Each message can have a Transaction Id called the id as an attribute. The id attribute is used by the XML Data server client to correlate request and response messages. The id attribute is optional and if specified, is an integer between 1 and 4294967295, expressed as a decimal number in ASCII. If the id attribute is specified in a request, the same id attribute and value are returned by the XML Data server in the corresponding response. A unique id value must be used in each request message to differentiate responses.