X-Git-Url: http://bilbo.iut-bm.univ-fcomte.fr/pub/gitweb/simgrid.git/blobdiff_plain/fbcf6ab31cae1988be858f9f894dafe529c575d7..889b415ad2d7c334222dfb9208bdffe2f5ab96ac:/docs/source/platform_howtos.rst

diff --git a/docs/source/platform_howtos.rst b/docs/source/platform_howtos.rst
index 5f1fe9a99c..0cd01b449e 100644
--- a/docs/source/platform_howtos.rst
+++ b/docs/source/platform_howtos.rst
@@ -1,8 +1,6 @@
-.. _platform:
-
 .. raw:: html
 
-   <object id="TOC" data="graphical-toc.svg" width="100%" type="image/svg+xml"></object>
+   <object id="TOC" data="graphical-toc.svg" type="image/svg+xml"></object>
    <script>
    window.onload=function() { // Wait for the SVG to be loaded before changing it
      var elem=document.querySelector("#TOC").contentDocument.getElementById("PlatformBox")
@@ -185,3 +183,60 @@ period and another one for the shutdown period.
 
 Of course, this is only one possible way to model these things. YMMV ;)
 
+.. _understanding_lv08
+
+Understanding the default TCP model
+***********************************
+When simulating a data transfer between two hosts, you may be surprised
+by the obtained simulation time. Lets consider the following platform:
+
+.. code-block:: xml
+
+   <host id="A" speed="1Gf" />
+   <host id="B" speed="1Gf" />
+
+   <link id="link1" latency="10ms" bandwidth="1Mbps" />
+
+   <route src="A" dst="B">
+     <link_ctn id="link1" />
+   </route>
+
+If host `A` sends `100kB` (a hundred kilobytes) to host `B`, one could expect
+that this communication would take `0.81` seconds to complete according to a
+simple latency-plus-size-divided-by-bandwidth model (0.01 + 8e5/1e6 = 0.81).
+However, the default TCP model of SimGrid is a bit more complex than that. It
+accounts for three phenomena that directly impact the simulation time even
+on such a simple example:
+
+  - The size of a message at the application level (i.e., 100kB in this
+    example) is not the size that will actually be transferred over the
+    network. To mimic the fact that TCP and IP headers are added to each packet of
+    the original payload, the TCP model of SimGrid empirically considers that
+    `only 97% of the nominal bandwidth` are available. In other words, the
+    size of your message is increased by a few percents, whatever this size be.
+
+  - In the real world, the TCP protocol is not able to fully exploit the
+    bandwidth of a link from the emission of the first packet. To reflect this
+    `slow start` phenomenon, the latency declared in the platform file is
+    multiplied by `a factor of 13.01`. Here again, this is an empirically
+    determined value that may not correspond to every TCP implementations on
+    every networks. It can be tuned when more realistic simulated times for
+    short messages are needed though.
+
+  - When data is transferred from A to B, some TCP ACK messages travel in the
+    opposite direction. To reflect the impact of this `cross-traffic`, SimGrid
+    simulates a flow from B to A that represents an additional bandwidth
+    consumption of `0.05`. The route from B to A is implicity declared in the
+    platfrom file and uses the same link `link1` as if the two hosts were
+    connected through a communication bus. The bandwidth share allocated to the
+    flow from A to B is then the available bandwidth of `link1` (i.e., 97% of
+    the nominal bandwidth of 1Mb/s) divided by 1.05 (i.e., the total consumption).
+    This feature, activated by default, can be disabled by adding the
+    `--cfg=network/crosstraffic:0` flag to command line.
+
+As a consequence, the time to transfer 100kB from A to B as simulated by the
+default TCP model of SimGrid is not 0.81 seconds but
+
+.. code-block:: python
+
+    0.01 * 13.01 + 800000 / ((0.97 * 1e6) / 1.05) =  0.996079 seconds.