HTTP is a stateless protocol, and VIEWSTATE is one mechanism used to persist client changes across multiple requests. Each control on a Web page contains a System.Web.UI.Control.ViewState property, which represents the accumulation of any changes made by the client. In a Web Forms page, these changes are encoded in postback data as the value of an HTML <input> element with a type attribute of hidden. For example:

<input type="hidden" name="__VIEWSTATE" value="t0PH_u56?cDxleHQ7P=" />

If true, non-alphabetic characters in the VIEWSTATE value will not be sent correctly by the browser, nor by an intermediate gateway. To correct this, server-control adapters replace non-alphabetic characters in the VIEWSTATE value with ones that do not require encoding in HTTP requests.