(wget.info.gz) 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.
 

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