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<body>Hi Thomas,<br><br>Thank you for your clarification.<br><br>There is a plausible reason why to keep the internal communities with the route. It is external debug info which you can get from BGP monitoring services. Anyway, there may be also cases of just misconfiguration. We all know, people are leaving trash everywhere and Internet is not a protected area of nature where you have to bring literally everything back when leaving. However, we can't clean up for others; the community attributes are transitive by definition. We must just pass them through. <br><br>The trade-off in BIRD of not keeping Adj-RIB-Out, should anyway not lead to what you are proposing. When the receiving site finds exactly the same route in their tables, they should not reannounce it further so the spreading stops at the first neighbor's ingress. At least BIRD does it this way; I don't remember exactly what is in RFC about this situation. <br><br>Maria<br><br><br><div class="gmail_quote">On October 19, 2020 11:04:41 PM GMT+02:00, Thomas Krenc <tkrenc@nps.edu> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div class="moz-cite-prefix">Hi Maria,</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">I agree with what you are saying.</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">One minor clarification in the
definitions: we observe (i) duplicates, i.e., AS path and
community attribute don't change, as well as (ii) announcement
where AS path don't change, but the community attribute does.
Interestingly, we show in the experiments that an AS-internal
change in the community attribute **only** can lead to these
observations.</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">In that regard, the tradeoff you are
describing is very interesting. Because the dicision of one AS not
to filter, in paricular (ii), has an impact on neighboring ASes;
and if they don't filter, they have an impact on their neighbors
etc.</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">It's relatively hard to tell how
frquent that happen, since a lot of updates are not visible to us.
But from the BGP collectors perspective, we observe that around
50% of all announcements fall into the category (i) and (ii), in
March 2020.<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Thanks for your feedback. It's very
insightful!</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">best regards</div>
<div class="moz-cite-prefix">Thomas<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">On 10/17/20 10:39 AM, Maria Matějka
wrote:<br>
</div>
<blockquote type="cite" cite="mid:934EBB95-9115-403C-8877-906167CE8DE1@nic.cz">
<div>Hello!<br>
<br>
If I read your paper correctly, I assume that this is just a
feature of the BGP itself. To make it clear: I read that in some
cases when a link inside one AS goes down, a route is
reannounced with the same AS path but different communities
because of e.g. some internal tagging which you call duplicate.
Please correct me if I'm wrong.<br>
<br>
This is definitely something for the admin of that AS whether
they want to share such an information outside or not. We can't
say from a global perspective whether they shall or shan't
reannounce that route. They may want to, they may not want to
reannounce. It depends on lots of things. <br>
<br>
So from my perspective, as a developer of BIRD, this is a matter
of configuration. If some of your routers do some tagging, it's
your configuration as well as the configuration of the outbound
router where you can strip these tags if you want.<br>
<br>
BIRD shall be never smarter than its user.<br>
<br>
There is also another thing that in case of egress filtering,
BIRD may send a duplicate announcement if it is not sure about
the contents of the previous route. From my point of view, it is
a tradeoff. We save memory by not keeping the Adj-RIB-Out by
default. This can be switched on by the "export table on"
setting in the appropriate bgp channel with the cost of some
memory overhead.<br>
<br>
I hope I've answered you well, feel free to discuss or correct
me if I misunderstood anything.
<br>
<br>
Thank you for the research. BTW, do you have an estimation on
how frequently does it happen?
<br>
<br>
Maria<br>
<br>
<br>
<div class="gmail_quote">On October 17, 2020 1:43:54 AM
GMT+02:00, Thomas Krenc <a class="moz-txt-link-rfc2396E" href="mailto:tkrenc@nps.edu"><tkrenc@nps.edu></a> wrote:
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<pre class="k9mail">Dear BIRD users and developers,
As a team of researchers from NPS and TU Berlin, we are investigating
the impact of BGP community attributes on the update behavior between ASes.
We find that when a route is associated with multiple distinct community
attributes it does not only lead to multiple announcement at the tagging
AS, but also at neighboring ASes, if communities are not filtered
properly. This behavior is wide-spread.
In order to better understand our observations, we have performed a
series of laboratory experiments using Cisco IOS, Junos OS, as well as
the BIRD daemon.
We find that - by default - all routers generate announcements with
changing community attributes, even when other attributes do not change.
In addition, when communities are filtered at egress, Cisco und BIRD
send duplicate announcements (Juniper does not).
Is this side-effect known to the BIRD community and would you
consider it a bug or a feature?
Since our findings are limited to observations in public data as well as
few router implementations, we would like to share our research and
kindly ask you to have a look at:
<a href="https://www.cmand.org/communityexploration/" moz-do-not-send="true">https://www.cmand.org/communityexploration/</a>
There, we provide some resources documenting our research, as well as
open questions. We greatly appreciate any feedback and insights you can
offer. Also, please don't hesitate to contact us directly:
communityexploration AT cmand DOT org
best regards
Thomas Krenc
Postdoctoral Researcher
Naval Postgraduate School
</pre>
</blockquote>
</div>
<br>
-- <br>
Sent from my Android device with K-9 Mail. Please excuse my
brevity.</div>
</blockquote>
<p><br>
</p>
</blockquote></div><br>-- <br>Sent from my Android device with K-9 Mail. Please excuse my brevity.</body></html>