fix a typo

[simgrid.git] / doc / doxygen / platform.doc

/*! \page platform Step 1: %Model the underlying platform

@tableofcontents

In order to run any simulation, SimGrid must be provided with three things:
something to run (i.e., your code), a description of the platform on which you
want to simulate your application and lastly information about the deployment
process: Which process should be deployed to which processor/core?

For the last two items, there are essentially two possible ways you can provide
this information as an input:
\li You can program it, either by using the Lua console (
    \ref MSG_Lua_funct) or, if you're using MSG, some of MSG's platform and
    deployment functions (\ref msg_simulation). If you want to use this,
    check the particular documentation. (You can also check the section
    \ref pf_flexml_bypassing, however, this documentation is deprecated;
    there is a new, but undocumented, way to do it properly).
\li You can use two XML files: one contains the platform description while
    the other contains the deployment instructions.

For more information on SimGrid's deployment features, please refer to
the \ref deployment documentation.

The platform description may be intricate. This documentation is all
about how to write this file: The basic concepts are introduced. Furthermore,
advanced options are explained. Additionally, some hints and tips on how to
write a good platform description are given.

\section pf_overview Some words about XML and DTD

We chose to use XML not only because it's extensible but also because many
tools (and plugins for existing tools) are available that facilitate editing and
validating XML files. Furthermore, libraries that parse XML are often already
available and very well tested.

The XML checking is done based on the Document Type Definition (DTD) file,
available at
<a href="http://simgrid.gforge.inria.fr/simgrid.dtd">http://simgrid.gforge.inria.fr/simgrid.dtd</a>.

If you read the DTD, you should notice the following:
\li The platform tags contain a version attribute; the current version is 3.
    This property might be used in the future to provide backwards
    compatibility.
\li The DTD contains definitions for the two files used by SimGrid (i.e.,
    platform description and deployment).

\section pf_basics Basic concepts

Nowadays, the Internet is composed of a bunch of independently managed
networks. Within each of those networks, there are entry and exit
points (most of the time, you can both enter and exit through the same
point); this allows to leave the current network and reach other
networks, possibly even in other locations.
At the upper level, such a network is called
<b>Autonomous System (AS)</b>, while at the lower level it is named
sub-network, or LAN (local area network).
They are indeed autonomous: routing is defined
(within the limits of his network) by the administrator, and so, those
networks can operate without a connection to other
networks. So-called gateways allow you to go from one network to
another, if such a (physical) connection exists. Every node in one network
that can be directly reached (i.e., without traversing other nodes) from
another network is called a gateway.
Each autonomous system consists of equipment such as cables (network links),
routers and switches as well as computers.

The structure of the SimGrid platform description relies exactly on the same
concept as a real-life platform (see above).  Every resource (computers,
network equipment etc.) belongs to an AS, which can be defined by using the
\<AS\> tag. Within an AS, the routing between its elements can be defined
abitrarily. There are several modes for routing, and exactly one mode must be
selected by specifying the routing attribute in the AS tag:

\verbatim
<AS id="AS0" routing="Full">
\endverbatim

\remark
  Other supported values for the routing attribute can be found below, Section
  \ref pf_raf.

There is also the ``<route>`` tag; this tag takes two attributes, ``src`` (source)
and ``dst`` (destination). Both source and destination must be valid identifiers
for routers (these will be introduced later). Contained by the ``<route>`` are
network links; these links must be used in order to communicate from the source
to the destination specified in the tag. Hence, a route merely describes
how to reach a router from another router.

\remark
  More information and (code-)examples can be found in Section \ref pf_rm.

An AS can also contain itself one or more AS; this allows you to
model the hierarchy of your platform.

### Within each AS, the following types of resources exist:

%Resource        | Documented in Section | Description
--------------- | --------------------- | -----------
AS              |                       | Every Autonomous System (AS) may contain one or more AS.
host            | \ref pf_host          | This entity carries out the actual computation. For this reason, it contains processors (with potentially multiple cores).
router          | \ref pf_router        | In SimGrid, routers are used to provide helpful information to routing algorithms.  Routers may also act as gateways, connecting several autonomous systems with each other.
link            | \ref pf_link          | In SimGrid, (network)links define a connection between two or potentially even more resources. Every link has a bandwidth and a latency and may potentially experience congestion.
cluster         | \ref pf_cluster       | In SimGrid, clusters were introduced to model large and homogenous environments. They are not really a resource by themselves - technically, they are only a shortcut, as they will internally set up all the hosts, network and routing for you, i.e., using this resource, one can easily setup thousands of hosts and links in a few lines of code. Each cluster is itself an AS.

%As it is desirable to interconnect these resources, a routing has to be
defined. The AS is supposed to be Autonomous, hence this has to be done at the
AS level. The AS handles two different types of entities (<b>host/router</b>
and <b>AS</b>). However, the user is responsible to define routes between those resources,
otherwise entities will be unconnected and therefore unreachable from other
entities. Although several routing algorithms are built into SimGrid (see
\ref pf_rm), you might encounter a case where you want to define routes
manually (for instance, due to specific requirements of your platform).

There are three tags to use:
\li <b>ASroute</b>: to define routes between two  <b>AS</b>
\li <b>route</b>: to define routes between two <b>host/router</b>
\li <b>bypassRoute</b>: to define routes between two <b>AS</b> that
    will bypass default routing (as specified by the ``routing`` attribute
    supplied to ``<AS>``, see above).

Here is an illustration of these concepts:

![An illustration of an AS hierarchy. Here, AS1 contains 5 other ASes who in turn may contain other ASes as well.](AS_hierarchy.png)
 Circles represent processing units and squares represent network routers. Bold
    lines represent communication links. 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_pftags Resource description

\subsection  pf_As Platform: The &lt;AS&gt; tag

The concept of an AS was already outlined above (Section \ref pf_basics);
recall that the AS is so important because it groups other resources (such
as routers/hosts) together (in fact, these resources must be contained by
an AS).

Available attributes :

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | String | The identifier of an AS; facilitates referring to this AS. ID must be unique.
routing         | yes       | Full\| Floyd\| Dijkstra\| DijkstraCache\| None\| Vivaldi\| Cluster | See Section \ref pf_rm for details.


<b>Example:</b>
\verbatim
<AS id="AS0" routing="Full">
   <host id="host1" power="1000000000"/>
   <host id="host2" power="1000000000"/>
   <link id="link1" bandwidth="125000000" latency="0.000100"/>
   <route src="host1" dst="host2"><link_ctn id="link1"/></route>
</AS>
\endverbatim

In this example, AS0 contains two hosts (host1 and host2). The route
between the hosts goes through link1.

\subsection pf_Cr Computing resources: hosts, clusters and peers.

\subsubsection pf_host &lt;host/&gt;

A <b>host</b> represents a computer/node card. Every host is able to execute
code and it can send and receive data to/from other hosts. Most importantly,
a host can contain more than 1 core.

### Attributes: ###

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | String | The identifier of the host. facilitates referring to this AS.
power           | yes       | double (must be > 0.0) | Computational power of every core of this host in FLOPS. Must be larger than 0.0.
core            | no        | int (Default: 1) | The number of cores of this host. If more than one core is specified, the "power" parameter refers to every core, i.e., the total computational power is no_of_cores*power.<br /> If 6 cores are specified, up to 6 tasks can be executed without sharing the computational power; if more than 6 tasks are executed, computational power will be shared among these tasks. <br /> <b>Warning:</b> Although functional, this model was never scientifically assessed.
availability    | no        | int    | <b>Specify if the percentage of power available.</b> (What? TODO)
availability_file| no       | string | (Relative or absolute) filename to use as input; must contain availability traces for this host. The syntax of this file is defined below. <br /> <b>Note:</b> The filename must be specified with your system's format.
state           | no        | ON\|OFF<br/> (Default: ON) | Is this host running or not?
state_file      | no        | string |  Same mechanism as availability_file.<br /> <b>Note:</b> The filename must be specified with your system's format.
coordinates     | no        | string | Must be provided when choosing the Vivaldi, coordinate-based routing model for the AS the host belongs to. More details can be found in the Section \ref pf_P2P_tags.

### Possible children: ###

Tag name        | Description | Documentation
------------    | ----------- | -------------
\<mount/\>        | Defines mounting points between some storage resource and the host. | \ref pf_storage_entity_mount
\<prop/\>         | The prop tag allows you to define additional information on this host following the attribute/value schema. You may want to use it to give information to the tool you use for rendering your simulation, for example. | N/A

### Example ###

\verbatim
   <host id="host1" power="1000000000"/>
   <host id="host2" power="1000000000">
        <prop id="color" value="blue"/>
        <prop id="rendershape" value="square"/>
   </host>
\endverbatim


\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 ####

\verbatim
<platform version="1">
  <host id="bob" power="500000000" availability_file="bob.trace" />
</platform>
\endverbatim

#### Example of "bob.trace" file ####

~~~~~~~~~~~~~~{.py}
PERIODICITY 1.0
  0.0 1.0
  11.0 0.5
  20.0 0.8
~~~~~~~~~~~~~~

Let us begin to explain this example by looking at line 2. (Line 1 will become clear soon).
The first column describes points in time, in this case, time 0. The second column
describes the relative amount of power this host is able to deliver (relative
to the maximum performance specified in the ``\<host\>`` tag). (Clearly, the
second column needs to contain values that are not smaller than 0 and not larger than 1).
In this example, our host will deliver 500 Mflop/s at time 0, as 500 Mflop/s is the
maximum performance of this host. At time 11.0, it will
deliver half of its maximum performance, i.e., 250 Mflop/s until time 20.0 when it will
will start delivering 80\% of its power. In this example, this amounts to 400 Mflop/s.

Since the periodicity in line 1 was set to be 1.0, i.e., 1 timestep, this host will
continue to provide 500 Mflop/s from time 21. From time 32 it will provide 250 MFlop/s and so on.

### Changing initial state ###

It is also possible to specify whether the host is up or down by setting the
``state`` attribute to either <b>ON</b> (default value) or <b>OFF</b>.

#### Example: Expliciting the default value "ON" ####

\verbatim
<platform version="1">
   <host id="bob" power="500000000" state="ON" />
</platform>
\endverbatim

If you want this host to be unavailable, simply substitute ON with OFF.

\anchor pf_host_churn
### Expressing churn ###

To express the fact that a host can change state over time (as in P2P
systems, for instance), it is possible to use a file describing the time
at which the host is turned on or off. An example of the content
of such a file is presented below.

#### Adding a state file ####

\verbatim
<platform version="1">
  <host id="bob" power="500000000" state_file="bob.fail" />
</platform>
\endverbatim

#### Example of "bob.fail" file ####

~~~{.py}
  PERIODICITY 10.0
  1.0 -1.0
  2.0 1.0
~~~

A negative value means <b>down</b> (i.e., OFF) while a positive one means <b>up and
  running</b> (i.e., ON). From time 0.0 to time 1.0, the host is on. At time 1.0, it is
turned off and at time 2.0, it is turned on again until time 12 (2.0 plus the
periodicity 10.0). It will be turned on again at time 13.0 until time 23.0, and
so on.


\subsubsection pf_cluster &lt;cluster&gt;

``<cluster />`` represents a machine-cluster. It is most commonly used
when one wants to define many hosts and a network quickly. Technically,
``cluster`` is a meta-tag: <b>from the inner SimGrid point of
view, a cluster is an AS where some optimized routing is defined</b>.
The default inner organization of the cluster is as follow:

\verbatim
                 __________
                |          |
                |  router  |
    ____________|__________|_____________ backbone
      |   |   |              |     |   |
    l0| l1| l2|           l97| l96 |   | l99
      |   |   |   ........   |     |   |
      |                                |
    c-0.me                             c-99.me
\endverbatim

Here, a set of <b>host</b>s is defined. Each of them has a <b>link</b>
to a central backbone (backbone is a link itself, as a link can
be used to represent a switch, see the switch / link section
below for more details about it). A <b>router</b> allows to connect a
<b>cluster</b> to the outside world. Internally,
SimGrid treats a cluster as an AS containing all hosts: the router is the default
gateway for the cluster.

There is an alternative organization, which is as follows:
\verbatim
                 __________
                |          |
                |  router  |
                |__________|
                    / | \
                   /  |  \
               l0 / l1|   \l2
                 /    |    \
                /     |     \
            host0   host1   host2
\endverbatim

The principle is the same, except that there is no backbone. This representation
can be obtained easily: just do not set the bb_* attributes.


Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the cluster. Facilitates referring to this cluster.
prefix          | yes       | string | Each node of the cluster has to have a name. This name will be prefixed with this prefix.
suffix          | yes       | string | Each node of the cluster will be suffixed with this suffix
radical         | yes       | string | Regexp used to generate cluster nodes name. Syntax: "10-20" will give you 11 machines numbered from 10 to 20, "10-20;2" will give you 12 machines, one with the number 2, others numbered as before. The produced number is concatenated between prefix and suffix to form machine names.
power           | yes       | int    | Same as the ``power`` attribute of the ``\<host\>`` tag.
core            | no        | int (default: 1) | Same as the ``core`` attribute of the ``\<host\>`` tag.
bw              | yes       | int    | Bandwidth for the links between nodes and backbone (if any). See the \ref pf_link "link section" for syntax/details.
lat             | yes       | int    | Latency for the links between nodes and backbone (if any). See <b>link</b> section for syntax/details.
sharing_policy  | no        | string | Sharing policy for the links between nodes and backbone (if any). See <b>link</b> section for syntax/details.
bb_bw           | no        | int    | Bandwidth for backbone (if any). See <b>link</b> section for syntax/details. If bb_bw and bb_lat (see below) attributes are omitted, no backbone is created (alternative cluster architecture <b>described before</b>).
bb_lat          | no        | int    | Latency for backbone (if any). See <b>link</b> section for syntax/details. If bb_lat and bb_bw (see above) attributes are omitted, no backbone is created (alternative cluster architecture <b>described before</b>).
bb_sharing_policy | no      | string | Sharing policy for the backbone (if any). See <b>link</b> section for syntax/details.
availability_file | no      | string | Allows you to use a file as input for availability. Similar to <b>hosts</b> attribute.
state_file        | no      | string | Allows you to use a file as input for states.  Similar to <b>hosts</b> attribute.
loopback_bw       | no      | int    | Bandwidth for loopback (if any). See <b>link</b> section for syntax/details. If loopback_bw and loopback_lat (see below) attributes are omitted, no loopback link is created and all intra-node communication will use the main network link of the node. Loopback link is a \ref pf_sharing_policy_fatpipe "\b FATPIPE".
loopback_lat      | no      | int    | Latency for loopback (if any). See <b>link</b> section for syntax/details. See loopback_bw for more info.
topology          | no      | FLAT\|TORUS\|FAT_TREE (default: FLAT) | Network topology to use. SimGrid currently supports FLAT (with or without backbone, as described before), <a href="http://en.wikipedia.org/wiki/Torus_interconnect">TORUS </a> and FAT_TREE attributes for this tag.
topo_parameters   | no      | string | Specific parameters to pass for the topology defined in the topology tag. For torus networks, comma-separated list of the number of nodes in each dimension of the torus. For fat trees, refer to \ref AsClusterFatTree "AsClusterFatTree documentation".

TODO

the router name is defined as the resulting String in the following
java line of code:

@verbatim
router_name = prefix + clusterId + _router + suffix;
@endverbatim


#### Cluster example ####

Consider the following two (and independent) uses of the ``cluster`` tag:

\verbatim
<cluster id="my_cluster_1" prefix="" suffix="" radical="0-262144"
         power="1e9" bw="125e6" lat="5E-5"/>

<cluster id="my_cluster_2" prefix="c-" suffix=".me" radical="0-99"
         power="1e9" bw="125e6" lat="5E-5"
         bb_bw="2.25e9" bb_lat="5E-4"/>
\endverbatim

The second example creates one router and 100 machines with the following names:
\verbatim
c-my_cluster_2_router.me
c-0.me
c-1.me
c-2.me
...
c-99.me
\endverbatim

\subsubsection pf_cabinet &lt;cabinet&gt;

\note
    This tag is only available when the routing mode of the AS
    is set to ``Cluster``.

The ``&lt;cabinet /&gt;`` tag is, like the \ref pf_cluster "&lt;cluster&gt;" tag,
a meta-tag. This means that it is simply a shortcut for creating a set of (homogenous) hosts and links quickly;
unsurprisingly, this tag was introduced to setup cabinets in data centers quickly. Unlike
&lt;cluster&gt;, however, the &lt;cabinet&gt; assumes that you create the backbone
and routers yourself; see our examples below.

#### Attributes ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the cabinet. Facilitates referring to this cluster.
prefix          | yes       | string | Each node of the cabinet has to have a name. This name will be prefixed with this prefix.
suffix          | yes       | string | Each node of the cabinet will be suffixed with this suffix
radical         | yes       | string | Regexp used to generate cabinet nodes name. Syntax: "10-20" will give you 11 machines numbered from 10 to 20, "10-20;2" will give you 12 machines, one with the number 2, others numbered as before. The produced number is concatenated between prefix and suffix to form machine names.
power           | yes       | int    | Same as the ``power`` attribute of the \ref pf_host "&lt;host&gt;" tag.
bw              | yes       | int    | Bandwidth for the links between nodes and backbone (if any). See the \ref pf_link "link section" for syntax/details.
lat             | yes       | int    | Latency for the links between nodes and backbone (if any). See the \ref pf_link "link section" for syntax/details.

\note
    Please note that as of now, it is impossible to change attributes such as,
    amount of cores (always set to 1), the initial state of hosts/links
    (always set to ON), the sharing policy of the links (always set to \ref pf_sharing_policy_fullduplex "FULLDUPLEX").

#### Example ####

The following example was taken from ``examples/platforms/meta_cluster.xml`` and
shows how to use the cabinet tag.

\verbatim
  <AS  id="my_cluster1"  routing="Cluster">
    <cabinet id="cabinet1" prefix="host-" suffix=".cluster1"
      power="1Gf" bw="125MBps" lat="100us" radical="1-10"/>
    <cabinet id="cabinet2" prefix="host-" suffix=".cluster1"
      power="1Gf" bw="125MBps" lat="100us" radical="11-20"/>
    <cabinet id="cabinet3" prefix="host-" suffix=".cluster1"
      power="1Gf" bw="125MBps" lat="100us" radical="21-30"/>

    <backbone id="backbone1" bandwidth="2.25GBps" latency="500us"/>
  </AS>
\endverbatim

\note
   Please note that you must specify the \ref pf_backbone "&lt;backbone&gt;"
   tag by yourself; this is not done automatically and there are no checks
   that ensure this backbone was defined.

The hosts generated in the above example are named host-1.cluster, host-2.cluster1
etc.


\subsubsection pf_peer The &lt;peer&gt; tag

This tag represents a peer, as in Peer-to-Peer (P2P) networks. However, internally,
SimGrid transforms a peer into an AS (similar to Cluster). Hence, this tag
is virtually only a shortcut that comes with some pre-defined resources
and values. These are:

\li A tiny AS whose routing type is cluster is created
\li A host
\li Two links: One for download and one for upload. This is
    convenient to use and simulate stuff under the last mile model (e.g., ADSL peers).
\li It connects the two links to the host
\li It creates a router (a gateway) that serves as an entry point for this peer zone.
    This router has coordinates.

#### Attributes ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the peer. Facilitates referring to this peer.
power           | yes       | int    | See the description of the ``host`` tag for this attribute
bw_in           | yes       | int    | Bandwidth downstream
bw_out          | yes       | int    | Bandwidth upstream
lat             | yes       | double | Latency for both up- and downstream, in seconds.
coordinates     | no        | string | Coordinates of the gateway for this peer. Example value: 12.8 14.4 6.4
sharing_policy  | no        | SHARED\|FULLDUPLEX (default: FULLDUPLEX) | Sharing policy for links. See <b>link</b> description for details.
availability_file| no       | string | Availability file for the peer. Same as host availability file. See <b>host</b> description for details.
state_file      | no        | string | State file for the peer. Same as host state file. See <b>host</b> description for details.

Internally, SimGrid transforms any ``\<peer/\>`` construct such as
\verbatim
<peer id="FOO"
  coordinates="12.8 14.4 6.4"
  power="1.5Gf"
  bw_in="2.25GBps"
  bw_out="2.25GBps"
  lat="500us" />
\endverbatim
into an ``\<AS\>`` (see Sections \ref pf_basics and \ref pf_As). In fact, this example of the ``\<peer/\>`` tag
is completely equivalent to the following declaration:

\verbatim
<AS id="as_FOO" routing="Cluster">
   <host id="peer_FOO" power="1.5Gf"/>
   <link id="link_FOO_UP" bandwidth="2.25GBps" latency="500us"/>
   <link id="link_FOO_DOWN" bandwidth="2.25GBps" latency="500us"/>
   <router id="router_FOO" coordinates="25.5 9.4 1.4"/>
   <host_link id="peer_FOO" up="link_FOO_UP" down="link_FOO_DOWN"/>
</AS>
\endverbatim


\subsection pf_ne Network equipments: links and routers

There are two tags at all times available to represent network entities and
several other tags that are available only in certain contexts.
1. ``<link>``: Represents a entity that has a limited bandwidth, a
    latency, and that can be shared according to TCP way to share this
    bandwidth.
\remark
  The concept of links in SimGrid may not be intuitive, as links are not
  limited to connecting (exactly) two entities; in fact, you can have more than
  two equipments connected to it. (In graph theoretical terms: A link in
  SimGrid is not an edge, but a hyperedge)

2. ``<router/>``: Represents an entity that a message can be routed
    to, but that is unable to execute any code. In SimGrid, routers have also
    no impact on the performance: Routers do not limit any bandwidth nor
    do they increase latency. As a matter of fact, routers are (almost) ignored
    by the simulator when the simulation has begun.

3. ``<backbone/>``: This tag is only available when the containing AS is
                    used as a cluster (i.e., mode="Cluster")

\remark
    If you want to represent an entity like a switch, you must use ``<link>`` (see section). Routers are used
    to run some routing algorithm and determine routes (see Section \ref pf_routing for details).

\subsubsection pf_router &lt;router/&gt;

%As said before, <b>router</b> is used only to give some information
for routing algorithms. So, it does not have any attributes except :

#### Attributes ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the router to be used when referring to it.
coordinates     | yes       | string | Must be provided when choosing the Vivaldi, coordinate-based routing model for the AS the router belongs to. More details can be found in the Section \ref pf_P2P_tags.

#### Example ####

\verbatim
 <router id="gw_dc1_horizdist"/>
\endverbatim

\subsubsection pf_link &lt;link/&gt;

Network links can represent one-hop network connections. They are
characterized by their id and their bandwidth; links can (but may not) be subject
to latency.

#### Attributes ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the link to be used when referring to it.
bandwidth       | yes       | int    | Maximum bandwidth for this link, given in bytes/s
latency         | no        | double (default: 0.0) | Latency for this link.
sharing_policy  | no        | \ref sharing_policy_shared "SHARED"\|\ref pf_sharing_policy_fatpipe "FATPIPE"\|\ref pf_sharing_policy_fullduplex "FULLDUPLEX" (default: SHARED) | Sharing policy for the link.
state           | no        | ON\|OFF (default: ON) | Allows you to to turn this link on or off (working / not working)
bandwidth_file  | no        | string | Allows you to use a file as input for bandwidth.
latency_file    | no        | string | Allows you to use a file as input for latency.
state_file      | no        | string | Allows you to use a file as input for states.


#### Possible shortcuts for ``latency`` ####

When using the latency attribute, you can specify the latency by using the scientific
notation or by using common abbreviations. For instance, the following three tags
are equivalent:

\verbatim
 <link id="LINK1" bandwidth="125000000" latency="5E-6"/>
 <link id="LINK1" bandwidth="125000000" latency="5us"/>
 <link id="LINK1" bandwidth="125000000" latency="0.000005"/>
\endverbatim

Here, the second tag uses "us", meaning "microseconds". Other shortcuts are:

Name | Abbreviation | Time (in seconds)
---- | ------------ | -----------------
Week | w | 7 * 24 * 60 * 60
Day  | d | 24 * 60 * 60
Hour | h | 60 * 60
Minute | m | 60
Second | s | 1
Millisecond | ms | 0.001 = 10^(-3)
Microsecond | us | 0.000001 = 10^(-6)
Nanosecond  | ns | 0.000000001 = 10^(-9)
Picosecond  | ps | 0.000000000001 = 10^(-12)

#### Sharing policy ####

\anchor sharing_policy_shared
By default a network link is \b SHARED, i.e., if two or more data flows go
through a link, the bandwidth is shared fairly among all data flows. This
is similar to the sharing policy TCP uses.

\anchor pf_sharing_policy_fatpipe
On the other hand, if a link is defined as a \b FATPIPE,
each flow going through this link will be provided with the complete bandwidth,
i.e., no sharing occurs and the bandwidth is only limiting each flow individually.
Please note that this is really on a per-flow basis, not only on a per-host basis!
The complete bandwidth provided by this link in this mode
is ``number_of_flows*bandwidth``, with at most ``bandwidth`` being available per flow.

Using the FATPIPE mode allows to model backbones that won't affect performance
(except latency).

\anchor pf_sharing_policy_fullduplex
The last mode available is \b FULLDUPLEX. This means that SimGrid will
automatically generate two links (one carrying the suffix _UP and the other the
suffix _DOWN) for each ``<link>`` tag. This models situations when the direction
of traffic is important.

\remark
  Transfers from one side to the other will interact similarly as
  TCP when ACK returning packets circulate on the other direction. More
  discussion about it is available in the description of link_ctn description.

In other words: The SHARED policy defines a physical limit for the bandwidth.
The FATPIPE mode defines a limit for each application,
with no upper total limit.

\remark
  Tip: By using the FATPIPE mode, you can model big backbones that
  won't affect performance (except latency).

#### Example ####

\verbatim
 <link id="SWITCH" bandwidth="125000000" latency="5E-5" sharing_policy="FATPIPE" />
\endverbatim

#### Expressing dynamism and failures ####

Similar to hosts, it is possible to declare links whose state, bandwidth
or latency changes over time (see Section \ref pf_host_dynamism for details).

In the case of network links, the ``bandwidth`` and ``latency`` attributes are
replaced by the ``bandwidth_file`` and ``latency_file`` attributes.
The following XML snippet demonstrates how to use this feature in the platform
file. The structure of the files "link1.bw" and "link1.lat" is shown below.

\verbatim
<link id="LINK1" state_file="link1.fail" bandwidth="80000000" latency=".0001" bandwidth_file="link1.bw" latency_file="link1.lat" />
\endverbatim

\note
  Even if the syntax is the same, the semantic of bandwidth and latency
  trace files differs from that of host availability files. For bandwidth and
  latency, the corresponding files do not
  express availability as a fraction of the available capacity but directly in
  bytes per seconds for the bandwidth and in seconds for the latency. This is
  because most tools allowing to capture traces on real platforms (such as NWS)
  express their results this way.

##### Example of "link1.bw" file #####

~~~{.py}
PERIODICITY 12.0
4.0 40000000
8.0 60000000
~~~

In this example, the bandwidth changes repeatedly, with all changes
being repeated every 12 seconds.

At the beginning of the the simulation, the link's bandwidth is 80,000,000
B/s (i.e., 80 Mb/s); this value was defined in the XML snippet above.
After four seconds, it drops to 40 Mb/s (line 2), and climbs
back to 60 Mb/s after another 4 seconds (line 3). The value does not change any
more until the end of the period, that is, after 12 seconds have been simulated).
At this point, periodicity kicks in and this behavior is repeated: Seconds
12-16 will experience 80 Mb/s, 16-20 40 Mb/s etc.).

##### Example of "link1.lat" file #####

~~~{.py}
PERIODICITY 5.0
1.0 0.001
2.0 0.01
3.0 0.001
~~~

In this example, the latency varies with a period of 5 seconds.
In the xml snippet above, the latency is initialized to be 0.0001s (100µs). This
value will be kept during the first second, since the latency_file contains
changes to this value at second one, two and three.
At second one, the value will be 0.001, i.e., 1ms. One second later it will
be adjusted to 0.01 (or 10ms) and one second later it will be set again to 1ms. The
value will not change until second 5, when the periodicity defined in line 1
kicks in. It then loops back, starting at 100µs (the initial value) for one second.


#### The ``<prop/>`` tag ####

Similar to ``\<host\>``, the link may also contain the ``<prop/>`` tag; see the host
documentation (Section \ref pf_host) for an example.


TODO

\subsubsection pf_backbone <backbone/>

\note
  This tag is <b>only available</b> when the containing AS uses the "Cluster" mode!

Using this tag, you can designate an already existing link to be a backbone.

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | Name of the link that is supposed to act as a backbone.

\subsection pf_storage Storage

\note
  This is a prototype version that should evolve quickly, this
  is just some doc valuable only at the time of writing this doc
  This section describes the storage management under SimGrid ; nowadays
  it's only usable with MSG. It relies basically on linux-like concepts.
  You also may want to have a look to its corresponding section in \ref
  msg_file_management ; functions access are organized as a POSIX-like
  interface.

\subsubsection pf_sto_conc Storage Main concepts
Basically there are 3 different entities available in SimGrid that
can be used to model storages:

Entity name     | Description
--------------- | -----------
\ref pf_storage_entity_storage_type "storage_type"    | Defines a template for a particular kind of storage (such as a hard-drive) and specifies important features of the storage, such as capacity, performance (read/write), content, ... Different models of hard-drives use different storage_types (because the difference between an SSD and an HDD does matter), as they differ in some specifications (e.g., different size or read/write performance).
\ref pf_storage_entity_storage "storage"        | Defines an actual instance of a storage (disk, RAM, ...); uses a ``storage_type`` template (see line above) so that you don't need to re-specify the same details over and over again.
\ref pf_storage_entity_mount "mount"          | Must be wrapped by a \ref pf_host tag; declares which storage(s) this host has mounted and where (i.e., the mountpoint).


\anchor pf_storage_content_file
### %Storage Content File ###

In order to assess exactly how much time is spent reading from the storage,
SimGrid needs to know what is stored in the storage device (identified by distinct (file-)name, like in a file system)
and what size this content has.

\note
    The content file is never changed by the simulation; it is parsed once
    per simulation and kept in memory afterwards. When the content of the
    storage changes, only the internal SimGrid data structures change.

\anchor pf_storage_content_file_structure
#### Structure of a %Storage File ####

Here is an excerpt from two storage file; if you want to see the whole file, check
the file ``examples/platforms/content/storage_content.txt`` that comes with the
SimGrid source code.

SimGrid essentially supports two different formats: UNIX-style filepaths should
follow the well known format:

\verbatim
/lib/libsimgrid.so.3.6.2  12710497
/bin/smpicc  918
/bin/smpirun  7292
/bin/smpif2c  1990
/bin/simgrid_update_xml  5018
/bin/graphicator  66986
/bin/simgrid-colorizer  2993
/bin/smpiff  820
/bin/tesh  356434
\endverbatim

Windows filepaths, unsurprisingly, use the windows style:

\verbatim
\Windows\avastSS.scr 41664
\Windows\bfsvc.exe 75264
\Windows\bootstat.dat 67584
\Windows\CoreSingleLanguage.xml 31497
\Windows\csup.txt 12
\Windows\dchcfg64.exe 335464
\Windows\dcmdev64.exe 93288
\endverbatim

\note
    The different file formats come at a cost; in version 3.12 (and most likely
    in later versions, too), copying files from windows-style storages to unix-style
    storages (and vice versa) is not supported.

\anchor pf_storage_content_file_create
#### Generate a %Storage Content File ####

If you want to generate a storage content file based on your own filesystem (or at least a filesystem you have access to),
try running this command (works only on unix systems):

\verbatim
find /path/you/want -type f -exec ls -l {} \; 2>/dev/null > ./content.txt
\endverbatim

\subsubsection pf_storage_entities The Storage Entities

These are the entities that you can use in your platform files to include
storage in your model. See also the list of our \ref pf_storage_example_files "example files";
these might also help you to get started.

\anchor pf_storage_entity_storage_type
#### \<storage_type\> ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | Identifier of this storage_type; used when referring to it
model           | yes       | string | For reasons of future backwards compatibility only; specifies the name of the model for the storage that should be used
size            | yes       | string | Specifies the amount of available storage space; you can specify storage like "500GiB" or "500GB" if you want. (TODO add a link to all the available abbreviations)
content         | yes       | string | Path to a \ref pf_storage_content_file "Storage Content File" on your system. This file must exist.
content_type    | no        | ("txt_unix"\|"txt_win") | Determines which kind of filesystem you're using; make sure the filenames (stored in that file, see \ref pf_storage_content_file_structure "Storage Content File Structure"!)

This tag must contain some predefined model properties, specified via the &lt;model_prop&gt; tag. Here is a list,
see below for an example:

Property id     | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
Bwrite          | yes       | string | Bandwidth for write access; in B/s (but you can also specify e.g. "30MBps")
Bread           | yes       | string | Bandwidth for read access; in B/s (but you can also specify e.g. "30MBps")
Bconnexion      | yes       | string | Throughput (of the storage connector) in B/s.

\note
     A storage_type can also contain the <b>&lt;prop&gt;</b> tag. The &lt;prop&gt; tag allows you
     to associate additional information to this &lt;storage_type&gt; and follows the
     attribute/value schema; see the example below. You may want to use it to give information to
     the tool you use for rendering your simulation, for example.

Here is a complete example for the ``storage_type`` tag:
\verbatim
<storage_type id="single_HDD" model="linear_no_lat" size="4000" content_type="txt_unix">
  <model_prop id="Bwrite" value="30MBps" />
  <model_prop id="Bread" value="100MBps" />
  <model_prop id="Bconnection" value="150MBps" />
  <prop id="Brand" value="Western Digital" />
</storage_type>
\endverbatim

\anchor pf_storage_entity_storage
#### &lt;storage&gt; ####

``storage`` attributes:

Attribute name | Mandatory | Values | Description
-------------- | --------- | ------ | -----------
id             | yes       | string | Identifier of this ``storage``; used when referring to it
typeId         | yes       | string | Here you need to refer to an already existing \ref pf_storage_entity_storage_type "\<storage_type\>"; the storage entity defined by this tag will then inherit the properties defined there.
attach         | yes       | string | Name of a host (see Section \ref pf_host) that this storage is <i>physically</i> attached to (e.g., a harddrive in a computer)
content        | no        | string | When specified, overwrites the content attribute of \ref pf_storage_entity_storage_type "\<storage_type\>"
content_type   | no        | string | When specified, overwrites the content_type attribute of \ref pf_storage_entity_storage_type "\<storage_type\>"

Here are two examples:

\verbatim
     <storage id="Disk1" typeId="single_HDD" attach="bob" />

     <storage id="Disk2" typeId="single_SSD"
              content="content/win_storage_content.txt"
              content_type="txt_windows" attach="alice" />
\endverbatim

The first example is straightforward: A disk is defined and called "Disk1"; it is
of type "single_HDD" (shown as an example of \ref pf_storage_entity_storage_type "\<storage_type\>" above) and attached
to a host called "bob" (the definition of this host is omitted here).

The second storage is called "Disk2", is still of the same type as Disk1 but
now specifies a new content file (so the content will be different from Disk1)
and the filesystem uses the windows style; finally, it is attached to a second host,
called alice (which is again not defined here).

\anchor pf_storage_entity_mount
#### &lt;mount&gt; ####

Attributes:
| Attribute name   | Mandatory   | Values   | Description                                                                                               |
| ---------------- | ----------- | -------- | -------------                                                                                             |
| id               | yes         | string   | Refers to a \ref pf_storage_entity_storage "&lt;storage&gt;" entity that will be mounted on that computer |
| name             | yes         | string   | Path/location to/of the logical reference (mount point) of this disk

This tag must be enclosed by a \ref pf_host tag. It then specifies where the mountpoint of a given storage device (defined by the ``id`` attribute)
is; this location is specified by the ``name`` attribute.

Here is a simple example, taken from the file ``examples/platform/storage.xml``:

\verbatim
    <storage_type id="single_SSD" model="linear_no_lat" size="500GiB">
       <model_prop id="Bwrite" value="60MBps" />
       <model_prop id="Bread" value="200MBps" />
       <model_prop id="Bconnection" value="220MBps" />
    </storage_type>

    <storage id="Disk2" typeId="single_SSD"
              content="content/win_storage_content.txt"
              content_type="txt_windows" attach="alice" />
    <storage id="Disk4" typeId="single_SSD"
             content="content/small_content.txt"
             content_type="txt_unix" attach="denise"/>

    <host id="alice" power="1Gf">
      <mount storageId="Disk2" name="c:"/>
    </host>

    <host id="denise" power="1Gf">
      <mount storageId="Disk2" name="c:"/>
      <mount storageId="Disk4" name="/home"/>
    </host>
\endverbatim

This example is quite interesting, as the same device, called "Disk2", is mounted by
two hosts at the same time! Note, however, that the host called ``alice`` is actually
attached to this storage, as can be seen in the \ref pf_storage_entity_storage "&lt;storage&gt;"
tag. This means that ``denise`` must access this storage via network, but SimGrid automatically takes
care of that for you.

Furthermore, this example shows that ``denise`` has mounted two storages with different
filesystem types (unix and windows). In general, a host can mount as many storage devices as
required.

\note
    Again, the difference between ``attach`` and ``mount`` is simply that
    an attached storage is always physically inside (or connected to) that machine;
    for instance, a USB stick is attached to one and only one machine (where it's plugged-in)
    but it can only be mounted on others, as mounted storage can also be a remote location.

###### Example files #####

\verbinclude example_filelist_xmltag_mount

\anchor pf_storage_entity_mstorage
#### &lt;mstorage&gt; ####
\note
    This is currently unused.

<b>mstorage</b> attributes :
\li <b>typeId (mandatory)</b>: the id of the <b>storage</b> that must
    be mounted on that computer.
\li <b>name (mandatory)</b>: the name that will be the logical
    reference to this disk (the mount point).

\subsubsection pf_storage_example_files Example files

Several examples were already discussed above; if you're interested in full examples,
check the the following platforms:

1. ``examples/platforms/storage.xml``
2. ``examples/platforms/remote_io.xml``

If you're looking for some examplary C code, you may find the source code
available in the directory ``examples/msg/io/`` useful.

\subsubsection pf_storage_examples_modelling Modelling different situations

The storage functionality of SimGrid is type-agnostic, that is, the implementation
does not presume any type of storagei, such as HDDs/SSDs, RAM,
CD/DVD devices, USB sticks etc.

This allows the user to apply the simulator for a wide variety of scenarios; one
common scenario would be the access of remote RAM.

#### Modelling the access of remote RAM ####

How can this be achieved in SimGrid? Let's assume we have a setup where three hosts
(HostA, HostB, HostC) need to access remote RAM:

\verbatim
      Host A
    /
RAM -- Host B
    \
      Host C
\endverbatim

An easy way to model this scenario is to setup and define the RAM via the
\ref pf_storage_entity_storage "storage" and \ref pf_storage_entity_storage_type "storage type"
entities and attach it to a remote dummy host; then, every host can have their own links
to this host (modelling for instance certain scenarios, such as PCIe ...)

\verbatim
              Host A
            /
RAM - Dummy -- Host B
            \
              Host C
\endverbatim

Now, if read from this storage, the host that mounts this storage
communicates to the dummy host which reads from RAM and
sends the information back.


\section pf_routing Routing

To achieve high performance, the routing tables used within SimGrid are
static. This means that routing between two nodes is calculated once
and will not change during execution. The SimGrid team chose to use this
approach as it is rare to have a real deficiency of a resource;
most of the time, a communication fails because the links experience too much
congestion and hence, your connection stops before the timeout or
because the computer designated to be the destination of that message
is not responding.

We also chose to use shortest paths algorithms in order to emulate
routing. Doing so is consistent with the reality: [RIP](https://en.wikipedia.org/wiki/Routing_Information_Protocol),
[OSPF](https://en.wikipedia.org/wiki/Open_Shortest_Path_First), [BGP](https://en.wikipedia.org/wiki/Border_Gateway_Protocol)
are all calculating shortest paths. They do require some time to converge, but
eventually, when the routing tables have stabilized, your packets will follow
the shortest paths.

\subsection pf_rm Routing models

For each AS, you must define explicitly which routing model will
be used. There are 3 different categories for routing models:

1. \ref pf_routing_model_shortest_path "Shortest-path" based models: SimGrid calculates shortest
   paths and manages them. Behaves more or less like most real life
   routing mechanisms.
2. \ref pf_routing_model_manual "Manually-entered" route models: you have to define all routes
   manually in the platform description file; this can become
   tedious very quickly, as it is very verbose.
   Consistent with some manually managed real life routing.
3. \ref pf_routing_model_simple "Simple/fast models": those models offer fast, low memory routing
   algorithms. You should consider to use this type of model if 
   you can make some assumptions about your AS. 
   Routing in this case is more or less ignored.

\subsubsection pf_raf The router affair

Using routers becomes mandatory when using shortest-path based
models or when using the bindings to the ns-3 packet-level
simulator instead of the native analytical network model implemented
in SimGrid.

For graph-based shortest path algorithms, routers are mandatory, because these
algorithms require a graph as input and so we need to have source and
destination for each edge.

Routers are naturally an important concept ns-3 since the
way routers run the packet routing algorithms is actually simulated.
SimGrid's analytical models however simply aggregate the routing time
with the transfer time. 

So why did we incorporate routers in SimGrid? Rebuilding a graph representation
only from the route information turns out to be a very difficult task, because
of the missing information about how routes intersect. That is why we
introduced routers, which are simply used to express these intersection points.
It is important to understand that routers are only used to provide topological
information.

To express this topological information, a <b>route</b> has to be
defined in order to declare which link is connected to a router. 


\subsubsection pf_routing_model_shortest_path Shortest-path based models

The following table shows all the models that compute routes using
shortest-paths algorithms are currently available in SimGrid. More detail on how
to choose the best routing model is given in the Section called \"\ref pf_routing_howto_choose_wisely\".

| Name                                                | Description                                                                |
| --------------------------------------------------- | -------------------------------------------------------------------------- |
| \ref pf_routing_model_floyd "Floyd"                 | Floyd routing data. Pre-calculates all routes once                         |
| \ref pf_routing_model_dijkstra "Dijkstra"           | Dijkstra routing data. Calculates routes only when needed                  |
| \ref pf_routing_model_dijkstracache "DijkstraCache" | Dijkstra routing data. Handles some cache for already calculated routes.   |

All those shortest-path models are instanciated in the same way and are
completely interchangeable. Here are some examples:

\anchor pf_routing_model_floyd
### Floyd ###

Floyd example:
\verbatim
<AS  id="AS0"  routing="Floyd">

  <cluster id="my_cluster_1" prefix="c-" suffix=""
           radical="0-1" power="1000000000" bw="125000000" lat="5E-5"
           router_id="router1"/>

  <AS id="AS1" routing="None">
    <host id="host1" power="1000000000"/>
  </AS>

  <link id="link1" bandwidth="100000" latency="0.01"/>

  <ASroute src="my_cluster_1" dst="AS1"
    gw_src="router1"
    gw_dst="host1">
    <link_ctn id="link1"/>
  </ASroute>

</AS>
\endverbatim

ASroute given at the end gives a topological information: link1 is
between router1 and host1.

#### Example platform files ####

This is an automatically generated list of example files that use the Floyd
routing model (the path is given relative to SimGrid's source directory)

\verbinclude example_filelist_routing_floyd

\anchor pf_routing_model_dijkstra
### Dijkstra ###

#### Example platform files ####

This is an automatically generated list of example files that use the Dijkstra
routing model (the path is given relative to SimGrid's source directory)

\verbinclude example_filelist_routing_dijkstra

Dijsktra example :
\verbatim
 <AS id="AS_2" routing="Dijsktra">
     <host id="AS_2_host1" power="1000000000"/>
     <host id="AS_2_host2" power="1000000000"/>
     <host id="AS_2_host3" power="1000000000"/>
     <link id="AS_2_link1" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link2" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link3" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link4" bandwidth="1250000000" latency="5E-4"/>
     <router id="central_router"/>
     <router id="AS_2_gateway"/>
     <!-- routes providing topological information -->
     <route src="central_router" dst="AS_2_host1"><link_ctn id="AS_2_link1"/></route>
     <route src="central_router" dst="AS_2_host2"><link_ctn id="AS_2_link2"/></route>
     <route src="central_router" dst="AS_2_host3"><link_ctn id="AS_2_link3"/></route>
     <route src="central_router" dst="AS_2_gateway"><link_ctn id="AS_2_link4"/></route>
  </AS>
\endverbatim

\anchor pf_routing_model_dijkstracache
### DijkstraCache ###

DijsktraCache example:
\verbatim
<AS id="AS_2" routing="DijsktraCache">
     <host id="AS_2_host1" power="1000000000"/>
     ...
(platform unchanged compared to upper example)
\endverbatim

#### Example platform files ####

This is an automatically generated list of example files that use the DijkstraCache
routing model (the path is given relative to SimGrid's source directory):

Editor's note: At the time of writing, no platform file used this routing model - so
if there are no example files listed here, this is likely to be correct.

\verbinclude example_filelist_routing_dijkstra_cache

\subsubsection pf_routing_model_manual Manually-entered route models

| Name                               | Description                                                                    |
| ---------------------------------- | ------------------------------------------------------------------------------ |
| \ref pf_routing_model_full "Full"  | You have to enter all necessary routers manually; that is, every single route. This may consume a lot of memory when the XML is parsed and might be tedious to write; i.e., this is only recommended (if at all) for small platforms. |

\anchor pf_routing_model_full
### Full ###

Full example :
\verbatim
<AS  id="AS0"  routing="Full">
   <host id="host1" power="1000000000"/>
   <host id="host2" power="1000000000"/>
   <link id="link1" bandwidth="125000000" latency="0.000100"/>
   <route src="host1" dst="host2"><link_ctn id="link1"/></route>
 </AS>
\endverbatim

#### Example platform files ####

This is an automatically generated list of example files that use the Full
routing model (the path is given relative to SimGrid's source directory):

\verbinclude example_filelist_routing_full

\subsubsection pf_routing_model_simple Simple/fast models

| Name                                     | Description                                                                                                                         |
| ---------------------------------------- | ------------------------------------------------------------------------------                                                      |
| \ref pf_routing_model_cluster "Cluster"  | This is specific to the \ref pf_cluster "&lt;cluster/&gt;" tag and should not be used by the user, as several assumptions are made. |
| \ref pf_routing_model_none    "None"     | No routing at all. Unless you know what you're doing, avoid using this mode in combination with a non-constant network model.       |
| \ref pf_routing_model_vivaldi "Vivaldi"  | Perfect when you want to use coordinates. Also see the corresponding \ref pf_P2P_tags "P2P section" below.                          |

\anchor pf_routing_model_cluster
### Cluster ###

\note
 In this mode, the \ref pf_cabinet "&lt;cabinet/&gt;" tag is available.

#### Example platform files ####

This is an automatically generated list of example files that use the Cluster
routing model (the path is given relative to SimGrid's source directory):

\verbinclude example_filelist_routing_cluster

\anchor pf_routing_model_none
### None ###

This model does exactly what it's name advertises: Nothing. There is no routing
available within this model and if you try to communicate within the AS that
uses this model, SimGrid will fail unless you have explicitly activated the
\ref options_model_select_network_constant "Constant Network Model" (this model charges
the same for every single communication). It should
be noted, however, that you can still attach an \ref pf_routing_tag_asroute "ASroute",
as is demonstrated in the example below:

\verbinclude platforms/cluster_and_one_host.xml

#### Example platform files ####

This is an automatically generated list of example files that use the None
routing model (the path is given relative to SimGrid's source directory):

\verbinclude example_filelist_routing_none


\anchor pf_routing_model_vivaldi
### Vivaldi ###

For more information on how to use the [Vivaldi Coordinates](https://en.wikipedia.org/wiki/Vivaldi_coordinates),
see also Section \ref pf_P2P_tags "P2P tags".

For documentation on how to activate this model (as some initialization must be done
in the simulator), see Section \ref options_model_network_coord "Activating Coordinate Based Routing".

Note that it is possible to combine the Vivaldi routing model with other routing models;
an example can be found in the file \c examples/platforms/cloud.xml. This
examples models an AS using Vivaldi that contains other ASes that use different
routing models.

#### Example platform files ####

This is an automatically generated list of example files that use the None
routing model (the path is given relative to SimGrid's source directory):

\verbinclude example_filelist_routing_vivaldi


\subsection ps_dec Defining routes

There are currently four different ways to define routes: 

| Name                                              | Description                                                                         |
| ------------------------------------------------- | ----------------------------------------------------------------------------------- |
| \ref pf_routing_tag_route "route"                 | Used to define route between host/router                                            |
| \ref pf_routing_tag_asroute "ASroute"             | Used to define route between different AS                                           |
| \ref pf_routing_tag_bypassroute "bypassRoute"     | Used to supersede normal routes as calculated by the network model between host/router; e.g., can be used to use a route that is not the shortest path for any of the shortest-path routing models. |
| \ref pf_routing_tag_bypassasroute "bypassASroute"  | Used in the same way as bypassRoute, but for AS                                     |

Basically all those tags will contain an (ordered) list of references
to link that compose the route you want to define.

Consider the example below:

\verbatim
<route src="Alice" dst="Bob">
        <link_ctn id="link1"/>
        <link_ctn id="link2"/>
        <link_ctn id="link3"/>
</route>
\endverbatim

The route here from host Alice to Bob will be first link1, then link2,
and finally link3. What about the reverse route? \ref pf_routing_tag_route "Route" and
\ref pf_routing_tag_asroute "ASroute" have an optional attribute \c symmetrical, that can
be either \c YES or \c NO. \c YES means that the reverse route is the same
route in the inverse order, and is set to \c YES by default. Note that
this is not the case for bypass*Route, as it is more probable that you
want to bypass only one default route.

For an \ref pf_routing_tag_asroute "ASroute", things are just slightly more complicated, as you have
to give the id of the gateway which is inside the AS you want to access ... 
So it looks like this:

\verbatim
<ASroute src="AS1" dst="AS2"
  gw_src="router1" gw_dst="router2">
  <link_ctn id="link1"/>
</ASroute>
\endverbatim

gw == gateway, so when any message are trying to go from AS1 to AS2,
it means that it must pass through router1 to get out of the AS, then
pass through link1, and get into AS2 by being received by router2.
router1 must belong to AS1 and router2 must belong to AS2.

\subsubsection pf_linkctn &lt;link_ctn/&gt;

This entity has only one purpose: Refer to an already existing
\ref pf_link "&lt;link/&gt;" when defining a route, i.e., it
can only occur as a child of \ref pf_routing_tag_route "&lt;route/&gt;"

| Attribute name  | Mandatory | Values | Description                                                   |
| --------------- | --------- | ------ | -----------                                                   |
| id              | yes       | String | The identifier of the link that should be added to the route. |
| direction       | maybe     | UP\|DOWN | If the link referenced by \c id has been declared as \ref pf_sharing_policy_fullduplex "FULLDUPLEX", this indicates which direction the route traverses through this link: UP or DOWN. If you don't use FULLDUPLEX, this attribute has no effect.

#### Example Files ####

This is an automatically generated list of example files that use the \c &lt;link_ctn/gt;
entity (the path is given relative to SimGrid's source directory):

\verbinclude example_filelist_xmltag_linkctn

\subsubsection pf_routing_tag_asroute ASroute

The purpose of this entity is to define a route between two ASes.
This is mainly useful when you're in the \ref pf_routing_model_full "Full routing model".

#### Attributes ####

| Attribute name  | Mandatory | Values | Description                                                                                                                                |
| --------------- | --------- | ------ | -----------                                                                                                                                |
| src             | yes       | String | The identifier of the source AS                                                                                                            |
| dst             | yes       | String | See the \c src attribute                                                                                                                   |
| gw_src          | yes       | String | The gateway that will be used within the src AS; this can be any \ref pf_host "Host" or \ref pf_router "Router" defined within the src AS. |
| gw_dst          | yes       | String | Same as \c gw_src, but with the dst AS instead.                                                                                            |
| symmetrical     | no        | YES\|NO (Default: YES) | If this route is symmetric, the opposite route (from dst to src) will also be declared implicitly.               | 

#### Example ####

\verbatim
<AS  id="AS0"  routing="Full">
  <cluster id="my_cluster_1" prefix="c-" suffix=".me"
                radical="0-149" power="1000000000"    bw="125000000"     lat="5E-5"
        bb_bw="2250000000" bb_lat="5E-4"/>

  <cluster id="my_cluster_2" prefix="c-" suffix=".me"
            radical="150-299" power="1000000000"        bw="125000000"  lat="5E-5"
            bb_bw="2250000000" bb_lat="5E-4"/>

     <link id="backbone" bandwidth="1250000000" latency="5E-4"/>

     <ASroute src="my_cluster_1" dst="my_cluster_2"
         gw_src="c-my_cluster_1_router.me"
         gw_dst="c-my_cluster_2_router.me">
                <link_ctn id="backbone"/>
     </ASroute>
     <ASroute src="my_cluster_2" dst="my_cluster_1"
         gw_src="c-my_cluster_2_router.me"
         gw_dst="c-my_cluster_1_router.me">
                <link_ctn id="backbone"/>
     </ASroute>
</AS>
\endverbatim

\subsubsection pf_routing_tag_route route 

The principle is the same as for 
\ref pf_routing_tag_asroute "ASroute": The route contains a list of links that
provide a path from \c src to \c dst. Here, \c src and \c dst can both be either a 
\ref pf_host "host" or \ref pf_router "router".  This is mostly useful for the 
\ref pf_routing_model_full "Full routing model" as well as for the 
\ref pf_routing_model_shortest_path "shortest-paths" based models (as they require 
topological information).


| Attribute name  | Mandatory | Values                 | Description                                                                                        |
| --------------- | --------- | ---------------------- | -----------                                                                                        |
| src             | yes       | String                 | The value given to the source's "id" attribute                                                     |
| dst             | yes       | String                 | The value given to the destination's "id" attribute.                                               |
| symmetrical     | no        | YES\| NO (Default: YES) | If this route is symmetric, the opposite route (from dst to src) will also be declared implicitly. |


#### Examples ####

A route in the \ref pf_routing_model_full "Full routing model" could look like this:
\verbatim
 <route src="Tremblay" dst="Bourassa">
     <link_ctn id="4"/><link_ctn id="3"/><link_ctn id="2"/><link_ctn id="0"/><link_ctn id="1"/><link_ctn id="6"/><link_ctn id="7"/>
 </route>
\endverbatim

A route in the \ref pf_routing_model_shortest_path "Shortest-Path routing model" could look like this:
\verbatim
<route src="Tremblay" dst="Bourassa">
  <link_ctn id="3"/>
</route>
\endverbatim
\note 
    You must only have one link in your routes when you're using them to provide
    topological information, as the routes here are simply the edges of the
    (network-)graph and the employed algorithms need to know which edge connects
    which pair of entities.

\subsubsection pf_routing_tag_bypassasroute bypassASroute

%As said before, once you choose
a model, it (most likely; the constant network model, for example, doesn't) calculates routes for you. But maybe you want to
define some of your routes, which will be specific. You may also want
to bypass some routes defined in lower level AS at an upper stage:
<b>bypassASroute</b> is the tag you're looking for. It allows to
bypass routes defined between already defined between AS (if you want
to bypass route for a specific host, you should just use byPassRoute).
The principle is the same as ASroute : <b>bypassASroute</b> contains
list of links that are in the path between src and dst.

#### Attributes ####

| Attribute name  | Mandatory | Values                  | Description                                                                                                  |
| --------------- | --------- | ----------------------  | -----------                                                                                                  |
| src             | yes       | String                  | The value given to the source AS's "id" attribute                                                            |
| dst             | yes       | String                  | The value given to the destination AS's "id" attribute.                                                      |
| gw_src          | yes       | String                  | The value given to the source gateway's "id" attribute; this can be any host or router within the src AS     |
| gw_dst          | yes       | String                  | The value given to the destination gateway's "id" attribute; this can be any host or router within the dst AS|
| symmetrical     | no        | YES\| NO (Default: YES) | If this route is symmetric, the opposite route (from dst to src) will also be declared implicitly. |

#### Example ####

\verbatim
<bypassASRoute src="my_cluster_1" dst="my_cluster_2"
  gw_src="my_cluster_1_router"
  gw_dst="my_cluster_2_router">
    <link_ctn id="link_tmp"/>
</bypassASroute>
\endverbatim

This example shows that link \c link_tmp (definition not displayed here) directly
connects the router \c my_cluster_1_router in the source cluster to the router
\c my_cluster_2_router in the destination router. Additionally, as the \c symmetrical
attribute was not given, this route is presumed to be symmetrical.

\subsubsection pf_routing_tag_bypassroute bypassRoute

%As said before, once you choose
a model, it (most likely; the constant network model, for example, doesn't) calculates routes for you. But maybe you want to
define some of your routes, which will be specific. You may also want
to bypass some routes defined in lower level AS at an upper stage :
<b>bypassRoute</b> is the tag you're looking for. It allows to bypass
routes defined between <b>host/router</b>. The principle is the same
as route : <b>bypassRoute</b> contains list of links references of
links that are in the path between src and dst.

#### Attributes ####

| Attribute name  | Mandatory | Values                  | Description                                                                                                  |
| --------------- | --------- | ----------------------  | -----------                                                                                                  |
| src             | yes       | String                  | The value given to the source AS's "id" attribute                                                            |
| dst             | yes       | String                  | The value given to the destination AS's "id" attribute.                                                      |
| symmetrical     | no        | YES \| NO (Default: YES) | If this route is symmetric, the opposite route (from dst to src) will also be declared implicitly. |

#### Examples ####

\verbatim
<bypassRoute src="host_1" dst="host_2">
   <link_ctn id="link_tmp"/>
</bypassRoute>
\endverbatim

This example shows that link \c link_tmp (definition not displayed here) directly
connects host \c host_1 to host \c host_2. Additionally, as the \c symmetrical
attribute was not given, this route is presumed to be symmetrical.

\subsection pb_baroex Basic Routing Example

Let's say you have an AS named AS_Big that contains two other AS, AS_1
and AS_2. If you want to make a host (h1) from AS_1 with another one
(h2) from AS_2 then you'll have to proceed as follows:
\li First, you have to ensure that a route is defined from h1 to the
    AS_1's exit gateway and from h2 to AS_2's exit gateway.
\li Then, you'll have to define a route between AS_1 to AS_2. %As those
    AS are both resources belonging to AS_Big, then it has to be done
    at AS_big level. To define such a route, you have to give the
    source AS (AS_1), the destination AS (AS_2), and their respective
    gateway (as the route is effectively defined between those two
    entry/exit points). Elements of this route can only be elements
    belonging to AS_Big, so links and routers in this route should be
    defined inside AS_Big. If you choose some shortest-path model,
    this route will be computed automatically.

%As said before, there are mainly 2 tags for routing :
\li <b>ASroute</b>: to define routes between two  <b>AS</b>
\li <b>route</b>: to define routes between two <b>host/router</b>

%As we are dealing with routes between AS, it means that those we'll
have some definition at AS_Big level. Let consider AS_1 contains 1
host, 1 link and one router and AS_2 3 hosts, 4 links and one router.
There will be a central router, and a cross-like topology. At the end
of the crosses arms, you'll find the 3 hosts and the router that will
act as a gateway. We have to define routes inside those two AS. Let
say that AS_1 contains full routes, and AS_2 contains some Floyd
routing (as we don't want to bother with defining all routes). %As
we're using some shortest path algorithms to route into AS_2, we'll
then have to define some <b>route</b> to gives some topological
information to SimGrid. Here is a file doing it all :

\verbatim
<AS  id="AS_Big"  routing="Dijsktra">
  <AS id="AS_1" routing="Full">
     <host id="AS_1_host1" power="1000000000"/>
     <link id="AS_1_link" bandwidth="1250000000" latency="5E-4"/>
     <router id="AS_1_gateway"/>
     <route src="AS_1_host1" dst="AS_1_gateway">
            <link_ctn id="AS_1_link"/>
     </route>
  </AS>
  <AS id="AS_2" routing="Floyd">
     <host id="AS_2_host1" power="1000000000"/>
     <host id="AS_2_host2" power="1000000000"/>
     <host id="AS_2_host3" power="1000000000"/>
     <link id="AS_2_link1" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link2" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link3" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link4" bandwidth="1250000000" latency="5E-4"/>
     <router id="central_router"/>
     <router id="AS_2_gateway"/>
     <!-- routes providing topological information -->
     <route src="central_router" dst="AS_2_host1"><link_ctn id="AS_2_link1"/></route>
     <route src="central_router" dst="AS_2_host2"><link_ctn id="AS_2_link2"/></route>
     <route src="central_router" dst="AS_2_host3"><link_ctn id="AS_2_link3"/></route>
     <route src="central_router" dst="AS_2_gateway"><link_ctn id="AS_2_link4"/></route>
  </AS>
    <link id="backbone" bandwidth="1250000000" latency="5E-4"/>

     <ASroute src="AS_1" dst="AS_2"
         gw_src="AS_1_gateway"
         gw_dst="AS_2_gateway">
                <link_ctn id="backbone"/>
     </ASroute>
</AS>
\endverbatim

\section pf_other_tags Tags not (directly) describing the platform

There are 3 tags, that you can use inside a \<platform\> tag that are
not describing the platform:
\li \ref pf_random "random": it allows you to define random generators you want to use
    for your simulation.
\li \ref pf_config "config": it allows you to pass some configuration stuff like, for
    example, the network model and so on. It follows the
\li \ref pf_include "include": allows you to include another file into the current one.

\subsection pf_config config

The only purpose of this tag is to contain the \c prop tags, as described below.
These tags will then configure the options as described by Section \ref options.
(See the example)

#### Attributes ####

| Attribute name  | Mandatory | Values                  | Description                                                                                                  |
| --------------- | --------- | ----------------------  | -----------                                                                                                  |
| id              | yes       | String                  | The identifier of the config tag when referring to id; this is basically useless, though.                    |

#### Possible children ####

Tag name        | Description | Documentation
------------    | ----------- | -------------
\<prop/\>       | The prop tag allows you to define different configuration options following the attribute/value schema. See the \ref options page. | N/A

#### Example ####

\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">
<config id="General">
        <prop id="maxmin/precision" value="0.000010"></prop>
        <prop id="cpu/optim" value="TI"></prop>
        <prop id="host/model" value="compound"></prop>
        <prop id="network/model" value="SMPI"></prop>
        <prop id="path" value="~/"></prop>
        <prop id="smpi/bw_factor" value="65472:0.940694;15424:0.697866;9376:0.58729"></prop>
</config>

<AS  id="AS0"  routing="Full">
...
\endverbatim


\subsection pf_random random

<b>This has not yet been implemented.</b>

\subsection pf_include include

The \c include tag allows you to import other platforms into your
local file. This is done with the intention to help people
combine their different AS and provide new platforms. Those files
should contain XML that consists of 
\ref pf_include "include", \ref pf_cluster "cluster", \ref pf_peer "peer", \ref pf_As "AS", \ref pf_trace "trace", \ref pf_trace "tags".

\note
    Due to some obscure technical reasons, you have to open
    and close the tag in order to make it work.

#### Attributes ####

| Attribute name  | Mandatory | Values                  | Description                                                                                                  |
| --------------- | --------- | ----------------------  | -----------                                                                                                  |
| file            | yes       | String                  | Filename of the path you want to include with either relative or absolute path. Syntax is defined by your OS |


#### Example ####

The following example includes two files, clusterA.xml and clusterB.xml and
combines them two one platform file; all hosts, routers etc. defined in 
each of them will then be usable.

\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">
        <AS id="main" routing="Full">
                <include file="clusterA.xml"></include>
                <include file="clusterB.xml"></include>
        </AS>
</platform>
\endverbatim

\subsection pf_trace trace and trace_connect

Both tags are an alternate way to pass files containing information on
availability, state etc. to an entity. (See also, for instance, Section \ref
pf_host_churn "Churn", as described for the host entity.) Instead of referring
to the file directly in the host, link, or cluster tag, you proceed by defining
a trace with an id corresponding to a file, later a host/link/cluster, and
finally using trace_connect you say that the file trace must be used by the
entity. 


#### Example #### 

\verbatim
<AS  id="AS0"  routing="Full">
  <host id="bob" power="1000000000"/>
</AS>
<trace id="myTrace" file="bob.trace" periodicity="1.0"/>
<trace_connect trace="myTrace" element="bob" kind="POWER"/>
\endverbatim

\note 
    The order here is important.  \c trace_connect must come 
    after the elements \c trace and \c host, as both the host
    and the trace definition must be known when \c trace_connect
    is parsed; the order of \c trace and \c host is arbitrary.


#### \c trace attributes ####


| Attribute name  | Mandatory | Values                 | Description                                                                                       |
| --------------- | --------- | ---------------------- | -----------                                                                                       |
| id              | yes       | String                 | Identifier of this trace; this is the name you pass on to \c trace_connect.                       |
| file            | no        | String                 | Filename of the file that contains the information - the path must follow the style of your OS. You can omit this, but then you must specifiy the values inside of &lt;trace&gt; and &lt;/trace&gt; - see the example below. |
| trace_periodicity | yes | String | This is the same as for \ref pf_host "hosts" (see there for details) |

Here is an example  of trace when no file name is provided:

\verbatim
 <trace id="myTrace" periodicity="1.0">
    0.0 1.0
    11.0 0.5
    20.0 0.8
 </trace>
\endverbatim

#### \c trace_connect attributes ####

| Attribute name  | Mandatory | Values                 | Description                                                                                       |
| --------------- | --------- | ---------------------- | -----------                                                                                       |
| kind            | no        | HOST_AVAIL\|POWER\|<br/>LINK_AVAIL\|BANDWIDTH\|LATENCY (Default: HOST_AVAIL)   | Describes the kind of trace.                   |
| trace           | yes       | String                 | Identifier of the referenced trace (specified of the trace's \c id attribute)                     |
| element         | yes       | String                 | The identifier of the referenced entity as given by its \c id attribute                           |

\section pf_hints Hints and tips, or how to write a platform efficiently

Now you should know at least the syntax and be able to create a
platform by your own. However, after having ourselves wrote some platforms, there
are some best practices you should pay attention to in order to
produce good platform and some choices you can make in order to have
faster simulations. Here's some hints and tips, then.

\subsection pf_as_h AS Hierarchy
The AS design allows SimGrid to go fast, because computing route is
done only for the set of resources defined in this AS. If you're using
only a big AS containing all resource with no AS into it and you're
using Full model, then ... you'll loose all interest into it. On the
other hand, designing a binary tree of AS with, at the lower level,
only one host, then you'll also loose all the good AS hierarchy can
give you. Remind you should always be "reasonable" in your platform
definition when choosing the hierarchy. A good choice if you try to
describe a real life platform is to follow the AS described in
reality, since this kind of trade-off works well for real life
platforms.

\subsection pf_exit_as Exit AS: why and how
Users that have looked at some of our platforms may have notice a
non-intuitive schema ... Something like that :


\verbatim
<AS id="AS_4"  routing="Full">
<AS id="exitAS_4"  routing="Full">
        <router id="router_4"/>
</AS>
<cluster id="cl_4_1" prefix="c_4_1-" suffix="" radical="1-20" power="1000000000" bw="125000000" lat="5E-5" bb_bw="2250000000" bb_lat="5E-4"/>
<cluster id="cl_4_2" prefix="c_4_2-" suffix="" radical="1-20" power="1000000000" bw="125000000" lat="5E-5" bb_bw="2250000000" bb_lat="5E-4"/>
<link id="4_1" bandwidth="2250000000" latency="5E-5"/>
<link id="4_2" bandwidth="2250000000" latency="5E-5"/>
<link id="bb_4" bandwidth="2250000000" latency="5E-4"/>
<ASroute src="cl_4_1"
        dst="cl_4_2"
        gw_src="c_4_1-cl_4_1_router"
        gw_dst="c_4_2-cl_4_2_router"
        symmetrical="YES">
                <link_ctn id="4_1"/>
                <link_ctn id="bb_4"/>
                <link_ctn id="4_2"/>
</ASroute>
<ASroute src="cl_4_1"
        dst="exitAS_4"
        gw_src="c_4_1-cl_4_1_router"
        gw_dst="router_4"
        symmetrical="YES">
                <link_ctn id="4_1"/>
                <link_ctn id="bb_4"/>
</ASroute>
<ASroute src="cl_4_2"
        dst="exitAS_4"
        gw_src="c_4_2-cl_4_2_router"
        gw_dst="router_4"
        symmetrical="YES">
                <link_ctn id="4_2"/>
                <link_ctn id="bb_4"/>
</ASroute>
</AS>
\endverbatim

In the AS_4, you have an exitAS_4 defined, containing only one router,
and routes defined to that AS from all other AS (as cluster is only a
shortcut for an AS, see cluster description for details). If there was
an upper AS, it would define routes to and from AS_4 with the gateway
router_4. It's just because, as we did not allowed (for performances
issues) to have routes from an AS to a single host/router, you have to
enclose your gateway, when you have AS included in your AS, within an
AS to define routes to it.

\subsection pf_P2P_tags P2P or how to use coordinates
SimGrid allows you to use some coordinated-based system, like vivaldi,
to describe a platform. The main concept is that you have some peers
that are located somewhere: this is the function of the
<b>coordinates</b> of the \<peer\> or \<host\> tag. There's nothing
complicated in using it, here is an example of it:

\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">

<config id="General">
        <prop id="network/coordinates" value="yes"></prop>
</config>
 <AS  id="AS0"  routing="Vivaldi">
        <host id="100030591" coordinates="25.5 9.4 1.4" power="1500000000.0" />
        <host id="100036570" coordinates="-12.7 -9.9 2.1" power="730000000.0" />
        ...
        <host id="100429957" coordinates="17.5 6.7 18.8" power="830000000.0" />
        </AS>
</platform>
\endverbatim

Coordinates are then used to calculate latency between two hosts by
calculating the euclidean distance between the two hosts coordinates.
The results express the latency in ms.

Note that the previous example defines a routing directly between hosts but it could be also used to define a routing between AS.
That is for example what is commonly done when using peers (see Section \ref pf_peer).
\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">

<config id="General">
        <prop id="network/coordinates" value="yes"></prop>
</config>
 <AS  id="AS0"  routing="Vivaldi">
   <peer id="peer-0" coordinates="173.0 96.8 0.1" power="730Mf" bw_in="13.38MBps" bw_out="1.024MBps" lat="500us"/>
   <peer id="peer-1" coordinates="247.0 57.3 0.6" power="730Mf" bw_in="13.38MBps" bw_out="1.024MBps" lat="500us" />
   <peer id="peer-2" coordinates="243.4 58.8 1.4" power="730Mf" bw_in="13.38MBps" bw_out="1.024MBps" lat="500us" />
</AS>
</platform>
\endverbatim
In such a case though, we connect the AS created by the <b>peer</b> tag with the Vivaldi routing mechanism.
This means that to route between AS1 and AS2, it will use the coordinates of router_AS1 and router_AS2.
This is currently a convention and we may offer to change this convention in the DTD later if needed.
You may have noted that conveniently, a peer named FOO defines an AS named FOO and a router named router_FOO, which is why it works seamlessly with the <b>peer</b> tag.


\subsection pf_routing_howto_choose_wisely Choosing wisely the routing model to use


Choosing wisely the routing model to use can significantly fasten your
simulation/save your time when writing the platform/save tremendous
disk space. Here is the list of available model and their
characteristics (lookup : time to resolve a route):

\li <b>Full</b>: Full routing data (fast, large memory requirements,
    fully expressive)
\li <b>Floyd</b>: Floyd routing data (slow initialization, fast
    lookup, lesser memory requirements, shortest path routing only).
    Calculates all routes at once at the beginning.
\li <b>Dijkstra</b>: Dijkstra routing data (fast initialization, slow
    lookup, small memory requirements, shortest path routing only).
    Calculates a route when necessary.
\li <b>DijkstraCache</b>: Dijkstra routing data (fast initialization,
    fast lookup, small memory requirements, shortest path routing
    only). Same as Dijkstra, except it handles a cache for latest used
    routes.
\li <b>None</b>: No routing (usable with Constant network only).
    Defines that there is no routes, so if you try to determine a
    route without constant network within this AS, SimGrid will raise
    an exception.
\li <b>Vivaldi</b>: Vivaldi routing, so when you want to use coordinates
\li <b>Cluster</b>: Cluster routing, specific to cluster tag, should
    not be used.

\subsection pf_switch Hey, I want to describe a switch but there is no switch tag !

Actually we did not include switch tag, ok. But when you're trying to
simulate a switch, the only major impact it has when you're using
fluid model (and SimGrid uses fluid model unless you activate 
ns-3 or constant network mode) is the impact of the upper limit of
the switch motherboard speed that will eventually be reached if you're
using intensively your switch. So, the switch impact is similar to a
link one. That's why we are used to describe a switch using a link tag
(as a link is not an edge by a hyperedge, you can connect more than 2
other links to it).

\subsection pf_platform_multipath How to express multipath routing in platform files?

It is unfortunately impossible to express the fact that there is more
than one routing path between two given hosts. Let's consider the
following platform file:

\verbatim
<route src="A" dst="B">
   <link_ctn id="1"/>
</route>
<route src="B" dst="C">
  <link_ctn id="2"/>
</route>
<route src="A" dst="C">
  <link_ctn id="3"/>
</route>
\endverbatim

Although it is perfectly valid, it does not mean that data traveling
from A to C can either go directly (using link 3) or through B (using
links 1 and 2). It simply means that the routing on the graph is not
trivial, and that data do not following the shortest path in number of
hops on this graph. Another way to say it is that there is no implicit
in these routing descriptions. The system will only use the routes you
declare (such as &lt;route src="A" dst="C"&gt;&lt;link_ctn
id="3"/&gt;&lt;/route&gt;), without trying to build new routes by aggregating
the provided ones.

You are also free to declare platform where the routing is not
symmetric. For example, add the following to the previous file:

\verbatim
<route src="C" dst="A">
  <link_ctn id="2"/>
  <link_ctn id="1"/>
</route>
\endverbatim

This makes sure that data from C to A go through B where data from A
to C go directly. Don't worry about realism of such settings since
we've seen ways more weird situation in real settings (in fact, that's
the realism of very regular platforms which is questionable, but
that's another story).

\section pf_flexml_bypassing Bypassing the XML parser with your own C functions
<b>NOTE THAT THIS DOCUMENTATION, WHILE STILL WORKING, IS STRONGLY DEPRECATED</b>

So you want to bypass the XML files parser, uh? Maybe doing some parameter
sweep experiments on your simulations or so? This is possible, and
it's not even really difficult (well. Such a brutal idea could be
harder to implement). Here is how it goes.

For this, you have to first remember that the XML parsing in SimGrid is done
using a tool called FleXML. Given a DTD, this gives a flex-based parser. If
you want to bypass the parser, you need to provide some code mimicking what
it does and replacing it in its interactions with the SURF code. So, let's
have a look at these interactions.

FleXML parser are close to classical SAX parsers. It means that a
well-formed SimGrid platform XML file might result in the following
"events":

  - start "platform_description" with attribute version="2"
  - start "host" with attributes id="host1" power="1.0"
  - end "host"
  - start "host" with attributes id="host2" power="2.0"
  - end "host"
  - start "link" with ...
  - end "link"
  - start "route" with ...
  - start "link_ctn" with ...
  - end "link_ctn"
  - end "route"
  - end "platform_description"

The communication from the parser to the SURF code uses two means:
Attributes get copied into some global variables, and a surf-provided
function gets called by the parser for each event. For example, the event
  - start "host" with attributes id="host1" power="1.0"

let the parser do something roughly equivalent to:
\verbatim
  strcpy(A_host_id,"host1");
  A_host_power = 1.0;
  STag_host();
\endverbatim

In SURF, we attach callbacks to the different events by initializing the
pointer functions to some the right surf functions. Since there can be
more than one callback attached to the same event (if more than one
model is in use, for example), they are stored in a dynar. Example in
host_ptask_L07.c:
\verbatim
  /* Adding callback functions */
  surf_parse_reset_parser();
  surfxml_add_callback(STag_surfxml_host_cb_list, &parse_cpu_init);
  surfxml_add_callback(STag_surfxml_prop_cb_list, &parse_properties);
  surfxml_add_callback(STag_surfxml_link_cb_list, &parse_link_init);
  surfxml_add_callback(STag_surfxml_route_cb_list, &parse_route_set_endpoints);
  surfxml_add_callback(ETag_surfxml_link_c_ctn_cb_list, &parse_route_elem);
  surfxml_add_callback(ETag_surfxml_route_cb_list, &parse_route_set_route);

  /* Parse the file */
  surf_parse_open(file);
  xbt_assert(!surf_parse(), "Parse error in %s", file);
  surf_parse_close();
\endverbatim

So, to bypass the FleXML parser, you need to write your own version of the
surf_parse function, which should do the following:
   - Fill the A_<tag>_<attribute> variables with the wanted values
   - Call the corresponding STag_<tag>_fun function to simulate tag start
   - Call the corresponding ETag_<tag>_fun function to simulate tag end
   - (do the same for the next set of values, and loop)

Then, tell SimGrid that you want to use your own "parser" instead of the stock one:
\verbatim
  surf_parse = surf_parse_bypass_environment;
  MSG_create_environment(NULL);
  surf_parse = surf_parse_bypass_application;
  MSG_launch_application(NULL);
\endverbatim

A set of macros are provided at the end of
include/surf/surfxml_parse.h to ease the writing of the bypass
functions. An example of this trick is distributed in the file
examples/msg/masterslave/masterslave_bypass.c


*/