Re: [codesprints] Who knows about web browser caching?

[Bill Fenner <[e-mail]>]

On Sun, Nov 7, 2010 at 6:40 PM, Julian Reschke <[e-mail]> wrote:
> On 07.11.2010 11:17, Bill Fenner wrote:
>>
>> On Nov 7, 2010, at 4:47 PM, Julian Reschke<[e-mail]>  wrote:
>>
>>> Bill,
>>>
>>> you may want to test with redbot, see
>>>
>>>
>>> <http://redbot.org/?uri=http://merlot.tools.ietf.org:31415/doc/draft-ietf-ipsecme-ikev2-redirect/history/>
>>>
>>> Also...
>>>>
>>>> I realize that the fact that the Etag varies here is going to be a
>>>> problem (it's because the Etag is calculated after the gzip), but this
>>>> is a separate problem.
>>>>
>>>
>>> ...the etag is *supposed* to be different across varying
>>> content-encodings...
>>
>> Yes, of course, but in this case it varies with the same encoding (because
>> we are just using an md5 of the returned data, and the metadata in the gzip
>> header is varying, even though the page contents are the same).
>>
>> Thanks for the redbot pointer!
>
> Um, if the payload changes the (strong) etag needs to change as well, right?

The data that you get as a result of decompressing the payload does
not change.  The bits in the stream that are different are thrown away
while decompressing.

> So *why* is the metadata in the gzip header changing?

The gzip encoder follows the "If the compressed data did not come from
a file..." part of the spec:

         MTIME (Modification TIME)
            This gives the most recent modification time of the original
            file being compressed.  The time is in Unix format, i.e.,
            seconds since 00:00:00 GMT, Jan.  1, 1970.  (Note that this
            may cause problems for MS-DOS and other systems that use
            local rather than Universal time.)  If the compressed data
            did not come from a file, MTIME is set to the time at which
            compression started.  MTIME = 0 means no time stamp is
            available.

so each time you get a gzip stream you get a different MTIME.

> (greetings to Bejing!)

(greetings from Beijing!)

  Bill