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Re: Detecting router id change in p2p-over-lan connection

Hi Sunil,

Sunil Patro wrote:

Hi Everyone,

Thanks for all the feedback.

Can I safely assume that the final behavior is what Acee's recommended
paragraph says below?

I believe that is the consensus and barring a very compelling argument otherwise, this will be
the final text.

Thanks,
Acee

"If the system ID or the router ID in an incoming hello packet
doesn't match the the system ID or router ID for an established
adjacency
over a p2p-over-lan circuit, the packet MUST be discarded. Futhermore,
if OSPF hello suppression as described in [DEMAND] is active for the
adjacency, it MUST be terminated and renegotiated."

Thanks,
Sunil



-----Original Message-----
From: Mailing List [mailto:OSPF at PEACH.EASE.LSOFT.COM] On Behalf Of


Naiming


Shen
Sent: Tuesday, February 21, 2006 9:43 PM
To: OSPF at PEACH.EASE.LSOFT.COM
Subject: Re: Detecting router id change in p2p-over-lan connection

Hello Acee,

Your suggested paragraph looks fine with me.

Even though this sentence assumes there is really a
router-id change from a DC neighbor. A router probably
should send a 'neighbor probing' [RFC 3883], if no
response, then reset the adjacency and start
the renegotiation; otherwise it should still ignore
the new join on the p2p-over-lan link.

But I would consider router-id change over a demand
circuit over a p2p-over-lan on OSPF link a
'misconfiguration';-) so it does not matter too much.
Your version is fine.

thanks.
- Naiming

Acee Lindem said the following on 02/21/2006 02:47 PM:


Hello Naiming,

Naiming Shen wrote:



Acee Lindem said the following on 02/21/2006 06:04 AM:



Naiming Shen wrote:



Agree with Dave's argument, and this is the same idea as in the
current draft of p2p-over-lan (section 4.5):

... If the system ID or the
router ID of incoming hello packet does not match the system


ID or


the router ID of already established adjacency over this
p2p-over-lan
circuit, it MUST discard the packet. The implementation should


offer


  logging and debugging information of the above events.

Do we need to change anything here?




Hi Naiming,

Given that this situation can happen in the case of


misconfiguration


or


router-ID change, I wouldn't optimize for the former. Especially
considering Kalyan's point that if hello's are suppressed, the
router-id change
will either not be recognized or require additional logic. I'd


vote


to either
leave it unspecified (consistent with RFC 2328's omission of


router-id


change P2P link handling) or discard the old neighbor and bring up
an adjacency with the new (which is what at least one


implementation


does
today).



Hello Acee,

The original idea in the draft was to prevent multiple routers


joinning


onto the same LAN rather than handing the router-ID change from the
same router. Even through there is a side effect to it.

For the router-ID change issue. If one is willing to change


router-ID


or system-ID during normal operation, there are more things broken
and there is nothing wrong to wait until the current adjacent


timeout.


Agreed.



If the hello is suppressed over the link, then the behaviour is not
bounded by this paragraph since it does not say the purpose is to
detect neighbor router-id change.


I disagree here. It says "MUST discard packet". That sounds pretty
bounding to me :^).



Also there is little reason to
suppress a hello over p2p-over-lan links. Thus I don't think
router-id is an issue with this sentence.


I agree - but there is nothing in this draft or RFC 1793 that


precludes


it.


Since p2p-over-lan is different from normal p2p circuit that


multiple


routers can easily join the same LAN unintentionally. If we ignore
the new station to join, then we can prevent the adjacency constant
flapping(this does not happen to normal p2p link), the worst it can
happen is slow to handle router-id change as mentioned above, which
is a non issue. Thus I think it's a good thing to ignore the extra
adjacency joinning. This sentence does not govern the normal p2p


link


behaviour so the implementation is ok(even prefered) to reset the
existing adjacency on a normal p2p link. Thus I don't think we
should say we must reset the current adjacency for p2p-over-lan.

My preference of this sentence in the draft is with this order:

- leave it as is
- change the MUST to SHOULD
- remove the router-id related sentence


How about leave it "almost" as is:

If the system ID or the router ID in an incoming hello packet


doesn't


match the the system ID or router ID for an established


adjacency


over


a p2p-over-lan circuit, the packet MUST be discarded.


Futhermore,


if OSPF hello suppression as described in [DEMAND] is active for


the


   adjacency, it MUST be terminated and renegotiated.

[DEMAND]   Moy, J., "Extending OSPF to Support Demand Circuits",
            RFC 1793, April 1995.

Thanks,
Acee



thanks.
- Naiming



Thanks,
Acee

Thanks,
Acee



thanks.
- Naiming

Dave Katz said the following on 02/20/2006 08:53 PM:



RFC2328 is silent on what to do if you see a different router ID


on


a P2P link; if you take it literally, you will create two
adjacencies (temporarily), advertise both in your router LSA,


and


your network will most likely black hole traffic until one of


them


times out.

On real P2P links, this will suffice (if you don't mind a


network


outage for the dead time of the old adjacency) and things will
eventually stabilize since it is "impossible" (cough) two get


two


neighbors in 2Way.  The fact that it is underspecified doesn't
matter  that much.

In the P2P-over-LAN case, there needs to be a mechanism to try


to


do something useful if there turn out to be more than two


routers


on the  link.  I don't think it's reasonable to say "gee it's
misconfigured,  we don't care what happens to the network."


It seems to me that there are two possible cases, namely, use


the


old adjacency until it times out (effectively ignoring the


hellos


from  the new guy) or to form an adjacency with the new guy (and
blow away  or otherwise ignore the old guy.)

The first case has the advantage of stability and


simplicity--the


third router that shows up for the party will be ignored by both
parties. There is a potential for a circle of one-way


adjacencies


to  stabilize, but this will still be stable (there won't be a
usable  adjacency across the subnetwork.)

The second case has the advantage that it adapts more quickly


when


something changes. Some implementations already do this for the
pure P2P case. However, it is destabilizing in the
misconfiguration case, as the router will flap its router LSA


each


time another Hello is received. Probably the only reasonable


way


out would be to detect the flap and then choose one neighbor


and


stick to it, ignoring the other (which degenerates into the


first


case.)

Given all of this, I'd propose that the right thing to do is the
first case, and suffer through the dead time (or use BFD and
suffer  through the detection time.)

(A third possible case would be to actually bring up the


adjacency


and end up running full-mesh multipoint, which would probably


even


work, but would be butt ugly.)

--Dave


On Feb 20, 2006, at 3:25 PM, Acee Lindem wrote:



Kalyan Bade wrote:



Acee,




This might not work if the interface is defined as demand


circuit.





Good point - We're in the process of discussing whether this
behavior
should be in the
draft. Possibly, handling detection of a router-id change on


a


P2P


over



LAN link
differently than a normal P2P link may be a bad idea.



How about bringing down the neighbor (rather than ignoring)


when


a third
neighbor on a P2P over LAN sends a new hello? This creates
unnecessary
flaps, but again having more than 2 routers on a p2p LAN is a
misconfig.
Atleast, the processing will be similar to a regular p2p


interface.


That would be fine and consistent with what most routers do on
normal P2P links. I think the
P2P over LAN specific router-ID change processing should be
removed  from the draft.

Thanks,
Acee



Thanks,
Kalyan.