How to handle multiple SSIDs with the same BSS

Marcel Holtmann marcel at holtmann.org
Wed Dec 23 17:24:53 EST 2009


Hi Witold,

> >>> BSS 00:1a:a1:53:50:a0 (on wlan0)
> >>> 	SSID: Private
> >>> BSS 00:1a:a1:53:50:a0 (on wlan0)
> >>> 	SSID: Public
> >>> When using the wpa_supplicant D-Bus interface, every BSS can only be
> >>> listed once and so we either see "Private" or "Public", but never
> >>> actually both of them. How should this be handled.
> >> I wasn't aware that there is this kind of limitation in the D-Bus
> >> interface. It is clearly broken if it cannot include both of those
> >> networks.
> > 
> > from what I have read so far while going through the code, it is not
> > possible. The BSSID is used as unique path in D-Bus terms and because of
> > that there is no way both networks could be listed at the same time.
> > 
> Precisely. Path of DBus object representing BSS is based on BSSID. Since
> the path must be unique we can't have both networks listed.
> I think that we can change a path format quite easy in the new DBus API,
> but I'm afraid that changes in the old API may may brake it.
> Actually, I was sure that BSSID is an unique identifier, but obviously
> we need to think out something else. Is the pair of BSSID and SSID
> unique? I'm not sure.

I think that BSSID + SSID should be enough. At least for now and we
could always make it more complicated later. Since it is only a string
in the end. And you should discover this string via D-Bus in the first
place and not guess it.

Since the SSID is a byte array, I would propose to convert the SSID into
a hex string representation and just append it to hex string you use for
the BSSID. This way we can support non-ASCII SSID and more important
don't have to do weird escaping because a D-Bus object path charset is
limited.

> >>> Also looking into the register_bss callbacks, you only get the BSS
> >>> address and not the set of IE.
> >> I don't know how this mechanism is supposed to be used, but there needs
> >> to be a way for fetching more information about the BSS and BSSID by
> >> itself is not a sufficient key for identifying which BSS is being
> >> queried for (need to have BSSID,SSID pair or some other internally
> >> generated identifier which can be guaranteed to be unique).
> > 
> > Does wpa_supplicant already generate such unique BSS based identifier
> > somewhere internally? If not, it might make sense to do so.
> 
> It doesn't AFAIK. We generate identifier for DBus usage only.

I still think we should add a generic function for this. It should
either generate a unique string and unique byte array for each BSS.

Regards

Marcel




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