Setting the SSID for point-to-point links

If you operate WPA-PSK, the PMK that is used for key generation is derived from the configured passphrase and the SSID (WLAN network name).

In the simplest case of a client connecting to an access point, it is clear that the applicable SSID is the one broadcast by the access point in its beacons. In the case of a point-to-point link where two access points intercommunicate, they each transmit beacons with their respective SSID. For this reason it is important to regulate which SSID is used to derive the PMK SSID, in order for both sides to arrive at the same PMK.

The basic rule is that the SSID of the master access point is used to derive the PMK for the P2P link. As long as the SSID broadcast is not suppressed on the master, the slave can read this and apply it.

However, if the master suppresses the broadcasting of the SSID in its beacons, then the slave assumes that the SSID of the master is identical to its own. If this is indeed the case, then the configuration must reflect this, otherwise no connection will be made. An SSID mismatch can be detected in the EAP trace, in that repeated 'MIC failure' messages will appear even though the passphrase is configured identically at both ends.

All versions of LCOS up to and including 8.8x are configured to use the SSID 'P2P-SSID' on the first logical network (WLAN-1 or WLAN-2 in the logical network settings). If SSID broadcasting is suppressed on the master, then the first logical network on the slave needs to be configured to have the same network name as that on the master.

With LCOS 8.90, the P2P link configuration was separated from the first logical network. These LCOS versions use the fixed setting '***P2P Info***' for the SSID, and this cannot be suppressed in the beacons. The rule still applies that the SSID of the master is used to derive the PMK for a P2P link, i.e. a slave with the newer version of LCOS continues to take the SSID from a master with an older version of LCOS—as long as this is not suppressed on the master.

Since it was formerly impossible to suppress the SSID '***P2P Info***', LCOS 9.10 and later allow this parameter to be configured.

As of LCOS 9.10 the new configuration item Network Name was introduced in the WLAN interpoint settings (LCOS menu tree → Setup → Interfaces → WLAN → Interpoint settings → <WLAN module>):

1) If the field Network Name is left blank (default configuration), the value ***P2P Info*** (or the SSID of the master) continues to be used for the PMK derivation, even though the broadcasting of this SSID is suppressed.

2) If an SSID is entered into the field Network Name, this is broadcast and is used to derive the PMK (unless the master dictates a different SSID).

For most practical scenarios the default configuration is reasonable and adequate, with the following exception:

If a device operating as a master with LCOS 9.10 is combined with a device with LCOS 8.8x or older operating as a slave, then there are two options: Either the master is explicitly set with an SSID entered into the new field, which is taken by the slave; or the slave needs the SSID of the first logical network to be set to the value ***P2P Info***.

The latter solution has the advantage that the P2P 'pseudo SSID' remains invisible to clients, although one disadvantage is that the first logical network on the slave no longer offers reasonable connections for clients.