From mboxrd@z Thu Jan 1 00:00:00 1970 From: Richard Sharpe Subject: Re: Correct radiotap header for 802.11ad Date: Sat, 22 Aug 2015 15:06:00 -0700 Message-ID: References: <38F46E1D-1C4A-48DC-A906-9522006E8474@alum.mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <38F46E1D-1C4A-48DC-A906-9522006E8474-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org> Sender: radiotap-owner-sUITvd46vNxg9hUCZPvPmw@public.gmane.org To: Guy Harris Cc: radiotap-S783fYmB3Ccdnm+yROfE0A@public.gmane.org List-Id: radiotap@radiotap.org On Sat, Aug 22, 2015 at 1:59 PM, Guy Harris wrote: > > 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. >> >> One thing I notice is that the radiotap header contains both a Channel f= ield >> 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"? I suspect so. The Present flags are 0x0a000800. The HT information present flag appears to indicate that MCS information is present. > 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. >> >> The MCS field says that the MCS index is present and that index is 0. >> >> 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-unr= easonable 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. > > From the *radiotap* point of view, I would say that the last field is inc= orrect 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-200= 9. > > which, if we update it to say "as in Clause 20 of IEEE 802.11-2012", mean= s it has values from 0 to 76, with modulations different from the ones in 1= 1ad'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-sp= ecific information. It would include its own mcs field, giving the MCS val= ues from Clause 21, and perhaps other information, such as a flag to indica= te whether "Static Tone Pairing" or "Dynamic Tone Pairing" was used. (Agai= n, I leave it up to people more familiar with 802.11ad to decide what infor= mation would be useful.) > > Would the existing A-MPDU field suffice for MPDUs inside an 11ad A-MPDU? I suspect that it would but I do not have any captures where the A-MPDU Status is listed as present. --=20 Regards, Richard Sharpe (=E4=BD=95=E4=BB=A5=E8=A7=A3=E6=86=82=EF=BC=9F=E5=94=AF=E6=9C=89=E6=9D=9C= =E5=BA=B7=E3=80=82--=E6=9B=B9=E6=93=8D)