Macro define_pdu 

macro_rules! define_pdu {
    (
        $(#[$meta:meta])*
        $vis:vis struct $name:ident {
            header: $header:ty,
            pdu_type: $pdu_type:expr,
            protocol_family: $protocol_family:expr,
            fields: {
                $(
                    $(#[len = $len_field:ident])? $fvis:vis $field:ident : $ftype:ty,
                )*
            }

        }
    ) => { ... };
}