+As @ref starting_components "explained in the introduction," any
+SimGrid study must entail the description of the platform on which you
+want to simulate your application. You have to describe **each element
+of your platform**, such as computing hosts, clusters, each disks,
+links, etc. You must also define the **routing on your platform**, ie
+which path is taken between two hosts. Finally, you may also describe
+an **experimental scenario**, with qualitative changes (e.g.,
+bandwidth changes representing an external load) and qualitative
+changes (representing how some elements fail and restart over time).
+
+You should really separate your application from the platform
+description, as it will ease your experimental campain afterward.
+Mixing them is seen as a really bad experimental practice. The easiest
+to enforce this split is to put the platform description in a XML
+file. Many example platforms are provided in the archive, and this
+page gives all needed details to write such files, as well as some
+hints and tricks about describing your platform.
+
+On the other side, XML is sometimes not expressive enough for some
+platforms, in particular large platforms exhibiting repetitive
+patterns that are not simply expressed in XML. In practice, many
+users end up generating their XML platform files from some sort of
+scripts. It is probably preferable to rewrite your XML @ref
+platform_lua "platform using the lua scripting language" instead.
+In the future, it should be possible to describe the platform directly
+in C++, but this is not possible yet.
+
+As usual, SimGrid is a versatile framework, and you should find the
+way of describing your platform that best fits your experimental
+practice.
+
+\section pf_overview Describing the platform with XML
+
+Your platform description should follow the specification presented in
+the [simgrid.dtd](http://simgrid.gforge.inria.fr/simgrid/simgrid.dtd)
+DTD file. The same DTD is used for both the platform and deployment
+files.
+
+From time to time, this DTD evolves to introduce possibly
+backward-incompatible changes. That is why each platform desciption is
+enclosed within a @c platform tag, that have a @c version attribute.
+The current version is <b>4.1</b>. The @c simgrid_update_xml program can
+upgrade most of the past platform files to the recent formalism.
+
+\section pf_first_example First Platform Example
+
+Here is a very simple platform file, containing 3 resources (two hosts
+and one link), and explicitly giving the route between the hosts.
+
+\code{.xml}
+<?xml version='1.0'?>
+<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid/simgrid.dtd">
+<platform version="4.1">
+ <zone id="first zone" routing="Full">
+ <!-- the resources -->
+ <host id="host1" speed="1Mf"/>
+ <host id="host2" speed="2Mf"/>
+ <link id="link1" bandwidth="125MBps" latency="100us"/>
+ <!-- the routing: specify how the hosts are interconnected -->
+ <route src="host1" dst="host2">
+ <link_ctn id="link1"/>
+ </route>
+ </zone>
+</platform>
+\endcode
+
+As we said, the englobing @ref pf_overview "<platform>" tag is
+used to specify the dtd version used for this file.
+
+Then, every resource (specified with @ref pf_tag_host, @ref
+pf_tag_link or others) must be located within a given **networking
+zone**. Each zone is in charge of the routing between its
+resources. It means that when an host wants to communicate with
+another host of the same zone, it is the zone's duty to find the list
+of links that are involved in the communication. Here, since the @ref
+pf_tag_zone tag has **Full** as a **routing attribute**, all routes
+must be explicitely given using the @ref pf_tag_route and @ref
+pf_tag_linkctn tags (this @ref pf_rm "routing model" is both simple
+and inefficient :) It is OK to not specify the route between two
+hosts, as long as the processes located on them never try to
+communicate together.
+
+A zone can contain several zones itself, leading to a hierarchical
+decomposition of the platform. This can be more efficient (as the
+inter-zone routing gets factorized with @ref pf_tag_zoneroute), and
+allows to have more than one routing model in your platform. For
+example, you could have a coordinate-based routing for the WAN parts
+of your platforms, a full routing within each datacenter, and a highly
+optimized routing within each cluster of the datacenter. In this
+case, determining the route between two given hosts gets @ref
+routing_basics "somewhat more complex" but SimGrid still computes
+these routes for you in a time- and space-efficient manner.
+Here is an illustration of these concepts:
+
+![A hierarchy of networking zones.](AS_hierarchy.png)
+
+Circles represent processing units and squares represent network
+routers. Bold lines represent communication links. The zone "AS2"
+models the core of a national network interconnecting a small flat
+cluster (AS4) and a larger hierarchical cluster (AS5), a subset of a
+LAN (AS6), and a set of peers scattered around the world (AS7).
+
+\section pf_res Resource description
+
+\subsection pf_res_computing Computing Resources
+
+\subsubsection pf_tag_host <host>
+
+An host is the computing resource on which an actor can execute.
+
+Attribute | Values | Description
+----------------- | -------------------------------------- | -----------
+id | String (mandatory) | The identifier of the host. facilitates referring to this AS.
+speed | double (mandatory) | Computational power of every core of this host in FLOPS (must be positive)
+core | int (defaults to 1) | Number of cores (see @ref howto_multicore)
+state | optionally "OFF" | If set to OFF, the host is initially turned off.
+availability_file | File name (optional) | (Relative or absolute) filename to use as input; must contain availability traces for this host. The syntax of this file is defined below.
+state_file | File name (optional) | File to use as a state profile (see @ref howto_churn)
+coordinates | String (mandatory when using Vivaldi routing) | The coordinates of this host (see @ref pf_P2P_tags).
+pstate | Double (Defaults to 0) | FIXME: Not yet documented.
+
+#### Included tags ####
+
+ - @ref pf_tag_mount Specifies the storages mounted on that host
+ - @ref pf_tag_prop Specifies a user-defined property of that host, that you can retrieve with MSG_host_get_property_value() or simgrid::s4u::Host::property().
+
+#### Examples ####
+
+\code{.xml}
+<host id="host1" speed="1000000000"/>
+<host id="host2" speed="1000000000">
+ <prop id="color" value="blue"/>
+ <prop id="rendershape" value="square"/>
+</host>
+\endcode
+
+\anchor pf_host_dynamism
+### Expressing dynamism ###
+
+SimGrid provides mechanisms to change a hosts' availability over
+time, using the ``availability_file`` attribute to the ``\<host\>`` tag
+and a separate text file whose syntax is exemplified below.
+
+#### Adding a trace file ####