u_xml.xsd

<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="node">
    <xs:complexType mixed="true">
      <xs:sequence minOccurs="0" maxOccurs="unbounded">
	<xs:element ref="node" minOccurs="0"/> 
      </xs:sequence>
      <xs:attribute name="id" type="xs:ID" use="required"/> 
      <xs:attribute name="type" type="xs:string"/> 
    </xs:complexType>
  </xs:element> 
  <xs:element name="arc">
    <xs:complexType>
      <xs:simpleContent>
	<xs:extension base="xs:string">
	  <xs:attribute name="src" type="xs:IDREF" use="required"/> 
	  <xs:attribute name="dst" type="xs:IDREF" use="required"/> 
	  <xs:attribute name="src_port" type="xs:integer"/>  
	  <xs:attribute name="dst_port" type="xs:integer"/> 
	  <xs:attribute name="type" type="xs:string"/> 
	</xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>
  <xs:element name="nodes">
    <xs:complexType>
      <xs:sequence minOccurs="0" maxOccurs="unbounded">
	<xs:element ref="node" minOccurs="0"/> 
      </xs:sequence>
      <xs:attribute name="nb" type="xs:integer"/> 
    </xs:complexType>
  </xs:element>
  <xs:element name="arcs">
    <xs:complexType>
      <xs:sequence minOccurs="0" maxOccurs="unbounded">
	<xs:element ref="arc" minOccurs="0"/> 
      </xs:sequence>
      <xs:attribute name="nb" type="xs:integer"/> 
    </xs:complexType>
  </xs:element>
  <xs:element name="u">
    <xs:complexType>
      <xs:sequence>
	<xs:element ref="nodes" minOccurs="0" maxOccurs="1"/> 
	<xs:element ref="arcs" minOccurs="0" maxOccurs="1"/> 
      </xs:sequence>
      <xs:attribute name="digest" type="xs:string" use="required"/>   
    </xs:complexType>
    <xs:key name="myId">
      <xs:selector xpath="./nodes/node"/>
      <xs:field xpath="@id"/>
    </xs:key>
    <xs:keyref name="mySrc" refer="myId">
      <xs:selector xpath="./arcs/arc"/>
      <xs:field xpath="@src"/>
    </xs:keyref>
    <xs:keyref name="myDst" refer="myId">
      <xs:selector xpath="./arcs/arc"/>
      <xs:field xpath="@dst"/>
    </xs:keyref>
  </xs:element>
</xs:schema>