Patent Publication Number: US-2023156270-A1

Title: Access Control Techniques for Media Playback Systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. Pat. Application No. 17/527,887, filed on Nov. 16, 2021, entitled “Access Control Techniques for Media Playback Systems,” the contents of which are incorporated by reference herein in its entirety. 
     U.S. Pat. Application No. 17/527,887 claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. Pat. Application No. 16/372,014, filed on Apr. 1, 2019, entitled “Access Control Techniques for Media Playback Systems,” and issued as U.S. Pat. No. 11,184,666 on Nov. 23, 2016, the contents of which are incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure is related to consumer goods and, more particularly, to methods, systems, products, features, services, and other elements directed to media playback or some aspect thereof. 
     BACKGROUND 
     Options for accessing and listening to digital audio in an out-loud setting were limited until in 2002, when SONOS, Inc. began development of a new type of playback system. Sonos then filed one of its first patent applications in 2003, titled “Method for Synchronizing Audio Playback between Multiple Networked Devices,” and began offering its first media playback systems for sale in 2005. The Sonos Wireless Home Sound System enables people to experience music from many sources via one or more networked playback devices. Through a software control application installed on a controller (e.g., smartphone, tablet, computer, voice input device), one can play what she wants in any room having a networked playback device. Media content (e.g., songs, podcasts, video sound) can be streamed to playback devices such that each room with a playback device can play back corresponding different media content. In addition, rooms can be grouped together for synchronous playback of the same media content, and/or the same media content can be heard in all rooms synchronously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, aspects, and advantages of the presently disclosed technology may be better understood with regard to the following description, appended claims, and accompanying drawings, as listed below. A person skilled in the relevant art will understand that the features shown in the drawings are for purposes of illustrations, and variations, including different and/or additional features and arrangements thereof, are possible. 
         FIG.  1 A  is a partial cutaway view of an environment having a media playback system configured in accordance with aspects of the disclosed technology. 
         FIG.  1 B  is a schematic diagram of the media playback system of  FIG.  1 A  and one or more networks. 
         FIG.  1 C  is a block diagram of a playback device. 
         FIG.  1 D  is a block diagram of a playback device. 
         FIG.  1 E  is a block diagram of a bonded playback device. 
         FIG.  1 F  is a block diagram of a network microphone device. 
         FIG.  1 G  is a block diagram of a playback device. 
         FIG.  1 H  is a partial schematic diagram of a control device. 
         FIG.  2    is a diagram of a control device obtaining access to (and controlling) a media playback system. 
         FIG.  3 A  is a flow diagram of an example method of operation for a computing device such as the control device in  FIG.  2   . 
         FIG.  3 B  is a flow diagram of another example method of operation for a computing device such as the control device in  FIG.  2   . 
         FIG.  4    is a flow diagram of an example method of operation for a media playback system such as the playback device in  FIG.  2   . 
         FIGS.  5 A and  5 B  are a flow diagram of an example method of operation for a computing system such as the computing system in  FIG.  2   . 
         FIG.  6    is a message flow diagram depicting example messages to detect a media playback system. 
         FIG.  7    is a message flow diagram depicting example messages to obtain a first identifier. 
         FIG.  8    is a message flow diagram depicting example messages to obtain a second identifier. 
         FIG.  9    is a message flow diagram depicting example messages to obtain a token. 
         FIG.  10    is a message flow diagram depicting example messages to cause a media playback system to perform an operation. 
         FIGS.  11 A and  11 B  each show an example sequence of user interface screens that may be displayed by a computing device such as the control device in  FIG.  2   . 
     
    
    
     The drawings are for the purpose of illustrating example embodiments, but those of ordinary skill in the art will understand that the technology disclosed herein is not limited to the arrangements and/or instrumentality shown in the drawings. 
     DETAILED DESCRIPTION 
     I. Overview 
     Playing media content, such as music, radio, audio books, and videos, can be a social activity that involves multiple individuals. For example, a household may play music via a media playback system during a social gathering such as a birthday party. In another example, a business may play music via a media playback system for patrons of the business. In such environments, individuals other than the owner of the media playback system may wish for certain media to be played on the media playback system. For example, a guest may visit a household and want to play a new song for the household on a media playback system that they recently heard on a music streaming service (e.g., SPOTFY, PANDORA, AMAZON MUSIC). Conventionally, only the owner of the media playback system is permitted to control playback of media on the media playback system. As a result, a guest would need to ask the owner of the media playback system to play the particular song. 
     Such conventional architectures where only the owner of the media playback system is permitted to control the media playback system may have several limitations. For example, a guest may ask the owner of the media playback system to play a piece of media that is unavailable to the owner of the media playback system (e.g., the owner neither owns the song nor has access to the song via any streaming service to which the owner subscribes). As a result, the owner of the media playback system may be unable to comply with the request and the guest may have to play the media on their own computing device (e.g., their smartphone). 
     One approach to empower guests of an establishment with a media playback system is to enable guest access for the media playback system. In such an approach, an owner may permit guests to directly control a subset of operations of the media playback system, such as adding media to a queue and changing playback volume. As a result, a guest may be able to play media from their own computing device instead of having to ask the owner of the media playback system to play particular media. Examples of such guest access for media playback systems are described in U.S. Pat. Publication No. 2018/0225012 titled “Guest Access to a Media Playback System,” filed on Apr. 5, 2018 and U.S. Pat. No. 9,374,607 titled “Media Playback System with Guest Access,” filed on Jun. 26, 2012, each of which is incorporated herein by reference in its entirety. 
     Enabling access to additional users separate and apart from the owner of the media playback system, such as guests, may raise security challenges. For example, blindly allowing any individual with a computing device to make changes to the operation of the media playback system may disadvantageously permit wrongdoers to tamper with the media playback system. While an unsophisticated wrongdoer may simply annoy other users of the media playback system by inappropriately modifying a playback queue and/or maximizing playback volume, a more sophisticated wrongdoer may cause significant damage. For example, a wrongdoer may install different firmware on the media playback system that includes known security vulnerabilities. The wrongdoer may, in turn, exploit such vulnerabilities to permanently damage one or more components of the media playback system and/or gain access to a network (e.g., a Wi-Fi network) to steal sensitive information from other devices on the network. 
     Accordingly, aspects of the present disclosure relate to new access control techniques to, for example, facilitate the introduction of other types of users separate and apart from the owner of the media playback system (e.g., guest users) without compromising the security of the media playback system. It should be appreciated that such access control techniques may also be readily implemented in media playback systems with only a single user (and/or a single type of user) to further enhance security of the media playback system. As a result, the access control techniques described herein may be advantageously implemented in media playback systems that support any number of users and/or user types. 
     The access control techniques described herein may employ a plurality of credentials (e.g., unique identifiers) that a computing device obtains from different sources in order to gain access to the media playback system. For example, a computing device seeking access to the media playback system may obtain each of the following credentials: (1) a first identifier that is associated with the media playback system (e.g., obtained from the media playback system itself over a network, such as a Wi-Fi network, that the media playback system is connected and/or an owner of the media playback system); (2) a second identifier obtained from a computing system (e.g., a cloud platform where the media playback system is registered) using the first identifier; and (3) a token that is obtained from the computing system using the second identifier (and/or the first identifier). By employing multiple credentials that are obtained from different sources in such a manner, a malicious user would need to compromise multiple distinct systems in order to gain the requisite credentials to access to the media playback system. For example, malicious users that are geographically remote from the media playback system are unlikely to be able to successfully obtain the first identifier because such malicious users are not on the same network (e.g., the same Wi-Fi network) as the media playback system or in communication with the owner of the media playback system. Even malicious users that have successfully obtained the first identifier may still be unable to obtain the second identifier because of one or more access restrictions stored by the computing system that limit access to the media playback system (e.g., require explicit approval by the owner before a new user is permitted, limit the number of concurrent users, forbid certain types of users). For illustration, the computing system may require explicit approval from the owner (e.g., in the form of a response to a notification, an email, or a text message) before generating a second identifier for a new computing device. As a result, malicious users are prevented from inappropriately gaining access to the media playback system while still permitting additional users and/or user types. 
     The access control techniques described herein may be readily applied to media playback systems (or other types of systems) that employ any of a variety of user permissions schemes. Such user permissions schemes may limit access to a single type of user (e.g., an owner user) or provide access to a plurality of different types of users (e.g., an owner user, a guest user, a child user). The media playback systems may be controlled, at least in part, by a computing device that has been successfully authenticated. Examples of such computing devices include smartphones, tablets, wearable computers (e.g., smartwatches and smartglasses), desktop computers, and laptop computers. The computing device may comprise one or more network interfaces that enable communication over one or more networks (e.g., wide area networks (WANs), local area networks (LANs), personal area networks (PANs)). The computing device may further comprise one or more processors and data storage having stored therein instructions executable by the one or more processors to cause the computing device to perform various operations. The data storage may comprise, for example, a non-transitory computer readable medium such as non-volatile memory (e.g., hard disk drives, solid state drives) or volatile memory (e.g., random-access memory (RAM)). The instructions may be organized into, for example, one or more applications including a native application made by the manufacturer of the media playback system and/or a third-party application made by a company other than the manufacturer of the media playback system (e.g., an application made by a music streaming company, such as SPOTIFY or PANDORA, selling a music streaming service that is playable on the media playback system). 
     In some embodiments, the instructions stored in the data storage may cause the computing device to obtain access to and/or control the media playback system. In these embodiments, the computing device may receive a request to control a media playback system comprising a playback device connected to a network. The computing device may receive the request from, for example, a user via a graphical user interface (GUI) displayed on the computing device. For illustration, the computing device may detect activation of a cast icon in the GUI for transferring playback from the computing device to the media playback system. After the computing device receives the request to control the media playback system, the computing device may obtain access to the media playback system by, for example, obtaining at least one credential from each of at least two different sources. 
     The computing device may obtain, as a first credential, a first identifier (e.g., a guest code). The first identifier may be, for example, a sequence of characters (e.g., numbers, letters, and/or symbols) that may be associated with the media playback system. The computing device may obtain the first identifier in any of a variety of ways. For example, the computing device may be on the same network (e.g., the same LAN and/or PAN) as the media playback system. In this example, the computing device may obtain the first identifier from the media playback system at least in part by transmitting a request for the first identifier to the media playback system over the network and receiving a response from the media playback system that comprises the first identifier. In another example, the computing device may obtain the first identifier from the user via one or more input fields the GUI displays on the computing device. In this example, the user may obtain the first identifier (e.g., from an owner of the media playback system or from a display on the media playback system) and type the first identifier into the one or more inputs fields in the GUI. 
     After the computing device has obtained the first identifier, the computing device may obtain, as a second credential, a second identifier (e.g., an authorization code) using the first identifier. The second identifier may be, for example, a sequence of characters (e.g., numbers, letters, and/or symbols) that may be associated with the computing device (and/or the user operating the computing device). The computing device may obtain the second identifier from a computing system comprising one or more computing devices. The computing system be, for example, one or more cloud platforms comprising one or more cloud servers. The computing device may communicate with the computing system over a WAN (e.g., the Internet) in implementations where the computing system is remote (e.g., the computing system comprises one or more cloud servers) relative to the computing device. In implementations where the computing system is not remote from the computing device (e.g., the computing system comprises one or more servers on the same LAN as the computing device), the computing device may communicate with the computing system over the LAN. 
     The computing device may obtain the second identifier from the computing system in any of a variety of ways. For example, the computing device may transmit a request for the second identifier to the computing system that comprises (or otherwise accompanied by) the first identifier. The computing device may receive, in response to the computing system determining the first identifier is authentic, the second identifier from the computing system. In some instances, the second request may comprise additional credentials separate and apart from the first identifier. For example, the user seeking access for their computing device to the media playback system may have an account registered with the computing system. In this example, the second request may additionally comprise user login credentials (e.g., a username, a password, and/or a biometric) associated with the users account. Additionally (or alternatively), the computing device may further transmit a value that is unique to the computing device, such as a client identifier. 
     After the computing device has obtained the second identifier, the computing device may obtain, as a third credential, a token (e.g., an access token) using the second identifier and/or the first identifier. The token may be employed by the computing device to cause the media playback system to perform various operations, such as playing media, pausing media, increasing volume, decreasing volume, adding media to a playlist, removing media from a playlist, reordering media in a playlist, and skipping media in a playlist. The computing device may obtain the token from the computing system (e.g., by communicating over a WAN and/or a LAN). For example, the computing device may transmit a request for the token to the computing system that comprises (or otherwise accompanied by) the second identifier and/or the first identifier. The computing device may receive, in response to the computing system determining that the second identifier and/or the first identifier are valid, the token. 
     After the computing device has obtained the token, the computing device may receive a request for the media playback system to perform an operation. The computing device may receive the request from, for example, a user via a GUI displayed on the computing device. For example, the computing device may detect selection of forward skip icon in the GUI for triggering the media playback system to forward skip media in a playlist. The computing device may, in turn, cause the media playback system to perform the requested operation using the token. For example, the computing device may transmit (e.g., over a LAN and/or a PAN) to the media playback system a request for the media playback system to perform the operation and the token. In turn, the media playback system may perform the operation in response to, for example, the token being valid. 
     While some examples described herein may refer to functions performed by given actors such as “users,” “listeners,” and/or other entities, it should be understood that this is for purposes of explanation only. The claims should not be interpreted to require action by any such example actor unless explicitly required by the language of the claims themselves. 
     In the Figures, identical reference numbers identify generally similar, and/or identical, elements. To facilitate the discussion of any particular element, the most significant digit or digits of a reference number refers to the Figure in which that element is first introduced. For example, element  110   a  is first introduced and discussed with reference to  FIG.  1 A . Many of the details, dimensions, angles and other features shown in the Figures are merely illustrative of particular embodiments of the disclosed technology. Accordingly, other embodiments can have other details, dimensions, angles and features without departing from the spirit or scope of the disclosure. In addition, those of ordinary skill in the art will appreciate that further embodiments of the various disclosed technologies can be practiced without several of the details described below. 
     II. Suitable Operating Environment 
       FIG.  1 A  is a partial cutaway view of a media playback system  100  distributed in an environment  101  (e.g., a house). The media playback system  100  comprises one or more playback devices  110  (identified individually as playback devices 110a-n), one or more network microphone devices  120  (“NMDs”) (identified individually as NMDs 120a-c), and one or more control devices  130  (identified individually as control devices  130   a  and  130   b ). 
     As used herein the term “playback device” can generally refer to a network device configured to receive, process, and output data of a media playback system. For example, a playback device can be a network device that receives and processes audio content. In some embodiments, a playback device includes one or more transducers or speakers powered by one or more amplifiers. In other embodiments, however, a playback device includes one of (or neither of) the speaker and the amplifier. For instance, a playback device can comprise one or more amplifiers configured to drive one or more speakers external to the playback device via a corresponding wire or cable. 
     Moreover, as used herein the term “NMD” (i.e., a “network microphone device”) can generally refer to a network device that is configured for audio detection. In some embodiments, an NMD is a stand-alone device configured primarily for audio detection. In other embodiments, an NMD is incorporated into a playback device (or vice versa). 
     The term “control device” can generally refer to a network device configured to perform functions relevant to facilitating user access, control, and/or configuration of the media playback system  100 . A control device may be implemented as, for example, a computing device having one or more applications installed thereon that enable control of the media playback system (e.g., the ability to cause the media control system to perform one or more operations). Such applications may include a native application made by the manufacturer of the media playback system and/or a third-party application made by a company other than the manufacturer of the media playback system. 
     Each of the playback devices  110  is configured to receive audio signals or data from one or more media sources (e.g., one or more remote servers, one or more local devices) and play back the received audio signals or data as sound. The one or more NMDs  120  are configured to receive spoken word commands, and the one or more control devices  130  are configured to receive user input. In response to the received spoken word commands and/or user input, the media playback system  100  can play back audio via one or more of the playback devices  110 . In certain embodiments, the playback devices  110  are configured to commence playback of media content in response to a trigger. For instance, one or more of the playback devices  110  can be configured to play back a morning playlist upon detection of an associated trigger condition (e.g., presence of a user in a kitchen, detection of a coffee machine operation). In some embodiments, for example, the media playback system  100  is configured to play back audio from a first playback device (e.g., the playback device  100   a ) in synchrony with a second playback device (e.g., the playback device  100   b ). Interactions between the playback devices  110 , NMDs  120 , and/or control devices  130  of the media playback system  100  configured in accordance with the various embodiments of the disclosure are described in greater detail below with respect to  FIGS.  1 B- 1 H . 
     In the illustrated embodiment of  FIG.  1 A , the environment  101  comprises a household having several rooms, spaces, and/or playback zones, including (clockwise from upper left) a master bathroom  101   a , a master bedroom  101   b , a second bedroom  101   c , a family room or den  101   d , an office  101   e , a living room  101   f , a dining room  101   g , a kitchen  101   h , and an outdoor patio  101   i . While certain embodiments and examples are described below in the context of a home environment, the technologies described herein may be implemented in other types of environments. In some embodiments, for example, the media playback system  100  can be implemented in one or more commercial settings (e.g., a restaurant, mall, airport, hotel, a retail or other store), one or more vehicles (e.g., a sports utility vehicle, bus, car, a ship, a boat, an airplane), multiple environments (e.g., a combination of home and vehicle environments), and/or another suitable environment where multi-zone audio may be desirable. 
     The media playback system  100  can comprise one or more playback zones, some of which may correspond to the rooms in the environment  101 . The media playback system  100  can be established with one or more playback zones, after which additional zones may be added, or removed, to form, for example, the configuration shown in  FIG.  1 A . Each zone may be given a name according to a different room or space such as the office  101   e , master bathroom  101   a , master bedroom  101   b , the second bedroom  101   c , kitchen  101   h , dining room  101   g , living room  101   f , and/or the balcony  101   i . In some aspects, a single playback zone may include multiple rooms or spaces. In certain aspects, a single room or space may include multiple playback zones. 
     In the illustrated embodiment of  FIG.  1 A , the master bathroom  101   a , the second bedroom  101   c , the office  101   e , the living room  101   f , the dining room  101   g , the kitchen  101   h , and the outdoor patio  101   i  each include one playback device  110 , and the master bedroom  101   b  and the den  101   d  include a plurality of playback devices  110 . In the master bedroom  101   b , the playback devices  1101  and  110   m  may be configured, for example, to play back audio content in synchrony as individual ones of playback devices  110 , as a bonded playback zone, as a consolidated playback device, and/or any combination thereof. Similarly, in the den  101   d , the playback devices  110   h - j  can be configured, for instance, to play back audio content in synchrony as individual ones of playback devices  110 , as one or more bonded playback devices, and/or as one or more consolidated playback devices. Additional details regarding bonded and consolidated playback devices are described below with respect to  FIGS.  1 B and  1 E . 
     In some aspects, one or more of the playback zones in the environment  101  may each be playing different audio content. For instance, a user may be grilling on the patio  101   i  and listening to hip hop music being played by the playback device  110   c  while another user is preparing food in the kitchen  101   h  and listening to classical music played by the playback device  110   b . In another example, a playback zone may play the same audio content in synchrony with another playback zone. For instance, the user may be in the office  101   e  listening to the playback device  110   f  playing back the same hip hop music being played back by playback device  110   c  on the patio  101   i . In some aspects, the playback devices  110   c  and  110   f  play back the hip hop music in synchrony such that the user perceives that the audio content is being played seamlessly (or at least substantially seamlessly) while moving between different playback zones. Additional details regarding audio playback synchronization among playback devices and/or zones can be found, for example, in U.S. Pat. No. 8,234,395 (hereinafter ‘“395 Patent”) filed Apr. 1, 2004 and titled, “System and method for synchronizing operations among a plurality of independently clocked digital data processing devices,” which is incorporated herein by reference in its entirety. 
     A. Suitable Media Playback System 
       FIG.  1 B  is a schematic diagram of the media playback system  100  and a cloud network  102 . For ease of illustration, certain devices of the media playback system  100  and the cloud network  102  are omitted from  FIG.  1 B . One or more communication links  103  (referred to hereinafter as “the links  103 ”) communicatively couple the media playback system  100  and the cloud network  102 . 
     The links  103  can comprise, for example, one or more wired networks, one or more wireless networks, one or more wide area networks (WAN) (e.g., the Internet), one or more local area networks (LAN) (e.g., one or more WiFi networks), one or more personal area networks (PAN) (e.g., one or more BLUETOOTH networks, Z-WAVE networks, wireless Universal Serial Bus (USB) networks, ZIGBEE networks, and/or IRDA networks), one or more telecommunication networks (e.g., one or more Global System for Mobiles (GSM) networks, Code Division Multiple Access (CDMA) networks, Long-Term Evolution (LTE) networks, 5G communication network networks, and/or other suitable data transmission protocol networks), etc. The cloud network  102   is configured to deliver media content (e.g., audio content, video content, photographs, social media content) to the media playback system  100  in response to a request transmitted from the media playback system  100  via the links  103 . In some embodiments, the cloud network  102  is further configured to receive data (e.g., voice input data) from the media playback system  100  and correspondingly transmit commands and/or media content to the media playback system  100 . 
     The cloud network  102  comprises computing devices  106  (identified separately as a first computing device  106   a , a second computing device  106   b , and a third computing device  106   c ). The computing devices  106  can comprise individual computers or servers, such as, for example, a media streaming service server storing audio and/or other media content, a voice service server, a social media server, a media playback system control server, etc. In some embodiments, one or more of the computing devices  106  comprise modules of a single computer or server. In certain embodiments, one or more of the computing devices  106  comprise one or more modules, computers, and/or servers. Moreover, while the cloud network  102  is described above in the context of a single cloud network, in some embodiments the cloud network  102  comprises a plurality of cloud networks comprising communicatively coupled computing devices. Furthermore, while the cloud network  102  is shown in  FIG.  1 B  as having three of the computing devices  106 , in some embodiments, the cloud network  102  comprises fewer (or more than) three computing devices  106 . 
     The media playback system  100  is configured to receive media content from the networks  102  via the links  103 . The received media content can comprise, for example, a Uniform Resource Identifier (URI) and/or a Uniform Resource Locator (URL). For instance, in some examples, the media playback system  100  can stream, download, or otherwise obtain data from a URI or a URL corresponding to the received media content. A network  104  communicatively couples the links  103  and at least a portion of the devices (e.g., one or more of the playback devices  110 , NMDs  120 , and/or control devices  130 ) of the media playback system  100 . The network  104  can include, for example, a wireless network (e.g., a WiFi network, a BLUETOOTH network, a Z-WAVE network, a ZIGBEE network, and/or other suitable wireless communication protocol network) and/or a wired network (e.g., a network comprising Ethernet, USB, and/or another suitable wired communication). As those of ordinary skill in the art will appreciate, as used herein, “WiFi” can refer to several different communication protocols including, for example, Institute of Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.11ac, 802.11ad, 802.11af, 802.11ah, 802.11ai, 802.11aj, 802.11aq, 802.11ax, 802.11ay, 802.15, etc. transmitted at 2.4 Gigahertz (GHz), 5 GHz, and/or another suitable frequency. 
     In some embodiments, the network  104  comprises a dedicated communication network that the media playback system  100  uses to transmit messages between individual devices and/or to transmit media content to and from media content sources (e.g., one or more of the computing devices  106 ). In certain embodiments, the network  104  is configured to be accessible only to devices in the media playback system  100 , thereby reducing interference and competition with other household devices. In other embodiments, however, the network  104  comprises an existing household communication network (e.g., a household WiFi network). In some embodiments, the links  103  and the network  104  comprise one or more of the same networks. In some aspects, for example, the links  103  and the network  104  comprise a telecommunication network (e.g., an LTE network, a 5G network). Moreover, in some embodiments, the media playback system  100  is implemented without the network  104 , and devices comprising the media playback system  100  can communicate with each other, for example, via one or more direct or indirect connections, PANs, LANs, telecommunication networks, and/or other suitable communication links. The network  104  may be referred to herein as a “local communication network” to differentiate the network  104  from the cloud network  102  that couples the media playback system  100  to remote devices, such as cloud services. 
     In some embodiments, audio content sources may be regularly added or removed from the media playback system  100 . In some embodiments, for example, the media playback system  100  performs an indexing of media items when one or more media content sources are updated, added to, and/or removed from the media playback system  100 . The media playback system  100  can scan identifiable media items in some or all folders and/or directories accessible to the playback devices  110 , and generate or update a media content database comprising metadata (e.g., title, artist, album, track length) and other associated information (e.g., URIs, URLs) for each identifiable media item found. In some embodiments, for example, the media content database is stored on one or more of the playback devices  110 , network microphone devices  120 , and/or control devices  130 . 
     In the illustrated embodiment of  FIG.  1 B , the playback devices  1101  and  110   m  comprise a group  107   a . The playback devices  1101  and  110   m  can be positioned in different rooms in a household and be grouped together in the group  107   a  on a temporary or permanent basis based on user input received at the control device  130   a  and/or another control device  130  in the media playback system  100 . When arranged in the group  107   a , the playback devices  1101  and  110   m  can be configured to play back the same or similar audio content in synchrony from one or more audio content sources. In certain embodiments, for example, the group  107   a  comprises a bonded zone in which the playback devices  1101  and  110   m  comprise left audio and right audio channels, respectively, of multi-channel audio content, thereby producing or enhancing a stereo effect of the audio content. In some embodiments, the group  107   a  includes additional playback devices  110 . In other embodiments, however, the media playback system  100  omits the group  107   a  and/or other grouped arrangements of the playback devices  110 . 
     The media playback system  100  includes the NMDs  120   a  and  120   d , each comprising one or more microphones configured to receive voice utterances from a user. In the illustrated embodiment of  FIG.  1 B , the NMD  120   a  is a standalone device and the NMD  120   d  is integrated into the playback device  110   n . The NMD  120   a , for example, is configured to receive voice input  121  from a user  123 . In some embodiments, the NMD  120   a  transmits data associated with the received voice input  121  to a voice assistant service (VAS) configured to (i) process the received voice input data and (ii) facilitate one or more operations on behalf of the media playback system  100 . 
     In some aspects, for example, the computing device  106   c  comprises one or more modules and/or servers of a VAS (e.g., a VAS operated by one or more of SONOS, AMAZON, GOOGLE, APPLE, MICROSOFT, NUANCE). The computing device  106   c  can receive the voice input data from the NMD  120   a  via the network  104  and the links  103 . 
     In response to receiving the voice input data, the computing device  106   c  processes the voice input data (i.e., “Play Hey Jude by The Beatles”), and determines that the processed voice input includes a command to play a song (e.g., “Hey Jude”). In some embodiments, after processing the voice input, the computing device  106   c  accordingly transmits commands to the media playback system  100  to play back “Hey Jude” by the Beatles from a suitable media service (e.g., via one or more of the computing devices  106 ) on one or more of the playback devices  110 . In other embodiments, the computing device  106   c  may be configured to interface with media services on behalf of the media playback system  100 . In such embodiments, after processing the voice input, instead of the computing device  106   c  transmitting commands to the media playback system  100  causing the media playback system  100  to retrieve the requested media from a suitable media service, the computing device  106   c  itself causes a suitable media service to provide the requested media to the media playback system  100  in accordance with the user’s voice utterance. 
     B. Suitable Playback Devices 
       FIG.  1 C  is a block diagram of the playback device  110   a  comprising an input/output  111 . The input/output  111  can include an analog I/O  111   a  (e.g., one or more wires, cables, and/or other suitable communication links configured to carry analog signals) and/or a digital I/O  111   b  (e.g., one or more wires, cables, or other suitable communication links configured to carry digital signals). In some embodiments, the analog I/O  111   a  is an audio line-in input connection comprising, for example, an auto-detecting 3.5 mm audio line-in connection. In some embodiments, the digital I/O  111   b  comprises a Sony/Philips Digital Interface Format (S/PDIF) communication interface and/or cable and/or a Toshiba Link (TOSLINK) cable. In some embodiments, the digital I/O  111   b  comprises an High-Definition Multimedia Interface (HDMI) interface and/or cable. In some embodiments, the digital I/O  111   b  includes one or more wireless communication links comprising, for example, a radio frequency (RF), infrared, WiFi, BLUETOOTH, or another suitable communication protocol. In certain embodiments, the analog I/O  111   a  and the digital  111   b  comprise interfaces (e.g., ports, plugs, jacks) configured to receive connectors of cables transmitting analog and digital signals, respectively, without necessarily including cables. 
     The playback device  110   a , for example, can receive media content (e.g., audio content comprising music and/or other sounds) from a local audio source  105  via the input/output  111  (e.g., a cable, a wire, a wired and/or wireless communication network such as a PAN and/or a LAN, and/or another suitable communication link). The local audio source  105  can comprise, for example, a mobile device (e.g., a smartphone, a tablet, a laptop computer) or another suitable audio component (e.g., a television, a desktop computer, an amplifier, a phonograph, a Blu-ray player, a memory storing digital media files). In some aspects, the local audio source  105  includes local music libraries on a smartphone, a computer, a networked-attached storage (NAS), and/or another suitable device configured to store media files. In certain embodiments, one or more of the playback devices  110 , NMDs  120 , and/or control devices  130  comprise the local audio source  105 . In other embodiments, however, the media playback system omits the local audio source  105  altogether. In some embodiments, the playback device  110   a  does not include an input/output  111  and receives all audio content via the network  104 . 
     The playback device  110   a  further comprises electronics  112 , a user interface  113  (e.g., one or more buttons, knobs, dials, touch-sensitive surfaces, displays, touchscreens), and one or more transducers  114  (referred to hereinafter as “the transducers  114 ”). The electronics  112  are configured to receive audio from an audio source (e.g., the local audio source  105 ) via the input/output  111  or one or more of the computing devices 106a-c via the network  104  ( FIG.  1 B ), amplify the received audio, and output the amplified audio for playback via one or more of the transducers  114 . In some embodiments, the playback device  110   a  optionally includes one or more microphones  115  (e.g., a single microphone, a plurality of microphones, a microphone array) (hereinafter referred to as “the microphones  115 ”). In certain embodiments, for example, the playback device  110   a  having one or more of the optional microphones  115  can operate as an NMD configured to receive voice input from a user and correspondingly perform one or more operations based on the received voice input. 
     In the illustrated embodiment of  FIG.  1 C , the electronics  112  comprise one or more processors  112   a  (referred to hereinafter as “the processors  112   a ”), memory  112   b , software components  112   c , a network interface  112   d , one or more audio processing components  112   g  (referred to hereinafter as “the audio components  112   g ”), one or more audio amplifiers  112   h  (referred to hereinafter as “the amplifiers  112   h ”), and power  112   i  (e.g., one or more power supplies, power cables, power receptacles, batteries, induction coils, Power-over Ethernet (POE) interfaces, and/or other suitable sources of electric power). In some embodiments, the electronics  112  optionally include one or more other components  112   j  (e.g., one or more sensors, video displays, touchscreens, battery charging bases). 
     The processors  112   a  can comprise clock-driven computing component(s) configured to process data, and the memory  112   b  can comprise a computer-readable medium (e.g., a tangible, non-transitory computer-readable medium loaded with one or more of the software components  112   c ) configured to store instructions for performing various operations and/or functions. The processors  112   a  are configured to execute the instructions stored on the memory  112   b  to perform one or more of the operations. The operations can include, for example, causing the playback device  110   a  to retrieve audio data from an audio source (e.g., one or more of the computing devices 106a-c ( FIG.  1 B )), and/or another one of the playback devices  110 . In some embodiments, the operations further include causing the playback device  110   a  to send audio data to another one of the playback devices  110   a  and/or another device (e.g., one of the NMDs  120 ). Certain embodiments include operations causing the playback device  110   a  to pair with another of the one or more playback devices  110  to enable a multi-channel audio environment (e.g., a stereo pair, a bonded zone). 
     The processors  112   a  can be further configured to perform operations causing the playback device  110   a  to synchronize playback of audio content with another of the one or more playback devices  110 . As those of ordinary skill in the art will appreciate, during synchronous playback of audio content on a plurality of playback devices, a listener will preferably be unable to perceive time-delay differences between playback of the audio content by the playback device  110   a  and the other one or more other playback devices  110 . Additional details regarding audio playback synchronization among playback devices can be found, for example, in the ‘395 Patent incorporated by reference above. 
     In some embodiments, the memory  112   b  is further configured to store data associated with the playback device  110   a , such as one or more zones and/or zone groups of which the playback device  110   a  is a member, audio sources accessible to the playback device  110   a , and/or a playback queue that the playback device  110   a  (and/or another of the one or more playback devices) can be associated with. The stored data can comprise one or more state variables that are periodically updated and used to describe a state of the playback device  110   a . The memory  112   b  can also include data associated with a state of one or more of the other devices (e.g., the playback devices  110 , NMDs  120 , control devices  130 ) of the media playback system  100 . In some aspects, for example, the state data is shared during predetermined intervals of time (e.g., every 5 seconds, every 10 seconds, every 60 seconds) among at least a portion of the devices of the media playback system  100 , so that one or more of the devices have the most recent data associated with the media playback system  100 . 
     The network interface  112   d  is configured to facilitate a transmission of data between the playback device  110   a  and one or more other devices on a data network such as, for example, the links  103  and/or the network  104  ( FIG.  1 B ). The network interface  112   d  is configured to transmit and receive data corresponding to media content (e.g., audio content, video content, text, photographs) and other signals (e.g., non-transitory signals) comprising digital packet data including an Internet Protocol (IP)-based source address and/or an IP-based destination address. The network interface  112   d  can parse the digital packet data such that the electronics  112  properly receives and processes the data destined for the playback device  110   a . 
     In the illustrated embodiment of  FIG.  1 C , the network interface  112   d  comprises one or more wireless interfaces  112   e  (referred to hereinafter as “the wireless interface  112   e ”). The wireless interface  112   e  (e.g., a suitable interface comprising one or more antennae) can be configured to wirelessly communicate with one or more other devices (e.g., one or more of the other playback devices  110 , NMDs  120 , and/or control devices  130 ) that are communicatively coupled to the network  104  ( FIG.  1 B ) in accordance with a suitable wireless communication protocol (e.g., WiFi, BLUETOOTH, LTE). In some embodiments, the network interface  112   d  optionally includes a wired interface  112   f  (e.g., an interface or receptacle configured to receive a network cable such as an Ethernet, a USB-A, USB-C, and/or Thunderbolt cable) configured to communicate over a wired connection with other devices in accordance with a suitable wired communication protocol. In certain embodiments, the network interface  112   d  includes the wired interface  112   f  and excludes the wireless interface  112   e . In some embodiments, the electronics  112  excludes the network interface  112   d  altogether and transmits and receives media content and/or other data via another communication path (e.g., the input/output  111 ). 
     The audio components  112   g  are configured to process and/or filter data comprising media content received by the electronics  112  (e.g., via the input/output  111  and/or the network interface  112   d ) to produce output audio signals. In some embodiments, the audio processing components  112   g  comprise, for example, one or more digital-to-analog converters (DAC), audio preprocessing components, audio enhancement components, digital signal processors (DSPs), and/or other suitable audio processing components, modules, circuits, etc. In certain embodiments, one or more of the audio processing components  112   g  can comprise one or more subcomponents of the processors  112   a . In some embodiments, the electronics  112  omits the audio processing components  112   g . In some aspects, for example, the processors  112   a  execute instructions stored on the memory  112   b  to perform audio processing operations to produce the output audio signals. 
     The amplifiers  112   h  are configured to receive and amplify the audio output signals produced by the audio processing components  112   g  and/or the processors  112   a . The amplifiers  112   h  can comprise electronic devices and/or components configured to amplify audio signals to levels sufficient for driving one or more of the transducers  114 . In some embodiments, for example, the amplifiers  112   h  include one or more switching or class-D power amplifiers. In other embodiments, however, the amplifiers include one or more other types of power amplifiers (e.g., linear gain power amplifiers, class-A amplifiers, class-B amplifiers, class-AB amplifiers, class-C amplifiers, class-D amplifiers, class-E amplifiers, class-F amplifiers, class-G and/or class H amplifiers, and/or another suitable type of power amplifier). In certain embodiments, the amplifiers  112   h  comprise a suitable combination of two or more of the foregoing types of power amplifiers. Moreover, in some embodiments, individual ones of the amplifiers  112   h  correspond to individual ones of the transducers  114 . In other embodiments, however, the electronics  112  includes a single one of the amplifiers  112   h  configured to output amplified audio signals to a plurality of the transducers  114 . In some other embodiments, the electronics  112  omits the amplifiers  112   h . 
     The transducers  114  (e.g., one or more speakers and/or speaker drivers) receive the amplified audio signals from the amplifier  112   h  and render or output the amplified audio signals as sound (e.g., audible sound waves having a frequency between about 20 Hertz (Hz) and 20 kilohertz (kHz)). In some embodiments, the transducers  114  can comprise a single transducer. In other embodiments, however, the transducers  114  comprise a plurality of audio transducers. In some embodiments, the transducers  114  comprise more than one type of transducer. For example, the transducers  114  can include one or more low frequency transducers (e.g., subwoofers, woofers), mid-range frequency transducers (e.g., mid-range transducers, mid-woofers), and one or more high frequency transducers (e.g., one or more tweeters). As used herein, “low frequency” can generally refer to audible frequencies below about 500 Hz, “mid-range frequency” can generally refer to audible frequencies between about 500 Hz and about 2 kHz, and “high frequency” can generally refer to audible frequencies above 2 kHz. In certain embodiments, however, one or more of the transducers  114  comprise transducers that do not adhere to the foregoing frequency ranges. For example, one of the transducers  114  may comprise a mid-woofer transducer configured to output sound at frequencies between about 200 Hz and about 5 kHz. 
     By way of illustration, SONOS, Inc. presently offers (or has offered) for sale certain playback devices including, for example, a “SONOS ONE,” “PLAY:1,” “PLAY:3,” “PLAY:5,” “PLAYBAR,” “PLAYBASE,” “BEAM,” “CONNECT:AMP,” “CONNECT,” “AMP,” and “SUB.” Other suitable playback devices may additionally or alternatively be used to implement the playback devices of example embodiments disclosed herein. Additionally, one of ordinary skilled in the art will appreciate that a playback device is not limited to the examples described herein or to SONOS product offerings. For example, one or more playback devices  110  comprises wired and/or wireless headphones (e.g., over-the-ear headphones, on-ear headphones, in-ear earphones). In some embodiments, the headphone may comprise a headband coupled to one or more earcups. For example, a first earcup may be coupled to a first end of the headband and a second earcup may be coupled to a second end of the headband that is opposite the first end. Each of the one or more earcups may house any portion of the electronic components in the playback device, such as one or more transducers. Further, the one or more of earcups may include a user interface for controlling operation of the headphone such as for controlling audio playback, volume level, and other functions. The user interface may include any of a variety of control elements such as buttons, knobs, dials, touch-sensitive surfaces, and/or touchscreens. An ear cushion may be coupled each of the one or more earcups. The ear cushions may provide a soft barrier between the head of a user and the one or more earcups to improve user comfort and/or provide acoustic isolation from the ambient (e.g., provide passive noise reduction (PNR)). Additionally (or alternatively), the headphone may employ active noise reduction (ANR) techniques to further reduce the user’s perception of outside noise during playback. 
     In some embodiments, one or more of the playback devices  110  comprise a docking station and/or an interface configured to interact with a docking station for personal mobile media playback devices. In certain embodiments, a playback device may be integral to another device or component such as a television, a lighting fixture, or some other device for indoor or outdoor use. In some embodiments, a playback device omits a user interface and/or one or more transducers. For example,  FIG.  1 D  is a block diagram of a playback device  110   p  comprising the input/output  111  and electronics  112  without the user interface  113  or transducers  114 . 
       FIG.  1 E  is a block diagram of a bonded playback device  110   q  comprising the playback device  110   a  ( FIG.  1 C ) sonically bonded with the playback device  110   i  (e.g., a subwoofer) ( FIG.  1 A ). In the illustrated embodiment, the playback devices  110   a  and  110   i  are separate ones of the playback devices  110  housed in separate enclosures. In some embodiments, however, the bonded playback device  110   q  comprises a single enclosure housing both the playback devices  110   a  and  110   i . The bonded playback device  110   q  can be configured to process and reproduce sound differently than an unbonded playback device (e.g., the playback device  110   a  of  FIG.  1 C ) and/or paired or bonded playback devices (e.g., the playback devices  1101  and 110 m of  FIG.  1 B ). In some embodiments, for example, the playback device  110   a  is full-range playback device configured to render low frequency, mid-range frequency, and high frequency audio content, and the playback device  1101  is a subwoofer configured to render low frequency audio content. In some aspects, the playback device  110   a , when bonded with the first playback device, is configured to render only the mid-range and high frequency components of a particular audio content, while the playback device  110   i  renders the low frequency component of the particular audio content. In some embodiments, the bonded playback device  110   q  includes additional playback devices and/or another bonded playback device. 
     C. Suitable Network Microphone Devices (NMDs) 
       FIG.  1 F  is a block diagram of the NMD  120   a  ( FIGS.  1 A and  1 B ). The NMD  120   a  includes one or more voice processing components  124  (hereinafter “the voice components 124”) and several components described with respect to the playback device  110   a  ( FIG.  1 C ) including the processors  112   a , the memory  112   b , and the microphones  115 . The NMD  120   a  optionally comprises other components also included in the playback device  110   a  ( FIG.  1 C ), such as the user interface  113  and/or the transducers  114 . In some embodiments, the NMD  120   a  is configured as a media playback device (e.g., one or more of the playback devices  110 ), and further includes, for example, one or more of the audio components 112 g ( FIG.  1 C ), the amplifiers  114 , and/or other playback device components. In certain embodiments, the NMD  120   a  comprises an Internet of Things (IoT) device such as, for example, a thermostat, alarm panel, fire and/or smoke detector, etc. In some embodiments, the NMD  120   a  comprises the microphones  115 , the voice processing  124 , and only a portion of the components of the electronics  112  described above with respect to  FIG.  1 B . In some aspects, for example, the NMD  120   a  includes the processor  112   a  and the memory  112   b  ( FIG.  1 B ), while omitting one or more other components of the electronics  112 . In some embodiments, the NMD  120   a  includes additional components (e.g., one or more sensors, cameras, thermometers, barometers, hygrometers). 
     In some embodiments, an NMD can be integrated into a playback device.  FIG.  1 G  is a block diagram of a playback device  110   r  comprising an NMD  120   d . The playback device  110   r  can comprise many or all of the components of the playback device  110   a  and further include the microphones  115  and voice processing  124  ( FIG.  1 F ). The playback device  110   r  optionally includes an integrated control device  130   c . The control device  130   c  can comprise, for example, a user interface (e.g., the user interface  113  of  FIG.  1 B ) configured to receive user input (e.g., touch input, voice input) without a separate control device. In other embodiments, however, the playback device  110   r  receives commands from another control device (e.g., the control device  130   a  of  FIG.  1 B ). 
     Referring again to  FIG.  1 F , the microphones  115  are configured to acquire, capture, and/or receive sound from an environment (e.g., the environment  101  of  FIG.  1 A ) and/or a room in which the NMD  120   a  is positioned. The received sound can include, for example, vocal utterances, audio played back by the NMD  120   a  and/or another playback device, background voices, ambient sounds, etc. The microphones  115  convert the received sound into electrical signals to produce microphone data. The voice processing  124  receives and analyzes the microphone data to determine whether a voice input is present in the microphone data. The voice input can comprise, for example, an activation word followed by an utterance including a user request. As those of ordinary skill in the art will appreciate, an activation word is a word or other audio cue signifying a user voice input. For instance, in querying the AMAZON VAS, a user might speak the activation word “Alexa.” Other examples include “Ok, Google” for invoking the GOOGLE VAS and “Hey, Siri” for invoking the APPLE VAS. 
     After detecting the activation word, voice processing  124  monitors the microphone data for an accompanying user request in the voice input. The user request may include, for example, a command to control a third-party device, such as a thermostat (e.g., NEST thermostat), an illumination device (e.g., a PHILIPS HUE lighting device), or a media playback device (e.g., a SONOS playback device). For example, a user might speak the activation word “Alexa” followed by the utterance “set the thermostat to 68 degrees” to set a temperature in a home (e.g., the environment  101  of  FIG.  1 A ). The user might speak the same activation word followed by the utterance “turn on the living room” to turn on illumination devices in a living room area of the home. The user may similarly speak an activation word followed by a request to play a particular song, an album, or a playlist of music on a playback device in the home. 
     D. Suitable Control Devices 
       FIG.  1 H  is a partial schematic diagram of the control device  130   a  ( FIGS.  1 A and  1 B ). As used herein, the term “control device” can be used interchangeably with “controller” or “control system.” Among other features, the control device  130   a  is configured to receive user input related to the media playback system  100  and, in response, cause one or more devices in the media playback system  100  to perform an action(s) or operation(s) corresponding to the user input. In the illustrated embodiment, the control device  130   a  comprises a smartphone (e.g., an IPHONE, an ANDROID phone) on which media playback system controller application software is installed. In some embodiments, the control device  130   a  comprises, for example, a tablet (e.g., an IPAD), a computer (e.g., a laptop computer, a desktop computer), and/or another suitable device (e.g., a television, an automobile audio head unit, an IoT device). In certain embodiments, the control device  130   a  comprises a dedicated controller for the media playback system  100 . In other embodiments, as described above with respect to  FIG.  1 G , the control device  130   a  is integrated into another device in the media playback system  100  (e.g., one more of the playback devices  110 , NMDs  120 , and/or other suitable devices configured to communicate over a network). 
     The control device  130   a  includes electronics  132 , a user interface  133 , one or more speakers  134 , and one or more microphones  135 . The electronics  132  comprise one or more processors  132   a  (referred to hereinafter as “the processors  132   a ”), a memory  132   b , software components  132   c , and a network interface  132   d . The processor  132   a  can be configured to perform functions relevant to facilitating user access, control, and configuration of the media playback system  100 . The memory  132   b  can comprise data storage that can be loaded with one or more of the software components executable by the processor  302  to perform those functions. The software components  132   c  can comprise applications and/or other executable software configured to facilitate control of the media playback system  100 . The memory  112   b  can be configured to store, for example, the software components  132   c , media playback system controller application software, and/or other data associated with the media playback system  100  and the user. 
     The network interface  132   d  is configured to facilitate network communications between the control device  130   a  and one or more other devices in the media playback system  100 , and/or one or more remote devices. In some embodiments, the network interface  132   d  is configured to operate according to one or more suitable communication industry standards (e.g., infrared, radio, wired standards including IEEE 802.3, wireless standards including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G, LTE). The network interface  132   d  can be configured, for example, to transmit data to and/or receive data from the playback devices  110 , the NMDs  120 , other ones of the control devices  130 , one of the computing devices  106  of  FIG.  1 B , devices comprising one or more other media playback systems, etc. The transmitted and/or received data can include, for example, playback device control commands, state variables, playback zone and/or zone group configurations. For instance, based on user input received at the user interface  133 , the network interface  132   d  can transmit a playback device control command (e.g., volume control, audio playback control, audio content selection) from the control device  304  to one or more of the playback devices  100 . The network interface  132   d  can also transmit and/or receive configuration changes such as, for example, adding/removing one or more playback devices  100  to/from a zone, adding/removing one or more zones to/from a zone group, forming a bonded or consolidated player, separating one or more playback devices from a bonded or consolidated player, among others. 
     The user interface  133  is configured to receive user input and can facilitate control of the media playback system  100 . The user interface  133  includes media content art  133   a  (e.g., album art, lyrics, videos), a playback status indicator  133   b  (e.g., an elapsed and/or remaining time indicator), media content information region  133   c , a playback control region  133   d , and a zone indicator  133   e . The media content information region  133   c  can include a display of relevant information (e.g., title, artist, album, genre, release year) about media content currently playing and/or media content in a queue or playlist. The playback control region  133   d  can include selectable (e.g., via touch input and/or via a cursor or another suitable selector) icons to cause one or more playback devices in a selected playback zone or zone group to perform playback actions such as, for example, play or pause, fast forward, rewind, skip to next, skip to previous, enter/exit shuffle mode, enter/exit repeat mode, enter/exit cross fade mode, etc. The playback control region  133   d  may also include selectable icons to modify equalization settings, playback volume, and/or other suitable playback actions. In the illustrated embodiment, the user interface  133  comprises a display presented on a touch screen interface of a smartphone (e.g., an IPHONE, an ANDROID phone). In some embodiments, however, user interfaces of varying formats, styles, and interactive sequences may alternatively be implemented on one or more network devices to provide comparable control access to a media playback system. 
     The one or more speakers  134  (e.g., one or more transducers) can be configured to output sound to the user of the control device  130   a . In some embodiments, the one or more speakers comprise individual transducers configured to correspondingly output low frequencies, mid-range frequencies, and/or high frequencies. In some aspects, for example, the control device  130   a  is configured as a playback device (e.g., one of the playback devices  110 ). Similarly, in some embodiments the control device  130   a  is configured as an NMD (e.g., one of the NMDs  120 ), receiving voice commands and other sounds via the one or more microphones  135 . 
     The one or more microphones  135  can comprise, for example, one or more condenser microphones, electret condenser microphones, dynamic microphones, and/or other suitable types of microphones or transducers. In some embodiments, two or more of the microphones  135  are arranged to capture location information of an audio source (e.g., voice, audible sound) and/or configured to facilitate filtering of background noise. Moreover, in certain embodiments, the control device  130   a  is configured to operate as a playback device and an NMD. In other embodiments, however, the control device  130   a  omits the one or more speakers  134  and/or the one or more microphones  135 . For instance, the control device  130   a  may comprise a device (e.g., a thermostat, an IoT device, a network device) comprising a portion of the electronics  132  and the user interface  133  (e.g., a touch screen) without any speakers or microphones. 
     III. Example Access Control Techniques for Media Playback Systems 
     As discussed above, a computing device (e.g., a control device) may obtain multiple distinct identifiers from different sources in order to obtain access to a media playback system. For example, the computing device may need to obtain a first identifier from the owner of the media playback system and/or the media playback system itself and obtain a second identifier from a computer system (e.g., a cloud platform of one or more cloud servers) where the media playback system has been registered. An example implementation of these access control techniques is shown in  FIG.  2    by the control device  130   a  obtaining access to a media playback system comprising playback device  110   a  by communicating with a computing system  201  comprising computing devices  106 ( a ),  106 ( b ), and/or  106 ( c ) and the playback device  110   a . 
     The control device  130   a  shown in  FIG.  2    may be associated with a user (e.g., an owner user or a guest user) that would like to control the media playback system that comprises the playback device  110   a . The control device  130   a  may receive a request from the user to obtain access to the media playback system via, for example, a GUI displayed on the control device  130   a . The GUI may be displayed in response to the control device  130   a  executing one or more applications, such as one or more native applications designed by the manufacturer of the media playback system and/or one or more third-party applications designed by another party. For example, the control device  130   a  may detect selection of an element in the GUI (e.g., a connect icon in a third-party application such as the SPOTIFY application), for controlling media playback on the media playback system. In another example, the control device  130   a  may detect activation of a native application for controlling the media playback system (e.g., opening the SONOS application). 
     After the control device  130   a  has received the request to obtain access to the media playback system, the control device  130   a  may begin the process of acquiring the appropriate credentials required to obtain access to the media playback system. The set of credentials required to obtain access to the media playback system may comprise a first identifier  202  that is obtained from a first source, a second identifier  204  that is obtained from a second source that is different from the first source, and a token  206  that is obtained using the first identifier  202  and/or the second identifier  204 . Any of the first identifier  202 , the second identifier  204 , and/or the token  206  may comprise a sequence of characters such as numbers, letters, and/or symbols. The token  206  may be employed by the control device  130   a  to cause the media playback system to perform various operations. Permissions  208  may be associated with the token that restrict which operations the control device  130   a  is permitted to cause the media playback system to perform. For example, the permissions  208  may indicate that the holder of a particular token is permitted to cause the playback device  110   a  to forward skip media in a playback queue but not to remove media from the playback queue. The control device  130   a  may obtain the credentials (e.g., the first identifier  202 , the second identifier  204 , and the token  206 ) in any of a variety of ways. 
     In some embodiments, the control device  130   a  may obtain the first identifier  202  from the playback device  110   a . In these embodiments, the control device  130   a  may detect the playback device  110   a  is connected to the same network as the control device  130   a , such as a LAN and/or a PAN, and send a request to the playback device  110   a  for the first identifier  202 . The playback device  110   a  may, in turn, generate the first identifier  202  and respond to the control device  130   a  in one or more messages  210   a . The playback device  110   a  may generate the first identifier  202  by itself or with the assistance of the computing system  201 . For example, the playback device  110   a  may store a unique identifier and generate the first identifier based on the unique identifier stored locally. In another example, the playback device  110   a  may transmit a request for the first identifier to the computing system  201  that causes, in turn, the computing system  201  to generate the first identifier and respond to the playback device  110   a  with the first identifier. 
     In other embodiments, the control device  130   a  may obtain the first identifier  202  from another source separate from the playback device 1 10a. For example, the user of the control device  130   a  may be a guest user and the control device  130   a  may display a graphical user interface screen that requests the first identifier to be typed into an input field by the guest user. In this example, the guest user may obtain the first identifier  202  from an owner of the media playback system. 
     After the control device  130  has obtained the first identifier  202 , the control device  130  may employ the first identifier  202  to obtain the second identifier  204  from the computing system  201 . In some embodiments, the control device  130   a  may transmit a request for the second identifier  204  and the first identifier  202  to the computing system  201  via one or more messages  210   b . The control device  130   a  may transmit the one or more messages  210   b  to the computing system  201  over, for example, a LAN and/or a WAN (e.g., the Internet). The computing system  201  may, in response to receiving the request for the second identifier  204  from the control device  130   a , the computing system  201  may validate the authenticity of the first identifier  202 . For example, the computing system  201  may store a list of known valid first identifiers and compare the first identifier  202  received from the control device  130   a  with the list of known valid first identifiers. If the computing system  201  determines that the first identifier  202  received from the control device  130   a  is valid, the computing system may generate the second identifier  204  and transmit the second identifier  204  to the control device  130   a  in one or more messages  210   c  over the LAN and/or WAN. 
     After the control device  130   a  has obtained the second identifier  204 , the control device  130   a  may obtain the token  206  from the computing system  201  using the second identifier  204  and/or the first identifier  204 . In some embodiments, the control device  130   a  may transmit a request for the token  206  and the second identifier  204  to the computing system  201  via one or more messages  210   d . Additionally (or alternatively), the one or more messages  210   d  may comprise the first identifier  202 . The control device  130   a  may transmit the one or more messages  210   d  to the computing system  201  over, for example, a LAN and/or a WAN (e.g., the Internet). The computing system  201  may, in response to receiving the request for the token  206  from the control device  130   a , validate the authenticity of the second identifier  204  (and/or the first identifier  202 ). For example, the computing system  201  may store a list of known valid identifiers and compare the identifier(s) received from the control device  130   a  with the list of known valid identifiers. If the computing system  201  determines that the identifier(s) received from the control device  130   a  are valid, the computing system may generate the token  206  and transmit the token  206  to the control device  130   a  in one or more messages  210   e . 
     After the control device  130   a  has obtained the token  206 , the control device  130   a  may employ the token  206  to cause the playback device  110   a  to perform one or more operations. In some embodiments, the control device  130   a  may cause the playback device  110   a  to perform one or more operations by transmitting a request to perform an operation and the token  206  in one or more messages  210   f . The one or more messages  210   f  may be, for example, transmitted over a network that both the control device  130   a  and the playback device  110  are connected to, such as a LAN or a PAN. 
     The playback device  110   a  may, in response to receipt of the request to perform an operation from the control device  130   a , validate the authenticity of the token  206 . The playback device  110   a  may validate the authenticity of the token alone and/or with the assistance of the computing system  201 . For example, the playback device  110   a  may locally store a list of valid tokens and may determine whether the token  206  received from the control device  130   a  matches a token in the list of valid tokens. If the playback device  110   a  determines that the token  206  does not match a token in the list of valid tokens, the playback device  110   a  may transmit a validation request and the token  206  to the computing system  201 . The computing system  201  may, in response to receiving the validation request, determine whether the token  206  is valid and transmit a response to the playback device  110   a  indicating whether the token  206  is valid. 
     Once the playback device  110   a  has determined that the token  206  is valid, the playback device  110   a  may determine whether the operation that the control device  130   a  requested is within the scope of permitted operations associated with the token  206 . The playback device  110   a  may determine whether the operation is within the scope of permitted operations at least in part by obtaining the permissions  208  associated with the token  206  from the computing system  201 . For example, the playback device  110   a  may transmit a request for the permissions  208  that comprises (or is accompanied by) the token  206  in one or more messages 210 g. The computing system  201  may, in response to receiving the request, identify the permissions  208  associated with the token  206  and transmit the permissions  208  in one or more messages  210   h  to the playback device  110   a . In turn, the playback device  110   a  may determine whether the request operation from the control device  130   a  matches one or more of the permitted operations in the permissions  208 . If the requested operation from the control device  130   a  matches the one or more permitted operations in the permissions  208 , the playback device  110   a  may perform the operation. Otherwise, the playback device  110   a  may reject the request from the control device  130   a  and not perform the operation. 
       FIG.  3 A  shows an example process 300A that may be performed by a computing device, such as the control device  130   a , to control operation of a media playback system comprising, for example, the playback device  110   a . As shown, the process 300A includes an act  302  of receiving a request for access to the media playback system, an act  303  of obtaining access to the media playback system, an act  310  of receiving a request for the media playback system to perform an operation, and an act  312  of causing the media playback system to perform the operation using the token. The act  303  of obtaining access to the media playback system may comprise, for example, an act  304  of obtaining a first identifier associated with the media playback system, an act  306  of obtaining a second identifier using the first identifier, and an act  308  of obtaining a token using the second identifier. 
     In act  302 , the computing device may receive a request for access to the media playback system. In some embodiments, the computing device may receive the request from a user via a GUI displayed on a display of the computing device. For example, the computing device may detect activation of a cast icon in the GUI of a third-party application, such as an application that provides audio content (e.g., songs, podcasts, video sound). Examples of such third-party applications that provide audio content include applications provided by media streaming services, such as music streaming services (e.g., SPOTIFY, PANDORA, AMAZON MUSIC, APPLE MUSIC, PLAY MUSIC, YOUTUBE MUSIC, IHEARTRADIO) and/or video streaming services (e.g., NETFLIX, HLTLU, AMAZON PRIME VIDEO, SLING, YOUTUBE TV). In another example, the computing device may detect startup of a native application associated with the media playback system (e.g., SONOS application). 
     In act  303 , the computing device may obtain access to the media playback system in order to, for example, control the media playback system (e.g., request the media playback system to perform one or more operations). The computing device may obtain access to the media playback system in any of a variety of ways. As shown in  FIG.  3 A , the computing device may obtain access to the media playback system by performing act  304  of identifying a first identifier associated with the media playback system, act  306  of obtaining the second identifier using the first identifier, and/or act  308  of obtaining the token using the second identifier. 
     In act  304 , the computing device may obtain the first identifier associated with the media playback system. In some embodiments, the computing device may obtain the first identifier from the media playback system, such as one or more playback devices in the media playback system. For example, the computing device may obtain the first identifier by transmitting a request for the first identifier to the media playback system and receiving, in response to the request, the first identifier from the media playback system. The mechanism of communication between the computing device and the media playback system may vary based on the particular implementation. For example, the computing device may be on the same network as the media playback system (e.g., the same LAN and/or PAN) and communicate with the media playback system over the network. In another example, the computing device may communicate with the media playback system using acoustic signals via one or more microphones and one or more speakers in each of the computing device and the media playback system. These acoustic signals may have any of a variety of frequencies depending on the particular implementation. For example, the acoustic signals have frequencies within the range of human hearing (e.g., between 20 Hz and 20 Khz), outside the range of human hearing (e.g., below 20 Hz or above 20 kHz), or near the edge of the range of human hearing (e.g., between 18 kHz and 20 kHz). 
     It should be appreciated that the computing device may obtain the first identifier via another source separate and apart from the media playback system. In some embodiments, the computing device may obtain the first identifier from the user via one or more input fields in a GUI. For example, the owner of the media playback system may be provided on their own computing device (e.g., during setup of the media playback system and/or upon request anytime after setup) a list of one or more first identifiers that the owner can provide at their discretion to other individuals (e.g., to guests). In this example, the owner may provide a first identifier to a guest user (e.g., verbally tell the guest user the first identifier, send the first identifier to the guest user in a written format) such that the guest user can enter the first identifier into an input field in the GUI displayed on their own computing device. 
     In act  306 , the computing device may obtain a second identifier using the first identifier. The computing device may obtain the second identifier from a source that is different from the source that provided the first identifier. In some embodiments, the computing device may obtain the second identifier from a computing system (e.g., computing system  201 ). For example, the computing device may transmit a request for the second identifier and the first identifier to the computing system. The computing device may receive, in response to the request for the second identifier, the second identifier from the computing system. 
     In some embodiments, the computing device may transmit additional information separate and apart from the first identifier to the computing system while obtaining the second identifier. For example, the computing device may obtain user login credentials (e.g., a username, a password, and/or a biometric) from the user for an account owned by the user that is maintained by the computing system. In this example, the computing device may provide the user login credentials to the computing system as part of the request for the second identifier and/or after transmitting the request for the second identifier. The user login credentials may be employed by the computing system to, for example, distinguish between different types of users. For example, the computing system may determine whether the received login credentials match the login credentials of the owner user (e.g., as established during the setup process for the media playback system) or are associated with another user, such as a registered guest user. In instances where the computing device fails to provide login credentials for a user account, the computing system may identify the user as simply an anonymous guest. The computing system may associate the determined user type (e.g., an owner user, a registered guest user, an anonymous guest user) with the second identifier provided to the user such that, for example, the user is provided a level of access to the media playback system that is tailored to their determined user type when subsequently interacting with the computing system and/or the media playback system. 
     In act  308 , the computing device may obtain the token using the second identifier. The computing device may obtain the token from, for example, the computing system. In some embodiments, the computing device may transmit a request for the token and the second identifier (and/or the first identifier) to the computing system. The computing device may, in turn, receive the token from the computing system. The token may be employed to request the media playback system to perform one or more operations. 
     In some embodiments, a token may only be employed to cause the media playback system to perform certain operations. The particular operations that a given token may be employed to request may vary based on, for example, the type of user. As mentioned above with respect to act  306 , the computing device may obtain user login credentials from the user that may be employed by the computing system to distinguish between different types of users and associate a user type with the second identifier. Such an association between the second identifier and user type may be employed by the computing system to identify a set of permitted operations to associate with a token. For example, an owner user may be permitted to perform more operations than a registered guest user, and the registered guest user may be permitted to perform more operations than an anonymous guest user. An example set of permitted operations associated with each of three different types users (e.g., an owner user, a registered guest user, and an anonymous guest user) are shown below in Table 1: 
     
       
         
          TABLE 1
           
               
               
             
               
                 Example Users Types and Permitted Operations 
               
               
                 Example User Type 
                 Example Permitted Operations 
               
             
            
               
                 First User Type (e.g., Owner User) 
                 Play Media; Pause Media; Change Volume; Forward Skip Media in a Playlist; Backward Skip Media in a Playlist; Reorder Media in a Playlist; Add Media to a Playlist; Remove Media from a Playlist; Change 
               
               
                   
                 Media playback system Settings (e.g., Change Groupings; Change Bonded Pairs; etc.) 
               
               
                 Second User Type (e.g., Registered guest User) 
                 Play Media; Pause Media; Change Volume; Forward Skip Media in a Playlist; Backward Skip Media in a Playlist; Reorder Media in a Playlist; Add Media to a Playlist; Remove Media from a Playlist 
               
               
                 Third User Type (e.g., Anonymous Guest User) 
                 Play Media; Pause Media; Change Volume; Forward Skip Media in a Playlist; Backward Skip Media in a Playlist 
               
            
           
         
       
     
     It should be appreciated that various operations may be associated with different types of users in any way and Table 1 above is only one example implementation. For example, any combination of the following operations: (1) play media, (2) pause media, (3) change volume, (4) forward skip media in a playlist, (5) backward skip media in a playlist, (6) reorder media in a playlist, (7) add media to a playlist, (8) remove media from a playlist, (9) change media playback system settings (e.g., change groupings, change bonded pairs, etc.), (10) shuffle media in a playlist, (11) favorite media, and (12) unfavorite media may be associated with a given user type. As discussed above, different combinations of operations may be associated with different types of users. In some implementations, the same combination of operations may be associated with at least two different types of users. Such implementations may be advantageous in scenarios where an owner may want to easily re-configure permissions for a set of non-owner users that may typically have the same level of access to the media playback system. For example, a media playback system in a commercial establishment may have an owner user type and a manager user type with the same associated set of permitted operations. In this example, the owner may easily re-configure the permissions associated with the all of the managers all at once by re-configuring the manager user type. 
     In act  310 , the computing device may receive a request for the media playback system to perform an operation. The computing device may, for example, receive the request for the media playback system to perform an operation from a user via a GUI displayed on the computing device. For example, the computing device may detect activation of a forward skip button in the GUI configured to trigger the media playback system to skip to the next song. 
     In act  312 , the computing device may cause the media playback system to perform the operation using the token. For example, the computing device may transmit a request for the media playback system to perform the operation and the token. Should the media playback system determine that the token is valid and/or the requested operation is within the set of permitted operations associated with the token, the media playback system may perform the operation. 
     It should be appreciated that one or more of the credentials including the first identifier, the second identifier, and/or the token may expire after a certain period of time. Limiting the lifespan of credentials may advantageously enhance the security of the media playback system. For example, a wrongdoer that steals an old token from a computing device of an authenticated user cannot control the media playback system. The period of time after which the particular credentials expire may vary based on the particular implementation. For example, the credentials may be set to expire between 1 and 48 hours after creation (e.g., 1 hour, 6 hours, 12 hours, 24 hours, 36 hours, 48 hours). Further, the expiration time of particular credentials may vary based on the type of user that the credentials are associated with. For example, a second identifier and/or a token associated with an owner user may last longer (including never expiring) than a second identifier and/or a token associated with a registered guest user and/or an anonymous guest user. Further, the second identifier and/or token associated with a registered guest user may last longer than a second identifier and/or token associated with an anonymous guest user. 
     In embodiments where one or more of the credentials expire, the computing device may perform one or more additional acts when a particular credential expires to automatically renew the expired credential (e.g., in situations where the user is still controlling the media playback system). For example, the computing device may detect that the token has expired and the user has recently requested the media playback system to perform an operation (e.g., request the media playback system to perform an operation within a threshold period of time, such as the last 10 minutes, the last 15 minutes, or the last 30 minutes) and, in response, automatically obtain a new token from the computing system by repeating one or more of acts  304 ,  306 , and/or  308 . Thus, the user may still be able to seamlessly control the media playback system without having to wait for the computing device to obtain a new token in response to requesting the media playback system perform another operation. 
     In some embodiments, the computing device may obtain access to the media playback system using different techniques based on the type of access requested. For example, the computing device may obtain a first level of access (e.g., owner user access) to the media playback system using a first technique and obtain a second, different level of access (e.g., registered guest user access) to the media playback system using a second technique that is different from the first technique. An example of such a process is shown in  FIG.  3 B  by process 300B that may be performed by a computing device (e.g., a control device). Relative to process 300A in  FIG.  3 A , process 300A adds an act  314  of determining a type of access requested and replaces acts  304 ,  306 , and  308  in act  303  of obtaining access to the media playback system with an act  305  of obtaining access to the media playback system using a first technique and an act  307  of obtaining access to the media playback system using an N th  technique. 
     In act  314 , the computing device may determine the type of access requested. In some embodiments, the computing device may determine the type of access requested based on input from the user via a GUI. For example, the computing device may display a GUI comprising a plurality of selectable elements that are each associated with a different level of access. In this example, the computing device may detect activation of one of the selectable elements and identify the type of access that is associated with the activated selectable element as the requested type of access. In another example, the computing device may display a GUI comprising input fields for user login credentials. In this example, the computing device may transmit the received user login credentials to a computing system along with information regarding the particular media playback system that is to be accessed. In this example, the computing system may determine the type of access that is appropriate given the user credentials (e.g., owner user access, registered guest user access, or anonymous guest access) and provide an indication of the type of access to the computing device. 
     In other embodiments, the computing device may determine the type of access requested without input from the user. For example, during setup of the media playback system, the computing device may store an identifier indicating that the computing device was used for setup of the media playback system and is associated with the owner. In this example, the computing device may determine whether the computing device has the identifier and, responsive to determining that the computing device has the identifier, obtain the highest level of access (e.g., owner user access). Otherwise, the computing device may obtain a lower level of access (e.g., registered guest user access). 
     Once the computing device has identified the type of access requested, the computing device may proceed to obtain access using a technique that is tailored to the particular type of access being requested. For example, the computing device may perform act  305  and obtain access to the media playback system using a first technique in response to determining that a first type of access to the media playback system was requested. Similarly, the computing device may perform act  307  and obtain access to the media playback system using an N th  technique that is different from the first technique. Any of the various techniques to obtain access to the media playback system may comprise one or more of acts  304 ,  306 , and/or  308  described above with reference to  FIG.  3 A . 
     In some embodiments, the techniques employed to gain access for owner users require fewer operations than the techniques employed to gain access for non-owner users (e.g., registered guest users and/or anonymous guest users). Thus, the owners (who may interact with the media playback system frequently) may be able to obtain access to the media playback devices faster than non-owner users. For example, the computing device may generate a token (e.g., during setup of the media playback system) for the owner that never expires. Thus, the computing device may obtain access to the media playback device simply by retrieving the token from memory. In contrast, non-owner users may only be able obtain tokens by performing one or more of acts  304 ,  306 , and/or  308  described above. Further, the tokens for non-owner users may expire after a set period of time (e.g., 12 hours) to force the non-owner users to perform one or more of acts  304 ,  306 , and/or  308  described above each time before obtaining access to the media playback system. 
       FIG.  4    shows an example process  400  that may be performed by a media playback system comprising the playback device  110   a  to authenticate a computing device (e.g., control device  130   a ) seeking to control the media playback system. The operations in process  400  may be performed by a single device within the media playback system (e.g., only playback device  110   a ) or distributed among a plurality of devices in the media playback system (e.g., distributed among multiple playback devices in the media playback system). For example, a first playback device may perform a first subset of acts in process  400  and a second playback device may perform a second, different subset of acts in process  400 . As shown, process  400  comprises an act  402  of receiving a request for a first identifier associated with the media playback system, an act  404  of obtaining the first identifier associated with the media playback system, an act  406  of transmitting the first identifier associated with the media playback system, an act  408  of receiving a request to perform an operation and a token, an act  410  of determining whether the token is valid, an act  412  of identifying permitted operations, an act  414  of determining whether the operation is permitted, an act  416  of performing the operation, and an act  418  of not performing the operation. 
     In act  402 , the media playback system may receive a request for a first identifier associated with the media playback system. The media playback system may receive the request for the first identifier from, for example, a computing device. The media playback system may receive the request via any of a variety of communication mechanisms. In some embodiments, the media playback system may be connected to the same network as the computing device, such as a LAN and/or a PAN. Additionally (or alternatively), the media playback system may receive the request via acoustic signals detected by one or more microphones in the media playback system. 
     In act  404 , the media playback system may obtain the first identifier associated with the media playback system. In some embodiments, the media playback system may generate the first identifier itself. In these embodiments, the media playback system may locally store a unique identifier associated with the media playback system (e.g., a household ID) and generate the first identifier based on the unique identifier. For example, the media playback system may transmit the unique identifier stored locally as the first identifier. In another example, the media playback system may generate the first identifier at least in part by combining the unique identifier with one or more other values such as an expiration time of the first identifier (e.g., in implementations where the first identifier expires) and/or a secret code. In embodiments where the media playback system generates the first identifier, the media playback system may additionally transmit the generated first identifier to the computing system such that the computing system can successfully validate the authenticity of a copy of the first identifier received from another source (e.g., a computing device attempting to obtain a second identifier and/or a token from the computing system). 
     In other embodiments, the media playback system may generate the first identifier with the assistance of another device. For example, the media playback system may transmit a request for the first identifier and a unique identifier associated with the media playback system to the computing system (e.g., over a WAN and/or a LAN). The media playback system may receive, in response to transmitting the request, the first identifier. 
     In act  406 , the media playback system may transmit the first identifier associated with the media playback system. The media playback system may transmit the first identifier over, for example, a LAN and/or a PAN to the computing device. In other examples, the media playback system may transmit the first identifier to the computing device via acoustic signals generated by one or more speakers in the media playback system. 
     In act  408 , the media playback system may receive a request to perform the operation and a token. The media playback system may receive the request to perform the operation and the token from, for example, the computing device. The media playback system may receive the request to perform the operation and/or the token over, for example, a LAN and/or a PAN from the computing device. For illustration, the media playback system may receive the request to perform the operation and/or the token via BLUETOOTH and/or a Wi-Fi network from the computing device. In other examples, the media playback system may receive the request to perform the operation and/or the token via acoustic signals detected by one or more microphones in the media playback system. In some embodiments, the token may be received via a different communication channel than the request to perform the operation. For example, the token may be transmitted via acoustic signals while the request to perform the operation may be transmitted over a LAN and/or a PAN. In another example, the token may be transmitted using BLUETOOTH while the request to perform the operation may be transmitted via a Wi-Fi network. 
     In act  410 , the media playback system may determine whether the token is valid. The media playback system may determine whether the token is valid using any of a variety of techniques. The media playback system may, for example, transmit the token to the computing system for validation. The media playback system may receive a response from the computing system indicative of whether the token is valid. If the token is valid, the media playback system may proceed to act  412  and identify the permitted operations. Otherwise, the media playback system may proceed to act  418  and not perform the operation. 
     In some embodiments, the media playback system may maintain a cache of valid tokens (e.g., tokens that have been recently validated and/or an expiration time associated with the valid tokens) in a local memory to reduce the communication traffic between the media playback system and the computing system. In these embodiments, the media playback system may store a table comprising a list of previously received tokens and the response from the computing system indicating whether each of the tokens is valid or invalid (and/or an expiration time for one or more valid tokens). As a result, the media playback system may, upon receipt of a token, first check the cache of tokens to see if the media playback system has previously received an indication from the computing system as to whether the token is valid or invalid. If the token matches one or more tokens in the cache of tokens, the media playback system may determine whether the token is valid based on the cache of tokens without contacting the computing system. For example, the media playback system may receive a token that, while previously valid, has since expired based on the expiration time associated with the token in the cache of tokens. In this example, the media playback system may determine that the token is not valid. If the token does not match one or more tokens in the cache of valid tokens, the media playback system may transmit the token to the computing system for validation as mentioned above. 
     In act  412 , the media playback system may identify the permitted operations associated with the token. The media playback system may identify the permitted operations associated with the token is valid using any of a variety of techniques. The media playback system may, for example, transmit a request for permissions and the token to the computing system for validation. The media playback system may receive a response from the computing system indicative of the permitted operations associated with the token. 
     In some embodiments, the media playback system may maintain a cache of permissions in a local memory to reduce the communication traffic between the media playback system and the computing system. In these embodiments, the media playback system may store a table comprising a list of valid tokens and permitted operations associated with each of the tokens. As a result, the media playback system may first check the cache of permissions to see if the media playback system has previously received an indication from the computing system as to the permitted operations associated with the token. If the token matches one or more tokens in the cache of permissions, the media playback system may identify the permitted operations based on the cache of permissions without contacting the computing system. For example, the media playback system may receive a token that matches one or more tokens in the cache of permissions and identify the set of permitted operations associated with the token from the cache of permissions. If the token does not match one or more tokens in the cache of permissions, the media playback system may transmit the token to the computing system to obtain the set of permissions operations as mentioned above. 
     In act  414 , the media playback system may determine whether the operation is permitted. The media playback system may determine whether the operation is valid at least in part by, for example, determining whether the operation matches one or more operations in the set of permitted operations. If the operation matches one or more operations in the set of permitted operations, the media playback system may determine that the operation is permitted and proceed to act  416  and perform the operation. Otherwise, the media playback system may determine that the operation is not permitted and proceed to act  418  and not perform the operation. 
     It should be appreciated that one or more acts may be omitted from process  400  without departing from the scope of the present disclosure. For example, acts  412  and/or  414  may be omitted in architectures where the media playback system only has a single type of user (and/or a single user). In such architectures, all of the tokens may have the same set of permitted operations (e.g., all operations). As a result, acts  412  and  414  may be redundant in such implementations. 
       FIGS.  5 A and  5 B  show an example process  500  that may be performed by computing system  201  to authenticate a computing device (e.g., control device  130   a ) attempting to obtain access to a media playback system comprising playback device  130   a . The operations in process  500  may be performed by a single computing device in the computing system  201  or distributed among a plurality of computing devices in the computing system  201 . For example, a first computing device may perform a first subset of acts in process  500  and a second computing device may perform a second, different subset of acts in process  500 . As shown in  FIGS.  5 A and  5 B , the process  500  comprises an act  502  of receiving a request for a first identifier, an act  503  of processing the request for the first identifier, an act  508  of receiving a request for a second identifier and first credential(s), an act  505  of processing the request for the second identifier, an act  516  of receiving a request for a token and second credential(s), an act  507  of processing the request for the token, an act  516  of receiving a request to validate third credentials, and an act  509  of processing the request to validate the third credentials. The act  503  of processing the request for the first identifier may comprise an act  504  of generating the first identifier and an act  506  of transmitting the first identifier. The act  505  of processing the request for the second identifier may comprise an act  510  of determining whether the first credentials are valid, an act  512  of generating the second identifier, an act  514  of transmitting the second identifier. The act  507  of processing the request for the token may comprise an act  518  of determining whether the second credentials are valid, an act  520  of generating the token, and an act  522  of transmitting the token. The act  509   of processing the request to validate the third credential(s) may comprise an act  526  of determining whether the third credential(s) are valid and an act  528  of transmitting the result. 
     In act  502 , the computing system may receive a request for a first identifier associated with the media playback system. The computing system may receive the request for the first identifier from, for example, the media playback system (e.g., over a WAN and/or a LAN). In some embodiments, the request for the first identifier may comprise (and/or be accompanied by) a unique identifier associated with the media playback system (e.g., a household ID). For example, the media playback system may have been assigned a household ID during a setup process where the media playback system is registered with the computing system. 
     In act  503 , the computing system may process the request for the first identifier. The computing system may process the request for the first identifier in any of a variety of ways. In some embodiments, the computing system may process the request for the first identifier at least in part by performing acts  504  and/or  506 . 
     In act  504 , the computing system may generate the first identifier. The computing device may generate the first identifier by, for example, generating a sequence of characters (e.g., a unique sequence of characters). The sequence of characters may, in some embodiments, be generated based on one or more parameters. For example, the sequence of characters may be generated based on a unique identifier associated with the media playback system received in act  502 . Additionally (or alternatively), the first identifier may be configured to expire after an expiration time is reached and the first identifier may be generated based on the expiration time of the first identifier. For illustration, the computing system may generate the first identifier at least in part based on one or more of: (1) the unique identifier associated with the media playback system (e.g., a household ID); (2) an expiration time of the first identifier; and (3) a secret code (e.g., a non-shared secret code such as a  128  random bits). For example, the first identifier may comprise a combination of a household ID, an expiration time, and a secret code. The first identifier may be, in some embodiments, digitally signed (e.g., cryptographically signed) to further enhance security. For example, digitally signing the first identifier may enable the computing system to subsequently validate the authenticity of the first identifier without having to directly store the first identifier. 
     In act  506 , the computing system may transmit the first identifier. For example, the computing system may transmit the first identifier to the media playback system, such as a playback device of the media playback system. The computing system may, in some embodiments, transmit the first identifier to the same device (e.g., playback device) within the media playback system that the computing system received the request for the first identifier from. In other embodiments, the computing system may transmit the first identifier to a different device within the media playback system than the device in the media playback system that the computing system received the request for the first identifier from in act  502 . For example, the computing system may receive the request for the first identifier from a first playback device in the media playback system in act  502  and transmit the first identifier to a second playback device in the media playback system that is different from the first playback device. 
     In act  508 , the computing system may receive the request for the second identifier and first credentials. The request for the second identifier may comprise the first credentials and/or the first credentials may be transmitted alongside the request for the second identifier (e.g., transmitted in separate messages). The computing system may receive the request for the second identifier and the first credentials from a computing device (e.g., over a WAN and/or a LAN). The first credentials may comprise any of a variety of credentials that may be employed to validate the authenticity of the request for the second identifier. For example, the first credentials may comprise the first identifier generated by the media playback system when the request for the second identifier is authentic. The computing system may validate the first credentials to, for example, prove that the computing system received the request for the second identifier from a computing device that successfully communicated with the media playback system. Additionally, the request for the first identifier may comprise (and/or be accompanied by) a unique identifier associated with the computing device, such as a client ID. 
     In act  505 , the computing system may process the request for the second identifier. The computing system may process the request for the first identifier in any of a variety of ways. In some embodiments, the computing system may process the request for the second identifier at least in part by performing acts  510 ,  512 , and/or  514 . 
     In act  510 , the computing system may determine whether the first credentials are valid. The computing system may, for example, determine whether the first credentials comprises the first identifier generated in act  504  to, for example, prove that the computing device that issued the request for the second identifier successfully communicated with the media playback system (e.g., over a Wi-Fi network). The computing system may determine whether the first credentials comprises the first identifier in any of a variety of ways. In some embodiments, the computing system may store the first identifier generated in act  503  (e.g., in a table) and compare the stored first identifier with a sequence of characters in the first credentials to determine whether there is a match. In other embodiments, the computing system may determine whether the first credentials comprise the first identifier without having to directly store the first identifier generated in act  503 . For example, the computing system may digitally sign the first identifier and store one or more keys. In this example, the computing system may employ the one or more keys (instead of the first identifier itself) to determine whether a received sequence of characters matches the first identifier. If the computing system determines that the first credentials are valid, the computing system may proceed to act  512  and generate the second identifier. Otherwise, the computing system may not perform acts  512  and/or  514 . 
     As mentioned above, in some embodiments, the first identifier may expire after an expiration time. In these embodiments, the computing system may determine whether the first credentials are valid by additionally determining whether the first identifier has expired. For example, the computing system may store an expiration time with the first identifier and/or one or more keys employed to cryptographically sign the first identifier (e.g., in a table). In this example, the computing may retrieve the expiration time from memory and determine whether the first identifier has expired. 
     In act  512 , the computing system may generate the second identifier. The computing device may generate the second identifier by, for example, generating a sequence of characters (e.g., a unique sequence of characters). The sequence of characters may, in some embodiments, be generated based on one or more parameters. For example, the sequence of characters may be generated based on a unique identifier associated with the computing system received in act  508  such as a client ID. Additionally (or alternatively), the second identifier may be configured to expire after an expiration time is reached and the second identifier may be generated based on the expiration time of the second identifier. For illustration, the computing system may generate the second identifier based at least in part on one or more of the following: (1) the unique identifier associated with the computing device (e.g., a client ID); (2) an expiration time of the second identifier; and (3) a secret code (e.g., a non-shared secret code such as a  128  random bits). For example, the second identifier may comprise a combination of the client ID, the expiration time, and the secret code. The second identifier may be, in some embodiments, digitally signed (e.g., cryptographically signed) to further enhance security. For example, cryptographically signing the second identifier may enable the computing system to subsequently validate the authenticity of the second identifier without having to directly store the second identifier. 
     In some embodiments, the computing system may identifier a user type and/or associated the identifier user type with the second identifier. Example user types include, for example, owner users, registered guest users, and anonymous users. Each type of user may be provided a different level of access to the media playback system as described above with reference to Table 1. For example, owner users may have a greater level of access than non-owner users. The computing system may distinguish between users based on information received from the computing device. In some embodiments, the computing system may receive user login credentials (e.g., a username, a password, and/or a biometric) for an account owned by the user from the computing device (e.g., received as part of the request for the second identifier and/or received separately). In these embodiments, the user login credentials may be employed by the computing system to, for example, distinguish between different types of users. For example, the computing system may determine whether the received login credentials match the login credentials of the owner user (e.g., as established during the setup process for the media playback system) or are associated another user, such as a registered guest user. In instances where no login credentials for a user account are provided by the computing device, the computing system may identify the user as simply an anonymous guest. Once the computing system has determined the user type, the determined user type may be associated with the second identifier such that, for example, the user is provided a level of access to the media playback system that is tailored to their determined user type when subsequently interacting with the computing system and/or the media playback system. 
     In act  514 , the computing system may transmit the second identifier. For example, the computing system may transmit the second identifier to the computing device, such as the computing device that the request for the second identifier originated. 
     In act  516 , the computing system may receive a request for a token and second credentials. The request for the token may comprise the second credentials and/or the second credentials may be transmitted alongside the request for the token (e.g., transmitted in separate messages). The computing system may receive the request for the token and the second credentials from a computing device. The second credentials may comprise any of a variety of credentials that may be employed to validate the authenticity of the request for the token. For example, the second credentials may comprise the second identifier previously generated by the computing system. Additionally (or alternatively), the second credentials may comprise the first identifier generated by the media playback system. 
     In act  507 , the computing system may process the request for the token. The computing system may process the request for the token in any of a variety of ways. In some embodiments, the computing system may process the request for the token at least in part by performing acts  518 ,  520 , and/or  522 . 
     In act  518 , the computing system may determine whether the second credentials are valid. For example, the computing system may determine whether the second credentials comprise a valid second identifier and/or a valid first identifier. In some embodiments, the computing system may store the second identifier generated in act  512  and/or the first identifier generated in act  504 . In these embodiments, the computing system may compare the stored second identifier and/or the first identifier with the sequence(s) of characters in the second credentials to determine whether there is a match. In other embodiments, the computing system may determine whether the second credentials comprise the second identifier and/or the first identifier without having to directly storing the identifier(s). For example, the computing system may cryptographically sign the second identifier and/or the first identifier and store one or more keys. In this example, the computing system may employ the one or more keys (instead of the identifier(s)) to determine whether a received sequence of characters matches the second identifier and/or the first identifier. If the computing system determines that the second credentials are valid, the computing system may proceed to act  520  and generate the token. Otherwise, the computing system may not perform acts  520  and/or  522 . 
     As mentioned above, in some embodiments, the second identifier and/or the first identifier may expire after an expiration time. In these embodiments, the computing system may determine whether the second credentials are valid by additionally determining whether the second identifier and/or the first identifier has expired. For example, the computing system may store an expiration time with the identifier(s) and/or one or more keys employed to digitally sign the identifier(s). In this example, the computing may retrieve the expiration time(s) from memory and determine whether the identifier(s) have expired. 
     In act  520 , the computing system may generate the token. The computing device may generate the token by, for example, generating a sequence of characters (e.g., a unique sequence of characters). The sequence of characters may, in some embodiments, be generated based on one or more parameters. For example, the sequence of characters may be generated based on the first and/or second identifier received in act  518 , a secret code, and/or an expiration time of the token (e.g., in instances where the token expires). In other embodiments, the token may be generated without any inputs. For example, the token may be a randomly generated sequence of characters with a fixed length or a variable length. 
     As mentioned above, in some embodiments, the computing system may provide different levels of access to the media playback system to different types of users. In these embodiments, the computing system may identify the type of user that requested the token and associate a set of permitted operations with the token that matches the user type. The computing system may use information received from the computing device in act  516 , such as the second identifier, to distinguish between different types of users. For example, the computing system may have identified a user type in act  512  and associated the identified user type with the second identifier (e.g., in a table). Thus, the computing system may readily identify the user type by identifying the user type associated with the particular second identifier received from the computing system. Once the computing system has identified the user type, the computing device may associate a set of permitted operations that corresponds to the identifier user type to the token. For example, the computing system may determine that a particular user is an owner user and associate all of the operations that the media playback system is capable of performing with the token. In another example, the computing system may determine that a particular user is a non-owner user and associate a portion of the operations that the media playback system is capable of performing with the token. 
     In act  522 , the computing system may transmit the token. For example, the computing system may transmit the token the computing device, such as the computing device that the request for the token originated. 
     In act  524 , the computing system may receive a request to validate third credentials. The request to validate the third credentials may comprise, for example, a token that needs to be validated to determine whether the token is valid or invalid. The computing system may receive the request to validate the third credentials from, for example, the media playback system. 
     In act  509 , the computing system may process the request to validate the third credentials. The computing system may process the request to validate the third credentials in any of a variety of ways. In some embodiments, the computing system may process the request to validate the third credentials at least in part by performing acts  526  and/or  528 . 
     In act  526 , the computing system may determine whether the third credentials are valid. For example, the computing system may determine whether the token is valid or invalid. In some embodiments, the computing system may compare the token with the sequence(s) of characters in the second credentials to determine whether there is a match. 
     In act  528 , the computing system may transmit the result. For example, the computing system may transmit an indication of whether the third credentials are valid to the media playback system. Additionally (or alternatively), the computing system may transmit permission information associated with the token in instances where the token is valid. For example, the computing system may transmit the list of permitted operations that are associated with the token (e.g., the permitted operations identified in act  520 ) to the media playback system. 
     It should be appreciated that, in some embodiments, the permissions provided to different types of users may be configurable by the owner user of the media playback system. For example, the owner user may forbid particular types of users (e.g., anonymous guest users and/or registered guest users) from accessing the media playback system. In this example, the computing system may reject all requests for credentials (e.g., first identifiers, second identifiers, and/or tokens) from such users. Additionally (or alternatively), the particular permitted operations associated with any given user type may be configurable by the owner user. For example, the owner user may add or remove permitted operations associated with registered guest users and/or anonymous users (e.g., as shown in Table 1). Thus, the owner user may control both the types of users that have access to the media playback system and the level of access that each type of user is granted to the media playback system. 
     In some embodiments, the control device  130   a  may seek to identify a playback device  110   a  in a media playback system that is connected to the same network as the control device  130   a , such as a LAN and/or a PAN. The control device  130   a  may attempt to identify the playback device  110   a  in response to any of a variety of stimuli such as detection of an application being opened, detection of a new network connection, and/or a certain amount of time expiring since the control device  130   a  previously attempted to identify the playback device  110 .  FIG.  6    shows an example message flow that may be employed between the control device  130   a  and the playback device  110   a  where the control device  130   a  detects a media playback system comprising the playback device  110   a . As shown in  FIG.  6   , the message flow comprises one or more broadcast messages  602 , one or more responses messages  604 , and a show media playback system availability action  606 . 
     The control device  130   a  may transmit a broadcast  602  over a network (e.g., a LAN and/or a PAN). The playback device  110   a  may detect the broadcast  602  and respond to the broadcast with a response  604 . The response messages  604  may comprise, for example, information that uniquely identifiers the media playback system to which the playback device  110   a  belongs (e.g., a household ID). Once the control device  130   a  receives the response  604 , the control device  130  may perform the action  606  and show the media playback system availability. For example, the control device  130   a  may present, via a GUI, an indication that the media playback system is available, such as displaying a cast icon. 
     After the control device  130   a  has detected a media playback system, the control device  130   a  may receive a request to connect to the media playback system. For example, the control device  130   a  may detect selection of a cast icon displayed in a GUI. Upon receipt of the request to connect to the playback device  110   a , the control device  130   a  may obtain a first identifier associated with the media playback system, as described above with respect to  FIGS.  2 - 5 B .  FIG.  7    shows an example message flow between the control device  130   a , a playback device  110   a  that is part of a media playback system, and a computer system  201  where the control device  130   a  obtains a first identifier. As shown, the message flow comprises first ID requests  702  and  704 , a generate first ID action  706 , and first ID responses  708  and  710 . 
     The control device  130   a  may transmit a first ID request  702  to the playback device  110   a  that requests a first identifier. The playback device  110   a  may, in turn, transmit a first ID request  704  to the computing system  201 . The first ID request  704  may, in some embodiments, contain additional information relative to the first ID request  702 . For example, the playback device  110   a  may incorporate a unique identifier associated with the media playback system to which the playback device  110   a  belongs, such as a household ID, into the first ID request  704  that may be employed by the computing system  201  to generate the first identifier. Upon receipt of the first ID request by the computing system  201 , the computing system may proceed to perform the action  706  to generate the first ID. The computing system may generate the first ID in any of a variety of ways including, for example, those techniques described above with respect to act  504  in  FIG.  5   . Once the computing system has generated the first ID, the computing system  201  may transmit a first ID response  708  to the playback device  110   a . The playback device  110   a  may, upon receipt of the first ID response  708 , transmit the first ID response  710  to the control device  130   a . 
     After the control device  130   a  has obtained the first identifier, the control device  130   a  may obtain a second identifier using the first identifier, as described above with respect to  FIGS.  2 - 5 B .  FIG.  8    shows an example message flow between the control device  130   a , a playback device  110   a  that is part of a media playback system, and a computer system  201  where the control device  130   a  obtains a second identifier. As shown, the message flow comprises a second ID request  802 , a validate credentials action  804 , a request for user input  806 , a user input response  808 , a validate user input action  810 , a consent request  812 , a consent response  814 , a generate second ID action  816 , and a second ID response  818 . 
     The control device  130   a  may transmit the second ID request  802  to the computing system  201 . The second ID request  802  may comprise one or more credentials, such as the first identifier. The computing system  201  may, in turn, validate the credentials in the second ID request  802  in the validate credentials action  804  using, for example, any of the techniques described above in act  510  in  FIG.  5   . If the credentials in the second ID request  802  are valid, the computing system  201  may transmit a user input request  806  to the control device  130   a  requesting input from the user, such as user login credentials for an account associated with the user. The user input request  806  may trigger the control device  130   a  to request input from the user in a GUI. For example, the user input request  806  may include an address (e.g., a URL) for a webpage (e.g., hosted by the computing system  201 ) that the control device  130   a  may access via a web browser where the user may input information, such as user login credentials. After the computing device  130   a  receives the user input, the control device  130   a  may transmit a user input response  808  to the computing system  201  that comprises the user input. The computing system  201  may, in response to receiving the user input response  808 , perform a validate user input action  810 . For example, the computing system  201  may determine whether the user account login credentials are valid (e.g., match the account login credentials of a user account) or invalid (e.g., do not match the account login credentials of a user account). Once the user input has been validated, the computer system may transmit a consent request  812  to the control device  130   a  requesting permission from the user. For example, the computing system  201  may need to make information associated with an account available to a third-party application and/or allow a third-party application to control the media playback system. In this example, the computing system  201  may request permission to make such information and/or access available to a third-party application. The control device  130   a  may receive input from the user via a GUI regarding whether the user grants the requested permission and transmit the consent response  814 . For illustration, the consent request  812  may include an address (e.g., a URL) for a webpage (e.g., hosted by the computing system  201 ) that the control device  130   a  may access via a web browser where the user may provide their consent (e.g., click a button indicating that the user provides consent). The computing system  201  may receive the consent response  814  and, responsive to the user providing consent, perform the generate second ID action  816  (e.g., using any of the techniques described above with reference to act  512  in  FIG.  5   ). The computing system  201  may, in turn, transmit a second ID response  818  that includes the second ID to the control device  130   a . 
     It should be appreciated that one or more portions of the message flow shown in  FIG.  8    may be omitted without departing from the scope of the present disclosure. For example, in some embodiments, the computing system  201  may not request input from the user (e.g., login credentials for an account). Further, in some embodiments, the computing system  201  may not request consent from the user (e.g., because a third-party application does not need information from the account). Thus, one or more of elements  806 ,  808 ,  810 ,  812 , and  814  may be omitted from the message flow in  FIG.  8    without departing from the scope of the present disclosure. 
     After the control device  130   a  has obtained the second identifier, the control device  130   a  may obtain a token using the second identifier (and/or the first identifier), as described above with respect to  FIGS.  2 - 5 B .  FIG.  9    shows an example message flow between the control device  130   a , a playback device  110   a  that is part of a media playback system, and a computer system  201  where the control device obtains a token. As shown, the message flow comprises a token request  902 , a validate credentials action  904 , a generate token action  906 , and a token response  908 . 
     The control device  902  transmit a token request  902  to the computing system  201 . The token request  902  may comprise one or more credentials such as the second identifier and/or the first identifier. The computing system  201  may, upon receipt of the token request  902 , perform a validate credentials action  904  and validate the credentials in the token request (e.g., using any of the techniques described above with reference to act  518  in  FIG.  5   ). If the computing system  201  determines that the credentials are valid, the computing system  201  may perform a generate token action (e.g., using any of the techniques described above with reference to act  518  in  FIG.  5   ) and transmit the token to the control device  130   a  in a token response  908 . 
     After the control device  130   a  has obtained the token, the control device  130   a  may receive a request for the media playback system to perform an operation from a user. For example, the control device  130   a  may detect activation of a GUI element for skipping tracks. Upon receiving the request for the media playback system to perform the operation, the control device  130   a  may cause the media playback system to perform the operation using the token, as described above with respect to  FIGS.  2 - 5 B .  FIG.  10    is an example message flow between the control device  130   a , a playback device  110   a  that is part of a media playback system, and a computer system  201  where the control device  130   a  causes the media playback system to perform an operation. As shown, the message flow comprises an operation request  1002 , a validation request  1004 , a validate credentials action  1006 , a validation response  1008 , a permissions request  1010 , an identify permissions operation  1012 , a permissions response  1014 , and an operation request response  1016 . 
     The control device  130   a  transmit an operation request  1002  to the playback device  110   a . The operation request  1002  may comprise an indication of an operation and one or more credentials, such as a token. The playback device  110   a  may, in turn, transmit a validation request  1004  that comprises the credentials received from the control device  130   a  (e.g., the token) to the computing system  201  for validation. The computing system  201  may perform a validate credentials action  1006  and validate the credentials (e.g., using any of the techniques described above with reference to act  526  in  FIG.  5   ). The computing system  201  may transmit a validation response  1008  to the playback device  110   a  that includes an indication of whether the credential(s) (e.g., token) are valid. The playback device  110   a  may, in response to the token being valid, transmit a permissions request  1010  to the computing system requesting an indication of the permitted operations associated with the token. The computing system  201  may then perform an identify permissions action  1012  and identify the permitted operations associated with the token. The identified set of permitted operations associated with the token may be transmitted in the permissions response  1014 . The playback device  110   a  may use the set of permitted operations received from the computing system  201  to determine whether the operation included in the operation request  1002  is permitted. If the operation is permitted, the playback device  110   a  may perform the operation and provide an indication that the operation was performed in the operation response  1016 . Otherwise, the playback device  110   a  may not perform the operation and provide an indication that the operation was not performed in the operation response  1016 . 
     It should be appreciated that one or more portions of the message flow shown in  FIG.  10    may be omitted without departing from the scope of the present disclosure. For example, in some embodiments, the playback device  110   a  may not send a permissions request to the computing system  201  (e.g., in architectures where all tokens have the same permissions). Thus, one or more of elements  1010 ,  1012 , and  1014  may be omitted from the message flow in  FIG.  10    without departing from the scope of the present disclosure. 
     As described above, a computing device (e.g., control device  130   a ) may display one or more user interface screens in a GUI while obtaining access to a media playback system. Example user interface screens that may be presented by the computing device are shown in  FIGS.  11 A and  11 B . Referring to  FIG.  11 A , a sequence of screens comprising a first screen  1102  and a second screen  1104  is shown. The first screen  1102  may be displayed after, for example, the computing device receives a request to obtain access to a media playback system. As shown, the first screen  1102  comprises a set of input fields  1106  where a user can input login credentials such as an email address and a password associated with an account. The first screen  1102  further comprises a “Sign In” button  1108  that triggers the computing device to submit the login credentials in the set of input fields  1106  to a computing system for validation. A user who, for example, owns the media playback system or is otherwise a registered user (e.g., a registered guest) may input their user login credentials into the set of input fields  1106  to obtain access to the media playback system. Upon activation of the “Sign In” button  1108 , the computing device may submit the login credentials in the set of input fields  1106  to a computing system for validation. The computing device may, in turn, receive a response from the computing system indicating that the login credentials are valid and transition to screen  1104 . As shown, the second screen  1104  includes a permission request  1112  for a third-party application (shown as “App”) to control the media playback system (e.g., request the media playback system to perform operations such as changing playback volume or skipping media in a playback queue). For example, the permission request  1112  may provide a third-party application associated with a media streaming service (e.g., a music streaming service such as SPOTIFY, PANDORA, AMAZON MUSIC) the ability to control playback of media from the media streaming service on the media playback system. The second screen  1104  includes an “Allow” button  1114  that, upon activation, allows the third-party application executed on the computing device to control the media playback system. For example, the computing system may permit the third-party application to request a token (or any other credential) that may be employed to control the media playback system. The screen  1104  further includes a “Don’t Allow” button  1116  that, upon activation, does not allow the third-party application to control the media playback system. For example, the computing system may not permit the third-party application to request a token (or any other credential) that may be employed to control the media playback system. 
     Returning to screen  1102 , the screen  1102  further comprises an “I’m a guest” button  1110  that may be activated to obtain anonymous guest access to the media playback system for those individuals who do not have an account. Upon activation of the “I’m a guest” button, the computing device may transition to screen  1118  shown in  FIG.  11 B . As shown, the screen  1118  includes a permission request  1122  for a third-party application (shown as “App”) to control the media playback system. The screen  1118  further includes a “Request Access” button  1124  that may trigger the computing device to request access from the owner of the media playback system. For example, the computing device may send one or more messages to the computing system that, in turn, causes the computing system to transmit a personal identification number (PIN) to the owner of the media playback system. The owner of the media playback system may, in turn, provide the PIN to the guest. The guest may enter the PIN by, for example, activating the “I have a PIN already” button  1126  that triggers the computing device to transition to screen  1120 . As shown, screen  1120  comprises an input field  1128  to receive the PIN and a “Continue” button  1130  that triggers the computing device to transmit the PIN from the input field to the computing system for validation to obtain access to the media playback system. The screen  1132  also includes an “I do not have a PIN” button that, upon activation, may cause the computing device to return to screen  1118 . 
     IV. Conclusion 
     The above discussions relating to playback devices, control devices, playback zone configurations, and media content sources provide only some examples of operating environments within which functions and methods described below may be implemented. Other operating environments and configurations of media playback systems, playback devices, and network devices not explicitly described herein may also be applicable and suitable for implementation of the functions and methods. 
     Further, the access control techniques described herein may be employed in systems separate and apart from media playback systems such as any Internet of Things (IoT) system. These IoT systems may comprise an IoT device, such as a smart thermostat, a smart doorbell, a smart lock (e.g., a smart door lock), a smart outlet, a smart light, a smart camera, a smart kitchen appliance (e.g., a smart oven, a smart coffee maker, a smart microwave), a smart speaker (including the network accessible and/or voice-enabled playback devices described above), that communicates with cloud servers (e.g., over the Internet) and a computing device (e.g., over a LAN and/or a PAN). For example, with reference to  FIGS.  2  and  6 - 10   , the playback device  110   a  may be replaced with the IoT device, the computing system  201  may be the cloud servers, and the control device  130   a  may be the computing device. Thus, the access control techniques described herein are not limited to media playback systems. 
     It should be appreciated that references to transmitting information to particular devices and/or systems herein should be understood to include transmitting information (e.g., messages, requests, responses) indirectly or directly to the particular devices and/or systems. Thus, the information being transmitted to the particular devices and/or systems may pass through any number of intermediary devices and/or systems prior to reaching its destination. For example, a control device may transmit information to a playback device by first transmitting the information to a computing system that, in turn, transmits the information to the playback device. Further, modifications may be made to the information by the intermediary devices. For example, intermediary devices may modify a portion of the information, reformat the information, and/or incorporate additional information. 
     Similarly, references to receiving information from particular devices and/or systems herein should be understood to include receiving information (e.g., messages, requests, responses) indirectly or directly from the particular devices and/or systems. Thus, the information being received from the particular devices and/or systems may pass through any number of intermediary devices and/or systems prior to being received. For example, a control device may receive information from a playback device indirectly by receiving information from a cloud server that originated from the playback device. Further, modifications may be made to the information by the intermediary devices. For example, intermediary devices may modify a portion of the information, reformat the information, and/or incorporate additional information. 
     The description above discloses, among other things, various example systems, methods, apparatus, and articles of manufacture including, among other components, firmware and/or software executed on hardware. It is understood that such examples are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of the firmware, hardware, and/or software aspects or components can be embodied exclusively in hardware, exclusively in software, exclusively in firmware, or in any combination of hardware, software, and/or firmware. Accordingly, the examples provided are not the only ways) to implement such systems, methods, apparatus, and/or articles of manufacture. 
     Additionally, references herein to “embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one example embodiment of an invention. The appearances of this phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. As such, the embodiments described herein, explicitly and implicitly understood by one skilled in the art, can be combined with other embodiments. 
     The specification is presented largely in terms of illustrative environments, systems, procedures, steps, logic blocks, processing, and other symbolic representations that directly or indirectly resemble the operations of data processing devices coupled to networks. These process descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. Numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it is understood to those skilled in the art that certain embodiments of the present disclosure can be practiced without certain, specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the embodiments. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing description of embodiments. 
     When any of the appended claims are read to cover a purely software and/or firmware implementation, at least one of the elements in at least one example is hereby expressly defined to include a tangible, non-transitory medium such as a memory, DVD, CD, Blu-ray, and so on, storing the software and/or firmware. 
     V. Example Features for Computing Devices 
     (Feature 1) A computing device (e.g., a control device) comprising: one or more network interfaces; one or more processors coupled to the one or more network interfaces; data storage having stored therein instructions executable by the one or more processors to cause the computing device to perform a method comprising: receiving a request to control a media playback system comprising a playback device; after receiving the request to control the media playback system, obtaining access to the media playback system, wherein obtaining access to the media playback system comprises: a) obtaining a first identifier associated with the media playback system; b) obtaining a second identifier using the first identifier, wherein obtaining the second identifier comprises communicating over a wide area network (WAN) (e.g., the Internet) with a computing system; and c) obtaining a token using the second identifier, wherein obtaining the token comprises communicating over the WAN with the computing system; receiving a request for the media playback system to perform an operation; and causing the media playback system to perform the operation using the token. 
     (Feature 2) The computing device of feature 1, wherein obtaining the first identifier comprises: transmitting a request for the first identifier to the playback device; and receiving the first identifier from the playback device. 
     (Feature 3) The computing device of feature 1, further comprising a display coupled to the one or more processors and wherein obtaining the first identifier associated with the media playback system comprises: displaying, via the display, a graphical user interface that comprises at least one input field; and receiving the first identifier in the at least one input field. 
     (Feature 4) The computing device of any of features 1-3, wherein obtaining the second identifier comprises: transmitting the first identifier to the computing system; and receiving the second identifier from the computing system. 
     (Feature 5) The computing device of feature 4, further comprising a display coupled to the one or more processors and wherein obtaining the second identifier further comprises: displaying, via the display, a graphical user interface that comprises at least one input field; receiving one or more credentials associated with a user in the at least one input field; and transmitting the one or more credentials associated with the user to the computing system. 
     (Feature 6) The computing device of feature 5, wherein the one or more credentials associated with the user include at least one member selected from the group consisting of: a username, a password, and a biometric (e.g., a fingerprint of a user, a photo of a user). 
     (Feature 7) The computing device of any of features 1-6, wherein obtaining the token comprises: obtaining the token using the first identifier and the second identifier. 
     (Feature 8) The computing device of any of features 1-6, wherein obtaining the token comprises: transmitting the second identifier to the computing system; and receiving the token from the computing system. 
     (Feature 9) The computing device of any of features 1-8, wherein causing the media playback system to perform the operation comprises: transmitting an indication of the operation and the token to the playback device. 
     (Feature 10) The computing device of any of features 1-9, wherein the operation comprises an operation selected from the group consisting of: playing media, pausing media, increasing volume, decreasing volume, adding media to a playlist, removing media from a playlist, reordering media in a playlist, and skipping media in a playlist. 
     (Feature 11) The computing device of any of features 1-10, wherein at least one member of the group consisting of: the first identifier, the second identifier, and the token comprises a sequence of characters and wherein the sequence of characters comprises at least one member selected from the group consisting of: a number, a letter, and a symbol. 
     (Feature 12) A method performed by a computing device (e.g., a control device), the method comprising: receiving a request to control a media playback system comprising a playback device; after receiving the request to control the media playback system, obtaining access to the media playback system, wherein obtaining access to the media playback system comprises: a) obtaining a first identifier associated with the media playback system; b) obtaining a second identifier using the first identifier, wherein obtaining the second identifier comprises communicating over a wide area network (WAN) (e.g., the Internet) with a computing system; and c) obtaining a token using the second identifier, wherein obtaining the token comprises communicating over the WAN with the computing system; receiving a request for the media playback system to perform an operation; and causing the media playback system to perform the operation using the token. 
     (Feature 13) The method of feature 12, wherein obtaining the first identifier comprises: transmitting a request for the first identifier to the playback device; and receiving the first identifier from the playback device. 
     (Feature 14) The method of feature 12, wherein obtaining the first identifier associated with the media playback system comprises: displaying, via a display in the computing device, a graphical user interface that comprises at least one input field; and receiving the first identifier in the at least one input field. 
     (Feature 15) The method of any of features 12-14, wherein obtaining the second identifier comprises: transmitting the first identifier to the computing system; and receiving the second identifier from the computing system. 
     (Feature 16) The method of feature 15, wherein obtaining the second identifier further comprises: displaying, via a display in the computing device, a graphical user interface that comprises at least one input field; receiving one or more credentials associated with a user in the at least one input field; and transmitting the one or more credentials associated with the user to the computing system. 
     (Feature 17) The method of any of features 12-16, wherein obtaining the token comprises: obtaining the token using the first identifier and the second identifier. 
     (Feature 18) The method of any of features 12-16, wherein obtaining the token comprises: transmitting the second identifier to the computing system; and receiving the token from the computing system. 
     (Feature 19) The method of any of features 12-18, wherein causing the media playback system to perform the operation comprises: transmitting an indication of the operation and the token to the playback device. 
     (Feature 20) A non-transitory computer readable medium having stored thereon instructions executable by one or more processors of a computing device (e.g., a control device) to cause the one or more processors to perform functions comprising: receiving a request to control a media playback system comprising a playback device; after receiving the request to control the media playback system, obtaining access to the media playback system, wherein obtaining access to the media playback system comprises: a) obtaining a first identifier associated with the media playback system; b) obtaining a second identifier using the first identifier, wherein obtaining the second identifier comprises communicating over a wide area network (WAN) (e.g., the Internet) with a computing system; and c) obtaining a token using the second identifier, wherein obtaining the token comprises communicating over the WAN with the computing system; receiving a request for the media playback system to perform an operation; and causing the media playback system to perform the operation using the token. 
     VI. Example Features for Media Playback Systems 
     (Feature 1) A media playback system comprising: one or more network interfaces; one or more amplifiers configured to drive one or more speakers; one or more processors coupled to the one or more network interfaces; data storage having stored therein instructions executable by the one or more processors to cause the media playback system to perform a method comprising: receiving a request for a first identifier from a computing device (e.g., a control device); obtaining the first identifier, wherein obtaining the first identifier comprises communicating with a computing system over a wide area network (WAN) (e.g., the Internet); transmitting the first identifier to the computing device; receiving a request to perform an operation and a token from the computing device; determining whether the token is valid; responsive to determining that the token is valid: a) identifying a set of permitted operations associated with the token; b) determining whether the operation is within the set of permitted operations associated with the token; and c) responsive to determining that the operation is within the set of permitted operations associated with the token, performing the operation. 
     (Feature 2) The media playback system of feature 1, wherein the method further comprises: responsive to determining that the token is invalid or determining that the operation is not within the set of permitted operations: a) not performing the operation; and b) transmitting an indication that the operation was not performed to the computing device. 
     (Feature 3) The media playback system of any of features 1-2, wherein obtaining the first identifier comprises: transmitting a second identifier associated with the media playback system to the computing system; and receiving the first identifier from the computing system. 
     (Feature 4) The media playback system of any of features 1-3, wherein determining whether the token is valid comprises: determining whether the token matches at least one token from a set of known valid tokens stored in the data storage; and responsive to determining that the token matches at least one token from the set of known valid tokens, determining that the token is valid. 
     (Feature 5) The media playback system of feature 4, wherein determining whether the token is valid further comprises: responsive to determining that the token does not match at least one token from the set of known valid tokens: a) transmitting the token to a computing system; and b) receiving an indication of whether the token is valid from the computing system. 
     (Feature 6) The media playback system of any of features 1-5, wherein identifying the set of permitted operations associated with the token comprises: retrieving from the data storage first permission information comprising permissions for a plurality of tokens; determining whether the token matches at least one token from the plurality of tokens; and responsive to determining that the token matches at least one token from the plurality of tokens, identifying the set of permitted operations associated with the token using the first permission information. 
     (Feature 7) The media playback system of feature 6, wherein identifying the set of permitted operations associated with the token further comprises: responsive to determining that the token does not match at least one token from the plurality of tokens: a) transmitting the token to the computing system; b) receiving second permissions information from the computing system; and c) identifying the set of permitted operations associated with the token using the second permissions information. 
     (Feature 8) The media playback system of any of features 1-7, further comprising a playback device including a housing and wherein each of the one or more processors, the one or more amplifiers, and the data storage are carried by the housing. 
     (Feature 9) The media playback system of any of features 1-7, further comprising a plurality of playback devices, wherein the data storage comprises a plurality of memory modules, and wherein the one or more processors and the plurality of memory modules are distributed among the plurality of playback devices. 
     (Feature 10) The media playback system of any of features 1-9, wherein the operation comprises an operation selected from the group consisting of: playing media, pausing media, increasing volume, decreasing volume, adding media to a playlist, removing media from a playlist, reordering media in a playlist, and skipping media in a playlist. 
     (Feature 11) The media playback system of any of features 1-10, wherein at least one member of the group consisting of: the first identifier and the token comprises a sequence of characters and wherein the sequence of characters comprises at least one member selected from the group consisting of: a number, a letter, and a symbol. 
     (Feature 12) A method performed by a media playback system, the method comprising: receiving a request for a first identifier from a computing device (e.g., a control device); obtaining the first identifier, wherein obtaining the first identifier comprises communicating with a computing system over a wide area network (WAN) (e.g., the Internet); transmitting the first identifier to the computing device; receiving a request to perform an operation and a token from the computing device; determining whether the token is valid; responsive to determining that the token is valid: a) identifying a set of permitted operations associated with the token; b) determining whether the operation is within the set of permitted operations associated with the token; and c) responsive to determining that the operation is within the set of permitted operations associated with the token, performing the operation. 
     (Feature 13) The method of feature 12, further comprising: responsive to determining that the token is invalid or determining that the operation is not within the set of permitted operations: a) not performing the operation; and b) transmitting an indication that the operation was not performed to the computing device. 
     (Feature 14) The method of any of features 12-13, wherein obtaining the first identifier comprises: transmitting a second identifier associated with the media playback system to the computing system; and receiving the first identifier from the computing system. 
     (Feature 15) The method of any of features 12-14, wherein determining whether the token is valid comprises: determining whether the token matches at least one token from a set of known valid tokens stored in the data storage; and responsive to determining that the token matches at least one token from the set of known valid tokens, determining that the token is valid. 
     (Feature 16) The method of feature 15, wherein determining whether the token is valid further comprises: responsive to determining that the token does not match at least one token from the set of known valid tokens: a) transmitting the token to a computing system; and b) receiving an indication of whether the token is valid from the computing system. 
     (Feature 17) The method of any of features 12-16, wherein identifying the set of permitted operations associated with the token comprises: retrieving from the data storage first permission information comprising permissions for a plurality of tokens; determining whether the token matches at least one token from the plurality of tokens; and responsive to determining that the token matches at least one token from the plurality of tokens, identifying the set of permitted operations associated with the token using the first permission information. 
     (Feature 18) The method of feature 17, wherein identifying the set of permitted operations associated with the token further comprises: responsive to determining that the token does not match at least one token from the plurality of tokens: a) transmitting the token to the computing system; b) receiving second permissions information from the computing system; and c) identifying the set of permitted operations associated with the token using the second permissions information. 
     (Feature 19) The method of any of features 12-18, wherein the operation comprises an operation selected from the group consisting of: playing media, pausing media, increasing volume, decreasing volume, adding media to a playlist, removing media from a playlist, reordering media in a playlist, and skipping media in a playlist. 
     (Feature 20) A non-transitory computer readable medium having stored thereon instructions executable by one or more processors of a media playback system to cause the one or more processors to perform functions comprising: receiving a request for a first identifier from a computing device (e.g., a control device); obtaining the first identifier, wherein obtaining the first identifier comprises communicating with a computing system over a wide area network (WAN) (e.g., the Internet); transmitting the first identifier to the computing device; receiving a request to perform an operation and a token from the computing device; determining whether the token is valid; responsive to determining that the token is valid: a) identifying a set of permitted operations associated with the token; b) determining whether the operation is within the set of permitted operations associated with the token; and c) responsive to determining that the operation is within the set of permitted operations associated with the token, performing the operation. 
     VII. Example Features for Computing Systems 
     (Feature 1) A computing system comprising: one or more network interfaces; one or more processors coupled to the one or more network interfaces; data storage having stored therein instructions executable by the one or more processors to cause the computing system to perform a method comprising: receiving, from a media playback system, a first request for a first identifier; processing the first request, wherein processing the first request comprises: a) generating the first identifier; and b) transmitting the first identifier to the media playback system; receiving, from a computing device (e.g., a control device), a second request for a second identifier and one or more first credentials; processing the second request, wherein processing the second request comprises: a) determining whether the one or more first credentials are valid, wherein determining whether the one or more first credentials are valid comprises determining whether the one or more first credentials comprises the first identifier; and b) responsive to determining that the one or more first credentials are valid, generating the second identifier and transmitting the second identifier to the computing device; receiving, from the computing device, a third request for a token to control one or more operations of the media playback system and one or more second credentials; and processing the third request, wherein processing the third request comprises: a) determining whether the one or more second credentials are valid, wherein determining whether the one or more second credentials are valid comprises determining whether the one or more second credentials comprise the second identifier; and b) responsive to determining that the one or more second credentials are valid, generating the token and transmitting the token to the computing device. 
     (Feature 2) The computing system of feature 1, wherein receiving the request for the first identifier comprises receiving a unique identifier associated with the media playback system and wherein generating the first identifier comprises generating the first identifier using the unique identifier associated with the media playback system. 
     (Feature 3) The computing system of any of features 1-2, wherein the first identifier has an expiration time after which the first identifier expires and wherein determining whether the one or more first credentials are valid further comprises: determining whether the first identifier has expired; and determining that the one or more first credentials are invalid responsive to determining that the first identifier has expired. 
     (Feature 4) The computing system of any of features 1-3, wherein determining whether the one or more second credentials are valid comprises: determining whether the one or more second credentials comprises the first identifier. 
     (Feature 5) The computing system of any of features 1-4, further comprising: receiving, from the media playback system, a fourth request for validation of one or more third credentials; determining whether the one or more third credentials are valid; and transmitting an indication of whether the one or more third credentials are valid. 
     (Feature 6) The computing system of feature 5, wherein the token has an expiration time after which the token expires, wherein the one or more third credentials comprises the token, and wherein determining whether the one or more third credentials are valid comprises: determining whether the token has expired; and determining that the one or more third credentials are invalid responsive to determining that the token has expired. 
     (Feature 7) The computing system of any of features 1-6, wherein generating the token comprises: identifying a set of permissions associated with the token; and storing the set of permissions associated with the token in the data storage. 
     (Feature 8) The computing system of feature 7, further comprising: receiving a fourth request for permission information associated with the token from the media playback system; and responsive to receiving the fourth request, transmitting the set of permissions associated with the token. 
     (Feature 9) The computing system of any of features 1-8, further comprising a plurality of servers and wherein the one or more processors comprises a plurality of processors distributed among the plurality of servers. 
     (Feature 10) The computing system of any of features 1-9, wherein the one or more operations comprises an operation selected from the group consisting of: playing media, pausing media, increasing volume, decreasing volume, adding media to a playlist, removing media from a playlist, reordering media in a playlist, and skipping media in a playlist. 
     (Feature 11) The computing system of any of features 1-10, wherein at least one member of the group consisting of: the first identifier, the second identifier, and the token comprises a sequence of characters and wherein the sequence of characters comprises at least one member selected from the group consisting of: a number, a letter, and a symbol. 
     (Feature 12) A method performed by a computing system, the method comprising: receiving, from a media playback system, a first request for a first identifier; processing the first request, wherein processing the first request comprises: a) generating the first identifier; and b) transmitting the first identifier to the media playback system; receiving, from a computing device (e.g., a control device), a second request for a second identifier and one or more first credentials; processing the second request, wherein processing the second request comprises: a) determining whether the one or more first credentials are valid, wherein determining whether the one or more first credentials are valid comprises determining whether the one or more first credentials comprise the first identifier; and b) responsive to determining that the one or more first credentials are valid, generating the second identifier and transmitting the second identifier to the computing device; receiving, from the computing device, a third request for a token to control one or more operations of the media playback system and one or more second credentials; and processing the third request, wherein processing the third request comprises: a) determining whether the one or more second credentials are valid, wherein determining whether the one or more credentials are valid comprises determining whether the one or more second credentials comprise the second identifier; and b) responsive to determining that the one or more second credentials are valid, generating the token and transmitting the token to the computing device. 
     (Feature 13) The method of feature 12, wherein the request for the first identifier comprises a unique identifier associated with the media playback system and wherein generating the first identifier comprises generating the first identifier using the unique identifier associated with the media playback system. 
     (Feature 14) The method of any of features 12-13, wherein the first identifier has an expiration time after which the first identifier expires and wherein determining whether the one or more first credentials are valid further comprises: determining whether the first identifier has expired; and determining that the one or more first credentials are invalid responsive to determining that the first identifier has expired. 
     (Feature 15) The method of any of features 12-14, wherein determining whether the one or more second credentials are valid comprises: determining whether the one or more second credentials comprises the first identifier. 
     (Feature 16) The method of any of features 12-15, further comprising: receiving, from the media playback system, a fourth request for validation of one or more third credentials; determining whether the one or more third credentials are valid; and transmitting an indication of whether the one or more third credentials are valid. 
     (Feature 17) The method of feature 16, wherein the token has an expiration time after which the token expires, wherein the one or more third credentials comprises the token, and wherein determining whether the one or more third credentials are valid comprises: determining whether the token has expired; and determining that the one or more third credentials are invalid responsive to determining that the token has expired. 
     (Feature 18) The method of any of features 12-17, wherein generating the token comprises: identifying a set of permissions associated with the token; and storing the set of permissions associated with the token in data storage. 
     (Feature 19) The method of feature 18, further comprising: receiving, from the media playback system, a fourth request for permission information associated with the token; and responsive to receiving the fourth request, transmitting the set of permitted operations associated with the token. 
     (Feature 20) A non-transitory computer readable medium having stored thereon instructions executable by one or more processors of a computing system to cause the one or more processors to perform functions comprising: receiving, from a media playback system, a first request for a first identifier; processing the first request, wherein processing the first request comprises: a) generating the first identifier; and b) transmitting the first identifier to the media playback system; receiving, from a computing device (e.g., a control device), a second request for a second identifier and one or more first credentials; processing the second request, wherein processing the second request comprises: a) determining whether the one or more first credentials are valid, wherein determining whether the one or more first credentials are valid comprises determining whether the one or more first credentials comprise the first identifier; and b) responsive to determining that the one or more first credentials are valid, generating the second identifier and transmitting the second identifier to the computing device; receiving, from the computing device, a third request for a token to control one or more operations of the media playback system and one or more second credentials; and processing the third request, wherein processing the third request comprises: a) determining whether the one or more second credentials are valid, wherein determining whether the one or more second credentials are valid comprises determining whether the one or more second credentials comprise the second identifier; and b) responsive to determining that the one or more second credentials are valid, generating the token and transmitting the token to the computing device.