[PATCH] Add 'preferred' route attribute for selective add-path

Sat Feb 27 11:05:21 CET 2021

Hello!
I have recently done some substantial changes in route exports (in separate branches). I'd like to summarize what we are looking for.

1) have 'primary' in the filter and decide based on it
2) export 'first N routes' with sorted table

I suppose this may be possible in the new code which is currently placed into the guernsey branch. I'll try to implement something of that while refactoring exports. 

Maria

On February 27, 2021 2:26:48 AM GMT+01:00, Trisha Biswas <tbiswas at fastly.com> wrote:
>Hi Ondrej,
>
>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.
>
>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?
>
>Thanks,
>Trisha
>--
>
>*Trisha Biswas* | Sr. Software Engineer, Network Systems
>fastly.com | @fastly <https://twitter.com/fastly> | LinkedIn
><http://www.linkedin.com/company/fastly>
>
>
>On Fri, Feb 26, 2021 at 3:53 PM Ondrej Zajicek <santiago at crfreenet.org>
>wrote:
>
>> On Fri, Feb 26, 2021 at 01:56:42PM -0800, Trisha Biswas wrote:
>> > Hi all,
>> >
>> > 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.
>>
>> Hi
>>
>> I agree this may be a useful feature, but there are two issues with
>the
>> approach used in the patch:
>>
>> First, we would like not to access rte->net from the filters (outside
>of
>> net_addr). We plan to remove this ptr and instead have ptr from rte
>> directly to net_addr.
>>
>> Second, RA_ANY mode of route announcement (which is used by BGP
>add-path
>> and pipes) would not really work with this change in a way how you
>expect
>> - consider this scenario: route A is added, preferred and accepted
>for
>> export, then route B is added, not-preferred and not-accepted for
>export.
>> Then route A is withdrawn and B is selected as preferred. Protocols
>with
>> RA_OPTIMAL received update with route B as new-preferred (as they
>accept
>> only preferred one and that changed), but protocols with RA_ANY
>receive
>> just withdraw for route A (as they accept all routes and only route A
>was
>> changed, while route B is the same as was announced before), so they
>do
>> not get notification that route B became preferred and export filter
>for
>> B is not re-evaluated.
>>
>> The one approach that avoids the first issue is to add REF_BEST flag
>> (in rte->flags) that would be added to mark preferred routes, and
>this
>> flag can be accessed from the filter code. But that does not fix the
>> second issue. We can add RA_ANY announcement for routes that changed
>> their REF_BEST, but that would add plenty of unnecessary
>announcements
>> (that are propagated as BGP updates) for BGP add-path that do not
>> use or care about this flag.
>>
>> In general, BGP add-path in BIRD was developed with assumption that
>> routes within one network are handled independently and would need
>some
>> deeper redesign for other cases. That might be useful (in order to be
>> able to implement things like 'announce best 4 routes for given
>> network'), but i do not see an easy solution to this.
>>
>> --
>> Elen sila lumenn' omentielvo
>>
>> Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
>> OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3,
>wwwkeys.pgp.net)
>> "To err is human -- to blame it on a computer is even more so."
>>
>
>> On Fri, Feb 26, 2021 at 1:56 PM Trisha Biswas <tbiswas at fastly.com>
>wrote:
>>
>>> Hi all,
>>>
>>> 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.
>>>
>>> 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.
>>>
>>> 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
>>> 172.16.0.0/16, 10.0.0.0/8{16,24} <http://10.0.0.0/8%7B16,24%7D>
>(regardless
>>> of best route status). In the case of 8.8.8.0/24, we export it if
>it's
>>> the best route, or if it additionally has the specified extended
>community
>>> attached to the route.
>>>
>>> protocol bgp {
>>> [....]
>>> add paths tx;
>>> export filter {
>>> if preferred || (net = [172.16.0.0/16, 10.0.0.0/8{16,24}
>>> <http://10.0.0.0/8%7B16,24%7D>]) then accept;
>>> if preferred || (net = 8.8.8.0/24 && (ro, 1, 2) ~ bgp_ext_community)
>>> then accept;
>>> #remaining filter code
>>> reject;
>>> }
>>> }
>>>
>>> Cheers,
>>> Trisha
>>> --
>>>
>>> *Trisha Biswas* | Sr. Software Engineer, Network Systems
>>> fastly.com | @fastly <https://twitter.com/fastly> | LinkedIn
>>> <http://www.linkedin.com/company/fastly>
>>>
>>

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://trubka.network.cz/pipermail/bird-users/attachments/20210227/a22f25e1/attachment.htm>