Hi, Joel,
The statement is that pure overlay VPNs cannot meet the requirement of some new 
services, and it would require integration between the underlay and the overlay 
networks.
As mentioned in this document, there is existing technology in the underlay to 
support enhanced VPNs , such as using a set of MPLS-TE based resource reserved 
point-to-point paths, while it scalability is the concern of many operators.
 
Thus VTN is introduced to provide the required topology and resource attribute 
in the underlay in a scalable manner. This is described in the introduction 
section.
 
Hope this helps.

Chongfeng

From: Joel M. Halpern
Date: 2020-03-25 21:52
To: xie...@chinatelecom.cn; lsr
Subject: Re: [Lsr] Using IS-IS Multi-Topology (MT) for Segment Routing based 
Virtual Transport Network
This drafts starts by asserting that there are limitations on what can 
be done with the existing technology.  As the description is quite 
vague, I can not be certain.  But I do not know of any difficulty in 
providing the described capabilities with current technology, without 
introducing a new, undescribed, construct called a VTN.
 
Yours,
Joel
 
On 3/25/2020 9:44 AM, xie...@chinatelecom.cn wrote:
> 
> Hello, folks,
> 
> we have submitted a new draft of 
>   https://tools.ietf.org/html/draft-xie-lsr-isis-sr-vtn-mt-00 .
> 
> It is about Using IS-IS Multi-Topology (MT) for Segment Routing based 
> Virtual Transport Network. Enhanced VPN (VPN+) as defined in 
> I-D.ietf-teas-enhanced-vpn aims to provide enhanced VPN service to 
> support some applications's needs of enhanced isolation and stringent 
> performance requirements.  VPN+ requries integration between the overlay 
> VPN and the underlay network.  A Virtual Transport Network (VTN) is a 
> virtual network which consists of a subset of the network toplogy and 
> network resources allocated from the underlay network.  A VTN could be 
> used as the underlay for one or a group of VPN+ services. This document 
> describes a simplified mechanism to build the SR based VTNs using IGP 
> multi- topology together with other well-defined IS-IS extensions.
> 
> Comments and suggestions are highly appreciated.
> 
> Chongfeng Xie
> 
> 
> 
> _______________________________________________
> Lsr mailing list
> Lsr@ietf.org
> https://www.ietf.org/mailman/listinfo/lsr
> 
 
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