+ *
+ * The @packet callback should adhere to the following rules:
+ * 1) it must return the number of bytes it consumed from the payload;
+ * 2) therefore, if it sent a packet that does not have payload (like FLAG),
+ * it must return zero;
+ * 3) if it does not support the requested packet type/flag combination,
+ * it must return -ENOTSUPP.
+ *
+ * The @unlink callback is called when there are no more active writers so
+ * that the master/channel can be quiesced.