**This release introduces fast fallback to better support mixed IPv4+IPv6 networks.** Fast fallback
is what we're calling our implementation of Happy Eyeballs, [RFC 8305][rfc_8305]. With this
feature OkHttp will attempt both IPv6 and IPv4 connections concurrently, keeping whichever connects
first. Fast fallback gives IPv6 connections a 250 ms head start so IPv6 is preferred on networks
where it's available.

    private set
  private var timeoutEarlyExit = false

  /**
   * This is the same value as [exchange], but scoped to the execution of the network interceptors.
   * The [exchange] field is assigned to null when its streams end, which may be before or after the
   * network interceptors return.
   */
  internal var interceptorScopedExchange: Exchange? = null
    private set

   * callback has returned. Synchronous calls become idle once [execute()][Call.execute] returns.
   * This means that if you are doing synchronous calls the network layer will not truly be idle
   * until every returned [Response] has been closed.
   */
  var idleCallback: Runnable? = null
    get() = this.withLock { field }
    set(value) {
      this.withLock { field = value }

      }

      val host = route.address.url.host
      if (!Platform.get().isCleartextTrafficPermitted(host)) {
        throw UnknownServiceException(
          "CLEARTEXT communication to $host not permitted by network security policy",
        )
      }
    } else {
      if (Protocol.H2_PRIOR_KNOWLEDGE in route.address.protocols) {
        throw UnknownServiceException("H2_PRIOR_KNOWLEDGE cannot be used with HTTPS")
      }

  fun receiveData(
    source: BufferedSource,
    length: Int,
  ) {
    lock.assertNotHeld()

    this.source.receive(source, length.toLong())
  }

  /** Accept headers from the network and store them until the client calls [takeHeaders]. */
  fun receiveHeaders(
    headers: Headers,
    inFinished: Boolean,
  ) {
    lock.assertNotHeld()

    val open: Boolean
    this.withLock {