2Friend.net, An XML scripting language for writing F2F software
- Discussion about F2F involving Ian Clarke of Freenet
- F2F page at altruists.org
- Adding simple and effective trust measurements to F2F P2P networks is a paper about using a time-based currency for trust in F2F.
- Ripple: P2P money for trusted social networks
- Bouillon project website at the Internet Archive