RFC 3568 Known CN Request-Routing Mechanisms July 2003 4. Application-Layer Request-Routing . . . . . . . . . . . . . 8 4.1. Header Inspection. . . . . . . . . . . . . . . . . . . 8 4.1.1. URL-Based Request-Routing. . . . . . . . . . . 8 4.1.2. Header-Based Request-Routing . . . . . . . . . 9 4.1.3. Site-Specific Identifiers. . . . . . . . . . .10 4.2. Content Modification . . . . . . . . . . . . . . . . .10 4.2.1. A-priori URL Rewriting . . . . . . . . . . . .11 4.2.2. On-Demand URL Rewriting. . . . . . . . . . . .11 4.2.3. Content Modification Limitations . . . . . . .11 5. Combination of Multiple Mechanisms . . . . . . . . . . . . .11 6. Security Considerations . . . . . . . . . . . . . . . . . .12 7. Additional Authors and Acknowledgements . . . . . . . . . .12 A. Measurements . . . . . . . . . . . . . . . . . . . . . . . .13 A.1. Proximity Measurements . . . . . . . . . . . . . . . .13 A.1.1. Active Probing . . . . . . . . . . . . . . . .13 A.1.2. Metric Types . . . . . . . . . . . . . . . . .14 A.1.3. Surrogate Feedback . . . . . . . . . . . . . .14 8. Normative References . . . . . . . . . . . . . . . . . . . .15 9. Informative References . . . . . . . . . . . . . . . . . . .15 10. Intellectual Property and Copyright Statements . . . . . . .17 11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . .18 12. Full Copyright Statement . . . . . . . . . . . . . . . . . .19 1. Introduction This document provides a summary of known request routing techniques that are used by the industry before December 2000. Request routing techniques are generally used to direct client requests to surrogates based on various policies and a possible set of metrics. The task of directing clients' requests to surrogates is also called Request-Routing, Content Routing or Content Redirection. Request-Routing techniques are commonly used in Content Networks (also known as Content Delivery Networks) [8]. Content Networks include network infrastructure that exists in layers 4 through 7. Content Networks deal with the routing and forwarding of requests and responses for content. Content Networks rely on layer 7 protocols such as HTTP [4] for transport. Request-Routing techniques are generally used to direct client requests for objects to a surrogate or a set of surrogates that could best serve that content. Request-Routing mechanisms could be used to direct client requests to surrogates that are within a Content Network (CN) [8]. Barbir, et al. Informational