conditional expression

Hi all!
I have this problem on validation with xsd.
The xml file has a particular tag: when his value is "TCP" the xml file
must have other two tags, when his value is "OUR" there should be no
other tags.
So the xml should be either

<special-tag  type="string" value="TCP"/>
<host type="string" value="192.168.72.26"/>	
<port type="int" value="10202"/>

or
<special-tag  type="string" value="OUR"/>

Is there some way to do this with xsd?
Thank you very much for your help!
Debora