Network Performance Effects of HTTP/1.1, CSS1, and PNG

One-line summary: Application-level HTTP request pipelining (using "keep-alive" or persistent HTTP), transport level compression, use of style sheets instead of "GIF text buttons", affords only a modest increase in performance (and, in this reviewer's opinion, is a band-aid over HTTP's egregious violation of the end-to-end argument).

Overview/Main Points

• HTTP/1.1 pipelining: software-pipeline the requests to same server using keepalive.
• CSS: use text and style sheets instead of "GIF text buttons".
• PNG: Portable Network Graphics, a somewhat better format than GIF.
• Net effect of pipelining is a savings of at most 30-40% compared to HTTP/1.0; usually savings is much less. Most pronounced savings on high bandwidth-delay product networks.
• Transport compression ("Content-encoding: deflate", i.e. running gzip on HTML) buys another 15% or so.
• Extensive hacks to client (Perl script) and somewhat fewer hacks to server (Apache) took about 4 person-months.
• State machine on client for keepalive is a mess; server can shut down connection at any time, client must know what do with requests that were issued but never acknowledged; etc. Not really appropriate for small clients (where it is most needed!)
• Style sheets: not surprisingly, in many cases a style-sheet description is better than a "GIF text button" (but not in all cases; CSS1 is pretty verbose) and clearly better from a metadata perspective.
• Some bullet-type icons could be converted into CSS1 or PNG predefined glyphs (Wink does something like this in their authoring tool). Byte savings is small, but savings in HTTP overhead is huge. ("The best HTTP connection is the one that does not exist") Most of these estimates are extrapolations; CSS1 and PNG are not widely deployed and lack some of the capabilities presupposed by the examples given.

Relevance

Flaws