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From: "Rao, Krishna" <kcrao-A+ZNKFmMK5xy9aJCnZT0Uw@public.gmane.org>
To: Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
Cc: "radiotap-sUITvd46vNxg9hUCZPvPmw@public.gmane.org"
	<radiotap-sUITvd46vNxg9hUCZPvPmw@public.gmane.org>
Subject: RE: Regarding A-MPDU status field
Date: Mon, 23 Jan 2012 16:16:14 +0000	[thread overview]
Message-ID: <21E1C3B49A18BA428D58607D5EEA5398C3D354@nasanexd02b.na.qualcomm.com> (raw)
In-Reply-To: <1326721599.3510.18.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>

Hi,

>> It would provide the position of the current subframe, in within the 
>> A-MPDU (irrespective of whether the subframe is zero or non-zero MPDU 
>> length). The index value itself is zero-based (i.e., it starts from 0 
>> and increments onwards).
>> 
>> a) If 0x01 is set, then the index for the current subframe takes into 
>> account previous zero MPDU length subframes (because these previous 
>> zero subframes have been reported individually and the index 
>> incremented for each of them).
>> b) If 0x01 is clear, then the index for the current subframe does not 
>> account for previous zero MPDU length subframes that might have been 
>> transmitted over the air.
>> 
>> Thus if two different drivers are observing the same traffic:
>> The driver that sets 0x01 will report a larger index value for a given 
>> non-zero subframe, than the driver that clears 0x01 (if zero padding 
>> occurs before that subframe).
>> 
>> If 0x01 is set, then the sniffer app can calculate zero padding by 
>> processing index information and MPDU length field values across 
>> subframes.
>Ok, but I think that then we still need to resolve a small ambiguity here. Imagine this:
>
>   [mpdu][pad][pad][mpdu][pad][mpdu]
>a) 0     1    2    3     4    5
>b) 0               1          2
>
>Right?
Correct.

>
>But now imagine a device misses a frame due to decode problem or so ...
>then which one would you want:
>1) subframe index skips that one
>2) subframe index just increases
>
>Say
>   [mpdu][pad][pad][----][pad][mpdu]
>1) 0     1    2    (3)   4    5
>2) 0     1    2          3    4
>
>but then the driver would have to guess at the padding? Not sure. In any case, we need to nail
>this down, because in the case 2) the driver can number the subframes (but it's less useful),
>and in the case 1) the device has to provide it (and not all do -- so we need a separate valid bit).
>In the case 1), even the application (wireshark) could provide the subframe index?

Good you brought this up.

I had basically case 2) in mind. Even the device itself may not always be able to provide the index for subframes totally skipped due to decode problems. It may sometimes be able to pass to the driver subframes with bad delimiter CRC and these would get indexed by the driver (with flag 0x10 set). So the principle is that only whatever subframes the device can pass to the driver, good or bad, get indexed.

But I now feel the application itself can provide the subframe indexing (as in case 2). It can use the subframe reference number and timestamp for this. The burden on the driver would be decreased, and we also save space in the Radiotap header by removing this field.

Regds,
Krishna


  parent reply	other threads:[~2012-01-23 16:16 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-10-31  9:13 Regarding A-MPDU status field Rao, Krishna
     [not found] ` <21E1C3B49A18BA428D58607D5EEA5398BB2E90-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2011-11-02  8:42   ` Johannes Berg
     [not found]     ` <1320223343.3950.29.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2011-11-03 15:02       ` Rao, Krishna
     [not found]         ` <21E1C3B49A18BA428D58607D5EEA5398BB67FA-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2011-11-09 14:02           ` Johannes Berg
     [not found]             ` <1320847366.3845.72.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2011-11-09 20:45               ` David Young
2011-11-13 16:49               ` Rao, Krishna
     [not found]                 ` <21E1C3B49A18BA428D58607D5EEA5398BBBD18-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2011-11-14  8:26                   ` Johannes Berg
     [not found]                     ` <1321259214.4472.16.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2011-11-20 16:37                       ` Rao, Krishna
     [not found]                         ` <21E1C3B49A18BA428D58607D5EEA5398BC8C00-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2011-11-21  9:27                           ` Johannes Berg
     [not found]                             ` <1321867620.3999.7.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2011-11-22 14:27                               ` Rao, Krishna
     [not found]                                 ` <21E1C3B49A18BA428D58607D5EEA5398BC971A-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2011-11-22 17:06                                   ` David Young
2011-11-22 14:33                               ` Rao, Krishna
2011-11-23 15:23                             ` Rao, Krishna
2012-01-03 17:25                             ` Rao, Krishna
     [not found]                               ` <21E1C3B49A18BA428D58607D5EEA5398C1F289-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2012-01-04 10:42                                 ` Johannes Berg
     [not found]                                   ` <1325673736.3339.27.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2012-01-08 13:43                                     ` Rao, Krishna
     [not found]                                       ` <21E1C3B49A18BA428D58607D5EEA5398C20149-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2012-01-09 10:16                                         ` Johannes Berg
     [not found]                                           ` <1326104166.3451.21.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2012-01-15 19:14                                             ` Rao, Krishna
     [not found]                                               ` <21E1C3B49A18BA428D58607D5EEA5398C36652-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2012-01-16 13:46                                                 ` Johannes Berg
     [not found]                                                   ` <1326721599.3510.18.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2012-01-23 16:16                                                     ` Rao, Krishna [this message]
     [not found]                                                       ` <21E1C3B49A18BA428D58607D5EEA5398C3D354-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2012-02-20 11:03                                                         ` Johannes Berg
     [not found]                                                           ` <1329735809.3458.14.camel-8upI4CBIZJIJvtFkdXX2HixXY32XiHfO@public.gmane.org>
2012-02-22  1:14                                                             ` Rao, Krishna
     [not found]                                                               ` <21E1C3B49A18BA428D58607D5EEA5398C55E21-RqevaKtL5OWBP98+muIrrKRtKmQZhJ7pQQ4Iyu8u01E@public.gmane.org>
2012-02-23 15:48                                                                 ` Johannes Berg
2012-07-05  9:49   ` Johannes Berg

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