Datatables sEcho

When the datas for the table are generated from the server side by using JSON, you must send a sEcho parameter with the datas. But what is sEcho ?

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sEcho : An unaltered copy of sEcho sent from the client side. This parameter will change with each draw (it is basically a draw count) - so it is important that this is implemented. Note that it is strongly recommended for security reasons that you 'cast' this parameter to an integer in order to prevent Cross Site Scripting (XSS) attacks.
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Source : http://datatables.net/usage/server-side