[Simh] Klh10 vs Simh

[Peter Svensson]

Hi all,

There is a lot of incorrect information about the 802.11 capabilities 
going around on this list lately.

Normally 802.11 (all versions) uses 3 mac addresses instead of 4 to 
conserve capacity. This is purely a legacy issue from way back when. These 
addresses are used as follows.

- Station to AP:
   - AP MAC address (BSSID)
   - Station MAC address
   - 802.1 destination MAC address
   - (802.1 source address is copied from Station MAC field by the AP)

- AP to Station
   - Station MAC address
   - AP MAC address (BSSID)
   - 802.1 source MAC address (SA)
   - (802.1 dest address is copied from the Station MAC filed by the STA)

See e.g. 
http://www.wildpackets.com/resources/compendium/wireless_lan/wlan_packets#wp1009742 
for more details.

The two first addresses are used by the 802.11 MAC to properly associate 
the packet with a AP+STA association. Unfortunately the mapping from the 
802.11 MAC fields to 802.1 does not work if there are more than 1 802.1 
MAC source/desitnation addresses on the STA end.

There are several solutions / work arounds to allow more than one device 
behind a STA. Unfortunately the various solutions are not blessed with 
standardized names. In fact several share names to ensure maximum 
confusion.

The frames can carry an additional address field. This is the so called 
four-address-mode. Sometimes also known as WDS (but WDS is used for other 
modes as well). Four-address-mode is only defined in the standard when 
using mesh mode, but is supported also in non-mesh-mode in many 
drivers. See 
https://wireless.wiki.kernel.org/en/users/documentation/iw#using_4-address_for_ap_and_client_mode
This is a fully 802.1-compatible transport, with no loss of functionallity 
or information.

Other WDS modes apparently stick to the 3-address format, but "borrow" one 
for the fields. This seems deprecated.

Yet other "wireless bridging" modes use proxy arp + layer 2 NAT or other 
tricks that are layer 3 protocol specific.

Another way that relies only on client side functions and is relatively 
portable is to have the STA create multiple associations with the AP, 
using different STA addresses that match each of the devices behind the 
STA. This works well for a few STA-side 802.1 source MAC address. In 
effect the AP sees multiple STAs, each with their own MAC address that is 
the same for the STA and the (possibley virtual) device behind it.
See https://wireless.wiki.kernel.org/en/users/documentation/iw/vif for 
Linux and search for "Microsoft Virtual WiFi Miniport Adapter" for 
Windwos.

The best functions for simulations or other virtualization technologies 
are the 4-address format or multi-virtual-adapters.

The