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+Reference Architecture
+----------------------
+
+This section describes the reference architecture, the function blocks,
+and the function entities of Escalator for the reader to well understand how
+the basic functions to be organized.
+
+1. Upgrade Scope
+~~~~~~~~~~~~~~~~
+Upgrade objects described in this document are software programs 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.
+Note that this upgrade process does not take into consideration the effects
+of Vi-Vnfm and Or-Vi. In other words, the unserviceability of the two
+interfaces during upgrade can  be accepted.
+
+.. figure:: images/figure1.png
+   :name: figure1
+   :width: 100%
+
+The software stack on each node is generally as shown in the table below.
+
+.. figure:: images/figure2.png
+   :name: figure2
+   :width: 100%
+
+Because the control node upgrade will not affect the business in the blue box,
+this scheme focuses on upgrading of compute nodes.
+
+2. Precondition of Upgrade
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+2.1 The environmental requirements
+1.  System is running normally. If there are any faults before the upgrade, it
+is difficult to distinguish between upgrade introduced and the environment
+itself.
+2.  The environment should have the redundant resources. Because the upgrade
+process is based on the business migration, in the absence of resource
+redundancy,it is impossible to realize the business migration, as well as to
+achieve a smooth upgrade.
+
+Resource redundancy in two levels:
+1)  NFVI level: This level is mainly the compute nodes resource redundancy.
+During the upgrade, the virtual machine on business can be migrated to another
+free compute node.
+2)  VNF level: This level depends on backup mechanism in VNF, such as:
+active-standby, load balance. In this case, as long as business of the target
+node on VMs is migrated to other free nodes, the migration of VM might not be
+necessary.
+
+The way of redundancy to be used is subject to the specific environment.
+Generally speaking, the impact of using NFVI redundancy on the VMs is larger
+than the rearrangement of the business on VNF level.
+
+2.2 The demand for version
+This is primarily a compatibility requirement. You can refer to Linux/Python
+Compatible Semantic Versioning 3.0.0:
+
+Given a version number MAJOR.MINOR.PATCH, increment the:
+1.  MAJOR version when you make incompatible API changes,
+2.  MINOR version when you add functionality in a backwards-compatible manner,
+3.  PATCH version when you make backwards-compatible bug fixes.
+
+The upgrade process needs to use some interfaces which require these
+interfaces to be backward compatible. Refer to "Interface" chapter for details.
+
+3.Upgrade related modules in VIM
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Upgrade operations are initiated by the user through the VIM. For VIM, upgrade
+management mainly contains the object:
+**Upgrade Manager**:Mainly responsible for upgrading process control. Physical
+nodes information of each node is saved in upgrade manager.
+**VIM Interface**:Mainly responsible for the external interface, include
+Vi-Vnfm, Or-Vi. This module stores VNFO and VNFM external information such as
+address and authentication.
+**Cloud Manager**:Mainly responsible for virtualization resources management,
+which might be considered made up of Openstack and SDN control node.
+**System Support**:Provide the upper software running environment, including:
+OS, HA, etc. To upgrade the upper software is based on this module.
+
+.. figure:: images/figure3.png
+   :name: figure3
+   :width: 100%
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