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Re: [STDS-802-11-TGAQ] Transmission of Multicast-Group Addressed Frames with no Listeners

Hi Mark:

Thanks for the clarification.

Regarding the forwarding of multicast frames, the proposal we discussed in the teleconference intends to forward multicast frames to devices that are not associated with the AP, and therefore not joined to the service. However, if none of  the associated clients has joined that service, or more specifically, if the AP has no associated clients at all, then this multicast service cannot be used to advertise services to unassociated clients on the channel, since the AP won't forward them if no one has joined that service.

I suppose we could define a "virtual client" that is always associated and always joined to that service, but that would cause other artifacts.

-- Ed R.


On Thu, Apr 25, 2013 at 8:38 PM, Hamilton, Mark <Mark.Hamilton@xxxxxxxxxxxxxxx> wrote:

Ed,

 

First, I’ll point out that both the quotes you gave are within the context of a receiver of the frame (although I admit that isn’t as clear as it could be in 9.3.6, especially).  So, yes, frames that don’t match the receiving STA’s GroupAddressTable are discarded, but _at the receiver_.  This is not meant to be talking about transmission rules.

 

While I agree with your summary about the difference between broadcast and multicast-group addresses, note that almost nowhere in 802.11 does the Standard make use of this difference, rather talking only about “group addressed frames” generally. 

 

Which leads to my next point – there is no mechanism in 802.11 for a non-AP STA to communicate to the AP that it is “requesting to join the multicast service [for a particular group address],” other than services like DMS or FMS which serve a different purpose that what you’re after.

 

The behavior you are suggesting about an AP not forwarding such frames can be accomplished at a higher layer, however.  A service like IGMP can be used, and the higher layer of the AP/DS/Portal/external bridge can filter out the multicast frames that way.  Thus, 802.11 never sees the frame at the AP’s MAC Service, and this is non-issue for the Standard.  (There are complications about knowing that all non-AP STAs are IGMP capable and participating, etc., so this isn’t a panacea, but it is an option.)

 

Mark

 

From: Edward Reuss [mailto:edreuss@xxxxxxxxx]
Sent: Thursday, April 25, 2013 7:41 PM
To: STDS-802-11-TGAQ@xxxxxxxxxxxxxxxxx
Subject: [STDS-802-11-TGAQ] Transmission of Multicast-Group Addressed Frames with no Listeners

 

Regarding the conversation in the teleconference of 22 April we had about the transmission of multicast-group packets when there are no listeners for that multicast service associated to the AP, I quote the third paragraph of section 9.2.8 of IEEE 802.11-2012:

If the Address 1 field of an MPDU carrying an A-MSDU does not match any address (i.e., individual or group address) at a receiving STA, then the entire A-MSDU is discarded. 

and the last paragraph of section 9.3.6:

A STA shall discard an MPDU with a group address in the Address 1 field if the value in the Address 1 field does not match any value in the dot11GroupAddressesTable and does not match the Broadcast address value. 

I believe that there are other entries in IEEE 802.11-2012 that relate to this issue, but these are the ones that I found. Keep in mind that each of these quotes are in subsections that are specific to particular scenarios, so they may not be a complete description of multicast transmission rules.

 

Note also the distinction between a broadcast address versus a multicast-group address, as described in section 8.2.4.3.3:

1.      b)  Group address. A multidestination address, which may be in use by one or more STAs on a given network. The two kinds of group addresses are as follows:

1.      1)  Multicast-group address. An address associated by higher level convention with a group of logically related STAs.

2.      2)  Broadcast address. A distinguished, predefined group address that always denotes the set of all STAs on a given LAN. All ones are interpreted to be the broadcast address. This group is predefined for each communication medium to consist of all STAs actively connected to that medium; it is used to broadcast to all the active STAs on that medium. 

So, if there are no client STAs associated with the AP that have requested to join that multicast service, the AP will not transmit the multicast-group data frame. This impacts the proposed solution discussed in the teleconference.

 

Again, since I can't attend the May interim, I'm trying to manipulate this discussion onto the e-mail reflector so that I can participate. ;-) More to the point, if we can clear up these issues before the May interim session, that should help make the discussion in the session more productive. (However, I will miss this more productive discussion...).

 

Mahalo.  -- Ed Reuss

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