[Babel-users] RTT stability inside a GRE tunnel

[Baptiste Jonglez]

On Tue, Jul 02, 2013 at 11:00:14AM -0400, Michael Richardson wrote:
> 
> Baptiste Jonglez <baptiste.jonglez at ens-lyon.fr> wrote:
>     > On Tue, Jul 02, 2013 at 09:35:55AM -0400, Michael Richardson wrote:
>     >>
>     >> Can you tell us a bit how about how you are measuring, and what the
>     >> remote side is?  I see that the two values are consistent with each
>     >> other --- it looks to me like the inside tunnel value is simply
>     >> truncated.
>     >>
>     >> I assume that you are measuring the RTT value from babeld, rather than
>     >> from the network itself (i.e. with tcpdump).
> 
>     > Both measures are done via ICMP; in each case, it is a simple,
>     > continuous ping with a 1-second interval.
> 
> Yes, but how are you observing the RTT?  Is it ping? tcpdump? something else?

ping.

>     > The hosts are running Debian stable; they are both behind a NAT, and
>     > the GRE tunnel is setup over IPv4 (which the firewall/NAT on front of
>     > each host seems to let through without any configuration).
> 
> That really seems weird to me.
> 1) Are you sure that the ICMP echo requests are really being answered by the
>    host rather than the NAT?

Hmm, that is a very valuable remark: outside of the tunnel, the ICMP
request is indeed answered by the CPE, and not by the host itself.

I think I implicitly assumed that the strange behaviour was caused by
a router somewhere along the path, and not because of the slight
difference of who is actually answering the ICMP request at the other
end; however, this might be wrong.

> 2) I would guess that the NAT has been setup with your host as the "DMZ" or
>    "bastion" host, and all unknown packets are being forwarded to that host.

The hosts are not in any kind of DMZ.

Modern NAT are intelligent enough to forward this type of tunnels
correctly (the same goes for IP-in-IP or SIT tunnels).
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