Why is wmm_param required for HT40- in mlme.c

DISCUSSION Ben Greear 4 participants 8 messages Jan 30, 2011 – Jan 31, 2011

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Ben Greear

author

2011-01-30 05:38 UTC #0 lore raw

I tried to set up hostapd to do HT40-, but for whatever reason, it's not sending any wmm_param options, so the stations will not associate with HT40- enabled due to the check below.

Is it really *reqired* that wmm_param exist to enable HT mode?

From mlme.c:

	if (elems.ht_info_elem && elems.wmm_param &&
	    (sdata->local->hw.queues >= 4) &&
	    !(ifmgd->flags & IEEE80211_STA_DISABLE_11N))
		changed |= ieee80211_enable_ht(sdata, elems.ht_info_elem,
					       cbss->bssid, ap_ht_cap_flags);

Thanks,
Ben

Signature · 2 lines

Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com

Johannes Berg

2011-01-30 09:36 UTC #1 lore raw

Quoted 4 lines · Ben Greear

I tried to set up hostapd to do HT40-, but for whatever reason, it's not sending any wmm_param options, so the stations will not associate with HT40- enabled due to the check below.

Quoted 8 lines

Is it really *reqired* that wmm_param exist to enable HT mode?

From mlme.c:

	if (elems.ht_info_elem && elems.wmm_param &&
	    (sdata->local->hw.queues >= 4) &&
	    !(ifmgd->flags & IEEE80211_STA_DISABLE_11N))
		changed |= ieee80211_enable_ht(sdata, elems.ht_info_elem,
					       cbss->bssid, ap_ht_cap_flags);

802.11n-2009 says it is required.

johannes

Ben Greear

author

2011-01-30 17:34 UTC #2 lore raw

Quoted 7 lines · Johannes Berg

Quoted 6 lines · Ben Greear
I tried to set up hostapd to do HT40-, but for whatever reason, it's not sending any wmm_param options, so the stations will not associate with HT40- enabled due to the check below.
Is it really *reqired* that wmm_param exist to enable HT mode?

Quoted 8 lines

Quoted 6 lines

From mlme.c:

	if (elems.ht_info_elem&&  elems.wmm_param&&
	(sdata->local->hw.queues>= 4)&&
	!(ifmgd->flags&  IEEE80211_STA_DISABLE_11N))
		changed |= ieee80211_enable_ht(sdata, elems.ht_info_elem,
					       cbss->bssid, ap_ht_cap_flags);

802.11n-2009 says it is required.

Do you know the pertinent section, or some keywords to search for? I'm unable to find anything useful.

Also, from looking at utils.c, it seems that this wmm_param is a vendor specific Microsoft thing? I don't see anywhere else that wmm_param is assigned...

		case WLAN_EID_VENDOR_SPECIFIC:
			if (elen >= 4 && pos[0] == 0x00 && pos[1] == 0x50 &&
			    pos[2] == 0xf2) {
				/* Microsoft OUI (00:50:F2) */

				if (calc_crc)
					crc = crc32_be(crc, pos - 2, elen + 2);

				if (pos[3] == 1) {
					/* OUI Type 1 - WPA IE */
					elems->wpa = pos;
					elems->wpa_len = elen;
				} else if (elen >= 5 && pos[3] == 2) {
					/* OUI Type 2 - WMM IE */
					if (pos[4] == 0) {
						elems->wmm_info = pos;
						elems->wmm_info_len = elen;
					} else if (pos[4] == 1) {
						elems->wmm_param = pos;
						elems->wmm_param_len = elen;
					}
				}
			}
			break;

Thanks,
Ben

Quoted 2 lines

johannes

Signature · 2 lines

Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com

Johannes Berg

2011-01-31 11:03 UTC #3 lore raw

Quoted 15 lines · Ben Greear

Quoted 14 lines · Johannes Berg
Quoted 12 lines · Ben Greear
I tried to set up hostapd to do HT40-, but for whatever reason, it's not sending any wmm_param options, so the stations will not associate with HT40- enabled due to the check below.
Is it really *reqired* that wmm_param exist to enable HT mode?
From mlme.c:
	if (elems.ht_info_elem&&  elems.wmm_param&&
	(sdata->local->hw.queues>= 4)&&
	!(ifmgd->flags&  IEEE80211_STA_DISABLE_11N))
		changed |= ieee80211_enable_ht(sdata, elems.ht_info_elem,
					       cbss->bssid, ap_ht_cap_flags);

Quoted 4 lines

Quoted 1 line
802.11n-2009 says it is required.
Do you know the pertinent section, or some keywords to search for? I'm unable to find anything useful.

No, I got confused. 802.11n says an HT STA is a QoS STA (5.2.9), but in practice WMM is tested/certified instead of QoS.

johannes

Ben Greear

author

2011-01-31 17:35 UTC #4 lore raw

Quoted 16 lines · Johannes Berg

Quoted 15 lines · Ben Greear
Quoted 14 lines · Johannes Berg
Quoted 12 lines · Ben Greear
I tried to set up hostapd to do HT40-, but for whatever reason, it's not sending any wmm_param options, so the stations will not associate with HT40- enabled due to the check below.
Is it really *reqired* that wmm_param exist to enable HT mode?
   From mlme.c:
	if (elems.ht_info_elem&&   elems.wmm_param&&
	(sdata->local->hw.queues>= 4)&&
	!(ifmgd->flags&   IEEE80211_STA_DISABLE_11N))
		changed |= ieee80211_enable_ht(sdata, elems.ht_info_elem,
					       cbss->bssid, ap_ht_cap_flags);

Quoted 7 lines

Quoted 4 lines
Quoted 1 line
802.11n-2009 says it is required.
Do you know the pertinent section, or some keywords to search for? I'm unable to find anything useful.
No, I got confused. 802.11n says an HT STA is a QoS STA (5.2.9), but in practice WMM is tested/certified instead of QoS.

So maybe we can remove that check for wmm_param, but put in a printk to warn users that their AP is probably mis-configured?

Thanks,
Ben

Quoted 2 lines

johannes

Signature · 2 lines

Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com

Johannes Berg

2011-01-31 17:40 UTC #5 lore raw

Quoted 5 lines · Ben Greear

Quoted 2 lines
No, I got confused. 802.11n says an HT STA is a QoS STA (5.2.9), but in practice WMM is tested/certified instead of QoS.
So maybe we can remove that check for wmm_param, but put in a printk to warn users that their AP is probably mis-configured?

Yes, but there are hidden assumptions that WMM is available throughout the code that we need to hunt down first -- I'm sure there are some in the aggregation code for example since aggregation can only work when QoS/WMM is enabled.

johannes

Pat Erley

2011-01-30 17:41 UTC #6 lore raw

In reply to Johannes Berg

Quoted 7 lines · Johannes Berg

Quoted 6 lines · Ben Greear
I tried to set up hostapd to do HT40-, but for whatever reason, it's not sending any wmm_param options, so the stations will not associate with HT40- enabled due to the check below.
Is it really *reqired* that wmm_param exist to enable HT mode?

Quoted 9 lines

Quoted 6 lines

From mlme.c:

	if (elems.ht_info_elem&&  elems.wmm_param&&
	(sdata->local->hw.queues>= 4)&&
	!(ifmgd->flags&  IEEE80211_STA_DISABLE_11N))
		changed |= ieee80211_enable_ht(sdata, elems.ht_info_elem,
					       cbss->bssid, ap_ht_cap_flags);

802.11n-2009 says it is required.

Having run into this same issue before, I considered adding some default values for the case that wmm is not configured, but someone tries to enable 802.11n, I just never decided where the best spot for that would be. Should this sort of thing be a hostapd default or somewhere in the cfg80211/mac80211 layer? I was thinking this should be something that hostapd does.

As I see it, there are a few ways to prevent this problem:

1. Hostapd doesn't allow 802.11n to be enabled without wmm
2. Hostapd does allow 802.11n to be enabled without wmm, but uses defaults
3. Hostapd doesn't care about this, and the kernel doesn't allow 802.11n without wmm
4. Hostapd doesn't care, but the kernel makes the correnctions

I believe 2 is the cleanest solution. So that the packets passed from hostapd can make it through unmolested, and new hostapd versions can work with older kernels. Any other solution I'm not thinking of?

Pat Erley

Jouni Malinen

2011-01-31 11:28 UTC #7 lore raw

Quoted 10 lines · Pat Erley

Having run into this same issue before, I considered adding some default values for the case that wmm is not configured, but someone tries to enable 802.11n, I just never decided where the best spot for that would be. Should this sort of thing be a hostapd default or somewhere in the cfg80211/mac80211 layer? I was thinking this should be something that hostapd does.
As I see it, there are a few ways to prevent this problem:
1. Hostapd doesn't allow 802.11n to be enabled without wmm

I don't think that that would ever happen since as far as I can tell, WMM is not required from 802.11n based on any standard or specification. (Sure, there may be some non-IEEE certifications that require it, but hostapd is not limited by such requirements; it just needs to allow one to implement a device to meet such needs.)

Quoted 2 lines

2. Hostapd does allow 802.11n to be enabled without wmm, but uses defaults

What do you mean with "defaults" here?

Quoted 2 lines

3. Hostapd doesn't care about this, and the kernel doesn't allow 802.11n without wmm

My comment on (1) would apply here.

Quoted 1 line

4. Hostapd doesn't care, but the kernel makes the correnctions

I don't think that that would be a good idea either and well, not really at all suitable for the current design of where the HT/WMM IEs come from for management frames in AP mode.

Quoted 3 lines

I believe 2 is the cleanest solution. So that the packets passed from hostapd can make it through unmolested, and new hostapd versions can work with older kernels. Any other solution I'm not thinking of?

I do not fully understand what (2) is, but it may be close enough to what I think would be a reasonable approach:
- for non-AP STA case, allows association with HT even if WMM (or IEEE 802.11 QoS) is not enabled (mac80211 change)
- for AP case, make hostapd default to enabling WMM with default EDCA parameters if HT is enabled and there is no explicit configuration for disabling WMM
- if WMM is explicitly disabled in hostapd.conf, allow IEEE 802.11n to be enabled without WMM/QoS

The main focus here is to make it less likely for users to end up configuring something they did not plan on doing while not removing flexibility in configuration. I don't know any particularly good reasons for disabling WMM/QoS/aggregation, but that is not good enough justification to make it impossible to do so.

Signature · 1 line

Jouni Malinen                                            PGP id EFC895FA