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Several major crawlers support a Crawl-delay parameter, which determines how long a spider must wait after retrieving a page before it can request another page from the site. This is a good security measure to implement as it reduces the likelyhood of a spider acting like a 'Denial of Service' attack as it prevents your site from being overloaded by a spider suddenly requesting a large number of pages in quick succession.
The crawl-delay directive accepts a value which represents the number of seconds to wait between successive requests to the same server:
#wait ten seconds between page requests
Another important function of the robots.txt file in particular for SEO is the sitemap directive which tells a webspider where the site map(s) is stored. Example as follows.
Sitemap: http://www.mysite.com/sitemaps/profiles-sitemap xml
Sitemap: http://www.mysite.com/sitemaps/blog-sitemap xml