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Re: Comparison of symmetric and asymmetric PLSN algorithms

Subject: Re: Comparison of symmetric and asymmetric PLSN algorithms
From: Alia Atlas
Date: Tue, 14 Jun 2005 12:23:59 -0400
At 11:47 AM 6/14/2005, mike shand wrote:
At 16:10 14/06/2005, Alia Atlas wrote:
What I was asking was whether you are seeing multi-hop micro-forwarding loops using the symmetric condition & PLSN during the transition on the real network topologies that you have.
Ah. I understand now. Yes I do see the occasional multi-hop loop in real
topologies using the symmetric condition.
OK. But the frequency (number of micro-loops * number of links affected)
is better with the symmetric condition than with the asymmetric?
I was asking about the max-costing because that is something that does occur in networks to cause asymmetric link costs.
If the networks with asymmetric link costs that you have seem to have
fewer unresolved micro-loops across fewer links using the symmetric
condition, then that's a good start to think that the symmetric condition
is the way to go. To be even more comfortable with that, it'd be very
good to see if similar behavior holds with links costed out causing
asymmetric costs. I imagine that the primary case to consider might be
when an entire router's links are all costed-out, as in RFC3137. An
operator taking a link out via costing would probably do it in both
directions; similarly, the costing out of a link due to lack of an LDP
session should occur in both directions - though it needn't.
Understand. So I should try going through each node, setting all its link
costs to be infinite in the direction from that node, and looking for
loops? Is that what you mean? I'll put it on the list of things to try.
I'm sure you've got a long list :-) That's basically what I mean/think
makes the most sense.
I just tried a randomly generated topology with all (very) asymmetric links and managed to produce a 9 hop loop, which traversed 6 type C nodes, 2 type As and a type B!!!
Excellent! I assume this was using the symmetric condition- because
there are type As and a type B in it?
Um... what am I missing here? It is still possible to get type A and B
with the asymmetric condition isn't it?
  What does it look like using the asymmetric one?
I get the loop with both sym and asym conditions.
I'm a bit confused then. A multi-hop loop with the asymmetric condition
shouldn't go through type A or type B nodes. That's the point of changing
the condition to be stricter.
Regardless of whether a node is using the old topology or the new topology,
with the asymmetric condition, a type A or a type B router will use a
next-hop between DELAY_SPF and DELAY_TYPEB will be closer to the
destination. This guarantees monotonically decreasing distance, so that
loops cannot form.
Thus, a multi-hop micro-loop could go through a lot of type C's, but as
soon as it hits a type B or type A, I'd think it would exit the loop.
Maybe I'm missing something. I guess it might be possible for a type C to
send back up , but I'm not seeing how. Consider a type A node A and a
type C node C - where C is the first type C node encountered on A's
next-hop path to D (where this path is that used between DELAY_SPF and
We know that:
         D_old(A, D) > D_old(C,D)
         D_new(A, D) > D_new(C, D)

Can you describe the topology you're using? I'm not picturing how one could get the multi-hop micro-loop between type A, B and C when using the asymmetric condition.
Do you think we can adequately describe the topology conditions that can lead to the multi-hop micro-loop problem with the symmetric condition?
Perhaps, but I'm not going to attmept it right now:-)
Not a 5-minute response-time email?  I'm shocked :-)




Testing path from node 8 (8) to node 7 (7) at time 1

Node 8 (8): Packet discarded because looped
Node 19 (19) New: Forwarded via adj 1 over link 42 to node 8
Node 3 (3) New: Forwarded via adj 1 over link 13 to node 19
Node 28 (28) New: Forwarded via adj 1 over link 37 to node 3
Node 18 (18) New: Forwarded via adj 1 over link 18 to node 28
Node 29 (29) New: Forwarded via adj 1 over link 47 to node 18
Node 22 (22) New: Forwarded via adj 1 over link 23 to node 29
Node 11 (11) Temp: Forwarded via adj 1 over link 44 to node 22
Node 26 (26) Old: Forwarded via adj 1 over link 8 to node 11
Node 8 (8) Old: Forwarded via adj 1 over link 15 to node 26

(read the trace going up)

Node 11 is type B and nodes 3 and 11 are type A for the destination node 7

So at time 1 (when the type C's change), type A MUST forward according to new topology, type B according to the temporary type B next hop, and the type C either according to old or new topology (since they are in the process of changing)
So, yes, we can get multi-hop loops with the asymmetric algorithm!


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