RadioTap Archive on
 help / color / Atom feed
From: Johannes Berg <>
To: Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/>
Subject: Re: [RFA] 0-length PSDU reporting
Date: Tue, 14 Aug 2018 10:30:04 +0200
Message-ID: <> (raw)
In-Reply-To: <>

On Mon, 2018-08-13 at 12:35 +0200, Johannes Berg wrote:

> Yes, one case for example is when you know the packet metadata because
> you received it, but were unable to capture the data because it was a
> multi-user transmission to a different user, or because it was a multi-
> user transmission with more streams than you have RX chains, e.g. In
> this case I'm not even sure you know the real length of the packet, to
> be able to indicate it in the "Original Packet Length" field.
> Additionally, I'm not sure it's possible to indicate an "Original Length
> Field" all the way through from the kernel while capturing, I believe
> only tcpdump/etc. will set this to long than the data frame from the
> kernel when they explicitly truncate data.

To clarify - I think this field will also be used (and I think that
should be valid) when you have a PPDU that did carry a PSDU, but you
don't know its length, and thus report

  frame length == captured length == radiotap header length.

Like I said, I don't currently see a way to - at least on Linux - report
frame length > captured length. Looking at libpcap's pcap-linux.c, we

        caplen = packet_len;
        if (caplen > handle->snapshot)
                caplen = handle->snapshot;

We do get packet_len as recvmsg(..., MSG_TRUNC), but inside the kernel
we cannot represent a message as simply not having the data and getting
truncated even without MSG_TRUNC, so that we can't actually get out of
recvmsg() with a packet_len bigger than the in-kernel buffer.

Guy, does that address your question sufficiently?


  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-18 14:37 [RFC] 0-length PPDU reporting Johannes Berg
     [not found] ` <>
2018-06-18 16:05   ` Simon Barber
     [not found]     ` <>
2018-06-18 19:20       ` Johannes Berg
     [not found]         ` <>
2018-06-19 21:20           ` Guy Harris
     [not found]             ` <3028EC8F-6CFD-4F52-8349-D49B1F37AC22-FrUbXkNCsVf2fBVCVOL8/>
2018-06-19 22:50               ` Guy Harris
     [not found]                 ` <BC6DCA08-4049-4B7A-A7C0-9FC545173FE5-FrUbXkNCsVf2fBVCVOL8/>
2018-06-29 10:49                   ` Johannes Berg
2018-07-02 14:33   ` Johannes Berg
     [not found]     ` <>
2018-07-03 12:13       ` Johannes Berg
2018-07-09  8:30   ` [RFA] 0-length PSDU reporting Johannes Berg
     [not found]     ` <>
2018-08-09  1:53       ` Guy Harris
     [not found]         ` <AEBBAD8F-A779-4E0A-B40D-659D55DFE847-FrUbXkNCsVf2fBVCVOL8/>
2018-08-13 10:35           ` Johannes Berg
     [not found]             ` <>
2018-08-14  8:30               ` Johannes Berg [this message]
2018-09-04 12:10   ` [ADOPTION] " Johannes Berg

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \
    --cc=guy-FrUbXkNCsVf2fBVCVOL8/ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

RadioTap Archive on

Archives are clonable:
	git clone --mirror radiotap/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 radiotap radiotap/ \
	public-inbox-index radiotap

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone