Re: [OSPF] OSPFv3 Autoconfiguration - draft-ietf-ospf-ospfv3-autoconfig-05.txt

[Acee Lindem <[e-mail]>]

Hi Les,
You’re certainly right about one thing - I still do not like the additional machinery and don’t believe it is worth the extra signaling and specification to handle an extremely unlikely event. Additionally, the consequences of a router-id change need not result in any traffic loss. It is all dependent on SPF convergence settings, timing, network size/diameter, and, most of all, implementation. We can discuss when I present at IETF 89. In the revision I just submitted, I’ve included the following text to section 8, Management Considerations:

    Since there is a small possibility of OSPFv3 Router ID collisions,
   manual configuration of OSPFv3 Router-IDs is RECOMMENDED in OSPFv3
   routing domains where route recovergence due to a router ID change is
   intolerable.


Thanks,
Acee

On Feb 11, 2014, at 1:29 AM, Les Ginsberg (ginsberg) <[e-mail]<mailto:[e-mail]>> wrote:

Acee -

One thing I have repeatedly emphasized is the simplicity of what I am proposing. You mention below that the proposal requires having fingerprint info available before LSDB exchange would normally occur in order to handle router-id collisions between neighbors - and you are understandably concerned about the complexity that would introduce.

There is no such requirement!! I am NOT proposing this!!

This is simply a misunderstanding on your part.

Here is all that is necessary:

Router-id collision between neighbors
--------------------------------------

I receive a hello with a duplicate router-id. The hello includes an options bit (or LLS if you prefer) indicating whether the neighbor considers itself "old/new". Because I cannot form a neighbor with "myself" I need to resolve the duplicate router-id. There is no need to have or wait to acquire the neighbor fingerprint information. The logic is solely based on the content of the received hello and my internal state (old/new). The logic is:

if ((I am old) && (neighbor is new)) {
 return; /* Neighbor will change its router-id */
} else if (I am new) && (neighbor is old)) {
  Change_my_router_id();
} else /* We are both old or both new */
 if (my link_local address < neighbor_link_local_address) {
   change_my_router_id();
 }
}

That's it! No additional logic required.
Similarly...

Router-id collision between non-neighbors
--------------------------------------

In this case we have acquired the LSDB so we detect a duplicate router-id as described in the draft Section 6.2. But fingerprint info includes a bit which indicates whether the originator is "old" or "new".
The logic is then:

if ((I am old) && (non-neighbor is new)) {
 return; /* Non-Neighbor will change its router-id */
} else if (I am new) && (non-neighbor is old)) {
  Change_my_router_id();
} else /* We are both old or both new */
 if (my fingerprint < non-neighbor fingerprint) {
   change_my_router_id();
 }
}

Setting of old/new state is done simply on the basis of a minimum amount of uptime (e.g. 5 minutes). The rather complex logic that Curtis proposed in an earlier reply is not needed - I will address why in a follow up response to his post.

You may still not like the idea - but complexity is not a basis for rejection. Hopefully what I have provided makes that clear.

  Les

-----Original Message-----
From: Acee Lindem [mailto:[e-mail]]
Sent: Monday, February 10, 2014 2:55 PM
Cc: [e-mail]<mailto:[e-mail]>; Les Ginsberg (ginsberg); OSPF List
Subject: Re: [OSPF] OSPFv3 Autoconfiguration - draft-ietf-ospf-ospfv3-
autoconfig-05.txt

One thing I neglected to mention is that a solution dependent having the
other router's hardware fingerprint available will greatly complicate
duplicate router-id detection for directly attached routers since the
resolution must either be deferred until the router's AC-LSA is received from
another router OR the hardware fingerprint must be signaled in the OSPFv3
Hello LLS. IMO, this complexity adds to the motivation of not attempting to
solve this problem.
Thanks,
Acee
On Feb 10, 2014, at 5:17 PM, Acee Lindem <[e-mail]<mailto:[e-mail]>> wrote:

Hi Curtis,

See inline.

On 2/9/14 12:38 PM, "Curtis Villamizar" <[e-mail]<mailto:[e-mail]>> wrote:


Les,

Perhaps you should read the abstract of the document you are
commenting about:

SPFv3 is a candidate for deployments in environments where auto-
configuration is a requirement.  One such environment is the IPv6
home network where users expect to simply plug in a router and have
it automatically use OSPFv3 for intra-domain routing.  This
document describes the necessary mechanisms for OSPFv3 to be
self-configuring.

Home network!

Or the introductio:

OSPFv3 [OSPFV3] is a candidate for deployments in environments
where auto-configuration is a requirement.

[...]

1.2. Acknowledgments

   This specification was inspired by the work presented in the
   Homenet working group meeting in October 2011 in Philadelphia,
   Pennsylvania.

The Homenet WG works on what?  Home networks!

So please keep that in mind when commenting.

Unless a provider were to be so stupid or lazy to use this on a SP
network then most of the comments from both of us don't apply,
*except* the few comments below about "in a home network".

Perhaps the draft should add text explicitly stating that the last
router-id used successfully should be used on a reboot rather than a
new random number.  I notice that only the Router-Hardware-Fingerprint
TLV is persistent across reboots.  This is insufficient if we want to
minimize disruption.

The only case then (if router-id is remembered across reboots) would
be a new router.  In that case your uptime rule would help.  So
perhaps two things could be reocmmended:

1.  In section 4, include a "SHOULD remember the most recent
   successfully used router-id across reboots and reuse that".
   Reword the rest so if that information is not available, then
   pick a random number.

I will do this.




2.  a.  In section 6, mention the uptime rule.  Modify the Router
       Uptime TLV as suggested.

   b.  Alternately add a flag to the Router-Hardware-Fingerprint
      TLV that indicates that since last reboot this router-id has
      been used and acheived a "full state".  A router just
      rebooting would not have ever reached the full state before
      noticing a conflict as long as the conflct check is run
      before considering itself in the full state.

       Note: A second flag bit indicating that this router-id had
      been used successfully in a past reboot might also help but
      would only matter among two routers both rebooting and
      neither having reached the full state.

I think #1 above is sufficient and does more to prevent surprises.

I agree and appreciate you arguments in previous messages in this thread.


I
think #2 above helps only in the new router case but #2a requires
adding a TLV and so isn't worth it IMHO.  Case #2b accomplished the
same thing with only a flag.  I would not object to #2b above if #1
above is also added.

I agree that this would be a better mechanism and would only represent a
single modification to the hardware fingerprint TLV. However, I really
don't think even this is necessary.

Thanks,
Acee




See inline anyway.

In message
<[e-mail]<mailto:[e-mail]>>
"Les Ginsberg (ginsberg)" writes:

Curtis -

-----Original Message-----
From: Curtis Villamizar [mailto:[e-mail]]
Sent: Saturday, February 08, 2014 7:30 AM
To: Les Ginsberg (ginsberg)
Cc: [e-mail]<mailto:[e-mail]>; Acee Lindem; OSPF List
Subject: Re: [OSPF] OSPFv3 Autoconfiguration - draft-ietf-ospf-ospfv3-
autoconfig-05.txt


In message
<[e-mail]<mailto:[e-mail]>>
"Les Ginsberg (ginsberg)" writes:

Curtis -

Your reply below is talking about things which I think do not
directly
bear on the value add of what I have proposed.

You mention various ways to insure that a given device assigns the
same router-id each time it starts up and ways to insure it picks
the
same sequence of second/third... choices in the event it has to
change
its router-id. All good suggestions, but what I am talking about is
what we do in the event a conflict occurs despite our best efforts
to
avoid it. With the current draft content preference is based solely
on
a fixed identifier (fingerprint) without regard to which choice
would
minimize disruption to the network. When preference is given to the
"old router" to retain its existing router-id this shortcoming is
addressed.

In the lifetime of a router it only gets added once.  In the lifetime
of a router we would hope it only reboots zero time but experience so
far has been that reboots over a router's lifetime tend to be > 0 and
in some cases >> 0.

So you are optimizing for a 1 in 4 billion occurance that can happen
only once in the lifetime of a router.

The entire duplicate router-id resolution logic is addressing the
improbable case. My proposal adds - literally - one line of code to
the logic used to decide whether "I" should change my router-id or
whether "you" should change your router-id.


We also need to look at the consequences of this very improbably
occurance.  Today's routers accomplish IGP convergence in large
networks in subsecond times, in some cases << 1 second.

Note that if flooding is completed (both withdraw old and install new)
in less than the SPF delay which is commonly implemented (some delay
after receiving the first flooded IGP change), then there is no impact
on routing.

Your analysis does not apply to this scenario. The router which
changes its router-id is effectively doing a cold start. All
adjacencies will go down. All LSAs originated by this router become
invalid. All routes will be removed from the forwarding plane. If
you are running BGP all the BGP nexthops will be gone on the router
which is changing its identity. Restoration of the adjacencies and
reacquisition of the LSDB will take multiple seconds. The best you
can hope for is several seconds of disruption - it could easily be
much longer.

For the new node which has usurped the old node's identity it will
have to purge/replace all of the LSAs generated by the old
node. While normal operation of the update process will insure that
this happens in a reliable way the amount of flooding network-wide
required to bringup a new node has now been roughly doubled
i.e. the old node must reissue all of its LSAs using a new identity
and the new node must purge/replace the old node's LSAs with its
own versions. This will result in multiple SPFs on all nodes in the
network and likely cause loops/blackholes during the transition
since some of the SPFs will be run on versions of the LSDB which
are inaccurate (part old node's old LSAs and part new node's
LSAs). Suggesting that this could be handled in the same way/time
as we typically handle a single link failure isn't credible.

All routers are supposed to keep a fixed router-id across reboots.  If
interfaces are changed when down, the last used router-id should be on
flash.  If flash is removed and replaced (rather than a new image
installed), then with the same set of interfaces, the same decision
should be made.  We are down to a very special case where both flash
and interfaces are removed and replaced yielding no history and a
different set of MACs to pick from.

Your statement that what I propose is only relevant when two routers
go down does not match the scenarios I envision. If I want to add a
new device to my network or if I need to replace an existing device
in
my network I am only affecting one device - but as I am introducing
a
device with a new fingerprint it is possible that it will introduce
a
conflict with an existing router-id.

In provider networks routers are generally added during maintenance
windows so should anything unexpected happen, impact is minimized.

In home nets, the home user isn't going to notice the convergence time
if there is any.  A 10 msec SPF delay is likely to be plenty.

As I stated above, disruption will be orders of magnitude longer than
10 ms.

In a home net?  With perhaps a half dozen routers and a default route?
Someone has a very bad OSPF implementation.  :-)  Or did you miss the
"In home nets" at the front of the paragraph.

For example, in a 10 node network with average degree 4, perhpas 40
links in 10 router LSA exist.  A few RTT (less than 1 msec for a
homenet) for each neighbor adjacency (which happen in parallel) and
ten packets from 4 sources is needed to reach the full state followed
by one SPF to be fully up and running.  Other routers get one
additional router LSA plus four new links in existing router LSA and
have to run an SPF.  Even on a software based homenet router using an
ARM, 10 msec is likely to be enough time and if it is "orders of
magnitude" longer, something is wrong with one of the implementations.
This would be an more complicated than usual home net or even soho,
more likely a small business.

In a subsequent reply you liked the idea of the new device delaying
advertising reachability until it is has determined that its
router-id
choice is not in conflict. The old/new router paradigm supports this
strategy by assuring that the old router will not consider changing
its router-id until enough time has elapsed for the new router to
transition to being an old router.

If it wins the coin toss, the router would advertise at least one LSA
to indicate its existance and could hold back on any additional
advertisements until the other router has withdrawn routes.


This suggestion does not alter the fact that if the old node changes
its router-id the network has to respond to three events:

1)Loss of the old node
2)Introduction of the old-node with a new identity
3)Introduction of the new node with the identity of the old-node

Again, the old node should remember the last router-id used and try to
reuse it.

If however we insure that the old-node does not change its identity
then the network only has to respond to a single event - the
introduction of the new-node.

Yes and if it were up and won the resolution last time, it will have
saved that router-id and will reuse it.  If it came up previously and
lost the resolution, then it will remember the router-id it used,
whether second or third pick, and use that.

Finally, what I propose is extremely simple to implement. I think it
isn't much of an exaggeration to say that any one of us could have
implemented the enhancement in the time it has taken to discuss its
merits. So we aren't overengineering for a case which is admittedly
very unlikely to occur - we are adding a modest extension to make
our
solution less disruptive.

Yes but it it *bad* for the more common case where routers go down
occasionally.

You are going to have to clarify exactly what "bad side effects" you
see for what I propose - because I don't see any - whereas I do
see benefits as described above.

If router-id is not remembered between reboots, then there is the one
in 4 billion time number of routers (less than 10 for a home net
today, but maybe more in the future).

If router-id is remembered between reboots, then no matter how long a
router has been down, if nothing else in the network changed, there is
zero chance of having a collision.

With either method, if router-id is remembered between reboots, then
there is zero chance of collision.

IMO should this ever be used on a managed network (including a home
net / soho / small business net that happens to be managed) then
having routers come back from a reboot with the same router-ids would
be a big plus.  For example, after a power outage NMS discovery would
not have to be repeated.

 Les



 Les

Curtis


-----Original Message-----
From: Curtis Villamizar [mailto:[e-mail]]
Sent: Friday, February 07, 2014 9:22 AM
To: Les Ginsberg (ginsberg)
Cc: Acee Lindem; Curtis Villamizar; OSPF List
Subject: Re: [OSPF] OSPFv3 Autoconfiguration -
draft-ietf-ospf-ospfv3-
autoconfig-05.txt


In message <F3ADE4747C9E124B89F0ED2180CC814F23C619A9@xmb-aln-
x02.cisco.com<http://x02.cisco.com>>
"Les Ginsberg (ginsberg)" writes:

So, I am one person who raised this concern to Acee - but the
proposal
outlined by Acee is not what I had in mind. There is no need to
use
"uptime" or to invent some unusual exchange of LSAs prior to
Exchange
state.

Also, in regards to Curtis's comment - it is not DOS attacks
that I am
trying to mitigate here. As he says if an attacker is in your
network
and able to originate credible packets no strategy is safe.

The motivating use case is to minimize disruption of a stable
network
when a new router is added or an existing router is
replaced/rebooted. In other words non-disruptive handling of the
common maintenance/upgrade scenarios.

What I have in mind is this:

1) A router needs a way to advertise that it has been up and
running
 for a minimum length of time - for the sake of discussion
let's say
 20 minutes. Routers then fall into two categories:

o Old routers (up >= minimum time)
o New routers (up < minimum time)

2) When a duplicate router-id is detected, the first tie
breaker is
 between old routers and new routers. The old router gets to
keep
 its router-id and the new router picks a new router-id.  If
both
 routers are "new" or both routers are "old" then we revert
to the
 existing tie breakers defined in the document (link local
address
 for directly connected routers and fingerprint info for
 non-neighbors).

3) Advertisement of the "old/new" state requires a single bit -
but it
 has to be available both in hellos and the new AC-LSA.
Adding it to
 the AC-LSA is easy to do. For hellos, there are two
possibilities:

 o Use one of the Options Bits
 o Use LLS

Be interested in how folks feel about this.

 Les


Les,

Excluding DoS attack, we are talking about a one in 4 billion case
(for any two routers, so with 400 routers, still well under one
in 1M)
where two routers hash a MAC address or pick a one time random
number
from out of nowhere and end up with the same number.

If that does happen (and one in 1M is certainly possible), then it
would be nice if the routers always ended up with the same
router-id.
This could be accomplished by some fixed method such as hashing a
constant with the first choice or router-id or using the
router-id as
a seed for the random number generator (which will pick the same
sequence of random numbers each time).  If this is done, then a
conflict would always produce the same set of next picks.  The
set of
routers in a given network would always end up with the same
router-ids once they all came up and if only one went down at a
time
then it would always end up with the same router-id when it came
up.

Zero conf was mainly intended for unmanaged networks (motivated by
work in the homenet WG).  In these small unmanaged networks it
doesn't
matter which router gets what router-id as long as they end up
unique
and convergence is in a reasonable time relative to keeping
eyeballs
happy.  It could be applied to enterprise or providers but in
either
case having the routers end up with the same router-ids would
make for
easier management.

For your scenario to matter at all with current rules, both
routers in
the conflict would have to go down.  If only the one that is
preferred
goes down, the other is not going to change its router-id as a
result
so when it comes up it gets its first pick with no conflict.  If
the
one that was not preferred goes down, it comes up, sees a
conflict and
takes its second pick (loses the conflict every time).  It is
only if
they both go down and the one that normally loses the conflict
comes
up first that there is a change in router-id.  That too can be
solved
with a rule that you always come up with the last router-id used.

Curtis


-----Original Message-----
From: OSPF [mailto:[e-mail]] On Behalf Of Acee
Lindem
Sent: Thursday, February 06, 2014 5:12 PM
To: Curtis Villamizar
Cc: OSPF List
Subject: Re: [OSPF] OSPFv3 Autoconfiguration -
draft-ietf-ospf-
ospfv3-
autoconfig-05.txt

Hi Curtis,
I agree and believe the significance of this use case where a
new
router
is
inserted into an auto-configured domain has been greater
exaggerated.
Thanks,
Acee
On Feb 5, 2014, at 3:58 PM, Curtis Villamizar
<[e-mail]<mailto:[e-mail]>>
wrote:


In message <[e-mail]<mailto:[e-mail]>>
Acee Lindem writes:

The OSPFv3 autoconfiguration draft was cloned and
presented in the
ISIS WG
(http://www.ietf.org/id/draft-liu-isis-auto-conf-00.txt).
In
the ISIS WG, there was a concern that the resolution of a
duplicate
system ID did not include the amount of time the router was
operational when determining which router would need to
choose a
new
router ID. With additional complexity, we could
incorporate router
uptime into the resolution process. One way to do this
would be
to:

  1. Add a Router Uptime TLV to the OSPFv3 AC-LSA. It
would
include
     the uptime in seconds.

  2. Use the Router Uptime TLV as the primary
determinant in
     deciding which router must choose a new OSPFv3
Router
     ID. Router uptimes less than 3600 (MaxAge) seconds
apart
are
     considered equal.

  3. When an OSPFv3 Hello is received with a different
link-
local
   source address but a different router-id, unicast the
OSPFv3
   AC-LSA to the neighbor so that OSPFv3 duplicate router
   resolution can proceed as in the case where it is
received
   through the normal flooding process. This is somewhat
of a
   hack as the we'd also need to accept OSPF Link State
Updates
   from a neighbor that is not in Exchange State or
greater.

An alternative to #3 would be to use Link-Local Signaling
(LLS)
for
signaling the contents of the OSPFv3 AC-LSA. However,
you'd only
want
to send the Router-Uptime and Router Hardware Fingerprint
when a
duplicate Router-ID is detected. This requires
implementing the
resolution two ways but may be preferable since it doesn't
require
violating the flooding rules.

In any case, I'd like to get other opinions as to whether
this
problem
is worth solving.

Thanks,
Acee


Acee,

If the basis for router-id on boot up results in a fixed
value, and
if
a duplicate will occur on a give network, then which of two
duplicate
routers gets that value may change after one of them
reboots.  If
uptime is not considered, it will never change as long as
one
router
stays up at any given time.

We are talking about a very low probability event (a
duplicate)
except
if this is a DoS attack and then either using or not using
uptime
won't matter since the attacker will claim an impossibly
long
uptime.

Curtis

_______________________________________________
OSPF mailing list
[e-mail]<mailto:[e-mail]>
https://www.ietf.org/mailman/listinfo/ospf

_______________________________________________
OSPF mailing list
[e-mail]<mailto:[e-mail]>
https://www.ietf.org/mailman/listinfo/ospf