// +---------------------------------------------------------------+
    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.