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].

   Request-Routing techniques are used as a vehicle to extend the reach
   and scale of Content Delivery Networks.  There exist multiple
   Request-Routing mechanisms.  At a high-level, these may be classified
   under: DNS Request-Routing, transport-layer Request-Routing, and
   application-layer Request-Routing.

   A request routing system uses a set of metrics in an attempt to
   direct users to surrogate that can best serve the request.  For
   example, the choice of the surrogate could be based on network
   proximity, bandwidth availability, surrogate load and availability of
   content.  Appendix A provides a summary of metrics and measurement
   techniques that could be used in the selection of the best surrogate.

   The memo is organized as follows: Section 2 provides a summary of
   known DNS based Request-Routing techniques.  Section 3 discusses
   transport-layer Request-Routing methods.  In section 4 application
   layer Request-Routing mechanisms are explored.  Section 5 provides
   insight on combining the various methods that were discussed in the
   earlier sections in order to optimize the performance of the
   Request-Routing System.  Appendix A provides a summary of possible
   metrics and measurements techniques that could be used by the
   Request-Routing system to choose a given surrogate.