Re: [trill] TRILL OAM Requirements: available paths

Erik,

The forwarding table is setup based on the control plane information, is-is, for all the ecmp paths. I am not clear when you say that requirement is not relevant for this. Even if the forwarding is local matter, one should be able test each of those ecmp paths using entropy field for crafting oam packets. This is how ecmp in other network types work as well, including trill.

If your understanding of the ecmp in trill is different to what I eluded to, I would like to hear it. 

Cheers
Sam
Sent from my iPhone

On Apr 27, 2012, at 4:07 PM, Erik Nordmark <nordmark@acm.org> wrote:

> On 4/27/12 2:15 PM, Santosh Rajagopalan wrote:
>> I agree, the wording needs to be tightened to say "all possible ecmp
>> paths" rather than "all available paths". An operator working out R1-R2
>> communication is interested in all possible paths data traffic could
>> take rather than all theoretically available paths.
> 
> The issue is that there is nothing in the TRILL dataplane nor IS-IS control plane which helps the operator determine that some paths (through RB121 and RB112 in the example) are not possible; the ECMP input and hashing is a completely local matter on each RBridge.
> 
> It isn't useful to state a requirement around something that can't be determined.
> 
>   Erik
> 
>> 
>> 
>> From: Erik Nordmark <nordmark@acm.org>
>> To: trill@ietf.org
>> Date: 04/27/2012 09:53 AM
>> Subject: [trill] TRILL OAM Requirements: available paths
>> Sent by: trill-bounces@ietf.org
>> ------------------------------------------------------------------------
>> 
>> 
>> 
>> 
>> 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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> 
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