1-1 Incentive Mechanisms for Large Collaborative Resource Sharing Objectives:  Why Resource harnessing  Resource sharing  Assumptions  Considerations  What are incentives?  Trust as a mechanism to provide incentives

1-2 Web Server Farms  All serious web sites are maintained using web server farms  A group of computers acting as servers and housed in a single location  ISP’s provide web hosting services using a web server farm  Hardware and software is used to load balance requests across the machines

1-3 Web Server Farms  Other issues addressed by web server farms include:  Redundancy eliminates single point of failure  Backup  Failover strategy is required  Security: secure areas are placed behind firewalls Monitor web traffic Network address translation Port translation SSL

1-4 Content Delivery Networks  Web Content can be replicated across the Internet  Reduces client access  Better manages network traffic  Such replication needs to be efficient  Replaces replicas closer to the clients  Clients are transparently redirected to the replica nearest them  A single administrative Web server does not own machines at every location  Web servers must cooperate We have a problem???

1-5 Content Delivery Networks  A single administrative Web server does not own machines at every location  Web servers must cooperate  A server that wants to replicate some of its contents access other servers MUST provide remote access to some of its local resources CPU, storage space, network bandwidth, etc.  We will have a server-to-server relationship  Such system presents some security risks

1-6 Content Delivery Networks  Some of these security risks are  A malicious server could accept to host Web contents and deliver modified versions  A malicious server could accepts to host replicas and refuse to answer any requests directed to them, causing a “denial of service”  A malicious server can allocate less resources to replicas than was negotiated

1-7 Resource Harnessing  Huge interest in linking up resources  Server Farms, Content Distribution Networks, Web Services  It is all about sharing  Quality of Service  Security  Participation versus Cost

1-8 Resource Sharing  Assumptions  Resource owners have committed their resources Honestly To be used efficiently To be used for the overall good of the community  Considerations  Free riding  Malicious entities  Non cooperative entities Incentives are needed for resources to cooperate honestly

1-9 Resource Harnessing:  Since, we deal with public resources, we need to address the following  How can we encourage resources to cooperate 70% of all users do not share files 50% of all requests are satisfied by the top 1% sharing hosts  How can we deal with security  We do not want security to become an overhead!  Can we use “trust” as an incentive?

1-10 Trust Considerations  How can we define “trust” in an operational way? Who will evaluate trust?  Trust maintenance can result in an efficient process especially in a very large-scale system. Hence, our task is to come up with an efficient model for maintaining trust  Techniques for managing and evolving trust in a large-scale distributed system  Mechanisms for maintaining trust from ongoing transactions

1-11 Overall Trust Model

1-12 Trust Terminology  Identity trust  Behavior trust  Honesty  Accuracy  Set of recommenders  Set of trusted allies

1-13  To make the trust model efficient  the overall NC system is divided into NCDs  trust is a slow varying attribute  the number of contexts is limited to printing, storage, and computing Trust Level (TL)Equivalent numerical valueDescription A1very low TL B2low TL C3medium TL D4high TL E5very high TL Trust Model Characteristics

1-14 Why Behavior Trust Trust Attributes IdentityBehavior Importancefoundationlayer Costfixedvariable Changeabilityvery seldomyes Naturegivengained Replacementyesno Propagationimmediatewith time Perceptionexistslearned

1-15 Notation  Let and represent recommenders set and trusted allies set, respectively  Let the honesty of recommender as observed by be denoted as  Let denote the recommendation for given by to at time for context  Let denote the recommendation for given by to where for the same and

1-16 Computing Honesty  Let  The value of will be less than a small value if recommender is honest  Therefore, is computed as

1-17 Computing Accuracy  Let denote the true trust level of obtained by as a results of monitoring the transaction  Let  The value of will be an integer value ranging from 0 to 4  Therefore, is computed as

1-18 Computing Trust & Reputation  Before can use the recommendation given by to calculate the reputation of, needs to be adjusted to reflect the accuracy of recommender  This shift is given by

1-19 Computing Trust & Reputation  Trust relationship expressed as  Direct trust relationship and the reputation of expressed as and,respectively.  The decay function is expressed as  Let and