RadioTap MCS is parsed wrong. PR #3963 seems to work for now.

Brief description

How i understand the problem:

• In Wireshark i see "Known MCS information: 0x00" which results in all Fields (Bandwith, MCS index, Guard interval...) are Absent. maybe odd of Wireshark too, it highlights 3 Bytes but only 7 bits (the previous mentioned Fields) are shown to me.
• dot11.py RadioTap is parsing to the Antenna signal correctly, but then everything is off by one byte. (Type, Subtype, MAC Address.....). I guess if known mcs information is 0x00 set everything to None or something.
• RadioTap documentation i don't understand it.

Scapy version

2.5.0+git20240324.2858b51

Python version

3.12.4

Operating system

Linux 6.9.9

Additional environment information

No response

How to reproduce

could take some time and compare it with Wireshark.

def write_mcs(pkt):
    wrpcap(filename='/tmp/mcs_issue.pcap', pkt=pkt, append=True)
    print(f'{pkt} written.', flush=True)
    

sniff(
    iface=interface,
    store=False,
    count=100,
    prn=write_mcs,
    lfilter=lambda x: x.present.value & 0x80000,
    )

Actual result

No response

Expected result

No response

Related resources

I think maybe PR #3963 is the Solution. Yes, parsing works.

Start in dot11.py and inspect the RadioTap MCS parsing path, then compare its behavior with PR #3963 and Wireshark. Reproduce with the provided sniffing script and captured packets; done means MCS fields and subsequent 802.11 fields remain correctly aligned when known MCS information is 0x00.