External Links
ATM based VPN - Comet Research Group, Columbia U.
SupraNet - Ferrari & Delgrossi
X-Bone - Touch/ISI
A collection of VPN related sites
VPN guide

Internal Collection

General
What is VPN? Paul Ferguson   (slides  - external)
Can Your Organization Benefit From VPNs?  Indus River Networks Inc. white paper
Verma's List of related docs
Ascend's VPN Resource Guide    (external link to Ascend's VPN site)
 

Architecture-Framework
A Framework for IP Based Virtual Private Networks - Gleeson et. al.
MPLS Mappings of Generic VPN Mechanisms - Heinanen et. al.
VPN support with MPLS - Heinanen and Rosen
MPLS VPN Architecture - Jamieson et. al.
CPE based VPNs using MPLS - Tony Li
Core IP VPN Architecture - Muthukrishnan and Malis
VPN Point to Multipoint Tunnel Protocol (VPMT) - Pegrum et. al.

Tunneling
IP in IP Tunneling - Simpson
Tunnel Establishment Protocol - Calhoun et. al.
Layer Two Tunneling Protocol - Valencia et. al.
Layer Two Tunneling Protocol "L2TP" IP Differential Services Extension - Calhoun et. al.
 

NAT

Misc