56 CHAPTER 1: WEB CACHING CONCEPTS AND DEPLOYMENTAccidental Webcache HierarchiesAn accidental Webcache hierarchy automatically exists in the networkingexample shown in Figure 15 if the parent Webcache is configured ineither Transparent cache, Inline cache or WCCP mode. In each of thesedeployment modes Web requests are transparently intercepted by adevice in your network and redirected to the Webcache. Therefore theparent Webcache will accidentally serve Web requests that it has receivedfrom the child Webcache, without the child being explicitly aware of theexistence of the parent.The main advantage of an explicit parent Webcache configuration, asopposed to an accidental hierarchy, is that it allows for a more flexiblenetwork topology between child and parent cache. The child Webachedirectly forwards its cache misses to the parent Webcache by using theparent Webcache’s IP address or domain name and TCP port number.Therefore you do not have to place the parent Webcache at a naturalpoint of convergence in your network and you do not have to add a newredirecting device to your network.ICP Caching ICP Caching is an open standard protocol allowing multiple proxy cachesto cooperate and appear as a single larger Webcache. It originallyappeared at a time when there was very little storage capacity on anindividual cache. Now that storage capacity is so much larger, in mostenvironments ICP is no longer used. A single Webcache, or a Webcacheoperating with Parent Caches offers better behavior and performance.Some environments may still require ICP in order to integrate with otherlegacy caches.ICP Caching creates an extended hierarchy similar to Parent Caching. ICPallows you to build hierarchies involving both ICP parent caches and ICPpeer caches. The Webcache requests content from its peer caches beforerequesting content from its parents.When a request is received from a browser, the Webcache firstdetermines if it has the content itself. If it does not, it will ask its peers(i.e. those caches at the same level in the ICP hierarchy). If none of thesepeers has the content it will ask its parents. If they do not have thecontent, the Webcache will go directly to the origin server.ICP Caching uses a different network protocol to Parent Caching. ICPCaching uses UDP/IP for communication between caches whereas ParentCaching uses TCP/IP.dua1611-5aaa04.book Page 56 Friday, November 29, 2002 8:56 PM
