The BlueZ towards a wireless world of penguins Marcel Holtmann BlueZ Project email@example.com Abstract Piconet 1 Piconet 2 S The Bluetooth wireless technology is getting more and more attention. There are a lot of devices available and most of them are working perfect with Linux, because Linux has the BlueZ. This is the codename of the official Bluetooth protocol stack for Linux. It is possible to use Bluetooth for simple cable free serial connections, dialup networks, TCP/IP networks, ISDN networks, human interface devices, printing, imaging, file transfers, contact and calendar synchronization etc. All these services are designed to integrate seamlessly into existing and established parts of Linux, like the kernel TTY layer, the network subsystem, the CUPS printing architecture, the OpenOBEX library and so on. 1 Introduction The Bluetooth technology was announced in May 1998 with the goal to create an easy usable cable replacement. Therefor it uses radio transmission within the 2.4 GHz ISM band to connect mobile devices like mobile phones, handhelds, notebooks, printer etc. from different manufactures without any cables. But Bluetooth is more than a simple cable replacement technology and with more and more devices using Bluetooth we see scenarios that are SB P P M S M S S P P S M S P SB = = = = Master Slave Parked Standby Figure 1: Bluetooth topology now making perfect sense. Examples for this are the communication of mobile phones with handhelds for exchanging contact and calendar information. Also the wireless printing of pictures without the interaction of a desktop computer. Many of these applications are also possible with other technologies like IrDA or IEEE 802.11 (WiFi), but Bluetooth make the use a lot easier and defines clear application profiles. The first steps into supporting Bluetooth with Linux are done by Axis Communications and they released their OpenBT Bluetooth Stack in April 1999. Also IBM released its BlueDrekar which was only available as binary modules. The problem of both stacks was that they are character device driven, but the Bluetooth technology is for connecting devices. So it is bet- • 239 • 240 • The BlueZ towards a wireless world of penguins ter to intergrate it into the Linux network layer and to use the socket interface as primary API. On May 3, 2001, the Bluetooth protocol stack called BlueZ which was written by Qualcomm was released under GPL. This new stack followed the socket based approach. One month later it was picked up by Linus Torvalds and integrated into the Linux 2.4.6-pre2 kernel. Another Bluetooth stack for Linux was released by Nokia Research Center in Helsinki and it is called Affix. The open source community already decided to support BlueZ as official Bluetooth protocol stack Linux and it became one of the best implementations of the Bluetooth specification. 2 Bluetooth architecture The Bluetooth architecture separates between three different core layers; hardware, host stack and applications. The hardware consists of radio, baseband and the link manager and this will be found in Bluetooth chips, dongles and notebooks. The control of the hardware is done via the host controller interface (HCI) and for the communication between the host stack and the Bluetooth hardware a hardware specific host transport driver is used. For the USB and UART transports it is possible to use general drivers, because these host transport are part of the Bluetooth specification. For PCMCIA or SDIO vendor specific driver are needed. BlueZ implements the host stack and also the applications. The lowest layer is the logical link control and adaptation protocol (L2CAP). This protocol uses segmentation and reassembly (SAR) and protocol and service multiplexing (PSM) to abstract from the Bluetooth packet types and low-level connection links. Starting with Bluetooth 1.2 this protocol layers was extended with retransmission and flow control (RFC). HID Netzwerke UDP TCP PIM Telefonie Verwaltung OBEX IP PPP AT-Modem CAPI HIDP BNEP CMTP RFCOMM TCS BIN Logical Link Control and Adaptation Protocol SDP Control Audio Host Controller Interface Link Manager Baseband Radio Figure 2: Bluetooth architecture All protocol layers above L2CAP are presenting the interaction with the applications. In general it is possible to use L2CAP directly (at least with Linux), but this is not specified within the Bluetooth specification. The real Bluetooth parts of the host stack are only the L2CAP layer and the adaption layer which integrates it into other subsystems or protocol suites, like TCP/IP and OBEX. 3 Design of BlueZ The main components of BlueZ are integrated into the Linux kernel as part of the network subsystem. It provides its own protocol family and uses the socket interface. This basic design makes it easy for application to adapt the Bluetooth technology and the integration is simple and straight forward. The use of different Bluetooth hardware is handled by the hardware abstraction inside the kernel. The BlueZ core supports the usage of 16 Bluetooth adapters at the same time. The list of supported devices is growing every day and currently over 300 different working adapters are known. Besides the BlueZ core and the hardware abstraction also the L2CAP layer is running inside the kernel. It provides a socket interface 2005 Linux Symposium • 241 Bluetooth Core Layer bcm203x bfusb hci_usb hci_uart dtl1_cs USB Subsystem bluecard_cs bt3c_cs PCMCIA Subsystem Serial Subsystem ery protocol (SDP) or the object exchange protocol (OBEX). Some of them are also directly integrated into applications. For example the hardcopy cable replacement protocol (HCRP) and the audio video distribution transport protocol (AVDTP). Almost every Linux distribution contains a Bluetooth enabled kernel and a decent version of the BlueZ library and the BlueZ utilities. Figure 3: BlueZ core with sequential packet characteristics and in future the RFC feature will add the stream interface. 4 With RFCOMM it is possible to emulate terminal devices. It is called cable replacement for legacy applications. With BlueZ it is possible to access this protocol layer from two levels. One is again a socket interface and this time with stream characteristics and the second is through the Linux TTY layer. RFCOMM emulates a full serial port and it is possible to use the point-to-point protocol (PPP) over it to create dialup or network connections. The Bluetooth network encapsulation protocol (BNEP), the CAPI message transport protocol (CMTP) and the human interface device protocol (HIDP) are transport protocols for the network layer, the CAPI subsystem and the HID driver. Their main job is to shrink the protocol overhead and keep the latency low. All of these protocols are implemented inside the kernel. Other Bluetooth protocols are implemented as libraries, like the service discovSerial Subsystem CAPI Subsystem Network Subsystem Input Subsystem HID Driver SOCK_STREAM SOCK_SEQPACKET RFCOMM CMTP BNEP HIDP L2CAP Layer Bluetooth Core Layer Figure 4: BlueZ protocols SCO Audio Bluetooth configuration After plugging in a Bluetooth USB dongle or inserting a Bluetooth PCMCIA card a call to hciconfig will show the new device. This device is still unconfigured and like a network card it needs to be activated first. This can be done via hciconfig hci0 up or in the background by hcid. The detailed output shows the Bluetooth device address (BD_ADDR) and additional information like name, class of device and manufacturer specific details. With hciconfig all of these settings can be changed. Now it is possible to scan for other Bluetooth devices in range. For this and some other actions hcitool is used. # hciconfig -a hci0: Type: USB BD Address: 00:02:5B:01:66:F5 ACL MTU: 384:8 SCO MTU: 64:8 UP RUNNING PSCAN ISCAN RX bytes:3217853 acl:79756 sco:0 events:199989 errors:0 TX bytes:77188889 acl:294284 sco:0 commands:206 errors:0 Features: 0xff 0xff 0x8f 0xfe 0x9b 0xf9 0x00 0x80 Packet type: DM1 DM3 DM5 DH1 DH3 DH5 HV1 HV2 HV3 Link policy: RSWITCH HOLD SNIFF PARK Link mode: SLAVE ACCEPT Name: ’Casira BlueCore4 module’ Class: 0x3e0100 Service Classes: Networking, Rendering, Capturing Device Class: Computer, Laptop HCI Ver: 2.0 (0x3) HCI Rev: 0x77b LMP Ver: 2.0 (0x3) LMP Subver: 0x77b Manufacturer: Cambridge Silicon Radio (10) Figure 5: Local Bluetooth adapter 242 • The BlueZ towards a wireless world of penguins # hcitool scan Scanning ... 00:80:37:25:55:96 00:E0:03:04:6D:36 00:90:02:63:E0:83 00:06:C6:C4:08:27 00:04:0E:21:06:FD 00:0A:95:98:37:18 00:A0:57:AD:22:0F 00:80:37:06:78:92 00:01:EC:3A:45:86 Pico Plug Nokia 6210 Bluetooth Printer Anycom LAP 00:06:C6:C4:08:27 Bluetooth ISDN Access Point Apple Wireless Keyboard ELSA Vianect Blue ISDN Ericsson T39m HBH-10 Figure 6: Scanning for Bluetooth devices After the scan the program sdptool can be used to retrieve the available services of a remote Bluetooth device. The service list identifies the supported Bluetooth profiles and reveals protocol specific information that are used by other tools. The example shows a dialup networking service and a fax service which both are using the RFCOMM channel 1. # sdptool browse 00:E0:03:04:6D:36 Browsing 00:E0:03:04:6D:36 ... Service Name: Dial-up networking Service RecHandle: 0x10000 Service Class ID List: "Dialup Networking" (0x1103) "Generic Networking" (0x1201) Protocol Descriptor List: "L2CAP" (0x0100) "RFCOMM" (0x0003) Channel: 1 Profile Descriptor List: "Dialup Networking" (0x1103) Version: 0x0100 Service Name: Fax Service RecHandle: 0x10001 Service Class ID List: "Fax" (0x1111) "Generic Telephony" (0x1204) Protocol Descriptor List: "L2CAP" (0x0100) "RFCOMM" (0x0003) Channel: 1 Profile Descriptor List: "Fax" (0x1111) Version: 0x0100 Figure 7: Requesting service information Some tools have integrated SDP browsing support and will determine the needed service information by themself. Others don’t have this capability, because it is not always useful. Every SDP request involves the creation of a piconet and this can fail or timeout. So for all Bluetooth tools running at boot time this is not a desired behavior. With the Bluetooth device address and the channel number it is possible to setup a RFCOMM TTY terminal connection for using AT commands or PPP for Internet access. The command rfcomm bind 0 00:E0: 03:04:6D:36 1 creates the device /dev/ rfcomm0 which is connected to the RFCOMM channel 1 on the mobile phone with the Bluetooth address 00:E0:03:04:6D: 36. The connection itself is not created by this command. It will first established when an application, like pppd, opens this device node and terminated when the last user closes it. 5 Bluetooth networks For creating network connection over Bluetooth the preferable method is using a personal area network (PAN) with BNEP. The old method was called LAN access using PPP and it used PPP over RFCOMM. This was a bad decision for the performance and now this profile is deprecated. A PAN connection can be created with the command pand --connect 00:06:C6:C4:08:27 and after a successful connect ifconfig will show a bnep0 device with the same MAC address as the BD_ADDR of the remote device. This network device is a virtual network card, # ifconfig -a bnep0 Link encap:Ethernet HWaddr 00:06:C6:C4:08:27 BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 RX bytes:4 (4.0 b) TX bytes:0 (0.0 b) Figure 8: Bluetooth network device 2005 Linux Symposium • 243 but not limited in its functionality. It is possible to use all Ethernet related commands on it and besides IPv4 and IPv6 it can also used inside an IPX network. Even methods like bridging or network address translation (NAT) are working without any problems. Input Subsystem HIDP Module Another possibility to create network connections is using ISDN and the CAPI subsystem. The Bluetooth part is called common ISDN profile (CIP) and it uses CMTP. Once a connection to an ISDN access point with ciptool connect 00:04:0E:21:06:FD has been created, a virtual ISDN card will be presented by the CAPI subsystem and the standard tools can be used. Parser Boot Report HID Protocol L2CAP Layer Figure 9: HID architecture 6 Printing over Bluetooth For accessing printers over Bluetooth it is possible to do this via RFCOMM or HCRP. The Bluetooth CUPS backend supports both methods and is able to choose the best one by itself. The setup of a Bluetooth printer is very easy. The only thing that it is needed is an URI and this is created from its BD_ADDR by removing the colons. For accessing a printer with the Bluetooth device address 00:90:02:63:E0:83 the URI bluetooth://00900263E083/ should be given to CUPS. 7 Bluetooth input devices With the human interface device specification for Bluetooth it is also possible to use wireless mice and keyboards. Since HID devices can disconnect and reconnect at any time it is necessary to run hidd --server to handle such events. To bind a mouse or keyboard to your system it is only needed to contact it once. This initial connection can be done with the command hidd --connect 00:0A:95: 98:37:18. All further connects are initiated by the device. 8 Bluetooth audio The Bluetooth technology can be used for data communication, but it also support audio connections. For example headsets for voice connection and headphones for high-quality stereo transmission. For both device types an integration into the ALSA sound system is planned. Like all other subsystem or library integrations done by BlueZ so far, this will be almost invisible for the end user. First beta versions of the ALSA plugins exists by now and a final version is expected very soon. 244 • The BlueZ towards a wireless world of penguins 9 Other applications Many more applications started to integrate native Bluetooth support. The popular examples are the contact and calendar synchronization program MultiSync and the Gnokii tool for accessing Nokia mobile phones. Both programs can also use IrDA or cable connections as transport and Bluetooth is only another access method. In most programs the lines of Bluetooth specific code are very small, but the mobility increases a lot. 10 Conclusion Since 2001 a lot of things have been improved and the current Bluetooth subsystem is ready for every day usage. But the development is not finished and the end user experience can be still be improved. The GNOME Bluetooth subsystem and the KDE Bluetooth framework are two projects to integrate Bluetooth into the desktop. With Bluetooth the need of cables is decreasing and BlueZ tries to paint the Linux world blue. References  Special Interest Group Bluetooth: Bluetooth Core Specification v1.2, 2003.  Special Interest Group Bluetooth: Bluetooth Network Encapsulation Protocol Specification, 2003.  Special Interest Group Bluetooth: Common ISDN Access Profile, 2002.  Special Interest Group Bluetooth: Human Interface Device Profile, 2003.  Infrared Data Association (IrDA): Object Exchange Protocol OBEX, Version 1.3, 2003. Proceedings of the Linux Symposium Volume One July 20nd–23th, 2005 Ottawa, Ontario Canada Conference Organizers Andrew J. Hutton, Steamballoon, Inc. C. Craig Ross, Linux Symposium Stephanie Donovan, Linux Symposium Review Committee Gerrit Huizenga, IBM Matthew Wilcox, HP Dirk Hohndel, Intel Val Henson, Sun Microsystems Jamal Hadi Salimi, Znyx Matt Domsch, Dell Andrew Hutton, Steamballoon, Inc. Proceedings Formatting Team John W. Lockhart, Red Hat, Inc. Authors retain copyright to all submitted papers, but have granted unlimited redistribution rights to all as a condition of submission.