The MPLS WG Archive

["Gray, Eric" <egray@celoxnetworks.com>]

Curtis,

	For the specific case where the destination IP
address is chosen at random from a large space and is
actually independent of the route followed by any path,
a destination IP address based hash algorithm results
in a fairly random distribution across all paths.  As
you probably know, this is not the case for destination
IP addresses in general - i.e. - the DIPA is not at all
independent of the routes followed by all equal cost 
paths.  Consequently, even assuming the implementation
does the deep dive and looks at the IP header for its
ECMP hashing algorithm, it will most likely not rely
on the DIPA exclusively for deriving the ECMP hash.  If
it did, there is likely to be insufficient entropy in
the derived hash values - resulting in an undesirable 
distribution among the equal cost paths.  

	This proposal will indirectly have the effect of 
requiring that any ECMP hash algorithm used in those 
implementations that use the IP header must include a 
part of the least significant bytes of the destination 
IP address in the key.  This in spite of the fact that
this is not actually helpful to the hash algorithm in
general.  Moreover, this may be interpreted by users as
justification for pressuring implementers to use the IP
header in their ECMP hash algorithm even though this is
clearly not going to scale in some LSR implementations. 

	My point is that the usefulness of this address
range depends on an assumption that ECMP hash algorithms 
will include some part of the least significant bytes of 
the destination IP address.  This is a grave restriction 
on the use of ECMP algorithms and should not be encouraged.
If you cannot make this assumption, then the existence of
this address range would not be useful and would in fact
be misleading.  

	If you cannot make this assumption, and the use of 
this address range is not useful, then I fail to see why 
it is necessary to propose something equally useless to 
replace this suggestion.  :-)

	However, if you proposed a more straight-forward
approach - requiring an ECMP origination point to (some
day) support the ability to recognize a Ping packet and
(presumably on it's own initiative) produce its own Ping
packets for all equal cost paths - that approach does 
seem useful and we should think about it some more.

Eric W. Gray
Systems Architect
Celox Networks, Inc.
egray@celoxnetworks.com
508 305 7214


> -----Original Message-----
> From: Curtis Villamizar [mailto:curtis@fictitious.org]
> Sent: Monday, December 09, 2002 2:13 PM
> To: Gray, Eric
> Cc: 'curtis@fictitious.org'; 'erosen@cisco.com'; mpls@UU.NET
> Subject: Re: Last call on LSP Ping
> 
> 
> In message <1117F7D44159934FB116E36F4ABF221B0267ED5B@celox-ma1-
> ems1.celoxnetwor
> ks.com>, "Gray, Eric" writes:
> > Curtis,
> >
> > 	I am suggesting that attempting to use any address range
> > as a mechanism for testing ECMP routes is a mistake.  I am also
> > suggesting that a single address is sufficient to satisfy the
> > need Eric Rosen points out.  Finally, if you were suggesting
> > that 127.0.0.1 should be specified (as opposed to 127.0.0.0/8)
> > for the same purpose, then I agree with you.
> >
> > 	Any address based approach to trying to test ECMP load
> > sharing paths implies prior knowledge of all ECMP algorithms
> > in use and is therefore broken.
> >
> > Eric W. Gray
> > Systems Architect
> > Celox Networks, Inc.
> > egray@celoxnetworks.com
> > 508 305 7214
> 
> 
> If you have ideas to address the prior message I sent on ECMP and in
> particular the problem of multiple branch points, I'd like to hear
> your solution.
> 
> One thing that came to mind was the midpoint enumerating the possible
> paths with indices and the ingress providing the index in subsequent
> messages.  To do this, the ingress may have to bundle the better part
> of an MPLS-ping message inside another so the top POP exposes an IP
> packet and ECMP index to use and the packet inside is injected into
> the stream.  If we go with this, then 127.0.0.1 would be fine.  This
> has the disadvantage of not actually exercising the forwarding, just
> the control plane.
> 
> The 127.0.0.1 address has a small advantage in that if the egress
> comes out in some unexpected place, then it is delivered to that
> router which can respond more intellegently than sending an ICMP TTL
> expired.
> 
> Curtis