blank.gif (43 bytes)

Church Of The
Swimming Elephant

14.24 If-Modified-Since Connected: An Internet Encyclopedia
14.24 If-Modified-Since

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 2068
Up: 14 Header Field Definitions
Prev: 14.23 Host
Next: 14.25 If-Match

14.24 If-Modified-Since

14.24 If-Modified-Since

The If-Modified-Since request-header field is used with the GET method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any message-body.

          If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

          If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases:

  • If the request would normally result in anything other than a 200 (OK) status, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.

  • If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.

  • If the variant has not been modified since a valid If-Modified-Since date, the server MUST return a 304 (Not Modified) response.

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

    Note that the Range request-header field modifies the meaning of If-Modified-Since; see section 14.36 for full details.

    Note that If-Modified-Since times are interpreted by the server, whose clock may not be synchronized with the client.

Note that if a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between the client and server. This includes the possibility of race conditions if the document has changed between the time it was first requested and the If-Modified- Since date of a subsequent request, and the possibility of clock- skew-related problems if the If-Modified-Since date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between client and server are at best approximate due to network latency.

Next: 14.25 If-Match

Connected: An Internet Encyclopedia
14.24 If-Modified-Since


Protect yourself from cyberstalkers, identity thieves, and those who would snoop on you.
Stop spam from invading your inbox without losing the mail you want. We give you more control over your e-mail than any other service.
Block popups, ads, and malicious scripts while you surf the net through our anonymous proxies.
Participate in Usenet, host your web files, easily send anonymous messages, and more, much more.
All private, all encrypted, all secure, all in an easy to use service, and all for only $5.95 a month!

Service Details

Have you gone to church today?
All pages ©1999, 2000, 2001, 2002, 2003 Church of the Swimming Elephant unless otherwise stated
Church of the Swimming Elephant©1999, 2000, 2001, 2002, 2003 is a wholly owned subsidiary of Packetderm, LLC.

Packetderm, LLC
210 Park Ave #308
Worcester, MA 01609