Use object id to identify a specific page

[knyjoh]

Now the canonical url is used to identify a specific page. This might lead to problems if the url of that page is changed. A suggestion is to also send an object id which is used to identify the page, regardless of the url.

[sirn-se]

Actually, any string can be used to identify a page. So this isn't a problem besides that the input says it should be an URL.