What should my Pragma HTTP response header be set to?

[a11WriniaNag]

I have my cache related headers set to expire at eight hours as follows...\[code\]header('Cache-Control: max-age=28800');header('Expires:' . gmdate('D, d M Y H:i:s T', strtotime('+8 hours')));\[/code\]With that being the case, what should my Pragma HTTP response header be set to?