size Layer

The <size> layer represents remaining serialization length of the frame until the end of <payload>. The frame definition must NOT contain more than one <size> layer.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <frame name="ProtocolFrame">
        <size name="Size">
            <int name="SizeField" type="uint16" />
        </size>
        <id name="Id">
            <int name="IdField" type="uint8" />  
        </id>
        <payload name="Data" />
    </frame>
</schema>

Some protocols may specify that the field of the <size> layer contains its own length as well. The CommsDSL allows implementation of such case by adding usage of serOffset property to the field of the <size> layer.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <frame name="ProtocolFrame">
        <size name="Size">
            <int name="SizeField" type="uint16" serOffset="2" displayOffset="2"/>
        </size>
        <id name="Id">
            <int name="IdField" type="uint8" />  
        </id>
        <payload name="Data" />
    </frame>
</schema>

The example below implements ID (2 bytes) | SIZE (2 bytes) | PAYLOAD framing where SIZE value includes length of the header (ID + SIZE) in addition to the length of PAYLOAD.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <frame name="ProtocolFrame">
        <id name="Id">
            <int name="IdField" type="uint16" />  
        </id>
        <size name="Size">
            <int name="SizeField" type="uint16" serOffset="4" displayOffset="4"/>
        </size>
        <payload name="Data" />
    </frame>
</schema>

Also NOTE that <size> layer specifies number of remaining bytes until the end of <payload> layer. There are protocols that append some kind of <checksum> after the payload. In order to include them in the value of the <size> layer, also use serOffset property.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <frame name="ProtocolFrame">
        <size name="Size">
            <int name="SizeField" type="uint16" serOffset="2" displayOffset="2"/>
        </size>
        <id name="Id">
            <int name="IdField" type="uint8" />  
        </id>
        <payload name="Data" />
        <checksum ...>
            <int name="ChecksumField" type="uint16" />
        </checksum>
    </frame>
</schema>

The <size> layer doesn't have any extra properties in addition to common ones.