If a field is marked OPTIONAL, do I have to encode it? (Q84)

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It depends. The OPTIONAL designation in the ASN.1 syntax tree just means that the component is syntactically optional. There could be normative text elsewhere that requires its presence, at least in some situations. For example, the H.245 mediaControlChannel is marked OPTIONAL. In H.323, it is mandatory for audio but not, for example, for a proprietary data channel.