How to use sed to extract substring

I have a file containing the following lines: <parameter name="PortMappingEnabled" access="readWrite" type="xsd:boolean"></parameter> <parameter name="PortMappingLeaseDuration" access="readWrite" activeNotify="canDeny" type="xsd:unsignedInt"></parameter> <parameter name="RemoteHost" access="readWrite"></parameter> <parameter name="ExternalPort" access="readWrite" type="xsd:unsignedInt"></parameter> <parameter name="ExternalPortEndRange" access="readWrite" type="xsd:unsignedInt"></parameter> <parameter name="InternalPort" access="readWrite" type="xsd:unsignedInt"></parameter> <parameter name="PortMappingProtocol" access="readWrite"></parameter> <parameter name="InternalClient" access="readWrite"></parameter> <parameter name="PortMappingDescription" access="readWrite"></parameter> I want to execute command on this file to extract only the parameter names as displayed in the following output: $sedcommand file.txt PortMappingEnabled PortMappingLeaseDuration RemoteHost ExternalPort ExternalPortEndRange InternalPort PortMappingProtocol InternalClient PortMappingDescription What could be this command?