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+This document describes the user requirements on the smooth upgrade
+function of the NFVI and VIM with respect to the upgrades of the OPNFV
+platform from one version to another. Smooth upgrade means that the
+upgrade results in no service outage for the end-users. This requires
+that the process of the upgrade is automatically carried out by a tool
+(code name: Escalator) with pre-configured data. The upgrade process
+includes preparation, validation, execution, monitoring and
+.. <MT> While it is good to have a tool for the entire upgrade process,
+ but it is a challenging task, so maybe we shouldn't require automation
+ for the entire process right away. Automation is essential at
+.. <hujie> Maybe we can analysis information flows of the upgrade tool,
+ abstract the basic / essential actions from the tool (or tools), and
+ map them to a command set of NFVI / VIM's interfaces.
+The requirements are defined in a stepwise approach, i.e. in the first
+phase focusing on the upgrade of the VIM then widening the scope to the
+The requirements may apply to different NFV functions (NFVI, or VIM, or
+both of them). They will be classified in the Appendix of this
+The objects being upgraded described in this document are software modules covered by
+red box in the picture below which includes: VIM and NFVI.
+The target of the upgrade is to reduce the impact on the applications in the
+blue box below as much as possible.
+Please keep in mind that the upgrade tool does not take Vi-Vnfm and Or-Vi into
+consideration. In other words, these two interfaces may not provided service normally
+during upgrade procedure.
+.. figure:: images/figure1.png
+ :name: figure1
+ :width: 100%