// +---------------------------------------------------------------+
    val length = source.readMedium()
    if (length > INITIAL_MAX_FRAME_SIZE) {
      throw IOException("FRAME_SIZE_ERROR: $length")
    }
    val type = source.readByte() and 0xff
    val flags = source.readByte() and 0xff
    val streamId = source.readInt() and 0x7fffffff // Ignore reserved bit.

  @Test fun tooLargeDataFrame() {
    assertFailsWith<IllegalArgumentException> {
      sendDataFrame(Buffer().write(ByteArray(0x1000000)))
    }.also { expected ->
      assertThat(expected.message).isEqualTo("FRAME_SIZE_ERROR length > 16384: 16777216")
    }
  }

  @Test fun windowUpdateRoundTrip() {
    val expectedWindowSizeIncrement: Long = 0x7fffffff
    writeMedium(frame, 4) // length

    flags: Int,
  ) {
    if (type != TYPE_WINDOW_UPDATE && logger.isLoggable(FINE)) {
      logger.fine(frameLog(false, streamId, length, type, flags))
    }
    require(length <= maxFrameSize) { "FRAME_SIZE_ERROR length > $maxFrameSize: $length" }
    require(streamId and 0x80000000.toInt() == 0) { "reserved bit set: $streamId" }
    sink.writeMedium(length)
    sink.writeByte(type and 0xff)
    sink.writeByte(flags and 0xff)