* identification, or may be used to obtain station status, or both.
   *
   * @since 8.0
   */
  public static final byte ENQ = 5;

  /**
   * Acknowledge: A communication control character transmitted by a receiver as an affirmative
   * response to a sender.
   *
   * @since 8.0
   */
  public static final byte ACK = 6;

  /**

		inbound:            nil,
		outBlock:           make(chan struct{}, outboundCap),
		LastPing:           time.Now().Unix(),
		BaseFlags:          c.baseFlags,
		clientPingInterval: c.clientPingInterval,
	}
	// Acknowledge Mux created.
	// Send async.
	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		defer wg.Done()
		var ack message
		ack.Op = OpAckMux
		ack.Flags = m.BaseFlags
		ack.MuxID = m.ID
		m.send(ack)

Framing rules make it impractical to implement http/2 correctly on a single blocking thread. The flow-control features introduce feedback between reads and writes, requiring writes to acknowledge reads and reads to throttle writes.

In OkHttp we expose a blocking API over a framed protocol. This document explains the code and policy that makes that work.

### Threads

#### Application's calling thread

     * do, when columnKeys is empty but rowKeys isn't, rowKeyList() can contain
     * elements but rowKeySet() will be empty and containsRow() won't
     * acknowledge them.
     */
    rowKeyToIndex = Maps.indexMap(rowList);
    columnKeyToIndex = Maps.indexMap(columnList);

    @SuppressWarnings("unchecked")

     * do, when columnKeys is empty but rowKeys isn't, rowKeyList() can contain
     * elements but rowKeySet() will be empty and containsRow() won't
     * acknowledge them.
     */
    rowKeyToIndex = Maps.indexMap(rowList);
    columnKeyToIndex = Maps.indexMap(columnList);

    @SuppressWarnings("unchecked")

 *  Fix: Don't lose HTTP/2 flow control bytes when incoming data races with a stream close. If this
    happened enough then eventually the connection would stall.

 *  Fix: Acknowledge and apply inbound HTTP/2 settings atomically. Previously we had a race where we
    could use new flow control capacity before acknowledging it, causing strict HTTP/2 servers to
    fail the call.


## Version 4.1.1