RFC 2187 (rfc2187) - Page 3 of 24

Application of Internet Cache Protocol (ICP), version 2

RFC 2187                          ICP                     September 1997


2.  Web Cache Hierarchies

   A single Web cache will reduce the amount of traffic generated by the
   clients behind it.  Similarly, a group of Web caches can benefit by
   sharing another cache in much the same way.  Researchers on the
   Harvest project envisioned that it would be important to connect Web
   caches hierarchically.  In a cache hierarchy (or mesh) one cache
   establishes peering relationships with its neighbor caches.  There
   are two types of relationship: parent and sibling.  A parent cache is
   essentially one level up in a cache hierarchy.  A sibling cache is on
   the same level.  The terms "neighbor" and "peer" are used to refer to
   either parents or siblings which are a single "cache-hop" away.
   Figure 1 shows a simple hierarchy configuration.

   But what does it mean to be "on the same level" or "one level up?"
   The general flow of document requests is up the hierarchy.  When a
   cache does not hold a requested object, it may ask via ICP whether
   any of its neighbor caches has the object.  If any of the neighbors
   does have the requested object (i.e., a "neighbor hit"), then the
   cache will request it from them.  If none of the neighbors has the
   object (a "neighbor miss"), then the cache must forward the request
   either to a parent, or directly to the origin server.  The essential
   difference between a parent and sibling is that a "neighbor hit" may
   be fetched from either one, but a "neighbor miss" may NOT be fetched
   from a sibling.  In other words, in a sibling relationship, a cache
   can only ask to retrieve objects that the sibling already has cached,
   whereas the same cache can ask a parent to retrieve any object
   regardless of whether or not it is cached.  A parent cache's role is























Wessels & Claffy             Informational