Unnumbered PtP links (Was: Re: OSPF: incorrect path computation for v2.0.5+?)

[Ondrej Zajicek]

On Sat, May 23, 2020 at 10:43:52AM +0000, Joakim Tjernlund wrote:
> On Fri, 2020-05-22 at 22:59 +0200, Ondrej Zajicek wrote:
> > On Fri, May 22, 2020 at 07:14:44PM +0000, Kenth Eriksson wrote:
> > > On Thu, 2020-05-21 at 12:43 +0200, Ondrej Zajicek wrote:
> > > > This patch should fix the issue, could you try it?
> > > 
> > > Looks promising, applied on top of 2.0.7, and a quick test on the 5
> > > node setup looks correct. Will do some more testing.
> > > 
> > > We definitely need this fix in the pending 2.0.8 :-)
> > 
> > This issue has a long history. In 2012, we changed data field for
> > unnumbered PtP links from iface id (specified by RFC) to IP address based
> > on reports of bugs in Quagga that required it, and we used out-of-band
> > information to distinquish unnumberred PtPs with the same local IP
> > address.
> > 
> > Then with OSPF graceful restart implementation, we found that we can no
> > longer use out-of-band information, and we need to use only LSAdb info
> > for routing table calculation, but i forgot to finish handling of this
> > case, so multiple unnumbered PtPs with the same local IP addresses were
> > broken.
> > 
> > This patch returned back iface id to data field for unnumbered PtP links
> > (i.e. reverted back the change from 2012), while doing computation just
> > from LSAdb info. It fixed your case (multiple unnumbered PtPs with the
> > same local IP address) and is correct per RFC, but it may trigger bugs
> > with other implementations (like the one that led to the 2012 change).
> 
> Not sure I follow here, have you done away with rt_pos_to_ifa() and friends now
> and gone back to the old way?

Yes, it does not use rt_pos_to_ifa(). The approach with rt_pos_to_ifa()
does not work with graceful restart - after restart, router learns its
own LSAs (generated by previous run) and needs to do routing table
calculation without stored pos info. And it is probably bad idea to have
different route calculation algorithms in these cases.


> The old way had several drawbacks, one of them was this
> dependency on interface ID. Does current impl. depend on a well behaved neighbor too?

The current (2.0.7) is broken with regard to multiple unnumbered PtPs
with the same local IP address (as it uses only IP address in the data
field), but does not depend on well behaved neighbors.

The offered patch uses interface IDs, like described in RFC. That patch
is reliable (i do not see any issue with using interface IDs, what do you
mean?), but depends on well behaved neighbors. And it seems that there
are significat badly behaved ones.


> Is it compatible with any other Bird release?

Yes, that is not an issue. BIRD (at least post-2012) does not use 'data'
field of PtP links from neighbor LSAs. This field is only relevant for
the router who originated that LSA. It would work (for BIRD peers) even
if we put some random numbers here.


My current idea how to make it work without interface-ids and without
stored pos info: The problem is to match Router-LSA records with OSPF
ifaces that generated them. Instead of using just 'data' field, we can
use all fields ('data' to mach local IP address, router-id to see if
there is established neighbor with that router-id, and matching
configured cost).

And for case with two parallel equal links that are described in
Router-LSA by two equal records, we would have flag (in ospf_iface) that
ensures one OSPF iface is matched with at most one record, so the first
record is matched with the first matching iface and the second record is
matched with the second matching iface.

I would be glad to hear any comments to this idea or suggestions of other
ideas how to solve it.

-- 
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."
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