Subject: RE: documenting ECMP - was on the mpls oam framework

From: "David Allan" <[email-masked]>
Date: Tue, 18 Nov 2003 03:06:12 -0500

Rahul:

You are missing the point, if we do not know how ECMP is implemented, we
cannot critique the solutions or participate in the design. IMHO properly
characterizing the dataplane is a pre-requisite to being able to design
maintenance procedures. For example VCCV proposes using the router alert
label as an alternative to the PW-CW **because** after much teasing out, the
proposing vendor's ECMP implementation only hashes bottom label and payload
and the RA label would be above the bottom label. Is that true of other
vendors???? Is this solution acceptable only because everyone else would
have to use the CW...(so there is an escape clause)?

IMHO the variations in ECMP that would be encountered by OAM are of
practical interest to both protocol design and independent of generating a
framework document. It is not an issue with the architecture, it is a
pre-requisite to working on it.

cheers
Dave

> -----Original Message-----
> From: Rahul Aggarwal [mailto:[email-masked]] 
> Sent: Monday, November 17, 2003 2:51 PM
> To: Allan, David [CAR:NS00:EXCH]
> Cc: [email-masked]'; [email-masked]
> Subject: RE: on the mpls oam framework 
> 
> 
> 
> Hi Dave,
> 
> >
> > BTW, would be nice to get some other opinions, IMHO this is rather 
> > fundamental....
> >
> 
> To reiterate what has already been said on this thread. A 
> MPLS OAM framework document must a) Follow the MPLS 
> architecture and b) Discuss an OAM framework that concerns 
> *practical* problems seen in MPLS deployments.
> 
> If folks have issues with MPLS architecture/usage, it should 
> be documented elsewhere, not in the OAM framework document.
> 
> Regards,
> rahul
> 
> 
> > cheers
> > Dave
> >
> >
>

---

From: Curtis Villamizar <[email-masked]>
Date: Tue, 18 Nov 2003 11:08:49 -0500

In message <[email-masked]>, "
> pre-requisite to working on it.

Dave,

We've described all the ECMP methods in use either explicitly in this
email thread or through reference to RFC2991 and RFC2992.  At least
one implementation will split based on the bandwidth configured for an
RSVP/TE LSP rather than an equal split.  Otherwise very few details
have not been discussed.

Some implementations use only labels.  Some implementations look for
an IP header *if configured to do so* and hash the IP src/dst, falling
back to labels if the header is not IP.  Most implementations
(probably all) limit label stack depth before using hashed labels.
Whether the bottom label of the whole stack is hashed will varry.

You are not going to get the actual hash function itself and the
method of seeding it.  You also won't get a list of companies and
which feature is released in what version and which line cards support
what (that would be a competitive analysis that is not needed for this
sort of technical discussion).

What *relevant* detail are you missing?

Curtis

---

From: Rahul Aggarwal <[email-masked]>
Date: Tue, 18 Nov 2003 10:53:47 -0800 (PST)

Hi Dave,

On Tue, 18 Nov 2003, David Allan wrote:

> cannot critique the solutions or participate in the design.

As has been articulated before on this thread, a MPLS OAM tool should be
able to work in the presence of ECMP, without prior knowledge of the hash
function. Lack of knowledge of the hash function does not preclude one
from designing an OAM solution or critiquing it. If certain tools have
constraints in an ECMP environment, the applicability of those tools
should be documented.

That said, if you really want to satisfy your curiosity and determine the
hash function used by a few major vendors, reverse engineering in the lab
is always possible :)

IMHO properly
> have to use the CW...(so there is an escape clause)?

The reason for having the router alert label is because a CW may not be
present or some h/w may not be able to support the CW solution. This has
the drawback of potentially breaking in the presence of ECMP and that is an
applicability of the router alert approach. We can clarify this in the
VCCV draft.

rahul