A session ID or session token is unique identifier that a website assigns to a specific user for some predetermined duration of time, or session, to keep track of visitor activity.
User authentication and shopping cart activity are two common uses for session IDs. The most common method of delivering and storing session IDs is with cookies; however, a session ID can also be embedded in a URL as a query string. Both Cookie and URL parameter session IDs can cause prevent search engines from properly crawling and indexing web content.
Session IDs stored in cookies are generally more secure than those transmitted through URL parameters.
Problem: Search engines ignore cookies.
Solution: Be sure to provide alternative methods for search engines to access web pages when cookies are used to present content, either through direct links or sitemaps.
When in doubt, "Fetch as Googlebot" from the site's associated Google Webmaster Tools account, to see if cookies are preventing search bots from accessing any pages.
When URL parameters are used for session IDs, as when tracking visits or referrers, the identifier is appended to the URL.
Problem: Because new URLs are created for one existing web page, duplicate content is created. For example, the following URLs all point to the exact same content:
http://www.example.com/products/women/dresses?sessionid=66704 http://www.example.com/products/tools?sessionid=45365 http://www.example.com/products/tools?sessionid=45365&source=google.ca
Solution: Duplicate content issues that arise from multiple URL parameters can often be solved with canonicalization, specifying to search engines which version of the URL should be indexed.
Google also offers a URL parameters tool from the 'Crawl' section of Webmaster Tools, which allows instructions on how Google should handle URLs that contain parameters.See also: Query string, clean URL, HTTP cookie Back to Springboard SEO's glossary