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Re: Pipelining and compression effect on HTTP/1.1 proxies

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Fri, 25 Apr 97 15:04:26 MDT
Message-Id: <9704252204.AA25408@acetes.pa.dec.com>
To: http-wg@cuckoo.hpl.hp.com
There seems to be some question about whether end-to-end compression
is useful in the Web.

Together with some people from AT&T Labs - Research, I wrote a
paper for this year's SIGCOMM conference that includes some 
Real Data that sheds some light on this issue.  SIGCOMM recently
accepted our paper, which means that we are now free to discuss
it in public (the SIGCOMM review process is supposed to be blind).

The paper is:
    Jeffrey C. Mogul, Fred Douglis, Anja Feldman, and Balachander
    Krishnamurthy.  Potential benefits of delta-encoding and data
    compression for HTTP.  In Proc. SIGCOMM '97 (to appear), Cannes,
    France, September 1997.
    
and you can retrieve an UNREVISED DRAFT copy from
	http://ftp.digital.com:80/~mogul/DRAFTsigcomm97.ps
Please do not treat this as a final draft of the paper!!!

The paper is primarily about a bandwidth-saving technique called
delta-encoding, which is not mature enough for IETF standardization
(so let's not discuss it here).  But we also looked at the potential
for improvement using simple data compression.

Other messages in this thread have pointed out that when evaluating
the utility of compression, it's not necessarily a good idea to look
at a static collection of URLs (since some URLs are referenced far
more often than others), and it's not a good idea to look at the
responses sitting in a cache (since this also mostly ignores the
relative reference rates, and completely ignores non-cachable
responses).

We instead looked at complete reference streams between a collection
of clients and the Internet, capturing the entire contents of the
requests and responses.  At Digital, I did this by modifying a
NON-CACHING proxy (wait - did he say "NON-CACHING?" Yes, I did)
and captured over 500,000 responses over a 2-day period, from 8K
clients and to 22K servers ... but (probably a mistake, in retrospect)
I tried to save space and so did not capture most of the image
URLs (.gif, .jpeg, etc.)

At AT&T, my co-authors used a packet-sniffing approach to capture
over a million responses, including images, over a longer period
of time, but from a much smaller set of clients.  (Again, no
proxy cache was involved.)

We looked at a few different compression algorithms, but the
winner was usually "gzip" (although I understand that "deflate",
which we did not try, is somewhat better than gzip). For the
Digital trace (which includes very few images), the overall
savings in bytes transferred was about 39% (75% of the responses
were improve at least a little by compression).  For the AT&T
trace, which includes images, the overall savings was about 20%
of the total bytes (but still we managed to improve 73% of the
responses).

The paper includes a table showing how compression improves
different HTTP content-types for the AT&T trace.  Overall,
99.7% of the text/html responses were compressible, and this
saved almost 69% of the text/html bytes.  So compression could
be quite useful in practice, even if the bulk of the responses
(as images) are not very compressible, for several reasons:
	(1) As Henrik and Jim have pointed out, compression
	of HTML files means that the IMG refs come sooner,
	which improves latency in retrieving them.
	(2) The increasing use of wireless or other slow
	networks, and of PDAs (or other small screens), means
	that there will be users who mostly care about HTML
	performance (because they are not going to load most
	images anyway).

One of the things that we could do with our traces (but 
haven't done yet) is to see if image responses have better
cache behavior than HTML responses.  We do have some evidence
that images change less often than HTML pages (i.e., once
an image is in a cache, it's highly likely that the next
reference to that URL will result in the same image; this
is not as true for HTML responses, since these often change
more rapidly over time.)  So it's possible that if we can
do various things to improve the caching of images, the
non-image (and therefore compressible) responses will become
more important as a fraction of total network load.

Regarding modem compression, Benjamin Franz wrote:
    Don't underestimate the modem compression. When moving highly
    compressible (textual) data it can easily double to triple
    throughput in my experience.
True.  However, modem compression seems to be less effective than
document-level compressors such as gzip, because (probably) it
has to operate more "locally".

We looked into this issue, and the paper presents results of
a fairly crude experiment, involving just 4 URLs (chosen
to show HTML files of a range of sizes and complexities).
We transferred the files over a 28.8K modem (with modem
compression enabled), with and without file-level compression.
For short documents, gzip seems to beat plain modem compression
by a moderate amount.  For longer documents, gzip does pretty
well, but another compressor ("vdelta") did even better ...
beating simple modem compression by as much as 69% of the
overall transfer time.  (And, as Bob Monsour pointed out,
high-level compression reduces the total number of packets
sent, which is usually a win).

The paper also looks at some other issues, such as the
computational cost of compressing and decompressing responses.
Even on a fairly slow machine, existing compression programs
run fast enough to provide some benefit, except when using
a fast LAN.

Bottom line: based on our results, we think that end-to-end
compression is a win, even though it's hard to compress
images.  It would be a real shame if HTTP/1.1 had some minor
flaws that make this impossible.  Henrik and I have an action
item, from the Memphis IETF meeting, to address the problems
that he has discovered when deploying "deflate" compression,
and we should be making a proposal soon.

-Jeff
Received on Friday, 25 April 1997 15:11:11 EDT

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