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salam every one, this is a topic from google web master centrale blog:

Webmaster level: Advanced

Based on your feedback, we’re happy to announce that Google web search now supports link rel="canonical" relationships specified in HTTP headers as per the syntax described in section 5 of IETF RFC 5988. Webmasters can use rel="canonical" HTTP headers to signal the canonical URL for both HTML documents and other types of content such as PDF files.

To see the rel="canonical" HTTP header in action, let’s look at the scenario of a website offering a white paper both as an HTML page and as a downloadable PDF alternative, under these two URLs:

• http://www.example.com/white-paper.html
• http://www.example.com/white-paper.pdf

In this case, the webmaster can signal to Google that the canonical URL for the PDF download is the HTML document by using a rel="canonical" HTTP header when the PDF file is requested; for example:

GET /white-paper.pdf HTTP/1.1
Host: www.example.com
(...rest of HTTP request headers...)
 
HTTP/1.1 200 OK
Content-Type: application/pdf
Link: <http://www.example.com/white-paper.html>; rel="canonical"
Content-Length: 785710
(... rest of HTTP response headers...)

Another common situation in which rel="canonical" HTTP headers may help is when a website serves the same file from multiple URLs (for example when using a content distribution network) and the webmaster wishes to signal to Google the preferred URL.

We currently support these link header elements for web search only. As we see how webmasters are using these elements, we're hoping to add support for them in our other properties. For more information, please see our Help Center articles about canonicalization and the rel="canonical" element. If you have any questions, please ask in our Webmaster Help Forum.

Posted by Pierre Far, Webmaster Trends Analyst

this is a topic published in 2013... to get contents for your blog or your forum, just contact me at: devnasser@gmail.com