Hi Thomas,

Thank you for your clarification.

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.

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.

Maria


On October 19, 2020 11:04:41 PM GMT+02:00, Thomas Krenc <tkrenc@nps.edu> wrote:
Hi Maria,

I agree with what you are saying.

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.

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.

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.

Thanks for your feedback. It's very insightful!

best regards
Thomas

On 10/17/20 10:39 AM, Maria Matějka wrote:
Hello!

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.

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.

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.

BIRD shall be never smarter than its user.

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.

I hope I've answered you well, feel free to discuss or correct me if I misunderstood anything.

Thank you for the research. BTW, do you have an estimation on how frequently does it happen?

Maria


On October 17, 2020 1:43:54 AM GMT+02:00, Thomas Krenc <tkrenc@nps.edu> wrote: 
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:

    https://www.cmand.org/communityexploration/

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

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Sent from my Android device with K-9 Mail. Please excuse my brevity.