Do I need to treat DEFAULT as optional or mandatory when transmitting a PDU?

At the abstract ASN.1 level the absence of a DEFAULT value in an encoding is the same as its being present. Contrast this with OPTIONAL, where a value being present in the encoding is semantically distinct from its being absent.

In some encoding rules (like BER/PER) it is at the whim of the sender whether a DEFAULT value is encoded or not (except for primitive type values in PER which are required by the PER standard to be absent in the encoding), while with others (like DER) the DEFAULT value is NEVER encoded. For all encoding rules, if the component that has a DEFAULT value is not encoded the receiving application must behave as though the DEFAULT value had been encoded.

For the sake of improving performance in PER the OSS ASN.1 Tools do not check if the value passed to the encoder is in fact a DEFAULT value. So it is your responsibility to not encode DEFAULT primitive type values, as required by the PER standard.

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