The MPLS WG Archive

[Vishal Sharma <Vishal.Sharma@tellabs.com>]

Hi Dave,

This is a follow-up to my earlier response to your email. After internally discussing
your point about the PML being an artificial restriction and the possibility
of multiple exit points out of an MPLS domain, we think
we understand what you were saying, but think that implementing that
may require stretching an MPLS-based solution (which was our goal)
a bit, since it may require information beyond what is easily available
to MPLS. This was one reason we had not expanded our initial solution
to include such cases.

However, your point is a valid one, and we'd like to explore to what extent
such alternatives can be incorporated into an MPLS-based recovery
mechanism such as ours. One of my colleagues, Ben Mack-Crane, who I believe
understands your point best, will be sending out an email explaining
what we think your point was, and we'd appreciate receving your feedback
on whether we've captured it right, and other follow-up thoughts.

Regards,
-Vishal

On Monday, April 17, 2000 11:40 AM, dallan@nortelnetworks.com [SMTP:dallan@nortelnetworks.com] 
wrote:
> Vishal:
>
> 	<snip>
>
> > I am not sure why you call it a "scalability hit". The RNT mechanism
> > cannot
> > be any worse than having one backup LSP per working LSP.
> > Also, how do you get (n+1)^2 meshing (and what is "n")?
> > In general if the backup paths form trees, you get savings.
> >
> 	My concern is not with the RNT mechanism. It is more with the
> construction of useful working and protection paths that are:
> 	- physically diverse
> 	- optimal
> 	That concern is not specific to your proposal.
>
> 	<snip>
>
> > First, I am not sure how you define a protection domain (and whether that
> > definition is the
> > same as the one we use in our draft).
> >
> 	Ultimately the problem I have is with the definition of protected
> domain. It limits the protection domain to being a set of LSRs with an
> explicitly identifed PSL and PML, which I take to mean it is a contiguous
> MPLS cloud or some administrative subset which has single egress point
> (which may be an artificial limitation). The statement that the PSL and PML
> are identified during the setting up of an LSP also bounds the granularity
> of the merge function to being that of the FEC that the LSP is set up to
> forward. I find this limiting.
>
> 	So why is there an explicit merge function inside the MPLS cloud?
> The whole network will not be MPLS, therefore merge can be deferred further
> towards the edge of the network and increased reliability can be obtained if
> the protection domain extends beyond MPLS domain. Where the merge function
> occurs for a specific packet will not be known to the PSL at LSP setup as it
> is a function of overall network state ands that's OK. How packets at some
> point downstream end up back on a common routing to a host doesn't have to
> happen at the granularity of the LSP originated at the PSL and may not
> happen until a true single point of attachment to the network is reached
> many hops beyond the edge of the MPLS cloud.
>
> 	If such a thing as the PML existed, then the PML would have to
> promiscuously accept packets from both the working and backup trees anyway
> as not all failures in a tree would be propagated to all PSLs. So it is
> really nothing more than an LSR which "by convention" stuff has to pass
> through. It may anchor a RNT for livliness messaging for each tree that ends
> there (which functionally can be disassociated from a promiscuous merge
> function as there is no reason that fast failure detection needs common
> state between working and protection paths either except at the PSL). What
> that means to me is that there is a "single point of failure" (the "per LSP"
> PML) which could be avoided "by convention" as well by allowing multiple
> points of egress from the MPLS protected domain (which is a subset of the
> network protected domain).
>
> 	As much as we would like a tidy, administrable and measurable
> definition of how far protection extended, IMHO the network would be more
> reliable if we didn't cap the definition of protected domain as being wholly
> within MPLS.
>
> 	regards
> 	Dave
>
>
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