+/*
+ * Set the `frozen' flag on a socket. A frozen socket is one in
+ * which all READABLE notifications are ignored, so that data is
+ * not accepted from the peer until the socket is unfrozen. This
+ * exists for two purposes:
+ *
+ * - Port forwarding: when a local listening port receives a
+ * connection, we do not want to receive data from the new
+ * socket until we have somewhere to send it. Hence, we freeze
+ * the socket until its associated SSH channel is ready; then we
+ * unfreeze it and pending data is delivered.
+ *
+ * - Socket buffering: if an SSH channel (or the whole connection)
+ * backs up or presents a zero window, we must freeze the
+ * associated local socket in order to avoid unbounded buffer
+ * growth.
+ */
+void sk_set_frozen(Socket sock, int is_frozen);