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<p>Hello Ralph,</p>
<blockquote>
<p>Yes, “I have no providers” is a much more accurate description of AS
0. It can be used by tier 1 networks as well as people trying to
depreciate their old ASN.</p>
</blockquote>
<p>Well, yes, a deprecated ASN has also no providers, yet it can still
be (maliciously) placed into a valid AS path if it is on the top place.
OTOH, with that, the attack surface is limited only to your downstream
networks.</p>
<blockquote>
<p>It looks like the source of my confusion was that I was under the
assumption that the transit ASPA entries could be used to auto-detect
upstream vs downstream as opposed to doing the check in the filter
script. Sorry about that!</p>
</blockquote>
<p>No problem, everybody is confused by ASPA. It’s hard to get it
right.</p>
<blockquote>
<p>I noticed in aspa_check() you check for confeds but AS_PATH_SET is
never checked for.</p>
</blockquote>
<p>Well, that looks like another oversight, thank you for reporting.</p>
<blockquote>
<p>The specs say they should return ASPA_INVALID however I noticed when
I did that I lost about 64 routes which caused some customer
complaints. I had to end up slightly changing the code to return
ASPA_INVALID if upstream and ASPA_UNKNOWN if downstream.</p>
</blockquote>
<p>Mhmmm. That’s definitely a problem. We can do various things with and
around that. First of all, the default behavior of
<code>aspa_check()</code> must conform to the RFC.</p>
<p>Brainstorming:</p>
<ul>
<li><p>something like <code>if bgp_path.contains_sets</code></p></li>
<li><p>allowing a more precise for-cycle over <code>bgp_path</code>,
e.g.</p>
<pre><code>for bgppath_segment bs in bgp_path do {
case bgppath_segment.type {
AS_PATH_SEQUENCE: for int a in bs do { ... }
AS_PATH_SET: for int a in bs do { ... }
AS_PATH_CONFED_SEQUENCE: ...
}
}</code></pre></li>
<li><p>adding an optional argument to <code>aspa_check()</code> to allow
sets, treting them as “any of the ASNs in the set”</p></li>
<li><p>adding an <code>aspa_is_customer(table, A, B)</code> function,
returning whether A can be a custormer of B according to the given
table</p></li>
</ul>
<p>Any other thoughts on that?</p>
<p>Thanks,<br />
Maria</p>
<p>–<br />
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.</p>
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