checksum Layer
The <checksum> layer represents checksum bytes, usually (but not always) present at the end of the frame.
<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
<frame name="ProtocolFrame">
<sync name="Sync">
...
</sync>
<size name="Size">
...
</size>
<id name="Id">
...
</id>
<payload name="Data" />
<checksum name="Checksum" ...>
<int name="ChecksumField" type="uint16" />
</checksum>
</frame>
</schema>
The <checksum> layer has all the common properties as well as extra properties and elements described below.
Checksum Algorithm
The checksum calculation algorithm must be specified using alg property. Supported values are:
sum - Sum of all the bytes.
crc-ccitt (aliases: crc_ccitt) - CRC-16-CCITT
where polynomial is
0x1021
, initial value is0xffff
, final XOR value is0
, and no reflection of bytes.crc-16 (aliases: crc_16) - CRC-16-IBM,
where polynomial is
0x8005
, initial value is0
, final XOR value is0
, reflection is performed on everybyte as well as final value.
crc-32 (aliases: crc_32) - CRC-32,
where polynomial is
0x04C11DB7
, initial value is0xffffffff
, final XOR value is0xffffffff
, reflection isperformed on every byte as well as final value.
custom - Custom algorithm, code for which needs to be provided to the
code generator, so it can copy it to the generated code.
<?xml version="1.0" encoding="UTF-8"?> <schema endian="big" ...> <frame name="ProtocolFrame"> ... <payload name="Data" /> <checksum name="Checksum" alg="crc-16" ...> <int name="ChecksumField" type="uint16" /> </checksum> </frame> </schema>
When custom algorithm is selected, its name must be provided using algName property.
<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
<frame name="ProtocolFrame">
...
<payload name="Data" />
<checksum name="Checksum" alg="custom" algName="MyCustomChecksumCalc" ...>
<int name="ChecksumField" type="uint16" />
</checksum>
</frame>
</schema>
The provided name of the custom algorithm can be used by the code generator to locate the required external implementation file and use appropriate class / function name when the calculation functionality needs to be invoked.
Calculation Area
The checksum layer definition must also specify the layers, data of which is used to calculate the checksum. It is done using from property that is expected to specify name of the layer where checksum calculation starts.
<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
<frame name="ProtocolFrame">
<sync name="Sync">
...
</sync>
<size name="Size">
...
</size>
<id name="Id">
...
</id>
<payload name="Data" />
<checksum name="Checksum" alg="crc-16" from="Size">
<int name="ChecksumField" type="uint16" />
</checksum>
</frame>
</schema>
The example above defines SYNC | SIZE | ID | PAYLOAD | CHECKSUM
frame where the checksum is calculated on SIZE
+ ID
+ PAYLOAD
bytes.
Some protocols may put checksum value as prefix to the area on which the checksum needs to be calculated. In this case use until property (instead of from) to specify layers for checksum calculation.
<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
<frame name="ProtocolFrame">
<sync name="Sync">
...
</sync>
<size name="Size">
...
</size>
<id name="Id">
...
</id>
<checksum name="Checksum" alg="crc-16" until="Data">
<int name="ChecksumField" type="uint16" />
</checksum>
<payload name="Data" />
</frame>
</schema>
Checksum Verification Order
The default behavior of the <checksum> layer is to perform calculation after all relevant layers and their fields have been successfully read and processed. However, it is possible to force the checksum verification right away (without reading fields of other layers and/or message payload). It is usually possible to do when value of the <size> field is already processed, so the right location of checksum value is known, and it is not included in checksum calculation. To force immediate checksum verification use verifyBeforeRead property with boolean value.
<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
<frame name="ProtocolFrame">
<sync name="Sync">
...
</sync>
<size name="Size">
...
</size>
<id name="Id">
...
</id>
<checksum name="Checksum" alg="crc-16" until="Data" verifyBeforeRead="true">
<int name="ChecksumField" type="uint16" />
</checksum>
<payload name="Data" />
</frame>
</schema>
Use properties table for future references.
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