KNX/EIB KNXNet/IP multicast

[fma]

Hi!

Sorry to bother your with english...

I'm working on a python KNX framework, and I need some help about group address communication.

As far as I understand, group address com. is done using multicast on IP side; am I right ?

I found that IP is 224.0.23.12 and port is 3671. But when I try to join this address, I can't see any frame. Even tcpdump does not see anything.

What am I doing wrong?

Thanks for you help!

PS: I'm using a Siemens N 148-21, and if I ping the above mutlicast address, the gateway answers...

[mmartin]

Hi,

groupaddress communication is not done via multicast. It is done via unicast. The multicast address you mentioned is used for discovering the features of your ip interface. If you send an search request to this address, your ip interface will answer with an search response that includes a device information block.
Tcpdump only logs tcp by default - the datagrams for knx use udp, so you explicitly have to set the udp parameter to see these datagrams in tcpdump.
For groupaddress communication you first have to setup a channel (Connection Request, answered by a connection response). If you received "no error" from the ip interface you can start sending knx telegrams. This looks rather easy - but is not :-)
For further information look for calimero sources.

best regards

Martin

[Gaston]

Hi,

groupaddress communication is not done via multicast. It is done via unicast.

Actually to be complete, Group communication it is done via both multicast (routing) and unicast (tunneling). In your case however you are using an IP Interface (as opposed to an IP Router) which uses tunneling, thus unicast.

Cheers,
Gaston

[fma]

Thanks for you answer.

Ok, this time, I'm really lost!

What I read everywhere is that GAD communication in KNX networks (whatever medium is used) is done via multicast (giving a group adress as destination). And IP gateways acting as routers also communicate via multicast on IP side. Right?

So, is it not possible to use multicast like routers do?

What I understood so far is that unicast is only used in tunneling mode, to configure devices...

[fma]

Sorry, I posted at the same time!

So, you confirm that routers use multicast. So, why is it not possible to use this mode?

[Gaston]

What I read everywhere is that GAD communication in KNX networks (whatever medium is used) is done via multicast (giving a group adress as destination).

This is a layer mixing. GA communication is multicast on the KNX layer. When KNX runs over IP it can be either routed, i.e. broadcast on the Network being similar to the Group communication arriving at each IP router at once. Or it can be tunneled where it is sent by Unicast to a Destination (router or Interface) which then forwards it to the KNX.

And IP gateways acting as routers also communicate via multicast on IP side. Right?

So, is it not possible to use multicast like routers do?

There are "KNX/IP Routers" (=routing=multicast) and "KNX/IP Interfaces" (=tunneling=unicast). Some Routers also offer tunnels as well thus acting as interface.

What I understood so far is that unicast is only used in tunneling mode, to configure devices...

No, Tunneling can be used for both GA communication and device configuration, as well as multicast routing.

Cheers,
Gaston

[Gaston]

Sorry, I posted at the same time!

So, you confirm that routers use multicast. So, why is it not possible to use this mode?

Because you are using an interface not a router.

[fma]

Ok, I see.

Can I do multicast adressing through 'eibd', if it is configured as router?