Re: [trill] TRILL OAM Requirements: available paths

We can make the wording uniform. It was intended to be all available
ECMP paths. Because with IS-IS in place you have only visibility to the
Equal cost paths. When using multi-level TRILL, still the packets get
forwarded along the Equal Cost Multipaths when it is available. How the
entropy is derived to cover different equal cost paths is in the scope
of a framework/solution.

-----Original Message-----
From: trill-bounces@ietf.org [mailto:trill-bounces@ietf.org] On Behalf
Of Erik Nordmark
Sent: Friday, April 27, 2012 9:53 AM
To: trill@ietf.org
Subject: [trill] TRILL OAM Requirements: available paths

Two requirements use the wording "all available paths." and "all
available ECMP paths.", respectively and I'm not sure I know what that
means.

Take the following example picture:

                R1
               /  \
              /    \
             /      \
            /        \
           /          \
        RB11          RB12
        /  \          /  \
       /    \        /    \
    RB111  RB112  RB121  RB122
       \      \    /      /
        \      \  /      /
         \      \/      /
          ------R2------

We can see that there are 4 paths with 3 hops to get from R1 to R2, with
two layers of ECMP decisions. But how many available paths are there?

Given that the RBridges make independent ECMP decisions, and how they do
that is a local matter, it could be the case that for any packet, if R1
picks the left link, then R11 would also pick the left link; if R1 picks
the right link, then R12 also picks the right link.
Thus there would be no packet (data or OAM) that would cause packets
between R1 and R2 to pass via RB112 or RB121.

Even if we ignore that as unlikely, there is still the fact that R1
can't easily tell how many (shortest) paths there might be between it
and R2. With single-level TRILL the LSDB could be used to tell, but if
we ever go to multi-level that wouldn't be the case any more.
And even if R1 knows that there are 4 potential paths, it can't format a
data packet (or format the flow entropy in an OAM packet) in such a way
that particular paths get chosen by downstream ECMP decisions.

Thus for any notion of "all available paths" that make sense to me, we'd
need an OAM approach that can explore the topology one hop at a time and
at each hop explore all the possible ECMP choices. Using the entropy
doesn't help with this (but the entropy is critical for following the
path taken by a particular flow of data packets).

Hence it would be good to clarify what the assumptions are behind this
notion of all available paths.

Thanks,
     Erik

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