Aliasing on the world wide web
Terence Kelly, Jeffrey C. Mogul · 2002
Aliasing occurs in Web transactions when requests containing different URLs elicit replies containing identical data payloads. Conventional caches associate stored data with URLs and can therefore suffer redundant payload transfers due to aliasing and other causes. Existing research literature, however, says little about the prevalence of aliasing in user-initiated transactions, or about redundant payload transfers in conventional Web cache hierarchies.This paper quantifies the extent of aliasing and the performance impact of URL-indexed cache management using a large client trace from WebTV Networks. Fewer than 5% of reply payloads are aliased (referenced via multiple URLs) but over 54% of successful transactions involve aliased payloads. Aliased payloads account for under 3.1% of the trace's "working set size" (sum of payload sizes) but over 36% of bytes transferred. For the WebTV workload, roughly 10% of payload transfers to browser caches and 23% of payload transfers to a shared proxy are redundant, assuming infinite-capacity conventional caches. Our analysis of a large proxy trace from Compaq Corporation yields similar results.URL-indexed caching does not entirely explain the large number of redundant proxy-to-browser payload transfers previously reported in the WebTV system. We consider other possible causes of redundant transfers (e.g., reply metadata and browser cache management policies) and discuss a simple hop-by-hop protocol extension that completely eliminates all redundant transfers, regardless of cause.