<div dir="ltr"><div dir="ltr">Hi Ondrej,</div><div dir="ltr"><br></div><div dir="ltr">Thanks for evaluating the patch. It makes sense to use rte->flags to store the preferred path and access that from the filter code to get around the first issue.<br><br>For the second issue, it almost feels like there should be a separate mode of route announcement called RA_SELECTIVE. This would be exactly the same as RA_ANY, but in addition it would announce when routes change their REF_BEST. The RA_SELECTIVE mode could be enabled with a config option such as 'add paths tx selective;', so that we don't end up with lots of unnecessary announcements for BGP add-path. Previously, we had tried this by making RA_SELECTIVE a clone of RA_OPTIMAL, but that did not work well. Do you see issues with this approach up front?</div><div dir="ltr"><br></div><div>Thanks,</div><div>Trisha</div><div dir="ltr"><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><span style="color:rgb(136,136,136)">--</span><br style="color:rgb(136,136,136)"><div dir="ltr" style="color:rgb(136,136,136)"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div style="margin:0px;padding:0px;color:rgb(51,51,51);font-family:Arial,sans-serif;font-size:14px;line-height:20px"><img src="https://www.fastly.com/img/sig.png"><br></div><div style="margin:0px;padding:0px;color:rgb(51,51,51);font-family:Arial,sans-serif;font-size:14px;line-height:20px"><b>Trisha Biswas</b> | Sr. Software Engineer, Network Systems</div><div style="margin:0px;padding:0px;color:rgb(51,51,51);font-family:Arial,sans-serif;font-size:14px;line-height:20px"><a href="http://fastly.com/" rel="nofollow" style="color:rgb(59,115,175)" target="_blank">fastly.com</a> | <a href="https://twitter.com/fastly" rel="nofollow" style="color:rgb(59,115,175)" target="_blank">@fastly</a> | <a href="http://www.linkedin.com/company/fastly" rel="nofollow" style="color:rgb(59,115,175)" target="_blank">LinkedIn</a></div></div></div></div></div></div></div></div><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Feb 26, 2021 at 3:53 PM Ondrej Zajicek <<a href="mailto:santiago@crfreenet.org">santiago@crfreenet.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On Fri, Feb 26, 2021 at 01:56:42PM -0800, Trisha Biswas wrote:<br>
> Hi all,<br>
> <br>
> The attached patch adds a 'preferred' route attribute to check whether the<br>
> route being processed has been selected as the best-path for the network it<br>
> belongs to. This attribute does not take in a value, and returns a boolean.<br>
> This is useful for allowing selection in an add-path enabled BGP session.<br>
<br>
Hi<br>
<br>
I agree this may be a useful feature, but there are two issues with the<br>
approach used in the patch:<br>
<br>
First, we would like not to access rte->net from the filters (outside of<br>
net_addr). We plan to remove this ptr and instead have ptr from rte<br>
directly to net_addr.<br>
<br>
Second, RA_ANY mode of route announcement (which is used by BGP add-path<br>
and pipes) would not really work with this change in a way how you expect<br>
- consider this scenario: route A is added, preferred and accepted for<br>
export, then route B is added, not-preferred and not-accepted for export.<br>
Then route A is withdrawn and B is selected as preferred. Protocols with<br>
RA_OPTIMAL received update with route B as new-preferred (as they accept<br>
only preferred one and that changed), but protocols with RA_ANY receive<br>
just withdraw for route A (as they accept all routes and only route A was<br>
changed, while route B is the same as was announced before), so they do<br>
not get notification that route B became preferred and export filter for<br>
B is not re-evaluated.<br>
<br>
The one approach that avoids the first issue is to add REF_BEST flag<br>
(in rte->flags) that would be added to mark preferred routes, and this<br>
flag can be accessed from the filter code. But that does not fix the<br>
second issue. We can add RA_ANY announcement for routes that changed<br>
their REF_BEST, but that would add plenty of unnecessary announcements<br>
(that are propagated as BGP updates) for BGP add-path that do not<br>
use or care about this flag.<br>
<br>
In general, BGP add-path in BIRD was developed with assumption that<br>
routes within one network are handled independently and would need some<br>
deeper redesign for other cases. That might be useful (in order to be<br>
able to implement things like 'announce best 4 routes for given<br>
network'), but i do not see an easy solution to this.<br>
<br>
-- <br>
Elen sila lumenn' omentielvo<br>
<br>
Ondrej 'Santiago' Zajicek (email: <a href="mailto:santiago@crfreenet.org" target="_blank">santiago@crfreenet.org</a>)<br>
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, <a href="http://wwwkeys.pgp.net" rel="noreferrer" target="_blank">wwwkeys.pgp.net</a>)<br>
"To err is human -- to blame it on a computer is even more so."<br></blockquote><div><div dir="auto"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><br></div><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Feb 26, 2021 at 1:56 PM Trisha Biswas <<a href="mailto:tbiswas@fastly.com">tbiswas@fastly.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr">Hi all,<div><br></div><div>The attached patch adds a 'preferred' route attribute to check whether the route being processed has been selected as the best-path for the network it belongs to. This attribute does not take in a value, and returns a boolean. This is useful for allowing selection in an add-path enabled BGP session. </div><div><br></div><div>The motivation behind selective add paths is to reduce the number of routes that are shared in an add-path enabled BGP session on a per-prefix basis. Instead of sending all paths to all destinations for an add-path session, we would like to specify prefixes (and/or other route attributes) for which all paths are to be shared. Further, if there are thousands of ways to reach a destination, simply negotiating add paths for a prefix would result in sharing all additional paths to the peer. This in turn would result in a flood of advertisements to be sent between the nodes, and a high memory footprint on the peer. To avoid this, we are considering a filter-based selection criteria that would unlock the potential of purposefully sending additional paths when desired. <br></div><div><br></div><div>Adding the 'preferred' keyword allows us to always send the best-path whether or not other criteria in the export filter match. In the following example, a route is exported when it is either the best route or one of <a href="http://172.16.0.0/16" target="_blank">172.16.0.0/16</a>, <a href="http://10.0.0.0/8%7B16,24%7D" target="_blank">10.0.0.0/8{16,24}</a> (regardless of best route status). In the case of <a href="http://8.8.8.0/24" target="_blank">8.8.8.0/24</a>, we export it if it's the best route, or if it additionally has the specified extended community attached to the route. </div><div><br></div><div><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap">protocol bgp { </span><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"><br></span><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap">[....]</span><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"><br></span><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"> </span><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap">add paths tx;</span></div><div><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"> export filter {</span></div><div><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"> if preferred || (net = [<a href="http://172.16.0.0/16" target="_blank">172.16.0.0/16</a>, <a href="http://10.0.0.0/8%7B16,24%7D" target="_blank">10.0.0.0/8{16,24}</a>]) then accept;<br></span></div><div><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"> if preferred || (net = <a href="http://8.8.8.0/24" target="_blank">8.8.8.0/24</a> && (ro, 1, 2) ~ bgp_ext_community) then accept;<br></span></div><div><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"> #remaining filter code</span></div><div><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"> reject;</span></div><div><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap"> }<br></span><span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-size:9pt;font-family:"Courier New";color:rgb(23,43,77);background-color:transparent;vertical-align:baseline;white-space:pre-wrap">}</span><br></div><div><br></div><div>Cheers,</div><div>Trisha</div><div><div><div dir="ltr"><div dir="ltr"><span style="color:rgb(136,136,136)">--</span><br style="color:rgb(136,136,136)"><div dir="ltr" style="color:rgb(136,136,136)"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div style="margin:0px;padding:0px;color:rgb(51,51,51);font-family:Arial,sans-serif;font-size:14px;line-height:20px"><img src="https://www.fastly.com/img/sig.png" class="gmail-CToWUd"><font color="#888888"><br></font></div><font color="#888888"><div style="margin:0px;padding:0px;color:rgb(51,51,51);font-family:Arial,sans-serif;font-size:14px;line-height:20px"><b>Trisha Biswas</b> | Sr. Software Engineer, Network Systems</div><div style="margin:0px;padding:0px;color:rgb(51,51,51);font-family:Arial,sans-serif;font-size:14px;line-height:20px"><a href="http://fastly.com/" rel="nofollow" target="_blank" style="color:rgb(59,115,175)">fastly.com</a> | <a href="https://twitter.com/fastly" rel="nofollow" target="_blank" style="color:rgb(59,115,175)">@fastly</a> | <a href="http://www.linkedin.com/company/fastly" rel="nofollow" target="_blank" style="color:rgb(59,115,175)">LinkedIn</a></div></font></div></div></div></div></div></div></div></div></div></blockquote></div></blockquote></div></div></div></div>