Re: [CCAMP] Overlay model framework and context

Lou Berger <> Mon, 17 December 2012 19:45 UTC

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Date: Mon, 17 Dec 2012 14:45:18 -0500
From: Lou Berger <>
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To: Daniele Ceccarelli <>
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Subject: Re: [CCAMP] Overlay model framework and context
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	Thanks for getting this on-list discussion going.  I have some comments
and questions:

- So what's a "client layer network" in this context?  Perhaps you mean
OC or "(overlay) customer layer"?

- So what's a "server layer network" in this context?  Perhaps you mean
OE or "(overlay) provider layer"?

- For OC, I'd thing referring back to a CE in the VPN context, and
likewise to a PE for an OE, is helpful context.

- As you mention in the Appendix, (from the OC perspective) there is no
difference between a virtual and real node (and presumably link as
well).  Given this and your comment in 8, that the ONI can take the form
of a UNI or include both signaling and routing (i.e., a peer/I-NNI or
E-NNI) what value is there in introducing the ONI term?  Said another
way, there's no specific term for the interface between a CE and PE in
L3VPNs, so why do we need to introduce one in this context?

I think this same comment probably holds for the O-NNI (e.g., what's the
name of the interface between providers which support L3VPN handoffs?)...

Much thanks,

On 12/17/2012 6:17 AM, Daniele Ceccarelli wrote:
> Dear CCAMPers,
> In the last weeks several off-line discussions on the Overlay model framework and related works took place. Some discussions led to some sort of agreemet among a small group of people, some others to a set a viable options, some others to totally open issues. I tried to summarize the output of such discussions below so to progress the discussions into a single thread on the WG ML.
> Please note that the aim of this mail is not to present a well shaped and conclusive idea to the WG but rather to provide the basis for starting a discussion from a barely shaped idea (step 1) instead of starting it from scratch (step 0).
> In addition you can find attached a slide depicting a proposal of the overlay scenario.
> Thanks,
> Daniele
> + Disclaimer:
>  1. Packet opto integration is often considered but the work can be extented to any type of SC. Eg. TDM over LSC.
> + Terminology:
>  1. Virtual Link: A virtual link is a potential path between two virtual or real network elements in a client layer network  that is maintained/controlled in and by the server domain control plane (and as such cannot transport any traffic/data and protected from being de-provisioned) and which can be instantiated in the data plane (and then can carry/transport/forward traffic/data) preserving previously advertised attributes such as fate sharing information.
>  2.  Virtual Node: Virtual node is a collection of zero or more server network  domain nodes that are collectively represented to the clients as a single node that exists in the client layer network and is capable of terminating of access, inter-domain and virtual links.
>  3.Virtual Topology: Virtual topology is a collection of one or more virtual or real server network domain nodes that exist in the client layer network and are interconnected via 0 or more virtual links.
>  4. Overlay topology:  is a superset of virtual topologies provided by each of server network domains, access and inter-domain links.
>  5. Access Link: Link between OC and OE. GMPLS runs on that link. It can support any of the SCs supported by the GMPLS.
>  6. Overlay Customer (OC): Something like the CN in RFC4208 teminology  but (i) receiving virtual topology from the core  network and requesting the set up of one of them or (ii) requesting the computation and establishment of a path accordingly to gien constraints in the core network and receiving the parameters characterizing such path. (ii) == UNI.
>  7. Overlay Edge (OE): Something like the EN in RFC4208 but able to deal with (i) and (ii) above.
>  8. ONI : Overlay network interface: Interface allowing for signaling and routing messages exchange between Overlay and Core network. Routing information consists on virtual topology advertisement. When there is no routing adjacency across the interface it is equivalent to the GMPLS UNI defined in 4208. Signaling messages are compliant with RFC4208. Information related to path carachteristics, e.g. TE-metrics, collected SRLG, path delay etc, either passed from OE to OC via signaling after the LSP establishment in the core network or from OC to OE to be used as path computation constraints, fall under the definition of signaling info and not routing info).
>  9. O-NNI (name to be found,maybe reused): Interface on the links between different core networks in the overlay model environment, i.e. Between border OEs. Same features of the ONI apply to this interface. Could it be an E-NNI? A ONI? A new name is needed?
> + Statements
>  1. In the context of overlay model we are aiming to build an overlay topology for the client network domains
>  2. The overlay topology is comprised of:
>     a) access links (links connecting client NEs to the server network domains). They can be PSC or LSC.
>     b) inter-domain links (links interconnecting server network domains)   
>     c) virtual topology provided by the server network domains. Virtual Links + Virtual Nodes (TBD) + Connectivity Matrix (with a set of parameters e.g. SRLG, optical impairments, delay etc for each entry) describing connectivity between access links and virtual links.
>  3. In the context of overlay model we manage  hierarchy  of overlay topologies with overlay/underlay relationships
>  4. In the context of overlay model multi-layering and inter-layer relationships are peripheral at best, it is all about horizontal network integration 
>  5. The overlay model assumes one instance for the client network and a separate instance for the server network and in the ONI case the server network also surreptitiously participates in the client network by injecting virtual topology information into it.
>  6. L1VPN (and LxVPN) in general is a service provided over the ONI (it falls under the UNI case as no routing adjacency is in place between OC and OE).
> + Open issues/questions
>  1. PCE-PCEP - do we need to include considerations about PCE and PCEP into the overlay framework context?
>  2. BGP-LS needs to be considered
>  3. Should potentials be included? E.g. I2RS?
> + Appendix:
> Some notes on the Virtual Node:
> 1.      Virtual Link Model along, sadly, does not scale because of N**2 problem. IP over ATM and single-segment PWs have the same issue, that's why people invented multi-segment PWs
> 2.      The only way to avoid full-mesh of Virtual Links is by having intermediate nodes interconnecting Virtual Links in the middle of the virtual topology
> 3.      These intermediate nodes cannot be real server domain switches, because, generally speaking:
>   a)Real switches belong to different layer network;
>   b)Real switches are named from different naming space
>   c)real switches individually may not have sufficient resources to terminate virtual links (while a group of real switches collectively will have)
>   d)Presenting a group of real switches as a single virtual node have better scalability qualities
> 4.      Even if you map a virtual node on a single real node, you need to keep in mind that real server domain switches are, generally speaking, blocking switches and as such must expose their connectivity matrices
> 5.      If you want to compute SRLG-disjoint paths that could potentially go through a real server domain switch, the latter's connectivity matrix must expose "internal" SRLGs, so that the two services traversing the switch will not simultaneously fail if a single internal element shared by the services fails
> 6.      If you walk through all cases that need to be addressed when you are traffic engineering topologies with blocking switches, you will understand that there is absolutely no difference between a virtual node and real blocking real node.
> 7.      Even in case of pure VL model, client NEs connected to server network domain must be upgraded so that they could understand the connectivity matrices advertised by the border nodes describing connectivity constraints between access links and virtual links they terminate.
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