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authorJouni Malinen <j@w1.fi>2009-11-28 19:19:48 (GMT)
committerJouni Malinen <j@w1.fi>2009-11-28 19:19:48 (GMT)
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+/**
+\page driver_wrapper Driver wrapper implementation (driver.h, drivers.c)
+
+All hardware and driver dependent functionality is in separate C files
+that implement defined wrapper functions. Other parts
+of the %wpa_supplicant are designed to be hardware, driver, and operating
+system independent.
+
+Driver wrappers need to implement whatever calls are used in the
+target operating system/driver for controlling wireless LAN
+devices. As an example, in case of Linux, these are mostly some glue
+code and ioctl() calls and netlink message parsing for Linux Wireless
+Extensions (WE). Since features required for WPA were added only recently to
+Linux Wireless Extensions (in version 18), some driver specific code is used
+in number of driver interface implementations. These driver dependent parts
+can be replaced with generic code in driver_wext.c once the target driver
+includes full support for WE-18. After that, all Linux drivers, at
+least in theory, could use the same driver wrapper code.
+
+A driver wrapper needs to implement some or all of the functions
+defined in driver.h. These functions are registered by filling struct
+wpa_driver_ops with function pointers. Hardware independent parts of
+%wpa_supplicant will call these functions to control the driver/wlan
+card. In addition, support for driver events is required. The event
+callback function, wpa_supplicant_event(), and its parameters are
+documented in driver.h. In addition, a pointer to the 'struct
+wpa_driver_ops' needs to be registered in drivers.c file.
+
+When porting to other operating systems, the driver wrapper should be
+modified to use the native interface of the target OS. It is possible
+that some extra requirements for the interface between the driver
+wrapper and generic %wpa_supplicant code are discovered during porting
+to a new operating system. These will be addressed on case by case
+basis by modifying the interface and updating the other driver
+wrappers for this. The goal is to avoid changing this interface
+without very good reasons in order to limit the number of changes
+needed to other wrappers and hardware independent parts of
+%wpa_supplicant. When changes are required, recommended way is to
+make them in backwards compatible way that allows existing driver
+interface implementations to be compiled without any modification.
+
+Generic Linux Wireless Extensions functions are implemented in
+driver_wext.c. All Linux driver wrappers can use these when the kernel
+driver supports the generic ioctl()s and wireless events. Driver
+specific functions are implemented in separate C files, e.g.,
+driver_hostap.c. These files need to define struct wpa_driver_ops
+entry that will be used in wpa_supplicant.c when calling driver
+functions. struct wpa_driver_ops entries are registered in drivers.c.
+
+In general, it is likely to be useful to first take a look at couple
+of driver interface examples before starting on implementing a new
+one. driver_hostap.c and driver_wext.c include a complete
+implementation for Linux drivers that use %wpa_supplicant-based control
+of WPA IE and roaming. driver_ndis.c (with help from driver_ndis_.c)
+is an example of a complete interface for Windows NDIS interface for
+drivers that generate WPA IE themselves and decide when to roam. These
+example implementations include full support for all security modes.
+
+
+\section driver_req Driver requirements for WPA
+
+WPA introduces new requirements for the device driver. At least some
+of these need to be implemented in order to provide enough support for
+%wpa_supplicant.
+
+\subsection driver_tkip_ccmp TKIP/CCMP
+
+WPA requires that the pairwise cipher suite (encryption algorithm for
+unicast data packets) is TKIP or CCMP. These are new encryption
+protocols and thus, the driver will need to be modified to support
+them. Depending on the used wlan hardware, some parts of these may be
+implemented by the hardware/firmware.
+
+Specification for both TKIP and CCMP is available from IEEE (IEEE
+802.11i amendment). Fully functional, hardware independent
+implementation of both encryption protocols is also available in Host
+AP driver (driver/modules/hostap_{tkip,ccmp}.c). In addition, Linux 2.6
+kernel tree has generic implementations for WEP, TKIP, and CCMP that can
+be used in Linux drivers.
+
+The driver will also need to provide configuration mechanism to allow
+user space programs to configure TKIP and CCMP. Linux Wireless Extensions
+v18 added support for configuring these algorithms and
+individual/non-default keys. If the target kernel does not include WE-18,
+private ioctls can be used to provide similar functionality.
+
+\subsection driver_roaming Roaming control and scanning support
+
+%wpa_supplicant can optionally control AP selection based on the
+information received from Beacon and/or Probe Response frames
+(ap_scan=1 mode in configuration). This means that the driver should
+support external control for scan process. In case of Linux, use of
+new Wireless Extensions scan support (i.e., 'iwlist wlan0 scan') is
+recommended. The current driver wrapper (driver_wext.c) uses this for
+scan results.
+
+Scan results must also include the WPA information element. Support for
+this was added in WE-18. With older versions, a custom event can be used
+to provide the full WPA IE (including element id and length) as a hex
+string that is included in the scan results.
+
+%wpa_supplicant needs to also be able to request the driver to
+associate with a specific BSS. Current Host AP driver and matching
+driver_hostap.c wrapper uses following sequence for this
+request. Similar/identical mechanism should be usable also with other
+drivers.
+
+- set WPA IE for AssocReq with private ioctl
+- set SSID with SIOCSIWESSID
+- set channel/frequency with SIOCSIWFREQ
+- set BSSID with SIOCSIWAP
+ (this last ioctl will trigger the driver to request association)
+
+\subsection driver_wpa_ie WPA IE generation
+
+%wpa_supplicant selects which cipher suites and key management suites
+are used. Based on this information, it generates a WPA IE. This is
+provided to the driver interface in the associate call. This does not
+match with Windows NDIS drivers which generate the WPA IE
+themselves.
+
+%wpa_supplicant allows Windows NDIS-like behavior by providing the
+selected cipher and key management suites in the associate call. If
+the driver generates its own WPA IE and that differs from the one
+generated by %wpa_supplicant, the driver has to inform %wpa_supplicant
+about the used WPA IE (i.e., the one it used in (Re)Associate
+Request). This notification is done using EVENT_ASSOCINFO event (see
+driver.h). %wpa_supplicant is normally configured to use
+ap_scan=2 mode with drivers that control WPA IE generation and roaming.
+
+\subsection driver_events Driver events
+
+%wpa_supplicant needs to receive event callbacks when certain events
+occur (association, disassociation, Michael MIC failure, scan results
+available, PMKSA caching candidate). These events and the callback
+details are defined in driver.h (wpa_supplicant_event() function
+and enum wpa_event_type).
+
+On Linux, association and disassociation can use existing Wireless
+Extensions event that is reporting new AP with SIOCGIWAP
+event. Similarly, completion of a scan can be reported with SIOCGIWSCAN
+event.
+
+Michael MIC failure event was added in WE-18. Older versions of Wireless
+Extensions will need to use a custom event. Host AP driver used a custom
+event with following contents: MLME-MICHAELMICFAILURE.indication(keyid=#
+broadcast/unicast addr=addr2). This is the recommended format until
+the driver can be moved to use WE-18 mechanism.
+
+\subsection driver_wext_summary Summary of Linux Wireless Extensions use
+
+AP selection depends on ap_scan configuration:
+
+ap_scan=1:
+
+- %wpa_supplicant requests scan with SIOCSIWSCAN
+- driver reports scan complete with wireless event SIOCGIWSCAN
+- %wpa_supplicant reads scan results with SIOCGIWSCAN (multiple call if
+ a larget buffer is needed)
+- %wpa_supplicant decides which AP to use based on scan results
+- %wpa_supplicant configures driver to associate with the selected BSS
+ (SIOCSIWMODE, SIOCSIWGENIE, SIOCSIWAUTH, SIOCSIWFREQ,
+ SIOCSIWESSID, SIOCSIWAP)
+
+ap_scan=2:
+
+- %wpa_supplicant configures driver to associate with an SSID
+ (SIOCSIWMODE, SIOCSIWGENIE, SIOCSIWAUTH, SIOCSIWESSID)
+
+
+After this, both modes use similar steps:
+
+- optionally (or required for drivers that generate WPA/RSN IE for
+ (Re)AssocReq), driver reports association parameters (AssocReq IEs)
+ with wireless event IWEVASSOCREQIE (and optionally IWEVASSOCRESPIE)
+- driver reports association with wireless event SIOCGIWAP
+- %wpa_supplicant takes care of EAPOL frame handling (validating
+ information from associnfo and if needed, from scan results if WPA/RSN
+ IE from the Beacon frame is not reported through associnfo)
+*/