}
    } else {
      // The server reads the CONNECTION_PREFACE byte string.
      val connectionPreface = source.readByteString(CONNECTION_PREFACE.size.toLong())
      if (logger.isLoggable(FINE)) logger.fine(format("<< CONNECTION ${connectionPreface.hex()}"))
      if (CONNECTION_PREFACE != connectionPreface) {
        throw IOException("Expected a connection header but was ${connectionPreface.utf8()}")
      }
    }
  }

  @Throws(IOException::class)
  fun connectionPreface() {
    this.withLock {
      if (closed) throw IOException("closed")
      if (!client) return // Nothing to write; servers don't send connection headers!
      if (logger.isLoggable(FINE)) {
        logger.fine(format(">> CONNECTION ${CONNECTION_PREFACE.hex()}"))
      }
      sink.write(CONNECTION_PREFACE)
      sink.flush()
    }
  }

   */
  @Throws(IOException::class)
  @JvmOverloads
  fun start(sendConnectionPreface: Boolean = true) {
    if (sendConnectionPreface) {
      writer.connectionPreface()
      writer.settings(okHttpSettings)
      val windowSize = okHttpSettings.initialWindowSize
      if (windowSize != DEFAULT_INITIAL_WINDOW_SIZE) {

 * limitations under the License.
 */
package okhttp3.internal.http2

import okhttp3.internal.format
import okio.ByteString.Companion.encodeUtf8

object Http2 {
  @JvmField
  val CONNECTION_PREFACE = "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n".encodeUtf8()

  /** The initial max frame size, applied independently writing to, or reading from the peer. */
  const val INITIAL_MAX_FRAME_SIZE = 0x4000 // 16384