On 09/11/2011 20:54, Gábor Sándor Enyedi wrote:
Hi Steward,
That's forced first hop won't help; I can always add an extra node against such
tricks. :)
4 4
[Src]-----[A]------[B]
| |
2| |1
| |
[C]-----[D]-----[Dest]
1 3
You cannot solve this without "directed forwarding" (forcing B with some magic to
forward packets to A instead of to Dest). Think on that phenomenon like this: until you have
only P chance to protect a failure, protecting it in both directions will always be P*P<P
until P<1. :)
Gabor
We would have solved that in the original tunnels draft with directed
forwarding at B.
However you have not answered my point that practical rings (as opposed
to ring fragments in mesh topologies) normally have equal cost per hop.
Stewart
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Stewart Bryant
Sent: Wednesday, November 09, 2011 6:56 PM
To: András Császár
Cc: [email protected]
Subject: Re: Charter Update (Discussion)
Hi András
Yes, it is true that for that topology fragment there is no bidirectional LFA
solution.
However you were talking about rings, and surely it's unusual for a ring
topology to have non-uniform costs.
If we consider tunnel solution with forced first hop the solution would work.
C tunnels to A with forced first hop (C-Dest), decap and A to Src.
Src LFAs to A, A sends to B, B sends to Dest
- Stewart
On 09/11/2011 10:59, András Császár wrote:
Hi Stewart, this phenomenon occurs with symmetric costs with LFA.
Trying to sketch an example:
2 2
[Src]-----[A]------[B]
| |
1| |1
| |
[C]-------------[Dest]
2
Now for Src->Dest traffic, the link failure of Src-C can be remedied with LFA,
as Src may pass the packet to A which is an LFA. For backwards traffic (e.g. TCP
Acks) this link failure cannot be solved by C with LFA, so ultimately the Src-Dest
traffic is screwed.
András
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Stewart Bryant
Sent: 2011. november 8. 18:54
To: [email protected]
Subject: Re: Charter Update (Discussion)
On 08/11/2011 17:20, András Császár wrote:
I think we should first see if there is a solution which
provides full coverage and practical (=reasonable complexity).
One problem with non-full-coverage solutions is
bidirectional coverage. I.e. even if a flow is covered for a failure
in one direction it may not be covered in the reverse direction which
is, for most applications, equal to not being covered at all. This
property is often neglected in coverage estimates. As an example
consider the LFA unfirendly sub-topologies like longer-than-triangle
rings. In those rings it might seem that the failures on the opposing
side to the exit point are covered by the LFA. But they are not
covered in the reverse direction. Might be a factor to consider for
PQ and co too.
Can you give me an example here?
I would expect a ring to have symmetric properties except when there
are asymmetric costs, but normally assymetric costs are a
configuration accident.
Stewart
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