The MPLS WG Archive

["David Allan" <dallan@nortelnetworks.com>]

Title: RE: draft-chang-mpls-path-protection Comments

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