Re: RTBH Support Across the Industry

[Phil Bedard via NANOG <nanog () lists nanog org>]

RTBH certainly has its place in the toolbox for DDoS mitigation but it is a bit of a hammer.

Flowspec can be used to simply remark traffic into scavenger and on their own network many do that today.   But like 
you mention the ability to put more granular guardrails on what gets accepted  from others else isn’t really baked into 
the standards.   FS can have more complex resource implications.  You create a FS policy that has 6 match conditions 
and it will take up a lot more TCAM space.  There were also some highly publicized outages caused by it early on when 
people did try using it downstream->provider.

Maybe orthogonal but I had a question from an operator recently about validating RTBH prefixes from a downstream.  They 
were being asked to simply accept anything based on a AS path check.  I would assume most are using strict prefix 
lists.  I saw the recent thread about using ROA as a check but that has some hurdles without config knobs or tricks to 
relax constraints.

Phil

From: Saku Ytti via NANOG <nanog () lists nanog org>
Date: Tuesday, July 28, 2026 at 3:23 AM
To: Barry Greene <bgreene () senki org>
Cc: North American Network Operators Group <nanog () lists nanog org>; Saku Ytti <saku () ytti fi>
Subject: Re: RTBH Support Across the Industry

Suggesting to replace RTBH with flowspec will not be marketable, many
people, rightly, are worried about flowspec, because it has a huge bug
surface and some serious design mistakes and implementation mistakes which
make it poor fit for environments which lack in trust.

Replacing blackhole community with QoS downgrade community is much more
marketable in comparison, and infact one large tier1 used to offer this on
a beta basis maybe 15-20 years ago. Sadly it is not commonly available.


On Tue, 28 Jul 2026 at 11:15, Barry Greene <bgreene () senki org> wrote:

> Suggestion …. Walk through the APRICOT 2022 talks with DDoS.
>
> [image: hqdefault.jpg]
>
> APRICOT 2022 - DDoS Resiliency Workshop
> <https://www.youtube.com/playlist?list=PLTAhO9aX5q8X5IS9M3m4fLtvQdjBp1UZ0>
> youtube.com
> <https://www.youtube.com/playlist?list=PLTAhO9aX5q8X5IS9M3m4fLtvQdjBp1UZ0>
> <https://www.youtube.com/playlist?list=PLTAhO9aX5q8X5IS9M3m4fLtvQdjBp1UZ0>
>
> What I’m seeing in this conversation is the missing tools in the DDoS
> Toolkit that get integrated into DDoS playbooks.
>
> RTBH was just the first element.
> We then had sRTBH when we created loose uRPF.
> Then we taught peers how to take dRTBH and sRTBH and redirect traffic to a
> network sinkhole set up to track the attacks once redirected.
> Then we had BGP community-based rate limiting.
> Chris Morrow (UUNET) and Job Snijders (NTT) then set up customer-based
> RTBH - where you, as a customer, can set up a BGP community and have it
> blocked at your upstream edge (giving you space to work the attack).
>
> Then we had work at Cisco and Arbor on industry-wide mitigation
> approaches. This would take time, so Flow-Spec was created as a stopgap.
> That Cisco/Arbor work was migrated into DOTS in the IETF.
>
> Listen to the sessions, especially the interviews.

--
  ++ytti
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