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However, nothing prevents to use UTF-8 in any text field. It is not a FIX-compliant way, but the only requirement for such a trick is that the counterparty should expect UTF-8 in such field, protocol requirements will not be violated in this case.
Regarding UTF-16 or Unicode: such trick will lead to protocol violations because the 0x01 symbol can be contained in the text body in these encodings.
About support of Encoded fields:
FA and FE supports and correctly processes Encoded fields, supports and correctly processes UTF-8 in non-encoded fields.
For FA, it is user responsibility to convert ASCII string with UTF-8 content to the UTF-8 string and vise versa.
Tag support tables for base TP ICAP FIX dictionaries can be found in the attachment.
Следующие поля для поддержки различных кодировок должны присутствовать в словаре:
вставить ссылки на FIXopedia
For custom tags, if the text tag has related pair tag with text length specified, the one can use UTF-8, Unicode or UTF-16 there, if the length should be specified in bytes. In case of the length should be specified in symbols, only UTF-8 can be used. In any scenario, counterparty should expect such encoding in such field.