ROV concern for hyper-specific prefixes (renamed from `Re: Smaller than a /24 for BGP?')

Amir Herzberg amir.lists at
Mon Jan 30 14:17:35 UTC 2023

Thanks to Lars for this interesting input and results (which I wasn't
familiar with).

I want to mention another concern with the possible use of hyper-specific
IP prefixes, i.e., longer than /24, which I haven't seen discussed in the
thread (maybe I missed it?). Namely, if you allow say /28 announcements,
then what would be the impact of ROV? Seems this can cause few problems:

- If origin, say AS 10, makes a ROA for the /28, this would allow an
attacker, e.g., AS 666,  to do origin-hijack (announce path 666-10 to the
/28), and attract traffic for the /28 from anybody accepting /28
announcements (that didn't receive the legit announcement). Plus, of
course, you get more overhead in ROA distribution and validation.

- If origin makes a ROA only for covering prefix (say /24) then the /28
announcement would be considered invalid by ROV and (even more likely)
dropped. Also you get more instances of `invalid' announcements, making
adoption of ROVs and ROAs harder.

Just a thought... Amir
Amir Herzberg

Comcast professor of Security Innovations, Computer Science and
Engineering, University of Connecticut
`Applied Introduction to Cryptography' textbook and lectures:

On Wed, Jan 25, 2023 at 1:17 PM Lars Prehn <lprehn at> wrote:

> We performed some high-level analyses on these hyper-specific prefixes
> about a year ago and pushed some insights into a blog post [1] and a paper
> [2].
> While not many ASes redistribute these prefixes, some accept and use them
> for their internal routing (e.g., NTT's IPv4 filtering policy [3]). Rob
> already pointed out that this is often sufficient for many traffic
> engineering tasks. In the remaining scenarios, announcing a covering /24
> and hyper-specific prefixes may result in some traffic engineering, even if
> the predictability of the routing impact is closer to path prepending than
> usual more-specific announcements. In contrast to John's claim, some
> transit ASes explicitly enabled redistributions of up to /28s for their
> customers upon request (at least, they told us so during interviews).
> Accepting and globally redistributing all hyper-specifics increases the
> routing table size by >100K routes (according to what route collectors
> see). There are also about 2-4 de-aggregation events every year in which
> some origin (accidentally) leaks some large number of hyper-specifics to
> its neighbours for a short time.
> Best regards,
> Lars
> [1]
> [2]
> [3]
> On 25.01.23 05:12, Forrest Christian (List Account) wrote:
> I have two thoughts in relation to this:
> 1) It's amazing how many threads end up ending in the (correct) summary
> that making an even minor global change to the way the internet works
> and/or is configured to enable some potentially useful feature isn't likely
> to happen.
> 2) I'd really like to be able to tag a BGP announcement with "only use
> this announcement as an absolute last resort" so I don't have to break my
> prefixes in half in those cases where I have a backup path that needs to
> only be used as a last resort.  (Today each prefix I have to do this with
> results in 3 prefixes in the table where one would do).
> And yes. I know #2 is precluded from actually ever happening because of
> #1.   The irony is not lost on me.
> On Tue, Jan 24, 2023, 7:54 PM John Levine <johnl at> wrote:
>> It appears that Chris J. Ruschmann <chris at> said:
>> >-=-=-=-=-=-
>> >How do you plan on getting rid of all the filters that don’t accept
>> anything less than a /24?
>> >
>> >In all seriousness If I have these, I’d imagine everyone else does too.
>> Right. Since the Internet has no settlements, there is no way to
>> persuade a network of whom you are not a customer to accept your
>> announcements if they don't want to, and even for the largest
>> networks, that is 99% of the other networks in the world. So no,
>> they're not going to accept your /25 no matter how deeply you believe
>> that they should.
>> I'm kind of surprised that we haven't seen pushback against sloppily
>> disaggregated announcements.  It is my impression that the route table
>> would be appreciably smaller if a few networks combined adjacent a
>> bunch of /24's into larger blocks.
>> R's,
>> John
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