From: "Henrique de Moraes Holschuh" <hmh@hmh.eng.br>
To: "Eric Dumazet" <eric.dumazet@gmail.com>,
linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: Unexpected timestamps in tcpdump with veth + tc qdisc netem delay
Date: Mon, 26 Apr 2021 23:34:55 -0300 [thread overview]
Message-ID: <52f1b7a1-ff3b-41d3-a84b-badcda8a6ad6@www.fastmail.com> (raw)
In-Reply-To: <8e0aa5a6-0457-ccd0-8984-9c9aaeab2228@gmail.com>
On Mon, Apr 26, 2021, at 14:07, Eric Dumazet wrote:
> On 4/26/21 4:35 PM, Henrique de Moraes Holschuh wrote:
[...]
> > [root netns]: tcpdump -i vec0 -s0 -n -p net 192.168.233.0/30
> > listening on vec0, link-type EN10MB (Ethernet), capture size 262144 bytes
> > 17:09:09.740681 IP 192.168.233.1 > 192.168.233.2: ICMP echo request, id 9327, seq 1, length 64
>
> Here you see the packet _after_ the 250ms delay
> > 17:09:09.990891 IP 192.168.233.2 > 192.168.233.1: ICMP echo reply, id 9327, seq 1, length 64
> Same here.
[...]
> > Adding more namespaces and VETH pairs + routing "in a row" so that the packet "exits" one veth tunnel and enters another one (after trivial routing) doesn't fix the tcpdump timestamps in the capture at the other end of the veth-veth->routing->veth-veth->routing->... chain.
> >
> > It looks like some sort of bug to me, but maybe I am missing something, in which case I would greatly appreciate an explanation of where I went wrong...
>
> That is only because you expect to see something, but you forgot that tcpdump captures TX packet _after_ netem.
That was it! Thank you very much for the quick reply, and direct, precise explanation.
I had completely forgotten about the qdisc running before interface timestamping, and I overlooked the fact that I did not try three netns in-a-chain with the qdiscs *in the middle netns*: I tried them in the two opposite edge netns, only.
Again, thank you!
--
Henrique Holschuh
prev parent reply other threads:[~2021-04-27 2:35 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-26 14:35 Unexpected timestamps in tcpdump with veth + tc qdisc netem delay Henrique de Moraes Holschuh
2021-04-26 17:07 ` Eric Dumazet
2021-04-27 2:34 ` Henrique de Moraes Holschuh [this message]
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:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=52f1b7a1-ff3b-41d3-a84b-badcda8a6ad6@www.fastmail.com \
--to=hmh@hmh.eng.br \
--cc=eric.dumazet@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=netdev@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).