title: What does "useinbandfec is a unidirectional receive-only parameter" mean in Opus?
body: <p>I am trying to implement a Opus transcoder in C++, and I have some conceptual questions.
  As for the useinbandfec parameter, I don't quite understand the meaning of &quot;unidirectional receive-only parameter&quot;, as written in <a href="https://www.rfc-editor.org/rfc/rfc7587#page-15" rel="nofollow noreferrer">RFC7587, section 7.1</a>:</p>
  <blockquote>
  <p>The &quot;useinbandfec&quot; parameter is a unidirectional receive-only
  parameter.</p>
  </blockquote>
  <p>For example, Alice is calling Bob. Alice uses Opus codec with useinbandfec=1, while Bob doesn't use Opus. Therefore, there should be a Opus transcoder between the two. If so, how can I configure the settings of Opus encoder and decoder?</p>
  <p>As for Alice-&gt;Bob where <code>opus_decode()</code> is needed, what value should be sent as the 5th parameter (fecFlag)? And as for Bob-&gt;Alice where a Opus encoder is needed, what value should be sent with <code>OPUS_SET_INBAND_FEC()</code> in <code>opus_encoder_ctl()</code>?</p>
  <p>Thank you!</p>