Re: [trill] TRILL OAM Requirements: available paths

Of course it is possible. The IS-IS topology table provide the
information on all available ECMP paths to a specific destination when
the egress RBRidge nickname is known. That is what TRILL unicast
forwarding table all about. Not sure why you are saying it is not
possible to know all ECMP paths.

What path among available paths to be selected for a specified is a
local matter. That is what all about flow entropy discovery. Like I
stated earlier how it is discovered is a framework and solution issue.
If you need some reference how it is done in the MPLS world RFC 4379 is
a good reference.

-----Original Message-----
From: trill-bounces@ietf.org [mailto:trill-bounces@ietf.org] On Behalf
Of Erik Nordmark
Sent: Friday, April 27, 2012 4:07 PM
To: Santosh Rajagopalan
Cc: trill@ietf.org
Subject: Re: [trill] TRILL OAM Requirements: available paths

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
>
> _______________________________________________
> trill mailing list
> trill@ietf.org
> https://www.ietf.org/mailman/listinfo/trill
>
>

_______________________________________________
trill mailing list
trill@ietf.org
https://www.ietf.org/mailman/listinfo/trill