What requests do browsers' "F5" and "Ctrl + F5" refreshes generate?

Question

Is there a standard for what actions F5 and Ctrl+F5 trigger in web browsers?

I once did experiment in IE6 and Firefox 2.x. The "F5" refresh would trigger a HTTP request sent to the server with an "If-Modified-Since" header, while "Ctrl+F5" would not have such a header. In my understanding, F5 will try to utilize cached content as much as possible, while "Ctrl+F5" is intended to abandon all cached content and just retrieve all content from the servers again.

But today, I noticed that in some of the latest browsers (Chrome, IE8) it doesn't work in this way anymore. Both "F5" and "Ctrl+F5" send the "If-Modified-Since" header.

So how is this supposed to work, or (if there is no standard) how do the major browsers differ in how they implement these refresh features?

Answer 1

Generally speaking:

F5 may give you the same page even if the content is changed, because it may load the page from cache. But Ctrl-F5 forces a cache refresh, and will guarantee that if the content is changed, you will get the new content.

Answer 2

I believe that ctrl-f5 additionally updates the mime-type from the server also.

Answer 3

It is up to the browser but they behave in similar ways.

I have tested FF, IE7, Opera and Chrome.

F5 usually updates the page only if it is modified. The browser usually tries to use all types of cache as much as possible and adds an "If-modified-since" header to the request. Opera differs by sending a "Cache-Control: no-cache"

CTRL-F5 is used to force an update, dissregaring any cahce. IE7 add an "Cache-Control: no-cache" as does FF who also add "Pragma: no-cache". Crome does a normal "If-modified-since" and Opera ignores the key.

If I remember correctly it was Netscape who was the first browser to add support for cache-control by adding "Pragma: No-cache" when you pressed CTRL-F5.

Edit: Updated table

The table below is updated with information on what will happen when the browsers refresh-button is clicked (after a request by Joel Coehoorn), and the "max-age=0" Cache-control-header.

Updated table, 27 September 2010

+------------+-----------------------------------------------+
|  UPDATED   |                Firefox 3.x                    |
|27 SEP 2010 |  +--------------------------------------------+
|            |  |             MSIE 8, 7                      |
| Version 3  |  |  +-----------------------------------------+
|            |  |  |          Chrome 6.0                     |
|            |  |  +  +--------------------------------------+
|            |  |  |  |       Chrome 1.0                     |
|            |  |  |  |  +-----------------------------------+
|            |  |  |  |  |    Opera 10, 9                    |
|            |  |  |  |  |  +--------------------------------+
|            |  |  |  |  |  |                                |
+------------+--+--+--|--+-----------------------------------+
|          F5|IM|I |IM|IM|C |                                |
|    SHIFT-F5|- |- |CP|IM|- | Legend:                        |
|     CTRL-F5|CP|C |CP|IM|- | I = "If-Modified-Since"        |
|      ALT-F5|- |- |- |- |*2| P = "Pragma: No-cache"         |
|    ALTGR-F5|- |I |- |- |- | C = "Cache-Control: no-cache"  |
+------------+--+--+--|--+--+ M = "Cache-Control: max-age=0" |
|      CTRL-R|IM|I |IM|IM|C | - = ignored                    |
|CTRL-SHIFT-R|CP|- |CP|- |- |                                |
+------------+--+--+--|--+--+                                |
|       Click|IM|I |IM|IM|C | With 'click' I refer to a      |
| Shift-Click|CP|I |CP|IM|C | mouse click on the browsers    |
|  Ctrl-Click|*1|C |CP|IM|C | refresh-icon.                  |
|   Alt-Click|IM|I |IM|IM|C |                                |
| AltGr-Click|IM|I |- |IM|- |                                |
+------------+--+--+--+--+--+--------------------------------+

Versions tested:

• Firefox 3.1.6 and 3.0.6 (WINXP)
• MSIE 8.0.6001 and 7.0.5730.11 (WINXP)
• Chrome 6.0.472.63 and 1.0.151.48 (WINXP)
• Opera 10.62 and 9.61 (WINXP)

Notes:

1. Version 3.0.6 sends I and C, but 3.1.6 opens the page in a new tab, making a normal request with only "I".

2. Version 10.62 does nothing. 9.61 might do C unless it was a typo in my old table.

Note about Chrome 6.0.472: If you do a forced reload (like CTRL-F5) it behaves like the url is internaly marked to always do a forced reload. The flag is cleared if you go to the adress bar and press enter.

Answer 4

When user press F5 although new request goes to web server and get a responce for the request as well. But when the responce header is Parsed it check the required information in browser cache. If the required information in cache has not expired then that information is restored from in cache itself.

When user click on Ctrl F5 even then new request goes to web server and get a responce. But this time when the responce header is Parsed it do not check any required information in cache, and bring all updated information form server only.

Answer 5

At least in Firefox (v3.5), cache seems to be disabled rather than simply cleared. If there are multiple instances of the same image on a page, it will be transferred multiple times. That is also the case for img tags that are added subsequently via Ajax/JavaScript.

So in case you're wondering why the browser keeps downloading the same little icon a few hundred times on your auto-refresh Ajax site, it's because you initially loaded the page using CTRL-F5.