> Date: Fri, 7 Oct 2016 15:28:19 +0200
> From: Stefan Sperling <s...@stsp.name>
>
> Currently tcpdump shows "0 Mbit/s" for any frame sent with 11n HT MCS.
> To make progress easier, I'd like to see which MCS are used on the air,
> by any device.
>
> The change below matches what FreeBSD did to pass an MCS index via radiotap.
> This simply writes the MCS index into a previously unused range of rate
> values.
> This format is already recognized by third party tools such as wireshark
> so let's just do it the same way.
>
> The diff below updates the radiotap docs, makes iwn pass the MCS index,
> shows it in tcpdump. I can now see MCS with iwn in monitor mode.
>
> ok?
ok kettenis@
> Index: sys/net80211/ieee80211_radiotap.h
> ===================================================================
> RCS file: /cvs/src/sys/net80211/ieee80211_radiotap.h,v
> retrieving revision 1.13
> diff -u -p -r1.13 ieee80211_radiotap.h
> --- sys/net80211/ieee80211_radiotap.h 12 Jan 2016 09:28:09 -0000 1.13
> +++ sys/net80211/ieee80211_radiotap.h 7 Oct 2016 13:15:59 -0000
> @@ -90,9 +90,10 @@ struct ieee80211_radiotap_header {
> * For frequency-hopping radios, the hop set (first byte)
> * and pattern (second byte).
> *
> - * IEEE80211_RADIOTAP_RATE u_int8_t 500kb/s
> + * IEEE80211_RADIOTAP_RATE u_int8_t 500kb/s or MCS index
> *
> - * Tx/Rx data rate
> + * Tx/Rx data rate in units of 500kb/s. If the high bit (0x80) is set
> + * the remaining bits contain an MCS index instead of a date rate.
> *
> * IEEE80211_RADIOTAP_DBM_ANTSIGNAL int8_t decibels from
> * one milliwatt (dBm)
> Index: sys/dev/pci/if_iwn.c
> ===================================================================
> RCS file: /cvs/src/sys/dev/pci/if_iwn.c,v
> retrieving revision 1.172
> diff -u -p -r1.172 if_iwn.c
> --- sys/dev/pci/if_iwn.c 5 Sep 2016 08:18:18 -0000 1.172
> +++ sys/dev/pci/if_iwn.c 7 Oct 2016 13:10:42 -0000
> @@ -2131,23 +2131,27 @@ iwn_rx_done(struct iwn_softc *sc, struct
> tap->wr_dbm_antsignal = (int8_t)rssi;
> tap->wr_dbm_antnoise = (int8_t)sc->noise;
> tap->wr_tsft = stat->tstamp;
> - switch (stat->rate) {
> - /* CCK rates. */
> - case 10: tap->wr_rate = 2; break;
> - case 20: tap->wr_rate = 4; break;
> - case 55: tap->wr_rate = 11; break;
> - case 110: tap->wr_rate = 22; break;
> - /* OFDM rates. */
> - case 0xd: tap->wr_rate = 12; break;
> - case 0xf: tap->wr_rate = 18; break;
> - case 0x5: tap->wr_rate = 24; break;
> - case 0x7: tap->wr_rate = 36; break;
> - case 0x9: tap->wr_rate = 48; break;
> - case 0xb: tap->wr_rate = 72; break;
> - case 0x1: tap->wr_rate = 96; break;
> - case 0x3: tap->wr_rate = 108; break;
> - /* Unknown rate: should not happen. */
> - default: tap->wr_rate = 0;
> + if (stat->rflags & IWN_RFLAG_MCS) {
> + tap->wr_rate = (0x80 | stat->rate); /* HT MCS index */
> + } else {
> + switch (stat->rate) {
> + /* CCK rates. */
> + case 10: tap->wr_rate = 2; break;
> + case 20: tap->wr_rate = 4; break;
> + case 55: tap->wr_rate = 11; break;
> + case 110: tap->wr_rate = 22; break;
> + /* OFDM rates. */
> + case 0xd: tap->wr_rate = 12; break;
> + case 0xf: tap->wr_rate = 18; break;
> + case 0x5: tap->wr_rate = 24; break;
> + case 0x7: tap->wr_rate = 36; break;
> + case 0x9: tap->wr_rate = 48; break;
> + case 0xb: tap->wr_rate = 72; break;
> + case 0x1: tap->wr_rate = 96; break;
> + case 0x3: tap->wr_rate = 108; break;
> + /* Unknown rate: should not happen. */
> + default: tap->wr_rate = 0;
> + }
> }
>
> mb.m_data = (caddr_t)tap;
> @@ -2876,8 +2880,7 @@ iwn_tx(struct iwn_softc *sc, struct mbuf
> if ((ni->ni_flags & IEEE80211_NODE_HT) &&
> !IEEE80211_IS_MULTICAST(wh->i_addr1) &&
> type == IEEE80211_FC0_TYPE_DATA) {
> - /* XXX need a way to pass current MCS in 11n mode */
> - tap->wt_rate = 0;
> + tap->wt_rate = (0x80 | ni->ni_txmcs);
> } else
> tap->wt_rate = rinfo->rate;
> tap->wt_hwqueue = ac;
> Index: usr.sbin/tcpdump/print-802_11.c
> ===================================================================
> RCS file: /cvs/src/usr.sbin/tcpdump/print-802_11.c,v
> retrieving revision 1.33
> diff -u -p -r1.33 print-802_11.c
> --- usr.sbin/tcpdump/print-802_11.c 2 Sep 2016 17:11:46 -0000 1.33
> +++ usr.sbin/tcpdump/print-802_11.c 7 Oct 2016 12:44:25 -0000
> @@ -1075,8 +1075,13 @@ ieee802_11_radio_if_print(u_char *user,
>
> if (RADIOTAP(RATE)) {
> TCHECK2(*t, 1);
> - if (vflag)
> - printf(", %uMbit/s", (*(u_int8_t*)t) / 2);
> + if (vflag) {
> + uint8_t rate = *(u_int8_t*)t;
> + if (rate & 0x80)
> + printf(", MCS %u", rate & 0x7f);
> + else
> + printf(", %uMbit/s", rate / 2);
> + }
> t += 1;
> }
>
>
>
