Linux Ethernet Bridging - Jan 2017 - [Bridge] [PATCH net-next] bridge: multicast to unicast

Am 09.01.2017 um 09:08 schrieb Johannes Berg:
> Does it make sense to implement the two in separate layers though?
> 
> Clearly, this part needs to be implemented in the bridge layer due to
> the snooping knowledge, but the code is very similar to what mac80211
> has now.
Does the bridge always know about all stations connected?

That is bridge fdb entries (need to) expire so the bridge might
"forget"
a still-connected station not sending but only consuming broadcast traffic.

E.g. there is a television broadcast station here that receives a video
stream (via wifi, udp packets) and then airs it (dvb-t) but (on its own)
would not send any data packet on wifi (static ip, etc.).

An other reason to implement this in mac80211 initially was that
mac80211 could encapsulate broacast/multicast ethernet packtes in
unicast A-MSDU packets in a way, so that the receiver would still see
process ethernet packets (after conversion) but have unicast wifi
frames. This cannot be done in bridge easily but one might want to add
this later to mac80211.

Michael

On Mon, 2017-01-09 at 12:44 +0100, M. Braun wrote:
> Am 09.01.2017 um 09:08 schrieb Johannes Berg:
> > Does it make sense to implement the two in separate layers though?
> > 
> > Clearly, this part needs to be implemented in the bridge layer due
> > to
> > the snooping knowledge, but the code is very similar to what
> > mac80211
> > has now.
> 
> Does the bridge always know about all stations connected?
> 
> That is bridge fdb entries (need to) expire so the bridge might
> "forget" a still-connected station not sending but only consuming
> broadcast traffic.
> 
> E.g. there is a television broadcast station here that receives a
> video stream (via wifi, udp packets) and then airs it (dvb-t) but (on
> its own) would not send any data packet on wifi (static ip, etc.).
Ok, that I don't know. Somehow if you address a unicast packet there
the bridge has to make a decision - so it really should know? Would it
query the port somehow to see if the device is behind it, if getting a
packet for a station it forgot about?
> An other reason to implement this in mac80211 initially was that
> mac80211 could encapsulate broacast/multicast ethernet packtes in
> unicast A-MSDU packets in a way, so that the receiver would still see
> process ethernet packets (after conversion) but have unicast wifi
> frames. This cannot be done in bridge easily but one might want to
> add this later to mac80211.
Yes, DMG would have to be done in mac80211, but that's a lot clearer
case too since it requires negotiation functionality etc.

johannes

On Mon, Jan 09, 2017 at 12:44:19PM +0100, M. Braun
wrote:
> Am 09.01.2017 um 09:08 schrieb Johannes Berg:
> > Does it make sense to implement the two in separate layers though?
> > 
> > Clearly, this part needs to be implemented in the bridge layer due to
> > the snooping knowledge, but the code is very similar to what mac80211
> > has now.
> 
> Does the bridge always know about all stations connected?
The bridge does not always know about all stations, especially the
silent ones like in your DVB-T example.

However, concerning IP multicast, there is IGMP/MLD. So the bridge
does know about all stations which are interested in a specific IP
multicast stream.

(As long as there is a querier on the link, which periodically
queriers for IGMP/MLD reports from any listener. If there is no
querier then the bridge multicast snooping, including the bridge
multicast-to-unicast will fall back to flooding)


So if your television example uses IP multicast properly, it is
completely doable with the bridge multicast-to-unicast, thanks to
IGMP/MLD.