[Ns-developers] bridging and wifi

[Mathieu Lacage]

On Mon, 2008-08-25 at 21:48 +0100, Gustavo Carneiro wrote:

 
>         For simplicity, let's say that wifi network is adhoc.
> 
> This is an assumption with serious implications.  I don't think this
> would ever work, conceptually, with wifi in adhoc mode because adhoc
> mode frames only use two addresses.  It has to be in infrastructure
> mode, because then frames have 3 addresses.

ok


>         At this point, I got a bit stomped and went back to some
>         serious wifi
>         spec reading and thinking but I still don't really know how
>         that is
>         expected to work. Gustavo, do you have any idea ?
> 
> I think I have an idea.
> 
> As I mentioned before, wifi has to be in infrastructure mode, adhoc
> won't work (incidentally trying to do bridging in Linux with cards in
> adhoc mode is a mistake I often used to make in the past).
> 
> The 802.11 frame has a total of four addresses, which have the
> following meaning in infrastructure mode (this table comes from
> somewhere deep in the 802.11 standard):
> 
> 
>   /*
>    *  To DS    From DS   Address 1    Address 2    Address 3
> Address 4 
> 
> *----------------------------------------------------------------------
>    *    0        0       Destination  Source        BSSID         N/A
>    *    0        1       Destination  BSSID         Source        N/A
>    *    1        0       BSSID        Source        Destination   N/A
>    *    1        1       Receiver     Transmitter   Destination
> Source
>    */
> 
> The ARP response, as transmitted by node 3, is of the type ToDS=1,
> FromDS=0, therefore Address 1 is the BSSID (AP netdevice MAC), Address
> 2 is mac3, Address 3 is mac1, and Address 4 is not used.  The Access
> Point (node2) will receive a frame whose BSSID is itself and so will
> send a Wifi ACK back to the source (Address 2 == mac3).

I more or less reached the same kind of conclusion here but, what
bothers me is that if I make node 3 be the AP and node 2 be a STA, it
won't work, right ?

> At this point, the promiscuous receive callback in node2 will send
> back to the BridgeNetDevice the original source and destination, i.e.
> Address 2 and 3.  Likewise, when the AP (node2) sends a frame being
> bridged from node 1 to node 3, via the new SendFrom API, it should
> consider the ToDS=0, FromDS=1 line in the table above, and so in this
> example Address1=mac3, Address2=mac2, Address3=mac1.