From mboxrd@z Thu Jan 1 00:00:00 1970 From: Guy Harris Subject: Re: Correct radiotap header for 802.11ad Date: Sat, 22 Aug 2015 13:59:38 -0700 Message-ID: <38F46E1D-1C4A-48DC-A906-9522006E8474@alum.mit.edu> References: Mime-Version: 1.0 (Mac OS X Mail 6.6 \(1510\)) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: Sender: radiotap-owner-sUITvd46vNxg9hUCZPvPmw@public.gmane.org To: Richard Sharpe Cc: radiotap-S783fYmB3Ccdnm+yROfE0A@public.gmane.org List-Id: radiotap@radiotap.org On Aug 22, 2015, at 12:41 PM, Richard Sharpe = wrote: > I have some 802.11ad captures because of some fixes to the Wireshark > 802.11 dissector I am looking at doing. >=20 > One thing I notice is that the radiotap header contains both a Channel = field > and an MCS field. That is to be expected for 11n packets. > The Present flags say that Flags are present, Channel is present and > HT Information is present. By "HT Information" do you mean "the MCS field"? The "MCS" field should, perhaps, have been called the "HT" field, as = it's information for the HT (11n) PHY, just as the VHT field is = information for the VHT (11ac) PHY. > The channel frequency is 60480MHz which seems to be reasonable. >=20 > The MCS field says that the MCS index is present and that index is 0. >=20 > It seems to me that this last field is perhaps incorrect, because the > radiotap information dissector claims that the PHY type is 802.11n, > and that seems to come about because the radiotap header use the MCS > index info to claim that the PHY type was 802.11n. Well, to be fair, that's the Wireshark code doing that, under the = non-unreasonable assumption that if there's an MCS field it's an HT = frame, just as it assumes that if there's a VHT field it's a VHT frame. =46rom the *radiotap* point of view, I would say that the last field is = incorrect because the page for the MCS field on the radiotap site says: The mcs field indicates the MCS rate index as in = IEEE_802.11n-2009. which, if we update it to say "as in Clause 20 of IEEE 802.11-2012", = means it has values from 0 to 76, with modulations different from the = ones in 11ad's Clause 21, i.e. the radiotap MCS field is *not* = appropriate for 11ad. > Should I be letting the capture hardware vendor know that they are > generating the wrong info? I'm not even remotely an expert on 802.11 at the PHY level, but I = suspect that what we really want for radiotap is a "DMG" field, = containing 11ad-specific information. It would include its own mcs = field, giving the MCS values from Clause 21, and perhaps other = information, such as a flag to indicate whether "Static Tone Pairing" or = "Dynamic Tone Pairing" was used. (Again, I leave it up to people more = familiar with 802.11ad to decide what information would be useful.) Would the existing A-MPDU field suffice for MPDUs inside an 11ad A-MPDU?=