Trust Horizons is an essay by Jack Rusher, published here Friday, October 24, 2003. It is part of Ideas, Big and Small.
A “trust horizon”-based routing scheme for use in peer to peer overlay networks.
One of the keys to building a file sharing network that uses instant messaging infrastructure to provide a private overlay network is the creation of a routing system designed to control request propagation. I propose the following mechanism that combines traditional TTL (time to live) methods with relationship-based trust metrics.
The underlying technological assumptions for this system include the existence of a request forwarding mechanism that uses instant messaging technology as a transport, a group of users who have installed a client for this system, and, lastly, that each user has created a list of “buddies” and assigned a level of trustworthiness to each of them.
When a search request is transmitted from one peer (P1) to another directly known peer (P2) — a neighbor in the relationship graph — a “trust value” (T) and a “trust horizon” (H) are included in the request header. The software on P2, upon receiving this request, iterates through its list of peers and multiplies the value of T contained in the request with the value of T it associates with its relationship with each of its peers and forwards the request, including the modified T, via those links for which the resultant T is above the originally specified H. This process is repeated by each node in the extended network until the request can no longer be transmitted without T falling below H.
The canonical trio of users, Alice, Bob and Charlie, will serve as an example. If Alice half trusts Bob (T = 0.5) and Bob one quarter trusts Charlie (T = 0.25) then Alice one eighth trusts Charlie (T = 0.125). Consequently, if Alice has set her client to send requests with a trust horizon of one quarter (H = 0.25) her request will never reach Charlie via Bob.
The trust horizon is thus analogous to the search horizon in a conventional peer to peer system.