<peek> Element Reference

The <peek> element provides access to data ahead in the message. This information can then be referenced in expressions.

Detail

In some protocols a field cannot be decoded correctly until a subsequent field is known. The <peek> element provides a solution for this situation.

    <?xml version="1.0" encoding="iso-8859-1" ?>
    <xddl>
     <comment>peek into switch element</comment>
     <structure>
      <peek name="pd" offset="4" length="4"/>
      <switch expr="pd">
        <case value="0">
          <field length="4" name="security header"/>
          <field length="4" name="protocol descriminator"/>
        </case>
        <case value="1">
          <field length="4" name="bearer identity"/>
          <field length="4" name="protocol descriminator"/>
        </case>
       </switch>
     </structure>
    </xddl>

    peek.xddl

The above example illustrates a typical use of the <peek> element. Notice the <peek> "looks into" each of the <case> elements to determine what its value should be. Then the <switch> can be properly evaluated.

There are some restrictions when using <peek>. Every possibly path must have a field that can be evaluated. This means in the above example if there were another <case> element that only had one 4-bit field then the document would not be valid.

Also, there cannot be any variable length attributes between the <peek> and the offset. This means all the length expressions must be integers as long as the <peek> is being evaluated:

    <?xml version="1.0" encoding="iso-8859-1" ?>
    <xddl>
     <comment>peek over expression -- error</comment>
     <structure>
      <field name="num" length="8"/>
      <peek name="pd" offset="4" length="4"/>
      <switch expr="pd">
        <case value="0">
          <!-- this next field causes validation error! -->
          <field length="4 * num" name="security header"/>
          <field length="4" name="protocol descriminator"/>
        </case>
        <case value="1">
          <field length="4" name="bearer identity"/>
          <field length="4" name="protocol descriminator"/>
        </case>
       </switch>
     </structure>
    </xddl>

    peek_ex.xddl

This example is a modification of the first example. Here the security header field has a variable length of4 * num, where num is a previously declared field. This causes the document to fail validation.