hostapd: Add README-MULTI-AP

Document what hostapd and wpa_supplicant do for Multi-AP.

This is only included in hostapd, since a Multi-AP device is always an
access point so it should have hostapd.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

diff --git a/hostapd/README-MULTI-AP b/hostapd/README-MULTI-AP
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+hostapd, wpa_supplicant and the Multi-AP Specification
+======================================================
+
+This document describes how hostapd and wpa_supplicant can be configured to
+support the Multi-AP Specification.
+
+Introduction to Multi-AP
+------------------------
+
+The Wi-Fi Alliance Multi-AP Specification is the technical specification for
+Wi-Fi CERTIFIED EasyMesh(TM) [1], the Wi-Fi Alliance® certification program for
+Multi-AP. It defines control protocols between Wi-Fi® access points (APs) to
+join them into a network with centralized control and operation. It is targeted
+only at routers (repeaters, gateways, ...), not at clients. Clients are not
+involved at all in the protocols.
+
+Most of the Multi-AP specification falls outside of the scope of
+hostapd/wpa_supplicant. hostapd/wpa_supplicant is only involved for the items
+summarized below. The rest of the protocol must be implemented by a separate
+daemon, e.g., prplMesh [2]. That daemon also needs to communicate with hostapd,
+e.g., to get a list of associated clients, but this can be done using the normal
+hostapd interfaces.
+
+hostapd/wpa_supplicant needs to be configured specifically to support:
+- the WPS onboarding process;
+- configuring backhaul links.
+
+The text below refers to "Multi-AP Specification v1.0" [3].
+
+
+Fronthaul and backhaul links
+----------------------------
+
+In a Multi-AP network, the central controller can configure the BSSs on the
+devices that are joined into the network. These are called fronthaul BSSs.
+From the point of view of hostapd, there is nothing special about these
+fronthaul BSSs.
+
+In addition to fronthaul BSSs, the controller can also configure backhaul
+links. A backhaul link is a link between two access point devices, giving
+internet access to access point devices that don't have a wired link. The
+Multi-AP specification doesn't dictate this, but typically the backhaul link
+will be bridged into a LAN together with (one of) the fronthaul BSS(s) and the
+wired Ethernet ports.
+
+A backhaul link must be treated specially by hostapd and wpa_supplicant. One
+side of the backhaul link is configured through the Multi-AP protocol as the
+"backhaul STA", i.e., the client side of the link. A backhaul STA is like any
+station and is handled appropriately by wpa_supplicant, but two additional
+features are required. It must send an additional information element in each
+(Re)Association Request frame ([3], section 5.2, paragraph 4). In addition, it
+must use 4-address mode for all frames sent over this link ([3], section 14).
+Therefore, wpa_supplicant must be configured explicitly as the backhaul STA
+role, by setting 'multi_ap_backhaul_sta=1' in the network configuration block
+or when configuring the network profile through the control interface. When
+'multi_ap_backhaul_sta=1', wpa_supplicant includes the Multi-AP IE in
+(Re)Association Request frame and verifies that it is included in the
+(Re)Association Response frame. If it is not, association fails. If it is,
+wpa_supplicant sets 4-address mode for this interface through a driver
+callback.
+
+The AP side of the backhaul link is called a "backhaul BSS". Such a BSS must
+be handled specially by hostapd, because it must add an additional information
+element in each (Re)Association Response frame, but only to stations that have
+identified themselves as backhaul stations ([3], section 5.2, paragraph 5-6).
+This is important because it is possible to use the same BSS and SSID for
+fronthaul and backhaul at the same time. The additional information element must
+only be used for frames sent to a backhaul STA, not to a normal STA. Also,
+frames sent to a backhaul STA must use 4-address mode, while frames sent to a
+normal STA (fronthaul, when it's a fronthaul and backhaul BSS) must use
+3-address mode.
+
+A BSS is configured in Multi-AP mode in hostapd by setting the 'multi_ap'
+configuration option to 1 (backhaul BSS), 2 (fronthaul BSS), or 3
+(simultaneous backhaul and fronthaul BSS). If this option is set, hostapd
+parses the Multi-AP information element in the Association Request frame. If the
+station is a backhaul STA and the BSS is configured as a backhaul BSS,
+hostapd sets up 4-address mode. Since there may be multiple stations connected
+simultaneously, and each of them has a different RA (receiver address), a VLAN
+is created for each backhaul STA and it is automatically added to a bridge.
+This is the same behavior as for WDS, and the relevant option ('bridge' or
+'wds_bridge') applies here as well.
+
+If 'multi_ap' is 1 (backhaul BSS only), any station that tries to associate
+without the Multi-AP information element will be denied.
+
+If 'multi_ap' is 2 (fronthaul BSS only), any station that tries to associate
+with the Multi-AP information element will be denied. That is also the only
+difference with 'multi_ap' set to 0: in the latter case, the Multi-AP
+information element is simply ignored.
+
+In summary, this is the end-to-end behavior for a backhaul BSS (i.e.,
+multi_ap_backhaul_sta=1 in wpa_supplicant on STA, and multi_ap=1 or 3 in
+hostapd on AP). Note that point 1 means that hostapd must not be configured
+with WPS support on the backhaul BSS (multi_ap=1). hostapd does not check for
+that.
+
+1. Backhaul BSS beacons do not advertise WPS support (other than that, nothing
+   Multi-AP specific).
+2. STA sends Authentication frame (nothing Multi-AP specific).
+3. AP sends Authentication frame (nothing Multi-AP specific).
+4. STA sends Association Request frame with Multi-AP IE.
+5. AP sends Association Response frame with Multi-AP IE.
+6. STA and AP both use 4-address mode for Data frames.
+
+
+WPS support
+-----------
+
+WPS requires more special handling. WPS must only be advertised on fronthaul
+BSSs, not on backhaul BSSs, so WPS should not be enabled on a backhaul-only
+BSS in hostapd.conf. The WPS configuration purely works on the fronthaul BSS.
+When a WPS M1 message has an additional subelement that indicates a request for
+a Multi-AP backhaul link, hostapd must not respond with the normal fronthaul
+BSS credentials; instead, it should respond with the (potentially different)
+backhaul BSS credentials.
+
+To support this, hostapd has the 'multi_ap_backhaul_ssid',
+'multi_ap_backhaul_wpa_psk' and 'multi_ap_backhaul_wpa_passphrase' options.
+When these are set on an BSS with WPS, they are used instead of the normal
+credentials when hostapd receives a WPS M1 message with the Multi-AP IE. Only
+WPA2-Personal is supported in the Multi-AP specification, so there is no need
+to specify authentication or encryption options. For the backhaul credentials,
+per-device PSK is not supported.
+
+If the BSS is a simultaneous backhaul and fronthaul BSS, there is no need to
+specify the backhaul credentials, since the backhaul and fronthaul credentials
+are identical.
+
+To enable the Multi-AP backhaul STA feature when it performs WPS, a new
+parameter has been introduced to the WPS_PBC control interface call. When this
+"multi_ap=1" option is set, it adds the Multi-AP backhaul subelement to the
+Association Request frame and the M1 message. It then configures the new network
+profile with 'multi_ap_backhaul_sta=1'. Note that this means that if the AP does
+not follow the Multi-AP specification, wpa_supplicant will fail to associate.
+
+In summary, this is the end-to-end behavior for WPS of a backhaul link (i.e.,
+multi_ap=1 option is given in the wps_pbc call on the STA side, and multi_ap=2
+and multi_ap_backhaul_ssid and either multi_ap_backhaul_wpa_psk or
+multi_ap_backhaul_wpa_passphrase are set to the credentials of a backhaul BSS
+in hostapd on Registrar AP).
+
+1. Fronthaul BSS Beacon frames advertise WPS support (nothing Multi-AP
+   specific).
+2. Enrollee sends Authentication frame (nothing Multi-AP specific).
+3. AP sends Authentication frame (nothing Multi-AP specific).
+4. Enrollee sends Association Request frame with Multi-AP IE.
+5. AP sends Association Response frame with Multi-AP IE.
+6. Enrollee sends M1 with additional Multi-AP subelement.
+7. AP sends M8 with backhaul instead of fronthaul credentials.
+8. Enrollee sends Deauthentication frame.
+
+
+References
+----------
+
+[1] https://www.wi-fi.org/discover-wi-fi/wi-fi-easymesh
+[2] https://github.com/prplfoundation/prplMesh
+[3] https://www.wi-fi.org/file/multi-ap-specification-v10
+    (requires registration)