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(wget.info)HTTP Time-Stamping Internals

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HTTP Time-Stamping Internals
============================

   Time-stamping in HTTP is implemented by checking of the
`Last-Modified' header.  If you wish to retrieve the file `foo.html'
through HTTP, Wget will check whether `foo.html' exists locally.  If it
doesn't, `foo.html' will be retrieved unconditionally.

   If the file does exist locally, Wget will first check its local
time-stamp (similar to the way `ls -l' checks it), and then send a
`HEAD' request to the remote server, demanding the information on the
remote file.

   The `Last-Modified' header is examined to find which file was
modified more recently (which makes it "newer").  If the remote file is
newer, it will be downloaded; if it is older, Wget will give up.(1)

   When `--backup-converted' (`-K') is specified in conjunction with
`-N', server file `X' is compared to local file `X.orig', if extant,
rather than being compared to local file `X', which will always differ
if it's been converted by `--convert-links' (`-k').

   Arguably, HTTP time-stamping should be implemented using the
`If-Modified-Since' request.

   ---------- Footnotes ----------

   (1) As an additional check, Wget will look at the `Content-Length'
header, and compare the sizes; if they are not the same, the remote
file will be downloaded no matter what the time-stamp says.